Corrosion Sl Part Two
- 1. Module 2 “The Forms of Corrosion”
- 2. Corrosion Forms on Metals General Localized (pitting and crevice) Galvanic- Stress Corrosion Cracking Flow-assisted Intergranular Dealloying Fretting Corrosion Fatigue High-temperature
- 3. General Corrosion Definition Characterized by chemical reactions, oxidation and reduction, that proceed uniformly over the entire wetted surface. Uniform attack, general corrosion, represents the greatest destruction of metal on a tonnage basis. This form of corrosion is easily monitored and the life of equipment accurately estimated.
- 5. General Corrosion due to Dissimilar Surfaces Mill Scale = Cathode New (Anode) –Old Pipe (Cathode) Stressed Surfaces= Anode Welded Sections (Heat Affected Zone)= Anode
- 6. New—Old Pipe Cell Old Pipe (cathode) New Pipe (anode) Old Pipe (cathode)
- 7. Different Stress The portion of the metal under higher stress, (eg. cold worked) will be anodic to the portion of the metal at a lower stress level. High Stress =Anode Low Stress = Cathode
- 8. General Corrosion due to Dissimilar Electrolytes In most cases, the metal immersed in the more conductive solution will be the anode, and the metal immersed in the less conductive solution will be the cathode. More Conductive = Anode Less Conductive = Cathode
- 9. General Corrosion due to Oxygen Concentration The metal immersed in the oxygen lean electrolyte will be the anode , and the metal immersed in the oxygen rich electrolyte will be the cathode . Oxygen lean = Anode Oxygen rich = Cathode
- 10. Metal Ion Concentration Cathode –in saturated solution Anode –in unsaturated solution
- 11. General Corrosion Remedies Coatings Linings Alloy selection Change the chemistry Corrosion inhibitors Cathodic or anodic protection
- 12. General Corrosion Spray Tower Severe corrosion of the inner carbon steel walls at the mid-section of a tower between the two spray levels was found. More than half of thickness was gone from the original 1/4 plate in 8 years. Notice support beams from upper level to the lower level. Cause high temperature and condensation water from sprayed detergent slurry.
- 13. Neutralization Mixers Reactors that see various concentrations of sulfuric acid should use Alloy 20 UNS No- N08020.
- 14. Heating Coils Heat transfer surfaces are very susceptible to general and localized pitting corrosion due to the higher temperature. Material selection for heat transfer surfaces is therefore important.
- 15. Atmospheric Corrosion Example Electrical Panels are an area where corrosion can play havoc. For example where the surrounding air is high in salt or acidity. Electrical fires have been associated with corroded contacts. Sealing these areas from the outside air and moisture is important.
- 16. Handouts Reference General Corrosion Carbon Steel- pp. 373-403 “Carbon and Low Alloy Steels” Stainless Steel- pp. 553-559 “Corrosion of Stainless Steels”
- 17. Localized Corrosion Definitions Pitting A deep, narrow corrosive attack in a localized region which often causes rapid penetration of the substrate thickness. Crevice Corrosion A form of localized attack in which the site of the attack is an area where free access to the surrounding environment is restricted.. Filiform Corrosion A special form of oxygen cell corrosion occurring beneath organic or metallic coatings on materials. The attack results in a fine network of random “threads” of corrosion product developed beneath the coating material.
- 18. Pitting on Stainless Steel 304 SS Micro view of pitting at 50x magnification
- 19. Copper Pitting
- 20. Pitting Remedies Use more alloyed material ( molybdenum is key alloying element) Reduce eliminate the pitting agents Lower the process temperature Coatings Cathodic protection In the case of SS increase process velocity
- 21. Electrochemical Lab Pitting Data First graph showing the effect of pH and Cl- at 35C Second graph showing the effect of increasing temp. with 4 diff. Cl- conc. The third graph showing pitting potentials with different alloys with increasing temperatures.
- 22. Heat Exchangers (above 140F with salt) 316L Stainless Steel dryer heat exchangers are susceptible to localized pitting of heat exchanger tubes . Past inspections showed shallow pits on the inner tube surfaces that eventually led to failure. Dryer heat exchangers should be fabricated out of a material with at least 4% molybdenum to avoid pitting. 316 has only about 2% molybdenum.
- 23. Handouts Reference Pitting Carbon Steel p. 385,389 “ Carbon and Low Alloy Steels” Stainless Steel p.113-114 “Localized Corrosion”
- 24. Exercise 2 Soft-soap Business Issue” Instructions: Form into groups of three with people from other functional groups if possible. Look at slide pictures Read Background Answer Questions Be prepared to share your answers with the rest of the class
- 25. Background “Soft-soap Pump Springs” A Soft-soap product experienced corrosion of 302 SS pump springs that could be seen by the consumer on the store shelf The corrosion occurred mainly on spring parts that were in contact with the plastic . Shown here is corrosion on last two winds
- 26. Crevice Corrosion Background M+Cl- + H2O = MOH + H+Cl- Corrosion Inside Crevice (Anode) Chemical Reaction with Salt NaCl
- 27. Electron Micrograph of 302 Spring Showing corrosion also occurring between spring coils
- 28. Results of Statistical Analysis Conditions: 120 F in Pump after 49 days
- 29. Results of Statistical Analysis
- 30. Another Example of Crevice Corrosion
- 31. Exercise 2-1 Soft-Soap Pump Springs What type of corrosion occurred on the 302 SS spring? What would be two possible solution based on the statistical data and remedies? Questions to answer
- 32. Crevice Corrosion Remedies Use continuous complete penetration welds Laps joints should be seal welded Remove deposits frequently Use proper packing materials in joints Avoid crevices in heat exchanger tube sheet Use higher alloyed material or plastic Use a corrosion inhibitor such as Sodium sulfate Alter the chemistry by adjusting the pH or lowering the salt concentration
- 33. Forms of Cracking Corrosion Definitions Stress Corrosion Cracking Refers to metal cracking caused by the simultaneous presence of tensile stress and a corrosive chemistry. Corrosion Fatigue Results from the combined action of a cyclic tensile stress and and a corrosive environment. Hydrogen-Induced Cracking (HIC) Results from the combined action of a tensile stress and hydrogen in metal.
- 34. Stress Corrosion Cracking Susceptibility
- 35. Chloride Stress Cracking Temperature Above 140 F Stress Corrosion Cracking occurs. Therefore if possible avoid continuous process operations above this temperature with 304/316 SS
- 36. Cracking - On A Microscopic Level Cracks that run across grains is called transgranular Cracks that follow grain boundaries are called intergranular
- 37. Stainless vs. Other Alloys Stress Cracking Lab Data
- 38. Lye Separators After about 9 years stress corrosion cracking was pervasive in a continuos lye seperation plant. A typical crack is shown here. The material of construction was 304 SS. Crack
- 39. Stress Corrosion Cracking 100x mag.
- 40. Area A Area B Area C Carbon steel Carbon steel stress relieve Nickel based alloys Carbon Steel Caustic Soda Service Chart Sodium hydroxide concentration 0 100 180 F Stress cracking line 120 F
- 41. Ammonium Chloride This is an example where a tank fluid is not compatible with 316 stainless steel. Pitting and stress corrosion cracking with quaternary ammonium chloride fabric softener.
- 42. Stress Cracking Remedies Evaluate a Duplex alloy- 2205 or 2507 Increase nickel concentration in the alloy Lower hardness of alloy through heat treat Lower process temperature (60C max) Cathodic Protection Remove the bad actor ions such as chloride
- 43. Lye Evaporators During the evaporation step, lyes can be corrosive to 316 SS alloy heat exchangers by pitting and stress corrosion cracking. In addition lyes can contain NaClO3. NaClO3, Sodium Chlorate, is corrosive to stainless steels. If this is the case, an upgrade in heat exchanger metallurgy is necessary to Inconel 625 for tubing.
- 44. Fatigue Cracking Expansion joints that undergo high stress and strain due to heat/mechanical forces are susceptible to corrosion fatigue cracking . In general the higher the nickel concentration in the alloy the better the fatigue properties.
- 45. Corrosion Fatigue Remedies Reduce the grain size Improve the surface finish Increase alloying elements ( Ni, Cr, Mo)
- 46. Surface Preparation and Corrosion Fatigue In general the fatigue life increases as the magnitude of surface roughness decreases. Best - A Metallographic finish, free of machining grooves and grinding scratches
- 48. Galvanic Corrosion Definition Corrosion due to the electrochemical potential difference between different metals. However, potential differences can be caused by differences in the homogeneity of the same metal.
- 49. Corrosion Galvanic
- 50. Galvanic Series More Reactive Less Reactive Potassium Magnesium Beryllium Aluminum Zinc Chromium Iron Nickel Tin Copper Silver Platinum Gold
- 51. Galvanic Example in Bottle Making Equipment that is used to make plastic bottles often times uses different metals to conduct heat at different rates. For example beryllium copper and aluminum. This alloy combination allows bottle thicknesses to differ for design purposes. Galvanic corrosion can and did occur at these joints. Replacement of the expensive parts were needed. In this case the water for cooling needs to be buffered to a pH of 8 and eliminate as much salt as possible.
- 52. Galvanic Corrosion Remedies Choose alloys of similar chemical potential Avoid bad area ratios: large cathode/small anode surface areas Insulate dissimilar alloys Coatings apply with caution ( pinhole free) Change the environment ( chemistry)
- 53. Flow-Assisted Corrosion Definitions Erosion-corrosion Occurs when the velocity of the fluid is sufficient to remove protective films from the metal surface. Impingement Corrosion Localized erosion-corrosion caused by turbulence or impinging flow. Cavitation Corrosion Mechanical damage process caused by collapsing bubbles in a flowing liquid.
- 55. Plodder Worms- Mixers Typically are silicon aluminum castings . These alloys are susceptible to erosion and pitting corrosion with time.
- 56. Erosion Corrosion Remedies Change the metallurgy ( adding chrome and moly 316 SS) Alter the design ( provide for large sweeps, eliminate 90 elbows) Lower the velocity ( less than 2 meters per second) Raise the pH ( pH above 9 helps) Lower the temperature Deaerate Filter Weld overlays or metallic coatings Rubber linings
- 57. Intergranular Corrosion Definition Corrosion due to the preferential attack at, or adjacent to, the grain boundaries of a metal. Most often associated with welds in low pH fluids. Caused by chromium being tied up by carbon.
- 58. Intergranular Corrosion 500 x mag. Dark areas are corroded
- 59. Intergranular Corrosion Remedies Use low carbon stainless steel (316L) Ti Stabilized steel in Europe Use higher alloy filler metals Note: Nitric acid can cause intergranular corrosion of sensitized (welded) 304 and 316 SS.
- 60. Fretting Corrosion Definition Fretting corrosion is defined as metal deterioration caused by repetitive slip at the interface between two surfaces in contact. It is not corrosion due to rotation or erosion. More exactly.. motion from vibration and corrosion effects during that time.
- 61. Fretting Corrosion Examples Axles Housings and shafts Spline connections Pin joints Riveted lap joints
- 62. Resistance to Fretting Corrosion Under dry conditions Steel on steel Low Nickel on steel Low Aluminum on Aluminum Low Zinc on steel Medium Copper on steel Medium Silver plate on steel High
- 63. High-temperature Corrosion A form of material degradation classified above ( 750+ F). General or pitting corrosion occurs.
- 64. High Temperature Corrosion Oxidation reactions include water vapor, hydrogen, hydrogen sulfide, ammonia, sulfur dioxide, chlorine, sulfur, carbon dioxide and oxygen Development of scales Corrosion products solids liquids gases
- 65. High Temperature Remedies Determine possible mechanism of corrosion - sulfidation, carburization, hydrogen effect Select alloy most resistant to above Carbon steel maximum temperature range is 800- 850 F in air.
- 66. Summary of Scaling Temperatures in Air 1010 Steel ... 900F Alloy Steel, 9Cr, 1.0 Mo...1200 F 410 SS, 12 Cr ...1400 F 304, 321, 347 18 Cr, 8 Ni... 1650 F 310 SS, 25 Cr, 20 Ni... 2100 F
- 67. Exercise 2-2 Sulfur Burner Exhaust Piping High temperature corrosion of 321 SS at 1300 F with SO2 The thickness of the plate was 1/8 inch and lasted less than two years.
- 68. Handout on High Temperature Corrosion See Corrosion by “Hot Gases and Molten Compounds” -- Specific to this case pp. 243-248 .
- 69. Corrosion of Stainless Steel in Sulfur vapor 1300 F Lab Data 314 16.9 mpy mils per year 310 18.9 309 22.3 304 27.0 302B 29.8 316 31.1 321 54.8 Haynes alloy 556 6
- 70. Exercise 2-2 Sulfur Burner Exhaust Review the previous data and the High Temperature Corrosion handout. What is the term used to describe this type of corrosion attack? What should be the recommended material of construction?
- 71. What did we learn? The most common forms of corrosion attack on metals. Metals and non-metals can become brittle and crack when misapplied. Carbon steel is attacked in a general fashion, making it easy to monitor and correct. Stainless steels on the other hand, are attacked in a localized fashion, making it difficult to monitor and correct. The critical operating temperature for stress corrosion is above 140 Fahrenheit for stainless steels 304 and 316. Stainless steels need to be selected carefully. There are different types of high temperature corrosion. That corrosion issues can be dealt with in different ways. Engineering processes and structures need to be inspected and protected from corrosion.
Editor's Notes
- #2: In module 2 we will take a look at what kinds of corrosion occur on different kinds of metals for example carbon steel vs. stainless steel. The types of corrosion damage will be related in pictures so that you can see what it looks like visually. You will be able to classify the corrosion this way . The remedies for each form of corrosion will be introduced. The remedies are the thought process for deciding direction in eliminating a corrosion issue.
- #3: As you can see there are several forms of corrosion. The most common ones for Colgate are general, crevice, pitting and stress corrosion cracking. General being mostly seen in carbon steel equipment such as tanks. Crevice, pitting and stress corrosion are localized forms of corrosion and is generally seen in stainless steel equipment.
- #4: General corrosion can be monitored using ultrasonics. Unexpected failure therefore is not common. Decreasing wall thickness can be monitored with time.
- #5: The typical bright red orange corrosion of carbon steel in the outdoor environment is shown here. Another word used for it is hematite which is a Iron oxide.
- #6: Mills scales is what is formed on the steel at the mill during processing. It is a tight adherent scale that forms during hot fabrication or heat treatment of metals. In a piping system if you weld in a new piece of pipe it is the anode. Oxidation occurs more readily on new pipe than old pipe because the old pipe already has an oxidation product on the surface. If a material is bent or welded it is at higher energy state therefore more ready to corrode to a lower energy state.
- #7: This shows the reason why a new pipe section could fail sooner than the old pipe. It becomes a smaller anode in a larger cathode pipe.
- #8: Additional stress can come from mechanical working or welding. Bending, hammering, is are examples of mechanical.
- #9: Conductance is a measure of the ease of electron flow through an electrolyte or a solid metal. Resistance to electrical flow is measured in ohms.
- #10: The gases present in the electrolyte and their concentration can cause localize corrosion cells. The main one being oxygen.
- #11: You can create a potential difference between high and low concentrations of metallic ions against a metallic surface. The areas in high concentration of metallic ions become cathodic to the lower concentration anodic areas. Therefore a corrosion cell is formed.
- #12: Typical coatings are paints and linings are thicker. The decision to use a coating or a lining depends on the operating conditions; i.e. chemistry and temperature. If the general corrosion rate is greater than 20 mils per year then alternative metals are generally used over steel , such as stainless steel. By adjusting the pH of the solution we can decrease the corrosion rate. Corrosion inhibitors slow either the anodic or cathodic reaction. Cathodic protection is using a sacrificial metal to protect the metal you do not want to corrode. Anodic protection is using reverse current through a rectifier etc. to cause electron flow in the opposite direction than what normally occur during corrosion.
- #13: It is important to note here that structures can be compromised by corrosion. The shell of this tower became so thin at the point of the red arrow that support beams had to be installed to take the load off this section. The solution was to fillet weld new carbon steel plate over the corroded areas. It was too hot in general to solve the problem with a coating or lining.
- #14: There are quite a few new alloys out there but the best still for sulfuric acid is Alloy 20. Sulfuric acid can be oxidizing or reducing depending on the concentration. And Alloy 20 has the right composition to handle both except the mid range about 75% sulfuric which generally needs other alternatives than metal.
- #15: We use nickel alloys for high temperature caustic for example because of cracking issues with carbon steel.
- #16: This slide is a statement to point out that the atmosphere can be corrosive and should be accounted for in plant site selection etc.
- #17: Handouts for reference in further reading.
- #18: Areas where we would see the most localized corrosion would be in our bleach and soap making business. The bleach and soap contain high amounts of salt. It is important to note that pitting and crevice corrosion are the most common forms of corrosion on stainless steels.
- #19: As you can see from the photograph, pitting is very localized in nature and the rest of the surface is not attacked. This type of corrosion could lead to product contamination. Microbes such as sulfate reducing bacteria can and do cause this type of corrosion.
- #20: Common in sulfates, moist chlorine, and chlorides.
- #21: Depending on when the pitting issue is determined will dictate the remedy. For example if after a launch you find you have a pitting issue, one of the quickest ways to slow it down is through adjusting the pH in the alkaline direction. Or if stagnant pockets are designed into the process , the best thing is to eliminate them or speed up , more agitation, added to the process.
- #22: Three effects are shown here Chloride concentration has a significant impact on the pitting potential Temperature has more of an effect on lower salt concentrations in that it raises the pitting susceptibility. As the temperature increases there is less differentiation between different metals and their associated pitting potentials.
- #23: Pitting is a concern mainly in areas where you have tubes that are thin walled. Tubes can be less than 1mm thick and the time perforate in a pitting environment can be a matter of weeks. In this case super fatted soap lowers the available NaOH making pitting easier to happen.
- #25: The purpose of this exercise is to give participants working knowledge on how to resolve a type of localized corrosion phenomenon on a low alloy stainless steel spring.
- #26: This is an example of corrosion that could be seen by the consumer.
- #27: Very common form of corrosion and it is what caused the corrosion of soft-soap pump springs. Cleaning of vessels and piping is one way of preventing crevice corrosion on stainless steel. The deposits on the surface act as crevice corrosion initiators.
- #28: A electron micrograph is a way to look at surfaces under very high magnification.
- #29: This slide shows data produced in Piscataway.
- #30: Interesting to note here the change in days for the onset of corrosion to begin from 25 to 45+ at 2.5% NaCl and pH.
- #31: Piping connections joined by flanges is one of the most common areas where crevice corrosion initiates.
- #32: 1. Crevice Corrosion 2. change the formula, lower the salt concentration below 1%. Raising the pH would help also. Adding an inhibitor such as sodium sulfate. Or changing the alloy for the spring material. Use a higher alloy for the spring
- #33: Packing materials that are best are those that do on adsorb moisture such as Teflon.
- #34: Cracking failures are dangerous. It could lead to a safety incident or disruption of plant production.
- #35: The important point here is that 304 and 316 SS are susceptible to cracking corrosion. Mostly in chlorides and hydroxides.
- #36: This is a very important slide and should be taken very seriously. When operating processes with salt above 140 F , do not use 304 or 316 SS. Use alloys with at least 25 Nickel and 4% Molybdenum if the pH is 7 or below.
- #37: The important point here that corrosion can cause metal to become brittle and break into small pieces. These pieces could get into our products and cause a significant business issue.
- #38: This shows the effect of adding nickel to an alloy to reduce the risk of stress corrosion cracking. Duplex alloys being on the outside danger region to the left of the green curve.
- #39: Stress cracking can cause a number of problems. Leaking from the process to the jacket side for example. It can eventually cause a plant system to just fall apart. The metal becomes brittle and cannot be rewelded.
- #40: Note the effect of corrosion here . It attacks the metal in a very localized fashion but eventually weakens the structure significantly.
- #41: This show the cracking potential of sodium hydroxide. At elevated temperatures or concentrations of NaOH it is wise to consult on materials of construction.
- #42: Example of tank issues with salt containing materials.
- #43: There could be a significant benefit to using a duplex alloy over 316 SS or a higher alloyed material with more nickel which is more costly. For example 2205 SS is the same basic cost as 316L SS and is not susceptible to stress corrosion cracking.
- #44: The reason for requiring a very alloyed material such as Inconel 625 is that Chlorate is an oxidizing salt that is very corrosive from a pitting standpoint as well as a stress corrosion standpoint. So you need over 30% Nickel and a minimum of 6% molybdenum.
- #45: Fatigue cracking has occurred in sulfur burning-SO2 piping.
- #46: Surface finish and using more corrosion resistant alloys are the easiest remedies for fatigue.
- #48: Found in Oil refineries.
- #49: Whenever you build a plant out of more than one material of construction you have the potential for galvanic corrosion.
- #50: Corrosion driven by the potential difference between copper and aluminum. Aluminum being anodic to copper and therefore corroded in this picture.
- #51: This chart shows the large number of elements apart that copper and aluminum are. It is also a basic chart and it is mostly used for teaching purposes.
- #52: A unique example how galvanic corrosion caused a considerable amount of problems for Colgate in bottle making.
- #53: Self explanatory slide.
- #54: In Colgate areas where erosion can be a concern are piping and pumps. Piping carrying high amounts of solids or abrasives for example.
- #55: The shows the elbow effect.. Particles crashing into the pipe wall just after the bend causing erosion of the metal at that point.
- #56: In the soap barrel there is pressure to move the soap out, this fact, and the fact the soap has salt and sometimes abrasive, the worm wears or erodes with time. The clearance changing from 1 to 3 mm over several years between the barrel and the worm.
- #57: Erosion is best controlled in the design.
- #58: Most common when using high carbon stainless steels.
- #59: You can see here that this corrosion is also quite serious. The grains of the metal are eaten away allowing it to basically crumble.
- #61: Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces when motion between the two surfaces is restricted or in other words machine components that are considered fixed and not expected to wear.
- #63: This basically shows that like materials do not have high fretting corrosion resistance. Like materials attract and unlike material repel each other which is good when designing for fretting corrosion resistance.
- #64: High temperature in general starts to occur around 750 degrees Fahrenheit.
- #71: Sulfidation 310 or 314 SS.