Fundamentals of metallic corrosion

Preface

Corrosion is both costly and dangerous. Billions of dollars are spent annually
for the replacement of corroded structures, machinery, and components,
including metal roofing, condenser tubes, pipelines, and many other items.
In addition to replacement costs are those associated with preventive
maintenance to prevent corrosion, inspections, and the upkeep of
cathodically protected structures and pipelines. Indirect costs of corrosion
result from shutdown, loss of efficiency, and product contamination or loss.
Although the actual replacement cost of an item may not be high, the loss
of production resulting from the need to shut down an operation to permit the
replacement may amount to hundreds of dollars per hour. When a tank
or pipeline develops a leak, product is lost. If the leak goes undetected for
a period of time, the value of the lost product could be considerable. In addition,
contamination can result from the leaking material, requiring cleanup, and this
can be quite expensive. When corrosion takes place, corrosion products build
up, resulting in reduced flow in pipelines and reduced efficiency of heat
transfer in heat exchangers. Both conditions increase operating costs. Corrosion
products may also be detrimental to the quality of the product being handled,
making it necessary to discard valuable materials.
Premature failure of bridges or structures because of corrosion can also
result in human injury or even loss of life. Failures of operating equipment
resulting from corrosion can have the same disastrous results.
When all of these factors are considered, it becomes obvious why the
potential problem of corrosion should be considered during the early design
stages of any project, and why it is necessary to constantly monitor the
integrity of structures, bridges, machinery, and equipment to prevent
premature failures.
To cope with the potential problems of corrosion, it is necessary to
understand

1. Mechanisms of corrosion
2. Corrosion resistant properties of various materials
3. Proper fabrication and installation techniques
4. Methods to prevent or control corrosion
5. Corrosion testing techniques
6. Corrosion monitoring techniques

Corrosion is not only limited to metallic materials but also to all materials
of construction. Consequently, this handbook covers not only metallic
materials but also all materials of construction.

Chapter 1 and Chapter 2 cover the mechanisms of corrosion and the effects
of atmospheric corrosion.
Chapter 3 through Chapter 27 cover metallic materials and their alloys.
Corrosion’s potential is discussed for each metal or alloy. Charts are provided
for the compatibility of each metal or alloy with selected corrodents.
References are provided for additional compatibility data.
It is the intention of this book that regardless of what is being built,
whether it be a bridge, tower, pipeline, storage tank, or processing vessel,
information for the designer/engineer/maintenance personnel/or whoever
is responsible for the selection of construction material, this book will enable
them to avoid unnecessary loss of material through corrosion.
Philip A. Schweitzer

Author

Philip A. Schweitzer is a consultant in corrosion prevention, materials of
construction, and chemical engineering based in York, Pennsylvania. A
former contract manager and material specialist for Chem-Pro Corporation,
Fairﬁeld, New Jersey, he is the editor of the Corrosion Engineering Handbook
and the Corrosion and Corrosion Protection Handbook, Second Edition; and the
author of Corrosion Resistance Tables, Fifth Edition; Encyclopedia of Corrosion
Technology, Second Edition; Metallic Materials; Corrosion Resistant Linings and
Coatings; Atmospheric Degradation and Corrosion Control; What Every Engineer
Should Know About Corrosion; Corrosion Resistance of Elastomers; Corrosion
Resistant Piping Systems; Mechanical and Corrosion Resistant Properties of
Plastics and Elastomers (all titles Marcel Dekker, Inc.); and Paint and Coatings,
Applications and Corrosion Resistance (Taylor & Francis). Schweitzer received
the BChE degree (1950) from Polytechnic University (formerly Polytechnic
Institute of Brooklyn), Brooklyn, New York.

Contents

Chapter 1 Fundamentals of Metallic Corrosion ......................................... 1
1.1 Forms of Corrosion...................................................................................... 2
1.1.1 Uniform Corrosion ......................................................................... 3
1.1.1.1 Passive Film on Iron......................................................... 3
1.1.1.2 Passive Film on Nickel..................................................... 4
1.1.1.3 Passive Film on Austenitic Stainless Steel.................... 4
1.1.1.4 Passive Film on Copper................................................... 4
1.1.1.5 Passive Film on Aluminum ............................................ 5
1.1.1.6 Passive Film on Titanium ................................................ 5
1.1.1.7 Passive Film on Tantalum ............................................... 5
1.1.1.8 Uniform Corrosion Rates................................................. 5
1.1.2 Intergranular Corrosion ................................................................. 7
1.1.3 Galvanic Corrosion ......................................................................... 8
1.1.4 Crevice Corrosion ......................................................................... 10
1.1.5 Pitting Corrosion........................................................................... 12
1.1.6 Erosion Corrosion ......................................................................... 15
1.1.7 Stress Corrosion Cracking (SCC) ............................................... 16
1.1.8 Biological Corrosion ..................................................................... 18
1.1.8.1 Corrosion of Speciﬁc Materials..................................... 21
1.1.9 Selective Leaching......................................................................... 23
1.2 Corrosion Mechanisms ............................................................................. 24
1.3 Measuring Polarization............................................................................. 31
1.3.1 Anodic Polarization...................................................................... 34
1.4 Other Factors Affecting Corrosion .......................................................... 35
Reference .............................................................................................................. 37

Chapter 2 Atmospheric Corrosion............................................................... 39
2.1 Atmospheric Types .................................................................................... 40
2.2 Factors Affecting Atmospheric Corrosion ............................................. 41
2.2.1 Time of Wetness ............................................................................ 42
2.2.1.1 Adsorption Layers .......................................................... 43
2.2.1.2 Phase Layers .................................................................... 43
2.2.1.3 Dew ................................................................................... 43
2.2.1.4 Rain ................................................................................... 43
2.2.1.5 Fog ..................................................................................... 44
2.2.1.6 Dust ................................................................................... 44
2.2.1.7 Measurement of Time of Wetness ................................ 44
2.2.2 Composition of Surface Electrolyte ........................................... 45
2.2.2.1 Oxygen.............................................................................. 45

2.2.2.2 SOX .................................................................................... 45
2.2.2.3 NOX ................................................................................... 45
2.2.2.4 Chlorides .......................................................................... 45
2.2.2.5 CO2 .................................................................................... 46
2.2.2.6 Concentrations of Different Species............................. 46
2.2.3 Temperature................................................................................... 46
2.2.4 Initial Exposure ............................................................................. 47
2.2.5 Sheltering........................................................................................ 47
2.2.6 Wind Velocity ................................................................................ 47
2.2.7 Nature of Corrosion Products .................................................... 47
2.2.8 Pollutants Present ......................................................................... 48
2.3 Mechanisms of Atmospheric Corrosion of Metals............................... 49
2.3.1 Damp Atmospheric Corrosion
(Adsorption Layers) ..................................................................... 52
2.3.2 Wet Atmospheric Corrosion (Phase Layers) ............................ 54
2.3.2.1 Dew ................................................................................... 54
2.3.2.2 Rain ................................................................................... 54
2.3.2.3 Fog ..................................................................................... 55
2.3.3 Deposit of Pollutants .................................................................... 55
2.4 Corrosion Products.................................................................................... 56
2.5 Speciﬁc Atmospheric Corrodents ........................................................... 58
2.5.1 Sulfur-Containing Compounds .................................................. 59
2.5.2 Nitrogen-Containing Compounds ............................................. 61
2.5.3 Chlorine-Containing Compounds.............................................. 62
2.5.4 Carbon Dioxide (CO2).................................................................. 62
2.5.5 Oxygen (O2) ................................................................................... 62
2.5.6 Indoor Atmospheric Compounds .............................................. 63
2.6 Summary ..................................................................................................... 63
2.7 Effects on Metals Used for Outdoor Applications ............................... 63
2.7.1 Carbon Steel................................................................................... 63
2.7.2 Weathering Steels.......................................................................... 64
2.7.3 Zinc.................................................................................................. 65
2.7.4 Aluminum...................................................................................... 65
2.7.5 Copper ............................................................................................ 65
2.7.6 Nickel 200....................................................................................... 66
2.7.7 Monel Alloy 400 ............................................................................ 66
2.7.8 Inconel Alloy 600 .......................................................................... 66
Reference .............................................................................................................. 66

Chapter 3 Corrosion of Carbon and Low-Alloy Steels........................... 67
3.1 Corrosion Data ........................................................................................... 67
3.2 Stress Corrosion Cracking ........................................................................ 78
3.3 Sulﬁde Stress Cracking ............................................................................. 78
3.4 Pitting........................................................................................................... 79
3.5 Hydrogen Damage .................................................................................... 79

3.5.1 Hydrogen Blistering ..................................................................... 80
3.5.2 Hydrogen Embrittlement ............................................................ 80
3.5.3 Decarburization............................................................................. 80
3.5.4 Hydrogen Attack .......................................................................... 80
3.6 Corrosion Fatigue ...................................................................................... 81
3.7 Microbiologically Inﬂuenced Corrosion ................................................ 81
Reference .............................................................................................................. 82

Chapter 4 Corrosion of Cast Iron and Cast Steel..................................... 83
4.1 Cast Irons .................................................................................................... 86
4.1.1 Gray Iron ........................................................................................ 86
4.1.2 Compacted Graphite Iron............................................................ 87
4.1.3 Ductile (Nodular) Iron ................................................................. 87
4.1.4 White Iron ...................................................................................... 88
4.1.5 Malleable Iron................................................................................ 88
4.2 High Alloy Cast Irons ............................................................................... 88
4.2.1 Austenitic Gray Cast Irons .......................................................... 88
4.2.2 Austenitic Ductile Cast Irons ...................................................... 89
4.2.3 High-Silicon Cast Irons ................................................................ 89
4.3 Carbon and Low-Alloy Carbon Steels ................................................... 96
References ............................................................................................................ 97

Chapter 5 Introduction to Stainless Steel.................................................. 99
5.1 Stainless Steel Classiﬁcation..................................................................... 99
5.1.1 Ferritic Family ............................................................................. 100
5.1.2 Martensitic Family ...................................................................... 102
5.1.3 Austenitic Family........................................................................ 102
5.1.4 Precipitation-Hardenable Stainless Steels............................... 103
5.1.5 Superferritic Stainless Steels ..................................................... 104
5.1.6 Duplex Stainless Steels............................................................... 104
5.1.7 Superaustenitic Stainless Steels ................................................ 105
5.2 Passivation ................................................................................................ 105
5.3 Sanitizing................................................................................................... 106
5.4 Preparing for Service............................................................................... 106
5.4.1 Iron Contamination .................................................................... 106
5.4.2 Organic Contamination.............................................................. 107
5.4.3 Welding Contamination............................................................. 107

Chapter 6 Corrosion of Stainless Steels................................................... 109
6.1 Pitting......................................................................................................... 111
6.2 Crevice Corrosion .................................................................................... 112
6.3 Stress Corrosion Cracking ...................................................................... 112
6.4 Intergranular Corrosion .......................................................................... 114
6.5 High-Temperature Corrosion................................................................. 116

6.6 Corrosion Fatigue .................................................................................... 122
6.7 Uniform Corrosion .................................................................................. 122

Chapter 7 Ferritic Stainless Steel Family................................................. 123
7.1 Type 405 (S40500)..................................................................................... 126
7.2 Type 409 (S40900)..................................................................................... 127
7.3 Type 430 (S43000)..................................................................................... 127
7.4 Type 439L (S43035) .................................................................................. 128
7.5 Type 444 (S44400)..................................................................................... 128
7.6 Type 446 (S44600)..................................................................................... 132
Reference ............................................................................................................ 132

Chapter 8 Superferritic Stainless Steel Family ...................................... 133
8.1 Type XM-27 (S44627) ............................................................................... 134
8.2 Alloy S44660 (Sea-Cure) ......................................................................... 134
8.3 Alloy S44735 (29-4C) ............................................................................... 136
8.4 Alloy S44800 (29-4-2)............................................................................... 136
8.5 Alloy S44700 (29-4) .................................................................................. 137
Reference ............................................................................................................ 137

Chapter 9 Martensitic Stainless Steel Family......................................... 139
9.1 Type 410 (S41000)..................................................................................... 139
9.2 Type 414 (S41400)..................................................................................... 144
9.3 Type 416 (S41600)..................................................................................... 144
9.4 Type 420 (S42000)..................................................................................... 145
9.5 Type 422 (S42200)..................................................................................... 146
9.6 Type 431 (S43100)..................................................................................... 147
9.7 Type 440A (S44002).................................................................................. 147
9.8 Type 440B (S44003) .................................................................................. 148
9.9 Type 440C (S44004).................................................................................. 148
9.10 Alloy 440-XH ............................................................................................ 149
9.11 13Cr-4N (F6NM) ...................................................................................... 149
Reference ............................................................................................................ 149

Chapter 10 Austenitic Stainless Steel Family......................................... 151
10.1 Type 201 (S20100)................................................................................... 155
10.2 Type 202 (S20200)................................................................................... 156
10.3 Type 22-13-5 (S20910) ............................................................................ 156
10.4 Type 216L (S21603) ................................................................................ 157
10.5 Type 301 (S30100)................................................................................... 158
10.6 Type 302 (S30200)................................................................................... 158
10.7 Type 303 (S30300)................................................................................... 158
10.8 Type 304 (S30400)................................................................................... 158
10.9 Type 305 (S30500)................................................................................... 159

10.10 Type 308 (S30800)................................................................................... 159
10.11 Type 309 (S30900) ................................................................................... 159
10.12 Type 310 (S31000)................................................................................... 164
10.13 Type 316 (S31600)................................................................................... 164
10.14 Type 317 (S31700)................................................................................... 169
10.15 Type 321 (S32100)................................................................................... 172
10.16 Type 329 (S32900)................................................................................... 174
10.17 Type 347 (S34700)................................................................................... 175
10.18 Type 348 (S34800)................................................................................... 175
Reference ............................................................................................................ 176

Chapter 11 Superaustenitic Family of Stainless Steel ......................... 177
11.1 Alloy 20Cb3 (N08020) ........................................................................... 180
11.2 Alloy 20Mo-4 (N08024) ......................................................................... 185
11.3 Alloy 20Mo-6 (N08026) ......................................................................... 185
11.4 Alloy 904L (N08904) .............................................................................. 186
11.5 Alloy 800 (N08800) ................................................................................ 186
11.6 Alloy 825 (N08825) ................................................................................ 187
11.7 Type 330 (N08330).................................................................................. 190
11.8 Al-6XN (N08367) .................................................................................... 191
11.9 Alloy 254SMo (S31254).......................................................................... 192
11.10 Alloy 25-6Mo (N08926)......................................................................... 193
11.11 Alloy 31 (N08031) .................................................................................. 194
11.12 Alloy 654SMo (S32654) ......................................................................... 194
11.13 Inconel Alloy 686 (N06686).................................................................. 195
Reference ............................................................................................................ 195

Chapter 12 Duplex Stainless Steel Family .............................................. 197
12.1 Alloy 2205 (S31803)................................................................................ 200
12.2 7-MoPlus (S32950).................................................................................. 201
12.3 Zeron 100 (S32760)................................................................................. 202
12.4 Ferralium 255 (S32550).......................................................................... 203

Chapter 13 Precipitation-Hardening Stainless Steel Family ............... 205
13.1 Alloy PH13-8Mo (S13800) .................................................................... 207
13.2 Alloy 15-5PH (S15500) .......................................................................... 207
13.3 Alloy 17-4PH (S17400) .......................................................................... 208
13.4 Alloy 17-7PH (S17700) .......................................................................... 209
13.5 Alloy 350 (S35000).................................................................................. 212
13.6 Alloy 355 (S35500).................................................................................. 212
13.7 Custom 450 (S45000) ............................................................................. 213
13.8 Custom 455 (S45500) ............................................................................. 214
13.9 Alloy 718 (N07718) ................................................................................ 214
13.10 Alloy A286 (S66286) .............................................................................. 215

13.11 Alloy X-750 (N07750) ............................................................................ 215
13.12 Pyromet Alloy 31................................................................................... 216
13.13 Pyromet Alloy CTX-1............................................................................ 217
13.14 Pyromet Alloy CTX-3............................................................................ 218
13.15 Pyromet Alloy CTX-909........................................................................ 218
13.16 Pyromet Alloy V-57............................................................................... 219
13.17 Thermospan Alloy................................................................................. 220
References .......................................................................................................... 220

Chapter 14 Cast Stainless Steel Alloys .................................................... 221
14.1 Martensitic Stainless Steels................................................................... 224
14.2 Ferritic Stainless Steels .......................................................................... 225
14.3 Austenitic Stainless Steels .................................................................... 226
14.4 Superaustenitic Stainless Steels ........................................................... 229
14.5 Precipitation-Hardening Stainless Steels ........................................... 231
14.6 Duplex Stainless Steels.......................................................................... 231
References .......................................................................................................... 233

Chapter 15 Nickel and High-Nickel Alloys ............................................ 235
15.1 Nickel 200 and Nickel 201.................................................................... 237
15.2 Monel Alloy 400 (N04400).................................................................... 243
15.3 Alloy B-2.................................................................................................. 245
15.4 Alloy 625 (N06625) ................................................................................ 252
15.5 Custom Age 625 Plus (N07716)........................................................... 257
15.6 Alloy C-276 (N10276) ............................................................................ 262
15.7 Alloy C-4 (N06455) ................................................................................ 263
15.8 Alloy C-22 (N06022) .............................................................................. 264
15.9 Hastelloy Alloy C-2000 ......................................................................... 265
15.10 Alloy X (N06002).................................................................................... 267
15.11 Alloy 600 (N06600) ................................................................................ 268
15.12 Alloy G (N06007) and Alloy G-3 (N06985) ....................................... 269
15.13 Alloy G-30 (N06030) .............................................................................. 270
15.14 Alloy H-9M ............................................................................................. 272
15.15 Alloys for High-Temperature Corrosion............................................ 272
15.15.1 Hastelloy Alloy S................................................................... 273
15.15.2 Haynes Alloy 556 (R30556).................................................. 273
15.15.3 Alloy 214................................................................................. 274
15.15.4 Alloy 230 (N06230)................................................................ 275
15.15.5 Alloy RA333 (N06333) .......................................................... 276
15.15.6 Alloy 102 (N06102)................................................................ 277
Reference ............................................................................................................ 277

Chapter 16 Cast Nickel and Nickel-Based Alloys ................................. 279
16.1 Commercially Pure Nickel ................................................................... 279
16.2 Nickel–Copper........................................................................................ 279

16.3 Nickel–Chromium ................................................................................. 281
16.4 Nickel–Chromium–Molybdenum ....................................................... 281
16.5 Other Nickel-Based Alloys................................................................... 282
References .......................................................................................................... 282

Chapter 17 Comparative Corrosion Resistance of Stainless
Steel and High-Nickel Alloys ............................................... 283

Chapter 18 Copper and Copper Alloys .................................................... 469
18.1 Coppers ................................................................................................... 472
18.2 High-Copper Alloys .............................................................................. 475
18.3 Copper–Zinc Alloys (Brasses).............................................................. 475
18.4 Copper–Tin Alloys................................................................................. 483
18.5 Copper–Aluminum Alloys................................................................... 483
18.6 Copper–Nickel Alloys ........................................................................... 485
18.7 Copper–Beryllium Alloys..................................................................... 488
18.8 Cast Copper Alloys ............................................................................... 488
18.8.1 Corrosion Resistance............................................................... 488
References .......................................................................................................... 490

Chapter 19 Aluminum and Aluminum Alloys....................................... 491
19.1 Classiﬁcations and Designations......................................................... 492
19.2 Temper Designations............................................................................. 493
19.3 Strain-Hardened Subdivisions............................................................. 494
19.3.1 H1X—Strain-Hardened Only ................................................ 494
19.3.2 H2X—Strain-Hardened and Partially Annealed................ 494
19.3.3 H3X—Strain-Hardened and Stabilized................................ 494
19.4 Heat-Treated Subdivisions ................................................................... 494
19.5 Chemical Composition.......................................................................... 495
19.6 General Corrosion Resistance .............................................................. 499
19.7 Pitting Corrosion.................................................................................... 500
19.8 Intergranular Corrosion ........................................................................ 506
19.8.1 Mechanism of Intergranular Corrosion
in 2XXX Alloys ........................................................................ 506
19.8.2 Mechanism of Intergranular Corrosion
in 7XXX Alloys ........................................................................ 508
19.9 Exfoliation Corrosion ............................................................................ 509
19.10 Stress Corrosion Cracking .................................................................... 509
19.11 Filiform Corrosion.................................................................................. 510
19.12 Crevice Corrosion .................................................................................. 510
19.13 Poultice Corrosion ................................................................................. 511
19.14 Galvanic Relations ................................................................................. 511
19.15 Reduction of Ions of Other Metals by Aluminum ........................... 512
19.16 Weathering .............................................................................................. 514

19.17 Waters (General)..................................................................................... 514
19.18 Relative Resistance of Aluminum and Alloys .................................. 514
19.19 Atmospheric Weathering...................................................................... 515
19.19.1 Seacoast Atmosphere............................................................ 515
19.19.2 Urban or Industrial Atmospheres ...................................... 516
19.19.3 Rural Atmosphere ................................................................. 517
19.19.4 Indoor Atmosphere............................................................... 517
19.20 Waters (Specific) .................................................................................... 518
19.20.1 Freshwaters............................................................................ 518
19.20.2 Seawater ................................................................................. 519
19.20.3 Piping Applications.............................................................. 519
19.21 Alclad Products ..................................................................................... 520
19.22 Cast Aluminum ..................................................................................... 520
References .......................................................................................................... 522

Chapter 20 Titanium..................................................................................... 525
20.1 Alloys ....................................................................................................... 526
20.2 Types of Corrosion................................................................................. 528
20.2.1 General Corrosion ................................................................... 529
20.2.2 Galvanic Corrosion ................................................................. 529
20.2.3 Hydrogen Embrittlement....................................................... 529
20.2.4 Crevice Corrosion.................................................................... 534
20.2.5 Stress Corrosion Cracking ..................................................... 536
20.3 Corrosion Resistance ............................................................................. 536
References .......................................................................................................... 538

Chapter 21 Tantalum .................................................................................... 539
21.1 The Oxide Film—A Protective Barrier ............................................... 540
21.2 Effect of Specific Corrosive Agents..................................................... 542
21.2.1 Water ......................................................................................... 542
21.2.2 Acids.......................................................................................... 542
21.2.2.1 Sulfuric Acid ............................................................ 545
21.2.2.2 Phosphoric Acid ...................................................... 545
21.2.2.3 Hydrochloric Acid .................................................. 546
21.2.2.4 Nitric Acid................................................................ 547
21.2.2.5 Hydrofluoric Acid ................................................... 547
21.2.2.6 Acid Mixtures and Other Acids ........................... 547
21.2.3 Alkali Salts, Organics, and Other Media............................. 548
21.2.4 Gases.......................................................................................... 549
21.2.4.1 Oxygen and Air ....................................................... 549
21.2.4.2 Nitrogen .................................................................... 550
21.2.4.3 Hydrogen.................................................................. 551
21.2.4.4 Halogens ................................................................... 554
21.2.4.5 Carbon Monoxide and Carbon Dioxide .............. 554
21.2.4.6 Nitrogen Monoxide and Nitrous Oxide .............. 554
21.2.4.7 Other Gases .............................................................. 554

21.2.5 Liquid Metals ........................................................................... 555
21.2.5.1 Aluminum ................................................................ 556
21.2.5.2 Antimony.................................................................. 556
21.2.5.3 Bismuth..................................................................... 556
21.2.5.4 Calcium..................................................................... 556
21.2.5.5 Cesium ...................................................................... 556
21.2.5.6 Gallium ..................................................................... 556
21.2.5.7 Lead ........................................................................... 556
21.2.5.8 Lithium ..................................................................... 556
21.2.5.9 Magnesium and Magnesium Alloys.................... 557
21.2.5.10 Mercury .................................................................... 557
21.2.5.11 Potassium ................................................................. 557
21.2.5.12 Silver ......................................................................... 557
21.2.5.13 Sodium ..................................................................... 557
21.2.5.14 Tellurium.................................................................. 558
21.2.5.15 Thorium–Magnesium............................................. 558
21.2.5.16 Uranium and Plutonium Alloys .......................... 558
21.2.5.17 Zinc ........................................................................... 558
21.2.6 General Corrosion Data.......................................................... 558
21.3 Corrosion Resistance of Tantalum-Based Alloys.............................. 561
21.3.1 Tantalum–Tungsten Alloys .................................................... 563
21.3.2 Tantalum–Molybdenum Alloys ............................................ 566
21.3.3 Tantalum–Niobium Alloys .................................................... 566
21.3.4 Tantalum–Titanium Alloys .................................................... 567
21.3.5 Other Alloys ............................................................................. 568
References .......................................................................................................... 568

Chapter 22 Zirconium .................................................................................. 571
22.1 Introduction ............................................................................................ 571
22.2 General Characteristics ......................................................................... 573
22.2.1 Physical Properties.................................................................. 574
22.2.2 Mechanical Properties ............................................................ 574
22.2.3 Chemical and Corrosion Properties..................................... 577
22.2.3.1 Water and Steam..................................................... 580
22.2.3.2 Salt Water ................................................................. 581
22.2.3.3 Halogen Acids......................................................... 582
22.2.3.4 Nitric Acid ............................................................... 586
22.2.3.5 Sulfuric Acid............................................................ 588
22.2.3.6 Phosphoric Acid ..................................................... 591
22.2.3.7 Other Acids.............................................................. 594
22.2.3.8 Alkalies..................................................................... 594
22.2.3.9 Salt Solutions ........................................................... 594
22.2.3.10 Organic Solutions ................................................... 596
22.2.3.11 Gases......................................................................... 597
22.2.3.12 Molten Salts and Metals........................................ 598

22.2.4Selected Corrosion Topics...................................................... 598
22.2.4.1 Pitting ....................................................................... 598
22.2.4.2 Stress Corrosion Cracking ..................................... 599
22.2.4.3 Fretting Corrosion .................................................. 600
22.2.4.4 Galvanic Corrosion................................................. 600
22.2.4.5 Crevice Corrosion ................................................... 601
22.2.5 Corrosion Protection............................................................... 601
22.2.5.1 Oxide Film Formation............................................ 601
22.2.5.1.1 Anodizing............................................ 601
22.2.5.1.2 Autoclave Film Formation ............... 602
22.2.5.1.3 Film Formation in Air or Oxygen ... 602
22.2.5.1.4 Film Formation in Molten Salts....... 602
22.2.5.2 Electrochemical Protection .................................... 603
22.2.5.3 Others ....................................................................... 604
22.3 Typical Applications.............................................................................. 605
22.3.1 Nuclear Industry ..................................................................... 605
22.3.2 Chemical Processing and Other Industries ........................ 606
22.3.2.1 Urea........................................................................... 607
22.3.2.2 Acetic Acid .............................................................. 608
22.3.2.3 Formic Acid ............................................................. 608
22.3.2.4 Sulfuric Acid-Containing Processes .................... 609
22.3.2.5 Halide-Containing Processes ................................ 612
22.3.2.6 Nitric Acid-Containing Processes ........................ 613
22.3.2.7 Others ....................................................................... 614
22.4 Zirconium Products............................................................................... 616
22.5 Health and Safety .................................................................................. 616
22.6 Concluding Remarks............................................................................. 617
References .......................................................................................................... 617

Chapter 23 Zinc and Zinc Alloys............................................................... 623
23.1 Corrosion of Zinc ................................................................................... 623
23.1.1 White Rust (Wet-Storage Stain)............................................. 623
23.1.2 Bimetallic Corrosion ............................................................... 624
23.1.3 Intergranular Corrosion ......................................................... 625
23.1.4 Corrosion Fatigue.................................................................... 625
23.1.5 Stress Corrosion....................................................................... 625
23.2 Zinc Coatings.......................................................................................... 626
23.2.1 Principle of Protection ............................................................ 626
23.3 Zinc Coatings.......................................................................................... 630
23.3.1 Hot Dipping ............................................................................. 630
23.3.2 Zinc Electroplating.................................................................. 631
23.3.3 Mechanical Coating ................................................................ 631
23.3.4 Sheradizing............................................................................... 632
23.3.5 Thermally Sprayed Coatings................................................. 632
23.4 Corrosion of Zinc Coatings .................................................................. 632
23.5 Zinc Alloys.............................................................................................. 637

23.5.1 Zinc–5% Aluminum Hot-Dip Coatings............................... 637
23.5.2 Zinc–55% Aluminum Hot-Dip Coatings............................. 639
23.5.3 Zinc–15% Aluminum Thermal Spray .................................. 640
23.5.4 Zinc–Iron Alloy Coating ........................................................ 641
23.6 Cast Zinc.................................................................................................. 643

Chapter 24 Niobium (Columbian) and Niobium Alloys ..................... 645
24.2 Niobium–Titanium Alloys.................................................................... 648
24.3 WC-103 Alloy ......................................................................................... 649
24.4 WC-1Zr Alloy ......................................................................................... 649
24.5 General Alloy Information ................................................................... 649

Chapter 25 Lead and Lead Alloys ............................................................. 651
Reference ............................................................................................................ 654

Chapter 26 Magnesium Alloys................................................................... 655

Chapter 27 Comparative Corrosion Resistance of Nonferrous
Metals and Alloys.................................................................... 657
Reference ............................................................................................................ 721

Index ................................................................................................................... 723

1
Fundamentals of Metallic Corrosion

There are three primary reasons for concern about and the study of
corrosion—safety, economics, and conservation. Premature failure of bridges
or structures due to corrosion can also result in human injury or even loss of
life. Failure of operating equipment can have the same disastrous results.
Several years ago, the National Institute of Standards and Technology
(formerly the National Bureau of Standards) estimated that the annual cost
of corrosion in the United States was in the range of $9 billion to $90 billion.
These figures were confirmed by various technical organizations, including
the National Association of Corrosion Engineers.
Included in this estimate was corrosion attributed to chemical processes;
corrosion of highways and bridges from deicing chemicals; atmospheric
corrosion of steel fences; atmospheric corrosion of various outdoor
structures such as buildings, bridges, towers, automobiles, and ships; and
innumerable other applications exposed to the atmospheric environment. It
has been further estimated that the cost of protection against atmospheric
corrosion is approximately 50% of the total cost of all corrosion-protection
methods.
Corrosion is the degradation of a material’s properties or mass over time
due to environmental effects. It is the natural tendency of a material’s
compositional elements to return to their most thermodynamically stable
state. For most metallic materials, this means the formation of oxides or
sulfides, or other basic metallic compounds generally considered to be ores.
Fortunately, the rate at which most of these processes progress is slow
enough to provide useful building materials. Only inert atmospheres and
vacuums can be considered free of corrosion for most metallic materials.
Under normal circumstances, iron and steel corrode in the presence of
both oxygen and water. If either of these materials is absent, corrosion
usually will not take place. Rapid corrosion may take place in water, in
which the rate of corrosion is increased by the acidity or velocity of the water,
by the motion of the metal, by an increase in the temperature or aeration, by
the presence of certain bacteria, or by other less prevalent factors.
Conversely, corrosion is generally retarded by films (or protective layers)

1

2 Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals

consisting of corrosion products or adsorbed oxygen; high alkalinity of the
water also reduces the rate of corrosion on steel surfaces. The amount of
corrosion is controlled by either water or oxygen, which are essential for the
process to take place. For example, steel will not corrode in dry air and
corrosion is negligible when the relative humidity of the air is below 30% at
normal or reduced temperatures. Prevention of corrosion by dehumidiﬁca-
tion is based on this.
All structural metals corrode to some degree in natural environments.
However, bronzes, brasses, zinc, stainless steels, and aluminum corrode so
slowly under the condition in which they are placed that they are expected to
survive for long periods of time without protection.
These corrosion processes follow the basic laws of thermodynamics.
Corrosion is an electrochemical process. Under controlled conditions it can
be measured, repeated, and predicted. Since it is governed by reactions on an
atomic level, corrosion processes can act on isolated regions, uniform surface
areas, or result in subsurface microscopic damage. Complicate these forms
of corrosion with further subdivisions, add just basic environmental
variables such as pH, temperature, and stress, and the predictability of
corrosion begins to suffer rapidly.

1.1 Forms of Corrosion
There are nine basic forms of corrosion that metallic materials may be
subject to:

1. Uniform corrosion
2. Intergranular corrosion
3. Galvanic corrosion
4. Crevice corrosion
5. Pitting
6. Erosion corrosion
7. Stress corrosion cracking
8. Biological corrosion
9. Selective leaching

In addition, there are other forms of corrosion that speciﬁc metals or
alloys are subject to. Prevention or control of corrosion can usually be
achieved by use of a suitable material of construction, use of proper design
and installation techniques, and by following in-plant procedures, or a
combination of these.

Fundamentals of Metallic Corrosion 3

1.1.1 Uniform Corrosion
Although other forms of attack must be considered in special circumstances,
uniform attack is one form most commonly confronting the user of metals
and alloys. Uniform or general corrosion, which is the simplest form of
corrosion, is an even rate of metal loss over the exposed surface. It is
generally thought of as metal loss due to chemical attack or dissolution of the
metallic component into metallic ions. In high-temperature situations,
uniform metal loss is usually preceded by its combination with another
element rather than its oxidation to a metallic ion. Combination with oxygen
to form metallic oxides, or scale, results in the loss of material in its useful
engineering form; scale ultimately flakes off to return to nature.
A metal resists corrosion by forming a passive film on the surface. This
film is naturally formed when the metal is exposed to the air for a period of
time. It can also be formed more quickly by chemical treatment. For example,
nitric acid, if applied to austenitic stainless steel, will form this protective
film. Such a film is actually a form of corrosion, but once formed it prevents
further degradation of the metal, provided that the film remains intact. It
does not provide an overall resistance to corrosion because it may be subject
to chemical attack. The immunity of the film to attack is a function of the film
composition, temperature, and the aggressiveness of the chemical. Examples
of such films are the patina formed on copper, the rusting of iron, the tarni-
shing of silver, the fogging of nickel, and the high-temperature oxidation
of metals.
There are two theories regarding the formation of these films. The first
theory states that the film formed is a metal oxide or other reaction
compound. This is known as the oxide film theory. The second theory states
that oxygen is adsorbed on the surface, forming a chemisorbed film.
However, all chemisorbed films react over a period of time with the
underlying metal to form metal oxides. Oxide films are formed at room
temperature. Metal oxides can be classified as network formers, intermedi-
ates, or modifiers. This division can be related to thin oxide films on metals.
The metals that fall into network-forming or intermediate classes tend to
grow protective oxides that support anion or mixed anion/cation move-
ment. The network formers are noncrystalline, whereas the intermediates
tend to be microcystalline at low temperatures.

1.1.1.1 Passive Film on Iron
Iron in iron oxides can assume a valence of two or three. The former acts as a
modifier and the latter is a network former. The iron is protected from the
corrosion environment by a thin oxide film l–4 mm in thickness with a
pffiffiffiffiffiffiffiffiffiffiffiffiffi
composition of Fe2 O3 =Fe3 O4 . This is the same type of film formed by the
reaction of clean iron with oxygen or dry air. The Fe2 O3 layer is responsible
for the passivity, while the Fe3O4 provides the basis for the formation of a
higher oxidizing state. Iron is more difficult to passivate than nickel, because


with iron it is not possible to go directly to the passivation species Fe2 O3 .
Instead, a lower oxidation state of Fe3O4 is required, and this film is highly
susceptible to chemical dissolution. The Fe2 O3 layer will not form until the
Fe3O4 phase has existed on the surface for a reasonable period of time.
During this time, Fe3O4 layer continues to form.

1.1.1.2 Passive Film on Nickel
The passive film on nickel can be formed quite readily in contrast to the
formation of the passive film on iron. Differences in the nature of the oxide
film on iron and nickel are responsible for this phenomenom. The film
thickness on nickel is between 0.9 and 1.2 mm, whereas the iron oxide film is
between 1 and 4 mm. There are two theories as to what the passive film on
nickel is. It is entirely NiO with a small amount of nonstoichiometry, giving
rise to Ni3C cation vacancies, or it consists of an inner layer of NiO and an
outer layer of anhydrous Ni(OH)2. The passive oxide film on nickel, once
formed, cannot be easily removed by either cathodic treatment or chemical
dissolution.
The passive film on nickel will not protect the nickel from corrosive attack
in oxidizing atmospheres such as nitric acid. When alloyed with chromium,
a much-improved stable film results, producing a greater corrosion
resistance to a variety of oxidizing media. However, these alloys are subject
to attack in environments containing chlorides or other halides, especially if
oxidizing agents are present. Corrosion will be in the form of pitting. The
addition of molybdenum or tungsten will improve the corrosion resistance.

1.1.1.3 Passive Film on Austenitic Stainless Steel
The passive film formed on austenitic stainless steel is duplex in nature,
consisting of an inner barrier oxide film and an outer deposit of hydroxide or
salt film. Passivation takes place by the rapid formation of surface-absorbed
hydrated complexes of metals that are sufficiently stable on the alloy surface
that further reaction with water enables the formation of a hydroxide phase
that rapidly deprotonates to form an insoluble surface oxide film. The three
most commonly used austenite stabilizers—nickel, manganese, and nitro-
gen—all contribute to the passivity. Chromium, a major alloying ingredient, is
in itself very corrosion resistant and is found in greater abundance in the
passive film than iron, which is the major element in the alloy.

1.1.1.4 Passive Film on Copper
When exposed to the atmosphere over long periods of time, copper will form
a coloration on the surface known as patina; in reality, the coloration is a
corrosion product that acts as a protective film against further corrosion.
When first formed, the patina exhibits a dark color that gradually turns green.
The length of time required to form the patina depends upon the atmosphere,


because the coloration is given by copper hydroxide compounds. In a marine
atmosphere, the compound is a mixture of copper/hydroxide/chloride; in
industrial atmospheres, it is copper/hydroxide/sulfate. These compounds
will form in approximately 7 years. When exposed in a clean rural atmo-
sphere, tens or hundreds of years may be required to form the patina.

1.1.1.5 Passive Film on Aluminum
Aluminum forms a thin, compact, and adherent oxide film on the surface
that limits further corrosion. When formed in air at atmospheric
temperatures it is approximately 5 mm thick. If formed at elevated
temperatures or in the presence of water or water vapor, it will be thicker.
This oxide film is stable in the pH range of 4–9. With a few exceptions, the
film will dissolve at lower or higher pH ranges. Exceptions are concentrated
nitric acid (pH 1) and concentrated ammonium hydroxide (pH 13). In both
cases, oxide film is stable.
The oxide film is not homogeneous and contains weak points. Breakdown
of the film at weak points leads to localized corrosion. With increasing
alloy content and on heat-treatable alloys, the oxide film becomes more
nonhomogeneous.

1.1.1.6 Passive Film on Titanium
Titanium forms a stable, protective, strongly adherent oxide film. This film
forms instantly when a fresh surface is exposed to air or moisture. Addition
of alloying elements to titanium affect the corrosion resistance because these
elements affect the composition of the oxide film.
The oxide film of titanium is very thin and is attacked by only a few
substances, the most notable of which is hydrofluoric acid. Because of its
strong affinity for oxygen, titanium is capable of healing ruptures in this film
almost instantly in any environment where a trace of moisture or oxygen
is present.

1.1.1.7 Passive Film on Tantalum
When exposed to oxidizing or slightly anodic conditions, tantalum forms
a thin impervious layer of tantalum oxide. This passivating oxide has the
broadest range of stability with regard to chemical attack or thermal
breakdown compared to other metallic films. Chemicals or conditions that
attack tantalum, such as hydrofluoric acid, are those which penetrate or
dissolve the film.

1.1.1.8 Uniform Corrosion Rates
When exposed to a corrosion medium, metals tend to enter into a chemical
union with the elements of the corrosion medium, forming stable
compounds similar to those found in nature. When metal loss occurs in
this manner, the compound formed is referred to as the corrosion product


and the surface is referred to as having been corroded. An example of such
an attack is that of halogens, particularly chlorides. They will react with and
penetrate the ﬁlm on stainless steel, resulting in general corrosion. Corrosion
tables are developed to indicate the interaction between a chemical and a
metal. This type of attack is termed uniform corrosion. It is one of the most
easily measured and predictable forms of corrosion. Many references exist
that report average or typical rates of corrosion for various metals in
common media. One such is Reference [1].
Because corrosion is so uniform, corrosion rates for materials are often
expressed in terms of metal thickness loss per unit time. The rate of uniform
attack is reported in various units. One common expression is mils per year
(mpy); sometimes millimeters per year is used. In the United States, it is
generally reported in inches penetration per year (ipy) and milligrams per
square decimeter per day (mdd). To convert from ipy to mpy, multiply the
ipy value by 1000 (i.e., 0.1 in.!1000Z100 mpy). Conversion of ipy to mdd or
vice versa requires knowledge of the metal density. Conversion factors are
given in Table 1.1.
Because of its predictability, low rates of corrosion are often tolerated and
catastrophic failures are rare if planned inspection and monitoring is
implemented. For most chemical process equipment and structures, general
corrosion rates of less than 3 mpy are considered acceptable. Rates between
3 and 20 mpy are routinely considered useful engineering materials for the

TABLE 1.1
Conversion Factors from ipy to mdd
0.00144
Metal Density (g/cc) Density!10L3 696!Density

Aluminum 2.72 0.529 1890
Brass (red) 8.75 0.164 6100
Brass (yellow) 8.47 0.170 5880
Cadmium 8.65 0.167 6020
Columbium 8.4 0.171 5850
Copper 8.92 0.161 6210
Copper–nickel (70–30) 8.95 0.161 6210
Iron 7.87 0.183 5480
Duriron 7.0 0.205 4870
Lead (chemical) 11.35 0.127 7900
Magnesium 1.74 0.826 1210
Nickel 8.89 0.162 6180
Monel 8.84 0.163 6140
Silver 10.50 0.137 7300
Tantalum 16.6 0.0868 11,550
Tin 7.29 0.198 5070
Titanium 4.54 0.317 3160
Zinc 7.14 0.202 4970
Zirconium 6.45 0.223 4490

Multiply ipy by (696!density) to obtain mdd. Multiply mdd by (0.00144/density) to obtain ipy.


given environment. In severe environments, materials exhibiting high
general corrosion rates between 20 and 50 mpy might be considered
economically justifiable. Materials that exhibit rates of general corrosion
beyond this are usually unacceptable. It should be remembered that not only
does the metal loss need to be considered, but where the metal is going must
also be considered. Contamination of product, even at low concentrations,
can be more costly than replacement of the corroded component.
Uniform corrosion is generally thought of in terms of metal loss due to
chemical attack or dissolution of the metallic component into metallic ions.
In high-temperature situations, uniform loss is more commonly preceded by
its combination with another element rather than its oxidation to a metallic
ion. Combination with oxygen to form metallic oxide or scale results in the
loss of the material in its useful engineering form as it ultimately flakes off to
return to nature.
To determine the corrosion rate, a prepared specimen is exposed to the test
environment for a period of time and then removed to determine how much
metal has been lost. The exposure time, weight loss, surface area exposed,
and density of the metal are used to calculate the corrosion rate of the metal
using the formula:
22:273WL
mpy Z ;
DAT
where

WL, weight loss, g
D, density, g/cm3
A, area, in.2
T, time, days.

The corrosion rates calculated from the formula or taken from the tables will
assist in determining how much corrosion allowance should be included in
the design based on the expected lifetime of the equipment.

1.1.2 Intergranular Corrosion
Intergranular corrosion is a localized form of corrosion. It is a preferential
attack on the grain boundary phases or the zones immediately adjacent to
them. Little or no attack is observed on the main body of the grain. This
results in the loss of strength and ductility. The attack is often rapid,
penetrating deeply into the metal and causing failure.
The factors that contribute to the increased reactivity of the grain
boundary area include:

1. Segregation of specific elements or compounds at the grain
boundary, as in aluminum alloys or nickel–chromium alloys


2. Enrichment of one of the alloying elements at the grain boundary,
as in brass
3. Depletion of the corrosion-resisting constituent at the grain
boundary, as in stainless steel

In the case of austenitic stainless steels, the attack is the result of carbide
precipitation during welding operations. Carbide precipitation can be
prevented by using alloys containing less than 0.03% carbon, by using
alloys that have been stabilized with columbium (niobium) or titanium, or
by specifying solution heat treatment followed by a rapid quench that will
keep carbides in solution. The most practical approach is to use either a low
carbon content or stabilized austenitic stainless steel.
Nickel-based alloys can also be subjected to carbide precipitation and
precipitation of intermetallic phases when exposed to temperatures lower
than their annealing temperatures. As with austenitic stainless steels, low-
carbon-content alloys are recommended to delay precipitation of carbides.
In some alloys, such as alloy 625, niobium, tantalum, or titanium is added
to stabilize the alloy against precipitation of chromium or molybdenum
carbides. Those elements combine with carbon instead of the chromium
or molybdenum.
All of these factors contributing to intergranular corrosion originate in the
thermal processing of materials, such as welding, stress relief, and other
heat treatments.

1.1.3 Galvanic Corrosion
This form of corrosion is sometimes referred to as dissimilar metal corrosion,
and is found in unusual places, often causing professionals the most
headaches. Galvanic corrosion is often experienced in older homes where
modern copper piping is connected to the older existing carbon steel lines.
The coupling of the carbon steel to the copper causes the carbon steel to
corrode. The galvanic series of metals provides details of how galvanic
current will flow between two metals and which metal will corrode when
they are in contact or near each other and an electrolyte is present (e.g.,
water). Table 1.2 lists the galvanic series.
When two different metallic materials are electrically connected and
placed in a conductive solution (electrolyte), an electric potential exists. This
potential difference will provide a stronger driving force for the dissolution
of the less noble (more electrically negative) material. It will also reduce the
tendency for the more noble metal to dissolve. Notice in Table 1.2 that the
precious metals gold and platinum are at the higher potential (more noble or
cathodic) end of the series (protected end), while zinc and magnesium are at
the lower potential (less noble or anodic) end. It is this principle that forms
the scientific basis for using such materials as zinc to sacrificially protect the
stainless steel drive shaft on a pleasure boat.


TABLE 1.2
Galvanic Series of Metals and Alloys
Corroded end (anodic)
Magnesium Muntz metal
Magnesium alloys Naval bronze
Zinc Nickel (active)
Galvanized steel Inconel (active)
Aluminum 6053 Hastelloy C (active)
Aluminum 3003 Yellow brass
Aluminum 2024 Admiralty brass
Aluminum Aluminum bronze
Alclad Red brass
Cadmium Copper
Mild steel Silicon bronze
Wrought iron 70/30 Cupro-nickel
Cast iron Nickel (passive)
Ni-resist Iconel (passive)
13% Chromium stainless steel Monel
(active)
50/50 Lead tin solder 18-8 Stainless steel type 304 (passive)
Ferretic stainless steel 400 series 18-8-3 Stainless steel type 316 (passive)
18-8 Stainless steel type 304 (active) Silver
18-8-3 Stainless steel type 316 Graphite
(active)
Lead Gold
Tin Platinum
Protected end (cathodic)

You will note that several materials are shown in two places in the
galvanic series, being indicated as either active or passive. This is the result
of the tendency of some metals and alloys to form surface films, especially in
oxidizing environments. This film shifts the measured potential in the noble
direction. In this state, the material is said to be passive.
The particular way in which a metal will react can be predicted from the
relative positions of the materials in the galvanic series. When it is necessary
to use dissimilar metals, two materials should be selected that are relatively
close in the galvanic series. The further apart the metals are in the galvanic
series, the greater the rate of corrosion.
The rate of corrosion is also affected by the relative areas between the
anode and cathode. Because the flow of current is from the anode to the
cathode, the combination of a large cathodic area and a small anodic area is
undesirable. Corrosion of the anode can be 100–1000 times greater than if
the two areas were equal. Ideally, the anode area should be larger than the
cathode area.
The passivity of stainless steel is the result of the presence of a corrosion-
resistant oxide film on the surface. In most material environments, it will
remain in the passive state and tend to be cathodic to ordinary iron or steel.
When chloride concentrations are high, such as in seawater or in reducing


solutions, a change to the active state will usually take place. Oxygen
starvation also causes a change to the active state. This occurs when there is
no free access to oxygen, such as in crevices and beneath contamination of
partially fouled surfaces.
Differences in soil concentrations, such as moisture content and resistivity,
can be responsible for creating anodic and cathodic areas. Where there is a
difference in concentrations of oxygen in the water or in moist soils in contact
with metal at different areas, cathodes will develop at relatively high oxygen
concentrations, and anodes will develop at points of low concentrations.
Strained portions of metals tend to be anodic and unstrained portions tend
to be cathodic.
Sometimes nonmetallic conductors may act as cathodes in galvanic
couples. Both carbon brick in vessels made of common structural metals
and impervious graphite in heat-exchanger applications are examples.
Conductive films, such as mill scale (Fe2O3) or iron sulfide on steel, or lead
sulfate on lead, are cathodic to the base metal or to some metallic
components in their contact.
When joining two dissimilar metals together, galvanic corrosion can be
prevented by insulating the two materials from each other. For example,
when bolting flanges of dissimilar metals together, plastic washers can be
used to separate the two metals.

1.1.4 Crevice Corrosion
Crevice corrosion is a localized type of corrosion occurring within or
adjacent to narrow gaps or openings formed by metal-to-metal-to-nonmetal
contact. It results from local differences in oxygen concentrations, associated
deposits on the metal surface, gaskets, lap joints, or crevices under a bolt
or around rivet heads where small amounts of liquid can collect and
become stagnant.
Crevice corrosion may take place on any metal and in any corrosive
environment. However, metals like aluminum and stainless steels that
depend on their surface oxide film for corrosion resistance are particularly
prone to crevice corrosion, especially in environments such as seawater that
contain chloride ions.
The gap defining a crevice is usually large enough for the entrapment of
a liquid but too small to permit flow of the liquid. The width is on the order
of a few thousandths of an inch, but not exceeding 3.18 mm.
The material responsible for forming the crevice need not be metallic.
Wood, plastic, rubber, glass, concrete, asbestos, wax, and living organisms
have been reported to cause crevice corrosion. After the attack begins within
the crevice, its progress is very rapid. It is frequently more intense in chloride
environments.
Prevention can be accomplished by proper design and operating pro-
cedures. Nonabsorbant gasketting material should be used at flanged joints,
while fully penetrated butt-welded joints are preferred to threaded joints.


TABLE 1.3
Critical Crevice Corrosion Temperatures in 10% Ferric
Chloride Solution
Alloy Temperature (8F/8C)

Type 316 27/K3
Alloy 825 27/K3
Type 317 36/2
Alloy 904L 59/15
Alloy 220S 68/20
E-Brite 70/21
Alloy G 86/30
Alloy 625 100/38
AL-6XN 100/38
Alloy 276 130/55

If lap joints are used, the laps should be filled with fillet welding or a suitable
caulking compound designed to prevent crevice corrosion.
The critical crevice corrosion temperature of an alloy is the temperature at
which crevice corrosion is first observed when immersed in a ferric chloride
solution. Table 1.3 lists the critical crevice corrosion temperature of several
alloys in 10% ferric chloride solution.
In a corrosive environment, the areas inside the crevice and outside the
crevice undergo corrosion in the same manner. In a neutral chloride solution,
the anodic dissolution is supported by the cathodic reduction of oxygen:

anodic M/ MnC C neK
cathodic O2 C 2H2 O C 4eK/ 4OHK

As the reactions proceed, the dissolved oxygen in the small volume of
stagnated solution inside the crevice is consumed. However, this does not
prevent the dissolution reaction inside the crevice because the electrons
reach outside the crevice through the metal, where plenty of oxygen is
available for reduction. A concentration cell (differential aeration) is set up
between the crevice area and the area outside the crevice.
When chloride ions are present, the situation is further aggravated. The
accumulated cations inside the crevice attract the negatively charged
chloride anions from the bulk solution. Hydroxide anions also migrate,
but they are less mobile than chloride ions. The metal chloride formed
hydrolyzes to produce metal hydroxide and hydrochloric acid:

MCl C H2 O/ MOH C HCl

The nascent hydrochloric acid destroys the passive film and accelerates the
rate of dissolution of the metal inside the crevice. The cathodic reduction
remains restricted to the areas outside the crevice that remain cathodically
protected.


Several steps can be taken to prevent and/or control crevice corrosion:

1. Proper design, avoiding crevices, will control crevice corrosion. If
lap joints are used, the crevices caused by such joints should be
closed by either welding or caulking. Welded butt joints are
preferable to bolted or riveted joints.
2. Porous gaskets should be avoided. Use an impervious gasket
material. During long shutdown periods, wet packing materials
should be removed.
3. The use of alloys resistant to crevice corrosion should be
considered. The resistance of stainless steels to crevice corrosion
can be improved by increasing the chromium, nickel, molyb-
denum, and nitrogen content. For example, type 316 stainless steel
containing 2–3% molybdenum is fairly resistant, whereas nickel
alloys are more resistant than stainless steels.
4. Reduction of crevice corrosion can be accomplished, when
possible, by reducing the temperature, decreasing the chloride
content, or decreasing the acidity.
5. The gaps along the periphery of tanks mounted on a masonry
platform should be closed with tar or bitumen to avoid seepage of
rainwater. Vessels and tanks should be designed to provide
complete drainage, thereby preventing the buildup of solid
deposits on the bottom.
6. Regular inspections and removal of deposits should be scheduled.

1.1.5 Pitting Corrosion
Pitting corrosion is in itself a corrosion mechanism, but it is also a form of
corrosion often associated with other types of corrosion mechanisms. It is
characterized by a highly localized loss of metal. In the extreme case, it appears
as a deep, tiny hole in an otherwise unaffected surface. The initiation of a pit
is associated with the breakdown of the protective ﬁlm on the metal surface.
The depth of the pit eventually leads to a thorough perforation or a
massive undercut in the thickness of the metal part. The width of the pit may
increase with time, but not to the extent to which the depth increases. Most
often, the pit opening remains covered with the corrosion product, making it
difﬁcult to detect during inspection. This, along with a negligible loss in
weight or absence of apparent reduction in the overall wall thickness, gives
little evidence as to the extent of the damage. Pitting may result in the
perforation of a water pipe, making it unusable even though a relatively
small percentage of the total metal has been lost due to rusting.
Pitting can also cause structural failure from localized weakening effects
even though there is considerable sound material remaining. Pits may also


assist in brittle failure, fatigue failure, environment-assisted cracking like
stress corrosion cracking (SCC), and corrosion fatigue, by providing sites
of stress concentration.
The main factor that causes and accelerates pitting is electrical contact
between dissimilar metals, or between what are termed concentration cells
(areas of the same metal where oxygen or conductive salt concentrations in
water differ). These couples cause a difference of potential that results in an
electric current ﬂowing through the water or across moist steel, from the
metallic anode to a nearby cathode. The cathode may be brass or copper, mill
scale, or any other portion of the metal surface that is cathodic to the more
active metal areas. However, when the anodic area is relatively large
compared with the cathodic area, the damage is spread out and is usually
negligible. When the anode area is relatively small, the metal loss is
concentrated and may be serious. For example, it can be expected when
large areas of the surface are generally covered by mill scale, applied coatings,
or deposits of various kinds, but breaks exist in the continuity of the protective
material. Pitting may also develop on bare clean metal surfaces because of
irregularities in the physical or chemical structure of the metal. Localized
dissimilar soil conditions at the surface of steel can also create conditions that
promote pitting. Figure 1.1 shows how a pit forms when a break in mill
scale occurs.
If an appreciable attack is conﬁned to a small area of metal acting as an
anode, the developed pits are described as deep. If the area of attack is rela-
tively large, the pits are called shallow. The ratio of deepest metal penetration
to average metal penetration, as determined by weight loss of the specimen,
is known as the pitting factor. A pitting factor of 1 represents uniform
corrosion. Pitting corrosion is characterized by the following features:

1. The attack is spread over small discrete areas. Pits are sometimes
isolated and sometimes close together, giving the area of attack a
rough appearance.

Electrolyte (water)
Fe2+ (rust)
Current flow
Cathode
(broken mill scale)

Anode steel

FIGURE 1.1
Formation of pit from break in mill scale.


2. Pits usually initiate on the upper surface of the horizontally placed
parts and grow in the direction of gravity.
3. Pitting usually requires an extended initiation period before visible
pits appear.
4. Conditions prevailing inside the pit make it self-propagating
without any external stimulus. Once initiated, the pit grows at an
ever-increasing rate.
5. Stagnant solution conditions lead to pitting.
6. Stainless steels and aluminum and its alloys are particularly
susceptable to pitting. Carbon steels are more resistant to pitting
than stainless steels. Most failure of stainless steels occurs in neutral-
to-acid chloride solutions. Aluminum and carbon steels pit in
alkaline chloride solutions.
7. Most pitting is associated with halide ions (chlorides, bromides),
and hypochlorites are particularly aggressive. Cupric, ferric, and
mercuric halides are extremely aggressive because their cations are
cathodically reduced and sustain the attack.

Performance in the area of pitting and crevice corrosion is often measured
using critical pitting temperature (CPT), critical crevice temperature (CCT),
and pitting resistance equivalent number (PREN). As a general rule, the
higher the PREN, the better the resistance. The PREN is determined by
the chromium, molybdenum, and nitrogen contents: PRENZ%CrC3.3
(%Mo)C30(%N). Table 1.4 lists the PRENs for various austenitic
stainless steels.
The CPT of an alloy is the temperature of a solution at which pitting is ﬁrst
observed. These temperatures are usually determined in ferric chloride
(10% FeCl3$6H2O) and an acidic mixture of chlorides and sulfates.

TABLE 1.4
Pitting Resistance Equivalent Numbers
Alloy PREN Alloy PREN

654 63.09 316LN 31.08
31 54.45 316 27.90
25-6Mo 47.45 20Cb3 27.26
Al-6XN 46.96 348 25.60
20Mo-6 42.81 347 19.0
317LN 39.60 331 19.0
904L 36.51 304N 18.3
20Mo-4 36.20 304 18.0
317 33.2


1.1.6 Erosion Corrosion
The term “erosion” applies to deterioration due to mechanical force. When
the factors contributing to erosion accelerate the rate of corrosion of a metal,
the attack is called “erosion corrosion.” Erosion corrosion is usually caused
by an aqueous or gaseous corrodent flowing over the metal surface or
impinging on it. The mechanical deterioration may be aggravated by the
presence of a corrodent, as in the case of fretting or corrosive wear.
The attack takes the form of grooves, i.e., scooped-out rounded areas,
horseshoe-shaped depressions, gullies, or waves, all of which often show
directionality. At times, attack may be an assembly of pits. Ultimate
perforation due to thinning or progression of pits, and rupture due to failure
of the thinned wall to resist the internal fluid pressure are common. All
equipment exposed to flowing fluid is subject to erosion corrosion, but
piping systems and heat exchangers are the most commonly affected.
Erosion corrosion is affected by velocity, turbulence, impingement,
presence of suspended solids, temperature, and prevailing cavitation
conditions. The acceleration of attack is due to the distribution or removal
of the protective surface film by mechanical forces exposing fresh metal
surfaces that are anodic to the uneroded neighboring film. A hard, dense
adherent and continuous film, such as on stainless steel, is more resistant
than a soft brittle film, as that on lead. The nature of the protective film
depends largely on the corrosive itself.
In most metals and alloys, corrosion rates increase with increased velocity,
but a marked increase is experienced only when a critical velocity is reached.
Turbulence is caused when the liquid flows from a larger area to a small-
diameter pipe, as in the inlet ends of tubing in heat exchangers. Internal
deposits in the pipes, or any obstruction to the flow inside a pipe by a foreign
body, such as a carried-in pebble, can also cause turbulence.
Impingement, direct contact of the corrodent on the metal surface, occurs
at bends, elbows and tees in a piping system and causes intense attack.
Impingement is also encountered on the surface of impellers and turbines in
areas in front of inlet pipes in tanks and in many other situations. The attack
appears as horseshoe-shaped pits with a deep undercut and the end
pointing in the direction of flow.
An additional subset of erosion corrosion is the case of cavitation that is
prevalent in pump impellers. This form of attack is caused by the formation
and collapse of tiny vapor bubbles near a metallic surface in the presence of a
corrodent. The protective film is again damaged, in this case by the high
pressures caused by the collapse of the bubbles.
When two metal surfaces are in contact and experience a very slight
relative motion that results in damage to one or both surfaces, fretting
corrosion, a special form of erosion corrosion, takes place. The movement
causes mechanical damage to the protective film; this can lead to erosion
corrosion when a corrodent is present. This corrosion usually takes the form
of a pitting attack.


Attack is further aggravated at higher temperatures and with solutions
containing solids in suspension. Steam carrying water condensate droplets
provides an aggressive medium for corrosion of steel and cast iron piping.
The impingement of water droplets at the return bends destroys the
protective oxide film and accelerates the attack on the substrate.
Soft and low-strength metals such as copper, aluminum, and lead are
especially susceptible to erosion corrosion, as are the metals and alloys that
are inherently less corrosion resistant, such as carbon steels.
Stainless steels of all grades, in general, are resistant to erosion corrosion.
The addition of nickel, chromium, and molybdenum further improves their
performance. Stainless steels and chromium steels are resistant as a result of
their tenacious surface films.
As a rule, solid solution alloys provide better resistance than alloys
hardened by heat treatment because the latter are heterogeneous in nature.
Cast irons usually perform better than steel. Alloy cast irons containing
nickel and chromium exhibit better performance. Duriron, containing 14.5%
silicon, gives excellent performance under severe corrosion conditions.
Prevention and/or reduction of erosion corrosion can be accomplished by
one or more means:

1. Reduce velocity
2. Reduce or eliminate turbulence
3. Select a harder material
4. Properly design the piping system or the condensers

1.1.7 Stress Corrosion Cracking (SCC)
SCC is defined as the delayed failure of alloys by cracking when exposed to
certain environments in the presence of static tensile stress. The importance
of a conjoint action of corrosion and stress is reflected in the definition; an
alternate application of stress and corrosive environment will not produce
SCC. The stress level at which the failure occurs is well below the stress
required for a mechanical failure in the absence of corrosion. The minimum
stress below which SCC will occur is called the threshold stress, but this may
be as low as 10% of the yield stress in some systems. Corrosion alone in the
absence of stress does not cause SCC.
SCC occurs at points of stress. Usually the metal or alloy is virtually free of
corrosion over most of its surface, yet fine cracks penetrate through the
surface at the points of stress. Depending on the alloy system and corrodent
combination, the cracking can be intergranular or transgranular. The rate of
propagation can vary greatly and is affected by stress levels, temperature,
and concentration of the corrodent. This type of attack takes place in certain
media. All metals are potentially subject to SCC. The conditions necessary
for stress corrosion are:


1. Suitable environment
2. Tensile stress
3. Sensitive metal
4. Appropriate temperature and pH values

An ammonia environment can induce SCC in copper-containing alloys,
whereas with low-alloy austenitic stainless steels a chloride-containing
environment is necessary. It is not necessary to have a high concentration of
corrodent to cause SCC. A solution containing only a few parts per million of
the critical ion is all that is necessary. Temperature and pH are also factors.
There is usually a threshold temperature below which SCC will not take
place and a maximum or minimum pH value before cracking will start.
Normally, SCC will not occur if the part is in compression. Fatigue is
triggered by a tensile stress that must approach the yield stress of the metal.
The stresses may be induced by faulty installation or they may be residual
stress from welding, straightening, bending, or accidental denting of the
component. Pits, which act as stress concentration sites, will often
initiate SCC.
The alloy content of stainless steel, particularly nickel, determines the
sensitivity of the metal to SCC. Ferritic stainless steels, which are nickel-free,
and the high-nickel alloys are not subject to SCC. An alloy with a nickel
content greater than 30% is immune to SCC. The most common grades of
stainless steel (304, 304L, 316, 316L, 321, 347, 303, and 301) have nickel
contents in the range of 7–10% and are the most susceptible to SCC.
Examples of SCC include the cracking of austenitic stainless steels in the
presence of chlorides; caustic embrittlement cracking of steel in caustic
solutions; cracking of cold-formed brass in ammonia environments, and
cracking on monel in hydrofluorosilicic acid. Table 1.5 provides partial
listing of alloy systems and the environments that will cause SCC.
In severe combinations, such as type 304 stainless steel in a boiling
magnesium chloride solution, extensive cracking can be generated in a
matter of hours.
Fortunately, in most industrial applications the progress of SCC is much
slower. However, because of the nature of the cracking, it is difficult to detect
until extensive corrosion has developed, which can lead to unexpected failure.
Tensile stresses can lead to other corrosion processes, such as the simple
mechanical fatigue process. Corrosion fatigue is difficult to differentiate
from simple mechanical fatigue, but it is recognized as a factor when the
environment is believed to have accelerated the normal fatigue process. Such
systems can also have the effect of lowering the endurance limit such that
fatigue will occur at a stress level below which it would normally
be expected.
It is important that any stresses that may have been induced during the
fabrication be removed by an appropriate stress-relief operation. Care should
be taken so as to not induce a stress as the result of installation. The design


TABLE 1.5
Alloy–Environment Combinations Causing Stress Corrosion Cracking
Alloy Environment

Aluminum alloys Air with water; potable waters; seawater NaCl
solutions; NaCl–H2O2 solutions
Carbon steels Caustic NaOH solutions; seawater; calcium,
ammonium, and sodium nitrate solutions; HCN
solutions; acidified H2S solutions; anhydrous
liquid ammonia; carbonate/bicarbonate; CO/CO2
solutions
Copper alloys Ammonical solutions; amines; nitrites
Nickel alloys Caustic alkaline solutions; high-temperature chloride
solutions; high-purity steam; hydrofluoric acid;
acid fluoride solutions
Stainless steels Hot acid chloride solutions; NaCl–H2O2 solutions;
austenitic NaOH–H2S solutions; seawater; concentrated
caustic solutions; neutral halides, BrK, IK, FK
Austenitic (sensitized) Polythionic acids; sulfurous acid; pressurized hot
water containing 2 ppm dissolved oxygen
Ferritic H2S; NH4Cl; NH4NO3; hypochlorite solutions
Martensitic Caustic NaOH solutions
Titanium alloys Red fuming nitric acid; hot salts; molten salts; N2O4;
methanol/halide

should also avoid stagnant areas that could lead to pitting and the initiation of
stress concentration sites.

1.1.8 Biological Corrosion
Corrosive conditions can be developed by living microorganisms as a result
of their influence on anodic and cathodic reactions. This metabolic activity
can directly or indirectly cause deterioration of a metal by the corrosion
process. This activity can

1. Produce a corrosive environment
2. Create electrolytic cells on the metal surface
3. Alter the resistance of surface films
4. Have an influence on the rate of anodic or cathodic reaction
5. Alter the environmental composition

Because this form of corrosion gives the appearance of pitting, it is first
necessary to diagnose the presence of bacteria. This is also referred to as
microbial corrosion.
The term microorganism covers a wide variety of life forms, including
bacteria, blue-green cyanobacteria, algae, lichens, fungi, and protozoa.
All microorganisms may be involved in the biodeterioration of metals.


Pure cultures never occur under natural conditions; rather, mixed cultures
prevail. Of the mixed cultures, only a few actually become actively involved
in the process of corrosion. The other organisms support the active ones
by adjusting the environmental conditions to support their growth. For
example, in the case of metal corrosion caused by sulfate-reducing bacteria
(SRB), the accompanying organisms remove oxygen and produce simple
carbon compounds, such as acetic acid and/or lactic acid, as nutrients
for SRB.
Bacteria are the smallest living organisms on this planet. Some can only
live with and others without oxygen. Some can adapt to changing conditions
and live either aerobically or anaerobically. There is a wide diversity with
regard to their metabolisms. They are classified as to their source of
metabolic energy as follows:

Energy Source Classification

Light Phototrophs
Chemical reactors Chemotrophs
Inorganic hydrogen donators Lithotrophs
Organic hydrogen donators Organotrophs
Carbon dioxide (cell source) Autotrophs
Organic molecules (cell source) Heterotrophs

These six terms may be combined to easily describe the nutritional
requirements of a bacterium. For example, if energy is derived from inorganic
hydrogen donators and biomass is derived from organic molecules, they are
called microtrophs (chemolithoorganotrophs).
An important feature of microbial life is the ability to degrade any
naturally occurring compound. Exceptions to this rule are a few manmade
materials such as highly polymerized and halogenated compounds.
In addition to energy and carbon sources, nitrogen, phosphorus, and trace
elements are needed by microorganisms. Nitrogen compounds may be
inorganic ammonium nitrate as well as organically bound nitrogen (e.g.,
amino acids, nucleotides). With the help of an enzyme called nitrogenase,
bacteria are able to fix nitrogen from atmospheric nitrogen, producing
ammonia that is incorporated into cell constituents.
Phosphorus is taken up as inorganic phosphate or as organically bound
phosphoroxylated compounds, such as phosphorus-containing sugars and
lipids. Phosphorus, in the form of adenosine triphosphate (ATP), is the main
energy-storing compound.
For many of the metabolic purposes, trace elements are needed. Cobalt
aids in the transfer of methyl groups from/to organic or inorganic
molecules. (Vitamin B12, cobalamin, is involved in the methylation of
heavy metals such as mercury.) Iron, as Fe2C or Fe3C, is required for the
electron transport system, where it acts as an oxidizable/reducible central
atom in cytochromes of nonheme iron-sulfur proteins. Magnesium acts in


a similar manner in the chlorophyll molecule. Copper is an essential part
of a cytachrome that, at the terminal end of the electron transport system, is
responsible for the reduction of oxygen to water.
Because life cannot exist without water, water is an essential requirement
for microbial life and growth. Requirements differ as to the amount of
water needed. A solid material is surrounded by three types of water;
hygroscopic, pellicular, and gravitational. Only gravitational and
pellicular water are biologically available and can be used by micro-
organisms. The biologically available water is usually measured as the water
activity:

Vs
aw Z ;
Pw

where Vs is the vapor pressure of the solution and Pw is the vapor pressure of
pure water at the same temperature. Most bacteria require an aw value in
excess of 0.90.
Hydrogen ion concentration is another important factor affecting growth.
Microorganisms are classified as to their ability to grow under acidic,
neutral, or alkaline conditions, being given such titles as acidophiles,
neutrophiles, or alkalophiles. Most microorganisms thrive in the neutral
pH of 6–8.
Microbial growth is also affected by redox potential. Under standard
conditions, hydrogen is assumed to have a redox potential of K42 mV and
oxygen has a redox potential of 820 mV. Metabolism can take place within
that range.
Available oxygen is another factor that influences microbial growth.
Microbial growth is possible under aerated as well as under totally oxygen-
free conditions. Those organisms living with the amount of oxygen
contained in the air are called aerobes, whereas those that perform their
metabolism without any trace of free oxygen are called anaerobes. The latter
are able to use bound oxygen (sulfate, carbon dioxide) or to ferment organic
compounds.
Temperature is another important factor affecting microbial growth.
Microbial life is possible within the range of K5 to 1108C (21 to 2308F).
Microorganisms are also classified as to the temperature range in which they
thrive, as in the following table.

Temperature Range
Microorganism 8C 8F

Psychrophiles K5 to 20 21 to 68
Pschotrophes 5 to 30 41 to 86
Mesophiles 20 to 45 68 to 113
Moderate thermophiles 40 to 55 104 to 131
Thermophiles 55 to 85 131 to 185
Extreme thermophiles up to 110 up to 230


Most of the organisms live in the mesophilic range of 20–458C (69–1108F),
which corresponds to the usual temperature range of the earth.

1.1.8.1 Corrosion of Specific Materials
Microbially induced corrosion (MIC) may occur for metallic materials in
many industrial applications. MIC has been reported in the following
industrial applications:

Industry Location of MIC

Chemical processing Pipelines, stainless steel tanks,
flanged joints, welded areas, after
hydro-testing with natural river or
well water
Nuclear power generating Copper–nickel, brass, stainless steel,
and aluminum–bronze cooling
water pipes, carbon and stainless
steel piping and tanks
Underground pipeline Water-saturated clay type soils of
near-neutral pH with decaying
organic matter and a source of
sulfate reducing bacteria
Metalworking Increased wear from breakdown of
machinery oils and emulsions
Onshore and offshore oil Mothballed and flooded systems, oil-
and gas plants and gas-handling systems,
particularly in environments
soured by sulfate reducing,
bacteria-produced sulfides
Water treatment sewage handling Heat exchangers and piping; concrete
and treatment and concrete-reinforced structures
Highway maintenance Culvert piping
Aviation Aluminum integral wiring, tanks,
including fuel storage tanks

MIC of metallic materials is not a new form of corrosion. The methods by
which microorganisms increase the rate of corrosion of metals and/or their
susceptibility to localized corrosion in an aqueous environment are:

1. Production of metabolites. Bacteria may produce inorganic acids,
organic acids, sulfides, and ammonia, all of which may be
corrosive to metallic materials.
2. Destruction of protective layers. Organic coatings may be attacked
by various microorganisms, leading to the corrosion of the
underlying metal.
3. Hydrogen embrittlement. By acting as a source of hydrogen and/or
through the production of hydrogen sulfide, microorganisms may
influence hydrogen embrittlement of metals.


4. Formation of concentration cells at the metal surface and, in particular,
oxygen centration cells. A concentration cell may be formed when
a biofilm or bacterial growth develops heterogenerously on the
metal surface. Some bacteria may tend to trap heavy metals such
as copper and cadmium within their extracelluar polymeric
substance, causing the formation of ionic concentration cells.
These lead to localized corrosion.
5. Modification of corrosion inhibitors. Certain bacteria may convert
nitrite corrosion inhibitors used to protect aluminum and
aluminum alloys to nitrate and ammonia.
6. Stimulation of electrochemical reactors. An example of this type is the
evolution of cathodic hydrogen from microbially produced
hydrogen sulfide.

MIC can result from:

1. Production of sulfuric acid by bacteria of the genus thiobacillus
through the oxidation of various inorganic sulfur compounds; the
concentration of sulfuric acid may be as high as 10–12%
2. Production of hydrogen sulfide by sulfate-reducing bacteria
3. Production or organic acids
4. Production of nitric acids
5. Production of ammonia

There are many approaches that may be used to prevent or minimize MIC.
Among the choices are:

1. Material change or modification
2. Environment or process parameter modification
3. Use of organic coatings
4. Cathodic protection
5. Use of biocides
6. Microbiological methods
7. Physical methods

Before any remedial action can be taken, it is necessary to identify the
type of bacteria involved in corrosion. Aeration of water in a closed
recirculating system reduces the activity of anerobic bacteria. Chlorination
and treatment with biocides help control populations of some bacteria,
though they are not effective in all cases. Also, the bacteriocides fail to reach
the areas underneath deposits where the bacteria thrive. Coating a buried
structure with tar, enamel, plastic, or the like is often an effective means to


preclude the bacteria from the metal surface. Cathodic protection in
combination with coatings can be used to prevent or arrest microbiological
corrosion.
During storage or after hydrotesting, water should not be allowed to stand
for a long period of time. Complete drainage and drying are advocated.
Inhibitors may be used in stagnating water and cutting-oil fluids. Periodic
cleaning of pipelines is also essential. In most affected soils, steel pipes may
be replaced by plastic pipes to avoid microbiological corrosion.
In areas where macrobiofouling may occur, these options can reduce or
prevent fouling. The application of antifouling paints is probably the most
effective and most widely used means of preventing biofouling in seawater.
Ships and piers are coated with specially formulated paints containing
compounds toxic to the organisms. Copper compounds are often used, as
the released copper ions poison the growth of barnacles and other marine
organisms. Periodic mechanical cleaning of surfaces of structures and inside
of pipelines helps the growth of bioorganisms and the creation of crevice
sites. In closed systems, fouling can be mitigated by chlorination and
periodic injection of suitable biocides, including copper compounds.
The approach to follow depends upon the type of bacteria present. A
technique that has gained importance, in addition to the preventative
methods, is that of a simulation of biogenic attack. A quick-motion effect can
be produced that will allow materials to be tested for their compatibility for a
specific application. To conduct the simulation properly, it is necessary that a
thorough knowledge of all of the processes and participating microorgan-
isms be known. The situation may be modified under conditions that will be
optimal for the microorganisms, resulting in a reduced time span for the
corrosion to become detectable.

1.1.9 Selective Leaching
When one element of a solid alloy is removed by corrosion, the process is
known as selective leaching, dealloying, or dezincification. The most
common example is the removal of zinc from brass alloys that contain
more than 15% zinc. When the zinc corrodes preferentially, a porous residue
of copper and corrosion products remains. The corroded part often retains
its original shape and may appear undamaged except for surface tarnish.
However, its tensile strength, and particularly its ductility, are seriously
reduced.
Dezincification of brasses takes place in either localized areas on the metal
surface, called “plug type,” or uniformly over the surface, called “layer
type.” Low-zinc alloys favor plug-type attack while layer-type attack is more
prevalent in high-zinc alloys. The nature of the environment seems to have a
greater effect in determining the type of attack. Uniform attack takes place in
slightly acidic water, low in salt content and at room temperature. Plug-type
attack is favored in neutral or alkaline water, high in salt content and above


room temperature. Crevice conditions under a deposit of scale or salt tend to
aggravate the condition.
A plug of dezincified brass may fall out, leaving a hole, whereas water
pipe having layer-type dezincification may split open.
Conditions that favor selective leaching are:

1. High temperatures
2. Stagnant solutions, especially if acidic
3. Porous inorganic scale formation

Brasses that contain 15% or less zinc are usually immune. Dezincification can
be suppressed by alloying additions of tin, aluminum, arsenic,
or phosphorus.
Corrective measures that may be taken include:

1. Use of a more resistant alloy. This is the more practical approach.
Red brass, with less than 15% zinc, is almost immune. Cupronickels
provide a better substitute in severely corrosive atmospheres.
2. Periodic removal of scales and deposits from the inside surfaces
of pipelines.
3. Removal of stagnation of corrosives, particularly acidic ones.
4. Use of cathodic protection.

Other alloy systems are also susceptible to this form of corrosion. Refer
to Table 1.6. Selective leaching of aluminum takes place in aluminum
bronze exposed to hydrofluoric acid or acid-containing chlorides. Copper–
aluminum alloys containing more than 80% aluminum are particularly
susceptible.
Severe leaching of tin in tin bronzes in hot brine or steam, and of silicon
from silicon bronzes in high-temperature steam, are other examples.
Selective leaching of iron from gray iron is termed “graphite corrosion.”
Iron will leach out selectively from gray iron pipe buried in soil. Graphite
corrosion does not occur in ductile iron or malleable iron.

1.2 Corrosion Mechanisms
Most of the commonly used metals are unstable in the atmosphere. These
unstable metals are produced by reducing ores artificially; therefore, they
tend to return to their original state or to similar metallic compounds when
exposed to the atmosphere. Exceptions to this are gold and platinum that are
already in their metallic state.


TABLE 1.6
Combination of Alloys and Environments for Selective Leaching
Alloy Environment Element Removed

Aluminum Hydrofluoric acid, acid Aluminum
chloride solutions
Bronzes, brasses Many waters Zinc
Cupronickels High heat flux and low water Nickel
velocity
Gray iron Soils, many waters Iron
Gold alloys Nitric, chromic, and sulfuric Copper or silver
acids, human saliva
High-nickel alloys Molten salts Chromium, iron,
molybdenum, tungsten
Iron–chromium alloys High-temperature oxidizing Chromium
atmosphere
Medium- and high- Oxidizing atmospheres, Carbon
carbon steels hydrogen at high
temperatures
Monel Hydrogen, and other acids Copper in some acids,
nickel in others
Nickel–molybdenum Oxygen at high temperatures Molybdenum
alloy
Silicon bronzes High-temperature steam Silicon
Tin bronzes Hot brine, steam Tin

Corrosion, by its simplest definition, is the process of a metal returning
to the material’s thermodynamic state. For most materials, this means the
formation of the oxides or sulfides from which they originally started when
they were taken from the earth, before being refined into useful engineering
materials. Most corrosion processes are electrochemical in nature, consisting
of two or more electrode reactions: the oxidation of a metal (anodic partial
reaction) and the reduction of an oxidizing agent (cathodic partial reaction).
The study of electrochemical thermodynamics and electrochemical kinetics
is necessary to understand corrosion reactions. For example, the corrosion of
zinc in an acidic medium proceeds according to the overall reaction

Zn C 2HC/ Zn2C C H2 : (1.1)

This breaks down into the anodic partial reaction

Zn/ Zn2C C 2e; (1.2)

and the cathodic partial reaction

2HC C 2e/ H2 : (1.3)


The corrosion rate depends on the electrode kinetics of both partial
reactions. If all of the electrochemical parameters of the anodic and cathodic
partial reactions are known, in principle the rate may be predicted.
According to Faraday’s law, a linear relationship exists between the metal
dissolution rate at any potential VM and the partial anodic current density
for metal dissolution iaM:
iaM
VM Z ; (1.4)
nF
where n is the charge number (dimensionless) that indicates the number of
electrons exchanged in the dissolution reaction and F is the Faraday constant
(FZ96,485 C/mol). In the absence of an external polarization, a metal in
contact with an oxidizing electrolytic environment spontaneously acquires a
certain potential, called the corrosion potential, Ecorr. The partial anodic
current density at the corrosion potential is equal to the corrosion current
density icorr . Equation (1.4) then becomes
icorr
Vcorr Z (1.5)
nF
The corrosion potential lies between the equilibrium potentials of the anodic
and cathodic partial reactions.
The equilibrium potential of the partial reactions is predicted by
electrochemical thermodynamics. The overall stoichiometry of any chemical
reaction can be expressed by

0 Z 3vi bi (1.6)

where b designates the reactants and the products. The stoichiometric
coefﬁcients, vi, of the products are positive and those of the reactants are
negative. The free enthalpy of reaction, DG, is

DG Z 3vi mi (1.7)

where mi is the chemical potential of the participating species. If Reaction
(1.6) is conducted in an electrochemical cell, the corresponding equilibrium
potential Erev is given by

DG ZKnFErev (1.8)

Under standard conditions (all activities equal to one),

DG0 ZKnFE0 (1.9)

where DG0 represents the standard free enthalpy and E0 represents the
standard potential of the reaction.


Electrode reactions are commonly written in the form

3vox;i box;i C ne Z 3vred;i bred;i (1.10)

where vox,i represents the stoichiometric coefficient of the “oxidized” species
with box,i appearing on the left side of the equality sign together with the free
electrons, and vred,i indicates the stoichiometric coefficients of the reducing
species with bred,i appearing on the right side of the equality sign, opposite to
the electrons. Equation (1.10) corresponds to a partial reduction reaction and
the stoichiometric coefficients vox,i and vred,i are both positive.
By setting the standard chemical potential of the solvated proton and
of the molecular hydrogen equal to zero, m0 C Z 0; m0 2 Z 0, it is possible to
H H
define the standard potential of the partial reduction reaction of Equation
(1.10) with respect to the standard hydrogen electrode. The standard
potential of an electrode reaction thus corresponds to the overall reaction

n
3vox;i box;i C H2ðPH2Z1 barÞ Z 3vred;i bred;i C nHC CZiÞ :
ðaH (1.11)
2

Table 1.7 indicates the standard potential of selected electrode reactions.
For a given reaction to take place, there must be a negative free energy
change, as calculated from the equation

DG ZKnFE: (1.12)

TABLE 1.7
Standard Potentials of Electrode Reactions at 258C
Electrode E8/V
C
Li CeZLi K3.045
Mg2CC2eZMg K2.34
Al3CC3eZAl K1.67
Ti2CC2eZTi K1.63
Cr2CC2eZCr K0.90
Zn2CC2eZZn K0.76
Cr3CC3eZCr K0.74
Fe2CC2eZFe K0.44
Ni2CC2eZNi K0.257
Pb2CC2eZPb K0.126
2HCC2eZH2 0
Cu2CC2eZCu 0.34
O2 C2H2OC4eZ4OH 0.401
Fe3CCeZFe2C 0.771
AgCCeZAg 0.799
Pt 2CC2eZPt 1.2
O2 C4HCC4eZ2H2O 1.229
Au3CC3eZAu 1.52


For this to occur the cell potential must be positive. The cell potential is taken
as the difference between two half-cell reactions, the one at the cathode
minus the one at the anode.
If we place pure iron in hydrochloric acid, the chemical reaction can be
expressed as

Fe C 2HCl/ FeCl2 C H2[ : (1.13)

On the electrochemical side,

Fe C 2HC C 2Cl2K/ Fe2C C Cl2 C H2[ : (1.14)

The cell potential is calculated to be

E Z Cathode half-cellKAnode half-cell
HC Fe
EZE KE
H2 Fe2C
E Z 0KðK0:440Þ

E Z C0:440

Because the cell is positive, the reaction can take place. The larger this
potential difference, the greater the driving force of the reaction. Other
factors will determine whether or not corrosion does take place and, if so, at
what rate. For corrosion to take place there must be a current flow and a
completed circuit that is then governed by Ohm’s law (IZE/R). The cell
potential calculated here represents the peak value for the case of two
independent reactions. If the resistance were infinite, the cell potential
would remain as calculated, but there would be no corrosion. If the
resistance of the circuit were zero, the potentials of each half-cell would
approach each other and the rate of corrosion would be infinite.
At an intermediate resistance in the circuit, some current begins to flow
and the potentials of both half-cells move slightly toward each other. This
change in potential is called polarization. The resistance in the circuit is
dependent on various factors, including the resistivity of the media, surface
films, and the metal itself. Figure 1.2 shows the relationship between
the polarization reactions at each half-cell. The intersection of the two
polarization curves closely approximate the corrosion current and the
combined cell potentials of the freely corroding situation.
The corrosion density can be calculated by determining the surface area
once the corrosion current is determined. A corrosion rate in terms of metal
loss per unit time can be determined using Faraday’s laws.
In addition to estimating corrosion rates, the extent of the polarization can
help predict the type and severity of corrosion. As polarization increases,
corrosion decreases. Understanding the influence of environmental changes


0.1
+
E [H2/H ] Cathodic polarization curve

− 0.1
Potential, E

++
Fe Fe + 2e

Ecorr
+
− 0.3 2H + 2e H2

++ Anodic polarization curve
E [Fe/Fe ]
− 0.5 i corr

Current, i

FIGURE 1.2
Polarization of iron in acid.

on polarization can aid in controlling corrosion. For example, in the iron–
hydrochloric acid example, hydrogen gas formation at the cathode can
actually slow the reaction by blocking access of hydrogen ions to the cathode
site, thereby increasing circuit resistance, resulting in cathodic polarization
and lowering the current flows and corrosion rate. If the hydrogen is
removed by bubbling oxygen through the solution, which combines with the
hydrogen to form water, the corrosion rate will increase significantly.
There are three basic causes of polarization: concentration, activation, and
potential drop. Concentration polarization is the effect resulting from the
excess of a species that impedes the corrosion process (as in the previous
hydrogen illustration), or with the depletion of a species critical to the
corrosion process.
Activation polarization is the result of a rate-controlling step within the
corrosion reaction. In the HC/H2 conversion reaction, the first step of the
process,

2HC C 2e/ 2H

proceeds rapidly, whereas the second step,

2H/ H2

takes place more slowly and can become a rate-controlling factor.
Potential drop is the change in voltage associated with effects of the
environment and the current circuit between the anode and cathode sites.
Included are the effects of surface films, corrosion products, resistivity of the
media, etc.


Other factors affecting corrosion include temperature, relative velocities
between the metal and the media, surface finish, grain orientation, stresses,
and time.
Because corrosion is an electrochemical reaction and reaction rates
increase with increasing temperature, it is logical that corrosion rates will
increase with increasing temperature.
In some instances, increasing the velocity of the corrodent over the
surface of the metal will increase the corrosion rates when con-
centration polarization occurs. However, with passive metals, increasing
the velocity can actually result in lower concentration rates because
the increased velocity shifts the cathodic polarization curve so that it
no longer intersects the anodic polarization curve in the active corrosion
region.
Rough surfaces or tight crevices can promote the formation of
concentration cells. Surface cleanliness is also a factor because deposits or
films can act as initiation sites. Biological growths can behave as deposits
or change the underlying surface chemistry to promote corrosion.
Variations within the metal surface on a microscopic level can influence
the corrosion process. Microstructural differences such as second phases
or grain orientation will affect the manner in which the corrosion process
will take. The grain size of the material plays an important role in deter-
mining how rapidly the material’s properties will deteriorate when the grain
boundaries are attacked by corrosive environments.
Stress is a requirement for SCC or fatigue, but can also influence the rate
of general corrosion. The severity of corrosion is affected by time. Corrosion
rates are expressed as a factor of time. Some corrosion rates are rapid and
violent, while most are slow and almost imperceptible on a day-to-day
basis.
Potential/pH diagrams (Pourbaix diagrams) graphically represent the
stability of a metal and its corrosion products as a function of the potential
and pH of an aqueous solution. The pH is shown on the horizontal axis and
the potential on the vertical axis. Pourbaix diagrams are widely used in
corrosion because they easily permit the identification of the predominant
species at equilibrium for a given potential and pH. However, being based
on thermodynamic data, they provide no information on the rate of possible
corrosion reactions.
To trace such a diagram, the concentration of the dissolved material
must be fixed. Figure 1.3 shows a simplified Pourbaix diagram for zinc. The
numbers indicate the H2CO3 concentration in the moisture film, for
example, 10K2, 10K4 mol/L. The diagram shown takes into account the
formation of zinc hydroxide, of Zn2C, and of the zincate ions HZnOK and 2
2K
ZnO2 . At high potentials, ZnO2 may possibly be formed, but because
the corresponding thermodynamic data are uncertain, they are not
presented in the diagram. The broken lines indicate the domain of
thermodynamic stability of water.


2
1.8
1.6 Zn O2
1.4
0 −2
1.2 −4 −6
−6
1
−4
−2
0.8
0
0.6

0.4

0.2 Zn (OH)2

0 Zn++
Zn CO3
−0.2
−0.4
−
H ZnO2
−0.6
2−
ZnO2
−0.8
−1 0 −2 −4 −8
−1.2
−1.4

−1.6

−1.8
−2 −1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

FIGURE 1.3
Potential/pH diagram for the system ZnCO2–H2O at 778F/258C.

1.3 Measuring Polarization
Measurement of corrosion rate is essential for the purpose of material
selection. The compatibility of a metal to its environment is a prime
requirement for its reliable performance. Corrosion rate measurement may
become necessary for the evaluation and selection of materials for a specific
environment or a given definite application, or for the evaluation of new or
old metals or alloys to determine the environments in which they are
suitable. Often the corrosive environment is treated to make it less
aggressive, and corrosion rate measurement of a specific material in the
untreated and treated environments will reflect the efficacy of the treatment.
Corrosion rate measurement is also essential in the study of the mechanisms
of corrosion.


Aqueous corrosion is electrochemical in nature. It is therefore possible
to measure corrosion rate by employing electrochemical techniques. Two
methods based on electrochemical polarization are available: The Tafel
extrapolation and linear polarization. Electrochemical methods permit rapid
and precise corrosion-rate measurement and may be used to measure
corrosion rate in systems that cannot be visually inspected or subject to
weight-loss tests. Measurement of the corrosion current while the corrosion
potential is varied is possible with the apparatus shown in Figure 1.4.
Using the example of iron corroding in a hydrochloric acid solution, if the
iron sample is maintained at the natural corrosion potential of K0.2 V, no
current will ﬂow through the auxiliary electrode. The plot of this data point
in the study would equate to that of A or C in Figure 1.5. As the potential is
raised, the current ﬂow will increase and curve AB will approximate the
behavior of the true anodic polarization curve. Alternatively, if the potential
were lowered below K0.2 V, current measurements would result in the
curve CD and approximate the nature of the cathodic polarization curve.
By using the straight line portions, or Tafel regions, of these curves,
an approximation of the corrosion current can be made.
Most often, it is the anodic polarization behavior that is useful in
understanding alloy systems in various environments. Anodic polarization
tests can be conducted with relatively simple equipment and the scans
themselves can be done in a short time. They are extremely useful in

Potentiostat

V
A

Salt bridge

Corrodent
Reference electrode
(Calomel)

Test specimen

Auxiliary electrode (Pt)

FIGURE 1.4
Anodic polarization measurement apparatus.


B
P
Measured
o
t
e
n
t A True
i
a
l C y
x Tafel
region

D

Current, log i

FIGURE 1.5
Anodic and cathodic polarization curves.

studying the active–passive behavior that many materials exhibit. As the
name suggests, these materials can exhibit both a highly corrosion-resistant
behavior and that of a material that corrodes actively, while in the same
corrodent. Metals that commonly exhibit this type of behavior include iron,
titanium, aluminum, chromium, and nickel. Alloys of these materials are
also subject to this type of behavior.
Active–passive behavior is dependent on the material–corrodent com-
bination and is a function of the anodic or cathodic polarization effects that
occur in that specific combination. In most situations where active–passive
behavior occurs, there is a thin layer at the metal surface that is more
resistant to the environment than the underlying metal. In stainless steels,
this layer is composed of various chromium and/or nickel oxides that
exhibit substantially different electrochemical characteristics than the
underlying alloy. If this resistant, or passive, layer is damaged while in the
aggressive environment, active corrosion of the freshly exposed surface will
occur. The damage to this layer can be either mechanical or electrochemical
in nature.
The behavior of iron in nitric acid underscores the importance of
recognizing the nature of passivity. Iron is resistant to corrosion in nitric
acid at concentrations around 70%. Once passivated under these conditions
it can also exhibit low rates of corrosion as the nitric acid is diluted.
However, if the passive film is disturbed, rapid corrosion will begin and
repassivation will not be possible until the nitric acid concentration is raised
to a sufficient level.


1.3.1 Anodic Polarization
Active–passive behavior is schematically represented by the anodic
polarization curve shown in Figure 1.6. Starting at the base of the plot, the
curve starts out with a gradually increasing current, as expected. However,
at point A, there is a dramatic polarizing effect that drops the current to a
point where corrosion is essentially halted. As the potential is increased
further, there is little change in current flow until the next critical stage B,
where the breakdown of the passive film occurs and the corrosion current
begins to increase.
Even with an established anodic polarization behavior, the performance of
a material can vary greatly with relatively minor changes in the corrodent.
This is also illustrated in Figure 1.7. Frame 1 illustrates the case where the
anodic and cathodic polarization curves intersect similar to the behavior of
materials with no active–passive behavior. The anode is actively corroding at
a high but predictable rate.
Frame 2 represents the condition that is often found perplexing when
using materials that exhibit active–passive behavior. With relatively minor
changes within the system, the corrosion current could be very low when the
material is in the passive state or very high when active corrosion begins.
Frame 3 typifies the condition sought when using materials in the passive
state. In this example, the cathodic polarization curve intersects only in
the passive region, resulting in a stable and low corrosion current. This
type of system can tolerate moderate upset conditions without the onset of
accelerated corrosion.
The anodic polarization technique is also useful in studying the effects
of variations in the environment and the benefits of alloy conditions. As

Transpassive
B

Ep
V
o Passive
l
t
s A
Epp

Active

ipass icrit
Log i

FIGURE 1.6
Anodic polarization curve for material exhibiting active–passive behavior.


Frame 1 Frame 2 Frame 3

ic ic ic

FIGURE 1.7
Schematic representation of a material with active–passive behavior in different corrosive
environments.

illustrated in Figure 1.8, temperature increases can cause a shift of the curve to
higher currents. Increasing chromium contents in steel expands the passive
region significantly; adding molybdenum raises the potential required for the
initiation of a pitting-type attack. The presence of chloride or other strong
oxidizing ions will shrink the passive region.

1.4 Other Factors Affecting Corrosion
As has been noted, temperature can have a significant influence on the
corrosion process. This is not surprising because it is an electrochemical
reaction, and reaction rates increase with increasing temperature. There are
additional influences on corrosion other than the corrodent itself.
The relative velocities between the component and the media can have a
direct effect on the corrosion rate. In some instances, increasing the velocity
of the corrodent over the surface of the metal will increase the corrosion rate.
When concentration polarization occurs, the increased velocity of the media
will disperse the concentrating species. However, with passive materials,
increasing the velocity can actually result in lower corrosion rates. This
occurs because the increasing velocity shifts the cathodic polarization curve
such that it no longer intersects the anodic polarization curve in the active
corrosion region, as shown in Figure 1.9.
The surface finish of the component also has an impact on the mode and
severity of the corrosion that can occur. Rough surfaces or tight crevices can
facilitate the formation of concentration cells. Surface cleanliness can also be
an issue with deposits or films acting as initiation sites. Biological growths
can behave as deposits or change the underlying surface chemistry to
promote corrosion.


V V

Increasing temperature
Increasing chloride

Log i Log i

V V

Increasing molybdenum

Increasing chromium

Log i Log i

FIGURE 1.8
Effects of environment and alloy content on anodic polarization behavior.

V

Decreasing
concentration
polarization

Log i

FIGURE 1.9
Increased corrodent velocity can shift the cathodic polarization curve such that passive behavior
can be induced.


Other variations within the metal surface on a microscopic level influence
the corrosion process. Microstructural differences, such as secondary phases
or grain orientation, will affect the way corrosion manifests itself. For
corrosive environments where grain boundaries are attacked, the grain size
of the material plays a significant role in how rapidly the material’s pro-
perties can deteriorate. Chemistry variations in the matrix of weld deposits
are also factors.
Radiation can have an effect on a material’s mechanical properties. The
effect on metallic materials is very gradual and not very pronounced.
Stresses, either residual or applied, impact the mode of corrosion and lower
the energy effect for corrosion to begin. Stress is a requirement for SCC or
corrosion fatigue, but can also influence the rate of general corrosion.
Finally, time is a factor in determining the severity of corrosion. Corrosion
rates are expressed using a time dimension. Some corrosion processes are
violent and rapid, whereas most are so slow as to be imperceptible on a day-
to-day basis. Equipment is planned to have a useful service life. A chief goal
in understanding corrosion is the proper selection of materials, equipment
processes, or controls to optimize our natural and financial resources.

Reference
1. P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed., New York:
Marcel Dekker.

2
Atmospheric Corrosion

Atmospheric corrosion, though not a separate form of corrosion, has
received considerable attention because of the staggering associated costs
that result. With the large number of outdoor structures such as buildings,
fences, bridges, towers, automobiles, ships, and innumerable other
applications exposed to the atmospheric environment, there is no wonder
that so much attention has been given to the subject.
Atmospheric corrosion is a complicated electrochemical process taking
place in corrosion cells consisting of base metal, metallic corrosion products,
surface electrolyte, and the atmosphere. Many variables influence the
corrosion characteristics of an atmosphere. Relative humidity, temperature,
sulfur dioxide content, hydrogen sulfide content, chloride content, amount
of rainfall, dust, and even the position of the exposed metal exhibit marked
influence on corrosion behavior. Geographic location is also a factor.
Because this is an electrochemical process, an electrolyte must be present
on the surface of the metal for corrosion to occur. In the absence of moisture,
which is the common electrolyte associated with atmospheric corrosion,
metals corrode at a negligible rate. For example, carbon steel parts left in the
desert remain bright and tarnish-free over long periods. Also, in climates
where the air temperature is below the freezing point of water or of aqueous
condensation on the metal surface, rusting is negligible because ice is a poor
conductor and does not function effectively as an electrolyte.
Atmospheric corrosion depends not only on the moisture content present
but also on the dust content and the presence of other impurities in the air, all
of which have an effect on the condensation of moisture on the metal surface
and the resulting corrosiveness. Air temperature can also be a factor.
All types of corrosion may take place, depending on the specific
contaminants present and the materials of construction. General corrosion
is the predominant form encountered because of the large quantities of steel
used. However, localized forms such as pitting, intergranular attack, and
stress corrosion cracking may be encountered with susceptible alloys.
Because the available electrolyte consists only of a thin film of condensed or
absorbed moisture, the possibility of galvanic corrosion is somewhat

39


minimized. However, this cannot be relied on and galvanic corrosion must
always be considered in designs for atmospheric exposures.
Synthetic materials as well as metals are also subject to atmospheric
corrosion, depending on the specific synthetic material and the conditions of
exposure. Synthetic materials, plastics, and elastomers can be subject to
degradation as a result of the action of ozone, oxygen, and sunlight. These
three weathering agents can greatly affect the properties and appearance of a
large number of synthetic materials. Surface cracking, discoloration of
colored stock, and serious loss of tensile strength are the result of this attack.
Elastomeric materials may also suffer loss of elongation and other rubber-
like properties.

2.1 Atmospheric Types
Because corrosion rates are affected by local conditions, atmospheres are
generally divided into the following major categories:

Rural
Industrial
Marine

Additional subdivisions, such as urban, arctic, and tropical (wet or dry), can
also be included. However, of main concern are the three major categories.
For all practical purposes, the more rural the area, with little or no heavy
manufacturing operations, or with very dry climatic conditions, the less will
be the problem of atmospheric corrosion.
In an industrial atmosphere, all types of contamination by sulfur in the
form of sulfur dioxide or hydrogen sulfide are important. The burning of
fossil fuels generates a large amount of sulfur dioxide that is converted to
sulfuric and sulfurous acid in the presence of moisture. Combustion of these
fossil fuels and hazardous waste products should produce only carbon
dioxide, water vapor, and inert gas as combustion products. This is seldom
the case. Depending on the impurities contained in the fossil fuel, the
chemical composition of the hazardous waste materials incinerated, and the
combustion conditions encountered, a multitude of other compounds may
be formed.
In addition to the most common contaminants previously mentioned,
pollutants such as hydrogen chloride, chlorine, hydrogen fluoride, and
hydrogen bromide are produced as combustion products from the burning
of chemical wastes. When organophosphorus compounds are incinerated,
corrosive phosphorous compounds are produced. Chlorides are also a
product of municipal incinerators.
Road traffic and energy production lead to the formation of NOx that may
be oxidized to HNO3. This reaction has a very low rate; therefore, in the

Atmospheric Corrosion 41

vicinity of the emission source, the contents of HNO3 and nitrates are very
low. The antipollution regulations that have been enacted do not prevent the
escape into the atmosphere of quantities of these materials sufficient to
prevent corrosion problems. The corrosivity of an industrial atmosphere
diminishes with increasing distance from the city.
Marine environments are subject to chloride attack resulting from the
deposition of fine droplets of crystals formed by evaporation of spray that
has been carried by the wind from the sea. The quantity of chloride
deposition from marine environment is directly proportional to the distance
from the shore. The closer to the shore, the greater the deposition and
corrosive effect. The atmospheric test station at Kure Beach, North Carolina
shows that steels exposed 80 feet from the ocean corrode 10–15 times faster
than steels exposed 800 feet from the ocean.
In addition to these general air contaminants, there may also be specific
pollutants found in a localized area. These may be emitted from a
manufacturing operation on a continuous or spasmodic basis and can result
in a much more serious corrosion problem than that caused by the presence
of general atmospheric pollutants.
Because of these varying conditions, a material that is resistant to
atmospheric corrosion in one area may not be satisfactory in another. For
example, galvanized iron is perfectly suitable for application in rural
atmospheres, but it is not suitable when exposed to industrial atmospheres.
To compound the problem, there is no clear line of demarcation between
these atmospheric types. In many cases there is no “pure” rural or urban
area. Contamination from industrial or marine areas can find its way into
these areas based on the prevailing winds and other atmospheric conditions.
Indoor atmospheres might be free of corrosion in “clean rooms” or subject
to severe corrosion, as near a pickling bath in a steel mill.
Atmospheric conditions should be defined in terms of temperature,
humidity, and contaminants, as well as their corrosivity to specific materials
of construction being considered. In addition to the general atmospheric
condition, special conditions such as cooling tower drift or spray, spills, or
releases of water or chemicals should not be overlooked and must be taken
into account.

2.2 Factors Affecting Atmospheric Corrosion
Atmospheric corrosion is an electrochemical process and, as such, depends
on the presence of an electrolyte. The usual electrolyte associated with
atmospheric corrosion is water resulting from rain, fog, dew, melting snow,
or high humidity. Because an electrolyte is not always present, atmospheric
corrosion is considered a discontinuous process. Corrosion takes place only


during the time of wetness. It can be described by the equation:

X
h
KZ tn Vk ðnÞ
L

where
KZaccumulated corrosion effect
tnZtime of wetness (the period with an electrolyte layer on
the surface)
VkZaverage corrosion rate during the individual periods of wetness,
the composition of the electrolyte, and the temperature determines
the corrosion rate; factors affecting the time of wetness, and the
composition of the electrolyte film will be discussed later.

In addition to the above, there are other factors that affect the corrosion
rate, including initial exposure conditions, sample mass, orientation, extent
of sheltering, wind velocity, nature of the corrosion products formed, and
pollutants present (both known and unknown).

2.2.1 Time of Wetness
The term time of wetness refers to the length of time during which the metal
surface is covered by a film of water that renders significant atmospheric
corrosion possible. The actual time of wetness will vary with climatic
conditions at the location. It is dependent on the relative humidity of the
atmosphere (being greatest when the relative humidity is R80%), the
temperature of the air and the metal surface above 328F/08C, the duration
and frequency of rain, fog, dew, and melting snow, as well as the hours of
sunshine and wind speed.
Moisture on the surface of the metal resulting from high humidity of the
atmosphere and the chemical and physical properties of the corrosion
products produces an adsorption layer of water. Phase layers of water are
the result of rain, fog, wet or melting snow, or dew formed by condensation
on cold metallic surfaces.
The total time of wetness (Ttw) may be divided into the periods when the
metal is moistened due to adsorption of water vapor on the surface (Taw) and
the periods when the surface is covered by a phase layer of water (Tph)
resulting from rain, fog, dew, or wet or melting snow:

Ttw Z Tad C Tph :

It is difficult to distinguish experimentally between the two categories of
time of wetness because there is no sharp boundary.


2.2.1.1 Adsorption Layers
The amount of water adsorbed on a metal surface depends on the relative
humidity of the atmosphere and on the chemical and physical properties of
the corrosion products. The metal surface may be wetted if hygroscopic salts,
deposited or formed by corrosion, absorb water from the atmosphere.

2.2.1.2 Phase Layers
Phase layers may arise from precipitation of rain, fog, or wet or melting
snow, or from dew formed by condensation on cold metallic surfaces.

2.2.1.3 Dew
Dew formation occurs when the temperature of the metal surface is below
the dew point of the atmosphere. This may occur outdoors during the night,
when the surface temperature may decrease by radiant heat transfer
between the structure and the sky. Another reason for dew formation may be
the conditions in the early morning, when the temperature of the air
increases faster than the temperature of the metal, especially if the mass, and
thus the heat capacity, of the metal is great. Dew may also form when metal
products are brought into warm storage after cold transport.

2.2.1.4 Rain
Rain creates even thicker layers of electrolyte on the surface than dew. The
thickness of the water layer retained on the surface has been estimated to be
approximately 100 g/m2. Precipitation in the form of rain affects corrosion
by giving rise to a phase layer of moisture on the material surface and by
adding corrosion stimulaters in the form of, e.g., HC and SO4 . On the other
2K

hand, rain also washes away pollutants deposited on the surface during the
preceding dry period. Whereas the first two processes promote corrosion,
the third—at least in the case of steel—decreases corrosion. The significance
of the two latter processes is dependent on the ratio between the dry and wet
deposition of pollutants.
In a strongly polluted atmosphere, corrosion on the skyward side of
steel plates is substantially lower than on the downward side. In a strongly
polluted atmosphere where dry deposition is considerably greater than
deposition of sulfur pollutants, the washing effect of rain predominates. In
a less-polluted area, the situation is reversed, which indicates that the
corrosive action of rain, in this case, is more important.
The pH value of precipitation seems to be of significance for metals whose
corrosion resistance may be ascribed to a protective layer of basic carbonates
or sulfates, as on zinc or copper. If the pH of rainwater falls to values close to
4 or even lower, this may lead to accelerated dissolution of the protective
coatings.


2.2.1.5 Fog
Especially high acidity and high concentrations of sulfates and nitrates can
be found in fog droplets in areas of high air pollution. In California, the pH
of fog water has been found to be in the range of 2.2–4.0. The processes
controlling the fog water chemistry appear to be condensation of water
vapor on, and its evaporation from, preexisting aerosol, and scavenging of
gas-phase nitric acid.

2.2.1.6 Dust
On a weight basis in many locations, dust is the primary air contaminant.
When in contact with metallic surfaces and combined with moisture, dust
can promote corrosion by forming galvanic or differential cells that, because
of their hygroscopic nature, form an electrolyte on the surface. Suspended
particles of carbon and carbon compounds, metal oxides, sulfuric acid,
ammonium sulfate, sodium chloride, and other salts will be found in
industrial atmospheres. It is these materials, when combined with moisture,
that initiate corrosion.
The settled dust may promote corrosion by absorbing sulfur dioxide and
water vapor from the air. Hygroscopic salts such as chlorides or sulfates
form a corrosive electrolyte on the surface. Carbonaceous particles can start
the corrosion process by forming cathodes in microcells with a steel surface.
Dust-free air is less likely to cause corrosion.

2.2.1.7 Measurement of Time of Wetness
For practical purposes, the time of wetness is usually determined on the
basis of meteorological measurements of temperature and relative humidity.
The period when the relative humidity is 80% at temperatures 08C/328F is
often used for estimating the actual time of wetness. The time of wetness
determined by this method may not necessarily be the same as the “actual”
time of wetness, because wetness is influenced by the type of metal,
pollution of the atmosphere, presence of corrosion products, and degree of
coverage against rain. The expression for time of wetness mentioned above,
although not based on a detailed theoretical model, usually shows good
correlation with corrosion data from field tests under outdoor conditions.
This implies that this parameter corresponds to the kinetically decisive time
periods during which corrosion proceeds. Under sheltered and indoor
conditions, however, other criteria seem to be valid, although they have not
yet been fully clarified.
The time of wetness may also be measured directly with electrochemical
cells. The cell consists of thin metal electrodes (0.5–1 mm) separated from
each other by a thin insulation (0.1–0.2 mm). When the surface becomes
wetted, a current starts to flow and the time of wetness is defined as the time
when the cell current or the electromotive force exceeds a certain value. Not
even the direct electrochemical measurement of the time of wetness will give


an unambiguous value because the result depends on, among other factors,
the type of cell, its dimensions, the presence of corrosion products, and the
definition of the threshold value of current or voltage that is considered the
lower limit for the time of wetness. Most of the electrochemical techniques
indicate mainly the time of wetness caused by phase layers of electrolyte,
and they usually give lower values than calculations from meteorological
data. Values between 1000 and 2700 h/year are often reported from sites in
the temperate climatic zone.

2.2.2 Composition of Surface Electrolyte
The electrolyte film on the surface will contain various species deposited
from the atmosphere or originating from the corroding metal. The
composition of the electrolyte is the determining factor of the corrosion
process. In the following, a brief survey will be presented of the origin,
transformation reactions, deposition mechanisms, and pollutants.

2.2.2.1 Oxygen
Oxygen is readily absorbed from the air; therefore, at least the outer region of
the thin water film on the metal surface may be considered saturated
with oxygen.

2.2.2.2 SOX
The main part of anthropogenic SOx pollution is caused by combustion of
fossil fuels, i.e., oil and coal in industrial regions that can cover less than 5%
of the earth’s surface.
Most of the sulfur derived from the burning of fossil fuels is emitted in
gaseous form as SO2. Both the chemical composition and the physical state of
the pollutants change during their transport in the atmosphere. The sulfur
dioxide is oxidized on moist particles or in droplets of water to sulfuric acid.

2.2.2.3 NOX
Emissions of NOx originate primarily from different combustion processes,
road traffic, and energy production. Since 1940, the atmospheric emission of
NOx relative to SO2 (in equivalents) has increased markedly, which stresses
the importance of NOx emissions. In combustion processes, most of the
nitrogen oxides are emitted as NO.

2.2.2.4 Chlorides
Chlorides are deposited mainly in the marine atmosphere as droplets or as
crystals formed by the evaporation of spray carried by the wind from the
seas. Other sources of chloride emission are coal burning and municipal


incinerators. Most coals have a chloride content of 0.09–0.15%. In high-
chlorine coals, values of 0.7% are found. In the burning of coal, most of the
chlorine is emitted as gaseous HCl.

2.2.2.5 CO2
Carbon dioxide occurs in the atmosphere in a concentration of 0.03–0.05% by
volume, varying slightly with the time of day and the season of the year due
to its cycle in nature.

2.2.2.6 Concentrations of Different Species
The concentrations of the various species in the electrolyte on the surface
vary greatly with respect to such parameters as deposition rates, corrosion
rate, intervals between rain washings, presence of rain shelter, and drying
conditions.
It would be expected that the concentration in the electrolyte film will be
low during a rainy period, while a highly concentrated solution may form
after a long period without washing.
The pH of the water film is difficult to specify. A moisture film in contact
with an atmosphere highly polluted with SOx may initially have a pH value
as low as 2. Due to acid rain or fog, the moisture film may also have a
low pH value. Because of reaction with the metal and the corrosion products,
the pH value will usually increase. When a steady state has been reached, the
pH is generally on the order of 5–6.

2.2.3 Temperature
The overall effect of temperature on corrosion rates is complex. During long-
term exposure in a temperate climatic zone, temperature appears to have little
or no effect on the corrosion rate. As the temperature increases, the rate
of corrosive attack will increase as a result of an increase in the rate of
electrochemical and chemical reactions as well as the diffusion rate.
Consequently, under constant humidity conditions, a temperature increase
will promote corrosion; conversely, an increase in temperature can cause a
decrease in the corrosion rate by causing a more rapid evaporation of the
surface moisture film created by rain or dew. This reduces the time of wetness
that in turn reduces the corrosion rate. In addition, as the temperature
increases, the solubility of oxygen and other corrosive gases in the electrolyte
film is reduced.
When the air temperature falls below 328F/08C, the electrolyte film might
freeze. As freezing occurs, there is a pronounced decrease in the corrosion
rate that is illustrated by the low corrosion rates in the subartic and arctic
regions.
In general, temperature is a factor influencing corrosion rates, but it is of
little importance except under extreme conditions.


2.2.4 Initial Exposure
Atmospheric corrosion rates proceed through three stages or periods: the
induction period, the transition period, and the stationary period. During
the induction period, the metal is covered with both a spontaneously formed
oxide and the aqueous layer. This oxide provides some degree of protection,
depending on the metal and the aggressiveness of the atmosphere. During
the transition period, the oxide layer transforms into a fully developed layer
of corrosion products. The final or stationary period is characterized by the
surface being fully covered by corrosion product and eventually reaching
constant properties with respect to chemical composition and stationary
corrosion rates. The more aggressive the exposure conditions, the shorter the
two initial periods. For example, in a benign (indoor) atmosphere, these
initial periods for steel will last for years, but in highly polluted industrial
areas, these initial periods might last only a few months.
The initial exposure conditions have a marked influence on the
subsequent corrosion rate. During the first days of exposure, wet conditions
(caused by high relative humidity or rainfall) cause higher corrosion rates
than dry conditions. These effects will vary from one material to another. For
example, zinc is more sensitive than steel. Differences are explained by the
fact that different materials form different corrosion products with different
protective properties. A wide variety of structurally related corrosion
products can be found on zinc, the nature of which depends on initial
exposure conditions. The seasonal dependence on the concentrations of
peroxide and ozone in the atmosphere might also be a contributing factor.
During the third period of exposure, constant corrosion rate is established,
thus the terminology “stationary period.”

2.2.5 Sheltering
The design of the installation should eliminate all possible areas where
water, dirt, and corrosion products can accumulate. The degree of shelter
from particle fallout and rainfall can affect initial and ultimate corrosion
rates. When pockets are present, the time of wetness increases, which leads
to corrosive conditions. The design should make maximum use of exposure
to the weather. Sheltering from rain should be avoided.

2.2.6 Wind Velocity
Wind speed and type of wind flow have a pronounced effect on the
atmospheric corrosion rate. This is illustrated by the dry deposition velocity
that is defined as the ratio of deposition rate of any gaseous compound and
the concentration of that compound in the atmosphere.

2.2.7 Nature of Corrosion Products
The atmospheric corrosion rate is influenced by many parameters, one of the
more important being the formation and protective ability of the corrosion


products formed. The composition of the corrosion products depends on the
participating dissolved metal ions and the access to anions solved in the
aqueous layer. The eventual thickening of the film of corrosion products can
be described in a sequence of consecutive steps—dissolution, coordination,
reprecipitation. Where the dissolution step is acid-dependent, the coordi-
nation is based on the hard and soft acid base (HSAB) principle, and
reprecipitation depends on the activities of the species involved.
Depending on the rate of crystallation and the rate of formation, the
corrosion products may be amorphous or crystalline. If the former is rate-
determining, one expects amorphous phases to form. From colloid
chemistry, it is known that aging, or slow growth, of amorphous phases
may result in a transition from the amorphous to the crystalline state, a
process that may occur through slow transformation in the solid state or
through the dissolution–reprecipitation process.
The corrosion products formed are the composition of the metal or alloy
and the contaminants present in the atmosphere. For example, carbon steel
does not have the ability to form its own protective coating except in a dry,
clean atmosphere. Under these conditions, a thick oxide film will form that
prevents further oxidation. Initiation of corrosion is the result of the presence
of solid particles on the surface. This settled airborne dust promotes
corrosion by absorbing SO2 and water vapor from the air. Even greater
corrosive effects result when particles of hygroscopic salts, such as chlorides
or sulfates, settle on the surface and form a corrosive electrolyte.
When small amounts of copper, chromium, nickel, phosphorus, silicon,
manganese, or various combinations thereof are added to conventional
carbon steel, low-alloy carbon steel results. These steels are known as
weathering steels. The corrosion resistance of these steels is dependent upon the
climatic conditions, the pollution levels, the degree of sheltering from the
atmosphere, and the specific composition of the steel. Upon exposure to most
atmospheres, the corrosion rate becomes stabilized within 3–5 years. A dark-
brown to violet patina, or protective film, develops over this period. This
patina is a rust formation that is tightly adhered to the surface and cannot be
wiped off. In rural areas with little or no pollution, a longer period may be
required to form this film. In areas that are highly polluted with SO2, the
weathering steels exhibit a much higher corrosion rate and loose rust particles
form. Under these conditions, the film formed offers little or no protection.
Additional information regarding the weathering steels and other metals
or alloys as to their resistance to atmospheric corrosion will be found in the
chapter dealing with the specific material.

2.2.8 Pollutants Present
One of the most important factors affecting atmospheric corrosion is the
presence of specific pollutants. In areas having low atmospheric pollution,
corrosion rates are correspondingly low. The presence of atmospheric


pollutants, such as the various oxides of nitrogen, sulfur-containing
compounds, chlorine-containing compounds, and other less common
pollutants, will stimulate corrosion.
It has been proven that the following gaseous constituents are of
significant importance in contributing to atmospheric corrosion: O2, H2O2,
SO2, H2S, COS, NO2, HNO3, NH3, HCl, Cl2, HCHO, and HCOOH. Typical
ranges of these materials as found under outdoor and indoor conditions are
given in Table 2.1 and Table 2.2. They may be present as the result of either
natural or anthropogenic processes, and may undergo a variety of chemical
changes during transport in the atmosphere.
All of the species are reactive and as such have a certain average lifetime
that is limited by the ability to react with atmospheric oxidizers, primarily
the hydroxyl radical, OHK, and O3. OHK is generated by photoinduced
dissociation of O3 (ozone) and the subsequent reaction of the electronically
excited, energy-rich oxygen atom O ( 0 D) and water vapor:

O3 C hn/ ðO 0 DÞ C O2 ðl! 310 nmÞ
Oð 0 DÞ C H2 O/ 2OH$

It is possible for the OH$ molecules to oxidize several of the species, such
as SO2, H2S, and NO. However, a large portion of the OH$ molecules are
consumed through reactions with hydrocarbon molecules, producing an
end-product of HO2$ (the hydroperoxyl radical). This radical converts to
hydrogen peroxide (H2O2) and O2 according to

HO2 $ C HO2 $/ H2 O2 C O2 :

H2O2 is highly soluble in aqueous environments and is a powerful
oxidizing agent.
However, the degree to which these pollutants will affect the corrosion
rate will be influenced by the other preceding factors that also have an
influence on the corrosion rate. The corrosive effect of these pollutants will
be dependent on the specific material with which they come into contact.
Succeeding chapters deal with the mechanisms of corrosion of these
pollutants and their effects on specific materials.

2.3 Mechanisms of Atmospheric Corrosion of Metals
Atmospheric corrosion takes place by means of an electrochemical process
occurring in corrosion cells. A corrosion cell must have the following
essential components:

1. A metal anode
2. A metal cathode

50

TABLE 2.1
Characteristics of Selected Gaseous Air Constituents
Equilibrium Concentration
(mM) Deposition Velocity (cm/s) Deposition Rate (ng/cm2)
Gas H M(atm) Outdoor Indoor Outdoor Indoor Outdoor Indoor

O3 1.8 (K2) 2.3 (K4) 1.7 (K4) 0.05–1 0.036 5.8 (K3) 6.8 (K4)
H2O2 2.4 (5) 4.2 (3) 1.2 (3) — 0.07 — 5.0 (K4)
SO2 1.4 — 1.1 (K2) 2.9 (K3) 0.1–1.2 0.05 7.5 (K3) 2.7 (K4)
H2S 1.5 (K1) 6.1 (K4) 4.0 (K5) 0.38 0.03 2.2 (K3) 1.1 (K5)
NO2 7.0 (K3) 1.9 (K4) 3.8 (K5) 0.2–0.8 0.006 2.0 (K2) 6.2 (K5)
HNO3 9.1 (4) 2.9 (2) 2.7 (2) 0.1–30 0.07 1.4 (K2) 5.5 (K4)
NH3 1.0 (1) 1.1 (K1) 5.8 (K1) 0.3–2.6 0.05 6.6 (K3) 2.1 (K3)
HCl 2.0 (1) 1.5 (K2) 1.9 (K3) — 0.04 — 5.8 (K6)
Cl2 6.2 (K2) 1.2 (K6) 1.4 (K7) 1.8–2.1 — 1.1 (K4) —
HCHO 1.4 (4) 1.1 (2) 1.4 (2) — 0.005 — 6.3 (K5)
HCOOH 3.7 (3) 3.3 (1) 7.4 (1) — 0.006 — 2.3 (K4)

The equilibrium solution concentration and deposition rate values were based on correlations from Table 2.2, and using geometric mean values for the
intervals 1.8 (K2) means 1.8!10K2.
Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals


TABLE 2.2
Concentration of Selected Gaseous Air Constituents (ppb)
Gas Outdoor Indoor

O3 4–42 3–30
H2O2 10–30 5
SO2 1–65 0.3–14
H2S 0.7–24 0.1–0.7
NO2 9–78 1–29
HNO3 1–10 3
NH3 7–16 13–259
HCl 0.18–3 0.05–0.18
Cl2 !0.005–0.08a 0.001–0.005
HCHO 4–15 10
HCOOH 4–20 20
a
Corresponding to 5 wt% HCl.

3. A metallic conductor between the anode and cathode
4. An electrolyte (water containing conductive salts) in contact with
the anode and cathode, but not necessarily of the same composition
at the two locations

In addition, oxygen will usually be present as a depolarizing agent. For the
cell to function, there must be a difference in potential between the anode
and cathode. This difference in potential is mainly due to contact between
dissimilar metallic conductors or differences in concentration of the solution,
generally related to dissolved oxygen in natural waters. Almost any lack of
homogeneity on the metal surface or in its environment may initiate attack
by causing a differential in potentials that results in more or less localization
of corrosion.
Atmospheric corrosion differs from the action that occurs in water or
underground in that a plentiful supply of oxygen is always present. In this
case, the formation of insoluble ﬁlms and the presence of moisture and
deposits from the atmosphere become the controlling factors. The presence
of contaminants such as sulfur compounds and salt particles also affects the
corrosion rate. Nevertheless, atmospheric corrosion is mainly electro-
chemical, rather than a direct chemical attack by the elements. The anodic
and cathodic areas are usually quite small and close together, so that
corrosion is apparently uniform rather than in the form of severe pitting as in
soil or water. Anodes and cathodes exist on all steel surfaces. Surface
imperfections, grain orientation, lack of homogeneity of the metal, variation
in the environment, localized shear and torque during manufacture, mill
scale, and existing red iron rust will produce anodes and cathodes. It is a
relatively complicated system consisting of a metal, corrosion products,


surface electrolyte, and the atmosphere. For an electrochemical reaction to
proceed, two or more electrode reactions must take place: the oxidation of a
metal (anodic partial reaction) and the reduction of an oxidizing agent
(cathodic partial reaction). For the electrode reactions to take place, it is
necessary that an electrolyte be present.
Most metals, if exposed to the atmosphere at room temperature with
virtually no humidity present, will form an oxide film. If the oxide film is
stable, the growth stops and the oxide film reaches a maximum thickness of
1–5 mm, protecting the metal.
Atmospheric corrosion can fall into two categories: “damp” atmospheric
corrosion that takes place in the presence of water vapor and traces of
pollutants, and “wet” atmospheric corrosion that occurs when rain or other
forms of bulk water are present with pollutants. For atmospheric corrosion
to proceed, water of some form must be present.
Damp atmospheric corrosion occurs when water is present on the
surface of the metal as an aqueous phase layer, as caused in humidity,
dew, or fog. Wet atmospheric corrosion occurs when bulk water is present,
such as rain.

2.3.1 Damp Atmospheric Corrosion (Adsorption Layers)
Aqueous phase layers consist of water adsorbed on the metal surface. The
amount formed is dependent on the relative humidity of the atmosphere and
the chemical and physical properties of the corrosion products.
Water may be absorbed from the atmosphere and wet the metal surface if
hygroscopic salts are deposited or formed by corrosion. This absorption will
take place when the relative humidity exceeds the critical relative humidity.
The value of the critical relative humidity is dependent on the specific metal
and the specific metal contaminants. When the relative humidity exceeds the
value at which the salt starts to absorb water and dissolve, the corrosion rate
increases sharply. This critical relative humidity corresponds to the vapor
pressure above a saturated solution of the salt present. Adsorption layers of
electrolyte on the surface of the metal may also be the result of capillary
condensation.
The corrosion rate is directly influenced by the amount of water present on
the corroding surface. Laboratory studies have shown that the corrosion rate
above the critical value sharply increases with increasing relative humidity.
The amount of water present on a metal surface has been roughly estimated
as follows:

Conditions Amount of Water (g/m2)

Critical relative humidity 0.01
100% relative humidity 1
Covered by dew 10
Wet from rain 100


Atmospheric corrosion resulting from the reaction of water vapor with a
metal surface is a serious problem. Most clean metal surfaces will permit the
bonding of water in molecular form. The oxygen atom bonds to the metal
surface and acts as a Lewis base (donating an electron pair) because the
bonding is connected with a net charge transfer from the water molecule to
the surface. The water adsorbs on electron-deficient adsorption sites.
It is also possible for water to bond in dissociated form. In this case, the
driving force is the formation of metal–oxygen or metal–hydroxyl bonds. The
end-products formed as a result of the water adsorption are adsorbed hydroxyl,
atomic oxygen, and atomic hydrogen. When metal oxides are present, water
may adsorb in either dissociative or molecular form. Lattice defect sites seem to
facilitate dissociation, as observed, for instance, on monocrystalline TiO2, NiO,
and a-Fe2O3. The dissociation of water forms a mono-molecular thick film of
surface hydroxyl groups that is relatively protective and reduces the
subsequent reaction rate of water. The first monolayer of water adsorbed to
the hydroxylated oxide surface is highly immobile, whereas the second and
third layers are more randomly oriented and less immobile.
Many different metals adsorb water in similar manners, forming metal
oxyhydroxides. The exact nature of the oxyhydroxide formed seems to have
only a minor influence on the water adsorption phenomena. The quantity of
reversibly adsorbed water increases with relative humidity and time.
Reference [1] refer to Table 2.3 for the approximate number of monolayers
of water at 778F/258C and steady-state conditions, which have been
experimentally determined by the quartz crystal microbalance method on a
number of metals.1
Gaseous constituents of the atmosphere dissolve in the aqueous layers
formed. Corrosive attack is generally found in areas where water adsorption
is favored, permitting easy dissolution of the gaseous molecules such as SO2
and NO2. The properties of wet atmospheric corrosion are approached when
the aqueous films are greater than approximately three monolayers. At this
point, the relative humidity is close to the “critical relative humidity.” At
values above the critical relative humidity, atmospheric corrosion rates
increase appreciably, whereas below this value atmospheric corrosion is
negligible. The critical relative humidity varies for different metals and
pollutants.

TABLE 2.3
Approximate Number of Water Monolayers on
Different Metals vs. Relative Humidity
Relative Humidity (%) Number of Monolayers

20 1
40 1.5–2
60 2–5
80 5–10


2.3.2 Wet Atmospheric Corrosion (Phase Layers)
Wet atmospheric corrosion results from repeated wet and dry cycles, the
presence of pollutants, and the formation of an aqueous layer in which the
atmospheric pollutants dissolve. The wet cycles result from dew, fog, rain, or
snow. In many cases, the dew, fog, rain, or snow may already contain the
dissolved corrodent that then deposits on the surface.

2.3.2.1 Dew
Dew is an important source of atmospheric corrosion—more so than rain—
and particularly under sheltered conditions. Dew forms when the
temperature of the metal surface falls below the dew point of the
atmosphere. This can occur outdoors during the night when the surface
temperature of the metal is lowered as a result of radiant heat transfer
between the metal and the sky. It is also common for dew to form during the
early morning hours when the air temperature rises more quickly than the
metal temperature. Dew may also form when metal products are brought
into warm storage after cold shipment.
Under sheltered conditions, dew is an important cause of corrosion. The
high corrosivity of dew is a result of several factors:

1. Relatively speaking, the concentration of contaminants in dew is
higher than in rainwater, which leads to lower pH values. Heavily
industrialized areas have reported pH values of dew in the range
of 3 or lower.
2. The washing effect that occurs with rain is usually slight or
negligible with dew.

With little or no run-off, the pollutants remain in the electrolyte and
continue their corrosive action. As the dew dries, these contaminants remain
on the surface to repeat their corrosive activity with subsequent dew
formation.

2.3.2.2 Rain
Depending on the conditions, rain can either increase or decrease the effects
of atmospheric corrosion. Corrosive action is caused by rain when a phase
layer of moisture is formed on the metal surface. Rain creates thicker layers
of electrolyte on the surface than dew. The corrosive activity is increased
when the rain washes corrosive promoters such as HC and SO2K from the air
4
(acid rain). Rain has the ability to decrease corrosive action on the surface of
the metal as a result of washing away the pollutants deposited during the
preceding dry spell.
Whether the rain will increase or decrease the corrosive action is
dependent on the ratio of deposition between the dry and wet contaminants.


When the dry period deposition of pollutants is greater than the wet period
deposition of surface compounds, the washing effect of the rain will
dominate and the corrosive action will be decreased.
In areas where the air is less heavily polluted, the corrosive action of the
rain will assume a much greater importance because it will increase the
corrosion rate.
Another factor affecting the relative corrosive rate resulting from rain is
the orientation of the metal surface. In areas of heavy industrial pollution,
skyward-facing metallic surfaces benefit from rain. In those areas where dry
deposition is considerably greater than wet deposition of sulfur pollutants,
the washing effect of rain predominates, and the corrosion rate is reduced. In
areas having less pollution, the situation is reversed and the corrosive action
of the rain predominates.

2.3.2.3 Fog
In areas having a high degree of air pollution, high acidity and high con-
centrations of sulfate and nitrate can be found in fog droplets. The pH of
fog water has been found to be in the range of 2.2–4.0 in highly contaminated
areas.

2.3.3 Deposit of Pollutants
Atmospheric pollutants can be deposited into the aqueous layer by either
wet or dry deposition. For wet deposition to take place, it is necessary for
rain, fog, dew, or snow to be present, whereas for dry deposition,
precipitation of any kind is not involved. Dry deposition is considered to
be predominant indoors or in highly polluted areas close to emission
sources. It is difficult to determine the relative importance of wet deposition
because of the incidental nature of the precipitation.
Dry deposition is controlled by two factors: aerodynamic processes and
surface processes. Aerodynamic processes relate to the actual depletion of
the gaseous constituent in the atmosphere (e.g., SO2), in the atmospheric
region adjacent to the aqueous phase, and the ability of the system to add
new SO2 into this region. Whether or not new SO2 can be introduced into the
region is dependent on the actual wind speed, type of wind flow, and shape
of the sample. The ability of the surface layer to accommodate the gaseous
constituent (e.g., SO2) is a phenonomen of the aqueous layer, which is a
surface process. Greater amounts of gaseous constituents (SO2) can be
accommodated with increases in the thickness of the aqueous layer, resulting
from increased relative humidity, the pH of the solution, and the alkalinity of
the solid surface.
The dry deposition velocity is defined as the ratio of deposition rate, or
surface flux, of any gaseous compound and the concentration of the


same compound in the atmosphere. It can be expressed as the inverse of
the sum of two resistances, namely, aerodynamic resistance and surface
resistance:

1
Vd Z
Ra C Rs

where
VdZdry deposition velocity
RaZaerodynamic resistance
RsZsurface resistance

In general, the dry deposition velocity will be the combined effect of both
resistances. However, at highly turbulent air flow conditions RaZ0 and the
dry deposition velocity is dependent only on the surface processes. Alkaline
surfaces, such as lead peroxide or triethanolamine, are ideal absorbers of SO2
for which RsZ0. In this case, the dry deposition velocity if dependent on the
aerodynamic processes. Typical ranges for dry deposition velocities onto
various materials under outdoor and indoor conditions are given in
Table 2.1.
In outdoor exposure conditions subject to wet–dry cycles, the actual
concentration of most corrosion-stimulating gases under many conditions is
not at equilibrium between the gas in the atmosphere and the same gas in the
aqueous layer. Even so, thermodynamic considerations have been used for
predicting the formation of different corrosion end-products and their
stability. Figure 2.1 is a schematic illustration of processes occurring at the
aqueous layer.

2.4 Corrosion Products
One of the most important factors influencing the corrosion rate is the
formation and protective ability of the corrosion products formed. The
specific corrosion products formed are dependent on the participating
dissolved metal ions and the access to anions solved in the aqueous layer.
Formation of the film of corrosion products take place in a sequence of
consecutive steps—dissolution, coordination, and reprecipitation. When
the dissolution step is acid-dependent, coordination is based on the hard
and soft acid base principle (i.e., hard acids are preferably coordinated
with hard bases and soft acids are preferably coordinated with soft bases).
Acids or bases with tightly held valence electrons that are not easily
distorted are hard acids or bases. Acids or bases having valence electrons


Process Result

H H H H H
Dissociative adsorption O O O O O Hydroxylated
of water. metal oxide.

Reversible adsorption Formation of
of water. aqueous layer.

Electrochemical reactions, Formation of
e.g., Mex(OH)z
Me→Men+ + ne− or MexOy(OH)z.
1 O + H O + 2e− → 2OH−.
2 2 2

SO2
Deposition of atmospheric
species, e.g., SO2. −
Dissolution of species, H2SO3 → H+ + HSO3 Acidification of
− + + SO 2−
Henry's law. HSO3 → H 3
aqueous layer.

SO2 2−
SO4
Alt: Oxidation of atmos- oxidants
pheric species, e.g., by H2SO3 → H2SO4 Stronger acidification of
oxidants or catalytic action H2SO4 → H+ + SO4 − aqueous layer.
or deposition of oxidized − + + SO 2−
HSO4 → H 4
species.

FIGURE 2.1
Schematic illustration of processes occurring in or at the aqueous layer.

that are easily polarized or removed are considered to be soft acids
or bases.
Based on experience with atmospheric corrosion, Table 2.4 indicates that
hard acids like Cr3C and Ti4C form oxygen-containing ﬁlms, whereas soft acids
such as CuC and AgC coordinate with reduced sulfur compounds. Inter-
mediate acids such as Fe2C, Cu2C, and Zn2C would be expected to coordinate
with a broader range of bases.
The corrosion products formed may be amorphous or crystalline,
depending on the rate of crystallization and formation. It is known that
slow growth or aging of amorphous phases may result in a change to the
crystalline state. This process can occur through slow transformation in
the solid state or through dissolution–reprecipitation processes. Such is the
case in the transition from amorphous to crystalline state of basic nickel
sulfates, with the former being less corrosion-resistant than the latter.


TABLE 2.4
Classification of Hard and Soft Acids and Bases
Hard Intermediate Soft

Acids
HC, NaC, Mn2C, Al3C, Cr3C, Fe3C, Ti4C Fe2C, Ni2C, Cu2C, Zn2C, Pb2C CuC, AgC
Bases
H2 O, OHK, O2K, SO2K, NOK, CU2K
4 3 3 SO2K, NOK
3 2 R2S, RSH, RS

2.5 Specific Atmospheric Corrodents
The atmospheric region closest to the earth is known as the troposphere and
contains nitrogen, oxygen, and the rare gases Ne, Kr, He, and Xe. Of all the
molecules involved, these make up 99.9% by weight and of these, only
oxygen plays a part in atmospheric corrosion. The remaining constituents,
nitrogen and the rare gases, due to their inability to react with metal surfaces,
are not of significant importance to atmospheric corrosion.
Oxygen, because of its ability to accept electrons and its involvement in
chemical transformations of the atmosphere, is particularly important to
atmospheric corrosion. Other materials present in the troposphere that play
a part in atmospheric corrosion are water and carbon dioxide. Water acts as
an electrolyte and carbon dioxide, which has a concentration of approxi-
mately 330 ppm and is highly soluble in water, contributes to the acidity of
the aqueous layer.
Other trace gases present with a total concentration of less than 10 ppm,
which are also of importance in atmospheric corrosion, are O3, H2O2, SO2,
H2S, COS, NO2, HNO3, NH3, HCl, Cl2, HCHO, and HCOOH. Either natural
or anthropogenic processes are responsible for their presence, and they
may undergo a variety of chemical changes during their presence in the
troposphere. All species are reactive and exhibit a specific average lifetime
that is limited by their ability to react with important atmospheric oxidizers,
specifically the hydroxyl radical OHK and O3. The hydroxyl radical is
formed by the photoinduced dissociation of ozone and the subsequent
reaction of the electronically excited, energy-rich oxygenation O( 0 D) and
water vapor:

O3 C hn/ Oð 0 DÞ C O2 ðl! 310 nmÞ
Oð 0 DÞ C H2 O/ 2OHK

While it is possible for OHK molecules to oxidize several of the corrosion-
stimulating materials such as SO2, H2S, and NO2, a large portion of the
hydroxyl molecules are consumed through reactions with hydrocarbon


molecules, producing the hydroperoxyl radical, HO2, that disproportionates
into hydrogen peroxide (H2O2) and oxygen (O2) according to:

HO2 C HO2 / H2 O2 C O2

Hydrogen peroxide is a powerful oxidizing agent that is highly soluble
in water.

2.5.1 Sulfur-Containing Compounds
The most important corrosive contaminant found in industrial atmospheres
is sulfur dioxide (SO2), which results from the combustion of sulfur-
containing coal and oil, and emission from metal, petrochemical, and pulp
and paper industries. Once in the atmosphere, SO2 undergoes physical and
chemical state changes. Depending on the environment, the sulfur dioxide is
capable of being oxidized in one or more of the following ways:

GM
SO2 C OH/ HSO3 $
HSO3 $ C O2 / SO3 C HO2 $
SO3 C H2 O/ H2 SO4

where GM represents another gaseous molecule, such as nitrogen or oxygen,
that collides with an HSO3$ molecule and removes some of the excess
energy released.
In the aqueous phase:

SO2 C XH2 O Z SO2 $XH2 O
SO2 $XH2 O Z HSOK C H3 OC C ðXK2ÞH2 O C H2 O2
3

HSOK/ SO2K
3 4

On moist particles or in droplets of water, the SO2 may be oxidized to
sulfuric acid:

SO2 C H2 O C 1 O2 / H2 SO4
2

Sulfur dioxide has a lifetime in the atmosphere of 0.5–2 days. This limits
the distance that the SO2 may be transported to a few hundred kilometers.
During this period, the sulfuric acid may be partly neutralized, particularly
with ammonia that results from the biological decomposition of organic
matter. When this occurs, particles containing ammonium sulfate (NH4)2SO4
and different forms of acid ammonium sulfate, such as NH4HSO4 and
(NH4)3H(SO4)2, are formed.


Atmospheric corrosion results from the deposition of these various
materials on metal surfaces. Deposition of the sulfur compounds can be
accomplished by:

1. Dry deposition
a. Adsorption of gas (SO2) on metal surfaces
b. Impaction of sulfate particles
2. Wet deposition
a. Removal of gas from the atmosphere by precipitation in the
form of rain or fog

The primary cause of atmospheric corrosion is dry deposition, which
consists mainly of the adsorption of sulfur dioxide. The amount deposited is
proportional to the concentration in the atmosphere. Different materials are
subject to different deposition rates. Rusty steel surfaces will adsorb SO2
quantitatively at high relative humidities, whereas the deposition on copper,
and particularly on aluminum, is much less. The rate of dry deposition of
other sulfur compounds is less than that of sulfur dioxide.
Sulfates are deposited primarily by wet deposition and experience a
lifetime of 3–5 days. This permits these particles to be transported a distance
on the order of 1000 km. Sulfate concentrations are usually less than SO2
concentrations close to the emission source.
The primary cause of atmospheric corrosion is the dry deposition of sulfur
dioxide on metallic surfaces. This type of corrosion is usually confined to
areas having a large population, many structures, and severe pollution.
Therefore, the atmospheric corrosion caused by sulfur pollutants is usually
restricted to an area close to the source.
Under these conditions, dry deposition of SO2 is considered to be the most
important sulfur deposition process for the corrosion of structural metals.
As previously stated, the deposition rate is dependent on the concen-
tration in the air. Because this concentration can vary considerably, it is
difficult to give ranges. Order-of-magnitude deposition rates for SO2 in
various types of atmospheres are as follows:

Type of Atmosphere Deposition Rate (mg SO2/m2 day)

Rural !10
Urban 10–100
Industrial Up to 200

Another atmospheric corrosion stimulant is hydrogen sulfide (H2S). Natural
biological sources, such as volcanoes, moss, or swamp areas, and
anthropogenic sources such as pulp and paper industries, catalyst
converters in motor vehicles, sewage plants, garbage dumps, animal
shelters, and geothermal plants, are responsible for the emission of H2S.
Hydrogen sulfide can cause the tarnishing of silver and copper by the


formation of tarnish films. Hydrogen sulfide can react with OHK to form
SO2 as follows:

H2 S C OHK/ HS$C H2 O
HS$ C 2O2 / HO2 $C SO2

2.5.2 Nitrogen-Containing Compounds
High-temperature combustion processes such as in power plants, vehicles,
etc., produce NO and NO2. The combustion gas produced has a much higher
percentage of NO than NO2; however, the NO is rapidly converted to NO2
according to:

2NO C O2 / 2NO2 $

At distances further from the emission source, the NO may also form NO2
through the influence of ozone (O3) as follows:

NO C O3 / NO2 C O2 $

The primary nitrogen pollutant near the emission source is nitrogen
dioxide (NO2). The ratio of NO2 to NO in the atmosphere varies with time
and distance from the emission source and is usually between 10 and 100.
Nitrogen dioxide can be oxidized to nitric acid according to:

NO2 C OHK/ HNO3 $

In addition, NO may be oxidized to nitric acid according to the total
reaction:

2NO C H2 O C 3 O2 / 2HNO3 $
2

Because this reaction has a very low rate, the concentrations of HNO3 and
nitrates in the immediate area of the emission source are low.
Nitrogen dioxide, by absorbing solar light and the subsequent formation
of ozone through:

NO2 C hn/ NO C O ðl! 420 nmÞ
O C O2 / O3

plays an important part in atmospheric chemistry.
The mechanisms by which nitrogen compounds are deposited are not
completely understood. Wet deposition seems to be the primary mechanism
at long distances from the emission source, whereas in the immediate area of


the emission source dry deposition of nitrates appears to dominate. This is
due to the fact that NO and NO2 have a low solubility in water, whereas
HNO3, which is highly soluble in water, has not yet formed.
Ammonia (NH3) is emitted primarily from animal shelters, cleaning
detergents, and fertilizer production. Ammonia in the aqueous phase
establishes equilibrium with NHK, which results in increased pH. NH3
4
affects the atmospheric corrosion chemistry by partly neutralizing acidifying
pollutants, forming particulate ammonium sulfate [(NH4)2SO4] and acid
ammonium sulfates such as NH4HSO4 and (NH4)3H(SO4)2.

2.5.3 Chlorine-Containing Compounds
In marine environments, chloride deposition is in the form of droplets or
crystals formed by evaporation of spray that has been carried by wind from
the sea. As the distance from the shore increases, this deposition decreases as
the droplets and crystals are filtered off when the wind passes through
vegetation or when the particles settle by gravity.
Other important sources of chloride emission are coal burning, municipal
incinerators, and deicers and dust binders on roads. Most coals have a
chlorine content of 0.09–0.15%. Values as high as 0.7% have been found in
high-chlorine coals. The combustion of these coals produces an emission of
gaseous hydrogen chloride (HCl) that is highly soluble in water and strongly
acidifies the aqueous phase.
Many industrial processes, such as bleaching plants in pulp and paper
industries, certain metal production facilities, and cleaning detergents, emit
chlorine (Cl2).
Cl2 can photodissociate into chlorine radicals that react with organic
compounds (RH) to form HCl:

Cl2 C hn/ Cl: C Cl: ðl! 430 nmÞ
RH C Cl:/ R C HCl$

2.5.4 Carbon Dioxide (CO2)
Carbon dioxide occurs naturally in the atmosphere in a concentration of
0.03–0.05% by volume. This concentration varies with the time of day and
the season of the year. The above percentages correspond to a concentration
on the order of 10K5 mol/L when at equilibrium in the water film, if the pH
value is 6 or lower.

2.5.5 Oxygen (O2)
Oxygen is a natural constituent of air and is readily absorbed from the air
into a water film on the metal surface, which is considered to be saturated,
thereby promoting any oxidation reactions.


2.5.6 Indoor Atmospheric Compounds
HCHO and HCOOH are important indoor corrosion stimulants that can
originate from tobacco smoke, combustion of biomass, adhesives, and
plastics. In general, the concentration of these stimulants is lower indoors
than outdoors, except for ammonia and the organic species that usually have
a higher concentration indoors than outdoors. This higher level is the result
of anthropogenic activity.

2.6 Summary
The concentrations of pollutants found in both indoor and outdoor
atmospheres can vary greatly as a result of the type of atmosphere and/or
the geographic location. It is almost impossible to provide a specific range
for a specific location unless air samples are taken and analyzed. Listed
below are some typical indoor and outdoor ranges of inorganic pollutants as
found in the United States.

Pollutant Outdoor Range (mg/m3) Indoor Range (mg/m3)

SO2 3–185 1–40
NO2 20–160 3–60
H2S 1–36 0.2–1
O3 10–90 7–65
HCl 0.3–5 0.08–0.3
Cl2 !5% of HCl levels except 0.004–0.015
where local source exists
NH3 6–12 10–150

2.7 Effects on Metals Used for Outdoor Applications
Carbon steel is the most widely used metal for outdoor applications,
although large quantities of zinc, aluminum, copper, and nickel-bearing
alloys are also used. Metals customarily used for outdoor installations will
be discussed.

2.7.1 Carbon Steel
Except in a dry, clean atmosphere, carbon steel does not have the ability to
form a protective coating as some other metals do. In such an atmosphere, a
thick oxide film forms that prevents further oxidation. Solid particles on
the surface are responsible for the start of corrosion. This settled airborne
dust promotes corrosion by absorbing SO2 and water vapor from the air.


Even greater corrosive effects result when particles of hygroscopic salts, such
as sulfates or chlorides, settle on the surface and form a corrosive electrolyte.
To protect the surface of unalloyed carbon steel, an additional surface
protection must be applied. This protection usually takes the form of an
antirust paint or other type of paint protection formulated for resistance
against a specific type of contaminant known to be present in the area. On
occasion, plastic or metallic coatings are used.

2.7.2 Weathering Steels
Weathering steels are steels to which small amounts of copper, chromium,
nickel, phosphorus, silicon, manganese, or various combinations thereof
have been added. This results in a low-alloy carbon steel that has improved
corrosion resistance in rural areas or in areas exhibiting relatively low
pollution levels. Factors that affect the corrosion resistance of these steels are:

Climatic conditions
Pollution levels
Degree of sheltering from the atmosphere
Specific composition of the steel

Exposure to most atmospheres results in a corrosion rate that becomes
stabilized in 3–5 years. Over this period, a protective film or patina, dark-
brown to violet in color, forms. This patina is a tightly adhering rust
formation on the surface of the steel that cannot be wiped off. Because the
formation of this film is dependent on the pollution in the air, in rural areas
where there may be little or no pollution, a longer time may be required to
form this film. In areas that have a high pollution level of SO2, loose rust
particles are formed with a much higher corrosion rate. This film of loose
particles offers little or no protection against continued corrosion.
When chlorides are present, such as in a marine environment, the
protective film will not be formed. Under these conditions, corrosion rates of
the weathering steels are equivalent to those of unalloyed carbon steel.
To form the patina, a series of wet and dry periods is required. If the steel is
installed in such a manner as to be sheltered from the rain, the dark patina
does not form. Instead, a rust lighter in color forms that provides the same
resistance. The corrosion rate of the weathering steels will be the same as the
corrosion rate of unalloyed steel when it is continuously exposed to wetness,
such as in water or soil.
Because the patina formed has a pleasant appearance, the weathering
steels can be used without the application of any protective coating of
antirust paint, zinc, or aluminum. This is particularly true in urban or
rural areas.
To receive the maximum benefit from the weathering steels, consideration
must be given to the design. The design should eliminate all possible areas


where water, dirt, and corrosion products can accumulate. When pockets are
present, the time of wetness increases, which leads to the development of
corrosive conditions. The design should make maximum use of exposure to
the weather. Sheltering from rain should be avoided.
While the protective film is forming, rusting will proceed at a relatively
high rate, during which time rusty water is produced. This rusty water may
stain masonry, pavements, and the like. Consequently, steps should be taken
to prevent detrimental staining effects, such as coloring the masonry brown,
so that any staining will not be obvious.

2.7.3 Zinc
Galvanized steel (zinc coating of steel) is used primarily in rural or urban
atmospheres for protection from atmospheric corrosion. Galvanizing will
also resist corrosion in marine atmospheres provided that saltwater spray
does not come into direct contact. In areas where SO2 is present in any
appreciable quantity, galvanized surfaces will be attacked.

2.7.4 Aluminum
Except for aluminum alloys that contain copper as a major alloying
ingredient, these alloys have a high resistance to weathering in most
atmospheres. When exposed to air, the surface of the aluminum becomes
covered with an amorphous oxide film that provides protection against
atmospheric corrosion, particularly that caused by SO2.
The shiny metal appearance of aluminum gradually disappears and
becomes rough when exposed to SO2. A gray patina of corrosion products
forms on the surface. If aesthetics are a consideration, the original surface
luster can be retained by anodizing. This anodic oxidation strengthens the
oxide coating and improves its protective properties.
It is important that the design utilizing aluminum eliminate rain-sheltered
pockets on which dust and other pollutants may collect. The formation of the
protective film will be disturbed and corrosion accelerated by the presence of
these pollutants.

2.7.5 Copper
When exposed to the atmosphere over long periods of time, copper will form
a coloration on the surface known as patina, which in reality is a corrosion
product that acts as a protective film against further corrosion. The length of
time required to form the patina depends on the atmosphere because the
color is due to the formation of copper hydroxide compounds. Initially, the
patina has a dark color, which gradually turns green. In urban or industrial
atmospheres, the compound is a mixture of copper/hydroxide/sulfate and
in marine atmospheres it is a mixture of copper/hydroxide/chloride. It
takes approximately 7 years for these compounds to form. When exposed to


clean or rural atmospheres, tens or hundreds of years may be required to
form the patina.
The corrosion resistance of copper is the result of the formation of this
patina or protective film. Copper roofs are still in existence on many castles
and monumental buildings that are hundreds of years old.

2.7.6 Nickel 200
When exposed to the atmosphere, a thin corrosion film (usually a sulfate)
forms dulling the surface. The rate of corrosion is extremely slow but will
increase as the SO2 content of the atmosphere increases. When exposed to
marine or rural atmospheres, the corrosion rate is very low.

2.7.7 Monel Alloy 400
The corrosion of Monel is negligible in all types of atmospheres. When
exposed to rain, a thin gray-green patina forms. In sulfurous atmospheres, a
smooth brown adherent film forms.

2.7.8 Inconel Alloy 600
In rural atmospheres, Inconel alloy 600 will remain bright for many years.
When exposed to sulfur-bearing atmospheres, a slight tarnish is likely to
develop. It is desirable to expose this alloy to atmospheres where the
beneficial effects of rain in washing the surface, and sun and wind in drying,
can be utilized. It is not recommended to design on the basis of a
sheltered exposure.

Reference
1. P.A. Schweitzer. 1999. Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker.

3
Corrosion of Carbon and Low-Alloy Steels

Smelting of iron to extract it from its ore is believed to have started around
1300 BC in Palestine. Tools of iron appeared about this time and an iron
furnace has been found. Steel is basically an alloy of iron and carbon with the
carbon content up to approximately 2 wt%.
Steel, because of its strength, formability, abundance, and low cost is the
primary metal used for structural applications. As the term “plain carbon
steel” implies, these are alloys of iron and carbon. These steels were the
ﬁrst developed, are the least expensive, and have the widest range
of applications.
The presence of carbon, without substantial amounts of other alloying
elements, is primarily responsible for the properties of carbon steel.
However, manganese is present to improve notch toughness at low
temperatures. The steels discussed in this chapter contain less than 0.35%
carbon to make them weldable.
Low-alloy steels are of two types:

1. Weathering steels that contain small additions of copper,
chromium, and nickel to form a more adherent oxide during
atmospheric exposures. An example is U.S. Steel’s Cor-Ten steel.
2. Hardenable steels that offer higher strength and hardness after
proper heat treatment and which contain additions of chromium or
molybdenum and possibly nickel. Common examples include 4130
and 4340 steels.

3.1 Corrosion Data
Carbon and low-alloy steels are primarily affected by general or uniform
corrosion. Iron occurs naturally in the form of various oxides, the ores of
which are reﬁned to produce steel. Therefore, in atmospheric service they

67


tend to return to their oxide form by a process known as rusting. The
corrosion of steel is very complex, with over a dozen variables determining
the rate of corrosion. Water is the most common solvent in everything from
dilute solutions to concentrated acids and salt solutions. Some organic
systems are also capable of causing some severe corrosion.
In dilute water solutions, the most important variable is acidity or solution
pH. Figure 3.1 shows the effect of pH on the corrosion of steel at 228C (778F)
and 408C (1048F). The diagram is suitable for water flowing at a moderate
flow rate. There is a range of pH from 5.5 to 10 where the corrosion rate is
constant at about 10–12 mpy (250–305 mm/year). In this range there is an
alkaline solution of saturated ferrous hydroxide covering the steel’s surface;
this hydroxide solution has a constant pH of about 9.5. The rate-determining
reaction in this corrosive range is the diffusion of oxygen through the ferrous
hydroxide film to feed the electrochemical cathodic reduction of the oxygen
to the hydroxyl ion. Thus, dissolved oxygen is another key variable in
aqueous corrosion.
At lower pH values, the cathodic reduction changes to the relatively rapid
reduction of hydrogen ions in the acidic solution to produce hydrogen gas
bubbles. Different acids have different values of pH where the onset of this
rapid reaction occurs. As shown, carbonic acid (dissolved carbon dioxide)
initiates it at pH 5.5. Hydrochloric acid starts the reaction at pH 4. The effect
is dramatic; at pH 2.7 the corrosion rate reaches 80 mpy (2 mm/year). Under
stagnant conditions the corrosion rate is lower. However, stagnant
conditions are to be avoided when possible because they are exactly
where the various forms of localized corrosion become serious, including
pitting, oxygen concentration cells, and microbiologically influenced
corrosion (MIC). These localized corrosions penetrate faster than overall
general corrosion.

40 Oxygen: 6 mL/L

CO2 76 mpy
30 HCI @
Corrosion pH = 2.67
40 C
rate
mpy 20

22 C
10

0
pH = 10 pH = 5 pH = 4
Water pH

FIGURE 3.1
Effect of pH on the corrosion of carbon steel.

Corrosion of Carbon and Low-Alloy Steels 69

The next important variable to consider is flow rate. Figure 3.2 shows the
effect of flow on the corrosion of steel from stagnant to 8 ft/s. Note that as
the flow rate rises from zero, the corrosion rate increases to a maximum
around 1 or 2 ft/s. This increase comes from an increase in the oxygen
supplied for the oxygen reduction process occurring on the cathodic areas of
the steel. Higher flow rates then supply enough oxygen so that the adsorbed
oxygen and the ferrous hydroxide layer can cover the entire steel surface, a
complete level of passivation. At 6–8 ft/s (1.8–2.4 m/s), which is the
common range of flow rates in the chemical industry, the corrosion rate
settles at 10–15 mpy (250–380 mm/year). Figure 3.2 also shows the effect of
roughness of the steel, another variable affecting corrosion.
With higher flow rates, the corrosion rate increases up to around 40 ft/s
(12 mm/s) where the attack changes to erosion–corrosion, which means that
any protective oxide or adsorbed layer is stripped away and bare steel is
open to accelerated attack. Turbulence has a similar effect. Figure 3.3 and
Figure 3.4 show the effects of increasing flow velocity for distilled water and
seawater. At 39 ft/s the corrosion rate in distilled water at 508C (1228F)
exceeds 200 mpy (5 mm/year).
With corrosion, as with other chemical reactions, temperature plays a
major role. Figure 3.1 shows the increase in corrosion from increasing
temperature. In neutral or alkaline waters, however, the temperature effect is
more complicated. In an open system, a higher temperature will drive off
oxygen, eventually to very low levels. Because oxygen provides the cathodic
reaction in the corrosion process, if there is no oxygen there will be no
corrosion. Figure 3.5 shows this effect, with the corrosion beginning to
decrease around 808C (1768F) and becoming very low above 1008C (2128F).
The behavior of weathering low-alloy steels in aqueous corrosion tests and
applications is unpredictable. In 1953, early tests on weathering steels
containing copper, chromium, phosphorus, and nickel showed superior

Municipal water @ 21°C in steel tubes
50

40
Corrosion
rate 30 Rough steel
mpy
20

10 Polished steel

0
2 4 6 8
Flow velocity, ft/s

FIGURE 3.2
Effect of water flow velocity on the corrosion of steel. Increased oxygen leads first to higher
corrosion, then to oxygen passivation that lowers corrosion.


400

300 Erosion
Corrosion
corrosion, 39 ft/s
rate
mpy
200

100
Mild swirling

0
4 6 8 10
pH of solution

FIGURE 3.3
Effect of pH of pure water on erosion–corrosion of steel at 508C at ﬂow velocity of 39 ft/s.

corrosion resistance in immersion tests in river water at a pH of 3.5–4. After 4
years immersion, for example, Cor-Ten steel showed an average corrosion
rate of 2.7 mpy (69 mm/year) with a maximum pit depth of 24 mils (360 mm).
For comparison, carbon steel corroded at an average of 4.1 mpy
(104 mm/year) and pitted to a maximum of 24 mils (530 mm).
However, in the laboratory tests described above involving carbon dioxide
and sodium sulfate, low-alloy steels performed worse than carbon steel.
Three alloy steels containing phosphorus, copper, chromium, and nickel in
various permutations corroded severely at 40–60 mpy (1.0–1.5 mm/year) in
a 0.5 M sodium sulfate solution containing 100 ppm oxygen with carbon

40

30
Corrosion
rate
mpy 20

10

0
0 5 10 15
Flow velocity, ft/s

FIGURE 3.4
Effect of seawater velocity on corrosion of steel (ambient temperature).


30

Closed
Corrosion system
rate 20
mpy

10
Open
system
0
0 20 40 60 80 100 120 140
Temperature, Celsius

FIGURE 3.5
Effect of temperature on corrosion of iron in water containing dissolved oxygen.

dioxide bubbling through at 258C (778F). For comparison, four carbon steels
corroded at rates of only 2 mpy (50 mm/year).
Table 3.1 shows corrosion data for several low-alloy steels compared with
carbon steels. The table includes results for solutions containing 200 ppm
propionic acid and 200 psig carbon dioxide at 1308F (548C). Again, as in
Figure 3.1, carbonic acid can be more aggressive that other acids, in this
instance exceeding an order of magnitude in some cases.
In Table 3.1, with the exception of some of the Cr–Mo steels in the
propionic acid tests, the alloy steels are generally more resistant than the
carbon steels, especially the 9% chromium steel. This is reasonable because
steels become “stainless” at the 11–13% chromium level. Furthermore, the

TABLE 3.1
Overall Corrosion Rate, 1308F (548C) (During Indicated Exposure
Time), mpy

Carbonic Acid (200 psig) Propionic Acid (200 ppm)
Alloy 7 Days 70 Days 7 Days 70 Days

Carbon steels
API J-55 62 5.8 1.8 4.6
API H-40 60 9.1 1.1 3.1
API N-80 77 5.5 1.5 4.7
Alloy steels
2.25 Cr–1 Mo 59 3.0 3.1 3.3
5 Cr–0.5 Mo 46 4.2 4.2 3.3
9 Cr–1 Mo 2 0.2 0.5 0.6
3.5 Ni 28 3.1 1.5 1.5
5 Ni 26 2.1 1.3 1.6
9 Ni 27 1.2 1.0 1.7


higher corrosion rates for the other Cr–Mo steels in the propionic acid
solution are still in a usable range for many applications.
The large decrease in corrosion rates in carbonic acid between 7 and
70 days is attributed to the eventual formation of protective surface films that
may be fragile under certain fluid flow conditions.
In general, carbon steel should not be used in contact with dilute acids. At
concentrations between 90 and 95% sulfuric acid, steel can be used up to the
boiling point; between 80 and 90% it is serviceable at room temperature. Carbon
steel is not normally used with hydrochloric, phosphoric, or nitric acids.
If iron contamination is permissible, steel can be used to handle caustic
soda, up to approximately 75% at 2128F (1008C). Stress relieving should be
employed to reduce caustic embrittlement.
Carbon steel is susceptible to stress corrosion cracking (SCC) in the
presence of hydroxides, gaseous hydrogen, hydrogen chloride, hydrogen
bromide, hydrogen sulfide gas, and aqueous nitrite solutions, even in low
concentrations.
In general, there are four types of organic compounds that can be corrosive
to carbon steel:

1. Organic acids such as acetic or formic
2. Compounds that hydrolyze to produce acids; this includes
chlorinated hydrocarbons (e.g., carbon tetrachloride) that react
with water to produce hydrochloric acid
3. Chelating agents that take up or combine with transition elements
4. Inorganic corrosives dissolved and dissociated in organic solvents;
for example, hydrochloric acid dissolved in methanol

Table 3.2 provides the compatibility of carbon steel with selected
corrodents. A more comprehensive list will be found in Reference [1].
Atmospheric corrosion of steel is a function of location. In country air, the
products of corrosion are either oxides or carbonates. In industrial
atmospheres, sulfuric acid is present, and near the ocean some salt is in
the air. Corrosion is more rapid in industrial areas because of the presence of
the acid, and it is higher both near cities and near the ocean because of the
higher electrical conductivity of the rain and the tendency to form soluble
chlorides or sulfates that cause the removal of protective scale.
When steel is exposed to a clean, dry atmosphere, the surface is covered
˚
with a 20–50-A thick oxide film consisting of a layer of Fe2O3. This film
practically prevents further corrosion. If small amounts of water are present,
FeOOH may also form.
In noncontaminated atmospheres, the initiation of corrosion on a clean
metal surface is a very slow process, even if the atmosphere is saturated with
water vapor. Under these conditions, initiation of corrosion may occur at
surface inclusions or MnS, which dissolves when the surface becomes wet.
However, the presence of solid particles on the surface is a more important


TABLE 3.2
Compatibility of Carbon Steel with Selected Corrodents

Chemical 8F/8C

Acetaldehyde 130/54
Acetic acid, all conc. X
Acetic acid vapors X
Acetone 300/149
Aluminum chloride, dry X
Aluminum fluoride X
Ammonium chloride X
Ammonium hydroxide, 25% 210/99
Aqua regia, 3:1 X
Benzene 140/60
Boric acid X
Bromine gas, dry X
Bromine gas, moist X
Calcium chloride 140/60
Calcium hydroxide, all conc. X
Citric acid, all conc. X
Diesel fuels 200/93
Ethanol 240/116
Ferric chloride X
Formaldehyde, to 50% X
Formic acid X
Glucose 170/77
Green liquor 400/204
Hydrobromic acid X
Hydrochloric acid, dil. X
Hydrochloric acid, 20% X
Hydrofluoric acid, dil X
Hydrofluoric acid, 30% X
Hydrofluoric acid, vapors X
Hydrogen sulfide, dry 90/32
Hydrogen sulfide, weta 450/232
Iodine X
Lactic acid X
Lard oil X
Linoleic acid X
Linseed oil 90/32
Magnesium chloride, 30% 80/27
Mercuric chloride X
Mercuric nitrate 100/38
Methyl alcohol 200/93
Methyl ethyl ketone 200/93
Methylene chloride 100/38
Mineral oil 100/38
Nitric acid X
Oil vegetable 160/71
(continued)


TABLE 3.2 Continued

Chemical 8F/8C

Oleum 80/27
Oxalic acid, all conc. X
Perchloric acid X
Petrolatum X
Phenol 210/99
Phosphoric acid X
Potassium chloride, 30% 210/99
Potassium hydroxide, 50% X
Propylene glycol 210/99
Sodium chloride, 30% 150/66
Sodium hydroxide, to 30% 210/99
Sulfur dioxide, wet X
Sulfuric acid, to 90% X
Water, demineralized X
Water, distilled X
Water, salt X
Water, sea X
Water, sewage 90/32
White liquor X
Wines X
Xylene 200/93

Note: The chemicals listed are in the pure state or in a saturated
solution unless otherwise indicated. Compatibility is shown to the
maximum allowable temperature for which data are available.
Incompatibility is shown by an X.
a
Hydrogen embrittlement may occur depending upon conditions.
Source: From P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols.
1–4, 5th ed., New York: Marcel Dekker.

factor for the start of corrosion. Airborne dust that has settled on the steel
surface may prompt corrosion by absorbing SO2 and water vapor from the
atmosphere. Of greater importance are particles of hygroscopic salts, such as
sulfates or chlorides, that form a corrosive electrolyte on the surface. Rusting
is rapidly initiated in SO2-polluted atmosphere.
Sulfur dioxide may be absorbed on steel surfaces under atmospheric
conditions. The rate of adsorption on rusty or polished steel surfaces
depends on the relative humidity; high concentrations of SO2 (O10 ppm)
create low pH in the surface ﬁlm. The sulfur dioxide is oxidized on moist
particles or in droplets of water to sulfuric acid. At concentrations below
90%, sulfuric acid is corrosive to steel.
Weathering steels were developed to provide protection from atmospheric
corrosion. They are produced by alloying carbon steel with copper,
chromium, nickel, phosphorus, silicon, and manganese. One such example
is U.S. Steel’s Cor-Ten steel.


These low-alloy steels have improved corrosion resistance in outdoor
atmospheres in rural areas, or in areas having relatively low pollution levels.
The protective action of copper and other alloying elements is due to a
resistant form of oxide that forms a protective coating under atmospheric
conditions, but has little or no favorable effect when immersed continuously
in water or when exposed to severe industrial corrosive conditions.
In an industrial atmosphere, steel with 0.32% copper will corrode only half
as much after 5 years as steel with 0.05% copper. A high-strength low-alloy
steel having the following composition (percentage) will corrode only half as
much as steel having 0.32% copper:

C 0.12 max
Mn 0.20–0.50
P 0.07–0.16
S 0.05 max
Si 0.75 max
Cu 0.30–0.50
Cr 0.50–1.25
Ni 0.55 max

It will be noted that in addition to copper, this high-strength alloy also
contains notable amounts of chromium and nickel, both of which are helpful
in increasing strength and adding resistance to corrosion. Phosphorus,
which it also contains, is another element that aids in providing protection
against atmospheric corrosion.
In general, the presence of oxygen or of acidic conditions promotes the
corrosion of carbon steel. Alkaline conditions inhibit corrosion. Factors that
affect the corrosion resistance of these steels are:

Climatic conditions
Population levels
Degree of sheltering from the atmosphere
Specific composition of the steel

Exposure to most atmospheres results in a corrosion rate that becomes
stabilized in 3–5 years. Over this period, a protective film or patina is formed
that is dark-brown to violet. The patina is a tightly adhering rust formation
on the surface of the steel that cannot be wiped off. Because the formation of
this film is dependent on pollution in the air, in rural areas where there is
little or no pollution, a longer time may be required to form this film. In areas
that have a high pollution level of SO2, loose particles are formed with a
much higher corrosion rate. This film of loose particles offers little or no
protection against continued corrosion.
When chlorides are present, such as in a marine environment, the
protective film will not be formed. Under these conditions, corrosion rates of
the weathering steels are equivalent to those of unalloyed carbon steel.


To form a patina, a series of wet and dry periods is required. If the steel is
installed in such a manner as to be sheltered from the rain, the dark patina
does not form. Instead, a rust, lighter in color, forms that provides the same
resistance. The corrosion rate of the weathering steels will be the same as the
corrosion rate of unalloyed steel when it is continuously exposed to wetness,
such as in soil or water.
Because the patina formed has a pleasant appearance, the weathering
steels can be used without the application of any protective coating of
antirust paint, zinc, or aluminum.
To achieve the maximum benefit from the weathering steels, consideration
must be given to the design. The design should eliminate all possible
areas where dirt, dust, water, and corrosion products can accumulate. When
pockets are present, the time of wetness increases; this leads to the
development of corrosive conditions. The design should make maximum
use of exposure to weather. Sheltering from the rain should be avoided.
While the protective film is forming, rusting will proceed at a relatively
high rate, during which time rusty water is produced. This rusty water may
stain masonry, pavements, and the like. Consequently, steps should be taken
to prevent detrimental staining effects, such as brown coloring of masonry,
so that staining effects will not be obvious. The ground area exposed to
staining can be covered with an easily exchangeable material such as gravel.
The corrosion mechanism for weathering steels is similar to that of
unalloyed carbon steels. The rust forms a more dense and compact layer on
the weathering steels than on unalloyed carbon steels. The rust layer more
effectively screens the steel surface from the corrosive environments of the
atmosphere. The corrosion process may be affected in several ways by this
rust layer. The cathodic reaction may be affected by the low diffusion rate of
oxygen, whereas the anodic reduction may be retarded by limiting the
supply of water and corrosion-stimulating ions that can reach the surface of
the steel. In addition, the increased electrolyte resistance may also decrease
the corrosion rate.
Each of the alloying ingredients reacts differently to improve the resistance
of weathering steels to atmospheric corrosion.
Copper has the most pronounced effect of any of the individual elements
in decreasing the corrosion rate. An increase in the copper content from 0.1
to 0.4% decreases the corrosion rate by up to 70%. Only a slight improvement
in the corrosion resistance results from an increase of copper in the range
0.2–0.5%. Several theories have been proposed regarding the mechanisms by
which copper improves the corrosion resistance. One theory is that the
beneficial effect is due to the formation of a surface coating of metallic copper
that acts either as protection on itself or promotes anodic passivation by
supporting the cathodic reaction. Another theory is that copper ions
dissolved from the base metal are able to precipitate sulfide ions originating
either from sulfide inclusions in the steel or from the atmospheric pollution,
and thus eliminate their detrimental effect. The most probable theory is that
copper forms basic sulfates with low solubility that precipitate within the


pores of the rust layer, thereby decreasing their porosity. Weathering steels
usually contain 0.2–0.5% copper.
When added in combination with copper, chromium and nickel further
increase the corrosion resistance of weathering steels. Chromium is usually
added to a content of 0.4–1% whereas nickel is usually added up to 0.65%.
Chromium appears to be more effective than nickel. The mechanical
properties of the steel are improved by both elements.
Chromium is enriched in the inner rust layer, together with copper and
phosphorus. They promote the formation of a dense layer of amorphous
FeOOH next to the steel surface. This layer acts as a barrier to the transport of
water, oxygen, and pollutants. Nickel is supposed to act by forming
insoluble basic sulfates on pores of the rust layer.
Phosphorus also helps to improve the corrosion resistance of weathering
steels. By increasing the phosphorus content from less than 0.01 to 0.1%, a
20–30% improvement in the corrosion resistance of copper-bearing steels is
realized. Phosphorus may form layers of insoluble phosphates in the rust,
acting as transportation barriers in the same manner as the basic sulfates
previously mentioned. A comparison of the corrosion rates of carbon steel, a
copper–phosphorus low-alloy steel, and a chromium–vanadium–copper
low-alloy steel is shown in Table 3.3.
As indicated previously, weathering steels possess no particular
advantage of corrosion protection in heavily polluted industrial atmos-
pheres or in direct contact with corrodents. They will suffer the same types of
corrosion as other low-alloy carbon steels.
Carbon steel and low-alloy carbon steels can be subjected to a number of
types of localized corrosion, including SCC, sulﬁde stress cracking (SSC),
pitting, hydrogen effects, and corrosion fatigue.

TABLE 3.3
Atmospheric Corrosion of Various Steels in Different Atmospheric Types

Average Reduction in Thickness (mil)
Atmospheric Exposure Carbon A242 (K11510) A558 (K11430)
Type Time (years) Steel Cu–P Steel Cr–V–Cu Steel

Urban 3.5 3.3 1.3 1.8
Industrial 7.5 4.1 1.5 2.1
Rural 3.5 2.0 1.1 1.4
7.5 3.0 1.3 1.5
Severe marine 0.5 7.2 2.2 3.8
80 ft from ocean 2.0 36.0 3.3 12.2
3.5 57.0 19.4 28.7
5.0 Destroyed 38.8


3.2 Stress Corrosion Cracking
Stress corrosion cracking (SCC) occurs at points of stress. Usually, the metal
or alloy is virtually free of corrosion over most of its surface, yet fine cracks
penetrate through the surface at points of stress. The conditions necessary
for SCC are:

1. A suitable environment (chemicals capable of causing SCC in
carbon steel and low-alloy carbon steels)
2. A tensile stress, either residual or operational
3. Appropriate temperature and pH values

One advantage of carbon steel is that SCC can be prevented by relieving
stress after fabrication.
Chemical species that induce SCC in carbon and low-alloy carbon steels,
even at low concentrations include: hydroxides, gaseous hydrogen, gaseous
chlorine, hydrogen chloride, hydrogen bromide, aqueous nitrate solutions,
hydrogen sulfide gas, MnS and MnSe inclusions in the alloy, As, Sb, and
Bi ions in aqueous solution, carbon monoxide–carbon dioxide–water gas
mixtures. Many of these chemical systems will crack steel at room
temperatures.
Another chemical that causes SCC in steels is anhydrous ammonia. Alloys
affected include carbon steel in storage tanks and ASTM A517 quenched and
tempered steel in motor vehicle cargo tanks. Various grades of A517 steel
contain small amounts of Cr, Ni, Mo, B, V, Ti, Zr, and Cu. This cracking
can be alleviated by adding 0.2% water to the ammonia, eliminating air
contamination from ammonia systems, and stress-relieving tanks or
fabricating with hot formed or stress-relieved heads.

3.3 Sulfide Stress Cracking
Many corrosion processes produce hydrogen ions that combine with
electrons from the base metal to form hydrogen atoms. Two such formed
hydrogen atoms may combine to form a hydrogen molecule. The majority of
such molecules will form hydrogen bubbles and float away harmlessly.
However, a percentage of the hydrogen atoms will diffuse into the base
metal and embrittle the crystalline structure. SSC will occur when a critical
concentration of hydrogen is reached while a tensile stress exceeding a
threshold level is present. Although H2S does not actively participate in the
SSC reaction, sulfides act to promote the entry of the hydrogen atoms into
the base metal.


The susceptibility of carbon steels to SSC is directly related to their
strength or hardness levels. As carbon of low-alloy carbon steel is heat-
treated to progressively higher levels of hardness, the time to failure
decreases rapidly for a given stress level.
Temperature is also a factor. The most severe SSC takes place in the
temperature range of 20–1208F (K7 to 498C). Above 1208F (498C), the diffusion
rate of hydrogen is so fast that the hydrogen passes through the material so
quickly that the critical concentration is not reached. Below 208F (K78C), the
diffusion rate is so slow that the critical concentration is never reached.
By carefully monitoring the processing of carbon and low-alloy carbon
steels, and keeping the hardness level below 22 HRC, these steels will have
acceptable resistance to SCC.

3.4 Pitting
Carbon and low-alloy carbon steels may pit under ﬂow or stagnant
conditions. These pits are generally shallow. In seawater, the pitting rate is
5–45 mpy, while the overall corrosion rate in the aerated splash zone is as
high as 17 mpy.
Soils will pit steels, which obviously affects buried pipelines. In one study
of 10 carbon and low-alloy carbon steels containing Cr, Ni, Cu, and Mo and
exposed to a variety of soils for 13 years, the conclusion was that factors such
as soil pH, resistivity and degree of aeration have more inﬂuence on the
severity of corrosion than the alloy content of the steel. In any case,
protective coatings and cathodic protection are the best means of reducing
corrosion in buried pipelines.

3.5 Hydrogen Damage
The body-centered crystal structures of carbon and low-alloy steels are
susceptible to four types of hydrogen damage, two of which are low-
temperature processes and two are high-temperature processes:

Low-temperature
Hydrogen blistering
Hydrogen embrittlement
High-temperature
Decarburization
Hydrogen attack

The diffusion of hydrogen through steels to harm mechanical properties
involves atomic or nascent hydrogen because molecular hydrogen cannot


diffuse through metals. Common sources of atomic hydrogen include corrosion
(including the acid pickling of steel), misapplied cathodic protection, high
temperature, moist atmospheres, electroplating, and welding.

3.5.1 Hydrogen Blistering
During some acid services, such as acid pickling of steels, hydrogen atoms
may penetrate the crystal lattice and collect in fissures or cavities in the steel.
These atoms then combine into hydrogen gas molecules, eventually reaching
pressures of several hundred thousand atmospheres and forming blisters on
the steel’s surface. In petroleum process streams, this problem is promoted
by so-called hydrogen evolution poisons such as sulfides, arsenic
compounds, cyanides, and phosphorus-containing ions. In closed systems
like pickling operations, chemical inhibitors are added to the acid to reduce
the hydrogen penetration.

3.5.2 Hydrogen Embrittlement
Another harmful effect of hydrogen penetration of steel is embrittlement,
which is a more complicated metallurgical effect, possibly involving the
interaction of hydrogen atoms with the tip of an advancing crack. For
low-alloy steels, the alloys are most susceptible in their highest strength
levels. Alloys containing nickel or molybdenum are less susceptible. If
hydrogen is initially present in a steel, for example from electroplating, the
hydrogen can be baked out. In fact, this embrittlement decreases with
increasing service temperature, especially above 1508F (658C). Generally,
hydrogen embrittlement is not usually a problem in steels with yield
strengths below about 1000 MPa (150 ksi), but if hydrofluoric acid or
hydrogen sulfide is present, the yield strength must be below 550 MPa
(80 ksi) for good resistance. Welding conditions should be dry and low-
hydrogen filler metal should be used to minimize hydrogen embrittlement.

3.5.3 Decarburization
The hardness and strength of a steel depends on its carbon content. A loss of
carbon (decarburization) lowers the tensile strength of steels and can be
caused by moist hydrogen at high pressures and temperatures. Figure 3.6
shows the Nelson diagram that depicts the limit of service conditions for
carbon and alloy steels in hydrogen services.

3.5.4 Hydrogen Attack
High-temperature hydrogen attack refers to a reaction between hydrogen
and a component of the alloy. For example, in steels, hydrogen reacts with
iron carbide at high temperatures to form methane gas according to the


1500 Decarburization

1200 1,25 Cr−0.5 Mo
600
2.25 Cr−1.0 Mo
Temperature
900
°F 1.0 Cr−0.5 Mo
°C
1.25 Cr−0.5 Mo
600 Carbon steel
1.0 Cr−0.5 Mo 300

300 Satisfactory
0 500 1000 1500 2000
Hydrogen partial pressure, psia

FIGURE 3.6
Schematic of hydrogen damage for low-alloy steels in hydrogen service. Decarburization and
hydrogen attack above alloy lines.

following reaction:

CðFeÞ C 4HðFeÞ Z CH4 :

Because methane cannot diffuse out of steel, it accumulates and causes
fissuring and blistering, thereby decreasing alloy strength and ductility. Alloy
steels containing chromium and molybdenum are beneficial in such services
because the carbides formed by the alloying elements are more stable than
iron carbide and therefore resist hydrogen attack. It is noteworthy that water
vapor and carbon dioxide at high temperatures can also decarburize steel.

3.6 Corrosion Fatigue
As the name implies, corrosion fatigue is affected by both the severity of
corrosive conditions and mechanical, cyclical stress factors. Stress raisers such
as notches, holes, weld defects, or corrosion pits can initiate fatigue cracks and
a corrosive environment can reduce crack initiation time. For many materials,
the stress range required to cause fatigue failure diminishes progressively
with increasing time and with the number of cycles of applied stress.

3.7 Microbiologically Influenced Corrosion
Under certain conditions, bacterial colonies change the chemistry of an
alloy’s surface and induce rapid corrosion. One common example involves


sulfate reducing bacteria (SRB) that produce acidic hydrogen sulfide, which
is highly corrosive to steel. Then other bacteria may act on the hydrogen
sulfide and produce sulfuric acid. Other instances of MIC report corrosion
due to the formation of acetic acid and formic acid. These acids may be
highly concentrated and cause pitting and rapid failure in the “slime”
nodules, or tubereles, where the bacteria live.
MIC is generally associated with stagnant or low-flow aqueous systems
over a range of pH values from 1.0 to 10.5 at temperatures of 328F (08C) to
2128F (1008C). There are 50–60 bacteria species believed to be associated with
MIC, both aerobic and anaerobic species. The corrosive attack can be rapid,
often occuring within weeks of introducing the bacteria. One solution that
suffered MIC involved dissolved polymeric organic materials and
ammonium phosphate compounds, so the affected solutions can be quite
varied.
There are various treatments used to prevent or alleviate MIC, including
the addition of biocides such as ozone or hydrogen peroxide to the water.
This, however, will be ineffectual if the bacteria have previously formed
protective nodules as their habitat. These nodules must be mechanically
removed to kill the bacteria.
Bacteria must always be assumed to be present in untreated water, so it is
obvious that untreated hydrotest water should be removed from a system or
vessel as soon as possible. If this is not possible, the water should be kept
flowing at velocities over 5 ft/s (1.5 m/s) because bacteria require low-flow
conditions to colonize.

Reference
Marcel Dekker.

4
Corrosion of Cast Iron and Cast Steel

The corrosion resistance of a cast alloy is often different from that of a
wrought equivalent—sometimes better, sometimes worse. Wrought metal is
metal that has been formed into a desired shape by working (rolling,
extruding, forging, etc.). This includes bar, plate, sheet, tubing, pipe, and
forgings. Most available corrosion data supplied by producers or in the
literature is for wrought material. Use of this data for selection of critical
castings may have undesirable results.
It is important for the end user to be familiar with how various
components are fabricated—from castings, forgings, bar stock, or whatever
the case may be. Cast and wrought alloys often behave differently in
identical service conditions. In nearly all cases, the cast compositions are
altered from the wrought alloys to improve castability. For example, silicon
is added to improve fluidity when pouring the molten metal into a mold.
It is also true that some cast alloys can be produced with superior
properties, but the composition cannot be produced in the wrought form.
Some cast alloys have high silicon and/or carbon contents for superior
corrosion or abrasion resistance; however, the low ductility and high
strength may make rolling and/or forging impossible.
Castings are produced by different molding processes: green sand, air-set
sand, resin-bonded sand, rammed graphite, investment, etc. The corrosion
resistance of an as-cast surface is a function of the moulding process, pouring
temperature, and mold surface treatments or mold washes. Carbon pick-up
and mold reactions are just two of the factors that influence corrosion
resistance. The corrosion resistance of most machined surfaces will be
independent of the molding process provided that 1/16–1/8 in. of material
is removed.
To make an accurate prediction of the corrosion resistance of a cast
material in a specific environment, laboratory or field corrosion testing is
needed. Cast coupons should be removed from larger cast pieces similar to
the equipment they are to represent. The minimum section thickness for
sand-type processes should be 1 in. Both as-cast and machined surfaces
should be tested.

83


At the present time there are three designation systems commonly used
for cast materials. They are (1) the Unified Numbering System (UNS), (2)
Alloy Casting Institute (ACI), and (3) the American Society for Testing and
Materials (ASTM).
The Unified Numbering System is the most widely accepted. In this
system, the metals and alloys are divided into 18 series. The UNS
designations start with a single letter that is followed by five numerical
digits. Where possible, the letter is suggestive of the family of metals (e.g., A
is for aluminum alloys, C for copper alloys, N for nickel alloys, etc.). Table 4.1
provides the complete listing of the letter designations. If possible, common
designations are used within the five numerical digits for user convenience.
Examples include:

A92024 Aluminum alloy 2024
C36000 Copper alloy 360
S31600 Type 316 stainless
steel
N10276 Nickel alloy C276

A complete listing of all UNS members assigned to date can be found in the
publication Metals and Alloys in the Unified Numbering System.
Designations in the ACI system begin with two letters that are followed
by two other numerical digits. Some also end with additional letters and/or
numerical digits. Usually, they begin with either a C (for corrosion-resistant
materials) or an H (for heat-resistant materials). The second letter ranges

TABLE 4.1
Letter Prefixes Used in UNS

Prefix Alloy Series

A Aluminum and aluminum alloys
C Copper and copper alloys
D Steels with special mechanical properties
E Rare earths and rate earth-like metals and alloys
F Cast irons
G Carbon and alloy steels
H AISI H-steels (hardenability controlled)
J Cast steels, except tool steels
K Miscellaneous steels and ferrous alloys
L Low melting temperature metals and alloys
M Miscellaneous nonferrous metals and alloys
N Nickel and nickel alloys
P Precious metals and alloys
R Reactive and refractory metals and alloys
S Heat- and corrosion-resistant (stainless) steels
T Tool steels
W Welding filler metals
Z Zinc and zinc alloys

Corrosion of Cast Iron and Cast Steel 85

from A to Z, depending upon the nickel content, and to a lesser degree upon
the chromium content. For example, an alloy containing 12% chromium and
no nickel would begin CA, whereas a material with 100% nickel would begin
CZ. Alloys in-between begin with intermediate letters. The maximum
content is indicated by the numerical digits (percent ! 100). The presence of
other alloying ingredients is indicated by additional letters. Examples are
shown in Table 4.2.
Nickel–copper alloys do not follow the scheme shown in Table 4.2. They
use M as the first letter (examples are M35-1 and M25.5). Nickel–
molybdenum alloys use the letter N as the beginning letter, such as N7Mn
and N12MV.
Because ACI no longer exists, ASTM has adopted the system and assigns
new designations as other alloys are developed. ASTM also has their own
system of designations for many special carbon and alloy steel products as
well as for cast iron. Some designations include the material type, such as
WCA, WCB, and WCC for welded carbon steel grades S, B, and C. Some
grades are numbered in sequence as added to a specification, and others
indicate a property such as strength. The UNS numbers have not been
adopted for these materials because they have no relation with any common
designation. Refer to Table 4.3 for examples.
Most cast alloys are derived from their wrought counterparts. Others are
proprietary alloys developed as casting alloys. The only alloys discussed
here are those covered by ASTM specifications. Use of industry specifications
is not a guarantee that the required casting quality will be obtained. The more
critical the applications, the more the end-user should know about the
material, the foundry, and any intermediate fabricators. When there are no
industry specifications, a private specification should be developed, either by
the end-user or by the foundry, and carefully reviewed by the end-user.
Extensive testing is required to develop a melting practice, compositional

TABLE 4.2
Examples of ACI Designations

Alloying Elements (%)
Other Alloying
Chromium, Nickel, Carbon, Elements,
Designation Nominal Nominal Maximum Nominal

CA15 12 — 0.15 —
CD4MCu 25 6 0.04 Mo 3; Cu 3
CF8M 19 10 0.08 Mo 2.5
CF3M 19 10 0.03 Mo 2.5
CN7M 21 29 0.07 Mo 2.5
CW2M 16 68 0.02 Mo 1.6
CZ100 0 100 1.0
HK40 25 20 0.40


TABLE 4.3
Examples of ASTM and UNS Designations

ASTM Designation UNS Designation

Grade WCC: welded carbon steel casting, grade C JO2503
Grade LCB: low temperature welded carbon steel JO3
casting, grade B
Class 3 cast iron F12802
Grade 135–125: 135 min tensile, 125 min yield None
strength; steel casting

modifications for castability, weld procedures, filler material, optimum heat
treatment, etc. All of these affect corrosion resistance and casting quality.

4.1 Cast Irons
The general term cast iron is inclusive of a number of alloys of iron, carbon,
and silicon. Typically, these alloys have carbon contents of approximately
1.8–4% and silicon contents of 0.5–3%. This composition describes all grades
of cast irons with properties ranging from highly wear-resistant hard
materials to ductile energy-absorbing alloys suitable for applications
involving high-energy and shock loads. The carbon content of the alloy
can be present in several different forms: graphite flakes, irregular graphite
modules, graphite spheres, iron carbides, cementite, and combinations of
these. The basic types of cast irons are gray iron, ductile (nodulur) iron,
malleable iron, and high-alloy cast irons.

4.1.1 Gray Iron
Gray iron is the most common cast iron, representing 59% of total worldwide
production in 1993. Gray cast iron has a relatively large percentage of the
carbon present as graphite flakes. The gray irons have good fluidity at
pouring temperatures, which makes them ideal for casting intricate shapes
in all sizes. Most show little or no shrinkage during solidification, so that
pattern making is simplified compared to other alloys. Gray iron has
relatively poor toughness because of the stress concentration effect of the
graphite flake tips. Gray irons are generally purchased to ASTM
specifications. ASTM A-48 and A-126, as well as other gray iron ASTM
specifications, use tensile strength as the main acceptance criteria.
Graphite is essentially an inert material and is cathodic to iron. This results
in rapid attack of the iron in even mildly corrosive environments. As the iron
is removed, the reducing graphite flakes and corrosion products may form a
barrier to further attack. This process is called graphitization because the


remaining film is often black. In the extreme case, the part may appear
unaffected, but the loss of iron may be so severe that significant structural
integrity is lost.
The corrosion resistance of gray iron is slightly better than carbon steel in
water, seawater, and various atmospheric environments. In general,
however, the corrosion properties of gray iron are similar to those for
carbon steel. Corrosion rates in rural, industrial, and seacoast environments
are generally acceptable. Gray iron is commonly used for flue-gas
applications, such as wood and coal-fired furnaces and heat exchangers.
The life of buried gray iron pipe is generally longer than that of steel, but
it is highly dependent on soil type, drainage, and other factors. Gray iron is
not resistant to corrosion in acid, except for certain concentrated acids where
a protective film is formed.
Gray iron has good resistance to alkaline solutions, such as sodium
hydroxide and molten caustic soda. Resistance is good in alkaline salt
solutions, such as cyanides, silicates, carbonates, and sulfides. Acids and
oxidizing salts rapidly attack gray iron. Gray iron is used to contain sulfur at
temperatures of 350–4008F (149–2058C). Molten sulfur must be air-free and
solid sulfur must be water-free. Gray iron melting pots are commonly used
for aluminum, cadmium, lead, magnesium, and zinc.

4.1.2 Compacted Graphite Iron
Compacted graphite iron is a relatively new type of cast iron. Its structure is
between that of gray and ductile iron. The graphite is present as blunt flakes
that are interconnected. Production is similar to ductile iron with an
additional alloying element such as titanium. Compacted graphite iron
retains many of the attractive casting properties of gray iron but has
improved strength and ductility. There is little difference in the corrosion
resistance of compacted graphite iron and gray iron.

4.1.3 Ductile (Nodular) Iron1
Ductile iron has basically the same chemical composition as gray iron with a
small chemical modification. Just prior to pouring the molten iron, an
appropriate innoculant such as magnesium is added. This alters the
structure of iron to produce a microstructure in which the graphite form
produced during the solidification process is spheroidal instead of flake
form. The flake form has better machineability, but the spheroidal form
yields much higher strength and ductility.
The corrosion resistance of ductile iron is comparable to that of gray iron
with one exception; under velocity conditions the resistance of ductile iron
may be slightly less than that of gray iron because it does not form the same
type of film that is present in gray iron.


4.1.4 White Iron
White iron solidifies with a “chilled” structure, meaning that instead of
forming free graphite, the carbon forms hard, abrasion-resistant, iron–
chromium carbides. Some white iron contains as much as 25% chromium to
permit casting thicker sections. White irons are primarily used for abrasive
applications. After final machining, the material is generally heat-treated to
form a martensitic matrix for maximum hardness and wear resistance.
White irons are very brittle. Elongation in the hardened condition is typically
about 2%. The only property required by the ASTM specification is the
Brinell hardness. In general, there is little difference in corrosion resistance of
gray iron and white iron. The high-chromium iron has only slightly better
corrosion resistance.

4.1.5 Malleable Iron
The properties of malleable iron and ductile iron are very similar, but
malleable iron is declining in use for economic reasons. Malleable iron is
only available in thin sections because rapid cooling in the mold is required
to form white iron. Irregularly shaped graphite nodules are formed from
white iron by a heat treatment of over 8 h. The costs involved with such a
long heat treatment in a controlled atmosphere furnace have become
prohibitive. The graphite nodules provide much better ductility than is
found in gray iron. ASTM A-47 and A-197 are the two most widely used
industry specifications. In general, there is little difference in the corrosion
resistance of gray iron and malleable iron. Under flowing conditions,
malleable iron may be inferior to gray iron. Without the graphite flakes to
hold the corrosion products in place, attack continues at a constant rate
rather than declining with time.

4.2 High Alloy Cast Irons
The high-alloy cast irons are generally divided into three groups: austenitic,
austenitic ductile, and high-silicon cast irons. Each will be discussed
separately.

4.2.1 Austenitic Gray Cast Irons
The austenitic gray irons are gray irons with additions of nickel (and in some
instances copper) to produce an austenitic matrix structure similar to the
300-series stainless steels. They have a flake graphite structure and
mechanical properties similar to the gray irons. The austenetic gray cast
irons (often called Ni-Resist, a trademark of Inco Alloys International)


offer better corrosion resistance and wear resistance, toughness, and
high-temperature properties than the standard gray irons. Austenitic gray
irons are purchased per ASTM A-436. The specification covers the chemistry,
minimum tensile strength, and Brinell hardness. Types 1 and 2 are the most
commonly used grades.
The corrosion resistance of austenitic gray iron falls between that of gray iron
and the 300-series stainless steels. The largest use is in hydrogen sulfide-
containing oil-field applications. A protective sulfide film is formed that
prevents excessive attack. Austenitic gray iron also resists erosion from sand
often entrained in crude oil. It is superior to gray iron in atmospheric exposure,
seawater, caustic soda (sodium hydroxide), and dilute and concentrated
(unaerated) sulfuric acid. The copper in type 1 provides the best resistance to
sulfuric acid.
Refer to Table 4.4 for the compatibility of Ni-Resist alloy with selected
corrodents.

4.2.2 Austenitic Ductile Cast Irons
These alloys are commonly called ductile Ni-Resist. They are similar to the
austenitic gray irons except that magnesium is added just prior to pouring
to produce a nodular graphite structure. As a result of this nodular structure,
higher strengths and greater ductility are produced as compared to the
flake-graphite structure. Although several different grades are produced,
type 2D is the most commonly used grade.
The corrosion resistance is similar to that of the austenitic gray iron,
although those containing 2% or more chromium are superior. Table 4.4
shows the compatibility of Ni-Resist with selected corrodents.

4.2.3 High-Silicon Cast Irons
High-silicon cast irons are sold under the trade name of Duriron and
Durichlor 51, which are tradenames of the Duriron Company. These alloys
contain 12–18% silicon, with 14.5% being nominal (14.2% minimum is
required for good corrosion resistance), 1% carbon, and the balance iron.
These alloys are particularly susceptible to thermal and mechanical shock.
They cannot be subjected to sudden fluctuations in temperature, nor can
they withstand any substantial stressing or impact. The high-silicon irons are
extremely brittle and difficult to machine.
When high-silicon cast irons are first exposed to a corrosive environment,
surface iron is removed, leaving behind a silicon oxide layer that is very
adherent and corrosion resistant. These alloys are extremely corrosion
resistant. One of the main uses is in the handling of sulfuric acid. It is
resistant to all concentrations of sulfuric acid up to and including the normal
boiling point. Below 30%, the temperature is limited to 1808F (828C).


TABLE 4.4
Compatibility of Ni-Resist Alloy with Selected Corrodents
Maximum Temperature
Chemical 8F 8C

Acetic anhydride X
Acetone 140 60
Acetylene 90 32
Alum 100 38
Aluminum hydroxide, 10% 470 243
Aluminum potassium sulfate 100 38
Ammonia, anhydrous 460 238
Ammonium carbonate, 1% 90 32
Ammonium chloride 210 99
Ammonium hydroxide 90 32
Ammonium nitrate, 60% 120 49
Ammonium persulfate, 60% 120 49
Ammonium phosphate X
Ammonium sulfate 130 54
Amyl acetate 300 149
Aniline 100 38
Arsenic acid X
Barium carbonate X
Barium chloride X
Barium hydroxide X
Barium sulfate X
Barium sulﬁde X
Benzene 400 204
Black liquor 90 32
Boric acid X
Bromine gas X
Butyl acetate X
Calcium carbonate 460 238
Calcium hydroxide 90 32
Calcium nitrate 210 99
Calcium sulfate 440 227
Carbon dioxide, dry 300 149
Carbon dioxide, wet X
Carbon monoxide 300 149
Carbon tetrachloride 170 77
Carbonic acid 460 238
Chlorine gas, dry 90 32
Chromic acid X
Cyclohexane 90 32
Diethylene glycol 300 149
Diphenyl 210 99
Ethanol amine 200 93
Ethyl acetate 90 32
Ethyl chloride, dry 90 32
Ethylene glycol 460 238
Ethylene oxide X
(continued)


TABLE 4.4 Continued
Maximum Temperature
Chemical 8F 8C

Ferric sulfate 460 238
Ferrous sulfate X
Fuel oil X
Furfural, 25% 210 99
Gallic acid 90 32
Gas, natural 90 32
Gasoline, leaded 400 204
Gasoline, unleaded 400 204
Glycerine 320 160
Hydrochloric acid X
Hydrogen chloride gas, dry X
Hydrogen sulfide, dry 460 238
Hydrogen sulfide, wet 460 238
Isooctane 90 32
Magnesium hydroxide X
Magnesium sulfate 150 66
Methyl alcohol 160 71
Methyl chloride X
Phosphoric acid X
Sodium borate 90 32
Sodium hydroxide, to 70% 170 77
Sodium nitrate 90 32
Sodium nitrite 90 32
Sodium peroxide, 10% 90 32
Sodium silicate 90 32
Sodium sulfate X
Sodium sulfide X
Steam, low pressure 350 177
Sulfate liquors 100 38
Sulfur 100 38
Sulfur dioxide, dry 90 32
Tartaric acid 100 38
Tomato juice 120 49
Vinegar 230 110
Water, acid mine 210 99
White liquor 90 32
The chemicals listed are in the pure state or in a saturated solution
unless otherwise indicated. Compatibility is shown to the maximum
allowable temperature for which data are available. Incompatibility is
shown by an X. When compatible, corrosion rate is less than 20 mpy.
Source: From P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4,
5th ed., New York: Marcel Dekker.

When 4.5% chromium is added to the alloy, it becomes resistant to severe
chloride-containing solutions and other strongly oxidizing environments.
The chromium-bearing grade (Durichlor) will handle hydrochloric acid up
to 808F (278C). Hydrofluoric acid causes rapid attack. Table 4.5 lists the
compatibility of high-silicon iron with selected corrodents.


TABLE 4.5
Compatibility of High-Silicon Irona with Selected Corrodents
Maximum Temperature
Chemical 8F 8C

Acetaldehyde 90 32
Acetamide
Acetic acid, 10% 200 93
Acetic acid, glacial 230 110
Acetic anhydride 120 49
Acetone 80 27
Acetyl chloride 80 27
Acrylic acid
Acrylonitrile 80 27
Adipic acid 80 27
Allyl alcohol 80 27
Allyl chloride 90 32
Alum 240 116
Aluminum acetate 200 93
Aluminum chloride, aqueous
Aluminum chloride, dry
Aluminum hydroxide 80 27
Aluminum nitrate 80 27
Aluminum oxychloride
Aluminum sulfate 80 27
Ammonia gas
Ammonium bifluoride X
Ammonium carbonate 200 93
Ammonium chloride, 10%
Ammonium chloride, 50% 200 93
Ammonium chloride, sat.
Ammonium fluoride, 10% X
Ammonium hydroxide, 25% 210 99
Ammonium hydroxide, sat.
Ammonium nitrate 90 32
Ammonium persulfate 80 27
Ammonium phosphate 90 32
Ammonium sulfate, 10–40% 80 27
Ammonium sulfide
Ammonium sulfite
Amyl acetate 90 32
Amyl alcohol 90 32
Amyl chloride 90 32
Aniline 250 121
Antimony trichloride 80 27
Aqua regia, 3:1 X
Barium carbonate 80 27
(continued)


TABLE 4.5 Continued
Maximum Temperature
Chemical 8F 8C

Barium chloride 80 27
Barium hydroxide
Barium sulfate 80 27
Barium sulfide 80 27
Benzaldehyde 120 49
Benzene 210 99
Benzene sulfonic acid, 10% 90 32
Benzoic acid 90 32
Benzyl alcohol 80 27
Benzyl chloride 90 32
Borax 90 32
Boric acid 80 27
Bromine gas, dry X
Bromine gas, moist 80 27
Bromine, liquid
Butadiene
Butyl acetate
Butyl alcohol 80 27
n-Butylamine
Butyl phthalate 80 27
Butyric acid 80 27
Calcium bisulfide
Calcium bisulfite X
Calcium chlorate 80 27
Calcium chloride 210 99
Calcium hydroxide, 10%
Calcium hydroxide, sat. 200 93
Calcium hypochlorite 80 27
Calcium nitrate
Calcium oxide
Caprylic acid 90 32
Carbon bisulfide 210 99
Carbon dioxide, wet 80 27
Carbon disulfide
Carbon monoxide
Carbonic acid 80 27
Cellosolve 80 27
Chloracetic acid, 50% water 80 27
Chloracetic acid 90 32
Chlorine gas, dry
Chlorine gas, wet
Chlorine, liquid
Chlorobenzene 80 27
Chloroform 90 32
(continued)


TABLE 4.5 Continued
Maximum Temperature
Chemical 8F 8C

Chlorosulfonic acid, dry
Chromic acid, 10% 200 93
Chromyl chloride 210 99
Citric acid, 15%
Citric acid, conc. 200 93
Copper acetate
Copper carbonate
Copper chloride X
Copper cyanide 80 27
Copper sulfate 100 38
Cresol
Cupric chloride, 5%
Cupric chloride, 50%
Cyclohexane 80 27
Cyclohexanol 80 27
Dichloroacetic acid
Dichloroethane (ethylene dichloride) 80 27
Ferric chloride X
Ferric chloride, 50% in water
Ferric nitrate, 10–50% 90 32
Ferrous chloride 100 38
Ferrous nitrate
Fluorine gas, dry X
Fluorine gas, moist
Hydrobromic acid, dilute X
Hydrobromic acid, 20%
Hydrobromic acid, 50% X
Hydrochloric acid, 20%b 80 27
Hydrochloric acid, 38%
Hydrocyanic acid, 10% X
Hypochlorous acid
Iodine solution, 10%
Ketones, general 90 32
Lactic acid, 25% 90 32
Lactic acid, conc. 90 32
Magnesium chloride, 30% 250 121
Malic acid 90 32
Manganese chloride
Methyl chloride
Methyl ethyl ketone 80 27
Methyl isobutyl ketone 80 27
Muriatic acid
Nitric acid, 5% 180 82
(continued)


TABLE 4.5 Continued
Maximum Temperature
Chemical 8F 8C

Nitric acid, anhydrous 150 66
Nitrous acid, conc. 80 27
Oleum X
Perchloric acid, 10% 80 27
Phenol 100 38
Phosphoric acid, 50–80% 210 99
Picric acid 80 27
Potassium bromide, 30% 100 38
Salicylic acid 80 27
Silver bromide, 10%
Sodium carbonate
Sodium chloride, to 30% 150 66
Sodium hydroxide, 10% 170 77
Sodium hydroxide, 50% X
Sodium hydroxide, conc. X
Sodium hypochlorite, 20% 60 16
Sodium hypochlorite, conc.
Sodium sulﬁde, to 50% 90 32
Stannic chloride X
Stannous chloride X
Sulfuric acid, 10% 212 100
Sulfuric acid, fuming
Sulfurous acid X
Thionyl chloride
Toluene
Trichloroacetic acid 80 27
White liquor
Zinc chloride
The chemicals listed are in the pure state or in a saturated solution unless
otherwise indicated. Compatibility is shown to the maximum allowable
temperature for which data are available. Incompatibility is shown by an X. A
blank space indicates that data are unavailable. When compatible, corrosion rate is
!20 mpy.
a
Resistance applies to Duriron unless otherwise noted.
b
Resistance applies only to Durichlor.
Source: From P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed.,
New York: Marcel Dekker.


4.3 Carbon and Low-Alloy Carbon Steels
Cast carbon and low-alloy steels are widely used because of their low cost,
versatile properties, and the wide range of available grades. The carbon
steels are alloys of iron and carbon with manganese (!1.65%), silicon, sulfur,
phosphorus, and other elements in small quantities. The latter elements are
present either for their desirable effects or because of the difficulty of
removing them. Steel castings are generally grouped into four categories:

1. Low-carbon castings with less than 0.20% carbon.
2. Medium carbon castings with 0.20–0.50 carbon.
3. High-carbon castings with more than 0.5% carbon.
4. Low-alloy castings with alloy content less than 8%.

There is no significant difference in the corrosion resistance of cast and
wrought carbon and low-alloy steels. Wrought “weathering” low-alloy steels
gradually form a protective rust layer after a few years of exposure to rural
and urban atmospheres. These steels contain both chromium and copper
and may also contain silicon, nickel, phosphorus, or other alloying
elements.3 ASTM G101 is a guide for estimating the atmospheric corrosion
rates of wrought weathering steels. Care should be used when utilizing these
estimating methods for cast steels. The natural segregation in cast steels may
produce different results. Weathering steels are of little benefit in
submersed service.
Cast carbon and low-alloy steels are usually protected from atmospheric
corrosion by painting and/or coating systems.4 Coatings may also be used to
prevent rust contamination where product purity is a requirement. Carbon
and low-alloy steels are used for water, steam, air, and many other mild
services. They are also resistant to many gases provided the moisture content
is below the saturation point. These applications include carbon dioxide,
carbon monoxide, hydrogen sulfide, hydrogen cyanide, sulfur dioxide,
chlorine, hydrogen chloride, fluorine, hydrogen fluoride, and nitrogen. It
must be emphasized that these gases must be dry. Contamination from air
and humidity will cause excessive attack and/or stress corrosion cracking
(SCC).
In certain corrosive environments, a protective surface layer may be
formed that will prevent excessive corrosion. Examples include concen-
trated sulfuric acid where a ferrous sulfate film protects the steel, and
concentrated hydrofluoric acid that forms a fluoride film. Extreme care must
be taken, however, to prevent conditions that may damage the film and lead
to extremely high corrosion rates. These conditions include high velocities,
condensing water (humidity from the air), and hydrogen bubbles floating
across a surface.


Steel is used for alkaline compounds such as sodium hydroxide and
potassium hydroxide. At temperatures above 1508F (668C), however, SCC
and excessive corrosion may develop.5 Neutral salts, brines, and organics
tend to be noncorrosive to steel. Acidic and alkaline salts are more corrosive.
The NACE Corrosion Data Survey is a good reference for these applications.6
There is generally little difference in the corrosion resistance of carbon
steels and low-alloy steels. In boiler feedwater, however, the Cr–Mo grades
such as WC6 and WC9 offer definite advantages over WCB and WCC. The
Cr–Mo grades form a more adherent iron oxide film that makes them more
resistant to erosion/corrosion. Very high velocities may erode the protective
film from WCB to WCC, whereas WC6 and WC9 are unaffected. When the
film is removed, corrosion will proceed at a high rate until the film is
reformed. Under conditions of high-velocity impingement, carbon steel may
be perforated in a few months, whereas a Cr–Mo replacement will last years.
Cast carbon and low-alloy steels are routinely used for hydrogen service.
As temperature and hydrogen partial pressure increase, however, a
phenomenon called hydrogen attack can occur. In hydrogen attack, atomic
hydrogen diffuses into the steel and combines with carbon to form methane.
Over time, high-pressure methane pockets are formed and structural
integrity is lost.7 Following the guidelines of the American Petroleum
Institute (API) will prevent this attack.8
Anhydrous ammonia is routinely handled by cast carbon and low-alloy
steels. To prevent SCC, however, small amounts of water are added. Oxygen
contamination should also be avoided. Other media known to cause SCC are
high-temperature hydroxides, nitrates, carbonates, moist gas mixtures of
carbon dioxide and carbon monoxide, hydrogen cyanide solutions, amine
solutions, and hydrogen sulfide.
Postweld heat treatment (PWHT) or stress relieving is a relatively
inexpensive process to minimize the occurrence of SCC. NACE International
committees have developed several standards and recommended practices
along these lines, including MR0175, RP0472, RP0590, and 8X194.9–11

References
1. P.A. Schweitzer. 2003. Metallic Materials, Physical, Mechanical and Corrosion
Properties, New York: Marcel Dekker.
2. P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed., New
York: Marcel Dekker.
3. ASTM. 1993. G101 Estimating the Atmospheric Corrosion Resistance of
Low-Allow Steels, in Metal Corrosion, Erosion and Wear, Vol. 32, Philadelphia:
American Society for Testing and Materials, pp. 408–413.
4. C.G. Munger. 1993. Marine and offshore corrosion control—past, present, and
future, Materials Performance, 32:9, 37–41.
5. R. Jones. 1986. Carbon and low-alloy steels, in Processing Industries Corrosion,
B.J. Moniz and W.I. Pollack, Eds, Houston: NACE International, p. 385.


6. NACE International. 1986. Corrosion Data Survey—Metals Section, 6th ed.,
Houston: NACE International.
7. D. Warren. 1987. Hydrogen effects on steel, in Materials Performance, 26:1, 38–48.
8. American Petroleum Institute. 1983. Steels for hydrogen service at elevated
temperatures and pressures in petroleum refineries and petrochemical plants,
API Publication 941, Washington, DC: American Petroleum Institute.
9. NACE International. 2005. Methods and controls to prevent in-service
environmental cracking of carbon steel weldments in corrosive petroleum
refining environments, NACE Standard RP0472-2005, Houston: NACE
International.
10. NACE International. 1996. Recommended practice for prevention, detection,
and correction of deaerator cracking, NACE Standard RP0590-96, Houston:
NACE International.
11. NACE International. 1994. Materials and fabrication practices for new pressure
vessels used in wet H2S refinery service, NACE Publication 8X194, Houston:
NACE International.

5
Introduction to Stainless Steel

In all probability the most widely known and most commonly used material
is stainless steel. Stainless steels are iron-based alloys containing 10.5% or
more chromium. There are currently over 70 types of stainless steels.
In the United States, annual stainless steel consumption is approaching 2
million metric tons. In addition to being an important factor in industrial
process equipment, it also finds application in a wide variety of household
items.
World-wide production of stainless steel exceeds 12.1 million metric tons.
The first trials of adding chromium to mild steel took place in the early 1900s.
This was apparently the result of the observation that chromium-plated steel
parts were highly corrosion resistant. This experiment resulted in the
production of the ferritic family of stainless steels. Documentation of this
class of steel began to appear in the 1920s. In 1935, the first American Society
for Testing and Materials (ASTM) specifications for stainless steel
were published.
Stainless steel is not a singular material, as the name might imply, but
rather a broad group of alloys, each of which exhibits its own physical,
mechanical, and corrosion resistance properties.
These steels are produced both as cast alloys (Alloy Casting Institute (ACI)
types) and wrought forms (American Iron and Steel Institute (AISI) types).
Generally, all are iron-based with 12–30% chromium, 0–22% nickel, and
minor amounts of carbon, nickel, niobium, copper, molybdenum, selenium,
tantalum, and titanium. They are corrosion resistant and heat resistant,
noncontaminating, and easily fabricated into complex shapes.

5.1 Stainless Steel Classification
There are three general classification systems used to identify stainless steels.
The first relates to metallurgical structure and places a particular stainless
steel into a family of stainless steels. The other two, namely, the AISI

99


TABLE 5.1
Ferritic Stainless Steels
AISI Type UNS Designation AISI Type UNS Designation

405 S40500 446 S44600
409 S40900 439 S43035
429 S42900 444 S44400
430 S43000 26-1 S44627
430F S43020 26-3-3 S44660
430FSe S43023 29-4 S44700
434 S43400 29-4C S44735
436 S43600 29-4-2 S44800
442 S44200

Numbering System and the Unified Numbering System (UNS) that were
developed by ASTM and SAE to apply to all commercial metals and alloys,
define specific alloy compositions. Table 5.1 through Table 5.3 provide a
comparison between ASI and UNS designation for stainless steels.
The various stainless steel alloys can be divided into seven basic families:

1. Ferritic
2. Martensitic
3. Austenitic
4. Precipitation-hardenable
5. Superferritic
6. Duplex (ferritic–austenitic)
7. Superaustenitic

5.1.1 Ferritic Family
The name is derived from the analogous ferrite phase, or relatively pure iron
component, of carbon steels, cooled slowly from the austenite region. The

TABLE 5.2
Martensitic Stainless Steels

403 S40300 420F S42020
410 S41000 422 S42200
414 S41400 431 S43100
416 S41600 440A S44002
416Se S41623 440B S44003
420 S42000 440 S44004

Introduction to Stainless Steel 101

TABLE 5.3
Austenitic Stainless Steels

201 S20100 308 S30800
202 S20200 309 S30900
204 S20400 309S S30908
204L S20403 310 S31000
205 320500 310S S31008
209 S20900 314 S31400
22-18-S S20910 316 S31600
18-8.8 plus S20220 316L S31603
301 S30100 316F S31620
302 S30200 316N S31651
302B S30215 317 S31700
303 S30300 317L S31703
303Se S30323 321 S32100
304 S30400 329 S32900
304L S30403 330 N08330
S30430 347 S34700
304N S30451 348 S34800
305 S30500 384 S38400

ferrite phase for pure iron in the stable phase existing below 16708F (9108C).
For low-carbon Cr–Fe alloys, the high-temperature austenite phase exists
only up to 12% Cr; immediately beyond this composition the alloys are
ferritic at all temperatures up to the melting point.
Chromium readily forms an oxide that is transparent and happens to be
extremely resistant to further degradation. It is less noble than iron and,
when alloyed with steel, tends to form its oxide first. Gradually increasing
the chromium content in steel above the 2% level steadily improves mild
atmospheric corrosion resistance up to approximately 12%, where corrosion
is essentially stopped. For exposure to mild wet environments, the addition
of approximately 11% chromium is sufficient to prevent rusting of steel,
hence the term stainless.
Ferritic stainless steels are magnetic, have body-centered cubic atomic
structures and possess mechanical properties similar to those of carbon steel,
though they are less ductile.
Continued additions of chromium will improve corrosion resistance in
more severe environments, particularly in terms of resistance in oxidizing
environments, at both moderate and elevated temperatures. Chromium
contents in ferritic stainless steels are limited to approximately 28%. These
alloys are known as 400-series stainless because they were identified with
numbers beginning with 400 when AISI had the authority to designate alloy
compositions. Specific members of the ferritic families will be covered in
Chapter 7.


5.1.2 Martensitic Family
The name is derived from the analogous martensite phase in carbon steels.
Martensite is produced by a shear-type phase transformation on cooling a
steel rapidly (quenching) from the austenitic region of the phase diagram.
These alloys are hardenable because of the phase transformation from body-
centered cubic to body-centered tetragonal. As with the alloy steels, this
transformation is thermally controlled. The martensitic stainless steels are
normally 11–13% chromium and are ferromagnetic.
Because the corrosion resistance of these stainless steels is dependent
upon the chromium content, and because the carbon contents are generally
higher than the ferritic alloys, it is logical that they are less corrosion
resistant. However, their useful corrosion resistance in mild environments
coupled with their high strengths made members of the martensitic family
useful for certain stainless steel applications. Details of the specific family
members are covered in Chapter 9.

5.1.3 Austenitic Family
The third group is named after the austenite phase, which for pure iron
exists as a stable structure between 1670 and 25528F (910 and 14008C). It is the
major or only phase of stainless steel at room temperature, existing as a
stable or metastable structure by virtue of its austenite-forming alloy
additions, notably nickel and manganese. These stainless steels have face-
centered austenite structure from below 328F (08C) up to near melting
temperatures.
This family of stainless steel accounts for the widest usage of all the
stainless steels. These materials are nonmagnetic, are not hardenable by heat
treatment. They can, however, be strain hardened by cold work, have face-
centered cubic structures, and possess mechanical properties similar to those
of mild steels, but with better formability. The strain hardening from cold
work induces a small amount of ferromagnetism.
It has been established that certain elements, specifically chromium,
molybdenum, and silicon, are ferrite formers. Aluminum and niobium may
also act as ferrite formers depending upon the alloy system. Other elements,
such as nickel, manganese, carbon, and nitrogen, tend to promote the
formation of austenite.
After the corrosion resistance plateau of 18% chromium is reached, the
addition of approximately 8% nickel is required to cause a transition from
ferritic to austenitic. This alloy is added primarily to form the austenitic
structure that is very tough, formable, and weldable. An additional benefit is
the increased corrosion resistance to mild corrodents. This includes
adequate resistance to most foods, a wide range of organic chemicals,
mild inorganic acids, and most natural environmental corrosion.
The corrosion resistance of the austenitic stainless steel is further
improved by the addition of molybdenum, titanium, and other elements.


Corrosion resistance properties of undivided members of the austenitic
family are discussed in Chapter 10.

5.1.4 Precipitation-Hardenable Stainless Steels
A thermal treatment is utilized to intentionally precipitate phases, causing a
strengthening of the alloy. An alloy addition of one or more of titanium,
niobium, molybdenum, copper, or aluminum generates the precipitating
phase. The ﬁnal alloy can be solution treated because all alloying elements
are in solid solution and the material is in its softest or annealed state. In
this condition, the material can be formed, machined, and welded. After
fabrication, the unit is exposed to an elevated temperature cycle (aging)
that precipitates the desired phases to cause an increase in mechanical
properties.
There are three types of precipitation-hardenable (pH) stainless steels:
martensitic, austenitic, and semiaustenitic. The relationship between these
alloys is shown in Figure 5.1. The semiaustenitic steels are supplied as an

S17400 S17700 S66286
less Ni 18−8 w/AI more Ni
add Cu Cb add Mo Ti

Austenitic

S15500 S15700
less Cr less Cr
add Mo

S35000
S45000
less Ni
more Ni
add N
add Mo
no AI

Semiaustenitic

S13800
less Cr
more Ni Mo
no Cu
add AI
Martensitic

FIGURE 5.1
Precipitation-hardening stainless steels.


unstable austenite that is in the workable condition and must be transformed
to martensite before aging. The martensitic and austenitic pH stainless steels
are directly hardened by thermal treatment.
These alloys possess high mechanical properties, but not as high as the
low-alloy martensitic steels, in conjunction with useful corrosion resistance
properties. In general, their corrosion resistance is below that of type 304
stainless steel, although certain specific alloys approach the corrosion
resistance of type 316 stainless steel.
The corrosion resistance properties of the individual family members will
be discussed in Chapter 13.

5.1.5 Superferritic Stainless Steels
During the 1970s, development efforts were directed at producing ferritic
materials that could exhibit a high level of general and localized pitting
resistance. The first commercially significant alloy that could meet these
specifications was an alloy containing 26% chromium and 19% molyb-
denum. To obtain the desired corrosion resistance and acceptable fabrication
characteristics, the material was electron-beam refined under a vacuum and
introsuced as E-Brite alloy. Carbon plus nitrogen contents were maintained
at levels below 0.020%. Other alloys were developed.
The superferritic alloys exhibit excellent localized corrosion resistance.
The superferritic materials alloyed with nickel exhibit improved mechanical
toughness and are less sensitive to contamination from interstitial elements.
However, their availability is still limited in thicknesses less than
approximately 0.20 in. This is related to the formation of embrittling phases
during cooling from annealing temperatures. Greater thicknesses cannot be
cooled quickly enough to avoid a loss of toughness.
Individual family members will be discussed in Chapter 8.

5.1.6 Duplex Stainless Steels
The duplex stainless steels contain roughly 50% austenite and 50% ferrite,
which provides improved corrosion resistance. These alloys contain
relatively high amounts of chromium with only enough nickel and
austenizers to develop 50% austenite.
The duplex stainless steels contain molybdenum as an alloying ingredient
that is responsible for the improved corrosion resistance in chloridic
environments. Molybdenum also reduces the susceptibility to chloride
pitting, crevice corrosion and stress corrosion cracking. The general
corrosion resistance of the duplex stainless steels is slightly greater than
that of 316 stainless steels in most media. These alloys also offer higher
strengths than those typically found with austenitic stainless steels.
Care must be taken when selecting these alloys because the boundary
between acceptable and poor performance is very sharp. They should


not be used under conditions that operate close to the limits of their
acceptability.
The duplex stainless steels are not as ductile as the austenitic family of
stainless steels. Welding requires more care than with the austenitic alloys
due to a greater tendency towards compositional segregation and sensitivity
to weld heat input.
Corrosion resistance properties of each family member will be discussed
in Chapter 12.

5.1.7 Superaustenitic Stainless Steels
The superaustenitic stainless steels were developed to provide alloys with
better resistance to localized corrosion. Included in this family of stainless
steels are those that have improved pitting resistance, those that have
improved crevice corrosion resistance, and those that have good general
corrosion resistance to strong acids.
Corrosion resistance properties of each family member will be discussed
in Chapter 11.

5.2 Passivation
The corrosion resistance of the steels is the result of the passive oxide film
that forms on the exposed surfaces. Under normal circumstances, this film
will form immediately upon exposure to oxygen. Some fabrication processes
can impede the formation of this film. To guarantee the formation of this
protective layer, stainless steels are subjected to passivation treatments.
The most common passivation treatments involve exposing the metal to
an oxidizing acid. Nitric and nitric/hydrochloric acid mixtures find the
widest usage. The nitric/hydrochloric acid mixtures are more aggressive
and are used to remove the oxide scales formed during thermal treatment.
This process provides two benefits. It removes the oxide scale and passivates
the underlying metal. Second, the passivation process will remove any
chromium-depleted layer that may have formed as the result of scale
formation.
For passivation treatments other than for scale removal, less aggressive acid
solutions are used. The purpose of these treatments is to remove any
contaminants that may be on the component’s surface that could prevent the
formation of the oxide layer locally. The most common contaminant is
embedded or free iron particles from forming or machining tools. A 10% nitric
acid solution is effective in removing free iron. For martensitic, ferretic, and
precipitation-hardening grades, a nitric acid solution inhibited with sodium
dichromate is used so as not to attack the stainless steel too aggressively.


A 1% phosphoric acid solution and 20% nitric acid solution are used for the
more resistant stainless alloys.

5.3 Sanitizing
When stainless steel is to be used in food service, it requires treatment to
remove bacteria or other microorganisms. It is quite common to use chlorine
water or hypochlorite solution for this purpose. These solutions should be
prepared using demineralized water. This process can be successful if the
solution is properly drained and ﬂushed. A conductivity test may be used on
the rinse water to ensure that the discharge is substantially equivalent to the
demineralized water used in formulating the sanitizing solutions. If not
thoroughly rinsed, chloride pitting, crevice corrosion, or stress corrosion
cracking may occur.
Other, safer alternative oxidizing solutions, such as ammonium persulfate,
hydrogen peroxide, dilute peracetic acid, or a citric nitrate solution, should
be considered. Another possible approach is the use of a nonoxidizing
biocide such as hexamethylene biguanide or other environmentally
safe biocides. These are free of the hazards associated with chlorine,
hypochlorite, chlorine dioxide, and other halogenated agents.

5.4 Preparing for Service
After fabrication is complete and the material is ready to be placed in service,
it is essential that steps are taken to preserve the protective ﬁlm of chromium
oxide. The most common causes of problems are:

Iron contamination
Organic contamination
Welding contamination

5.4.1 Iron Contamination
Embedded iron can be removed by pickling. This is primarily an operation
required on fabricated vessels. However, care must be exercised in the
storage and handling of stainless steel sheet or plate to prevent the surface
from becoming contaminated with embedded iron. If cleanliness on the
surface is extremely important, as in pharmaceutical or food environments
where product contamination would be detrimental, the sheet or plate can
be ordered with a protective adhesive paper on the surface. Leaving this


paper in place during fabrication will reduce the amount of time required for
cleanup after fabrication. The sheet and plate should be stored upright, not
lain on the floor.
During fabrication, it is good practice to use cardboard or plastic sheets on
carbon steel layout and cutting tables, forming roll aprons and rollout
benches. This will go a long way towards reducing or preventing iron
embedment. The use of plastic, wood, or aluminum guards on slings, hooks,
and the forks of forklift trucks will further reduce the chance of
iron embedding.

5.4.2 Organic Contamination
Organic contamination is the result of grease, construction markings
(crayon) oil, paint, adhesive tapes, sediment, and other sticky substances
being allowed to remain on the stainless steel. If not removed, they may
cause crevice corrosion of the stainless steel if exposed to extremely
corrosive atmospheres. During fabrication, there is little that can be done to
prevent this contamination from occurring. The only solution is to insure
that all such deposits are removed during final cleanup.
The cleanup procedures to be followed will depend somewhat on the
service to which the vessel will be put. In very corrosive media, a greater
degree of cleanup will be required than in relatively mild media.
Good commercial practice will always include degreasing and removal of
embedded iron. A complete specification for the procurement of a vessel
should include the desired cleanup procedures to be followed, even if only
degreasing and removal of embedded iron are required.

5.4.3 Welding Contamination
In corrosive environments, corrosion will be initiated by surface imperfec-
tions in stainless steel plate. This corrosion can occur in the presence of
media to which stainless steel is normally resistant. Such imperfections can
be caused by:

Weld splatter
Welding slag from coated electrode arc strikes
Welding stop points
Heat tint

Weld splatter produces small particles of metal that adhere to the surface,
at which point the protective film is penetrated, forming minute crevices
where the film has been weakened the most. If a splatter-prevention
paste is applied to either side of the joint to be welded, this problem will
be eliminated. Splatter will then easily wash off with the paste
during cleanup.


Whenever coated electrodes are used, there will be some slag around the
welded joints. This slag is somewhat difficult to remove, but if it is not done,
the small crevices formed will be points of initiation of corrosion.
Arc strikes and weld stop points are more damaging to stainless steel than
embedded iron because they occur in the area where the protective film has
already been weakened by the heat of welding. Weld stop points create
pinpoint defects in the metal, whereas arc strikes form crevice-like
imperfections in or adjacent to the heat-affected zone.
It is possible to avoid weld-stop defects by employing extensions at the
beginning and end of a weld (runout tabs) and by beginning just before each
stop point and welding over each intermediate stop point.
An arc strike can be struck initially on a runout tab or on weld metal,
provided that the filler metal will tolerate this. If the filler metal will not
tolerate striking of an arc, then the arc must be struck adjacent to it, in it, or
near the heat-affected zone when it is necessary to strike an arc between
runout tabs.
Heat tint results in the weakening of the protective film beneath it and can
be the result of the welding of internals in a vessel or the welding of external
attachments. The heat tint must be removed to prevent corrosion from taking
place in the tinted area.
Welding contamination removal is best accomplished using abrasive discs
and flapper wheels. Although grinding has been used, this procedure tends
to overheat the surface, thereby reducing its corrosion resistance. Its use
should be avoided.

6
Corrosion of Stainless Steels

The first mention of the corrosion resistance of various alloys that had been
formulated in which chromium–iron alloys were prepared appeared in 1820
in a published report by Stodart and M. Faraday. However, the maximum
chromium content was below that required for passivity. Consequently, they
narrowly missed discovering stainless steels.
In 1821 in France, Berthier found that iron alloyed with large amounts of
chromium was more resistant to acids than unalloyed iron. However, the
alloys were high in carbon, brittle, and had no value as structural materials.
During subsequent years, a variety of chromium–iron alloys were
developed by several investigators who took advantage of the high strength
and high hardness imparted by chromium. The inherent corrosion resistance
of the alloys was not observed, primarily because the accompanying high
carbon content impaired the corrosion properties. In 1904, Guillot of France
produced low-carbon–chromium alloys overlapping the passive composition
range. Although he studied the metallurgical structure and mechanical
properties of the chromium–iron alloys and the chromium–iron–nickel alloys,
he did not recognize the outstanding property of passivity.
The property of passivity, starting at a minimum of 12% chromium, was
first described by Monnartz of Germany in 1908. He published a detailed
account of the chemical properties of the chromium–iron alloys in 1911.
Meanwhile, H. Brearly in England was attempting to develop iron-based
chromium alloys to prevent erosion and fouling in rifle barrels. During
his experiments, he noted their resistance to etching for metallographic
examination. He observed that the 12% chromium–iron alloys did not etch
with the usual nitric acid and other etching reagents, and that they did not
rust over long periods of exposure to the atmosphere. He called these ferritic
alloys “stainless steel” and recognized their possible use as cutlery materials.
Simultaneously, Benno Strauss and Edward Maurer in Germany were
investigating iron–chromium–nickel compositions. They observed that the
austenitic alloys containing 8% nickel were resistant to acid fumes, but the
alloys were impractical because they cracked during any metalworking
operation. However, Strauss restored ductility when he developed an

109


annealing heat treatment, followed by a water quench that dissolved the
chromium carbides.
Based on his experiments, Monnarty postulated that the passivity in
stainless steels was caused by an invisible oxide film. This theory was not
universally accepted. It was not until 1930 that his theory was proven
electrochemically by H.H. Uhlig at the Massachusetts Institute of
Technology.
Stainless steels and similar chromium-rich alloys are characterized by
their passivity. The general concept of passivity involves a base metal
exhibiting the corrosion behavior of a more noble metal or alloy. For
example, a piece of bare steel immersed in a copper sulfate solution develops
a flash plating of metallic copper by a process known as cementation. If the
bare steel is first immersed in a strong nitric acid solution, an invisible
protective oxide layer is formed that prevents cementation and the steel is
said to have been passivated. Passivation of ferrous alloys containing more
than 10.5% chromium is by the chromium addition.
There are many stainless steel compositions, all of which have their own
set of physical, mechanical, and corrosion resistance properties. In this
chapter, we discuss the various types of corrosion to which stainless steels
may be susceptible, keeping in mind that all compositions are not affected to
the same degree, if at all. Specific corrosion problems and resistance of
specific compositions will be discussed in succeeding chapters.
Stainless steels are alloys of iron to which a minimum of 11% chromium
has been added to provide a passive film to resist “rusting” when the
material is exposed to weather. This film is self-forming and self-healing in
environments where stainless steel is resistant. As more chromium is added
to the alloy, improved corrosion resistance results. Consequently, there are
stainless steels with chromium contents of 15, 17, and 20%, and even higher.
Chromium provides resistance to oxidizing environments such as nitric acid
and also provides resistance to pitting and crevice attack.
Other alloying ingredients are added to further improve the corrosion
resistance and mechanical strength. Molybdenum is extremely effective in
improving pitting and crevice corrosion resistance.
By the addition of copper, improved resistance to general corrosion in
sulfuric acid is obtained. This will also strengthen some precipitation-
hardening grades. In sufficient amounts, however, copper will reduce the
pitting resistance of some alloys.
The addition of nickel will provide improved resistance in reducing
environments and to stress corrosion cracking (SCC). Nitrogen can also be
added to improve corrosion resistance to pitting and crevice attack and to
improve strength.
Niobium and titanium are added to stabilize carbon. They form carbides
and reduce the amount of carbon available to form chromium carbide that
can be deleterious to corrosion resistance.
It is because of all of these alloying possibilities that so many types of
stainless steel exist. It should also be kept in mind that the more alloying

Corrosion of Stainless Steels 111

elements used in the formulation, the greater will be the cost. Consequently,
it is prudent to select the specific stainless steel composition that will meet
the needs of the application. For example, it is not necessary to provide
additional pitting resistance if the environment of the application does not
promote pitting.

6.1 Pitting
Pitting corrosion is a form of localized attack. It occurs when the protective
film breaks down in small isolated spots, such as when halide salts contact
the surface. Once started, the attack may accelerate because of the difference
in electric potential between the large area of passive surface and the
active pit.
If appreciable attack is confined to a small area of metal acting as an anode,
the developed pits are described as deep. If the area of attack is relatively
large, the pits are called shallow. The ratio of deepest metal penetration to
average metal penetration, as determined by weight loss of the specimen, is
known as the pitting factor. A pitting factor of 1 represents uniform corrosion.
and pitting resistance equivalent number (PREN). As a general rule, the
higher the PREN, the better the resistance. Alloys having similar values may
differ in actual service. The PREN is determined by the chromium,
molybdenum, and nitrogen contents:

PREN Z %Cr C 3:3ð%MoÞ C 30ð%NÞ:

Table 6.1 lists the PREN for various stainless steels.

TABLE 6.1
Pitting Resistance Equivalent Numbers
Alloy PREN Alloy PREN

654 63.09 316LN 31.08
31 54.45 316 27.90
25-6Mo 47.45 20Cb3 27.26
Al-6XN 46.96 348 25.60
20Mo-6 42.81 347 19.0
317LN 39.60 331 19.0
904L 36.51 304N 18.3
20Mo-4 36.20 304 18.0
317 33.2


The CPT of an alloy is the temperature of a solution at which pitting is ﬁrst
observed. These temperatures are usually determined in ferric chloride (10%
FeCl3$6H2O) and in an acidic mixture of chlorides and sulfates.
Pitting on boldly exposed surfaces may occur in high-chloride environ-
ments, such as brackish cooling waters or marine atmospheres. In some
cases, a tiny pinhole at the surface may have a substantial cavity beneath it,
analogous to a dental caries.

6.2 Crevice Corrosion
Crevice corrosion is a localized type of corrosion resulting from local
differences in oxygen concentration associated with deposits on the metal
surface, gaskets, lap joints, or crevices under bolt or rivet heads where small
amounts of liquid can collect and become stagnant.
The material responsible for the crevice need not be metallic. Wood,
plastics, rubber, glass, concrete, asbestos, wax, and living organisms have all
been reported to cause crevice corrosion. Once the attack begins within the
crevice, its progress is very rapid and it is frequently more intense in chloride
environments. For this reason, the stainless steels containing molybdenum
are often used to minimize the problem. However, the best solution to
crevice corrosion is a design that eliminates crevices.
The CCT of an alloy is that temperature at which crevice corrosion is ﬁrst
observed when immersed in a ferric chloride solution. The critical corrosion
temperatures of several alloys in 10% ferric chloride solution are as follows:


Type 316 27/3
Alloy 825 27/3
Type 317 36/2
Alloy 904L 59/15
Alloy 220S 68/20
E-Brite 70/21
Alloy G 86/30
Alloy 625 100/38
Alloy 6NX 100/38
Alloy 276 130/55

6.3 Stress Corrosion Cracking
Stress corrosion cracking (SCC) of stainless steels is caused by the combined
effects of tensile stress, corrosion, temperature, and the presence of chlorides.


Wet–dry or heat-transfer conditions that promote the concentration of
chlorides are particularly aggressive with respect to initiating SSC.
Alloy contents of stainless steels, particularly nickel, determine the
sensitivity of the metal to SCC. Ferritic stainless steels, which are nickel-
free, and the high-nickel alloys are not subject to SCC. An alloy with a nickel
content of greater than 30% is immune to SCC. The most common grades of
stainless steel (304, 304L, 316, 316L, 321, 347, 303, 302, and 301) have nickel
contents in the range of 7–10% and are the most susceptible to SCC.
The ferritic stainless steels, such as types 405 and 430, should be
considered when the potential exists for SCC. The corrosion resistance of
ferritic stainless steels is improved by the increased addition of chromium
and molybdenum, whereas ductility, toughness, and weldability are
improved by reducing carbon and nitrogen content.
Other related corrosion phenomena are corrosion fatigue, delayed brittle
fatigue, and hydrogen stress cracking. Corrosion fatigue is the result of cycle
loading in a corrosive environment. Brittle fatigue is caused by hydrogen
impregnation of an alloy during processing, which leads to brittle failure
when subsequently loaded. Hydrogen stress cracking results from a
cathodic reaction in service.
The austenitic stainless steels resist hydrogen effects, but martensitic and
precipitation-hardening alloys may be susceptible to both hydrogen stress
cracking and chloride stress cracking.
Sulﬁde ions, selenium, phosphorus, and arsenic compounds increase the
likelihood of hydrogen stress cracking. Their presence should warn of a
possible failure.
Cathodic protection can also cause hydrogen stress cracking of high-
strength alloys in service if “overprotective.” The use of cathodic protection
(the coupling of hardenable stainless steels to lees noble metals in corrosive
environments) should be done with caution.
Only ferritic stainless steels are generally immune to both hydrogen and
chloride stress cracking.
Precipitation-hardening grades (S17400 and S17700) are subject to SCC in
sulfate solutions, as well as in neutral or acid chlorides.
The most common agents for SCC in stainless steels are chlorides, hot
caustic solutions, and, with simultaneous sensitization, polythionic acids
(a mixture of H2S and SO2 in aqueous media) and dissolved oxygen (DO)
in supercritical water.
The most common form of SCC for 18-8 austenitic grades is from
chloride contamination. The ubiquitous chloride is sodium chloride, as
found in water and atmospheric exposures. Sodium chloride usually causes
SCC only between about 1208F (508C) and 3908F (2008C). However, SCC can
occur at room temperature and even at cryogenic temperatures in the
presence of other halides (e.g., sulfuric/NaCl mixtures, aqueous solutions
of hydrogen sulﬁde/NaCl, HCl). For boldly exposed surfaces, there are
rough correlations between pH and chloride concentration at which SCC
may be anticipated. There is no minimum chloride concentration below


which SCC will not occur if there is a possibility of concentration by
evaporation or by occlusion or adsorption ﬁlms adhering to the stainless
surface (e.g., mill scale, heat tints, welding slag, rust deposits, calcareous
deposits, and biomasses).
Extreme stress corrosion cracking (ESCC) occurs when chlorides from the
atmosphere, water leaks, or insulation concentrate on the surface of an 18-8
type stainless steel. Ferric ions from rusted steel ﬂanges will aggravate the
situation. ESCC is best combatted by coating vessels and piping over 4 in.
using a zinc-free and chloride-free paint system.
Hot caustic can also cause SCC of 18-8 stainless steels, as frequently
happens when high-pressure steam is contaminated by carryover of alkaline
boiler-treating chemicals.

6.4 Intergranular Corrosion
When austenitic stainless steels are heated or cooled through the
temperature range of about 800–16508F (427–8998C), the chromium along
grain boundaries tends to combine with carbon to form carbon carbides.
Called sensitization, or carbide precipitation, the effect is a depletion of
chromium and the lowering of corrosion resistance in areas adjacent to the
grain boundary. This is a time- and temperature-dependent phenomenon, as
shown in Figure 6.1.

°F °C 0.062%C

900
1600 0.058%C
0.056%C
0.08%C
800
1400
0.052%C
700
1200 0.042%C 0.030%C
600

1000 0.019%C
500

800

10 1 10 1 10 100 1000 10,000
sec min min h h h h h

FIGURE 6.1
Effect of carbon content on carbide precipitation. Carbide precipitation forms in the areas to the
right of the various carbon-content curves.


Slowly cooling from annealing temperature, relieving stress in the
sensitizing range, or welding may cause carbide precipitation. Due to the
longer times at the temperature of annealing or stress relieving, it is possible
that the entire piece of material will be sensitized, whereas the shorter times at
welding temperature can result in sensitization of a band, usually 1/8–1/4 in.
wide, adjacent to but slightly removed from the weld. This region is known
as the heat-affected zone (HAZ).
Intergranular corrosion depends upon the magnitude of the sensitized
material exposed and the aggressiveness of the environment to which the
sensitized material is exposed. Many environments do not cause inter-
granular corrosion in sensitized austenitic stainless steels.
For this form of attack to occur, there must be a specific environment.
Acids containing oxidizing agents, such as sulfuric or phosphoric acid
containing ferric or cupric ions and nitric acid, as well as hot organic acids,
such as acetic or formic, are highly specific for this type of attack. Seawater
and other high-chloride waters cause severe pitting in sensitized areas, but
low-chloride waters (e.g., potable water) do not, except in specific situations
such as might occur under the influence of microbiological corrosion.
If the carbon content is held close to less than 0.30%, chromium carbide
precipitation can still occur upon sensitization, but in such small amounts
that no significant depletion occurs. Such low-carbon grades are practically
immune to weld decay. However, sensitization can occur under prolonged
heating in the critical temperature range, such as during service at elevated
temperatures or during very prolonged thermal stress relief (refer to
Figure 6.1). For all practicality, the low-carbon grades can be welded, hot
formed, and even thermally stress relieved without sensitization occurring.
Sensitization can also be prevented by using stabilized stainless steels. These
are stainless steels to which titanium, niobium, or niobium–titanium
mixtures have been added.
Titanium and niobium additions equal to five or ten times the carbon
content, respectively, permit the carbon to precipitate as titanium or niobium
carbides during a sensitizing heat treatment. The carbon precipitation does
not reduce the chromium content of the grain boundaries.
Three problems are presented by this approach. First, titanium-stabilized
grades such as type 321 require a stabilizing anneal to tie up the carbon in the
form of titanium carbides before welding. Second, titanium does not transfer
well across a welding arc and thus loses much of its effectiveness in
multipass welding or cross-welding. Third, although niobium does not have
this drawback, the niobium carbides (as well as the titanium carbides) can be
redissolved by the heat or welding. Consequently, multipass welding or
cross-welding can first redissolve titanium or niobium carbides and then
permit chromium carbide precipitation in the fusion zone (not the heat-
affected zone). This can cause a highly localized form of intergranular
corrosion known as knife-line attack (KLA), seen particularly in alloys, such
as type 347, alloy 20Cb3, and alloy 825.


Intergranular attack may also occur due to mechanisms other than carbide
precipitation. The ferrite phase, if present, may be selectively attacked by
reducing acids, such as hydrochloric or sulfuric. Its thermal conversion
product, the sigma phase, is selectively attacked by oxidizing acids such
as nitric.

6.5 High-Temperature Corrosion
The term high temperature is relative. In practical terms, it usually means a
temperature about 35% of the absolute melting range of a given metal or
alloy (or up to 60% for some nickel- and cobalt-based alloys). For the
conventional austenitic grades of stainless steel, such as type 304, this would
be any temperature above 10508F (5758C).
In general, the straight chromium and austenitic varieties of stainless steel
have an upper limit of about 16008F (8708C), except the more highly alloyed
grades (O20% Cr) that will tolerate slightly higher temperatures—about
20008F (11008C) in continuous service.
Changes can occur in the nature of the surface film of stainless steel when
exposed to high temperatures. For example, at mildly elevated temperatures
in an oxidizing gas, a protective oxide film is formed. In an environment
containing sulfur-bearing gases, the film will be in the form of sulfides that
may also be protective.
In more aggressive environments, with temperatures above 16008F
(8718C), the surface film may breakdown with a sudden increase in scaling.
Depending on alloy content and environment, the film may be self-healing
for a period of time followed by another breakdown.
Under extreme conditions of high temperature and corrosion, the surface
film may not be protective at all. Based on this, service tests are recommended.
High-temperature corrosion is considered to be electrochemical in nature,
with the high-temperature scale formed acting as an electrolyte. Corrosion
is usually uniform in nature. The predominant effects are oxidation and
carburization/decarburization. Changes in mechanical properties, speci-
fically a loss of ductility due to phase changes, also take place.
Most high-temperature reactions involve oxidation because oxides are
common products in the many applications where air or oxygen-rich
environments are present. In clean atmospheres, a thick oxide film forms
that develops into a thicker scale. Oxidation phenomena are controlled by
thermodynamic and kinetic factors, notably gas composition and tempera-
ture. High-temperature grades of austenitic stainless steels contain at least
12% nickel. Alloys, such as types 309 and 310 are widely used for good creep
strength and ductility, in addition to scaling resistance, at high temperatures.
The nickel-rich type 310 alloy is less susceptible to sigma-phase formation.
Above 15988F (8708C), there is little risk of embrittlement. Alloy compositions


are more critical in temperature ranges of 1220–15988F (650–8708C). Fully
annealed wrought alloys are preferred.
A practical option for high-temperature applications are nickel-rich alloys
such as alloy 800/800H. Sigma phase may still be a problem for some nickel-
based alloys in the range of 1450–17008F (770–9278C).
Alloys containing molybdenum, specifically types 316 and 317 austenitic
stainless steels containing 2 and 3% molybdenum, respectively, are subject to
catastrophic oxidation. Under some conditions, there is a selective oxidation
of molybdenum with rapid loss of volatile Mo3.
In strongly carbon-reducing atmospheres (e.g., carbon monoxide) at
high temperatures, carburization of stainless steel takes place. In oxidizing
atmospheres, such as steam or carbon dioxide, carbon may be selectively
removed (decarburization). Usually, complex gas mixtures are involved and
the net result of the H2/H2O and CO/CO2 is critical. Under some conditions
of environment and temperature, a pitting-type phenomenon called “metal
dusting” occurs.
Many high-temperature applications involve oxidizing conditions in
which stainless steels usually perform well, within specific parameters of
temperature and environment, based on their chromium content. The oxide
film that causes passivation in conventional service becomes a visible scale,
rich in chromic oxide, Cr2O3. This oxide or mixed oxide provides protection
against further oxidation of the substrate. However, spalling of the
protective film will permit continued oxidation. High-temperature reducing
conditions can cause direct attack by preventing, or causing the loss of, the
oxide film.
High-temperature environments, such as specific liquid chemical
solutions, may be oxidizing or reducing. The overall nature is determined
by the ratio of specific gases, vapors, or molten material in the environment.
The net effect is an algebraic sum resulting from the concentrations of specific
oxidizing or reducing components. The common species encountered in
gaseous media are shown in Table 6.2.

TABLE 6.2
Materials Found in Gaseous Media
Oxidizing Reducing

Oxygen Hydrogen
Steam
Sulfurous oxides (SO2 and SO3) Hydrogen sulfide
Sulfur Carbon disulfide
Carbon dioxide Carbon monoxide
Carbon
Hydrocarbons
Chlorine Hydrogen chloride
Oxides of nitrogen Ammonia


45

40

35
Scaling − % weight loss

30

25

20

15

10

5

0
0 2 4 6 8 10 12 14 16 18 20 22 24
% Chromium

FIGURE 6.2
Effect of chromium content on scaling resistance at 18008F (9828C).

In nonfluctuating temperature service, the oxidation resistance (scaling
resistance) of stainless steel depends on the chromium content as shown
in Figure 6.2. Steels with less than 18% chromium, primarily ferretic grades,
are limited to temperatures below 15008F (8168C). Those containing 10–20%
chromium can be used up to 18008F (9828C), whereas steels having
chromium content of at least 25% can be used up to 20008F (10938C). Typical
of these latter steels are types 309, 310, and 416.
Based on an oxidation rate of 10 mg/cm3 in 1000 h, Table 6.3 provides the
maximum service temperature for several stainless steels, for both
nonfluctuating and intermittent service.
In many processes, constant temperature conditions are not maintained.
Expansion and contraction differences between the base metal and the
protective film (scale) during heating and cooling can cause cracking and
spalling of the protective scale. This permits the oxidizing media to attack
the exposed metal surface.
Higher nickel levels improve the spalling resistance of the austenitic
stainless steels. This is shown in Figure 6.3. Nickel reduces the thermal
expansion differential between the alloy and the oxide film, thereby
reducing stresses at the alloy–oxide interface during cooling. The cycling
temperature conditions in Figure 6.3 at 18008F (9828C) consisted of 15 min in
the furnace and 5 min in air. Sheet specimens 0.031-in. (0.787 mm) thick were
exposed on both sides.


TABLE 6.3
Suggested Maximum Service Temperatures in Air
Service
Intermittent Continuous
AISI Type 8F 8C 8F 8C

201 1500 815 1550 845
202 1500 815 1550 845
301 1550 845 1650 900
302 1600 870 1700 925
304 1600 870 1700 925
308 1700 925 1800 980
309 1800 980 2000 1095
310 1900 1035 2100 1150
316 1600 870 1700 925
317 1600 870 1700 925
321 1600 870 1700 925
330 1900 1035 2100 1150
347 1600 870 1700 925
410 1500 815 1300 705
416 1400 760 1250 675
420 1350 735 1150 620
440 1500 815 1400 760
405 1500 815 1300 705
430 1600 870 1500 815
442 1900 1035 1800 980
446 2150 1175 1000 1095

As discussed previously, reducing conditions can result from a high ratio
of reducing to oxidizing species or from inherently reducing environments.
Carburization can occur when there is an excess of carbon monoxide over
carbon dioxide.
Carburization, as such, is not a corrosion phenomenon. However, it forms
chromium carbides that reduce the chromium matrix, and it reduces the
efficacy of the prior oxide film. Higher silicon contents will reduce the rate of
carburization. Nickel in the iron–chromium–nickel alloys will improve the
resistance to carburization by lowering the solubility of carbon, but not to the
same degree that silicon and chromium do.
The most common corrosive condition associated with carburization is
general absorption. Metal dusting, however, is a more serious form of attack,
where under alternating oxidizing and reducing conditions localized high-
carbon areas are burned out during the oxidation period.
When hot surfaces react with active nitrogen, nitriding occurs. Because
elements, such as aluminum, chromium, and titanium readily form nitrides,
the integrity of the oxide film is at risk. To form a stable protective oxide film,
a nickel content on the order of 35–40% is required.
Halogens form films on stainless alloys, but their efficiency is limited
because of the high volatility of metal chlorides. Chlorine in oxidizing flue


70
304

60 347

50
Weight loss, %

40

30 309

20

10 310

0
0 200 400 600 800 1000
Hours of cycles

FIGURE 6.3
Effect of nickel on scaling resistance.

gases in air will increase the corrosion compared to air alone. The attack
usually entails internal corrosion and voids, as well as surface attack.
Sulfur, in small quantities and in various forms, accelerates corrosion in
many environments. The most corrosive forms are sulfur dioxide, hydrogen
sulfide, and sulfur vapor, with the latter two being the most aggressive.
Sulfur attack is more severe than oxidation. Metal sulfides melt at lower
temperatures than comparable oxides, and they may fuse to metal surfaces.
In addition, sulfides are less likely to form tenacious, continuous protective
films. Accelerated corrosion is the result of fusion and lack of adherence. The
chromium content determines the resistance of stainless steel to sulfidation.
Type 316 stainless steel, when subjected to mixtures of oxygen and sulfur
dioxide, in compositions ranging from 100% oxygen to 100% sulfur dioxide
at 1100 and 16008F (593 and 8718C), did not develop a scale, only a heavy
tarnish. The rate of attack was largely independent of gas composition.


Low concentrations of hydrogen sulfide can be handled satisfactorily in
low-chromium stainless steels. However, hydrogen under high pressure
results in rapid corrosion. Under these conditions, a minimum of 17%
chromium is required to obtain satisfactory corrosion resistance. Type 304
stainless steel is used for this service.
Austenitic stainless steels are readily attacked by sulfur vapors. High
corrosion rates are encountered at 10608F (5718C). Liquid sulfur can be
handled by austenitic stainless steel up to a temperature of 4008F (2048C),
whereas stabilized grades types 321 and 347 give satisfactory service up to
8328F (4448C).
Flue gases containing sulfur dioxide or hydrogen sulfide exhibit the same
corrosiveness as most sulfur-bearing gases. Consequently, an increase in the
chromium content will improve the corrosion resistance of the stainless
steels, as shown in Figure 6.4. Corrosion rates of 1–2 mils per year (mpy)
have been reported for types 304, 321, 347, and 316 in the temperature range
of 1200–14008F (649–7608C). Service tests must be conducted for reducing
flue-gas environments.
Stainless steels are not resistant to molten hydroxides, particularly sodium
and potassium hydroxides, because of chromium dissolution related to
peroxide formation. However, they do perform well in molten carbonates up
to 9308F (5008C). Above 12908F (7008C), nickel-based alloys containing
chromium are required.

°C °F
1200 2200
Maximum mean temperature for oxidation resistance

2000

1000
1800

1600

800
1400

Normal combustion
1200
atmosphere
600

1000
0 4 8 12 16 20 24 28
Chromium in steel %

FIGURE 6.4
Effect of chromium in normal combustion atmosphere.


When stainless steel alloys are exposed to specific molten metals, there are
potential problems of liquid metal embrittlement (LME) and liquid metal
cracking (LMC) development. Molten tin at 2488F (1208C) has induced LME
in austenitic stainless steels. At 5708F (3008C), the fatigue limit was lowered.
At about 785–10608F (420–5708C), zinc slowly eroded unstressed 18-8
stainless steel. At about 1060–13808F (570–7508C), zinc penetrated to the
matrix via a Zn–Ni compound. Molten cadmium can also cause LMC of
austenitic grades above 5708F (3008C).

6.6 Corrosion Fatigue
Fatigue is a phenomenon that leads to cracking of a metal under repeated
or fluctuating stresses at values below the tensile strength. Such fractures
are progressive in nature, growing with the time exposure to the stress
fluctuation. In corrosion fatigue, the fracture occurs sooner because of the
combined effect of cyclic loading and corrosion at lower stress levels.
Corrosion fatigue is difficult to predict because it varies with both alloy and
environment; there is no specific environment that affects a particular alloy
or alloy system.
Occasional failures are experienced in applications in which austenitic
stainless steel has replaced, for example, a carbon steel nozzle or pipe
section. When a lighter gage of stainless steel replaces a heavier-walled steel
section, either fatigue or corrosion fatigue may ensue because of vibrations
that did not damage the carbon steel.

6.7 Uniform Corrosion
General or uniform corrosion, as found in other metals, is not to be expected
in the stainless steels. The many sets of corrosion data and charts found in
the literature that show various corrosion rates of stainless steel in certain
environments are actually indicating that the stainless alloy, under those
conditions, is fluctuating between an active and passive condition with a net
result of so many mils per year loss. These may or may not be reliable
figures. Consequently, recommendations should be based on rates of less
than 5 mpy and preferably less than 1 mpy. Under these conditions, no
corrosion allowance need be specified.

7
Ferritic Stainless Steel Family

Chromium is a metal that readily forms an oxide that is transparent and
happens to be extremely resistant to further degradation. As a further benefit
to alloying with steel, it is less noble than iron and thus tends to form its
oxide first. Increasing the chromium content in steel gradually above about
2% improves mild atmospheric corrosion resistance steadily up to a level of
about 12% where corrosion is essentially arrested. For exposure to mild, wet
environments the addition of about 11% chromium is sufficient to prevent
“rusting” of steel components, hence the term stainless.
Ferritic stainless steels are magnetic, have body-centered cubic atomic
structures, and possess mechanical properties similar to those of carbon
steel, though less ductile. Continued additions of chromium will also
continue to improve corrosion resistance in more severe environments.
Chromium additions are particularly beneficial in terms of resistance in
oxidizing environments, at both moderate, and elevated temperatures.
Addition of chromium is the most cost-effective means of increasing the
corrosion resistance of steel, with chromium costing less than a dollar per
pound. Chromium contents in the ferritic stainless alloys top out around
28%. These materials are historically known as 400 series stainless as they
were identified with numbers beginning with 400 when the American
Institute for Iron and Steel (AISI) had the authority to designate alloy
compositions. Alloy identification is now formally handled by the Unified
Numbering System (UNS), whereby stainless alloy identification numbers
generally begin with “S” followed by a five-digit number. Most of the old
AISI designations were retained as the first three digits of the UNS number
such that the old Stainless 405, a basic 12% Cr balance iron material, is
designated UNS S40500.
Table 7.1 lists some of the more common ferritic stainless steels. Type 430
stainless (UNS S43000) is alloyed with 18% chromium. This is the next
logical step in alloy additions because steady improvement in corrosion
resistance is obtained by increasing the chromium content over 11%.
Incremental additions over 18% become less effective, particularly for

123


TABLE 7.1
Selected Ferritic Stainless Steels
CTEa Density
Alloy C Cr Ni Mo N Other (min./in.) bb (#/in.3)

S40500 0.06 12.0 — — — 6.8 600 0.279
S40900 0.06 11.0 — — — Ti-0.4 6.7 600 0.280
S43000 0.07 16.5 — — — 6.6 600 0.278
S43035 0.05 18.0 — — 0.03 Ti-0.7 6.6 600 0.280
S44300 0.12 21.0 — — — Cu-1.0 6.7 600 0.277
S44600 0.07 25.5 — — — 6.3 600 0.270
Room Room Room Elevated
Temperature Temperature Temperature Toughness Temperature
Alloy Yield (KSI) Tensile (KSI) Elong. (%) (ft–lb.@8F) Strength (KSI@8F)c

S40500 40 70 30 75@RT
S40900 35 60 30
S43000 45 74 28
S43900 40 70 30
S44300 50 90 22 2@12008 CRP
S44600 50 70 25 25@RT 2@12008 CRP

Values are approximate.
a
Coefﬁcient of thermal expansion for range of 72–12008F.
b
Magnetic permeability.
c
CRP is stress required to produce 1% creep strain in 10,000 h.

aqueous corrosion. Beyond this, the highest practical level of chromium
content in iron is afforded by type 446 (UNS S44600). This stainless alloy is
used primarily for high-temperature oxidation resistance.
Examination of the compositions of stainless types 409 and 439 introduce
an additional approach to improving corrosion resistance. It also under-
scores the importance of carbon in stainless alloys. The role of carbon as an
alloy addition to steel is primarily that of increasing strength. Increasing
carbon content, because it is an interstitial element, pins the movement of
atoms within the matrix, resulting in higher stresses required to cause
deformation. This is also a factor in stainless steels, but increasing carbon
content can have a deleterious effect on corrosion resistance.
During melting and high-temperature working operations, the carbon
content in stainless steel is generally in solid solution, i.e., uniformly
distributed within the steel matrix, analogous to sugar in solution in warm
water. As the steel cools from a temperature of around 16008F, there is a
preference for the formation of a chromium carbide compound that
precipitates preferentially at grain boundaries. This is somewhat analogous
to the sugar-water solution as the stage where cooling has caused the sugar
to crystallize and precipitate to the bottom of the container. The solubility
limit of carbon in austenitic steel is illustrated in Figure 7.1. The solubility of
carbon in ferrite is slightly higher.

Ferritic Stainless Steel Family 125

1100

1000

900 Austenite

800
Temperature, °C

Austenite+Carbide
700

600

500

400

300
0 0.02 0.04 0.06 0.08 0.1
Weight percent carbon

FIGURE 7.1
Solubility of carbon in austenite.

Chromium carbides in themselves do not suffer from poor corrosion
resistance. The detrimental effect is in the fact that chromium is depleted
from the surrounding matrix. In fact, the chromium depletion can be so
severe as to lower the chromium content locally to below the 11% content
considered to be the minimum for stainless steel. In actuality, any depletion
can be significant if the environment is severe enough to cause the depleted
zone to become anodic to the matrix. In high-temperature service, even
where the component is used at a temperature that will cause chromium
carbide precipitation, grain boundary chromium depletion is usually not a
concern. Due to diffusion of chromium from within the grain toward the
grain boundary, chromium depletion at elevated temperatures is short-lived.
One way to avoid the precipitation of chromium carbides is to force the
precipitation of another carbide first. Two elements, titanium and niobium
(columbium), are particularly effective. Titanium will tie up carbon in the
ratio of about five times its weight. Niobium is more efficient, tying up about
15 times its own weight. In both types 409 and 439, titanium is used as the
stabilizer. In other alloys, such as some of “superferritic” materials, both
elements are used because in higher concentrations each element can
produce detrimental side effects.
Ferritic stainless steels offer useful resistance to mild atmospheric corrosion
and most freshwaters. They will corrode with exposure to seawater atmo-
spheres. These alloys are also useful in high-temperature situations, with 446


exhibiting useful oxidation (scaling) resistance through about 21008F. Ferritic
materials that contain more than about 18% chromium are also susceptible to
an embrittlement phenomenon when exposed to temperatures in the range
of 600–11008F. This is due to the formation of a secondary phase and is
termed 8858 embrittlement after the temperature that causes the most rapid
formation. These materials are not brittle in this temperature range but lose
ductility when cooled to room temperature.
Consequently, these alloys are limited to a maximum operating
temperature of 6508F (3438C).
Corrosion resistance is rated good, although ferritic alloys do not resist
reducing acids such as hydrochloric. Mildly corrosive conditions and
oxidizing media are handled satisfactorily. Type 430 ﬁnds wide application
in nitric acid plants. Increasing the chromium content to 24 and 30% improves
the resistance to oxidizing conditions at elevated temperatures. These alloys
are useful for all types of furnace parts not subject to high stress. Because the
oxidation resistance is independent of the carbon content, soft forgeable
alloys that are low in carbon can be rolled into plates, shapes, and sheets.
Ferretic stainless steels offer useful resistance to mild atmospheric
corrosion and most fresh waters. They will corrode with exposure to
seawater atmospheres.

7.1 Type 405 (S40500)
This is a nonhardenable 12% chromium stainless steel. The chemical
composition is given in Table 7.2. Type 405 stainless is designed for use in the
as-welded condition; however, heat treatment improves corrosion resistance.

TABLE 7.2
Chemical Composition of Ferritic Stainless Steels
Nominal Composition (%)
AISI Type C max Mn max Si max Cr Othera

405 0.08 1.00 1.00 11.50–14.50 0.10–0.30 Al
430 0.12 1.00 1.00 14.00–18.00
430F 0.12 1.25 1.00 14.00–18.00 0.15 S min
430(Se) 0.12 1.25 1.00 14.00–18.00 0.15 Se min
444 0.025 1.00 1.00 17.5–19.5 1.75–2.50 Mo
446 0.20 1.50 1.00 23.00–17.00 0.25 max N
XM-27b 0.002 0.10 0.20 26.00
a
Elements in addition to those shown are as follows: phophorus—0.06% max in type 430F and
430(Se), 0.015% in XM-27; sulfur—0.03% max in types 405, 430, 444, and 446; 0.15% min type
430F, 0.01% in XM-27; nickel—1.00% max in type 444, 0.15% in XM-27; titaniumCniobium—
0.80% max in type 444; copper—0.02% in XM-27; nitrogen—0.010% in XM-27.
b
E-Brite 26-1 trademark of Allegheny Ludium Industries, Inc.


The low chromium favors less sensitivity to 8558F (4758C) embrittlement and
sigma phase formation.
Type 405 stainless steel is resistant to nitric acid, organic acids, and
alkalies. It will be attacked by sulfuric, hydrochloric, and phosphoric acids,
as well as seawater. It is resistant to chloride stress corrosion cracking.
Applications include heat, exchanger tubes in the refining industry and other
areas where exposure may result in the 8558F (4758C) or sigma temperature
range. It has an allowable maximum continuous operating temperature of
13008F (7058C) with an intermittent allowable operating temperature of 15008F
(8158C).

7.2 Type 409 (S40900)
Type 409 stainless is an 11% chromium alloy stabilized with titanium. Its
chemical composition will be found in Table 7.3. The material can be welded
in the field, however, heat treatment improves corrosion resistance. It has a
maximum allowable continuous operating temperature of 13008F (7058C)
with an intermittent operating temperature of 15008F (8158C). It cannot be
hardened by heat treatment.
The primary application for alloy 409 is in the automotive industry as
mufflers, catalytic convertors, and tail pipes. It has proven an attractive
replacement for carbon steel because it combines economy and good
resistance to oxidation and corrosion.

7.3 Type 430 (S43000)
This is the most widely used of the ferritic stainless steels. It combines good
heat resistance and mechanical properties. The chemical composition will be

TABLE 7.3
Chemical Composition of Alloy 409 (S40900)
Chemical Weight Percent

Carbon 0.08
Manganese 1
Silicon 1
Chromium 10.5–11.751
Nickel 0.5
Phosphorus 0.045
Sulfur 0.045
Titanium 6!%C min–0.75% max
Iron Balance


found in Table 7.1. In continuous service, type 430 may be operated to a
maximum temperature of 15008F (8158C) and 16008F (8708C) in intermittent
service. However, it is subject to 8858F (4758C) embrittlement and loss of
ductility at subzero temperatures.
Type 430 stainless is resistant to chloride stress corrosion cracking and
elevated sulfide attack. Applications are found in nitric acid services, water
and food processing, automobile trim, heat exchangers in petroleum and
chemical processing industries, reboilers for desulfurized naphtha, heat
exchangers in sour-water strippers and hydrogen plant efluent coolers. The
compatibility of type 430 stainless steel with selected corrodents is provided
in Table 7.4, which is taken from Reference [1].
Stainless steel type 430F is a modification of type 430. The carbon content
is reduced to 0.065%, manganese to 0.80%, and silicon to 0.3/0.7%, while
0.5% molybdenum and 0.60% nickel have been added. This is an alloy used
extensively in solenoid armatures and top plugs. It has also been used in
solenoid coils and housings operating in corrosive environments.
Type 430FR alloy has the same chemical composition as type 430F except
for an increase in the silicon content to 1.00/1.50 wt%. The alloy has been
used for solenoid valve magnetic core components that must combat
corrosion from atmospheric fresh water and corrosive environments. This
grade has a higher electrical resistivity than 430F solenoid quality, which
includes eddy-current loss of the material.

7.4 Type 439L (S43035)
The composition of type 439L will be found in Table 7.5. This alloy is
nonhardenable through heat treatment and has excellent ductility and
weldability. It resists intergranular attack and formation of martensite in the
as-welded, heat-affected zone, but is subject to 8858F (4758C) embrittlement.
Alloy 439L is resistant to chloride stress corrosion cracking, organic acids,
alkalies, and nitric acid. It will be attacked by sulfuric, hydrochloric, and
phosphoric acids, as well as seawater.
Applications include heat exchangers, condensers, feed-water heaters,
tube-oil coolers, and moisture-separator reheaters.

7.5 Type 444 (S44400)
Table 7.2 provides the chemical composition of this alloy. This is a low-carbon
alloy with molybdenum added to improve chloride pitting resistance. It is
virtually immune to chloride stress corrosion cracking. The alloy is subject to
8858F (4758C) embrittlement and loss of ductility at subzero temperatures.


TABLE 7.4
Compatibility of Ferritic Stainless Steels with Selected Corrodents
Type of Alloy
Chemical 430 (8F/8C) 444 (8F/8C) XM-27 (8F/8C)

Acetic acid, 10% 70/21 200/93 200/93
Acetic acid, 50% X 200/93 200/93
Acetic acid, 80% 70/21 200/93 130/54
Acetic acid, glacial 70/21 140/60
Acetic anhydride, 90% 150/66 300/149
Aluminum chloride, aqueous X 110/43
Aluminum hydroxide 70/21
Aluminum sulfate X
Ammonia gas 212/100
Ammonium carbonate 70/21
Ammonium chloride, 10% 200/93
Ammonium hydroxide, sat. 70/21
Ammonium nitrate 212/100
Ammonium persulfate, 5% 70/21
Ammonium phosphate 70/21
Ammonium sulfate, 10–40% X
Amyl acetate 70/21
Amyl chloride X
Aniline 70/21
Antimony trichloride X
Aqua regia, 3:1 X
Barium carbonate 70/21
Barium chloride 70/21a
Barium sulfate 70/21
Barium sulﬁde 70/21
Benzaldehyde 210/99
Benzene 70/21
Benzoic acid 70/21
Borax, 5% 200/93
Boric acid 200/93a
Bromine gas, dry X
Bromine, liquid X
Butyric acid 200/93
Calcium carbonate 200/93
Calcium chloride X
Calcium hypochlorite X
Calcium sulfate 70/21
Carbon bisulﬁde 70/21
Carbon dioxide, dry 70/21
Carbon monoxide 1600/871
Carbon tetrachloride, dry 212/100
Carbonic acid X
Chloracetic acid, 50% water X
Chloracetic acid X
(continued)


TABLE 7.4 Continued

Type of Alloy

Chlorine gas, dry X
Chlorine gas, wet X
Chloroform, dry 70/21
Chromic acid, 10% 70/21 120/49
Chromic acid, 50% X X
Citric acid, 15% 70/21 200/93 200/93
Citric acid, concentrated X
Copper acetate 70/21
Copper carbonate 70/21
Copper chloride X X
Copper cyanide 212/100
Copper sulfate 212/100
Cupric chloride, 5% X
Ethylene glycol 70/21
Ferric chloride X 80/27
Ferric chloride, 10% in water 75/25
Ferric nitrate, 10–50% 70/21
Ferrous chloride X
Fluorine gas, dry X
Fluorine gas, moist X
Hydrocyanic acid, 10% X
Hydroﬂuoric acid, 30% X X
Iodine solution, 10% X
Lactic acid, 20% X 200/93 200/93
Lactic acid, conc. X
Magnesium chloride 200/93
Malic acid 200/93
Muriatic acid X
Nitric acid, 5% 70/21 200/93 320/160
Nitric acid, 20% 200/93 200/93 320/160
Nitric acid, 70% 70/21 X 210/99
Nitric acid, anhydrous X X
Nitrous acid, 5% 70/21
Phenol 200/93
Phosphoric acid, 50–80% X 200/93 200/93
Picric acid X
Silver bromide, 10% X
Sodium chloride 70/21a
Sodium hydroxide, 10% 70/21 212/100 200/93
Sodium hydroxide, 50% X 180/82
(continued)


TABLE 7.4 Continued

Type of Alloy

Sodium hypochlorite, 30% 90/32
Sodium sulﬁde, to 50% X
Stannic chloride X
Stannous chloride, 10% 90/32
Sulfuric acid, 10% X X X
Sulfuric acid, 50% X X X
Sulfuric acid, 70% X X
Sulfuric acid, 90% X X
Sulfuric acid, 98% X 280/138
Sulfuric acid, 100% 70/21 X
Sulfuric acid, fuming X
Sulfurous acid, 5% X 360/182
Toluene 210/99
Trichloroacetic acid X
Zinc chloride, 20% 70/21a 200/93
The chemicals listed are in the pure state or in a saturated solution unless otherwise indicated.
Compatibility is shown to the maximum allowable temperature for which data are available.
Incompatibility is shown by an X. A blank space indicates that data are unavailable. When
compatible, the corrosion rate is !20 mpy.
a
Pitting may occur.
Source: From P.A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed., New York:
Marcel Dekker.

The chloride pitting resistance of this alloy is similar to that of type 316
stainless steel and superior to that of types 430 and 439L. Like all ferritic
stainless steels, type 444 relies on a passive ﬁlm to resist corrosion, but
exhibits rather high corrosion rates when activated. This characteristic
explains the abrupt transition in corrosion rates that occur at particular acid
concentrations. For example, it is resistant to very dilute solutions of sulfuric
acid at boiling temperature, but corrodes rapidly at higher concentrations.

TABLE 7.5
Chemical Composition of Alloy 439L (S43035)

Carbon 0.07 max
Manganese 1.00 max
Silicon 1.00 max
Nitrogen 0.50
Titanium 12!%C min
Aluminum 0.15 max


The corrosion rates of type 444 in strongly concentrated sodium hydroxide
solutions are also higher than those of austenitic stainless steels. The
compatibility of type 444 alloy with selected corrodents will be found in
Table 7.4. The data has been extracted from Reference [1]. The corrosion
resistance of type 444 is generally considered equal to that of type 304.
This alloy is used for heat exchangers in chemical-, petroleum-, and
food-processing industries, as well as piping.

7.6 Type 446 (S44600)
Type 446 is a heat-resisting grade of ferritic stainless steel. It has a maximum
temperature rating of 20008F (10958C) for continuous service and a
maximum temperature rating of 21508F (11758C) for intermittent service.
Table 7.2 lists the chemical composition.
This nonhardenable chromium steel exhibits good resistance to reducing
sulfurous gases and fuel-ash corrosion. Alloy S44600 has good general
corrosion resistance in mild atmospheric environments, fresh water, mild
chemicals, and mild oxidizing conditions.
Applications have included furnance parts, kiln linings, and annealing
boxes.

Reference
Marcel Dekker.

8
Superferritic Stainless Steel Family

Ferritic stainless alloys are noted for their ability to resist chloride stress
corrosion cracking, which is one of their most useful features in terms of
corrosion resistance. Consequently, development efforts during the 1970s
were undertaken to produce ferritic stainlesses that would also possess a
high level of general and localized pitting resistance.
The first significant alloy developed commercially to meet these
requirements contained 26% chromium and 1% molybdenum. To obtain the
desired corrosion resistance and acceptable fabrication characteristics,
the material had to have very low interstitial element contents. To achieve
these levels, the material was electron-beam rerefined under a vacuum. It
was known as E-Brite alloy. Carbon plus nitrogen contents were maintained
at levels below 0.02%.
The E-Brite alloy (S44627) was termed a “superferritic” because of its high
level of corrosion resistance for a ferritic material and partly because it is
located so far into the ferritic zone on the Schaeffler diagram. For a period of
years, the usage of this alloy grew. Finally, its benefits for the construction of
pressure vessels were overshadowed by the difficult nature of fabrication
and a concern over its toughness. Due to the very low level of interstitial
elements, the alloy had a tendency to absorb these elements during welding
processes. Increases in oxygen plus nitrogen to levels much over 100 ppm
resulted in poor toughness. Even without these effects, the alloy could
exhibit a ductile-to-brittle transition temperature (DBIT) around room
temperature. Other superferritic alloys were also developed.
The chemical composition of selected superferritic alloys are shown in
Table 8.1.
These alloys exhibit excellent localized corrosion resistance. Although the
superferritic materials alloyed with some nickel have improved mechanical
toughness and are less sensitive to contamination from interstitial elements,
their availability is still limited to thicknesses below approximately 0.200 in.
This is related to the formation of embrittling phases during cooling from
annealing temperatures. Section thicknesses over these levels cannot be
cooled quickly enough to avoid a loss of toughness.

133


TABLE 8.1
Chemical Composition of Selected Superferritic Stainless Steels
Alloy C Cr Ni Mo N Other

S44627 0.002 26.0 — 1.0 0.010
S44660 0.02 26.0 2.5 3.0 0.025 TiCCb 0.5
S44800 0.005 29.0 2.2 4.0 0.01

Values are in wt%.

8.1 Type XM-27 (S44627)
This alloy is manufactured under the trade name of E-Brite by Allegheny
Ludlum Industries, Inc. It is a high-chromium alloy. Refer to Table 8.1 for the
chemical composition. Compared to the 300 series of stainless steel, alloy
S44627 has a high thermal conductivity and a low coefficient of thermal
expansion.
In general, E-Brite has good general corrosion resistance in most oxidizing
acids, organic acids, and caustics. It is resistant to pitting and crevice
corrosion and free from chloride stress corrosion cracking. Refer to Table 8.2
for the compatibility of alloy S44627 with selected corrodents.
This alloy also resists intergranular corrosion and is approved for use in
contact with foods. Applications include heat exchanger tubing, overhead
condensers, reboilers, and feed heaters (petroleum refining), pulp and paper
liquid heaters, organic acid heaters and condensers, and nitric acid
cooler condensers.

8.2 Alloy S44660 (Sea-Cure)
Sea-Cure is a trademark of Trent Tube. It is a chromium–nickel–molybdenum
superferritic alloy. The chemical composition is shown in Table 8.1.
Because of its chromium, nickel, and molybdenum contents, it possesses
excellent resistance to chloride-induced pitting, crevice corrosion, and stress
corrosion cracking. It has better resistance than austenitic stainless steels to
general corrosion in diverse conditions. Good to excellent resistance is
shown to organic acids, alkalies, salts, and seawater, with good resistance
shown to sulfuric, phosphoric, and nitric acids.
Sea-Cure is used in electric power plant condensers and feedwater
heaters, and heat exchangers in the chemical, petrochemical, and refining
applications.

Superferritic Stainless Steel Family 135

TABLE 8.2
Compatibility of E-Brite Alloy S44627 with Selected Corrodents
Chemical 8F/8C

Acetic acid, 10% 200/93
Acetic acid, glacial 140/60
Acetic anhydride* 300/149
Ammonium chloride, 10%* 200/93
Aqua regia, 3:1 X
Beer 160/71
Beet sugar liquors 120/49
Benzaldehyde* 210/99
Bromine water, 1% 80/27
Calcium hydroxide, 50%* 210/99
Chromic acid, 10% 130/54
Chromic acid, 50% X
Citric acid, 10% 200/93
Copper chloride, 5% 100/38
Ethylene chloride* 210/99
Ferric chloride 80/27
Fluosilicic acid X
Formic acid, 80% 210/99
Hydrochloric acid X
Lactic acid, 80% 200/93
Methylene chloride X
Nitric acid, 5%* 310/154
Oxalic acid, 10% X
Phosphoric acid, 25–50%* 210/99
Sodium chlorite 90/32
Sodium hydroxide, 10% 200/93
Sodium hypochlorite, 30%* 90/32
Stearic acid 210/99
Sulfamic acid 100/38
Sulfur dioxide, wet 550/293
Sulfuric acid, 10% X
Sulfuric acid, 30–90% X
Sulfuric acid, 95% 150/66
(continued)


TABLE 8.2 Continued

Chemical 8F/8C

Sulfurous acid, 5%* 210/99
Tartaric acid, 50% 210/99
Toluene 210/99
temperature for which data are available. Incompatibility is shown by an
X. When compatible, corrosion rate is !2 mpy except for those marked
with an *, whose corrosion rate is !20 mpy.
Source: From P. A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4,

8.3 Alloy S44735 (29-4C)
The chemical composition of alloy 29-4C is shown in Table 8.3. This alloy has
improved general corrosion resistance to chloride pitting and stress corrosion
cracking in some environments. The absence of nickel reduces the cost.
Applications are found in the utility industry, chemical processing
equipment, household condensing furnaces, and vent pipes.

8.4 Alloy S44800 (29-4-2)
The chemical composition of alloy S44800 is shown in Table 8.1. Applications
are found in chemical processing equipment and the utility industry for use
in corrosive environments.

TABLE 8.3
Chemical Composition of Alloy S44735 (29-4C)

Carbon 0.03 max
Manganese 0.30 max
Silicon 1.0 max
Nickel 1.0
Phosphorus 0.03
Molybdenum 3.60–4.20
TitaniumCniobium 6 (%CC%N): 0.045 N min
Iron Balance

Superferritic Stainless Steel Family 137

TABLE 8.4
Chemical Composition of Alloy S44700 (29-4)

Carbon 0.010 max
Manganese 0.30 max
Nickel 0.15
Silicon 0.02 max
Copper 0.15
Nitrogen 0.02
Iron Balance

This alloy has improved resistance to chloride pitting and stress corrosion
cracking and improved general corrosion resistance in some environments.

8.5 Alloy S44700 (29-4)
This is a chromium–nickel–molybdenum alloy, with its composition shown
in Table 8.4. It has excellent resistance to chloride pitting and stress corrosion
cracking environments. It ﬁnds use in the chemical processing and utility
industries.

Reference
1. P. A. Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed., New York:
Marcel Dekker.

9
Martensitic Stainless Steel Family

Within a certain range of compositions based on ferritic stainless steels, as
indicated by the diagram in Figure 9.1, martensitic structures can be
developed. The martensitic grades are so named because, when heated
above the critical temperature of 16008F (8708C) and cooled rapidly, a
metallurgical structure known as martensite is obtained. In the hardened
condition, the steel has very high strength and hardness, but to obtain
optimal corrosion resistance, ductility, and impact strength, the steel is given
a stress-relieving or tempering treatment, usually in the range of 300–7008F
(149–3718C). These alloys are hardenable because of the phase transfor-
mation from body-centered cubic to body-centered tetragonal. As with the
low-alloy steels, this transformation is thermally controlled. The Fe–Cr
phase diagram suggests that the maximum chromium content would be
about 12.7%. But the carbon content expands the g region to the extent that
larger chromium contents are possible. Common alloys are 410, containing
12% chromium and low carbon, and alloy 440 of 17% chromium with a high
carbon content. The martensitic stainless steels are the strongest of all
stainless steels, having strength to 275 ksi. However, at such high strength
levels they lack ductility.
Tempering at 8008F (4258C) does not reduce the hardness of the part and in
this condition these alloys show an exceptional resistance to fruit and
vegetable acids, lye, ammonia, and other corrodents to which cutlery may
be subjected.
Moderate corrosion resistance, relatively high strength, and good fatigue
properties after suitable heat treatment are usually the reasons for selecting
the martensitic stainless steels.

9.1 Type 410 (S41000)
Type 410 stainless steel is heat treatable and is the most widely used of the
martensitic stainless steels. Its chemical composition is shown in Table 9.1.

139


30
Austenite
Ni % + 0.5 Mn % + 30 (C % + N %)

25 0%

20 5%

A+M
15
A+F
40%

10
A+M+F 100%
Martensite
5
M+F
Ferrite
0
0 10 20 30 40
Cr % + Mo + % 1.5 Si % + 0.5 Cb %

FIGURE 9.1
Schaefﬂer constitutional diagram for stainless steel.

This alloy, when heat treated, has high-strength properties with good
ductility. Type 410 stainless steel has a maximum operating temperature of
13008F (7058C) for continuous service, but for intermittent service may be
operated at a maximum of 15008F (8158C).
With time and temperature, changes in metallurgical structure can be
expected for almost any steel or alloy. In martensitic stainless steels,
softening occurs when exposed to temperatures approaching or exceeding
the original tempering temperature. Type 410 stainless, which is a
12%-chromium alloy, has been known to display brittle tendencies after
extended periods in the same temperature range. This phenomenon is called
8858F embrittlement, which has been discussed previously.
Type 410 stainless steel is used where corrosion is not severe, such as air,
fresh water, some chemicals, and food acids. Table 9.2 provides the
compatibility of type 410 stainless steel with selected corrodents.

TABLE 9.1
Chemical Composition of Type 410 Stainless Steel

Carbon 0.15
Manganese 1.00
Phosphorus 0.040
Sulfur 0.030
Silicon 1.00
Iron Balance

Martensitic Stainless Steel Family 141

TABLE 9.2
Compatibility of Type 410 Stainless Steel with Selected Corrodents
Maximum Temperature
Chemical 8F 8C

Acetaldehyde 60 16
Acetamide 60 16
Acetic acid, glacial X
Acetic anhydride X
Acetone 210 99
Acrylonitrile 110 43
Allyl alcohol 90 27
Alum X
Aluminum chloride, aqueous X
Aluminum chloride, dry 150 66
Aluminum oxychloride X
Aluminum sulfate X
Ammonium chloride, 10%a 230 110
Ammonium chloride, 50% X
Ammonium chloride, sat. X
Ammonium hydroxide, sat. 70 21
Ammonium persulfate, 5% 60 16
Ammonium phosphate, 5% 90 32
Ammonium sulﬁte X
Amyl acetatea 60 16
Amyl alcohol 110 43
Amyl chloride X
Aniline 210 99
Barium carbonate, 10% 210 99
Barium chloridea 60 16
Barium hydroxide 230 110
Benzaldehyde
Benzene 230 110
Benzoic acid 210 99
Borax 150 66
Boric acid 130 54
Bromine gas, dry X
Bromine, liquid X
Butadiene 60 16

(continued)


TABLE 9.2 Continued

Maximum Temperature
Chemical 8F 8C

Butyl acetate 90 32
Butyl alcohol 60 16
Butyric acid 150 66
Calcium chloridea 150 66
Calcium hydroxide, 10% 210 99
Carbon disulﬁde 60 16
Carbon tetrachloridea 210 99
Carbonic acid 60 16
Chloracetic acid X
Chlorine gas, dry X
Chlorine gas, wet X
Chlorine, liquid X
Chlorobenzene, dry 60 16
Chloroform 150 66
Chlorosulfonic acid X
Chromic acid, 10% X
Chromic acid, 50% X
Citric acid, 15% 210 99
Copper acetate 90 32
Copper carbonate 80 27
Copper chloride X
Cyclohexane 80 27
Cyclohexanol 90 32
Ferric chloride X
Ferric chloride, 50% in water X
Ferrous chloride X
Fluorine gas, dry 570 299
Hydrocyanic acid, 10% 210 99

(continued)


TABLE 9.2 Continued

Maximum Temperature
Chemical 8F 8C

Malic acid 210 99
Methyl chloride, dry 210 99
Muriatic acid X
Nitric acid, anhydrous X
Perchloric acid, 10% X
Phenola 210 99
Phosphoric acid, 50–80% X
Picric acid 60 16
Sodium carbonate, 10–30% 210 99
Sodium chloridea 210 99
Sodium hypochlorite, 20% X
Sodium hypochlorite, conc. X
Stannic chloride X
Stannous chloride X
Sulfurous acid X
Toluene 210 99
Trichlorocetic acid X
Zinc chloride X

temperature for which data are available. Incompatibility is shown by an X.
When compatible, the corrosion rate is !20 mpy.
a
Material is subject to pitting.
Source: P.A. Schweitzer, 2004. Corrosion Resistance Tables, Vols. 1–4, 5th ed.,
New York: Marcel Dekker.


TABLE 9.3

Carbon 0.15
Manganese 1.00
Phosphorus 0.040
Sulfur 0.030
Silicon 1.00
Nickel 1.25–2.50
Iron Balance

Applications include valve and pump parts, fasteners, cutlery, turbine
parts, bushings, and heat exchangers.
Type 410 double tempered is a quenched and double-tempered variation
conforming to NACE and API speciﬁcations for parts used in hydrogen
sulﬁde service. Type 410S has a lower carbon content (0.8%) and a nitrogen
content of 0.60%.

9.2 Type 414 (S41400)
Type 414 stainless steel is a nickel-bearing chromium stainless steel. The
composition is shown in Table 9.3. By adding nickel, the hardenability is
increased, but not enough to make it austenitic at ambient temperatures.
By adding nickel, the chromium content can be increased, which leads to
improved corrosion resistance. The nickel addition also increases notch
toughness. Type 414 can be heat treated to somewhat higher tensile and
impact strengths than type 410.
Type 414 stainless steel is resistant to mild atmospheric corrosion, fresh
water, and mild chemical exposures. Applications include high-strength
nuts and bolts.

9.3 Type 416 (S41600)
Type 416 stainless steel is a low-carbon-class martensitic alloy, a free-machining
variation of type 410 stainless steel. The chemical composition is shown in
Table 9.4. It has a maximum continuous operating temperature of 12508F
(6758C) and an intermittent maximum operating temperature of 4008F (7608C).
Type 416Se has selenium added to the composition and the sulfur quantity
reduced to improve the machinability. Refer Table 9.5 for the chemical
composition of type 416Se.


TABLE 9.4

Carbon 0.15
Manganese 1.25
Phosphorus 0.060
Silicon 1.00
Molybdenum 0.60a
Iron Balance
a
May be added at manufacturer’s option.

TABLE 9.5
Chemical Composition of Type 416Se Stainless Steel

Carbon 0.15
Manganese 1.25
Phosphorus 0.060
Sulfur 0.060
Silicon 1.00
Selenium 0.15 min
Iron Balance

These alloys exhibit useful corrosion resistance to natural food acids, basic
salts, water, and most natural atmospheres.

9.4 Type 420 (S42000)
Type 420 stainless steel is a hardenable, 12%-chrome stainless steel with
higher strength and wear resistance than type 410. Table 9.6 shows the

TABLE 9.6

Carbon 0.15 min
Manganese 1.50
Phosphorus 0.040
Sulfur 0.030
Silicon 1.50
Iron Balance


TABLE 9.7
Chemical Composition of Type 420F (S42020) Stainless
Steel

Carbon 0.15 min
Manganese 1.25
Phosphorus 0.060
Sulfur 0.15 min
Silicon 1.00
Molybdenum 0.60
Iron Balance

chemical composition. This alloy has been used for cutlery, surgical
instruments, magnets, molds, shafts, valves, and other products.
Type 420F stainless is a free-machining version of type 420. It is hardenable
and also exhibits higher strength, hardness, and wear resistance than type
410. The chemical composition will be found in Table 9.7.

9.5 Type 422 (S42200)
This alloy is designed for service temperatures to 12008F (6498C). It is a high-
carbon martensitic alloy whose composition is shown in Table 9.8. It exhibits
good resistance to scaling and oxidation in continuous service at 12008F
(6498C), with high strength and toughness.
Type 422 is used in steam turbines for blades and bolts.

TABLE 9.8

Carbon 0.2–0.25
Manganese 1.00
Phosphorus 0.025
Sulfur 0.025
Silicon 0.75
Nickel 0.5–1.00
Vanadium 0.15–0.30
Tungsten 0.75–1.25
Iron Balance


TABLE 9.9

Carbon 0.20
Manganese 1.00
Phosphorus 0.040
Sulfur 0.030
Silicon 1.00
Nickel 1.25–2.50
Iron Balance

9.6 Type 431 (S43100)
The addition of nickel to type 431 provides improved corrosion resistance
and toughness (impact strength). Table 9.9 shows the chemical composition.
This alloy ﬁnds application as fasteners and ﬁttings for structural
components exposed to marine atmospheres, and for highly stressed aircraft
components.

9.7 Type 440A (S44002)
Type 440A is a high-carbon chromium steel providing stainless properties
with excellent hardness. Because of the high carbon content, type 440A
exhibits lower toughness than type 410. The chemical composition is shown
in Table 9.10. Type 440A has lower carbon content than type 440B or 440C
and consequently exhibits a lower hardness but greater toughness.

TABLE 9.10
Chemical Composition of Type 440A Stainless Steel

Carbon 0.60–0.75
Manganese 1.00
Phosphorus 0.040
Sulfur 0.030
Silicon 1.00
Molybdenum 0.75
Iron Balance


TABLE 9.11
Chemical Composition of Type 440B Stainless Steel

Carbon 0.75–0.95
Manganese 1.00
Phosphorus 0.040
Sulfur 0.030
Silicon 1.00
Molybdenum 0.75
Iron Balance

9.8 Type 440B (S44003)
When heat-treated, this high-carbon chromium steel attains a hardness of
Rockwell C 58, intermediate between types 440A and 440C with comparable
intermediate toughness. Table 9.11 shows the chemical composition.
Type 440B has been used for cutlery, hardened balls, and similar parts.

9.9 Type 440C (S44004)
Type 440C is a high-carbon chromium steel that can attain the highest
hardness (Rockwell C 60) of the 400-series stainless steels. In the hardened
and stress-relieved condition, type 440C has maximum hardness together
with high strength and corrosion resistance. It also has good abrasion
resistance. The chemical composition is shown in Table 9.12.
This stainless steel is used principally in bearing assemblies, including
bearing balls and races.

TABLE 9.12
Chemical Composition of Type 440C Stainless Steel

Carbon 0.95–1.2
Manganese 1.00
Phosphorus 0.040
Sulfur 0.030
Silicon 1.00
Molybdenum 0.75
Iron Balance


9.10 Alloy 440-XH
This product is produced by Carpenter Technology, having a nominal
composition as follows:


Carbon 1.60
Manganese 0.50
Silicon 0.40
Chromium 16.00
Nickel 0.35
Molybdenum 0.80
Vanadium 0.45
Iron Balance

This is a high-carbon, high-chromium, corrosion-resistant alloy that can be
described as either a high-hardness type 440C or a corrosion resistant, D2 tool
steel. It possesses corrosion resistance equivalent to type 440C stainless but
can attain a maximum hardness of Rockwell C 64, approaching that of
tool steel.

9.11 13Cr-4N (F6NM)
F6NM is a high-nickel, low-carbon, martensitic stainless with higher
toughness and corrosion resistance than type 410 and superior weldability.
It has been used in oilﬁeld applications as a replacement for type 410.
F6NM has a chemical composition as follows:


Carbon 0.05
Manganese 0.50–1.00
Phosphorus 0.030
Sulfur 0.030
Silicon 0.30–0.60
Nickel 3.50–4.50
Iron Balance

Reference
Marcel Dekker.

10
Austenitic Stainless Steel Family

This family of stainless accounts for the widest usage of all the stainless
steels. These materials are nonmagnetic, have face-centered cubic structures,
and possess mechanical properties similar to those of the mild steels, but
with better formability. The AISI designation system identifies the most
common of these alloys with numbers beginning with 300 and resulted in
the term 300 series stainless.
The relationship between alloying elements and alloy types illustrated in
the Schaeffler diagram (Figure 9.1) is an important concept in understanding
stainless steels. It has been established that certain elements, specifically
chromium, molybdenum and silicon, are ferrite formers. Aluminum and
niobium are also ferrite formers, although their effect is dependent
on the alloy system. There are also elements that tend to promote the
formation of austenite. The most often used are nickel, manganese, carbon,
and nitrogen.
Examination of the Schaeffler diagram offers insight into the reason for the
composition of type 304, the cornerstone of the austenitic alloy family. After
the corrosion resistance plateau of 18% chromium is reached, the addition of
about 8% nickel is required to cause a transition from ferritic to austenitic.
The primary benefit of this alloy addition is to achieve the austenitic
structure that relative to the ferritics, is very tough, formable, and weldable.
The added benefit, of course, is the improved corrosion resistance to mild
corrodents. This includes adequate resistance to most foods, a wide range of
organic chemicals, mild inorganic chemicals, and most natural environ-
mental corrosion.
Nickel is used judiciously as an alloying element because its cost is
substantially higher than chromium. However, type 304 is balanced near the
austenite-ferrite boundary for another reason. Compositions similar to type
304 that can form no ferrite when solidifying after welding are prone to
cracking during solidification and are more difficult to hot-work. As a result,
adding more nickel to the 18-8 composition offers little benefit from a
corrosion standpoint and would be detrimental in other regards.
The next major step in alloying additions comes from molybdenum. This
element also provides excellent corrosion resistance in oxidizing

151


environments, particularly in aqueous corrosion. It participates in strength-
ening the passive film that forms on the stainless steel surface along
with chromium and nickel. A significant benefit is realized with the
addition of only about 2% molybdenum. Added directly to the 18-8
composition, the alloy would contain too much ferrite, so it must be
rebalanced. The resulting chemistry is roughly 16% chromium, 10% nickel,
and 2% molybdenum.
Anodic polarization studies, such as the examples in Figure 10.1, can be
useful in understanding the benefits of different alloy additions. Chromium
significantly increases the area of passivity, as indicated in the adjacent
diagram. It lowers the potential required for the onset of passivity and raises
the pitting potential. A further increase in chromium also shifts the current
in the passive region to lower levels. Molybdenum has the particular benefit
of raising the pitting potential. It is about three times more efficient at this
than chromium.
Austenitic alloys also make use of the concept of stabilization. Stainless
types 321 and 347 are versions of type 304 stabilized with titanium and
niobium, respectively. The austenitic family of stainless also prompted
another approach to avoiding the effects of chromium carbide precipitation.

Corrosion of stainless steels

v v

Increasing temperature
Increasing chloride

Log1 Log1

v v

Increasing molybdenum

Increasing chromium

Log1 Log1

FIGURE 10.1
Effects of environment and alloy content on anodic polarization behavior.

Austenitic Stainless Steel Family 153

Because the amount of chromium that precipitated was proportional to the
carbon content, lowering the carbon could prevent sensitization. As shown
in Figure 9.1, maintaining the carbon content to below about 0.035%, vs. the
usual 0.08% maximum, will avoid the precipitation of harmful levels of
chromium carbide. This discovery, along with improvements in melting
technology, resulted in the development of the low-carbon version of many
of these alloys. When first introduced, extra-low carbon (ELC) grades
required premiums on pricing due to higher production costs. This
differential has essentially disappeared in the face of modern argon–oxygen
decarburization (AOD) furnaces.
AOD furnaces, utilized as a final refining stage in melting, are designed to
permit the bubbling of the molten steel with oxygen, which facilitates the
removal of carbon and sulfur. During this process the exposed surface of the
melt is protected with an inert argon atmosphere. This arrangement also
permits bubbling with nitrogen gas that will dissolve as atomic nitrogen into
the steel. Nitrogen acts in a fashion similar to carbon by pinning slip planes,
thus leading to higher-strength materials.
Modern melting technology is also responsible for another trend in
stainless metallurgy. At one time, the permissible chemistry ranges for
alloying elements needed to be broad to accommodate inhomogeneity in
electric furnace melts, chemical analysis variations, and raw material quality.
For example, the chromium range for type 304 was 18.0–20.0% and still heats
were occasionally missed. With current technology, it is possible to maintain
G3s limits on chromium to 0.5% or better. The result is that alloys are
currently being produced with 0.50–0.75% less of an alloying element than
they were just 15 years ago.
Chemistries of the 300-series alloys are listed in Table 10.1. An overview of
these and other nonmagnetic stainless steel families is shown in Figure 10.2.
Further improvements in general, localized, and high-temperature corrosion
resistance are gained by additions of chromium, nickel, molybdenum, or
other more minor alloying elements. These modifications have led to other
austenitic alloys such as type 310, used primarily for high-temperature
(above 11008F) applications due to oxidation and sulfidation resistance. More
recent austenitic alloys include materials such as S30815 (253MAw) and
S30615 (RA85Hw). S30815 is a highly oxidation-resistant material with
exceptional elevated-temperature mechanical properties. The oxidation
resistance is a result of the 22% chromium content combined with a small
cerium addition that helps form a tightly adherent scale. The strength is
enhanced by a nitrogen alloy addition, without a deleterious effect on
corrosion resistance.
Even with alloying additions such as molybdenum to improve localized
corrosion resistance to halogens, the workhorse 304 and 316 alloys are
susceptible to chloride stress corrosion cracking (SCC). This cracking
mechanism manifests itself as branched, generally transgranular cracks that
are so fine as to be virtually undetectable until it has progressed to catastrophic
proportions. This mode of failure can occur when the austenitic alloy is under


TABLE 10.1
Chemical Composition of Austenitic Stainless Steels
Nominal Composition (%)
AISI C Mn Si
Type Max Max Max Cr Ni Othersa

201 0.15 7.5b 1.00 16.00–18.00 3.50–5.50 0.25 max N
202 0.15 10.00c 1.00 17.00–19.00 4.00–6.00 0.25 max N
205 0.25 15.50d 0.50 16.50–18.00 1.00–1.75 0.32/0.4 max N
301 0.15 2.00 1.00 16.00–18.00 6.00–8.00
302 0.15 2.00 1.00 17.00–19.00 8.00–10.00
302B 0.15 2.00 3.00e 17.00–19.00 8.00–10.00
303 0.15 2.00 1.00 17.00–19.00 8.00–10.00 0.15 min S
303(Se) 0.15 2.00 1.00 17.00–19.00 8.00–10.00 0.15 min Se
304 0.08 2.00 1.00 18.00–20.00 8.00–12.00
304L 0.03 2.00 1.00 18.00–20.00 8.00–12.00
304N 0.08 2.00 1.00 18.00–20.00 8.00–10.50 0.1/0.16 N
305 0.12 2.00 1.00 17.00–19.00 10.00–13.00
308 0.08 2.00 1.00 19.00–21.00 10.00–12.00
309 0.20 2.00 1.00 22.00–24.00 12.00–15.00
309S 0.08 2.00 1.00 22.00–24.00 12.00–15.00
310 0.25 2.00 1.50 24.00–26.00 19.00–22.00
310S 0.08 2.00 1.50 24.00–26.00 19.00–22.00
314 0.25 2.00 3.00f 23.00–26.00 19.00–22.00
316 0.08 2.00 1.00 16.00–18.00 10.00–14.00 2.00–3.00 Mo
316F 0.08 2.00 1.00 16.00–18.00 10.00–14.00 1.75–2.50 Mo
316L 0.03 2.00 1.00 16.00–18.00 10.00–14.00 2.00/3.00 Mo
316N 0.08 2.00 1.00 16.00–18.00 10.00–14.00 2.00–3.00 Mo
317 0.08 2.00 1.00 18.00–20.00 11.00–15.00 3.00–4.00 Mo
317L 0.03 2.00 1.00 18.00–20.00 11.00–15.00 3.00–4.00 Mo
321 0.08 2.00 1.00 17.00–19.00 9.00–12.00 5 ! C min Cb–Ta
330 0.08 2.00 1.5g 17.00–20.00 34.00–37.00 0.10 TA
0.20 Cb
347 0.08 2.00 1.00 17.00–19.00 9.00–13.00 10!C min Cb–Ta
348 0.08 2.00 1.00 17.00–19.00 9.00–13.00 10C min Cb–Ta
2.0 Mo
3.0 Cu
20Cb3 0.07 0.75 1.00 20.0 29.0 8 ! C min Cb–Ta
904L 0.02 — — 21.0 25.5 4.7 Mo
1.5 Cu
a
Other elements in addition to those shown are as follows: phosphorus is 0.03% max in type
205; 0.06% max in types 201 and 202; 0.045% max in types 301, 302, 302b, 304, 304L, 304N, 305,
308, 309, 309S, 310, 310S, 314, 316, 316N, 316L, 317, 317L, 321, 330, 347, and 348; 0.20% max in
types 303, 303(Se), and 316D. Sulfur is 0.030% max in types 201, 202, 205, 301, 302, 302B, 304,
304L, 304N, 305, 308, 309, 309S, 310, 310X, 314, 316, 316L, 316N, 317, 317L, 321, 330, 347, and 348;
0.15% min in type 303; and 0.10% min in type 316D.
b
Mn range 4.40–7.50.
c
Mn range 7.50–10.00.
d
Mn range 14.00–15.50.
e
Si range 2.00–3.00.
f
Si range 1.50–3.00.
g
Si range 1.0–1.5.


S30400 S20200
"18.8" Add Mn N
S30403 Less Ni
S31600 S31603 Low C
Add Mo Low C
S30409
Add C S24100
S31651 S30451 More Mn N
Add N Add N Less Ni
Add Mo
S31703 S30908
More Mo S31726 More Cr/Ni S32100
Low C More Ni Mo Add Ti S21900
Add N More Cr Mn

S30815 S34700
N08020 More Si Add Cb
More Cr Ni Add Ce N S20910
Add Cu Cb More Cr Ni
Less Mn
S30300
S31008 Add S
N08904 More Cr Ni
S20161
More Mo
Less Mn
Loss Ni S30500 Add Si
More Ni
N08330
More Cr Ni
N08367 Add Si
More Mo S30430
Add N Add Cu

FIGURE 10.2
Austenitic stainless steels.

stress in the presence of halogen ions at temperatures above about 1208F.
Studies by Copsen, summarized in Figure 10.3, underscored the benefit of very
low-nickel contents, such as the ferritic stainless steels, or nickel levels in excess
of about 20%. In fact, the nickel contents in these two alloys are in the range that
tend to crack most quickly in chloride-bearing environments.
Another group of austenitic alloys is based on the substitution of
manganese for nickel. Manganese has about half the austenitizing power of
nickel. This approach was first used during and shortly after World War II in
response to nickel shortages. Stainless type 201 was developed as a substitute
for type 304 stainless. By adding about 4% manganese and 0.2% nitrogen, the
nickel content could be lowered to about 5%. Although the strength of this
alloy is higher than that of type 304, its corrosion resistance is inferior.
Other alloys in this line have been developed. These include Nitronic 40
(S21900) and Nitronic 50 (S20910). The corrosion resistance of S20910
exceeds that of type 316 stainless with the additional benefit of higher
mechanical properties.

10.1 Type 201 (S20100)
This is one of the alloys based on the substitution of manganese for nickel
because of the shortage of nickel during and shortly after World War II. It
was developed as a substitute for type 304 stainless steel. By adding about


1000

100
Time to failure

10

1
0 10 20 30 40 50
Weight percent nickel

FIGURE 10.3
Chloride stress cracking vs. nickel content.

4% manganese and 0.2% nitrogen, the nickel content could be lowered to
about 5%. The chemical composition is shown in Table 10.1. Although the
strength of this alloy is higher than that of type 304, its corrosion resistance is
inferior. It exhibits a corrosion resistance comparable to type 301.
This alloy can be cold-worked to high strength levels. It is nonmagnetic as
annealed and becomes somewhat magnetic after cold work.

10.2 Type 202 (S20200)
Alloy type 202 is one of the series of alloys using manganese as a
replacement for nickel. As can be seen in Table 10.1, the manganese content
of this alloy is greater than that of type 201, as is the chromium and nickel
content. This provides improved corrosion resistance.
The mechanical properties of alloy 202 improved to the same degree as
type 201.

10.3 Type 22-13-5 (S20910)
This is a nitrogen-strengthened stainless alloy having the following
composition:


Carbon 0.06
Manganese 4.00/6.00%
Phosphorus 0.040%
Sulfur 0.030%
Silicon 1.00%
Chromium 20.50/23.50%
Nickel 11.50/13.50%
Molybdenum 1.50/3.00%
Columbium 0.10/0.30%
Vanadium 0.10/0.30%
Nitrogen 0.20/0.40%
Iron Balance

It is superior in corrosion resistance to type 316 stainless steel with twice
the yield strength; it can be welded, machined, and cold-worked using the
same equipment and methods used for the conventional 300-series stainless
steels. It remains nonmagnetic after severe cold work.
Type 22-13-5 stainless steel has very good corrosion resistance in many
reducing and oxidizing acids, chlorides, and pitting environments. It has a
pitting resistance equivalent number (PREN) of 45.5. In particular, the alloy
provides an excellent level of resistance to pitting and crevice corrosion in
seawater. Resistance to intergranular attack in boiling 65% nitric acid and in
ferric sulfate–sulfuric acid is excellent for both the annealed and sensitized
conditions. Like other austenitic stainless steels, S20910 under certain
conditions my suffer stress corrosion cracking in hot chloride environments.
This alloy also demonstrates good resistance to sulﬁde stress cracking at
ambient temperatures.
Applications for this alloy has included such items as valve shafts, taper
pins, pumps, fasteners, cables, chains, screens, wire cloth, marine hardware,
boat shafting, heat exchanger parts, and springs.
This alloy is sometimes referred to as “nitronic 50.”

10.4 Type 216L (S21603)
This is a low-carbon alloy in which a portion of the nickel has been replaced
by molybdenum. It has the following composition:

Carbon 0.03%
Manganese 7.50/9.00%
Chromium 17.5/22.0%
Nickel 5.00/7.00%
Molybdenum 2.00/3.00%
Silicon 1.00%

This alloy ﬁnds application as aircraft hydraulic lines, heat-exchanger
tubes, pollution-control equipment, and particle-accelerator tubes.


10.5 Type 301 (S30100)
This is a nitrogen-strengthened alloy that has the ability to work-harden. As
with the 200-series alloys, it forms martensite while deforming, but retains
the contained strain to higher levels. The chemical composition is shown in
Table 10.1. Types 301L and 301LN ﬁnd application in passenger rail cars,
buses, and light rail vehicles. The chemical composition of type 301L
(S30103) and type 301LN (S30153) are as follows.

Alloy
Alloying Element 301L 301LN

Carbon 0.030 max 0.030 max
Chromium 16.0–18.0 16.0–18.0
Manganese 2.0 max 2.0 max
Nitrogen 0.20 max 0.07–0.20
Nickel 5.0–8.0 5.0–8.0
Phosphorus 0.045 max 0.045 max
Sulfur 0.030 max 0.030 max
Silicon 1.0 max 1.0 max

10.6 Type 302 (S30200)
Type 302 and type 302B are nonmagnetic, extremely tough and ductile and
are two of the most widely used of the chromium–nickel stainless and heat-
resisting steels. They are nonhardenable by heat-treating. The chemical
compositions are shown in Table 10.1.

10.7 Type 303 (S30300)
This is free-machining version of type 304 stainless steel for automatic
machining. It is corrosion resistant to atmospheric exposures, sterilizing
solutions, most organic and many inorganic chemicals, most dyes, nitric
acid, and foods. The chemical composition is given in Table 10.1.

10.8 Type 304 (S30400)
Type 304 stainless steels are the most widely used of any stainless steels.
Although they have a wide range of corrosion resistance, they are not the


most corrosion resistant of the austenitic stainlesses. The chemical
composition of various types of 304 alloys are shown in Table 10.1.
Type 304 stainless steel is subject to intergranular corrosion as a result of
carbide precipitation. Welding can cause this phenomenon, but competent
welders using good welding techniques can control the problem. Depending
upon the particular corrodent being handled, the effect of carbide
precipitation may or may not present a problem. If the corrodent being
handled will attack through intergranular corrosion, another alloy should
be used.
If the carbon content of the alloy is not allowed to exceed 0.03%, carbide
precipitation can be controlled. Type 304L is such an alloy. This alloy can be
used for welded sections without danger of carbide precipitation.
Type 304N has nitrogen added to the alloy, which improves its resistance
to pitting and crevice corrosion.
Types 304 and 304L stainless steels exhibit good overall corrosion
resistance. They are used extensively in the handling of nitric acid. Refer
to Table 10.2 for the compatibility of these alloys with selected corrodents.

10.9 Type 305 (S30500)
Type 305 stainless steel is used extensively for cold heading, severe deep
drawing, and spinning operations. A high nickel content slows work-
hardening. This alloy maintains low magnetic permeability after cold
working. The chemical composition is shown in Table 10.1.
Table 305 stainless steel has the equivalent corrosion resistance of type 304
stainless steel.

10.10 Type 308 (S30800)
The chemical composition of type 308 stainless steel is shown in Table 10.1. It
will be noted that this alloy has an increased chromium and nickel content
over that of type 304 stainless steel. In the annealed conditions, type 308
exhibits greater tensile and yield strengths than annealed type 304.
The corrosion resistance of type 308 is slightly better than that of type
304 stainless.

10.11 Type 309 (S30900)
Types 309 and 309S are superior heat-resisting stainless alloys. They are
applicable for continuous exposure to 20008F (10938C). These are


TABLE 10.2
Compatibility of Types 304, 304L, and 347 Stainless Steel with Selected
Corrodents
Maximum Temperature
Chemical 8F 8C

Acetaldehyde 200 93
Acetamide 100 38
Acetone 190 88
Acrylic acid 130 54
Adipic acid 210 99
Allyl alcohol 220 104
Alum X
Aluminum sulfatea 210 99
Ammonia gas 90 32
Ammonium nitrateb 210 99
Ammonium persulfate X
Ammonium sulfate, 10–40% X
Ammonium sulﬁde 210 99
Ammonium sulﬁte 210 99
Amyl alcohol 80 27
Amyl chloride 150 66
Aniline 500 260
Aqua regia, 3:1 X
Barium chloride X
(continued)


TABLE 10.2 Continued

Maximum Temperature
Chemical 8F 8C

Benzaldehyde 210 99
Benzene 230 110
Benzoic acid 400 204
Borax 150 66
Boric acida 400 204
Bromine gas, dry X
Bromine, liquid X
Butadiene 180 82
Butyl acetate 80 27
Butyl alcohol 200 93
Butyric acid 180 82
Calcium bisulﬁtec 300 149
Calcium chlorate, 10% 210 99
Calcium chloridea,b 80 27
Calcium oxide 90 32
Caprylic acidb 210 99
Cellosolve 210 99
Chloracetic acid X
Chlorine gas, dry X
Chlorine gas, wet X
Chlorine, liquida 110 43
Chlorobenzene 210 99
Chloroformb 210 99
(continued)



Maximum Temperature
Chemical 8F 8C

Copper carbonate, 10% 80 27
Copper chloride X
Copper sulfatec 210 99
Cresol 160 71
Cyclohexane 100 38
Cyclohexanol 80 27
Dichloroethane 210 99
(ethylene dichloride)
Ferric chloride X
Ferrous chloride X
Hypochlorous acid X
Lactic acid, 25%a,c 120 49
Lactic acid, conc.a,c 80 27
Magnesium chloride X
Malic acid, 50% 120 49
Manganese chloride X
Methyl chloridea 210 99
Muriatic acid X
Oleum 100 38
(continued)



Maximum Temperature
Chemical 8F 8C

Phenola 560 293
Phosphoric acid, 50–80%c 120 49
Picric acida 300 149
Sodium carbonate, 30% 210 99
Sodium chloride, to 30%a 210 99
Sodium hydroxide, conc. 90 32
Sodium sulﬁde, to 50%a 210 99
Stannic chloride X
Stannous chloride X
Sulfuric acid, 90%c 80 27
Sulfuric acid, fuming 90 32
Sulfurous acid X
Thionyl chloride X
Toluene 210 99
White liquor 100 38
Zinc chloride X
a
Subject to pitting.
b
Subject to stress cracking.
c
Subject to intergranular attack (type 304).

modiﬁcations of type 304 stainless steel. The chemical composition is
shown in Table 10.1. These alloys have better creep strength than the
304 alloys.
Types 309 and 309S alloys have slightly better corrosion resistance than
type 304 stainless steel. They are resistant to corrosive action of high-sulfur


gases if they are oxidizing, but poor in reducing gases like hydrogen sulfide.
These alloys are excellent in resisting sulfite liquors, nitric acid, nitric–
sulfuric acid mixtures, and acetic, citric, and lactic acids. Type 309S with a
maximum of 0.08% carbon resists corrosion in welded parts. They may be
susceptible to stress corrosion cracking.
Applications include sulfur-bearing gas atmospheres, furnace parts, fire
boxes, high-temperature containers, and heat-exchanger tubing.

10.12 Type 310 (S31000)
This is an alloy for high temperatures. It is an improvement over types 309
and 309S. The 310 and 310S alloys have a maximum allowable temperature
of 21008F (11498C) at continuous operation. Chemical compositions are
shown in Table 10.1.
These alloys have better general corrosion resistance than type 304 and
type 309. They have excellent high-temperature oxidation resistance and
good resistance to both carburizing and reducing environments. Chloride
stress corrosion cracking may cause a problem under the right conditions.
Type 310S with 0.08% maximum carbon content offers improved resistance
in welded components.

10.13 Type 316 (S31600)
These chromium–nickel grades of stainless steel have molybdenum added
in the range of 2–3%. The molybdenum substantially increases resistance to
pitting and crevice corrosion in systems containing chlorides and improves
overall resistance to most types of corrosion in chemically reducing
neutral solutions.
In general, these alloys are more corrosion resistant than type 304 stainless
steels. With the exception of oxidizing acids, such as nitric, the type 316
alloys will provide satisfactory resistance to corrodents handled by type 304
with the added ability to handle some that type 304 alloy cannot handle.
Type 316L stainless steel is the low-carbon version of type 316 and offers
the additional feature of preventing excessive intergranular precipitation of
chromium carbides during welding and stress relieving.
Table 10.3 provides the compatibility of types 316 and 316L stainless steel
with selected corrodents. The chemical composition of types 316 and 316L
stainless steel are shown in Table 10.1.
In the annealed condition, these alloys are nonhardenable and non-
magnetic, but are slightly magnetic when cold worked.
Type 316H stainless steel has a higher carbon content for better high-
temperature creep properties to meet the requirements of ASME Section VIII,


TABLE 10.3
Compatibility of Types 316, and 316L Stainless Steel with
Selected Corrodents
Maximum Temperature
Chemical 8F 8C

Acetaldehyde 210 99
Acetamide 340 171
Acetone 400 204
Acrylic acid 120 49
Adipic acid 210 99
Alum 200 93
Aluminum fluoride 90 32
Aluminum sulfatea 210 99
Ammonia gas 90 32
Ammonium bifluoride, 10% 90 32
Ammonium fluoride, 10% 90 32
Ammonium nitratea 300 149
Amyl alcohol 400 204
Aniline 500 260
Aqua regia, 3:1 X
Barium chlorideb 210 99
(continued)



Maximum Temperature
Chemical 8F 8C

Benzaldehyde 400 204
Benzene 400 204
Borax 400 204
Boric acid 400 204
Bromine gas, dry X
Bromine, liquid X
Butadiene 400 204
Butyl acetate 380 193
n-butylamine 400 204
Butyric acid 400 204
Calcium bisulﬁde 60 16
Calcium bisulﬁte 350 177
Calcium chloridea 210 99
Calcium oxide 80 27
Caprylic acida 400 204
Carbon tetrachloridea,b 400 204
Cellosolve 400 204
Chloracetic acid X
Chlorine gas, wet X
Chlorine, liquid dry 120 49
Chlorobenzene, ELC only 260 127
Chloroforma 210 99
Chromic acid, 10%c 400 204
Chromic acid, 15%c 150 49
(continued)



Maximum Temperature
Chemical 8F 8C

Citric acid, 15%b 200 93
Citric acid, conc.b 380 193
Copper carbonate, 10% 80 27
Copper chloride X
Cresol 100 38
Cyclohexane 400 204
Cyclohexanol 80 27
Dichloroethane (ethylene dichloride) 400 204
Ferric chloride X
Ferrous chloride X
Hydroﬂuoric acid, 100% 80 27
Hypochlorous acid X
Lactic acid, conc.b,d 300 149
Magnesium chloride, 50%a,b 210 99
Malic acid 250 121
Manganese chloride, 30% 210 99
Methyl chloride, dry 350 177
Muriatic acid X
Nitric acid, 5%d 210 99
Nitric acid, anhydrousd 110 43
Nitric acid, conc. 80 27
Oleum 80 27
(continued)



Maximum Temperature
Chemical 8F 8C

Phenol 570 299
Phosphoric acid, 50–80%d 400 204
Picric acid 400 204
Potassium bromide, 30%b 350 177
Sodium carbonate 350 177
Sodium hydroxide, 50%a 350 177
Stannic chloride X
Stannous chloride, 10% 210 99
Sulfuric acid, 90%d 80 27
Sulfurous acidd 150 66
Thionyl chloride X
Toluene 350 177
White liquor 100 38
Zinc chloride 200 93
X. When compatible, the corrosion rate is !20 mpy.
a
Subject to stress cracking.
b
Subject to pitting.
c
Subject to crevice attack.


Table UHA-21 Footnote 8. This alloy is used in temperatures over 18328F
(10008C). It has a chemical composition as follows:

Chromium 16.0/18.0%
Nickel 10.0/14.0%
Molybdenum 2.0/3.0%
Carbon 0.04/0.10%
Iron Balance

The corrosion resistance of type 316H stainless steel is the same as type 316
stainless steel, except after long exposure to elevated temperatures where
intergranular corrosion may be more severe. It may also be susceptible to
chloride stress corrosion cracking.
Type 316N is a high-nitrogen type 316 stainless steel. The chemical
composition is shown in Table 10.1. It has a higher strength than type 316
and greater ASME section VIII allowables. Corrosion resistance is the same
as type 316 and it may be susceptible to chloride stress corrosion cracking.
It is used in tubing and applications where minimum wall pressure
requirements are critical.
Type 316LN stainless steel is a low-carbon, high-nitrogen type 316
stainless. It has the following composition:

Chromium 16.0/180%
Nickel 10.0/15.0%
Molybdenum 2.0/3.0%
Carbon 0.035%
Nitrogen 0.10/0.16%
Iron Balance

Type 316LN stainless has the same high-temperature strength and ASME
allowables as type 316, the weldability of type 316L. The corrosion resistance
is the same as type 316 stainless and may be susceptible to chloride stress
corrosion cracking.

10.14 Type 317 (S31700)
Type 317 stainless steel contains greater amounts of molybdenum,
chromium, and nickel than type 316. The chemical composition is shown
in Table 10.1. As a result of the increased alloying elements, these alloys offer
higher resistance to pitting and crevice corrosion than type 316 in various
process environments encountered in the process industry. However, they
may still be subject to chloride stress corrosion cracking. The alloy is


nonmagnetic and nonhardenable in the annealed condition, but becomes
slightly magnetic when cold worked. Type 317 stainless steel ﬁnds
application in the chemical, petroleum, and pulp and paper industries for
heat exchangers, evaporators, and condenser tubes.
Type 317L is a low-carbon version of the basic alloy that offers the
additional advantage of preventing intergranular precipitation of chromium
carbide during welding and stress-relieving. The chemical composition is
Type 317L has improved pitting resistance over that of type 316L, but may
still be subject to chloride stress corrosion cracking. The compatibility of type
317 and type 317L stainless steel with selected corrodents is shown in Table 10.4.
Type 317L stainless steel is used for welding, brazing and other short-term
exposures to high temperatures.
Type 317LM stainless steel is a low-carbon, high-molybdenum form of
type 317. It has better corrosion resistance than types 317L, 316L, or 304L,
and the best chloride resistance of the 300-series stainless steels. It may be
susceptible to chloride stress corrosion cracking. The chemical composition
is as follows:

Chromium 18.0/20.0%
Nickel 13.0/17.0%
Molybdenum 4.0/5.0%
Nitrogen 0.1% max
Carbon 0.03% max
Iron Balance

This alloy is used for ﬂue-gas applications and other heat-exchanger
tubing subject to higher-acid chlorides.
Type 317LMN is a low-carbon, high-molybdenum, high-nitrogen type 317
stainless steel. It has a composition of

Chromium 17.0/20.0%
Nickel 13.0/17.0%
Molybdenum 4.0/5.0%
Nitrogen 0.1/0.2%
Carbon 0.03% max
Iron Balance

The corrosion resistance of this alloy is the same as for type 317LM with
the advantage of preventing chromium carbide precipitation during
welding or stress-relieving, and it has the strength of type 317 stainless. It
is used where weldability and strength are important.


TABLE 10.4
Compatibility of Types 317 and 317L Stainless Steel with
Selected Corrodents
Chemical Maximum Temperature (8F/8C)

Acetaldehyde 150/66
Acetic acid, glacial 240/116
Acetic anhydride 70/21
Acetone 70/21
Aluminum sulfate, 50–55% 225/107
Ammonium nitrate, 66% 70/21
Ammonium sulfate, 10–40% 100/38
Benzene 100/38
Boric acid 210/99
Bromine gas, dry X
Bromine liquid X
Butyl alcohol, 5% 195/91
Calcium hypochlorite 70/21
Carbon tetrachloride 70/21
Carbonic acid 70/21
Chloracetic acid, 78% 122/50
Chlorine, liquid X
Chlorobenzene 265/129
Chromic acid, 10% X
Chromic acid, 50% X
Citric acid, conc. 210/99
Ferric chloride 70/21
Iodine solution, 10% 70/21
Lactic acid, conc. 330/166
Magnesium chloride, 30% 70/21
Nitric acid, 5% 70/21
Phenol 70/21
Phosphoric acid, 50–80% 140/60
(continued)




Sodium carbonate 210/99
Sodium chloride, 30% X
Sodium hydrochlorite, 20% 70/21
Sodium hypochlorite, conc. 70/21
Sodium sulfide, to 50% 210/99
Sulfurous acid X
allowable temperature for which data are available. Incompatibility is
shown by an X. When compatible, the corrosion rate is !20 mpy.

10.15 Type 321 (S32100)
By alloying austenitic alloys with a small amount of an element having a
higher affinity for carbon than does chromium, carbon is restrained from
diffusing to the grain boundaries, and any carbon that reaches the boundary
reacts with the element instead of with the chromium. These are known as
stabilized grades. Type 321 is such an alloy that is stabilized by the addition
of titanium. Its chemical composition is shown in Table 10.1.
Type 321 stainless steel can be used with improved corrosion
resistance wherever type 316 is suitable, particularly in the presence
of nitric acid. This alloy is particularly useful in high-temperature service in
the carbide precipitation range and for parts heated intermittently between
800 and 16508F (428–8998C). Even with the overall improved corrosion
resistance it still may be susceptible to chloride stress corrosion cracking.
Table 10.5 provides the compatibility of type 321 stainless with selected
corrodents.
This alloy has excellent weldability in the field. Applications include use
in exhaust manifolds, expansion joints, high-temperature chemical process
heat-exchanger tubes, and recuperator tubes.
Type 321H is a high-carbon type stainless steel with better high-
temperature creep properties; it meets the requirements of ASME Section
VIII Table UHA-21, Footnote 8. It has good weldability in the field.


TABLE 10.5
Compatibility of Type 321 Stainless Steel with Selected
Corrodents

Acetic acid, 10% X
Acetic acid, 50% X
Acetic acid, 80% X
Acetic anhydride 70/21
Alum X
Aluminum sulfate 70/21
Ammonium sulfate, 10–40% 70/21
Benzene 100/38
Boric acid 210/99
Bromine gas, dry X
Bromine, liquid X
Calcium chloride X
Carbon tetrachloride X
Carbonic acid 70/21
Chloracetic acid, 78% X
Chlorine, liquid X
Chromic acid, 10% X
Chromic acid, 50% X
Citric acid, conc. 70/21
Ferric chloride X
Lactic acid, conc. 70/21
Phenol X
Sodium carbonate 70/21
Sodium chloride, 30% X
(continued)




Sodium hydrochlorite, 20% X
Sodium hypochorite, conc. X
Sodium sulﬁde, to 50% 70/21
Sulfurous acid X
X. When compatible, the corrosion rate is !20 mpy.

The corrosion resistance of type 321H is the same as the corrosion
resistance of type 321 and may be susceptible to chloride stress corrosion
cracking. It has the following chemical composition:

Chromium 17.0/20.0%
Nickel 9.0/13.0%
Carbon 0.04/0.10%
Titanium 4 ! Carbon min, 0.60% max
Iron Balance

Type 321H stainless is used in applications where temperatures exceed
1008F (5388C).

10.16 Type 329 (S32900)
Type 329 stainless steel is listed under the austenitic stainless steels; in
actuality, it is really the basic material of duplex stainless steels. It has the
following composition:

Chromium 26.5%
Nickel 4.5%
Molybdenum 1.5%
Carbon 0.05%
Iron Balance

Type 329 stainless possesses higher strength values than those typically
found in the austenitic stainlesses.
The general corrosion resistance of type 329 stainless is slightly above that
of type 316 stainless in most media. In addition, because the nickel content is
low, it has good resistance to chloride stress cracking.


10.17 Type 347 (S34700)
Type 347 stainless steel is a columbium (niobium)-stabilized alloy. Its
chemical composition will be found in Table 10.1. Being stabilized, it will
resist carbide precipitation during welding and intermittent heating to
800–16508F (427–8998C) and has good high-temperature scale resistance.
This alloy is equivalent to type 304 stainless steel with the added protection
against carbide precipitation. Type 304L also offers this protection, but is
limited to a maximum operating temperature of 8008F (4278C), whereas type
347 can be operated to 10008F (5388C).
In general, the corrosion resistance of type 347 is equivalent to that of type
304 stainless steel and may be susceptible to chloride stress corrosion
cracking. Refer to Table 10.2.
Applications include exhaust manifolds, expansion joints, high-tempera-
ture heat-exchanger tubes, and recuperation tubes.
Type 347H is a high-carbon type 347 for better high-temperature creep
properties to meet the requirements of ASME Section VIII, Table UHA-21,
Footnote 8. The chemical composition is as follows:

Chromium 17/20%
Nickel 9/13%
Carbon 0.04/0.01%
ColumbiumCtantalum 8!carbon min, 1.0% max
Iron Balance

Type 347H has the same corrosion resistance as type 347 and may be
susceptible to chloride stress corrosion cracking. It has excellent
weldability.

10.18 Type 348 (S34800)
Type 348 stainless is the same as type 347 except that the tantalum content
is restricted to a maximum of 0.10%. The chemical composition is as
follows:

Chromium 17.0/20.0%
Nickel 9.0/13.0%
Carbon 0.08%
NiobiumCtantalum 10!carbon min 1.0% max
(0.1% max tantalum)
Iron Balance


In general, the corrosion resistance is the same as that of type 347 stainless
and may be subject to chloride stress corrosion cracking.
This material is used in nuclear applications where tantalum is
undesirable because of high neutron cross-section.
Type 348H stainless steel is a high-carbon version of type 348 designed to
provide better high-temperature creep properties and to meet the
requirements of ASME Section VIII, Table UHA-21, Footnote 8.
It ﬁnds application in nuclear environments at temperatures over 10008F
(5388C), where tantalum is undesirable because of high neutron cross-section.

Reference
Marcel Dekker.

11
Superaustenitic Family of Stainless Steel

During the 1970s and into the 1980s, much attention was focused on a family
of stainless alloys that came to be identified as superaustenitic. The
foundation for the development of this class of materials was in the
development of Carpenter No. 20 stainless, introduced in 1951. Consisting of
28% nickel and 19% chromium with additions of molybdenum and copper,
this alloy was first produced as a cast material. Development of the process
to produce this material as a wrought product and later refinements in
chemistry ultimately resulted in the introduction of 20Cb-3 stainless in 1965.
Superaustenitic materials are tabulated in Table 11.1.
20Cb-3 stainless became popular in the chemical process industry as an
intermediate step between type 316 stainless and the more highly alloyed
nickel-based materials. In particular, it was a cost-effective way to combat
chloride stress corrosion cracking (SCC). This form of cracking is particularly
difficult to combat by means other than alloy selection. Because of the high
nickel content of 20Cb-3 stainless, it received a nickel-based alloy UNS
designation as UNS N08020. However, because the major constituent is iron,
it is truly a stainless steel. The superaustenitic term is derived from the fact
this composition plots high above the austenite–ferrite boundary on the
Schaeffler diagram. Unlike the 300-series stainless alloys, there is no chance
of developing ferrite in this material.
In a similar time frame, another superaustenitic alloy was introduced
based on the wrought version of the heat-resistant cast alloy, HT. This alloy,
identified as RA330 stainless, contains about 35% nickel and 20% chromium
with an addition of silicon. This superaustenitic stainless also was assigned a
nickel-based UNS number (N08330). N08330 offers excellent oxidation and
carburization resistance in combination with good elevated temperature
Other superaustenitic stainless alloys with long histories include Inconel
825 (N08825) and Inconel 800 (N08800), which have similarities with N08020
and N08330, respectively. The driving force for the development of newer
superaustenitic stainless materials lay primarily in the desire for alloys with
better resistance to localized corrosion. While alloys N08020 and N08825

177


TABLE 11.1
Selected Superaustenitic Stainless Steels
CTEa Density
Alloy C Cr Ni Mo N Other (min./in.) bb (#/in.3)

N08020 0.02 19.5 33.0 2.2 — Cu-3.2 8.9 !1.02 0.292
N08330 0.05 19.5 35.0 — — Si-1.2 9.5 !1.02 0.287
N08367 0.02 20.5 25.0 6.1 0.22 9.5 !1.02 0.291
N08800 0.08 19.5 32.0 — — Al-0.4, Ti-0.4 9.6 !1.02 0.287
N08825 0.02 20.0 38.5 3.0 Cu-2.0, Ti-0.8 9.1 !1.02 0.294
N08904 0.02 20.0 25.0 4.5 Cu-1.5 9.4 !1.02 0.289
S31254 0.02 20.0 18.0 6.1 0.2 Cu-0.7 9.4 !1.02 0.289
S31654 0.02 24.0 22 7.3 0.5 Cu-0.5, Mn-3.0 10.0 !1.02 0.289
S35315 0.05 25.0 35.0 — 0.15 Si-1.8, Ce-0.05 9.5 !1.02 0.285
Room Room Elevated
Room Temperature Temperature Temperature
Temperature Tensile Elongation Toughness Strength
Alloy Yield (KSI) (KSI) (%) (ft-lb@8F) (KSI@8F)c

N08020 48 90 45 145@K3008 1.5@13008 CRP
N08330 37 86 48 240@RT 5.3@13008 CRP
N08367 55 110 50 85@K3008 22@9008 STYS
N08800 36 85 45 105@RT 5.5@13008 CRP
N08825 44 100 43 70@K3008
N08904 36 85 40 125@RT
S31254 44 94 35 88@RT 23@7508 STYS
S31654 62 108 40 130@RT 43@7508 (STYS)
S35315 46 103 48 142@RT 4.3@14008
Note: Values are approximate.
a
Coefficient of thermal expansion for range of 72–12008F.
b
Magnetic permeability.
c
CRP is stress required to produce 1% creep strain in 10,000 h. STYS represents a short-time
tensile yield strength.

exhibit good general corrosion resistance to strong acids, their pitting
resistance is only slightly better than that of type 316L. Their performance in
seawater or brackish water is marginal at best.
The main approach to improving the pitting and crevice corrosion
resistance of the basic 35% nickel, 19% chromium, and 2% molybdenum
alloy was to increase the molybdenum content. Among the first of the newer
alloys introduced was 904L (UNS N08904), with a boosted molybdenum
content of 4% and reduced nickel content of 25%. The reduction in nickel
content was beneficial as a cost-saving factor, with minimal loss of general
corrosion resistance and sufficient resistance to chloride SCC.
The next progression was an increase in the molybdenum content to a
higher level, 6%, which offset the tendency for the formation of s phase by
the alloying addition of nitrogen. This concept was introduced with two
alloys, 254SMOw (UNS S31254) and Al-6XNw (UNS N08367). The major

Superaustenitic Family of Stainless Steel 179

beneﬁt of the addition of nitrogen was the ability to produce these alloys in
heavy product sections such as plate, bar, and forgings. An additional beneﬁt
was derived from alloying with nitrogen in terms of increased pitting
resistance.
and pitting resistance equivalent numbers (PREN). As a general rule, the
higher the PREN, the better resistance to pitting. The pitting resistance number
is determined by the chromium, molybdenum, and nitrogen contents.
PRENZ%CrC3.3X%MoC30X%N. The PREN for various austenitic
stainless steels can be found in Table 11.2.
Another method used to compare the resistance of alloys to localized attack
is to compare their respective CPTs and CCTs. These are the temperatures at
which pitting and crevice attack are initiated. Critical temperatures for several
alloys are given in Table 11.3.
A PREN value in excess of 33 is considered necessary for pitting and
crevice resistance to ambient seawater.

TABLE 11.2
Pitting Resistance of Selected Alloys
Alloy UNS Pitting Resistance Equivalent

654 S32654 63.09
31 N08031 54.45
825 N08825 51.9
686 51.0
625 N06625 50.7
25-6Mo N08926 47.45
A16XN N08367 46.96
926 N08926 46.45
254SMo S31254 45.8
20Mo6 N08026 42.81
317LN S31753 39.6
904L N08904 36.51
20Mo4 N08024 36.2
317 S31700 33.12
316LN S31653 31.08
315 S35315 29.5
316 S31600 27.9
20Cb3 N08020 27.26
348 S34800 25.6
800 N08800 21.0
810 N08810 21.0
347 S34700 19.0
331 N08331 19.0
330 N08330 18.5
304 S30400 18.0


TABLE 11.3
Critical Pitting and Crevice Temperatures of Selected Alloys
Critical Temperature (8F/8C)
Alloy Pitting Attack Crevice Attack
a
Inconel alloy 686 O185/O85 O185/O85
UNS N06059 O185/O85 O185/O85
Inconela alloy 622 R185/R85 R185/R85
UNS N06022 O185/O85 O136/O58
Alloy C-276 O185/O85 113/44
Alloy 625 O185/O85 95/35
Alloy 25-6Mo 158/70 86/30
Alloy 825 86/30 41/5
UNS S31600 68/20 !32/!0
317LM 36/2.5
a
Inconel is the trademark of Inco Alloys International.

The various superaustenitic alloys are discussed in detail in the
following section.

11.1 Alloy 20Cb3 (N08020)
This alloy was originally developed to provide improved corrosion
resistance to sulfuric acid. However, it has found wide application
throughout the chemical process industry. The alloy’s composition is
The alloy is stabilized with niobium and tantalum and has a high nickel
content, approximately 33%. Alloy 20Cb3 is weldable, machinable, and cold
formable, and has minimum carbide precipitation due to welding.
This alloy is particularly useful in the handling of sulfuric acid. It is
resistant to SCC in sulfuric acid at a variety of temperatures and
concentrations. The resistance of 20Cb3 to chloride SCC is also increased
over type 304 and type 316 stainless steels. The alloy also exhibits excellent
resistance to sulfide stress cracking and consequently finds many
applications in the oil industry.
In high concentrations of chlorides, alloy 20Cb3 is vulnerable to pitting
and crevice attack. For improved resistance to these types of corrosion, the
2% molybdenum must be increased to 4 or 6% as has been done in alloy
20Mo-4 and 20Mo-6. Table 11.4 contains the compatibility of alloy 20Cb3
with selected corrodents.
This alloy finds application in the manufacture of synthetic fibers, heavy
chemicals, organic chemicals, pharmaceuticals, and food processing
equipment.


TABLE 11.4
Compatibility of Type 20Cb3 Stainless Steel with
Selected Corrodents
Maximum
Temperature
Chemical 8F 8C

Acetaldehyde 200 93
Acetamide 60 16
Acetone 220 104
Adipic acid 210 99
Alum 200 93
Aluminum chloride, aqueous 120 43
Ammonia gas 90 32
Ammonium biﬂuoride 90 32
Ammonium chloride, sat.a 210 99
Ammonium nitratea 210 99
Ammonium phosphate 210 99
Amyl alcohol 160 71
Aniline 500 260
(continued)



Maximum
Temperature
Chemical 8F 8C

Aqua regia, 3:1 X
Barium chloride, 40% 210 99
Barium hydroxide, 50% 230 110
Benzaldehyde 210 99
Benzene 230 110
Borax 100 38
Boric acid 130 54
Bromine gas, dry 80 27
Butadiene 180 82
Butyl alcohol 90 32
Butyric acid 300 149
Calcium oxide 80 27
Caprylic acid 400 204
Cellosolve 210 99
Chlorine gas, wet X
Chloroform 210 99
(continued)



Maximum
Temperature
Chemical 8F 8C

Chlorosulfonic acid 130 54
Copper chloride X
Cupric chloride, 5% 60 16
Cyclohexane 200 93
Cyclohexanol 80 27
Dichloroethane (ethylene 210 99
dichloride)
Ferric chloride X
Ferrous chloride X
Lactic acid, 25%a 210 99
Lactic acid, conc., air free 300 149
Magnesium chloride 200 93
Malic acid, 50% 160 71
Methyl chloride 210 99
Muriatic acid X
(continued)



Maximum
Temperature
Chemical 8F 8C

Oleum 110 43
Phenol 570 299
Picric acid 300 149
Silver bromide, 10% 90 32
Sodium hydroxide, 50%b 300 149
Sodium hypochlorite, 30% 90 32
Stannic chloride X
Sulfurous acida 360 182
Toluene 210 99
White liquor 100 38
Zinc chloride 210 99

The chemicals listed are in the pure state or in a saturated
solution unless otherwise indicated. Compatibility is shown
to the maximum allowable temperature for which data are
available. Incompatibility is shown by an X. When compatible
the corrosion rate is !20 mpy.
a
Material subject to intergranular corrosion.
b
Material subject to stress cracking.
Source: From P.A. Schweitzer. 2004. Corrosion Resistance Tables,
Vols. 1–4, 5th ed., New York: Marcel Dekker.


11.2 Alloy 20Mo-4 (N08024)
This alloy is similar to alloy 20Cb3 but with 4% molybdenum content instead
of 2%, providing improved pitting and crevice corrosion resistance over
alloy 20Cb3. The chemical composition is as follows:


Nickel 35/40
Chromium 22.5/25.0
Molybdenum 3.5/5.0
Copper 0.5/1.5
Columbium 0.15/0.35
Carbon 0.03 max
Iron Balance

Alloy 20Mo-4 has outstanding corrosion resistance to chloride pitting and
crevice corrosion with good resistance to sulfuric acid and various other
acidic environments.
Applications include heat exchangers, chemical process equipment, and
wet phosphoric acid environments.

11.3 Alloy 20Mo-6 (N08026)
Of the three grades of alloy 20Cb3, this offers the highest level of pitting and
crevice corrosion resistance.
Alloy 20Mo-6 is resistant to corrosion in hot chloride environments and is
also resistant to oxidizing media. This alloy is designed for applications
where better pitting and crevice corrosion resistance is required than that
offered by 20Cb3.
This alloy is melted with low carbon to provide a high level of resistance to
intergranular corrosion. It also possesses excellent resistance to chloride
SCC. When in contact with sulfuric acid, excellent resistance is shown at
1768F (808C), with the exception of concentrations in the range of
approximately 75–97 wt%. In boiling sulfuric acid, 20Mo-6 stainless has
good resistance to general corrosion only in relatively dilute solutions. At
approximately 10% concentration of boiling sulfuric acid, the corrosion rate
becomes excessive.
This alloy is highly resistant to phosphoric acid, both wet process-plant
acid and reagent-grade concentrated phosphoric acid.
20Mo-6 has the following chemical composition:



Chromium 22.00/26.00
Nickel 33.00/37.20
Molybdenum 5.00/6.70
Silicon 0.03/0.50
Manganese 1.00
Phosphorus 0.03
Carbon 0.03
Iron Balance

11.4 Alloy 904L (N08904)
This is a fully austenitic, low-carbon, chromium stainless steel with additives
of molybdenum and copper. The chemical composition will be found in
Table 11.1. Its high nickel and chromium contents make alloy 904L resistant
to corrosion in a wide variety of both oxidizing and reducing environments.
Molybdenum and copper are included in the alloy for increased resistance to
pitting and crevice corrosion and to general corrosion in reducing acids.
Other advantages of the alloy’s composition are sufficient nickel for
resistance to chloride SCC and low carbon content for resistance to
intergranular corrosion.
The alloy’s outstanding attributes are resistance to nonoxidizing acids,
along with resistance to pitting, crevice corrosion, and SCC in such media as
stack-gas concentrate and brackish water.
Alloy 904L is especially suited for handling sulfuric acid; hot solutions at
moderate concentrations represent the most corrosive conditions. It also has
excellent resistance to phosphoric acid. At high temperatures, 904L may be
subject to stress corrosion cracking.
Alloy 904L finds applications in piping systems, pollution control
equipment, heat exchangers, and bleaching systems.

11.5 Alloy 800 (N08800)
The composition of this alloy is shown in Table 11.1. This alloy is used
primarily for its oxidation resistance and strength at elevated temperatures.
It is particularly useful for high-temperature applications because it does not
form the embrittling sigma phase after long exposures at 1200–16008F
(640–8718C). High creep and rupture strengths are other factors that
contribute to its performance in many other applications. It resists
sulfidation, internal oxidation, scaling, and carburization.


At moderate temperatures, the general corrosion resistance of alloy 800 is
similar to that of other austenitic nickel–iron–chromium alloys. However, as
the temperature increases, alloy 800 continues to exhibit good corrosion
resistance, whereas other austenitic alloys are unsatisfactory for the service.
Alloy 800 has excellent resistance to nitric acid at concentrations up to
about 70%. It resists a variety of oxidizing salts, but not halide salts. It also
has good resistance to organic acids such as formic, acetic, and propionic.
Alloy 800 is particularly suited for the handling of hot corrosive gases such
as hydrogen sulfide.
In aqueous service, alloy 800 has general resistance that falls between type
304 and type 316 stainless steels. Thus, the alloy is not widely used for
aqueous service. The stress corrosion cracking resistance of alloy 800, while
not immune, is better than that of the 300 series of stainless steels and may be
substituted on that basis. Table 11.5 provides the compatibility of alloy 800
with selected corrodents.
Applications include heat exchanger and heating-element cladding.
Alloy 800H is a controlled version of alloy 800. The carbon content is
maintained between 0.05 and 0.1% to provide the alloy with better elevated-
temperature creep and rupture properties. It is solution-annealed to assure
the improved creep and stress-to-rupture properties.
Applications include superheater and reheater tubing, headers, and furnace
tubing, as well as applications in the refining and heat treatment industries.
Alloy 800AT is similar to alloy 800 but has higher levels of titanium and
aluminum. It is used for thermal processing applications, chemical and
petrochemical piping, pigtails, and outlet manifolds.

11.6 Alloy 825 (N08825)
Alloy 825 is very similar to alloy 800 but the composition has been modified
to improve its aqueous corrosion resistance. Refer to Table 11.1 for the
chemical composition of alloy 825.
The higher nickel content of alloy 825 compared to alloy 800 makes it
resistant to chloride SCC. Addition of molybdenum and copper gives
resistance to pitting and to corrosion in reducing acid environments, such as
sulfuric and phosphoric acid solutions. Alloy 825 is resistant to pure sulfuric
acid solutions up to 40% by weight at boiling temperatures and at all
concentrations at a maximum temperature of 1508F (608C). In dilute
solutions, the presence of oxidizing salts such as cupric or ferric actually
reduces the corrosion rates. It has limited use in hydrochloric or hydrofluoric
acids.
The chromium content of alloy 825 gives it resistance to various oxidizing
environments such as nitrates, nitric acid solutions, and oxidizing salts. The


TABLE 11.5
Compatibility of Alloy 800 and Alloy 825 with Selected
Corrodents
Maximum
Temperature
Chemical 8F 8C

Acetic acid, 10%a 200 93
Acetic acid, glaciala 220 104
Acetone 210 99
Aluminum ﬂuoride, 5% 80 27
Ammonium chloride, sat. 200 93
Amyl acetatea 200 93
Amyl chloride 90 32
Aniline 90 32
Benzene 190 88
Benzoic acid, 5% 90 32
Borax 190 88
Boric acid, 5% 210 99
Bromine gas, drya 90 32
Butyl acetatea 90 32
Butyric acid, 5% 90 32
Calcium chloridea,b 60 16
Carbonic acid 90 32
Chloracetic acid X
Chlorine gas, drya 90 32
Chlorine gas, wet X
(continued)



Maximum
Temperature
Chemical 8F 8C

Chlorobenzene 90 32
Chloroform 90 32
Chromic acid, 10%a 210 99
Chromic acid, 50% X
Citric acid, conc.a 210 99
Copper chloride, 5%a 80 27
Ferric chloride X
Ferrous chloridea,b 90 32
Fluorine gas, dry X
Hydrochloric acid, 20%a 90 32
Magnesium chloride, 1–5% 170 77
Malic acid 170 77
Magnanese chloride, 10–50% 210 99
Muriatic acida 90 32
Phenol 90 32
Picric acid 90 32
Silver bromide, 10%a 90 32
Sodium chlorideb 200 93
Stannic chloride X
Sulfuric acid, 10%a 230 110
(continued)



Maximum
Temperature
Chemical 8F 8C

Sulfurous acida 370 188
Zinc chloride, 5% 140 60
The chemicals listed are in the pure state or in a saturated
solution unless otherwise indicated. Compatibility is shown to
the maximum allowable temperature for which data are
available. Incompatibility is shown by an X. When compatible
the corrosion rate is !20 mpy.
a
Applicable to alloy 825 only.
b
Material subject to printing.

alloy is not fully resistant to SCC when tested in magnesium chloride, but it
has good resistance in neutral chloride environments.
If localized corrosion is a problem with the 300 series stainless steels, alloy
825 may be substituted. Alloy 825 also provides excellent resistance to
corrosion by seawater. The compatibility of alloy 825 with selected
corrodents is shown in Table 11.5.
Applications include the nuclear industry, chemical processing, and
pollution control systems.

11.7 Type 330 (N08330)
This is a nickel–chromium–iron alloy with the addition of silicon. Refer to
Table 11.1 for its chemical composition. Type 330 stainless has good strength
at elevated temperatures, good thermal stability, and excellent resistance to
carburizing and oxidizing atmospheres. It is weldable and machinable. This
alloy has been used in low-stress applications to temperatures as high as
22508F (12308C) and has moderate creep to 16008F (8708C).
Type 330 stainless steel resists the absorption of carbon and nitrogen,
making it an excellent choice for furnace components. Overall, it exhibits a
good corrosion resistance.


11.8 Al-6XN (N08367)
Al-6XN is the registered trademark of Allegheny Ludlum Corporation and
has the UNS designation of N08367. The typical and specified chemical
compositions of this alloy are given in Table 11.6.
Alloy Al-6XN was originally designed to resist seawater. However, it has
proven to be resistant to a wide range of corrosive environments.
The high strength and corrosion resistance of this alloy make it a better
choice than more expensive nickel-based alloys in applications where
excellent formability, weldability, strength, and corrosion resistance
are essential.
It is also a cost-effective alternative to less expensive alloys, such as type
316, that do not have the strength or corrosion resistance required to
minimize life-cycle costs in certain applications.
The high nickel and molybdenum contents provide improved resistance
to chloride SCC. Copper has been kept to a residual level for improved
performance in seawater. The high alloy composition resists crevice
corrosion and pitting in oxidizing chloride solutions.
The low carbon content of the alloy defines it as an L grade, providing
resistance to intergranular corrosion in the as-welded condition.
Wrought alloy Al-6XN is approved by the ASME for use to 8008F (4278C)
in unfired pressure vessels under the ASME Boiler and Pressure Vessel
Code, Section 11.8, Division 1.
The corrosion resistant properties of alloy Al-6XN show exceptional
resistance to pitting, crevice attack, and stress cracking in high chloride
concentrations and general resistance in various acid, alkaline, and salt
solutions found in chemical processing and other industrial environments.

TABLE 11.6
Typical and Specified Chemical Composition of Alloy Al-6XN
Composition (wt%)
Chemical Element Typical Al-6XN Alloy UNS N08367 Specification

Carbon 0.02 0.03 max
Manganese 0.40 2.00 max
Phosphorus 0.020 0.040 max
Sulfur 0.001 0.030 max
Silicon 0.40 1.00 max
Chromium 20.5 20.00/22.00
Nickel 24.0 23.50/25.00
Molybdenum 6.2 6.00/7.00
Nitrogen 0.22 0.18/0.25
Copper 0.2 0.75 max
Iron Balance Balance


TABLE 11.7
Compatibility of Al-6XN Stainless Steel with Selected
Corrodents

Oxalic acid, 10% 210/99
Phosphoric acid, 20% 210/99
Sulfamic acid, 10% 210/99
Sulfuric acid, 10% X/X
Sodium bisulfate, 10% 210/99

Compatibility is shown to the maximum allowable temperature
for which data are available. Incompatibility is shown by an X.

Excellent resistance is shown to oxidizing chlorides, reducing solutions, and
seawater corrosion. Sulfuric, nitric, phosphoric, acetic, and formic acids can
be handled at various concentrations and a variety of temperatures. The
material is also approved for contact with foods. Refer to Table 11.7 for the
compatibility of alloy Al-6XN with selected corrodents.
Alloy Al-6XN ﬁnds applications as chemical process vessels and
pipelines, condensers, heat exchangers, power plant ﬂue-gas scrubbers,
distillation columns, service-water piping in nuclear plants, and food-
processing equipment.

11.9 Alloy 254SMo (S31254)
This is a superaustenitic stainless steel in the 6-moly alloy family that is
designed for maximum resistance to pitting and crevice corrosion. Its
chemical makeup will be found in Table 11.1.
The alloy has a PREN of 45.8. A value above 33 is considered necessary for
pitting and crevice resistance to ambient seawater. With its high levels of
chromium, molybdenum, and nitrogen, S31254 is especially suited for high-
chloride environments, such as brackish water, seawater, pulp mill bleach
plants, and other high-chloride process streams.


11.10 Alloy 25-6Mo (N08926)
This alloy is produced by Inco International. It is also known as 1925hMo
and has been assigned UNS N08926. Typical and specified compositions of
this alloy are shown in Table 11.8.
These alloys have higher mechanical properties than those of the
austenitic stainless steels such as 316L. They also have higher design values
than lower-strength materials, enabling the use of thinner sections.
One of the outstanding attributes of alloy 25-6Mo is its resistance to
environments containing chlorides or other halides. It is especially suited for
applications in high-chloride environments such as brackish water,
seawater, caustic chlorides, and pulp mill bleach systems.
The alloy offers excellent resistance to pitting and crevice corrosion,
having a PREN of 47.45. The critical pitting temperature for alloy 25-6Mo is
1408F (608C) or higher while the critical crevice temperature for alloy 25-6Mo
is 908F (32.58C).
In brackish and wastewater systems, microbially influenced corrosion
(MIC) can occur, especially in systems where equipment has been idle for
extended periods. A 6% molybdenum alloy offers protection from
manganese-bearing, sulfur-bearing, and generally reducing types of
bacteria. Because of its resistance to MIC, Alloy 25-6Mo is being used in
the wastewater piping systems of power plants.
In saturated sodium chloride environments and pH values of 6–8, alloy
25-6Mo exhibits a corrosion rate of less than 1 mpy. Even under more
aggressive oxidizing conditions involving sodium chlorate, alloy 25-6Mo
maintains a corrosion rate of less than 1 mpy and shows no pitting, even at
temperatures up to boiling.

TABLE 11.8
Typical and Specified Composition of Alloy 25-6Mo
Chemical Alloy 25-6Mo (wt%) UNS N08926 (wt%)

Carbon 0.02 max 0.02 max
Chromium 19.0–21.0 20.0–21.0
Nickel 24.0–26.0 24.5–25.5
Molybdenum 6.0–7.0 6.0–6.8
Nitrogen 0.15–0.25 0.18–0.20
Copper 0.5–1.5 0.8–1.0
Manganese 2.0 max 2.0 max
Phosphorus 0.030 max 0.030 max
Sulfur 0.010 max 0.010 max
Silicon 0.050 max 0.050 max


11.11 Alloy 31 (N08031)
The chemical composition of this alloy is:


Carbon 0.02 max
Nickel 31
Chromium 27
Molybdenum 6.5
Copper 1.8
Nitrogen 0.20
Iron Balance

With a 6.5% molybdenum content, alloy 31 exhibits excellent resistance to
pitting and crevice corrosion in neutral and acid solutions. The high
chromium content of 27% imparts superior resistance to corrosive attack by
oxidizing media. It has PREN of 54.45.

11.12 Alloy 654SMo (S32654)
Alloy 654 has about double the strength of type 316L stainless steel. This
alloy contains 7.4% molybdenum, which provides it with a corrosion
resistance associated with nickel-based alloys. The composition will be
found in Table 11.9.
Alloy 654 has better resistance to localized corrosion than other
superaustenitic alloys. Indications are that alloy 654 is as corrosion resistant
as alloy C276, based on tests in ﬁltered seawater, bleach plants, and other
aggressive chloride environments. It is intended to compete with titanium in
the handling of high-chloride environments.

TABLE 11.9
Chemical Composition of Alloy 654SMo (S32654)

Carbon 0.02 max
Chromium 24.0
Nickel 22.0
Molybdenum 7.3
Nitrogen 0.5
Copper 0.5
Manganese 3.0
Iron Balance


TABLE 11.10
Chemical Composition of Alloy 686 (N06686)

Tungsten 3.0–4.0
Titanium 0.02–0.25
Iron 5.0 max
Carbon 0.01 max
Manganese 0.75 max
Sulfur 0.02 max
Silicon 0.08 max
Phosphorus 0.04 max
Nickel Balance

11.13 Inconel Alloy 686 (N06686)
Inconel alloy 686 is an austenitic, nickel–chromium–molybdenum–tungsten
alloy. The chemical composition will be found in Table 11.10.
The alloy’s composition provides resistance to general corrosion, SCC,
pitting, and crevice corrosion in a broad range of aggressive environments.
The high nickel and molybdenum contents provide good corrosion resistance
in reducing environments, while the high chromium level imparts resistance
to oxidizing media. The molybdenum and tungsten also aid resistance to
localized corrosion such as pitting, while the low carbon content and other
composition controls helps minimize grain-boundary precipitates to
maintain resistance to corrosion in heat-affected zones of welded joints.
The ability of alloy 686 to resist pitting can be seen from its PREN of 51.
Alloy 686 has excellent resistance to mixed acids as well as reducing and
oxidizing acids, and to mixed acids containing high concentrations of
halides. Good resistance has been shown to mixed acid media having pH
levels of 1 or less and chloride levels in excess of 100,000 ppm.

Reference
Marcel Dekker.

12
Duplex Stainless Steel Family

The duplex stainless steels are those alloys whose microstructures are a
mixture of austenite and ferrite. These alloys were developed to improve
the corrosion resistance of the austenitic stainlesses, particularly in the areas
of chloride stress corrosion cracking (SCC) and in maintaining corrosion
resistance after welding. The original duplex stainlesses developed did not
meet all of the criteria desired. Consequently, additional research
was undertaken.
Duplex stainless steels have been available since the 1930s. The first-
generation duplex stainless steels, such as type 329 (S32900), exhibit good
general corrosion resistance because of their high chromium and molyb-
denum contents. When welded, however, these grades lose the optimal
balance of austenite and ferrite, and consequently corrosion resistance and
toughness are reduced. While these properties can be restored by a postweld
heat treatment, most of the applications of the first-generation duplexes use
fully annealed material without further welding. Because these materials do
not meet all of the criteria of duplex stainless steels, they have been included
in the chapter on austenitic stainless steels.
In the 1970s, this problem was made manageable through the use of
nitrogen as an alloy addition. The introduction of argon–oxygen decarbur-
ization (AOD) technology permitted the precise and economical control of
nitrogen in stainless steel. Although nitrogen was first used because it was
an inexpensive austenite former, replacing some nickel, it was quickly found
that it had other benefits. These include improved tensile properties and
pitting and crevice corrosion resistance.
The original duplex stainless steels did not have nitrogen added
specifically as an alloying ingredient. By adding 0.15–0.25% nitrogen, the
chromium partitioning between the two phases is reduced, resulting in the
pitting and crevice corrosion resistance of the austenite being improved. This
nitrogen addition also improves the weldability of the stainless steel without
losing any of its corrosion resistance.
Nitrogen also causes austenite to form from ferrite at a higher
temperature, allowing for restoration of an acceptable balance of austenite

197


and ferrite after a rapid thermal cycle in the heat-affected zone (HAZ) after
welding. This nitrogen enables the use of duplex grades in the as-welded
condition and has created the second generation of duplex stainless steels.
The duplex grades characteristically contain molybdenum and have a
structure approximately 50% ferrite and 50% austenite because of the excess
of ferrite-forming elements such as chromium and molybdenum. The duplex
structure, in combination with molybdenum, gives them improved
resistance to chloride-induced corrosion (pitting, crevice corrosion, and
SCC), in aqueous environments particularly.
However, the presence of ferrite is not an unmixed blessing. Ferrite may be
attacked selectively in reducing acids, sometimes aggravated by a galvanic
influence of the austenite phase, while the sigma phase produced by thermal
transformation (as by heat of welding) is susceptible to attack by strong
oxidizing acids. The duplex structure is subject to 8858F (4758C) embrittle-
ment and has poor NDIT properties. Except for temper embrittlement, these
problems can be minimized through corrosion testing and impact testing.
Because the stainless steels are a mixture of austenite and ferrite, it is only
logical that their physical properties would lie between the comparable
properties of these microstructures. The duplexes have better toughness
than ferritic grades and higher yield strengths than the austenitics.
Because the duplexes contain a large amount of ferrite, they are magnetic.
However, unlike the ferritics, they have a high degree of toughness along
with their high strength.
Because the duplexes have a higher yield strength than the austenitics,
they can provide certain economic advantages. Money can be saved using
thinner-walled sections for piping and vessels without sacrificing operating
pressures. Conversely, piping and equipment manufactured from these
stainless steels using conventional wall thicknesses can be operated at
higher pressures.
Although more formable than the ferritic alloys, they are not as ductile as
the austenitic family of alloys. Welding requires more care than with the
austenitic alloys due to a greater tendency to compositional segregation and
sensitivity to weld heat input.
Due to the high chromium contents, duplex alloys are sensitive to 8858F
(4758C) embrittlement. This generally limits their usage to 6008F (3138C)
maximum for pressure vessels. Due to the presence of nickel, chromium, and
molybdenum they are also susceptible to the formation of s phase. This is a
brittle phase that forms islands in the matrix and will affect mechanical
properties and corrosion resistance due to alloy depletion. The s phase
forms in the temperature range of 11008F (5938C)–16008F (8828C) and most
rapidly at about 14508F (7888C). The deleterious effects of s phase formation
are not obvious at the elevated temperature but can become a factor at room
temperature. The formation of s phase in these alloys is sufficiently rapid to
have an effect on properties due to slow cooling (air) after anneal. A
measurable effect as a result of exposure in this temperature range due to
welding has been demonstrated.

Duplex Stainless Steel Family 199

The high chromium and molybdenum contents of the duplex stainless
steels are particularly important in providing resistance in oxidizing
environments and are also responsible for the exceptionally good pitting
and crevice corrosion resistance, especially in chloride environments. In
general, these stainless steels have greater pitting resistance than type 316,
and several have an even greater resistance than alloy 904L. The critical
crevice corrosion temperature (CCT) of selected duplex stainless steels in
10% FeCl3$6H2O having a pH of 1 are shown below:

UNS Number Temperature (8F/8C)

S32900 41/5
S31200 41/5
S31260 50/10
S32950 60/15
S31803 63.5/17.5
S32250 72.5/22.5

The resistance to crevice corrosion of the duplexes is superior to the
resistance of the 300-series austenitics. They also provide an appreciably
greater resistance to SCC. Like 20Cb3, the duplexes are resistant to chloride
SCC in chloride-containing process streams and cooling water. However,
under very severe conditions, such as boiling magnesium chloride, the
duplexes will crack, as will alloy 20Cb3.
To achieve the desired microstructure, the nickel content of the duplexes is
below that of the austenitics. Because the nickel content is a factor for
providing corrosion resistance in reducing environments, the duplexes show
less resistance in these environments than do the austenitics. However, the
high chromium and molybdenum contents partially offset this loss, and
consequently they can be used in some reducing environments, particularly
dilute and cooler solutions. Although their corrosion resistance is good, the
boundary between acceptable and poor performance is sharper than with
austenitic materials. As a result, they should not be used under conditions
that operate close to the limits of their acceptability.
Duplex stainless steels are known best for the following performance
characteristics:

1. Lower life-cycle cost
2. High resistance to SCC
3. Excellent resistance to pitting and crevice corrosion
4. High resistance to erosion and general corrosion in many
environments
5. Very high mechanical strength
6. Low thermal expansion
7. Good weldability


Included among the duplex stainless steels are the following:

Alloy UNS Alloy UNS

2206 S31803 329 S32900
3RE60 S31500 7-MoPlus S32950
255 S32550 Z100 S32760
44LN S31200 DP3W S32740
DP-3 S31260 45D J93345
2304 S32304 CD4MCu J93370
2507 S32750 U-50 S32404

Of these, the four most commonly used are alloy 2205 (S31803), 7-MoPlus
(S32950), Z100 (S32760), and 255 (S32550). Each of these alloys will be
discussed in detail.

12.1 Alloy 2205 (S31803)
Alloy 2205 exhibits an excellent combination of both strength and corrosion
resistance. The chemical composition is shown in Table 12.1.
The approximate 50/50 ferrite–austenite structure provides excellent
chloride pitting and SCC resistance, with roughly twice the yield strength of
the standard austenitic grades.
The high chromium and molybdenum contents, coupled with the nitrogen
addition, provide general corrosion, pitting, and crevice corrosion resistance,
superior to those of type 316 and 317L.
When compared to type 316 stainless steel, alloy 2205 demonstrates
superior erosion–corrosion resistance. It is not subject to intergranular
corrosion in the welded condition.
Alloy 2205 resists oxidizing mineral acids and most organic acids in
addition to reducing acids, chloride environments, and hydrogen sulﬁde.

TABLE 12.1
Chemical Composition of Alloy 2205 Stainless Steel

Carbon 0.03 max
Manganese 2.00 max
Phosphorus 0.03 max
Sulfur 0.02 max
Silicon 1.00 max
Nickel 4.50–6.50
Nitrogen 0.14–0.20
Iron Balance


To achieve the desired microstructure, the nickel content of the duplex is
below that of the austenitics. Because the nickel content is a factor for
providing corrosion resistance in reducing environments the duplexes
show less resistance in these environments than do the austenitics.
However, the high chromium and molybdenum contents partially offset
the loss and consequently they can be used in some reducing
environments, particularly dilute and cooler solutions. Although their
corrosion resistance is good, the boundary between acceptable and poor
performance is sharper than with austenitic materials. As a result, they
should not be used under conditions that operate close to the limits of
their acceptability.
The following corrosion rates have been reported for alloy 2205:

Solution Corrosion Rate (mpy)

1% Hydrochloric acid, boiling 0.1
10% Sulfuric acid, 1508F/668C 1.2
10% Sulfuric acid, boiling 206
30% Phosphoric acid, boiling 1.6
85% Phosphoric acid, 1508F/668C 0.4
65% Nitric acid, boiling 21
10% Acetic acid, boiling 0.1
20% Formic acid, boiling 1.3
3% Sodium chloride, boiling 0.1

Alloy 2205 will be attacked by hydrochloric and hydroﬂuoric acids.
Applications are found primarily in oil and gas ﬁeld piping systems,
condensers, reboilers, and heat exchangers.

12.2 7-MoPlus (S32950)
7-MoPlus stainless steel is a trademark of Carpenter Technology. It is a two-
phase (duplex) alloy with approximately 45% austenite distributed within a
ferrite matrix. Alloy S32950 displays good resistance to chloride SCC, pitting
corrosion, and general corrosion in many severe environments. The chemical
composition is shown in Table 12.2.
This alloy is subject to 8558F (4758C) embrittlement when exposed for
extended periods of time between about 700–10008F (371–5388C).
7-MoPlus is also subject to precipitation of sigma phase when exposed
between 1250–15508F (677–8438C) for extended periods. Sigma phase
increases strength and hardness but decreases ductility and corrosion
resistance.


TABLE 12.2
Chemical Composition of Type 7-MoPlus Stainless Steel

Carbon 0.03 max
Manganese 2.00 max
Phosphorus 0.035 max
Sulfur 0.010 max
Silicon 0.60 max
Nickel 3.50–5.20
Iron Balance

The general corrosion resistance of 7-MoPlus stainless is superior to that
of stainless steels such as type 304 and type 316 in many environments.
Because of its high chromium content, it has good corrosion resistance in
strongly oxidizing media such as nitric acid. Molybdenum extends the
corrosion resistance into the less oxidizing environments. Chromium and
molybdenum impart a high level of resistance to pitting and crevice
corrosion. It has a PREN of 40.

12.3 Zeron 100 (S32760)
Zeron 100 is the trademark of Weir Materials Limited of Manchester,
England. Table 12.3 details the chemical composition of Zeron 100, which is

TABLE 12.3
Chemical Composition of Zeron 100 (S32760) Stainless Steel

Carbon 0.03 max
Manganese 1.00 max
Phosphorus 0.03 max
Sulfur 0.01 max
Silicon 1.00 max
Nickel 6.0–8.0
Copper 0.5–1.0
Nitrogen 0.2–0.3
Tungsten 0.5–1.0
Iron Balance


tightly controlled by Weir Materials, while the chemical composition of
S32760 is a broad compositional range.
Zeron 100 is a highly alloyed duplex stainless steel for use in aggressive
environments. In general, its properties include high resistance to pitting
and crevice corrosion, resistance to SCC in both chloride and sour
environments, resistance to erosion–corrosion and corrosion fatigue, and
excellent mechanical properties.
Zeron 100 is highly resistant to corrosion in a wide range of organic and
inorganic acids. Its excellent resistance to many nonoxidizing acids is the
result of the copper content.
A high resistance to pitting and crevice corrosion is also exhibited by
Zeron 100. It has a PREN of 48.2. Intergranular corrosion is not a
problem because the alloy is produced to a low carbon speciﬁcation and
water-quenched from solution annealing, which prevents the formation
of any harmful precipitates and eliminates the risk of intergranular
corrosion.
Resistance is also exhibited to SCC in chloride environments and process
environments containing hydrogen sulﬁde and carbon dioxide.

12.4 Ferralium 255 (S32550)
The chemical composition of Ferralium 255 is shown in Table 12.4. This is a
duplex alloy with austenite distributed within a ferrite matrix. This alloy has
a maximum service temperature of 5008F (2608C).

TABLE 12.4
Chemical Composition of Ferralium 255 (S32550)
Stainless Steel

Carbon 0.04
Manganese 1.50
Phosphorus 0.04
Sulfur 0.03
Silicon 1.00
Nickel 4.5–6.5
Copper 1.5–2.5
Nitrogen 0.1–0.25
Iron Balance


Ferralium exhibits good corrosion resistance to a variety of media, with a
level of resistance to chloride pitting and SCC. The following corrosion rates
of Ferralium 255 have been reported:

Solution Corrosion Rate (mpy)

1% Hydrochloric acid, boiling 0.1
10% Sulfuric acid, 1508F (668C) 0.2
10% Sulfuric acid, boiling 40
30% Phosphoric acid, boiling 0.2
85% Phosphoric acid, 1508F (668C) 0.1
65% Nitric acid, boiling 5
3% Sodium chloride, boiling 0.4

13
Precipitation-Hardening Stainless
Steel Family

This family of stainless alloys utilizes a thermal treatment to intentionally
precipitate phases that cause a strengthening of the alloy. The principle of
precipitation hardening is that a supercooled solid solution (solution-
annealed) changes its metallurgical structure on aging. The principal
advantage is that products can be fabricated in the annealed condition and
then strengthened by a relatively low temperature (900–15008F/462–6208C)
treatment, minimizing the problems associated with high temperature
treatments. Strength levels of up to 260 ksi (tensile) can be achieved—
exceeding even those of the martensitic stainless steels—while corrosion
resistance is usually superior—approaching that of type 304 stainless steel.
Ductility is similar to corresponding martensitic grades at the same
strength level.
The precipitating phase is generated through an alloy addition of one
or more of the following: niobium, titanium, copper, molybdenum, or
aluminum. The metallurgy is such that the material can be solution treated,
i.e., all alloying elements are in solid solution and the material is in its softest
or annealed state. In this condition the material can be machined, formed,
and welded to desired conﬁguration. After fabrication, the unit is exposed
to an elevated temperature cycle (aging) that precipitates the desired phases
to cause an increase in mechanical properties.
Precipitation-hardening stainless steels have high strength and relatively
good ductility and corrosion resistance at high temperatures. These steels
can attain very high strength levels. They reach these high strengths by
precipitation of intermetallic compounds via the same mechanism as that
found in aluminum alloys. These compounds are usually formed from iron
or nickel with titanium, aluminum, molybdenum, and copper. Typical
compounds are Ni3Al, Ni3Ti, and Ni3Mo. Chromium contents are in the
range of 13–17%. These steels have been around for several decades but are
now being recognized as a real alternative to the other stainless steels. They
have the good characteristics of the austenitic steels plus strength
approaching that of the martensitic steels. One of the early problems

205


centered around forging difﬁculties, but these problems have been overcome
to some extent.
Precipitation-hardenable (PH) stainless steels are themselves divided into
three alloy types: martensitic, austenitic, and semicaustenitic. An illustration
of the relationship between these alloys is shown in Figure 13.1. The
martensitic and austenitic PH stainless steels are directly hardened by
thermal treatment. The semiaustenitic stainless steels are supplied as an
unstable austenitic, which is the workable condition, and must be
transformed to martensite before aging.
On average, the general corrosion resistance is below that of type 304
stainless. However, the corrosion resistance of type PH 15-7 Mo alloy
approaches that of type 316 stainless. The martensitic and semiaustenitic
grades are resistant to chloride stress cracking. These materials are
susceptible to hydrogen embrittlement.
The PH steels ﬁnd a myriad of uses in small forged parts and even in
larger support members in aircraft designs. They have been considered for
landing gears. Many golf club heads are made from these steels by
investment casting techniques, and the manufacturers proudly advertise

S17400 S17700 S66286
Less Ni 18−8 with More Ni
Add Cu + Cb aluminum Add Mo + Ti

S15500 S15700
Less Cr Less Cr
Add Mo

S45000 S35000
More Ni Less Ni
Add Mo Add N,
Mo, Al

S13800
Less Cr
More Ni, Mo
No Cu
Add Al

Martensitic Semiaustenitic Austenitic

FIGURE 13.1
Precipitation-hardening stainless steels.

Precipitation-Hardening Stainless Steel Family 207

these clubs as being made from 17-4 stainless steel. Applications also include
fuel tanks, landing gear covers, pump parts, shafting bolts, saws, knives, and
ﬂexible bellows type expansion joints.

13.1 Alloy PHI3-8Mo (S13800)
PH13-8Mo is a registered trademark of Armco, Inc. It has a martensitic
precipitation/age-hardening stainless steel capable of high strength and
hardness along with good levels of resistance to both general corrosion and
stress corrosion cracking (SCC). The chemical composition is shown in
Table 13.1.
Generally, this alloy should be considered where high strength, toughness,
corrosion resistance, and resistance to SCC are required in a stainless steel
showing minimal direction-ability in properties.

13.2 Alloy 15-5PH (S15500)
Alloy 15-5PH, a martensitic precipitation-hardening stainless steel, is a
trademark of Armco, Inc. It provides a combination of high strength, good
corrosion resistance, good mechanical properties at temperatures up to 6008F
(3168C), and good toughness in both the longitudinal and transverse
directions in both the base metal and welds. The chemical composition is
As supplied from the mill in condition A, 15-5PH stainless steel can be
heat-treated at a variety of temperatures to develop a wide range
of properties.

TABLE 13.1
Chemical Composition of Alloy PH-13-8Mo (S13800)

Carbon 0.05
Manganese 0.10
Phosphorus 0.010
Sulfur 0.008
Silicon 0.10
Nickel 7.5–8.50
Aluminum 0.90–1.35
Nitrogen 0.010
Iron Balance


TABLE 13.2
Chemical Composition of Alloy 15-5PH (S15500)

Carbon 0.07 max
Manganese 1.00 max
Phosphorus 0.04 max
Sulfur 0.03 max
Silicon 1.00 max
Nickel 3.50–5.50
Copper 2.50–4.50
ColumbiumCtantalum 0.15–0.45
Iron Balance

Alloy 15-5PH in condition A exhibits useful mechanical properties. Tests
at Kure Beach, N.C. show excellent stress corrosion resistance after 14 years
of exposure. Condition A material has been used successfully in numerous
applications.
However, in critical applications, alloy 15-5PH should be used in the
precipitation-hardened condition rather than in condition A. Heat-treating
to the hardened condition, especially at the higher end of the temperature
range, stress-relieves the structure and may provide more reliable resistance
to SCC than condition A.
The general level of corrosion resistance of alloy 15-5PH exceeds that of
types 410 and 431, and is approximately equal to that of alloy-4PH. Very little
rusting is experienced when exposed to 5% salt fog at 958F (358C) for a period
of 500 h. When exposed to seacoast atmospheres, rust gradually develops. This
is similar to other precipitation-hardening stainless steels. The general level of
corrosion resistance of alloy 15-5PH stainless steel is best in the fully hardened
condition, and decreases slightly as the aging temperature is increased.

13.3 Alloy 17-4PH (S17400)
Alloy 17-4PH is a trademark of Armco, Inc. It is a martensitic-hardening
stainless steel that has a combination of high strength, good corrosion
resistance, good mechanical properties at temperatures up to 6008F (3168C),
good toughness in both base metal and welds, and short-time, low-
temperature heat treatments that minimize warpage and scaling. The
chemical composition will be found in Table 13.3.
As supplied from the mill in condition A, 17-4PH stainless steel can be heat-
treated at a variety of temperatures to develop a wide range of properties.
Alloy 17-4PH stainless steel exhibits useful mechanical properties in
condition A. Excellent stress corrosion resistance has been exhibited by this


TABLE 13.3

Carbon 0.07 max
Manganese 1.00 max
Phosphorus 0.04 max
Sulfur 0.03 max
Silicon 1.00 max
Nickel 3.00–5.00
Copper 3.00–5.00
Iron Balance

alloy after 14 years of exposure at Kure Beach, N.C. Condition A material has
been used successfully in numerous applications. However, in critical
applications, alloy 17-4PH stainless steel should be used in the precipitation-
hardened condition rather than in condition A. Heat-treating to the
hardened condition, especially at the higher end of the temperature range,
stress-relieves the structure and may provide more reliable resistance to SCC
than in condition A.
Alloy 17-4PH has excellent corrosion resistance. It withstands attacks
better than any of the standard hardenable stainless steels and is comparable
to type 304 in most media. It is equivalent to type 304 when exposed in rural
or mild atmospheres. However, when exposed in a seacoast atmosphere it
will gradually develop overall light rusting and pitting in all heat-treated
conditions.
This alloy is suitable for use in pump and motor shafting, provided that it
is operated continuously. As with other stainless steels, crevice attack will
occur when exposed to stagnant seawater for any length of time.
Table 13.4 shows the compatibility of alloy 17-4PH with selected
corrodents. A more comprehensive listing will be found in Reference [1].

13.4 Alloy 17-7PH (S17700)
This a semiaustenitic stainless steel. In the annealed or solution-annealed
condition it is austenitic (nonmagnetic), and in the aged or coldworked
condition it is martensitic (magnetic). The chemical composition is shown in
Table 13.5.
The alloy exhibits high strength in all conditions. Service over 10508F
(5658C) will cause overaging. Overaging may occur at lower temperatures,
depending on the temperature selected.


TABLE 13.4
Compatibility of 17-4PH Stainless Steel with Selected
Corrodents
Chemical (8F/8C)

Acetyl chloride 110/43
Acetylene 110/43
Allyl alcohol 90/32
Aluminum hydroxide 80/27
Aluminum nitrate 110/43
Aluminum potassium sulfate X
Aluminum sulfate X
Ammonia, anhydrous 270/132
Ammonium carbonate 110/43
Ammonium chloride X
Ammonium nitrate 130/54
Ammonium persulfate 130/54
Amyl acetate 90/32
Amyl alcohol 90/32
Amyl chloride 90/32
Aniline 170/71
Aniline hydrochloride X
Argon 210/99
Arsenic acid 130/54
Barium hydroxide 110/43
Beer 110/43
Beet sugar liquors 110/43
Benzene 130/54
Benzene sulfonic acid X
Benzoic acid 150/66
Benzyl alcohol 110/43
Boric acid 110/43
Bromine gas, dry X
Bromine liquid X
Butyl cellosolve 140/66
Calcium sulfate 150/54
Carbon dioxide, dry 210/99
Carbon dioxide, wet 210/99
Carbon monoxide 230/110
Carbon tetrachloride 150/66
Chloric acid, 20% X
(continued)



Chemical (8F/8C)

Chlorine liquid X
Chromic acid, 10% X
Chromic acid, 30% X
Chromic acid, 40% X
Chromic acid, 50% X
Ethyl alcohol 170/77
Ethyl chloride, dry 210/99
Ferric nitrate 150/66
Ferrous chloride X
Fluorine gas, dry 230/110
Heptane 130/54
Hydrobromic acid X
Hydrochloric acid X
Hydrocyanic acid X
Hydrogen sulﬁde, wet X
Iodine X
Magnesium hydroxide 140/66
Magnesium nitrate 130/54
Magnesium sulfate 130/54
Methylene chloride 130/54
Phenol 130/54
Phosphoric acid, 70% X
Phthalic acid 270/132
allowable temperature for which data are available. Incompatibility
is shown by X. When compatible, the corrosion rate is less than
20 mpy.

TABLE 13.5

Carbon 0.09 max
Aluminum 0.75–1.5
Nickel 6.5–7.75
Iron Balance


In the aged condition, the alloy is resistant to chloride cracking. Its corrosion
resistance, in general, is on a par with that of type 304 stainless steel.

13.5 Alloy 350 (S35000)
This is a chromium–nickel–molybdenum stainless alloy hardenable by
transformation and precipitation hardening. The chemical composition is
Alloy 350 normally contains 5–10% delta-ferrite, which aids weldability.
When heat-treated it has high strength. However, to achieve optimum
properties, a complex heat treatment is required including two subzero
exposures at K1008F (K738C). Unless cooled to subzero temperatures prior
to aging, the alloy may be subject to intergranular attack.
In general, the corrosion resistance of alloy 350 is similar to that of type
304 stainless steel. This alloy is used where high strength and corrosion
resistance at room temperatures are essential.

13.6 Alloy 355 (S35500)
Alloy 355 is a chromium–nickel–molybdenum stainless alloy hardenable by
martensitic transformation and precipitation hardening. The chemical
Depending on the heat treatment, the alloy may be austenitic with
formability similar to other austenitic stainless steels. Other heat treatments
yield a martensitic structure with high strength.
The alloy exhibits better corrosion resistance than other quench-hardenable
martensitic stainless steels. Service over 10008F (5388C) will cause overaging.

TABLE 13.6

Carbon 0.07–0.11
Phosphorus 0.04
Sulfur 0.03
Silicon 0.50
Nickel 4.00–5.00
Iron Balance


TABLE 13.7

Carbon 0.10–0.15
Phosphorus 0.04
Sulfur 0.03
Silicon 0.05
Nickel 4.00–5.00
Iron Balance

Overaging may occur at lower temperatures, depending on the tempering
temperature selected. Over-aged material is subject to intergranular
corrosion. A subzero treatment during heat treatment removes this
susceptibility.
Alloy 355 ﬁnds application where high strength is required at
intermediate temperatures.

13.7 Custom 450 (S45000)
Custom 450 is a trademark of Carpenter Technology Corp. It is a martensitic,
age-hardenable stainless steel with very good corrosion resistance and
moderate strength. Table 13.8 contains its chemical composition.
This alloy has high strength, good ductility and toughness, and is easily
fabricated. Unlike alloy 17-4, custom 450 can be used in the solution-
annealed condition.

TABLE 13.8
Chemical Composition of Custom 450 (S45000)

Carbon 0.05
Manganese 2.00
Phosphorus 0.03
Sulfur 0.03
Silicon 1.00
Nickel 5.00–7.00
Copper 1.25–1.75
Columbium 8!%C min
Iron Balance


The corrosion resistance of custom 450 stainless is similar to that of type
304 stainless steel. Custom 450 alloy is used in applications where type 304 is
not strong enough or where type 410 is insufﬁciently corrosion resistant.

13.8 Custom 455 (S45500)
Custom 455 is a registered trademark of Carpenter Technology Corp. It is a
martensitic, age-hardenable stainless steel, which is relatively soft and
formable in the annealed condition. A single-step aging treatment develops
exceptionally high yield strength with good ductility and toughness. The
chemical composition is shown in Table 13.9.
Custom 455 exhibits high strength with corrosion resistance better than
type 410 and approaching type 430. Service over 10508F (5658C) will cause
overaging. Overaging may occur at lower temperatures, depending on the
temperature selected.
This alloy may be susceptible to hydrogen embrittlement under some
conditions. Custom 455 should be considered when ease of fabrication, high
strength, and corrosion resistance are required. Custom 455 alloy is suitable
to be used in contact with nitric acid and alkalies. It also resists chloride SCC.
Materials, such as sulfuric acid, phosphoric acid, hydrochloric acid,
hydroﬂuoric acid, and seawater will attack custom 455.

13.9 Alloy 718 (N07718)
Alloy 718 is a precipitation-hardened, nickel-based alloy, designed to display
exceptionally high yield, tensile, and creep rupture properties up to 13008F

TABLE 13.9
Chemical Composition of Custom 455 (S45500)

Carbon 0.05
Manganese 0.50
Phosphorus 0.040
Sulfur 0.030
Silicon 0.50
Nickel 7.50–9.50
Copper 1.50–2.50
Molybdenum 0.50
Iron Balance


TABLE 13.10

Carbon 0.10
Manganese 0.35
Silicon 0.35
Phosphorus 0.015
Sulfur 0.015
NickelCcobalt 50.00–55.00
Boron 0.001–0.006
Copper 0.015
Iron Balance

(7048C). It can also be used as low as K4238F (K2538C). Table 13.10 shows the
chemical composition.
This alloy is readily fabricated and has excellent resistance to post-weld
cracking. Excellent oxidation resistance is displayed up to 18008F (9528C).
Alloy 718 is resistant to sulfuric acid, organic acids, and alkalies. It is also
resistant to chloride SCC. Hydrochloric, hydroﬂuoric, phosphoric and nitric
acids, and seawater will attack the alloy.
This alloy has been used for jet engines and high-speed airframe parts, such
as wheels, brackets, and spacers, and high-temperature bolts and fasteners.

13.10 Alloy A286 (S66286)
Alloy A286 is an austenitic, precipitation-hardenable stainless steel. Its
chemical composition will be found in Table 13.11.
Alloy A286 has excellent resistance to sulfuric and phosphoric acids and
good resistance to nitric acid and organic acids. It is also satisfactory for use
with salts, seawater, and alkalies.
This alloy has been used for gas turbine components and applications
requiring high strength and corrosion resistance.

13.11 Alloy X-750 (N07750)
This is a precipitation-hardening alloy that is highly resistant to chemical
corrosion and oxidation. The chemical composition is shown in Table 13.12.
Alloy NO7750 exhibits excellent properties down to cryogenic temperature


TABLE 13.11
Chemical Composition of Alloy A286 (S66286)

Carbon 0.08
Manganese 2.00
Silicon 1.00
Nickel 24.00–27.00
Vanadium 0.10–0.50
Aluminum 0.35
Boron 0.003–0.010
Iron Balance

and good corrosion and oxidation resistance up to 13008F (7048C). When
exposed to temperatures above 13008F (7048C), overaging results with a loss
of strength.
Alloy X-750 is resistant to sulfuric, hydrochloric, phosphoric, and organic
acids, as well as alkalies, salts, and seawater. It is also resistant to chloride SCC.
Hydrofluoric and nitric acids will attack the alloy.
The alloy finds applications where strength and corrosion resistance are
important, for example, as high-temperature structural members for jet
engine parts, heat-treating fixtures, and forming tools.

13.12 Pyromet Alloy 31
Pyromet alloy 31 is a trademark of Carpenter Technology Corp. It is a
precipitation-hardenable superalloy that exhibits corrosion resistance and

TABLE 13.12
Chemical Composition of Alloy X-750 (N07750)

Carbon 0.08
NickelCcolumbium 70.00
Manganese 0.30
Sulfur 0.010
Silicon 0.50
Copper 0.05
Iron 5.0–9.0


strength to 15008F (8168C). It is resistant to sour brines and hot sulfidation
attack.
Applications include hardware in coal gasification units. It has a chemical
composition as follows:


Carbon 0.04
Manganese 0.20
Silicon 0.20
Phosphorus 0.015
Sulfur 0.015
Chromium 27.7
Nickel 55.5
Molybdenum 2.0
Titanium 2.5
Aluminum 1.5
Columbium 1.1
Boron 0.005
Iron Balance

13.13 Pyromet Alloy CTX-1
Pryomet alloy CTX-1 is a trademark of Carpenter Technology Corp. The
alloy is a high-strength, precipitation-hardening superalloy having a low
coefficient of expansion with high strength at temperatures to 12008F (6498C).
Applications include gas turbine components and hot work dies.
If exposed to atmospheric conditions above 10008F (5388C), a protective
coating must be applied to the alloy. The chemical composition is as follows:


Carbon 0.05
Manganese 0.50
Silicon 0.50
Phosphorus 0.015
Sulfur 0.015
Chromium 0.50
Molybdenum 0.20
Copper 0.50
Nickel 38.00–40.00
Columbium and tantalum 2.50–3.50
Boron 0.0075
Cobalt 14.00–16.00
Iron Balance


This is a low-expansion, high-strength, precipitation-hardenable superalloy.
It has significant improvement in notch stress-rupture strength over
pyromet CTX-1. As with alloy CTX-1, a protective coating must be applied
if the alloy is to be exposed at atmospheric conditions above 10008F (5388C).
Applications include gas turbine components. It has the following chemical
composition:


Carbon 0.05
Manganese 0.50
Silicon 0.50
Phosphorus 0.015
Sulfur 0.015
Chromium 0.50
Nickel 37.00–39.00
Copper 0.50
Cobalt 13.00–15.00
Columbium and tantalum 4.50–5.50
Aluminum 0.25
Boron 0.012
Iron Balance

Alloy CTX-909 is a high-strength, precipitation-hardenable super-alloy that
offers significant improvements over alloys CTX-1 and CTX-3 due to its
combination of tensile properties and stress-rupture strength to 12008F
(6498C) in the recrystallized condition combined with the use of common
age-hardening treatments.
The alloy exhibits a low and relatively constant coefficient of thermal
expansion over a broad temperature range, a high hot hardness, and good
thermal fatigue resistance. As with other CTX alloys, a protective coating is
required if the alloy is exposed to atmospheric conditions above 10008F
(5388C).


The chemical composition is as follows:


Carbon 0.06
Manganese 0.50
Silicon 0.40 nom.
Phosphorus 0.015
Sulfur 0.015
Chromium 0.50
Nickel 38.00 nom.
Cobalt 14.00 nom.
Titanium 1.60 nom.
ColumbiumCtantalum 4.90 nom.
Aluminum 0.15
Copper 0.50
Boron 0.012
Iron Balance

13.16 Pyromet Alloy V-57
This is an iron-based, austenitic, precipitation-hardening alloy for parts
requiring high strength and good corrosion resistance at operating temper-
atures to 14008F (7608). It is produced by Carpenter Technology Corp.
Chemically, it has the following composition:


Carbon 0.08
Manganese 0.35
Silicon 050
Phosphorus 0.015
Sulfur 0.015
Nickel 22.50–28.50
Vanadium 0.50
Boron 0.005–0.012
Iron Balance


13.17 Thermospan Alloy
Thermospan alloy is a trademark of Carpenter Technology Corp. It is a
precipitation-hardenable superalloy having an excellent combination of
tensile properties and stress-rupture strength in the recrystallized condition
with the use of common solution and age-hardening treatments. The alloy
also exhibits a low coefﬁcient of expansion over a broad temperature range.
As a result of the chromium addition, signiﬁcant improvements in
environmental resistance over that of the CTX alloys is realized.
The alloy should be considered for applications in which other current
low-expansion superalloys are presently used, such as compressor and
exhaust casings, seals, and other gas turbine engine components.
The alloy has the following composition:


Carbon 0.05
Manganese 0.50
Silicon 0.30
Phosphorus 0.015
Sulfur 0.015
Chromium 5.50
Nickel 25.0
Cobalt 29.0
Titanium 0.80
Columbium 4.80
Aluminum 0.50
Copper 0.50
Boron 0.01
Iron Balance

References
Marcel Dekker.
2. G.T. Murray. 1993. Introduction to Engineering Materials, New York: Marcel
Dekker.

14
Cast Stainless Steel Alloys

Most wrought alloy compositions are also available in an equivalent grade
casting. In addition, there are many alloy castings available in compositions
that are not available as wrought materials. This is the result of the design
freedom associated with the various casting processes. The compositions of
many cast grades are modified relative to their wrought equivalents to take
advantage of the casting process because little or no mechanical working of
the cast component will be necessary. Because of this ability to modify the
alloy compositions of the cast grades, improved and unique properties may
be imparted to some cast alloys compared to their wrought equivalents.
Various alloy designation systems have been developed to separate the
wrought alloys from the cast alloys because of the variations in compositions
and the resulting variations in physical and mechanical properties. Three
designation systems are presently used for the identification of alloys:
Unified Numbering System (UNS), Alloy Casting Institute (ACI), and
American Society for Testing and Materials (ASTM).
The UNS was developed by the Society of Automotive Engineers (SAE)
and ASTM. Under the UNS system, metals and alloys are divided into 18
series. The designations start with a single letter followed by five numerical
digits. As much as possible, the letter is suggestive of the family of metals it
identifies, for example, A is for aluminum alloys, C is for copper alloys, N is
for nickel alloys, and S is for stainless alloys. A complete listing of the letters
used will be found in Table 14.1. If possible, common designations are used
within the five numerical digits for user convenience, for example, A92024 is
for 2024 aluminum, C36000 for copper alloy 360, and S31600 for type 316
stainless steel.
The UNS system is more commonly used for wrought materials. ACI
designations are more appropriate for cast alloys because the designations
are more indicative of the compositions. Most ACI designations begin with
two letters followed by two or three numerical digits. Some may also end
with additional letters and/or numerical digits. In general, the designations
begin with either a C for corrosion-resistant materials or an H for heat-
resistant materials. The second letter in the designation ranges from A to Z,

221


TABLE 14.1
Letter Prefixes Used in the Unified Numbering System
Prefix Alloy Series

A Aluminum and aluminum alloys
C Copper and copper alloys
D Steels with specified mechanical properties
E Rare earths and rare earth-like metals and alloys
F Cast irons
G Carbon and alloy steels
H AISI H-steels (hardenability controlled)
J Cast steels (except tool steels)
K Miscellaneous steels and ferrous alloys
L Low-melting metals and alloys
M Miscellaneous nonferrous metals and alloys
N Nickel and nickel alloys
P Precious metals and alloys
R Reactive and refractory metals and alloys
S Heat and corrosion resistant (stainless) steels
T Tool steels
W Welding filler materials
Z Zinc and zinc alloys

depending upon the nickel and, to a lesser degree, the chromium content. For
example, a corrosion resistant material with 12% chromium and no nickel
begins with CA; an alloy with 100% nickel begins with CZ; alloys inbetween
have intermediate letters. The maximum carbon content is indicated by the
numerical digits (percent!100). Additional letters following the numerical
digits indicate the presence of other alloying ingredients. Table 14.2
gives examples.
There are two groups of materials whose designations do not follow the
scheme shown in Section 17 Table 2. Nickel–copper materials use M as

TABLE 14.2
Examples of ACI Designations
Alloying Elements (wt%)
Other Alloying
Chromium, Nickel, Elements,
Designation Nominal Nominal Carbon, Max Nominal

CA 15 12 — 0.15 —
CD 4MCu 25 6 0.04 Mo 3, Cu 3
CF 8M 19 10 0.08 Mo 2.5
CF 3M 19 10 0.03 Mo 2.5
CN 7M 21 29 0.07 Mo 2.5
CW 2M 16 68 0.02 Mo 16
CZ 100 0 100 1.0 —
HK 40 25 20 0.40 —

Cast Stainless Steel Alloys 223

the first letter (examples are M35-1 and M25S). Nickel–molybdenum alloys
begin with the letter N, such as N7M and N12MV. Although the ACI is no
longer in existence, the system has been adopted by ASTM and appropriate
ASTM committees assign designations for new cast alloys.
In addition to the UNS designations previously described, there is also a
series of UNS designations specifically for cast materials. Table 14.3 lists
several alloys giving both ACI and UNS designations.
ASTM designations are used for many special carbon and alloy steel
products and for cast iron.
Castings have several advantages over wrought materials. Among the
advantages are:

1. Unlimited freedom on design configuration
2. Minimization or elimination of machining and material waste
3. Wide range of alloy choice
4. Mechanical property isotropy
5. Production economies

However, there are limitations—the most serious of which is variations in
quality from casting to casting and foundry to foundry. Potential quality
shortcomings involve:

1. Surface finish
2. Compositional purity
3. Internal integrity
4. Dimensional control

TABLE 14.3
ACI and UNS Designations for Stainless Steel Castings (Nominal Weight Percent)
ACI UNS Cr Ni (Max) Other

CAa — 12 1 C
CBa — 19 2 C
CD 4MCu J93370 26 6 C 0.04 max, Mo 2, Cu 3
CF 8 J92600 19 11 C 0.08 max
CF 3 J92500 18 12 C 0.03 max
CF 8M J92900 19 12 C 0.08 max, Mo 2
CF 3M J92800 18 13 C 0.03 max, Mo 2
CH 20 J93402 23 15 C 0.20 max
CK 20 J94202 24 22 C 0.20 max
CN 7M N08007 20 21 C 0.07 max, Mo 2, Cu 3
HKb — 24 20 C 0.60 max
a
Maximum carbon.
b
G0.05% Carbon.


These limitations can be overcome through application of sound foundry
practices.
It should be kept in mind that specifications for castings should be based
on ACI designations. A specification such as cast type 316 stainless steel
should never be used because a foundry might pour just that from bar stock
(to meet your specification) without regard to the proper balance
of constituents.
Stainless steels (SST) are ferrous alloys with a minimum of 12% chromium.
The chromium forms a uniform, adherent chromium oxide film, providing
greatly improved corrosion resistance compared to carbon and low-alloy
steel. SSTs also contain varying amounts of nickel, molybdenum, nitrogen,
copper, and/or other elements. The widely different compositions result in a
range of properties. To review their corrosion properties, they will be
grouped as follows: martensitic, ferritic, austenitic, superaustenitic, precipi-
tation-hardenable, and duplex stainless steels. The Avesta Sheffield Corrosion
Handbook is a good general reference for SSTs.1

14.1 Martensitic Stainless Steels
Martensitic stainless steels were the original SST’s developed in the early
1990s. Since then, a range of martensitic grades have been developed.
The main advantages they offer are low cost and the ability to be hardened
for wear resistance. The martensitic grades can be heat-treated similar to
the low-alloy steels to produce hardnesses, varying by grade, as high as
60 HRC.
Cast CA15 is the modern version of the original 12%-chromium stainless
steel. CA15 is often replaced by a newer grade called CA6NM. CA6NM
is modified (with additions of nickel and molybdenum) for improved
castability, mechanical properties, low-temperature toughness, and resist-
ance to sulfide stress cracking (SSC). CA28MWV is also a modified 410 with
improved high temperature strength. CA40F is the free machining version of
420 SST.
The martensitic grades are resistant to corrosion in mild atmospheres,
water, steam, and other nonsevere environments (Table 14.4). They will
quickly rust in marine and humid industrial atmospheres, and are attacked
by most inorganic acids. They are susceptible to several forms of Stress
Corrosion Cracking (SCC) when used at high hardness levels. Hardened
martensitic SSTs have poor resistance to sour environments and may crack in
humid industrial atmospheres. In the quenched and fully tempered condition
(usually below 25 HRC), SCC resistance is greatly improved, especially for
CA6NM. The martensitic grades are generally less corrosion-resistant than
the austenitic grades.


TABLE 14.4
Chemical Composition of Cast Martensitic Stainless Alloys
Alloy (wt%)
Chemical CA 6NM CA 15 CA 15M CA 28MWV CA 40

Carbon 0.06 0.05 0.15 0.2–0.28 0.20–0.40
Manganese 1.00 1.00 1.00 — 1.00
Silicon 1.00 1.50 0.65 — 1.50
Phosphorus 0.04 0.04 0.04 — 0.04
Sulfur 0.03 0.04 0.04 — 0.04
Chromium 11.5–14.0 11.5–14.0 11.5–14.0 11.0–12.5 11.5–14.0
Nickel 3.5–4.5 1.00 1.00 — 1.0
Molybdenum 0.40–1.0 0.50 0.15–1.0 0.9–1.25 0.5
Tungsten — — — 0.9–1.25 —
Vanadium — — — 0.2–0.3 —
Iron Balance Balance Balance Balance Balance

Maximum unless otherwise indicated.

14.2 Ferritic Stainless Steels
When the chemistry of stainless steel is properly balanced, the structure will
be ferritic at room temperature just like a plain carbon steel. The ferritic SSTs
have properties much different from those of the austenitic SSTs (see
Table 14.5), some of which can be very advantageous in certain applications.
The two most common cast ferritic SSTs are CB30 and CC50. These alloys
have very poor impact resistance compared to the cast austenitic grades.
Due to the formation of a brittle s phase at elevated temperatures, most
ferritic SSTs are limited to use below about 6508F (3438C). In general, ferritic

TABLE 14.5
Chemical Composition of Cast Ferritic Stainless Steels
Alloy (wt%)
Chemical CB 30 CC 50

Carbon 0.3 0.5
Manganese 1.00 1.00
Silicon 1.50 1.50
Phosphorus 0.04 0.04
Sulfur 0.04 0.04
Chromium 18.0–21.0 26.0–30.0
Nickel 2.00 4.00
Maximum unless otherwise noted.


SSTs have poor weldability. There are few instances where these materials
would be preferred over an austenitic SST.
CB30 is resistant to nitric acid, alkaline solutions, and many inorganic
chemicals. CC50 is used for dilute sulfuric acid, mixed nitric and sulfuric
acids, and various oxidizing acids. Their resistance to chloride SCC is better
than austenitic SSTs due to their low nickel contents.

14.3 Austenitic Stainless Steels
The early austenitic stainless steels had compositions of approximately 18%
chromium and 8% nickel, and were commonly called “18-8” SST. Austenitic
SSTs have much better general corrosion resistance than the 12%-chromium
SSTs (Table 14.6). While the wrought austenitic SSTs have complete
austenitic structures in the annealed condition, the castings are chemically
balanced to form some ferrite as they solidify. The ferrite is necessary to
prevent hot cracking of the castings. While most grades contain at least 5%
ferrite and weakly attract a magnet, it is not unusual for CG8M to contain as
much as 30% ferrite and strongly attract a magnet. Another benefit is that the
ferrite phase is resistant to SCC in some environments, and its presence can
retard cracking.
CF8M is the most widely used cast stainless steel. CF8M is the cast
equivalent of 316. CF8M and 316 have a good balance of corrosion resistance,
availability, strength, and cost. Although 304 is considered the standard
wrought SST, CF8M is the standard cast SST. CF8 castings are more
expensive than CF8M and should only be specified when CF8M cannot be
used. Most other cast SSTs are used for specific niches where a small
compositional difference gives better performance in that application.
CF8M has excellent corrosion resistance in normal atmospheric con-
ditions, including seacoast exposure. At worst, some slight staining may
develop. It resists most water and brines at ambient temperature. Seawater
may cause pitting corrosion particularly under low-flow or stagnant
conditions, or at elevated temperatures. CF8M is used for 80–100% sulfuric
acid at ambient temperature.
Corrosion is reduced further under oxidizing conditions, such as small
additions of nitric acid, air, or copper salts. CF8M has good resistance to
phosphoric acid at all concentrations up to 1708F (778C). It is used for nitric
acid up to boiling at all concentrations to 65%. CF8M resists attack by most
organic acids including acetic, formic, and oxalic acids, at all concentrations
at ambient temperature. It is used for citric acid at all concentrations. It is not
attacked by organic solvents; however, chlorinated organics may attack
CF8M, especially under condensing conditions or when water is present.
CF8M resists many alkaline solutions and alkaline salts; ammonium
hydroxide at all concentrations to boiling and sodium hydroxide at all
concentrations up to 1508F (658C) above which SCC may occur.2

TABLE 14.6
Chemical Composition of Cast Austenitic Stainless Steels
Chemical (wt%)
Alloy C Mn Si P S Cr Ni Mo Other

CE 30 0.30 1.50 2.00 0.04 0.04 26.0–30.0 8.0–11.0 — —
Cast Stainless Steel Alloys

CF 3 0.03 1.50 2.00 0.04 0.04 12.0–21.0 8.0–12.0 0.5 —
CF 3A 0.03 1.50 2.00 0.04 0.04 17.0–21.0 8.0–12.0 0.5 —
CF 3M 0.03 1.50 1.50 0.04 0.04 17.0–21.0 9.0–13.0 2.0–3.0
CF 8 0.08 1.50 2.00 0.04 0.04 18.0–21.0 8.0–11.0 0.5 —
CF 8A 0.08 1.50 2.00 0.04 0.04 18.0–21.0 8.0–11.0 0.5 —
CF 20 0.20 1.50 2.00 0.04 0.04 18.0–21.0 8.0–11.0 — —
CF 3MA 0.03 1.50 1.50 0.04 0.04 12.0–21.0 9.0–13.0 2.0–3.0 —
CF 8M 0.08 1.50 2.00 0.04 0.04 18.0–21.0 9.0–12.0 2.0–3.0 —
CF 8C 0.08 1.50 2.00 0.04 0.04 18.0–21.0 9.0–12.0 0.5 8 ! C Cb, 1.0 Cb
CF 10MC 0.10 1.50 1.50 0.04 0.04 15.0–18.0 13.0–16.0 1.75–2.25 10 ! C Cba, 1.2 Cb
CF 10SMnN 0.1 7–9 3.5–4.5 — — 16.0–18.0 8.0–9.0 — 0.08–0.18 N
CF 16F 0.16 1.50 2.00 0.17 0.04 18.0–21.0 9.0–12.0 1.50 0.20–0.35 Se
CG 6MMn 0.06 4–6 — — — 20.5–23.5 11.5–13.5 1.5–3 0.1–0.3 Cb, 0.1–0.3 V,
0.2–0.4 N
CG 8M 0.08 1.50 1.50 0.04 0.04 18.0–21.0 9.0–13.0 3.0–4.0
CG 12 0.12 1.50 2.00 0.04 0.04 20.0–23.0 10.0–13.0 — —
CH 20 0.20 1.50 2.00 0.04 0.04 22.0–26.0 12.0–15.0 0.05 —
CK 20 0.20 2.00 2.00 0.04 0.04 23.0–27.0 19.0–22.0 0.05 —
Maximum unless otherwise speciﬁed; iron balance in all cases.
a
Minimum.
227


Metallic chloride salts, such as ferric chloride and cupric chloride, can be
very corrosive to CF8M. Above 1608F (718C), chloride can also cause SCC.
The combination of chlorides, water, oxygen, and surface tensile stress can
result in cracking at stresses far below the tensile strength of all austenitic
SSTs. Although a threshold chloride level may exist, one is difficult to set
because chlorides concentrate in pits, crevices, and under deposits until the
minimum concentration is reached. One must be concerned about SCC any
time a few hundred ppm chlorides is present and the temperature exceeds
about 1608F (718C). SCC may develop at lower temperatures if the pH is low.
Sensitization of austenitic SSTs develops from exposure to temperatures
between 950 and 14508F (510 and 7888C). Chromium carbides form at the
grain boundaries, leaving a zone that is chromium-depleted. In aggressive
environments, the grain boundaries are corroded. This is called intergranular
attack (IGA). When attack surrounds an entire grain, grain dropping occurs,
resulting in extremely high rates of attack. Welding can also produce
sensitization in the weld and in the heat-affected zone (HAZ). In most
applications, this attack can be prevented by welding with low-carbon filler
material and using minimal heat input. Only the most aggressive
environments will produce IGA.
CF3M is the cast equivalent of 316L. It has a maximum carbon content of
0.03% vs. 0.08% for CF8M. With !0.03% carbon, sensitization is largely
eliminated. CF3M can be specified for applications where IGA has been a
problem. With today’s improved foundry technology, many heats of CF8M
are at or near the 0.03% carbon limit.
Molybdenum is added to SSTs to increase pitting resistance. Molybdenum
makes the surface oxide layer tougher, so that chlorides and other pitting
agents are less likely to break it down. CF3 and CF8 are the cast equivalents
of 304L and 304, respectively. CF3 and CF8 contain a maximum
molybdenum content of 0.5% vs. the 2–3% molybdenum of CF3M and
CF8M, which sacrifices pitting resistance and general corrosion resistance in
some environments. In strongly oxidizing environments, the lower
molybdenum of CF8 provides superior corrosion resistance. CF3 and 304L
are the standard materials in hot, concentrated nitric acid. CF8 and 304 are
not generally used because they are more susceptible to IGA in nitric acid.
Cast CF20 (cast equivalent of 302) is the modern version of the original
18-8 composition. CF16F (cast equivalent of 303) is a free-machining version
of CF20. The lower alloy content in these grades sacrifices some corrosion
resistance. The added sulfur reduces resistance further due to the galvanic
effects between the matrix and the manganese sulfide inclusions.
CF8C (cast equivalent of 347) contains columbium to stabilize the material
against chromium carbide formation. A narrow line of attack adjacent
to a weld can occur if the casting is not properly heat-treated. CF8C must
be solution heat-treated at 1950–20488F (1066–11208C) and stabilized at
1598–16528F (870–9008C).3 The corrosion resistance of CF8C is about the
same as that for CF3 and CF8.


CG8M (cast equivalent of type 317) is essentially a modified CF8M. The
chromium, nickel, and molybdenum contents are all increased slightly,
imparting better overall corrosion and pitting resistance. CG8M is widely
used in the pulp and paper industry, where it better resists the attack
from pulping liquors and bleach-containing water. These applications
are becoming increasingly corrosive and even higher grades of SST are
often needed.
CG6MMN is the cast equivalent of Nitronic 50 (trademark of Armco, Inc.).
It is a nitrogen-strengthened alloy with 22% chromium, 13% nickel, 5%
manganese, and 2.2% molybdenum. The material is used in place of CF8M
when higher strength and/or better corrosion resistance is needed.
CF10SMnN is the cast equivalent of Nitronic 60 (trademark of Armco,
Inc.). It has better galling resistance than the other CF grades. The corrosion
resistance is similar to CF8 but not as good in hot, nitric acid.
Austenitic SST castings are purchased to three specifications. ASTM A743
and A744 are used for general applications and A351 is used for pressure-
retaining castings. For critical applications, additional specifications may be
necessary. Items that may be addressed include filler material, interpass
temperature, solution heat-treating temperature, quench method, surface
condition, nondestructive examination, etc.4

14.4 Superaustenitic Stainless Steels
Austenitic SSTs with alloying element contents (particularly nickel and/or
molybdenum) higher than the conventional 300-series SSTs are commonly
categorized as “superaustenitic” SSTs (see Table 14.7). In some cases, they

TABLE 14.7
Chemical Composition of Cast Superaustenitic Stainless Steel
Alloy (wt%)
Chemical CD 7M CN 7MS CK 3MCuN CE 3MN CUSMCuC

Carbon 0.07 0.07 0.025 0.03 0.05
Manganese 1.50 — — — —
Silicon 1.50 — — — —
Phosphorus 0.04 — — — —
Sulfur 0.04 — — — —
Chromium 19.0–22.0 18.0–20.0 19.5–20.5 20.0–22.0 19.5–23.5
Nickel 27.5–30.5 22.0–25.0 17.5–19.5 23.5–25.5 38.0–46.0
Molybdenum 2.0–3.0 2.5–3.0 6.0–7.0 6.0–7.0 2.50–3.50
Copper 3.0–4.0 1.5–2.0 0.5–1.0 — 1.50–3.50
Nitrogen — — 0.18–0.24 0.18–0.26 —
Columbium — — — — 0.6–1.2
Iron Balance Balance Balance — —



have even been classified as nickel alloys. These alloys typically contain
16–25% Cr, 30–35% Ni, Mo, and N; some also contain Cu. No single element
exceeds 50%.5 The additional nickel provides added resistance to reducing
environments and the additional molybdenum, copper, and nitrogen boost
the resistance to pitting in chlorides. Even in the cast form, these alloys are
fully austenitic, making them considerably more difficult to cast than the
ferrite-containing austenitic grades. Foundry experience and expertise is
critical in casting superaustenitics.
CK3MCuN and CE3MN are the cast equivalents of Avesta 254SMO
(trademark of Avesta AB) and AL6XN (trademark of Allegheny Ludlum,
Inc.), respectively. They are part of the “6 Mo” superaustenitic family. These
alloys have complete resistance to freshwater, steam, boiler feed water,
atmospheric and marine environments. They also have excellent resistance
to phosphoric, dilute sulfuric, and many other acids and salts. They are
highly resistant to acetic, formic, and other organic acids and compounds.6
Superaustenitics are particularly suitable for high-temperature, chloride-
containing environments where pitting and SCC are common causes of
failure with other SSTs. Resistance to chloride SCC extends beyond 2508F
(1218C). They also have excellent resistance to sulfide stress cracking.
CK3MCuN will resist pitting in 6% FeCl3 (60,000 ppm Cl) at 1048F (408C)
while the conventional SSTs will pit at ambient temperature.7 In some
applications, superaustenitic SSTs can be used instead of nickel-based alloys
at a lower cost.8
CN7M, commonly called alloy 20, is the cast equivalent of Carpenter
20Cb3 (trademark of Carpenter Technology). This is the industry-standard
alloy for sulfuric acid. CN7MS is a modified version.9 They have useful
resistance over most of the sulfuric acid concentration range below 1608F
(718C) and below 10% to the boiling point. They have excellent resistance to
chloride SCC. Although the ASTM specifications permit up to 0.07% carbon,
0.03% maximum is recommended.10 CK3MCuN and CE3MN are superior
for chloride environments.
CU5MCuC is the cast version of Incoloy 825 (trademark of Inco Alloys
International) although columbium is substituted for titanium. Titanium will
oxidize rapidly during air melting; columbium will not. CU5MCuC has
corrosion resistance and weldability similar to CN7M. It has equal corrosion
resistance in sulfuric, nitric, and phosphoric acids, seawater, and other
environments. It is also highly resistant to chloride SCC.
Weld procedures for superaustenitic SSTs must be carefully developed to
preserve the special corrosion properties. Heat input must be kept to a
minimum, and interpass temperatures must be in the 250–3508F (121–1778C)
range. Overmatching weld filler materials are generally used for weld repairs
and fabrication welds.11 American Welding Society (AWS) filler metal
grades NiCrMo-3, NiCrMo-7, NiCrMo-10, and NiCrMo-12 are the most
commonly used grades.12,13 Welding with matching filler requires re-solution
heat treatment after all welding. Autogenous welding (without filler material)


should never be performed on these materials. AWS 320LR weld filler is
normally used on CN7M.

14.5 Precipitation-Hardening Stainless Steels
CB7Cu-1 and CB7Cu-2 are the cast versions of 17-4PH and 15-5PH
(trademarks of Armco Steel). These are high-strength, precipitation-
hardening, martensitic SSTs (Table 14.8). Although there are many other
wrought precipitation-hardening SSTs, these are the only two cast alloys
covered by ASTM specifications. Typically, these materials are cast, solution
heat-treated, machined, and then aged. CB7Cu-1 is more commonly cast
than CB7Cu-2.
Cast CB7Cu-1 was recently added to the NACE standard MR0175 for
nonpressure-containing, internal valve and pressure regulator components.
It is acceptable for sour service in the H1150 DBL condition to a maximum
hardness of 310 HB (30 HRC). For both alloys, the higher hardness
conditions are quite susceptible to SCC. SCC resistance improves with
increasing aging temperature and decreasing strength and hardness.
The corrosion resistance of these alloys is similar to CF8 and 304 and better
than the 400-series SSTs.14 CB7Cu-1 and CB7Cu-2 resist atmospheric attack
in all but the most severe environments. They are resistant to natural water,
except seawater, where pitting can be expected. They are widely used in
steam, boiler feed water, condensate, and dry gases.

14.6 Duplex Stainless Steels
When the chemistry of a stainless steel is adjusted properly, both ferrite
and austenite will be present at room temperature. SSTs with approximately
50% austenite and 50% ferrite are called duplex SSTs (see Table 14.9).
The popularity of these materials has increased rapidly in recent years

TABLE 14.8
Cast Precipitation Hardening Stainless Steels
Specification and Wrought Other
Grade Equivalent C Max Cr Ni Elements

ASTM A747 Grade 17-4PH 0.07 15.5–17.7 3.6–4.6 Cu 2.5–3.2, Cb
CB7Cu-1 0.15–0.35
ASTM A747 Grade 15-5PH 0.07 14–15.5 4.5–5.5 Cu 2.5–3.2, Cb
CB7Cu-2 0.15–0.35


TABLE 14.9
Chemical Composition of Cast Duplex Stainless Steel
Alloy (wt %)
Chemical CD 4MCu CD 3MN CD 3MWN Z6CNDU20.08M

Carbon 0.04 0.03 0.03 0.08
Manganese 1.00 — — —
Silicon 1.00 — — —
Phosphorus 0.04 — — —
Sulfur 0.04 — — —
Chromium 24.5–26.5 21–23.5 24–26 19–23
Nickel 4.75–6.00 4.5–6.5 6.5–8.5 7–9
Molybdenum 1.75–2.25 2.5–3.5 3–4 2.3
Copper 2.75–3.25 — 0.0–1 1.2
Nitrogen — 0.1–0.3 0.2–0.3 —
Tungsten — — 0.5–1 —
Iron Balance Balance Balance Balance


because they offer superior corrosion resistance and higher yield strength
than the austenitic SSTs with a lower alloy content. Due to the formation of s
phase at elevated temperatures, duplex SSTs are limited to a maximum
service temperature of 5008F (2608C). The formation of s phase adversely
affects both toughness and corrosion resistance. Use of s-phase formation as
a hardening mechanism is occasionally done, but is not recommended.
Welding of duplex alloys can also be somewhat difﬁcult due to the
potential for forming the s phase. Welding ﬁller material containing about
1–2% more nickel than the casting is normally used when the castings will be
re-solution heat-treated. Filler material with 3% additional nickel is used
when castings are not re-solution heat-treated.15
Duplex stainless steels have complete resistance to freshwater, brine,
steam, boiler feed water, atmospheric, and marine environments. They are
particularly suitable for high-temperature, chloride-containing environ-
ments where pitting and SCC are common causes of failure with other SSTs.
Duplex alloys have inherently better SCC resistance than single-phase alloys
because at least one of the phases is generally resistant to cracking in a given
environment. These alloys have good resistance to urea and sulfuric,
phosphoric, and nitric acids.16 They are also highly resistant to acetic, formic,
and other organic acids and compounds.
Alloy Z 6CNDU20.08M to French National Standard NF A 320-55 is the
cast version of Uranus 50M (trademark of Creusot-Loire). It is the only cast
duplex SST grade which is currently acceptable per NACE MR0175
for general use.15 Unlike other duplex SSTs, Z 6CNDU20.08M is limited to
25–40% ferrite in NACE MR0175, which means it is only a borderline
“duplex” SST. Its corrosion resistance is slightly better than CF8M, but inferior
to the other duplex SSTs.


CD3MN is the cast version of wrought UNS S31803 or 2205. It is actually
listed in ASTM A890 as grade 4A. This is a nonproprietary duplex SST
available from many sources world-wide. With its lower alloy content
compared to other duplex grades, its cost is lower, but some corrosion
resistance is sacrificed.
CD4MCu is a cast duplex SST that has been in use for many years. It is
used for environments that are too corrosive for the commonly used
austenitic SSTs or where SCC may be a problem. It is similar to wrought
Ferralium 255 (trademark of Bonar Langley Alloys Ltd.). Its corrosion
resistance is better than that of CF8M.
CD3MWN is a new duplex recently added to ASTM. It is the wrought
equivalent of Zeron 100 (trademark of Weir Materials, Ltd.). It has higher
alloy content than the other duplex grades, giving corrosion resistance
nearly as good as that of the superaustenitic alloys.

References
1. A.B. Avesta Sheffield. 1994. Sheffield Corrosion Handbook, Stockhold: Avesta
Sheffield AB.
2. ASM International. 1979. ASM Handbook, Vol. 3, 9th ed., Metals Park, OH: ASM
International, pp. 78–93.
3. W.H. Herrnstein. 1977. Structure and constitution of cast iron–chromium–
nickel alloys, in Handbook of Stainless Steels, D. Peckner and I.M. Bernstein, Eds,
New York: McGraw Hill, pp. 10–12.
4. A.H. Tuthill. 1990. Practical guide for procurement of quality stainless steel
castings, Materials Performance, May, 55–60.
5. W. Grinthal, Ed. 1992. High-tech steels to the rescue, Chemical Engineering,
New York (January).
6. Allegheny Ludlum Corp. 1991. Al-6XN Alloy, Pittsburgh, PA: Allegheny
Ludlum Corp.
7. J.L. Gossett. 1994. Unpublished work, Marshalltown, IA: Fisher Controls, Inc.
8. ´
B. Wallen. 1981. Seawater resistance of a high molybdenum stainless steel, in
Proceedings of the Second BSE-NACE Corrosion Conference, Bahrain, 140–151.
9. C. McCaul. 1991. Evaluation of intergranular corrosion susceptibility in as
as-welded high alloy austenitic stainless steel casting, British Corrosion Journal,
December.
10. J.L. Gossett. 1988. New and improved, high nickel alloy castings. Paper 322,
Corrosion, 88th Conference, National Association of Corrosion Engineers,
Houston.
11. R.J. Davison and J.D. Redmond. 1988. Practical guide to using 6 Mo austenitic
stainless steels, Materials Performance, December, 39–43.
12. American Welding Society. AWS A5.11, Specification for Nickel and Nickel Alloy
Electrodes for Shielded Metal arc Welding, Miami: American Welding Society.
13. American Welding Society. AWS A5.14, Specification for Nickel and Nickel Alloy
Bare Welding Electrodes and Rods, Miami: American Welding Society.
14. Armco, Inc. 1982. Product Data, Bulletin no. S-6d, Armco 17-4PH, Middletown,
OH: Armco, Inc.


15. J.C.M. Farrar. 1992. Welding castings of duplex stainless steel, Welding Design
and Fabrication, November, 48–51.
16. F. Dupoiron, L. Renaud, M. Verneau, and J. Charles. 1994. Industrial
Applications and Experience of Duplex and Superduplex Stainless Steels in
Chemical Industries. Paper 386, Proceedings of Corrosion 94, Houston, TX: NACE
International.

15
Nickel and High-Nickel Alloys

The nickel-based alloys show a wider range of application than any other
class of alloys. These alloys are used as corrosion-resistant alloys, heating
elements, controlled expansion alloys, creep-resistant alloys in turbines and
jet engines, and high-temperature, corrosion-resistant alloys.
The austenitic stainless steels were developed and utilized early in the
1900s, whereas the development of the nickel-based alloys did not begin
until about 1930. Initially, some of the alloys were produced only as castings
and later the wrought versions were developed. Since that time, there has
been a steady progression of different or improved alloys emerging from the
laboratories of nickel-based alloy producers. Many of these find their major
usage in the high-temperature world of gas turbines and furnaces, but
several are used primarily by the chemical industry for aqueous corrosion
service.
Historically, the use of these alloys was typically reserved for those
applications where it was adjudged that nothing else would work. At one
time, the primary factor in the selection of construction materials was initial
cost. Very little thought was given to the possible maintenance and
downtime associated with the equipment. Today, the increasing costs of
maintenance and downtime have placed greater emphasis on the reliable
performance of the process equipment. The annual amortized cost of the
equipment over the expected life is now important with regards to the
material selection.
In the electrochemical series, nickel is nobler than iron but more active
than copper. Reducing environments, such as dilute sulfuric acid, find nickel
more corrosion-resistant than iron but not as resistant as copper or nickel–
copper alloys. The nickel–molybdenum alloys are more corrosion-resistant
to reducing environments than nickel or nickel–copper alloys.
Although nickel can form a passive film in some environments, it is not a
particularly stable film; therefore, nickel cannot generally be used in
oxidizing media, such as nitric acid. When alloyed with chromium, a
much improved stable passive film results, producing a greater corrosion
resistance to a variety of oxidizing environments. However, these alloys are

235


subject to attack in media containing chloride or other halides, especially if
oxidizing agents are present. Corrosion will be in the form of pitting. The
corrosion resistance can be improved by adding molybdenum and tungsten.
One of the most important attributes of nickel with respect to the for-
mation of corrosion-resistant alloys is its metallurgical compatibility with a
number of other metals, such as copper, chromium, molybdenum, and iron.
A survey of the binary phase diagrams for nickel and these other elements
shows considerable solid solubility, and thus one can make alloys with a
wide variety of composition. Nickel alloys are, in general, all austenitic
alloys; however, they can be subject to precipitation of intermetallic and
carbide phases when aged. In some alloys designed for high-temperature
service, intermetallic and carbide precipitation reactions are encouraged to
improve properties. However, for corrosion applications, the precipitation
of second phases usually promotes corrosion attack. The problem is rarely
encountered because the alloys are supplied in the annealed condition
and the service temperatures rarely approach the level required for
sensitization.
In iron–chromium–nickel stainless steels, minimization of carbide
precipitation can be achieved by lowering the carbon content to a maximum
of about 0.03%. As the nickel content is increased from the nominal 8% in
these alloys to that of the majority element (i.e., more that 50%), the nature of
the carbide changes predominantly from M23C6 to M6C, and the carbon
solubility decreases by a factor of 10. It was therefore very difficult in the past
to produce an L-grade material because of the state of the art of melting.
Many alloys were produced with carbon stabilizers to tie up the carbon,
but with varying degrees of success. Changes in melting techniques
were developed to overcome the problem. The transfer of alloys from
air induction or vacuum induction melting to air-arc plus argon–oxygen
decarburization has provided a means for producing nickel alloys
comparable to the L grades of stainless steels.
While general corrosion resistance is important, one of the major reasons
that nickel-based alloys are specified for many applications is their excellent
resistance to localized corrosion, such as pitting, crevice corrosion, and stress
corrosion cracking. In many environments, austenitic stainless steels do not
exhibit general attack but suffer from significant localized attack, resulting in
excessive downtime and/or expensive repair and replacement.
In general, the localized corrosion resistance of alloys is improved by the
addition of molybdenum. However, molybdenum content alone does not
solve the problem. For example, alloy B-2 has the highest molybdenum
content (26.5%) and is not recommended for most localized corrosion
service. Chromium, which is present in alloy B-2 in residual quantities, also
plays an important role because the environments are normally oxidizing
in nature.
The nickel-based alloys are sometimes referred as superalloys. They have
been defined as those possessing good high-temperature strength and
oxidation resistance and are alloys of nickel, cobalt, and iron that contain

Nickel and High-Nickel Alloys 237

larger amounts of chromium (25–30%) for oxidation resistance. Classifi-
cations include iron–nickel, nickel, and cobalt-based alloys. For many years,
cobalt-based superalloys held the edge, but because of the precarious
availability of cobalt from South Africa, the nickel-based superalloys have
replaced many of the cobalt-based alloys.
The physical metallurgy of these alloys is the result of the precipitation of a
very fine distribution of small particles, primarily Ni3Al and Ni3Ti, which
have the generic name of gamma prime in a gamma matrix. The nickel–iron
alloys also have a phase in the form of a Ni3Nb compound. In actuality, the
superalloys are really dispersion-hardened alloys because they achieve
their strength by a fine dispersion of these compounds. Even though these
compound particles are often obtained via aging or precipitation heat
treatment, they do not develop the coherency strains that the true
precipitation-hardening alloys do. These particles resist dislocation motion
and thereby strengthen the base metal. In addition, these particles resist
growth at elevated temperatures. For this reason, they have found
application in the turbines and hot compartments of jet aircraft.
Dispersion-hardening alloys do not overage as readily as precipitation-
hardening alloys.
Not all nickel-based alloys are used for high-temperature applications; the
monels and some solid-solution inconels are the most notable exceptions.
The nickel and high-nickel alloys will be discussed individually.

15.1 Nickel 200 and Nickel 201
This family is represented by nickel alloys 200 (N0220) and 201 (N02201).
The chemical composition is shown in Table 15.1. Commercially pure nickel
is a white magnetic metal very similar to copper in its other physical and
The Curie point—the temperature at which it loses its magnetism—varies
with the type and quantity of alloy additions, rising with increased iron and

TABLE 15.1
Chemical Composition of Nickel 200 and Nickel 201

Weight Percent (max)
Chemical Nickel 200 Nickel 201

Carbon 0.1 0.02
Copper 0.25 0.25
Iron 0.4 0.4
Nickel 99.2 99.0
Silicon 0.15 0.15
Titanium 0.1 0.1


cobalt additions and falling as copper, silicon, and most other elements are
added. Nickel is also an important alloying element in other families of
corrosion-resistant materials.
Alloy 201 is a low-carbon version of alloy 200. Alloy 200 is subject to the
formation of a grain boundary graphitic phase that tremendously reduces
ductility. Consequently, nickel alloy 200 is limited to a maximum operating
temperature of 6008F (3158C). For applications above this temperature, alloy
201 should be used.
There are two basic pure nickel alloys, each containing a minimum of 99%
nickel: alloy 200 and alloy 201. Alloy 201 is the low-carbon version of alloy
200. Alloy 200 is subject to the formation of a grain-boundary graphitic phase
that reduces ductility tremendously. Consequently, nickel alloy 200 is limited
to a maximum operating temperature of 6008F (3158C). For application above
this temperature, alloy 201 should be used.
The corrosion resistance of alloys 200 and 201 are the same. They exhibit
outstanding resistance to hot alkalies, particularly caustic soda. Excellent
resistance is shown at all concentrations at temperatures up to and including
the molten state. Below 50%, the corrosion rates are negligible, usually being
less than 0.2 mil/year (mpy) even in boiling solutions. As concentrations
and temperatures increase, corrosion rates increase very slowly. Impurities
in the caustic, such as chlorates and hypochlorites, will determine the
corrosion rate.
Nickel is not subject to stress corrosion cracking in any of the chloride
salts and it exhibits excellent general resistance to nonoxidizing halides.
Oxidizing acid chlorides such as ferric, cupric, and mercuric are very
corrosive and should be avoided.
Nickel 201 also finds application in the handling of hot, dry chlorine and
hydrogen chloride gas on a continuous basis up to 10008F (5408C). The
resistance is attributed to the formation of a nickel chloride film. Dry fluorine
and bromine can be handled in the same manner. The resistance will
decrease when moisture is present.
Nickel exhibits excellent resistance to most organic acids, particularly fatty
acids such as stearic and oleic, if aeration is not high. Nickel is not attacked
by anhydrous ammonia or ammonium hydroxide in concentrations of 1%
or less. Stronger concentrations cause rapid attack.
Nickel also finds application in the handling of food and synthetic fibers
because of its ability to maintain product purity. The presence of nickel ions
is not detrimental to the flavor of food products and it is nontoxic. Unlike
iron and copper, nickel will not discolor organic chemicals such as phenol
and viscose rayon.
Refer to Table 15.2 for the compatibility of nickel 200 and nickel 201 with
selected corrodents.
In addition to alloy 200, there are a number of alloy modifications
developed for increased strength, hardness, resistance to galling, and
improved corrosion resistance. Other alloys in this family are not specifically
used for their corrosion resistance.


TABLE 15.2
Compatibility of Nickel 200 and Nickel 201 with Selected
Corrodents

Maximum Temperature
Chemical 8F 8C

Acetaldehyde 200 93
Acetone 190 88
Acrylic acid
Adipic acid 210 99
Alum 170 77
Aluminum acetate
Aluminum nitrate
Aluminum oxychloride
Ammonia gas 90 32
Ammonium bifluoride
Ammonium hydroxide, 25% X
Ammonium sulfide
Ammonium sulfite X
Amyl alcohol
Amyl chloride 90 32
Aniline 210 99
Aqua regia, 3:1 X
(continued)



Maximum Temperature
Chemical 8F 8C

Benzaldehyde 210 99
Benzene 210 99
Borax 200 93
Boric acid 210 99
Bromine, liquid
Butadiene 80 27
Butyl acetate 80 27
n-Butylamine
Butyric acid X
Calcium bisulfide
Calcium carbonate
Calcium nitrate
Calcium oxide 90 32
Carbon bisulfide X
Carbon disulfide X
Carbonic acid 80 27
Cellosolve 210 99
Chloracetic acid, 50% water
Chlorine gas, wet X
Chlorine, liquid
Chloroform 210 99
(continued)



Maximum Temperature
Chemical 8F 8C

Chromic acid, 50% X
Copper carbonate X
Copper chloride X
Copper cyanide X
Copper sulfate X
Cresol 100 38
Cyclohexane 80 27
Cyclohexanol 80 27
Dichloroacetic acid
Dichloroethane (ethylene dichloride) X
Ferric chloride X
Ferric nitrate, 10–50% X
Ferrous chloride X
Ferrous nitrate
Fluorine gas, moist 60 16
Hydrochloric acid, 20% 80 27
Hydrocyanic acid, 10%
Hydroﬂuoric acid, 30%b 170 77
Hypochlorous acid X
Iodine solution, 10%
Lactic acid, 25% X
Magnesium chloride 300 149
Malic acid 210 99
Methyl ethyl ketone
Muriatic acid X
Nitric acid, 5% X
(continued)



Maximum Temperature
Chemical 8F 8C

Nitric acid, 20% X
Nitric acid, 70% X
Nitrous acid, conc. X
Oleum
Perchloric acid, 70%
Phenol, sulfur-free 570 299
Picric acid 80 27
Potassium bromide, 30%
Salicyclic acid 80 27
Silver bromide, 10%
Sodium carbonate, to 30% 210 99
Stannic chloride X
Stannous chloride, dry 570 299
Sulfurous acid X
Thionyl chloride 210 99
Toluene 210 99
White liquor
Zinc chloride, to 80% 200 93
is shown by an X. When compatible, corrosion rate is !20 mpy.
a
Material subject to pitting.
b


Alloy 270 is a high-purity, low-inclusion version of alloy 200. Alloy 301
(also referred to by tradename Duranickel) is a precipitation-hardenable
alloy containing aluminum and titanium. Alloy 300 (also called by the
tradename Permanickel) is a moderately precipitation-hardenable alloy
containing titanium and magnesium that also possesses higher thermal and
electrical conductivity.

15.2 Monel Alloy 400 (N04400)
The first nickel alloy, invented in 1905, was approximately two-thirds nickel
and one-third copper. The present equivalent of the alloy, Monel 400,
remains one of the widely used nickel alloys. Refer to Table 15.3 for the
chemical composition.
Nickel–copper alloys offer somewhat higher strength than un-alloyed
nickel, with no sacrifice of ductility. The thermal conductivity of alloy 400,
although lower than that of nickel is significantly higher than that of nickel
alloys containing substantial amounts of chromium or iron. The alloying of
30–33% copper with nickel, producing alloy 400, provides an alloy with
many of the characteristics of pure nickel but improves other characteristics.
Nickel–copper alloy 400 is a solid solution binary alloy, combining high
strength (comparable to structural steel) and toughness over a wide range
with excellent resistance to many corrosive environments. The alloy can be
used at temperatures up to 8008F (4278C) and as high as 10008F (5388C) in
sulfur-free oxidizing atmospheres. It has excellent mechanical properties
at subzero temperatures. The alloy is readily fabricated and is virtually
immune to chloride ion stress corrosion cracking in typical environments.
Generally, its corrosion resistance is very good in reducing environments,
but poor in oxidizing conditions.

TABLE 15.3
Chemical Composition of Monel Alloys

Weight Percent
Chemical 400 (N04400) 405 (N04405) K-500 (N05500)

Carbon 0.2 max 0.3 max 0.1 max
Manganese 2.0 max 2.0 max 0.8 max
Silicon 0.5 max 0.5 max 0.2 max
Sulfur 0.015 max 0.020–0.060 —
Nickel 63.0–70.0 63.0–70.0 63.0 min
Iron 2.50 max 2.50 max 1.0
Copper Balance Balance 27.0–33.0
Columbium — — 2.3–3.15
Titanium — — 0.35–0.85


The general corrosion resistance of alloy 400 in the nonoxidizing acids,
such as sulfuric, hydrochloric, and phosphoric is improved over that of pure
nickel. The influence of oxidizers is the same as for nickel. The alloy is not
resistant to oxidizing media such as nitric acid, ferric chloride, chromic acid,
wet chlorine, sulfur dioxide, or ammonia.
Alloy 400 exhibits excellent resistance to hydrofluoric acid solutions at all
concentrations and temperatures, as shown in Figure 15.1. Again, aeration
or the presence of oxidizing salts increases the corrosion rate. This alloy is
widely used in HF alkylation, is comparatively insensitive to velocity
effects, and is widely used for critical parts such as bubble caps or valves
that are in contact with flowing acid. Monel 400 is subject to stress
corrosion cracking in moist, aerated hydrofluoric or hydrofluorosilicic acid
vapor. However, cracking is unlikely if the metal is completely immersed in
the acid.
Water handling, including seawater and brackish waters, is a major area of
application. It gives excellent service under high-velocity conditions, as in

250

Atmospheric
boiling
point curve
200
Air-free,
<20 mpy

Aerated,
>20 mpy
150
Temperature, °F

Air-free, Aerated,
<10 mpy <25 mpy
100

Air-free, Aerated,
<1 mpy <10 mpy
50

0
0 10 20 30 40 50 60 70 80 90 100
Acid concentration, weight percent

FIGURE 15.1
Isocorrosion diagram for alloy 400 in hydrofluoric acid. (From G.F. Hodge. 1999. in Corrosion and
Corrosion Protection Handbook, P.A. Schweitzer, Ed., 1st ed., New York: Marcel Dekker.)


propellers, propeller shafts, pump shafts, impellers, and condenser tubes.
The addition of iron to the composition improves the resistance to cavitation
and erosion in condenser tube applications. Alloy 400 can pit in stagnant
seawater, as does nickel 200; however, the rates are considerably lower. The
absence of chloride stress corrosion cracking is also a factor in the selection of
the alloy for this service.
Alloy 400 undergoes negligible corrosion in all types of natural
atmospheres. Indoor exposures produce a very light tarnish that is easily
removed by occasional wiping. Outdoor surfaces that are exposed to rain
produce a thin gray–green patina. In sulfurous atmospheres, a smooth
brown adherent ﬁlm forms.
Monel 400 exhibits stress corrosion cracking in high temperatures,
concentrated caustic, and in mercury. Refer to Table 15.4. A more detailed
compilation will be found in Reference [6].
Monel alloy 405 is a higher sulfur grade in which the sulfur content is
increased over that of alloy 400 to improve machinability. Refer to Table 15.3
for the chemical composition. The corrosion resistance of this alloy is
essentially the same as alloy 400.
Monel alloy K-500 is an age-hardenable alloy that combines the excellent
corrosion resistance characteristics of alloy 400 with the added advantage of
increased strength and hardness. Chemical composition will be found in
Table 15.3.
Typical applications include pump shafts, impellers, electronic com-
ponents, doctor blades, and scrapers, oil well drill collars and instruments,
springs and valve trim.

15.3 Alloy B-2
Alloy B was originally developed to resist hydrochloric acid up to the
atmospheric boiling point. However, because of susceptibility to inter-
granular attack in the heat-affected zone after welding in some environ-
ments, a low-carbon variant, alloy B-2, was developed and is replacing
alloy B in most applications. The chemical composition is shown in
Table 15.5.
This alloy is different from other corrosion-resistant alloys because it does
not contain chromium. Molybdenum is the primary alloying element and
provides signiﬁcant corrosion resistance to reducing environments.
Alloy B-2 has improved resistance to knifeline and heat-affected zone
attack. It also resists formation of grain-boundary precipitates in weld-heat-
affected zone.
Alloy B-2 has excellent elevated-temperature (16508F (9008C)) mechanical
properties because of the high molybdenum content and has been used for
mechanical components in reducing environments and vacuum furnaces.
Because of the formation of the intermetallic phases Ni3Mo and Ni4Mo after


TABLE 15.4
Compatibility of Monel 400 with Selected Corrodents

Maximum Temperature
Chemical 8F 8C

Acetaldehyde 170 77
Acetamide 340 171
Acetone 190 88
Adipic acid 210 99
Alum 100 38
Ammonia gas X
Ammonium biﬂuoride 400 204
Ammonium hydroxide, 25% X
Ammonium hydroxide, sat. X
Ammonium nitrate X
Amyl alcohol 180 82
Aniline 210 99
Aqua regia, 3:1 X
Barium sulﬁde X
(continued)



Maximum Temperature
Chemical 8F 8C

Benzaldehyde 210 99
Benzene 210 99
Benzoic acid 210 99
Borax 90 32
Boric acid 210 99
Butadiene 180 82
Butyric acid 210 99
Calcium oxide 90 32
Carbon bisulﬁde X
Carbon dioxide, weta 400 204
Carbon disulﬁde X
Carbonic acid X
Cellosolve 210 99
Chloracetic acid, 50% water 180 82
Chloracetic acid X
Chlorine gas, wet X
Chlorine, liquid 150 66
Chloroform 210 99
Chromic acid, 50% X
Copper acetate X
(continued)



Maximum Temperature
Chemical 8F 8C

Copper carbonate X
Copper chloride X
Copper cyanide X
Copper sulfate X
Cresol 100 38
Cyclohexane 180 82
Cyclohexanol 80 27
dichloride)
Ferric chloride X
Ferrous chloride X
Ferrous nitrate
Hypochlorous acid X
Lactic acid, 25% X
Malic acid 210 99
Muriatic acid X
Nitric acid, 5% X
Nitric acid, 20% X
Nitric acid, 70% X
Oleum X
(continued)



Maximum Temperature
Chemical 8F 8C

Phenol 570 299
Picric acid X
Potassium bromide, 30%, 210 99
air-free
Silver bromide, 10% 80 27
Stannic chloride X
Sulfurous acid X
Thionyl chloride 300 149
Toluene 210 99
White liquor X
Zinc chloride, to 80% 200 93
X. When compatible, corrosion rate is !20 mpy.
a
Not for use with carbonated beverages.
b

long aging, the use of alloy B-2 in the temperature range 1110–15608F
(600–8008C) is not recommended, irrespective of environment.
Alloy B-2 is recommended for service in handling all concentrations of
hydrochloric acid in the temperature range of 158–2128F (70–1008C) and for
handling wet hydrogen chloride gas, as shown in Figure 15.2.
Alloy B-2 has excellent resistance to pure sulfuric acid at all concentrations
and temperatures below 60% acid and good resistance to 2128F (1008C)


TABLE 15.5
Chemical Composition of Alloy B-2


Chromium 1.0 max
Iron 2.0 max
Nickel Balance

above 60% acid, as shown in Figure 15.3. The alloy is resistant to a number of
phosphoric acids and numerous organic acids, such as acetic, formic, and
cresylic. It is also resistant to many chloride-bearing salts (nonoxidizing),
such as aluminum chloride, magnesium chloride, and antimony chloride.
Because alloy B-2 is nickel rich (approximately 70%), it is resistant to
chloride-induced stress corrosion cracking. Because of its high molybdenum
content, it is highly resistant to pitting attack in most acid chloride
environments.
Alloy B-2 is not recommended for elevated temperature service except in
very speciﬁc circumstances. Because there is no chromium in the alloy, it

125
250
Corrosion rates in parentheses are in mm/year.

Boiling point curve

0−5 mpy 100
200 5 mpy
Temperature, °C
Temperature, °F

(0.13)

5−20 mpy 75
(0.13−0.51)
150

5 mpy 50
(0.13)
0−5 mpy
100 (0−0.13)

0 10 20 30 40
Concentration, weight percent

FIGURE 15.2
Isocorrosion diagram for alloy B-2 in hydrochloric acid. (From G.F. Hodge. 1999. in Corrosion and


400 Corrosion rates in parentheses are in mm/year
Over 50 mpy 200
(Over 1.27)
350 50 mpy
(1.27)
20 mpy
Boiling point curve 20−50
300 (0.51) mpy 150
5 mpy

Temperature, °C
Temperature, °F

(0.13) 5−20 mpy
250
0−5 mpy
(0−0.13)
100
200

5−10 mpy
5 mpy
(0.13−0.25)
150 (0.13)
50
100

0 10 20 30 40 50 60 70 80 90 100

FIGURE 15.3
Isocorrosion diagram for alloy B-2 in sulfuric acid. (From G.F. Hodge. 1999. in Corrosion and

scales heavily at temperatures above 14008F (7608C). A nonprotective layer
of molybdenum trioxide forms and results in a heavy green oxidation scale.
In a chloride-containing environment, alloy B-2 has demonstrated
good resistance.
The major factor limiting the use of alloy B-2 is the poor corrosion
resistance in oxidizing environments. Alloy B-2 has virtually no corrosion
resistance to oxidizing acids, such as nitric and chromic, or to oxidizing salts,
such as ferric chloride or cupric chloride. The presence of oxidizing salts in
reducing acids must also be considered. Oxidizing salts, such as ferric
chloride, ferric sulfate or cupric chloride, even when present in the parts
per million range, can signiﬁcantly accelerate the attack in hydrochloric or
sulfuric acids as shown in Figure 15.4. Even dissolved oxygen has sufﬁcient
oxidizing power to affect the corrosion rates for alloy B-2 in hydrochloric
acid. Alloy B-2 exhibits excellent resistance to pure phosphoric acid.
Stress corrosion cracking has been observed in alloy B-2 in 20%
magnesium chloride solution at temperatures exceeding 5008F (2608C).
Other environments in which stress corrosion cracking of this alloy has been
observed, include high purity water at 3508F (1708C), molten lithium at 3158F
(1578C), oxygenated deionized water at 4008F (2048C), 1% hydrogen iodide at
62–4508F (17–2308C), and 10% hydrochloric acid at 4008F (2048C). In some
environments, such as concentrated ammonia at 77–1408F (25–608C),
cracking has been observed if the alloy was aged at 13828F (7508C) for 24 h


10,000
Corrosion rates in parentheses are in mm/year

4000 Boiling 20% HCI

Boiling 30% H2SO4

1000
Corrosion rate, MPY

400

100

50 mpy
40 (1.27)

10
1 2 4 6 8 10 20 40 60 80100 200 400 600 800
Ferric ion concentration, PPM

FIGURE 15.4
Effect of ferric ions on corrosion rate of alloy B-2. (From G.F. Hodge. 1999. in Corrosion and

before the test. Precipitation of an ordered intermetallic phase, Ni4Mo, has
been hypothesized to be the cause of the increased embrittlement.
Table 15.6 shows the compatibility of alloy B-2 with selected corrodents.
Reference [6] contains a more extensive listing.

15.4 Alloy 625 (N06625)
Alloy 625, also known as Inconel alloy 625, is used both for its high strength
and aqueous corrosion resistance. The strength of alloy 625 is primarily a
solid solution effect from molybdenum and niobium (Columbian). Alloy 625
has excellent weldability. The chemical composition is shown in Table 15.7.
Because of its combination of chromium, molybdenum, carbon, and
niobiumCtantalum, the alloy retains its strength and oxidation resistance
at elevated temperatures.
This alloy ﬁnds application where strength and corrosion resistance are
required. It exhibits exceptional fatigue strength and superior strength and


TABLE 15.6
Compatibility of Alloy B-2 and Alloy C-276 with Selected
Corrodents

Maximum Temperature (8F/8C)
Chemical Alloy B-2 Alloy C-276

Acetaldehyde 80/27 140/60
Acetamide 60/16
Acetic acid, 10% 300/149 300/149
Acetic acid, 50% 300/149 300/149
Acetic acid, 80% 300/149 300/149
Acetic acid, glacial 560/293 560/293
Acetic anhydride 280/138 280/138
Acetone 200/93 200/93
Acetyl chloride 80/27
Acrylic acid 210/99
Acrylonitrile 210/99 210/99
Adipic acid 210/99
Allyl alcohol 570/299
Allyl chloride 200/93
Alum 150/66 150/66
Aluminum acetate 60/16 60/16
Aluminum chloride, aqueous 300/149 210/99
Aluminum chloride, dry 210/99 210/99
Aluminum fluoride 80/27 80/27
Aluminum sulfate 210/99 210/99
Ammonia gas 200/93 200/93
Ammonium bifluoride 380/193
Ammonium carbonate 300/149 300/149
Ammonium chloride, 10% 210/99 210/99
Ammonium chloride, 50% 210/99 210/99
Ammonium chloride, sat. 570/299 570/299
Ammonium fluoride, 10% 210/99 210/99
Ammonium fluoride, 25% 210/99
Ammonium hydroxide, 25% 210/99 570/299
Ammonium hydroxide, sat. 210/99 570/299
Ammonium sulfate, 10–40% 80/27 200/93
Ammonium sulfite 100/38
Amyl acetate 340/171 340/171
Amyl alcohol 180/82
Amyl chloride 210/99 90/32
Aniline 570/299 570/299
Antimony trichloride 210/99 210/99
Aqua regia, 3:1 X X
Barium carbonate 570/299 570/299
Barium chloride 570/299 210/99
Barium hydroxide 270/132 270/132
Benzaldehyde 210/99 210/99
(continued)




Benzene 210/99 210/99
Benzene sulfonic acid, 10% 210/99 210/99
Benzoic acid 210/99
Benzyl alcohol 210/99 210/99
Benzyl chloride 210/99
Borax 120/49 120/49
Boric acid 570/299 570/299
Bromine gas, dry 60/16 60/16
Bromine gas, moist 60/16
Bromine liquid 180/82
Butadiene 300/149 300/149
Butyl acetate 200/93 200/93
Butyl alcohol 210/99 200/93
n-Butylamine 210/99 210/99
Butyric acid 280/138 280/138
Calcium bisulfite 80/27
Calcium carbonate 210/99 210/99
Calcium chlorate 210/99
Calcium chloride 350/177 350/177
Calcium hydroxide, 10% 210/99 170/77
Calcium hydroxide, sat. 210/99
Calcium nitrate 210/99 210/99
Calcium oxide 90/32
Calcium sulfate, 10% 320/160 320/160
Caprylic acid 300/149 300/149
Carbon bisulfide 180/82 210/99
Carbon dioxide, dry 570/299 570/299
Carbon dioxide, wet 570/299 200/93
Carbon disulfide 180/82 300/149
Carbon monoxide 570/299 570/299
Carbon tetrachloride 300/149 300/149
Carbonic acid 80/27 80/27
Cellosolve 210/99 210/99
Chloracetic acid, 50% water 210/99
Chloracetic acid 370/188 300/149
Chlorine gas, dry 200/93 570/299
Chlorine gas, wet X 220/104
Chlorine, liquid 110/43
Chlorobenzene 350/177 350/177
Chloroform 210/99 210/99
Chlorosulfonic acid 230/110 230/110
Chromic acid, 10% 130/54 210/99
Chromic acid, 50% X 210/99
Chromyl chloride 210/99 210/99
Citric acid, 15% 210/99 210/99
Citric acid, conc. 210/99 210/99
(continued)




Copper acetate 100/38 100/38
Copper carbonate 90/32 90/32
Copper chloride 200/93 200/93
Copper cyanide 150/66 150/66
Copper sulfate 210/99 210/99
Cresol 210/99 210/99
Cupric chloride, 5% 60/16 210/99
Cupric chloride, 50% 210/99 210/99
Cyclohexane 210/99 210/99
Cyclohexanol 80/27 80/27
Dichloroethane 230/110 230/110
Ethylene glycol 570/299 570/299
Ferric chloride 90/32 90/32
Ferrous chloride 280/138 280/138
Fluorine gas, dry 80/27 150/66
Fluorine gas, moist 570/299
Hydrobromic acid, dilute 210/99
Hydrobromic acid, 20% 210/99 90/32
Hydrobromic acid, 50% 260/127 90/32
Hydrochloric acid, 20% 140/60 150/66
Hydrochloric acid, 38% 140/60 90/32
Hydroﬂuoric acid, 30% 140/60 210/99
Hypochlorous acid 90/32 80/27
Iodine solution, 10% 180/82
Ketones, general 180/82 100/38
Lactic acid, 25% 250/121 210/99
Lactic acid, conc. 250/121 210/99
Magnesium chloride 300/149 300/149
Malic acid 210/99 210/99
Manganese chloride, 40% 210/99 210/99
Methyl chloride 210/99 90/32
Methyl ethyl ketone 210/99 210/99
Methyl isobutyl ketone 200/93 200/93
Muriatic acid 90/32 90/32
Nitric acid, 5% X 210/99
Nitric acid, anhydrous X 80/27
Nitrous acid, conc. X X
Oleum, to 25% 110/43 140/60
Perchloric acid, 70% 220/104
Phenol 570/299 570/299
Phosphoric acid, 50–80% 210/99 210/99
Picric acid 220/104 300/149
(continued)




Potassium bromide, 30% 90/32 90/32
Salicylic acid 80/27 250/121
Silver bromide, 10% 90/32 90/32
Sodium carbonate 570/299 210/99
Sodium chloride, to 30% 210/99 210/99
Sodium hydroxide, 10%a 240/116 230/110
Sodium hydroxide, 50% 250/121 210/99
Sodium hydroxide, conc. 200/93 120/49
Sodium hypochlorite, 20% X X
Sodium hypochlorite, conc. X X
Sodium sulfide, to 50% 210/99 210/99
Stannic chloride, to 50% 210/99 210/99
Stannous chlorideb 570/299 210/99
Sulfuric acid, 10% 210/99 200/93
Sulfuric acid, 50% 230/110 230/110
Sulfuric acid, 70% 290/143 290/143
Sulfuric acid, 90% 190/88 190/88
Sulfuric acid, 98% 280/138 210/99
Sulfuric acid, 100% 290/143 190/88
Sulfuric acid, fuming 210/99 90/32
Sulfurous acid 210/99 370/188
Toluene 210/99 210/99
Trichloroacetic acid 210/99 210/99
White liquor 100/38 100/38
Zinc chloride 60/16 250/121
is shown by an X. A blank space indicates that data are unavailable.
When compatible, corrosion rate is !20 mpy.
a
Alloy B-2 is subject to stress cracking.
b
Alloy B-2 is subject to pitting.

toughness at temperatures varying from cryogenic to 20008F (10938C). The
niobium and tantalum stabilization makes the alloy suitable for corrosion
service in the as-welded condition. It has excellent resistance to chloride
corrosion cracking.
Resistance to aqueous solutions is good in organic acids, sulfuric and
hydrochloric acid at temperatures below 1508F (658C), as well as a variety of
other applications. Satisfactory resistance has also been exhibited to
hydrofluoric acid. Although nickel-based alloys are not normally used in
nitric acid service, alloy 625 is resistant to mixtures of nitric-hydrofluoric
acids, in which stainless steel loses its resistance.


TABLE 15.7


Cobalt 1.00 max
Aluminum 0.40 max
Titanium 0.40 max
Carbon 0.10 max
Iron 5.00 max
Manganese 0.50 max
Silicon 0.50 max
Sulfur 0.015 max
Nickel Balance

Field-operating experience has shown that alloy 625 exhibits excellent
resistance to phosphoric acid solutions, including commercial grades that
contain fluorides, sulfates, and chlorides that are used in the production of
superphosphoric acid (72% P205).
Refer to Table 15.8 for the compatibility of alloy 625 with selected
corrodents. Reference [1] contains a more extensive listing.
Elevated temperature applications include ducting systems, thrust
reverser assemblies, and afterburners. Use of this alloy has been considered
in the high-temperature, gas-cooled reactor; however, after long aging in the
temperature range of 1100–14008F (590–7608C), the room temperature
ductility is significantly reduced.
Alloy 625 has also been used in preheaters for sulfur dioxide scrubbing
systems in coal-fired power plants and bottoms of electrostatic precipitators
that are flushed with seawater.

15.5 Custom Age 625 Plus (N07716)
Custom Age 625 is a trademark of Carpenter Technology. It is a precipitation-
hardenable nickel-based alloy that, in many environments, displays
corrosion resistance similar to that of alloy 625 and superior to that of
alloy 718. The chemical composition is given in Table 15.9.
This alloy provides high levels of strength while maintaining corrosion
resistance, even in applications where large section size or intricate shape
precludes warm or cold working. It offers exceptional resistance to pitting,
crevice corrosion, and general corrosion, as well as stress corrosion cracking
in the age-hardened (high strength) condition. It has good strength up to
about 10008F (5388C).


TABLE 15.8
Compatibility of Alloy 600 and Alloy 625 with Selected
Corrodents

Maximum Temperature
Chemical 8F 8C

Acetaldehyde 140 60
Acetic acid, 50% X
Acetic acid, 80% X
Acetone 190 88
Adipic acid 210 99
Alum 200 93
Aluminum sulfate X
Ammonium nitrate X
Ammonium sulfate, 10–40%b 210 99
Ammonium sulﬁde
Amyl chloride X
Aniline 210 99
Aqua regia, 3:1 X
Benzaldehyde 210 99
Benzene 210 99
Benzoic acid, 10% 90 32
(continued)



Maximum Temperature
Chemical 8F 8C

Borax 90 32
Boric acid 80 27
Butadiene 80 27
Butyl acetate 80 27
Butyl alcohol 80 27
n-Butylamine
Butyric acid X
Calcium sulfateb 210 99
Caprylic acid 230 110
Cellosolve 210 99
Chloracetic acid X
Chlorine gas, wet X
Chloroform 210 99
Copper chloride X
Cresol 100 38
Cyclohexanol 80 27
(continued)



Maximum Temperature
Chemical 8F 8C

dichloride)
Ferric chloride X
Ferrous chloride X
Fluorine gas, moist 60 16
Hydrobromic acid, dilute 90 32
Hydrobromic acid, 20% 80 27
Malic acid 210 99
Manganese chloride, 37% X
Muriatic acid X
Nitric acid, 70% X
Oleum X
Phenol 570 299
Picric acid X
Sodium carbonate, to 30% 210 99
Sodium hydroxide, 50%a 300 149
Stannic chloride X
(continued)



Maximum Temperature
Chemical 8F 8C

Sulfurous acid 90 32
Toluene 210 99
Zinc chloride, dry 80 27

is shown by an X. When compatible, corrosion rate is !20 mpy.
a
Material is subject to stress cracking.
b
Material subject to pitting.

Applications include:
Deep sour gas wells
Reﬁneries
Chemical process plant environments
High-temperature, high-purity nuclear water

TABLE 15.9
Chemical Composition of Custom Age 625 Plus Alloy
(N07716)


Carbon 0.03 max
Manganese 0.020 max
Sulfur 0.010 max
Silicon 0.20 max
Nickel 59.00–63.00
Columbium 2.75–4.00
Aluminum 0.35 max
Iron Balance


15.6 Alloy C-276 (N10276)
Hastelloy alloy C-276 is a low carbon (0.01% maximum) and silicon
(0.08% maximum) version of Hastelloy C. The chemical composition is given
in Table 15.10. Alloy C-276 was developed to overcome the corrosion
problem associated with the welding of alloy C. When used in the as-welded
condition, alloy C was often susceptible to serious intergranular corrosion
attack in many oxidizing and chloride-containing environments. The low
carbon and silicon content of alloy C-276 prevents precipitation of
continuous grain-boundary precipitates in the weld heat-affected zone.
Thus, alloy C-276 can be used in most applications in the as-welded
condition without suffering severe intergranular attack.
Alloy C-276 is extremely versatile because it possesses good resistance to
both oxidizing and reducing media, including conditions with ion
contamination. When dealing with acid chloride salts, the pitting and
crevice corrosion resistance of the alloy make it an excellent choice.
Alloy C-276 has exceptional corrosion resistance to many process
materials, including oxidizing, neutral, and acid chlorides, solvents, chlorine,
formic and acetic acids, and acetic anhydride. It also resists highly corrosive
agents, such as wet chlorine gas, hypochlorite, and chlorine solutions.
Exceptional corrosion resistance is exhibited in the presence of phosphoric
acid at all temperatures below the boiling point of phosphoric acid, when
concentrations are less than 65% by weight. Corrosion rates of less than
5 mpy were recorded. At concentrations above 65% by weight and up to
85%, alloy C-276 displays similar corrosion rates, except at temperatures
between 2408F (1168C) and the boiling point, where corrosion rates may be
erratic and may reach 25 mpy.
Isocorrosion diagrams for alloy C-276 have been developed for a number
of inorganic acids, including sulfuric, see Figure 15.5. Rather than having
one or two acid systems in which the corrosion resistance is exceptional, as

TABLE 15.10
Chemical Composition of Alloy C-276 (N10276)


Carbon 0.01 max
Manganese 0.5
Silicon 0.08 max
Chromium 15.5
Nickel 57
Molybdenum 16
Tungsten 3.5
Iron 5.5


400 Corrosion rates in parentheses are in mm/year. 200

350 Boiling point curve
Over 200 mpy
(Over 5.08)
300 150

Temperature, °C
Temperature, °F

200 mpy
250 (5.08)

20−50 mpy 5−200 mpy
100
200 (0.51−1.27) 20 mpy (1.27−5.08)
5−20 mpy (0.51)
(0.13−0.51) 50 mpy
5 mpy
150 (1.27)
(0.13)

0−5 mpy 50
100 (0−0.13)

0 10 20 30 40 50 60 70 80 90 100

FIGURE 15.5
Isocorrosion diagram for Hastelloy C-276 in sulfuric acid. (From G.F. Hodge. 1999. in Corrosion
and Corrosion Protection Handbook, P.A. Schweitzer, Ed., 1st ed., New York: Marcel Dekker.)

with alloy B-2, alloy C-276 is a good compromise material for a number of
systems. For example, in sulfuric acid coolers handling 98% acid from the
absorption tower, alloy C-276 is not the optimum alloy for the process-side
corrosion, but it is excellent for the water-side corrosion and allows the use
of brackish water or seawater. Concentrated sulfuric acid is used to dry
chlorine gas. The dissolved chlorine will accelerate the corrosion of alloy B-2,
but alloy C-276 has performed quite satisfactorily in a number of chlorine-
drying installations.
Alloy C-276 has been indicated as a satisfactory material for scrubber
construction where problems of localized attack have occurred with other
alloys because of pH, temperature, or chloride content. Refer to Table 15.6 for
the compatibility of alloy C-276 with selected corrodents, and Reference [6]
for a more comprehensive listing.

15.7 Alloy C-4 (N06455)
Alloy C-4 was developed for improved stability relative to precipitation of
both carbides and intermetallic phases. The chemical composition is shown
in Table 15.11. By controlling these secondary phases, excellent high-
temperature stability is achieved to the point that the corrosion resistance
and mechanical properties in the thermally-aged condition are similar to the
annealed condition properties.


TABLE 15.11


Titanium 0.07 max
Iron 3.0 max
Nickel Balance

Examples can be taken from various chemical processing applications in
which oxidizing and reducing conditions can cause serious intergranular
corrosion of a sensitized (precipitated) microstructure. This sensitization can
be the result of welding, improper anneal, stress relief, thermochemical
processing, or operation of process equipment in the sensitizing range.
Alloy C-4 alleviates this problem because it can be subjected to
temperatures in the normal sensitizing range of 1022–19948F (550–10908C)
for extended periods without experiencing the severe corrosion attack that is
found with the common austenitic alloys.
With the exception of iron and tungsten, the composition of alloy C-4 and
C-276 are approximately the same. Consequently, the corrosion resistance of
the two alloys is approximately the same. In a strongly reducing medium,
such as hydrochloric acid, alloy C-4 has a slightly higher rate of corrosion
than alloy C-276, but in an oxidizing medium the rates are reversed.
Alloy C-4 offers excellent corrosion resistance to nitric acid, hydrochloric
acid, organic acids, alkalies, salts, seawater, and chloride stress corrosion
cracking. Good to excellent resistance is exhibited in sulfuric, hydroﬂuoric
and phosphoric acids.

15.8 Alloy C-22 (N06022)
Hastelloy alloy C-22 is a versatile nickel–chromium–molybdenum alloy
with better overall corrosion resistance than other nickel–chromium–
molybdenum alloys, including C-276, C-4, and alloy 625. The chemical
Alloy C-22 resists the formation of grain boundary precipitates in the
weld-heat-affected zone. Consequently, it is suitable for most chemical
process applications in the as-welded condition.
Although alloy C-276 is a versatile alloy, its main limitations are in
oxidizing environments containing low amounts of halides and in
environments containing nitric acid. In addition, the thermal stability of
the alloy was not sufﬁcient to enable it to be used as a casting.


TABLE 15.12


Carbon 0.015 max
Manganese 0.50 max
Sulfur 0.010 max
Cobalt 2.5 max
Tungsten 2.5–3.5
Iron 2.0–6.0
Silicon 0.08 max
Vanadium 0.35 max
Nickel Balance

Alloy C-22 was developed to improve the resistance to oxidizing
environments, such as nitric acid, and also to improve the thermal stability
sufﬁciently to enable it to be used as a casting. The higher chromium level in
this alloy not only makes it superior in oxidizing environments containing
nitric acid, but also improves the pitting resistance over that of alloy C-276.
Alloy C-22 has outstanding resistance to pitting, crevice corrosion, and
stress corrosion cracking. It has excellent resistance to oxidizing aqueous
media, including acids with oxidizing agents, wet chlorine, and mixtures
containing nitric or oxidizing acids with chloride ions. The alloy also has
outstanding resistance to both reducing and oxidizing media and because of
its versatility can be used where upset conditions are likely to occur or in
multipurpose plants.
Alloy C-22 has exceptional resistance to a wide variety of chemical process
environments, including strong process environments, strong oxidizers
such as ferric and cupric chlorides, hot contaminated media (organic and
inorganic), chlorine, formic and acetic acids, acetic anhydride, seawater, and
brine solutions. The compatibility of alloy C-22 with selected corrodents will
be found in Table 15.13.
The areas of application of alloy C-22 are many of the same as those for alloy
C-276. It is being used in the pulp and paper bleaching systems, pollution
control systems, and various areas in the chemical process industry.

15.9 Hastelloy Alloy C-2000
Hastelloy alloy C-2000 is a trademark of Haynes International. It is one of
the nickel–chromium–molybdenum alloys. The chemical composition is


TABLE 15.13
Compatibility of Alloy C-22 with Selected Corrodents

Average
Weight Temperature Corrosion
Corrodent Percent (8F/8C) Rate (mpy)

Acetic acid 99 Boiling Nil
Ferric chloride 10 Boiling 1.0
Formic acid 88 Boiling 0.9
Hydrochloric acid 1 Boiling 2.5
Hydrochloric acid 1.5 Boiling 11
Hydrochloric acid 2 194/90 Nil
Hydrochloric acid 2 Boiling 61
Hydrochloric acid 2.5 194/90 0.3
Hydrochloric acid 2.5 Boiling 84
Hydrochloric acid 10 Boiling 400
Hydrofluoric acid 2 158/70 9.4
Hydrofluoric acid 5 158/70 19
Phosphoric acid, reagent grade 55 Boiling 12
Phosphoric acid, reagent grade 85 Boiling 94
Nitric acid 10 Boiling 0.8
Nitric acid 65 Boiling 5.3
Nitric acidC1% HCl 5 Boiling 0.5
Nitric acidC2.5% HCl 5 Boiling 1.6
Sulfuric acid 10 Boiling 11
Sulfuric acid 20 150/66 0.2
Sulfuric acid 50 174/79 16
Sulfuric acid 70 100/38 Nil
Sulfuric acid 80 100/38 Nil

Alloy C-2000 exhibits outstanding resistance to oxidizing media with
superior resistance to reducing environments. In the family of nickel–
chromium–molybdenum alloys, a high chromium content is required for
resistance to oxidizing media, such as ferric ions, cupric ions, or dissolved
oxygen. Reducing environments, however, such as dilute hydrochloric or
sulfuric acids, require a high content of molybdenum plus tungsten.
Metallurgical stability limitations dictate that you cannot optimize both.
Alloy C-2000 solves this dilemma. A high chromium content is combined
with both molybdenum and copper contents, sufficient to provide resistance
to reducing environments, with no sacrifice of metallurgical stability.


TABLE 15.14
Chemical Composition of Alloy C-2000


Carbon 0.01 max
Manganese 0.050 max
Sulfur 0.010 max
Silicon 0.080 max
Copper 1.30–1.90
Cobalt 2.00 max
Iron 3.00 max
Aluminum 0.50 max
Nickel Balance

Alloy C-200 also exhibits pitting resistance and crevice corrosion
resistance superior to that of alloy C-276. Its critical pitting temperature is
2308F (1108C) and its critical crevice temperature is 2038F (958C). Some typical
uniform corrosion rates are as follows:

Weight Temperature Corrosion Rate
Chemical Percent (8F/8C) (mpy)

Hydroﬂuoric acid 20 174/79 19
Phosphoric acid 50 Boiling 1
Acetic acid 75 Boiling 33
99 Boiling 0.1
Formic acid 88 Boiling 0.4
Chromic acid 10 Boiling 44

15.10 Alloy X (N06002)
Alloy X is a nonmagnetic heat- and corrosion-resistant nickel-based alloy.
The chemical composition will be found in Table 15.15.
Alloy X possesses a combination of high strength and excellent oxidation
resistance. Its oxidation resistance is due to the formation of a complex
chromium oxide spinel that provides good resistance up to temperatures of
21508F (11778C). The high-temperature strength and resistance to warpage
and distortion provide outstanding performance as distributor plates and
catalyst support grids.
Alloy X has excellent resistance to nitric acid, organic acids, alkalies, salts,
seawater, chloride cracking, and good to excellent resistance to phosphoric
and sulfuric acids, with good resistance in hydrochloric and hydroﬂuoric
acids.


TABLE 15.15
Chemical Composition of Alloy X (N06002)


Iron 17.0–20.0
Tungsten 0.2–1.0
Carbon 0.05–0.15
Cobalt 0.5–2.5
Nickel Balance

The catalyst regenerator for high density polyethylene is constructed of
alloy X because high temperatures and pressures are required to revitalize
the catalysts. Unfortunately, this continuous temperature cycling eventually
reduces the room temperature ductility in alloy X so that repair welding
becomes difﬁcult without solution annealing.
Alloy X also ﬁnds application in gas turbine components, high-temperature
heat exchangers, afterburner components, and furnace hardware.

15.11 Alloy 600 (N06600)
Alloy 600, also known as Inconel, is a nickel-based alloy with about 16%
chromium and 7% iron that is used primarily to resist corrosive atmospheres
at elevated temperatures. The chemical composition will be found in
Table 15.16.
Alloy 600 has excellent mechanical properties and a combination of high
strength and good workability. It performs well in temperatures from
cryogenic to 12008F (6498C) and is readily fabricated and welded.

TABLE 15.16


Nickel 72.0 min
Iron 6.0–10.0
Carbon 0.15 max
Copper 0.50 max
Manganese 1.0 max
Sulfur 0.015 max
Silicon 0.5 max


Although the alloy is resistant to oxidation, the presence of sulfur in
the environment can signiﬁcantly increase the rate of attack. The mode of
attack is generally intergranular and therefore the attack proceeds
more rapidly. The maximum use temperature is restricted to about 6008F
(3158C).
Inconel has excellent resistance to dry halogens at elevated temperatures
and has been used successfully for chlorination equipment at temperatures
up to 10008F (5388C). Where arrangements can be made for cooling the metal
surface, the alloy can be used at high gas temperatures.
Resistance to stress corrosion cracking is imparted to alloy 600 by virtue of
its nickel base. The alloy therefore ﬁnds considerable use in handling water
environments where stainless steels fail by cracking. Because of its resistance
to corrosion in high-purity water, it has a number of uses in nuclear reactors,
including steam generator tubing and primary water piping. The lack of
molybdenum in the alloy precludes its use in applications where pitting is
the primary mode of failure.
In certain high-temperature caustic applications where sulfur is
present, alloy 600 is substituted for alloy 201 because of its improved
resistance. Inconel, is however, subject to stress corrosion cracking in
high-temperature, high-concentration alkalies. For this reason, the alloy
should be stress-relieved prior to use and the operating stresses should
be kept to a minimum. Alloy 600 is almost entirely resistant to attack
by solutions of ammonia over the complete range of temperatures
and concentrations.
The alloy exhibits greater resistance to sulfuric acid under oxidizing
conditions than nickel 200 or alloy 400. The addition of oxidizing salts to
sulfuric acid tends to passivate alloy 600, making it suitable for use with acid
mine waters or brass pickling solutions where alloy 400 cannot be used.
Table 15.8 provides the compatibility of alloy 600 with selected corrodents.
Reference [1] provides a more comprehensive listing.

15.12 Alloy G (N06007) and Alloy G-3 (N06985)
Alloy G is a high-nickel austenitic stainless steel having the following
chemical composition:


Chromium 22
Nickel 45
Iron 20
Molybdenum 6.5
Copper 2
Carbon 0.05 max
Columbium 2.0


Alloy G is intended for use in the as-welded condition, even under
the circumstances of multipass welding. The Columbian addition
provides better resistance in highly oxidizing environments than does
titanium additions. Because of the nickel base, the alloy is resistant to
chloride-induced stress corrosion cracking. The 2% copper addition
improves the corrosion resistance of the alloy in reducing acids, such as
sulfuric and phosphoric. Alloy G will resist combinations of sulfuric acid
and halides.
Alloy G resists pitting, crevice corrosion, and intergranular corrosion.
Applications include heat exchangers, pollution control equipment,
and various applications in the manufacture of phosphoric and sulfuric acids.
Alloy G-3 was developed with a lower carbon content than alloy G to
prevent precipitation at the welds. Its chemical composition is as follows:


Chromium 22/23.5
Molybdenum 6.0/8.0
Tungsten 1.5% max
Iron 18/21
Copper 1.5/2.5
Carbon 0.015 max
Columbium 0.8
Nickel 44
Silicon 1.0 max

Although columbium (niobium) stabilized alloy G from formation of
chromium-rich carbides in the heat-affected zones of the welds, secondary
carbide precipitation still occurred when the primary columbium
carbides dissolved at high temperatures, and the increased carbon in the
matrix increases the tendency of the alloy to precipitate intermetallic phases.
Alloy G-3 has lower carbon (0.015% maximum vs. 0.05% maximum for alloy
G) and lower columbium (0.3% maximum vs. 2% for alloy G). The alloy also
possesses slightly higher molybdenum (7% vs. 5% for alloy G).
The corrosion resistance of alloy G-3 is about the same as that of alloy G,
however, thermal stability is much better. Refer to Table 15.17 for the
compatibility of alloy G and alloy G-3 with selected corrodents.

15.13 Alloy G-30 (N06030)
This alloy has a higher chromium content than alloy G, which gives it a
higher resistance to oxidizing environments than other alloys in this series. It
has the following composition:



Chromium 28.0/31.5
Molybdenum 4.0/6.0
Tungsten 1.5/4.0
Iron 13.0/17.0
Copper 1.0/2.4
Columbium 0.30/1.50
NickelCcobalt Balance

TABLE 15.17
Compatibility of Alloy G and Alloy G-3 with Selected Corrodents

Chemical Temperature (8F/8C)

Ammonium chloride, 28% 180/82
Calcium carbonate 120/49
Calcium chloride, 3–20% 220/104
Chlorine gas, wet 80/27
Chlorobenzene, 3–60% 100/38
Fluorosilicic acid, 3–12% 180/82
Hydrofluoric acid X
Hydrofluorosilicic acid, 10–50% 160/71
Kraft liquor 80/27
Lime slurry 140/60
Lithium chloride, 30% 260/127
Magnesium hydroxide 210/99
Magnesium sulfate 210/99
Mercury 250/121
Nitrous oxide 560/293
Oleum 240/116
Potassium chloride, 10% 230/110
Sodium chlorate 80/27
Sodium chloride 210/99
Sodium hypochlorite, conc. 90/32
Sodium sulfide, 3–20% 120/49
Sodium dioxide, wet 130/54
When compatible, the corrosion rate is less that 20 mpy.


Alloy G-30 possesses excellent corrosion resistance in the as-welded
condition. In acid mixtures, such as nitric plus hydrofluoric and sulfuric
plus nitric acids, alloy G-30 shows the highest resistance of this class
of alloys.
Applications include pipe and tubing in phosphoric acid manufacture,
sulfuric acid manufacture, and fertilizer and pesticide manufacture. The
alloy is also used in the evaporators of commercial wet process phosphoric
manufacturing systems. This process contains complex mixtures of
phosphoric, sulfuric, and hydrofluoric acids and various oxides. Under
these conditions, the corrosion rate for alloy G-30 was 6 mpy as compared to
16 mpy for alloy G-3 and alloy 625.

15.14 Alloy H-9M
This alloy can be considered as either a modification of alloy G-3 or of alloy
625. The purpose of the modification is to improve the localized corrosion
resistance of both alloys G-3 and 625. Alloy H-9M has higher molybdenum
content than alloy G-3 and higher tungsten content than alloy 625. To
increase the localized corrosion resistance, copper has been eliminated from
alloy H-9M. The critical pitting temperatures of these alloys in an oxidizing
acidic chloride mixture are shown in Table 15.18. The critical pitting
temperature indicates the temperature above which pitting is observed in
the solution, and the higher the temperature, the better the alloy in
pitting resistance.

15.15 Alloys for High-Temperature Corrosion
Alloys designed to resist high-temperature corrosion are basically oxidation-
resistant materials because all forms of attack at elevated temperatures are

TABLE 15.18
Critical Pitting Temperature of Alloy H-9M in
Comparison to Other Alloys

Critical Pitting

H-9M 203/95
625 194/90
G-3 167/75
Solution 4% NaClC0.1% Fe2(SO4)3C0.01 M HCl.


considered to be oxidation. As with aqueous corrosion, a protective film is
formed. The rate at which the metal oxidizes will depend on the stability of
the film. If the film is stable and remains in place, the rate will be logarithmic,
diminishing with time.
Cycling temperatures will tend to spall off the surface film, leading to a
stepwise oxidation of the alloy. Changes in the environment can have the
same effect.
Although all high-temperature corrosion is considered oxidation, there
are other terms that are also encountered, such as oxidation–reduction,
sulfidation, fuel ash corrosion, carburization, and nitridation, to name
a few.
Although many of the high-nickel alloys previously discussed can be
utilized at elevated temperatures, there are some instances where the
materials are not satisfactory. Consequently, other alloys have been
developed to overcome these shortcomings.

15.15.1 Hastelloy Alloy S
In 1973, Hastelloy S was developed for gas turbine applications requiring
oxidation resistance, good alloy stability, and a low thermal expansion. The
chemical composition will be found in Table 15.19. Its composition is similar
to that of alloys C-4 and C-276, and it has similar corrosion resistance.
However, the carbon content may prevent its use in some aqueous media in
the as-welded condition. After 10,000 h of aging in the temperature ranges
encountered in this application, the alloy S welds exhibited 80% of their
original ductility.

15.15.2 Haynes Alloy 556 (R30556)
Haynes alloy 556 exhibits useful resistance to a wide variety of high-
temperature corrosive atmospheres as well as molten salts. The presence of
approximately 18% cobalt results in more resistance to sulfidation than

TABLE 15.19
Chemical Composition of Hastelloy Alloy S


Chromium 15.5
Molybdenum 14.5
Iron 1.0
Carbon 0.01
Silicon 0.4
Manganese 0.5
Lanthanum 0.02
NickelCcobalt Balance


TABLE 15.20
Chemical Composition of Haynes Alloy 556
(R30556)


Nickel 19.0–22.5
Tungsten 2.0–3.5
Carbon 0.05–0.15
Silicon 0.2–0.8
Cobalt 16.0–21.0
Aluminum 0.1–0.5
Tantalum 0.3–1.25
Zirconium 0.001–0.1
Lanthanum 0.005
Nitrogen 0.1–0.3
Iron Balance

many nickel-based alloys. Table 15.20 shows the chemical composition of
alloy 556.
The alloy has good oxidation resistance and fabrication properties and
excellent high-temperature strength. In pure oxidation, the alloy shows
good resistance, but it is superseded in performance by other alloys such as
alloy X and 214. In chlorine-bearing oxidizing environments, the alloy shows
better resistance than alloys 800H and X, but not as good as alloy 214.
Alloy 556 exhibits excellent resistance to hydrochloric, phosphoric, and
organic acids, alkalies, salts, seawater, and chlorine stress corrosion cracking.
It also offers good resistance to sulfuric acid, hydrofluoric acid, and
nitric acid.
Typical applications include internals of municipal waste incinerators and
refractory anchors in refinery tail gas burning units.

15.15.3 Alloy 214
The chemical composition of alloy 214 will be found in Table 15.21. This is a
nickel-based alloy with excellent resistance to 22008F (12048C). The excellent
oxidation resistance is the result of the tenacious aluminum oxide film that
protects the metal during prolonged exposure.
The alloy possesses the highest oxidation resistance to both static and
dynamic environments among the nickel-based alloys. The alumina film
also lends superior resistance to carburizing environments and complex
environments containing chlorine and oxygen. However, as typical of many
high-temperature alloys, alloy 214 does not possess good resistance to
aqueous chloride solutions; therefore dew point conditions must be avoided.


TABLE 15.21
Chemical Composition of Alloy 214


Chromium 16.0
Iron 3.0
Aluminum 4.5
Yttrium Trace
Nickel Balance

Alloy 214 exhibits excellent resistance to nitric and organic acids and
alkalies. It is not recommended for use with sulfuric, hydrochloric,
hydroﬂuoric or phosphoric acids or salts or seawater.
Applications include radiant tubes, high-temperature heat exchangers,
honeycomb seals in turbine engines, and mesh belts for supporting
chinaware being heated in a kiln.

15.15.4 Alloy 230 (N06230)
Alloy 230 has excellent high-temperature strength and outstanding
resistance to oxidizing environments up to 21008F (11508C). Refer to
Table 15.22 for the chemical composition.
This alloy exhibits resistance to nitriding, excellent long-term thermal
stability, and low thermal expansion. It is also resistant to grain coarsening at
high temperatures. Good resistance to carburization is also displayed.
Because of its nickel matrix, the alloy does not possess adequate resistance to
sulfadizing environments.
Excellent resistance is shown to phosphoric acid, organic acids, alkalies,
salts, seawater and chloride stress corrosion cracking, while good to

TABLE 15.22


Chromium 22.0
Tungsten 14.0
Molybdenum 2.0
Iron 3.0 max
Cobalt 5.0 max
Aluminum 0.3
Carbon 0.10
Lanthanum 0.02
Boron 0.005
Nickel Balance


TABLE 15.23
Chemical Composition of Alloy RA333 (N06333)


Nickel 44.0–47.0
Cobalt 2.50–4.00
Carbon 0.08 max
Silicon 0.75–1.50
Manganese 2.00 max
Phosphorus 0.03 max
Sulfur 0.03 max
Iron Balance

excellent resistance is shown to sulfuric and nitric acids. It is not suitable for
use with hydrochloric acid.
Because of its nitridizing resistance and high creep strength, it has found
application as a catalyst grid support in the manufacture of nitric acid.

15.15.5 Alloy RA333 (N06333)
Alloy RA333 is a registered trademark of Rolled Alloys Inc. It is a high-
chromium, nickel-based alloy with extreme temperature corrosion resist-
ance and strength. The chemical composition is shown in Table 15.23.
Alloy RA333 is one of the few materials that can withstand corrosive
conditions ranging from aqueous to white heat. The alloy has been used
for dampers and refractory anchors in 13% SO2/SO3 at 18008F (9828C) and
for reﬁnery ﬂare tips. Other features include resistance to high-temperature

TABLE 15.24


Columbium 2.75–3.25
Iron 5.0–9.0
Aluminum 0.3–0.6
Titanium 0.4–0.7
Boron 0.003–0.008
Carbon 0.08 max
Nickel Balance


SOx, hot salt resistance, practical immunity to chloride ion and to polythionic
acid stress corrosion cracking, good resistance to sulfuric acid, and excellent
oxidation and carburization resistance at elevated temperatures.

15.15.6 Alloy 102 (N06102)
This is a nonmagnetic nickel–chromium-based alloy strengthened with
refractory metals. The chemical composition is given in Table 15.24. It
possesses excellent corrosion properties, strength, ductility, and toughness,
and has outstanding structural stability.
Alloy 102 exhibits excellent resistance to phosphoric, nitric, and organic
acids, alkalies, salts, seawater and chloride stress corrosion cracking. It has
good resistance to sulfuric acid and acceptable resistance to hydrochloric
and hydroﬂuoric acids.

Reference
Marcel Dekker.

16
Cast Nickel and Nickel-Based Alloys

Most cast nickel-based alloys are derived from wrought alloys (Table 16.1).
The nickel-based alloys are considerably more difficult to cast than austenitic
stainless steels (SSTs). Nickel-alloy castings should never be purchased
using a wrought-alloy trade name. Foundry selection is critical in obtaining
high-quality, corrosion-resistant castings. To develop the required expertise,
a foundry must pour nickel alloys on a daily basis. Other important factors
are dedicated high-alloy patterns, careful alloy selection, and additional
specifications beyond the normal American Society for Testing and Materials
(ASTM) requirements. Items covered would include foundry qualification,
heat qualification using a weldability test, raw material restriction, heat
treating, non destructive examination and repair welding.1–3

16.1 Commercially Pure Nickel
CZ100 is the cast commercially pure nickel grade. The wrought equivalent is
nickel 200. CZ100 has higher carbon and silicon for castability and is
generally used in the as-cast condition. Its properties are not affected by heat
treatment. Nickel is used for dry halogen gases and liquids (F2, HF, Cl2, and
HCl) and ambient temperature hydrofluoric acid. Nickel is used for caustics,
including sodium hydroxide and potassium hydroxide, over a wide range of
temperatures and concentrations. Ammonium hydroxide rapidly corrodes
nickel.4

16.2 Nickel–Copper
Monel is the Inco trademark of the original nickel–copper alloy developed in
the 1930s. Monel has excellent resistance to organic fouling and corrosion
in seawater. The most common cast grade is M35-1. Other cast grades are

279

TABLE 16.1
280

Cast Nickel-Based Alloys
Minimum Strength (ksi/MPa)
Speciﬁcation Wrought
and Grade Equivalent C Max Cr Ni Fe Mo Others Tensile Yield

ASTM A494 Nickel 200 1 — 95a 3a — — 50/345 18/125
Grade CZ100
ASTM A494 Monel 400 0.35 — Balance 3.5a — Si 1.25a 65/450 25/170
Grade M35-1
ASTM A494 Monel 400 0.35 — Balance 3.5a — Si 2a 65/450 30/205
Grade M35-2
ASTM A494 Monel 400 0.3 — Balance 3.5a — Si 1–2, Cb 1–3 65/450 32.5/225
Grade M30C
ASTM A494 S-Monel 0.25 — Balance 3.5a — Si 3.5–4.5 300 HB min —
Grade M25S aged
condition
ASTM A494 Inconel 600 0.4 14–17 Balance 11a — — 70/485 28/195
Grade CY40
ASTM A494 Inconel 625 0.06 20–23 Balance 5a 8–10 Cb 3.15–4.5 70/485 40/275
Grade
CW6MC
ASTM A494 Hastelloy C 0.02 15–17.5 Balance 2a 15–17.5 — 72/495 40/275
Grade CW2M
ASTM A494 Hastelloy C22 0.02 20–22.5 Balance 2–6 12.5–14.5 W 2.5–3.6 80/550 45/280
Grade CX2MW
ASTM A494 Chlorimet 3 0.07 17–20 Balance 3a 17–20 — 72/495 40/275
Grade CW6M
ASTM A494 Hastelloy B2 0.07 1a Balance 3a 30–33 — 76/525 40/275
Grade N7M
ASTM A494 Waukesha 88 0.05 11–14 Balance 2a 2–3.5 Bi 3–5, Sn 3–5 — —
Grade
CY5SnBiM
a
Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals

Maximum content.

Cast Nickel and Nickel-Based Alloys 281

M35-2 and M30C. M25S is a high-silicon nickel–copper alloy with superior
wear and galling resistance, but is the most difficult of all to cast. It is also
known as S-monel (trademark of Inco Alloys). These grades are used in the
as-cast condition, except M25.S that does respond to heat treatment. They are
the industry standards for oxygen, dry chlorine, fluorine, and hydrogen
fluoride gases (no water vapor present). They are also used for hydrofluoric
acid, neutral and alkaline salts, and sodium hydroxide.5 Other common uses
are brine and seawater.

16.3 Nickel–Chromium
CY40 is the cast equivalent of Inconel 600 (trademark of Inco Alloys). CY40 is
a nickel–chromium alloy without the molybdenum content of most other
nickel–chromium alloys. In most environments, the corrosion resistance of
CY40 is poor compared to the nickel–chromium–molybdenum alloys. Pitting
can occur in moist, humid conditions, seawater, chloride environments, and
salts. CY40 is used in steam, boiler feedwater, and alkaline solutions,
including ammonium hydroxide. The resistance to chloride Stress Corrosion
Cracking (SCC) is good.6

16.4 Nickel–Chromium–Molybdenum
Nickel–chromium–molybdenum alloys offer excellent corrosion resistance
and good mechanical properties over a wide range of environments and
temperatures. CW2M, the cast version of Hastelloy C, is the workhorse of the
group. Castings should not be called “Hastelloy.”7,8 The properties of the
different Hastelloy alloys vary widely in specific applications. Disaster can
result from use of the wrong grade.
CW2M has excellent corrosion resistance in many chemical process
environments, including hydrochloric and sulfuric acids at temperatures
below 1258F (528C). At low concentrations, the useful temperature range is
much higher. Corrosion resistance is excellent in organic acids. Contami-
nation by strong oxidizing species, such as ferric and cupric ions, will not
cause the accelerated attack common with other alloys such as Hastelloy B2.
CW2M is resistant to most forms of SCC, including chloride, caustic, and
H2S.9 AWS filler materials NiCrMo-7 or NiCrMo-10 maintain good
as-welded corrosion resistance.10 CW12MW is the original Hastelloy C
type of casting grade. Segregation problems inherent with the alloy resulted
in corrosion resistance inferior to wrought C276. CW12MW has been largely
replaced by CW2M. The casting characteristics, weldability, and ductility are
all greatly enhanced.


In addition to CW2M, there are a number of other nickel–chromium–
molybdenum casting alloys. Some of the alloys are CX2MW (cast Hastelloy
C22). CW6MC (cast Inconel 625), and CW6M (Chlorimet 3, trademark of
Duriron Co.).

16.5 Other Nickel-Based Alloys
N7M is the cast equivalent of Hastelloy B2. This nickel–molybdenum alloy
has excellent corrosion resistance in all concentrations and temperatures of
hydrochloric acid. If ferric or cupric ions are present, however, severe attack
will occur. It is also good for sulfuric, acetic, and phosphoric acids.11
CY5SnBiM is a proprietary alloy known as Waukesha 88 (trademark of
Waukesha Foundry). Tin and bismuth are added as solid metal lubricants for
improved galling resistance. It is primarily used in the food industry to
prevent galling against SST. Weld repairs are prohibited. It is not as
corrosion-resistant as other nickel-based alloys; however, it performs well in
food-industry applications.

References
1. J.L. Gossett. 1988. Improved high-nickel alloy castings, Materials Performance,
27:12, 44–47.
2. J.L. Gossett. 1989. Speciﬁcations for obtaining high-quality, high-nickel alloy
castings, Materials Performance, 28:1, 64–66.
3. J.L. Gossett. 1989. Reliable base for high nickel equipment, Chemical
Engineering, November, 145–148.
4. Inco Alloys. 1972. Nickel Alloys Bulletin, Huntington, WV: Inco Alloys, pp. 7–12.
5. E.C. Hoxie. 1986. Nickel and nickel-base alloys, in Process Industries Corrosion,
B.J. Moniz and W.I. Pollock, Eds., Houston: NACE international, pp. 463–464.
6. Inco Alloys. 1973. Inconel 600 Bulletin, Huntington, WV: Inco Alloys, pp. 14–18.
7. F.G. Hodge. 1981. Hastelloy C-4C, an Improved Nickel Base Casting Alloy for the
CPI.
8. F.G. Hodge. 1983. Cast alloy resists corrosion by hot acids and oxidizing agent,
Industrial Research and Development, 25:July, 82–85.
9. Haynes International. 1978. Corrosion Resistance of Hastelloy Alloys, Kokomo, IN:
Haynes International.
10. J.L. Gossett. 1988. New and improved, high nickel alloy castings, in Proceedings
of the BHRA Conference, Development in Valves and Actuators for Fluid Control,
Manchester, UK, pp. 134–154.
11. F.G. Hodge and R.W. Kirchner. 1976. An improved Ni–Mo alloy for
hydrochloric acid service, Materials Performance, 15:8, 40–45.

17
Comparative Corrosion Resistance of Stainless
Steel and High-Nickel Alloys

The corrosion tables on the following pages are arranged alphabetically
according to corrodent. The chemicals listed are in the pure state or in a
saturated solution unless otherwise indicated. Compatibility is shown to
the maximum allowable temperature for which data are available. Symbols
used to designate speciﬁc corrosion rates are as follows:

E indicates that the corrosion rate rate is !2 mpy.
G indicates that the corrosion rate is between 2 and 20 mpy.
S indicates that the corrosion rate is between 20 and 50 mpy.
U indicates that the corrosion rate is O50 mpy and therefore not
recommended for this service.

Further information regarding the corrosion of speciﬁc materials by
certain corrodents is provided by the following symbols. In the tables, the
symbols follow the applicable material:

Symbol Meaning

1 Material is subject to pitting
2 Material is subject to stress cracking
3 Material is subject to crevice attack
4 Applicable to alloy 825 only
5 Material is subject to intergranular corrosion
6 Material not to be used with carbonated
beverages
7 Corrodent must be acid free
8 Corrodent must be acid free and the material
passivated
9 Corrodent must be alkaline
10 Material is subject to stress cracking when
corrodent is wet
11 Corrodent must be sulfur free
ELC Material must be low carbon grade

283


Corrosion rate is shown as a function of temperature. The use of the
temperature scale is explained by the following example.

Acetic acid, 80%
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560
Metals ˚F

Monel E G

Stainless steels

Type 316 E G S U
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440
˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227
˚C

From the above it is seen that Monel has a corrosion rate varying with
temperature as follows:

!2 mpy between 60 and 1208F (E—)
!20 mpy between 120 and 2108F (G—)
No data beyond 2108F

Type 316 stainless steel has a corrosion rate varying with temperature as
follows:

!2 mpy between 60 and 1008F (E—)
!20 mpy between 100 and 1808F (G—)
!50 mpy between 180 and 2408F (S—)
O50 mpy above 2408F (U)

In reading the temperature scale, note that the vertical lines refer to
temperatures midway between the temperatures cited (refer to example
given above).

Comparative Corrosion Resistance of Stainless Steel and High-Nickel Alloys 285

Acetaldehyde

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560
Stainless steels ˚F

Type 304/347 E G

Type 316 E G

Type 317 G

Type 321

Alloy 20Cb3 E G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetamide

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201

Alloy B -2

Alloy C-276 G

Alloy 600/625

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetic acid, 10%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317 G

Type 321 U

Alloy 20Cb3 E

Alloy 800/825 4 E

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1 E

Type 410 G

Type 430 G

Type 444 G

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E G

Alloy C-276 E

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetic acid, 20%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 E

Type 317 G

Type 321 U

Alloy 20Cb3 E

Alloy 800/825 4 E

Alloy Al6XN G

Type 904L

Type 17-4PH G

E-Brite 26-1 E

Type 410 G

Type 430

Type 444 G

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E G

Alloy C-276 E

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetic acid, 50%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G S

Type 316 E

Type 317 G

Type 321 U

Alloy 20Cb3 E G

Alloy 800/825 4 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1 E

Type 410 G

Type 430 U

Type 444 G

Nickel and high-
nickel alloys

Nickel 200/201 G S

Alloy B -2 E

Alloy C-276 E G

Alloy 600/625 S U

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetic acid, 80%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E G S U

Type 316 E G S U

Type 317 G

Type 321 U

Alloy 20Cb3 E G

Alloy 800/825 4 E

Alloy Al6XN G

Type 904L G

Type 17-4PH G

E-Brite 26-1 E

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E G

Alloy C-276 E

Alloy 600/625 S U

Monel 400 E G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetic acid, glacial

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E G U

Type 316 E

Type 317 G

Type 321 U

Alloy 20Cb3 E G

Alloy 800/825 4 E

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1 E

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 G

Monel 400 E G U

Alloy G/G3

Alloy C-22, 99% E

Alloy D E
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetic anhydride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317 G

Type 321 G

Alloy 20Cb3 G

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1 G

Type 410 U

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G S

Alloy B -2 G

Alloy C-276 E

Alloy 600/625 G

Monel 400 G S

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetone

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317 G

Type 321

Alloy 20Cb3 E

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 E

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2 E G

Alloy C-276 E G

Alloy 600/625 E

Monel 400 E

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acetyl chloride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347, dry G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Acrylonitrile

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Adipic acid

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2

Alloy C-276 E

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Allyl alcohol

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2

Alloy C-276 G

Alloy 600/625 G

Monel 400 E

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Alum

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 G

Type 317

Type 321 U

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G S U

Alloy B -2 G U

Alloy C-276 G U

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aluminum chloride, aqueous

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G U

Type 316 G U

Type 317 U

Type 321 U

Alloy 20Cb3 G U

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 U

Monel 400 G S U

Alloy G/G3

Alloy D E G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aluminum chloride, dry

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G U

Type 316 G U

Type 317 U

Type 321 U

Alloy 20Cb3 G U

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 U

Monel 400 G S U

Alloy G/G3

Alloy D E G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aluminum fluoride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 G

Type 317

Type 321

Alloy 20Cb3 U

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2, 5% E

Alloy C-276, 10% G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aluminum hydroxide

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aluminum nitrate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 E

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201

Alloy B -2

Alloy C-276

Alloy 600/625

Monel 400 U

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aluminum sulfate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 1 G U

Type 316 G U

Type 317 50 − 55% G

Type 321 G

Alloy 20Cb3 G

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G U

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 U

Monel 400 G U

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonia, anhydrous

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 E

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 E

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 E

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium bifluoride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347

Type 316, 10% G

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201

Alloy B -2 E

Alloy C-276 G

Alloy 600/625

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium carbonate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 E

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C



15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825 4 E

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1 G

Type 410 1 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 2 E

Monel 400 E G

Alloy G/G3 G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C



15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 E

Monel 400 E

Alloy G/G3, 28% G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium chloride, saturated

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317

Type 321

Alloy 20Cb3 1 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium hydroxide, 10%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E G

Type 316 E G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH E

E-Brite 26-1

Type 410

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2 G

Alloy C-276 E G

Alloy 600/625 G

Monel 400 U

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium hydroxide, saturated

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 U

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium nitrate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 2 E

Type 316 2 E

Type 317 G

Type 321

Alloy 20Cb3 E

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 E G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2, 40% G

Alloy C-276, 10% E

Alloy 600/625 S

Monel 400 U

Alloy G/G3

Alloy D U
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium persulfate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347, 10% U

Type 316, 10% G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410, 5% E

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2 U

Alloy C-276, 10% E

Alloy 600/625 E

Monel 400 U

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium phosphate, 5%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347, 40% E

Type 316, 40% G

Type 317 G

Type 321 G

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201, 30% G

Alloy B -2 E

Alloy C-276, 10% G

Alloy 600/625, 10% G

Monel 400, 30% G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium sulfate, 10–40%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 G

Type 317 G

Type 321 G

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 1 G

Monel 400 G

Alloy G/G3

Alloy D G

˚F
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Ammonium sulfite

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 U

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2

Alloy C-276 E

Alloy 600/625 G U

Monel 400 G U

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Amyl acetate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825 4 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 1 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 E

Monel 400 E

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Amyl alcohol

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201

Alloy B -2

Alloy C-276 G

Alloy 600/625

Monel 400 E G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Amyl chloride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 U

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 U

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aniline

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aniline hydrochloride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444 U

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 U

Alloy C-276 U

Alloy 600/625 U

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Antimony trichloride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317 U

Type 321 U

Alloy 20Cb3 E

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Aqua regia 3:1

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317 U

Type 321 U

Alloy 20Cb3 U

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1 U

Type 410

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2 U

Alloy C-276 U

Alloy 600/625 U

Monel 400 U

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Arsenic acid

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825, 90% G

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2 G

Alloy C-276 G

Alloy 600/625

Monel 400 U

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Barium carbonate, 10%

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Barium chloride

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 1 G

Type 317

Type 321

Alloy 20Cb3, 40% G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 1 G

Type 430 1 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E G

Alloy C-276 E

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Barium hydroxide

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3, 50% E

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 E

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Barium sulfate

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Barium sulfide

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2

Alloy C-276 G

Alloy 600/625

Monel 400 S

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Beet sugar liquors

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1 E

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2

Alloy C-276

Alloy 600/625 E

Monel 400 E

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Benzaldehyde

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH

E-Brite 26-1 G

Type 410

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Benzene

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317 G

Type 321 E

Alloy 20Cb3 E

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Benzene sulfonic acid

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 E

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444 U

Nickel and high-
nickel alloys

Nickel 200/201, 10% G U

Alloy B -2 G

Alloy C-276 G

Alloy 600/625

Monel 400 G

Alloy G/G3

Alloy D, 10% G
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Benzoic acid

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 G

Type 316 G

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276, 10% E

Alloy 600/625, 10% G

Monel 400 G

Alloy G/G3

Alloy D, 10% E
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Benzyl alcohol

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 G

Alloy C-276 G

Alloy 600/625 G

Monel 400 E

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Borax

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 E

Type 316 E

Type 317

Type 321

Alloy 20Cb3 G

Alloy 800/825 G

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 E

Type 430 5% G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 G

Monel 400 E

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Boric acid

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 1 G U

Type 316 1 G U

Type 317 G

Type 321 G

Alloy 20Cb3 G

Alloy 800/825, 5% E

Alloy Al6XN

Type 904L

Type 17-4PH G

E-Brite 26-1

Type 410 G

Type 430 1 G

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 G

Alloy B -2 E

Alloy C-276 E

Alloy 600/625 G

Monel 400 G

Alloy G/G3

Alloy D E
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Bromine gas, dry

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317 U

Type 321 U

Alloy 20Cb3 E

Alloy 800/825 4 E

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 E

Alloy B -2 E

Alloy C-276 E G

Alloy 600/625 G

Monel 400 E

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Bromine gas, moist

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317 U

Type 321 U

Alloy 20Cb3 U

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201 U

Alloy B -2

Alloy C-276 E

Alloy 600/625 U

Monel 400 U

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216

238
249
260
271
282
293
227

˚C


Bromine liquid

15
26
38
49
60
71
82
93
104
116
127
138
149
160
171
182
193
204
216
227
238
249
260
271
282
293
˚C

60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560

Type 304/347 U

Type 316 U

Type 317 U

Type 321 U

Alloy 20Cb3

Alloy 800/825

Alloy Al6XN

Type 904L

Type 17-4PH U

E-Brite 26-1

Type 410 U

Type 430 U

Type 444

Nickel and high-
nickel alloys

Nickel 200/201

Alloy B -2

Alloy C-276

Alloy 600/625

Monel 400

Alloy G/G3
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420

460
480
500
520
540
560
440

˚F
15
26
38
49
60
71
82
93
104
116
127
138
149
160
171

Fundamentals of metallic corrosion

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Fundamentals of metallic corrosion