2-Corrosion-Its-Control.ppt

 INTRODUCTION
 CAUSES OF CORROSION
 THEORIES OF CORROSION
 CORROSION REACTIONS
 FACTORS EFFECTING THE CORROSION
 TYPES OF CORROSION
 CORROSION CONTROL METHODS

 Destruction of a metal by chemical or electrochemical reaction
with its environment.
 Physical or mechanical wearing away of a metal is not called
corrosion but is called erosion
 If corrosion and erosion take place together the destruction of
the metal takes place very much faster
 Corrosion is an unintentional attack on a material by reaction
with a surrounding medium
 The corrosion problem should be very seriously considered in
all those cases where the structure or the equipment is meant
to last many years say 20 years…

Corrosion is the deterioration of materials by chemical interaction
with their environment. The term corrosion is sometimes also
applied to the degradation of plastics, concrete and wood, but
generally refers to metals.

 The chemical combined state of the metal is known as “ore”
 The ore has low energy in its thermodynamical state.
 The extracted metal has high energy ,thermodynamically
unstable .
 Metal converts itself into stable state by reacting with the
environment and undergoes corrosion.
 The corroded metal is thermodynamically stable.

 The valuable metallic properties like conductivity,malleability,
ductility etc are lost due to corrosion.
 Life span of the metallic parts of the machineries is reduced.
 The process of corrosion is very harmful and is responsible for
the enormous wastage of metal in the form of its compound.
 The failure of the machinery takes place due to lose of useful
properties of metals.
 The approximate estimate of loss of metal due to corrosion is
2 to 2.5 billion dollars per annum all over the world.

Different types of electrochemical corrosions are
 Galvanic corrosion
 Pitting corrosion
 Intergrannular corrosion
 Water line corrosion

 This corrosion occurs when different metals are in contact and
exposed to corrosive atmosphere.
 The metal which has higher electrode potential value will form
anode and undergoes corrosion. Example: Zn-Cu galvanic
cell zinc behaves as anode and corrosion occurs, copper
behaves as cathode and protected.

 When corrosion starts on a metal surface
at certain spots they become less open to
oxygen with the result that corrosion
proceeds faster at those very spots making
deeper and deeper cavities.
 Pitting corrosion can be explained on the
basis of differential aeration.
 The pit created becomes deeper, its
bottom becomes less open to oxygen
which makes it more anodic.
 The part which has higher oxygen
concentration is cathodic.

 The concentration of oxygen dissolved in
water is greater at the surface than deeper
down. This causes formation of a
concentration cell.
 Anode is the lower portion .
 Cathode is at the water level.
 Due to the poor conductivity of water the
ions just below the water level are
available for reaction, the metal corrodes
just below the water level.

 In atmospheric corrosion the metals tend to revert to the states in
which they occur in nature.
 Several theories of corrosion have been proposed from time to
time..
 The three theories are
i) ACID THEORY
ii) CHEMICAL CORROSION (DRY CORROSION)
iii) ELECTROCHEMICAL CORROSION (WET
CORROSION)

Acid theory of corrosion considers acid to be the main cause.
Rusting takes place faster in acid solutions than in neutral or basic
solutions.
Oxygen and water are necessary for rusting of iron.
This is not true for all the metals like Zn etc..
Rate of corrosion in absence of Carbon di oxide is also seen.

 The direct chemical reaction of environment/atmospheric
gases or inorganic liquids with metal surfaces .
 There are three types of chemical corrosion
i) Oxidation corrosion
ii) Corrosion by other gases
iii) Liquid metal corrosion

(1.) Oxidation Corrosion: This is carried out by the direct action
of oxygen low or high temperatures on metals in absence of
moisture.
Alkali metals and Alkaline earth metals are rapidly oxidized at low
temperatures. At high temperature all metals are oxidized (except
Ag, Au, Pt).
M  M2
+ + 2e- (Oxidation)
O2 + 2e -  2O2
- (Reduction)
M + O2  M2
+ + 2O2- (Metal oxide)

Mechanism:-
1. When temp increases the metal undergoes oxidation and losses e-
2M → 2M+n + 2ne-
Metal Ion
2. Electron are gained by the oxygen molecules forms oxide ions
nO2 + 4ne- → 2n O2-
Oxide Ion
3. Scale of metal oxide formed 2M + nO2 → 2M + 2n O2
Metal Oxide

2.) Corrosion due to other gases: This type of corrosion is due
to gases like SO2, CO2, Cl2, H2S, F2 etc. In this corrosion, the
extent of corrosive effect depends mainly on the chemical affinity
between the metal and the gas involved. The degree of attack
depends on the formation of protective or non protective films on
the metal surface which is explained on the basis of Pilling
Bedworth rule.
(i) If the volume of the corrosion film formed is more than the
underlying metal, it is strongly adherent, non-porous does not
allow the penetration of corrosive gases.
Ag + Cl2  2AgCl (protective film)

3.) Liquid metal corrosion: This corrosion is due to chemical
action of flowing liquid metal at high temperatures on solid metal
or alloy. The corrosion reaction involves either dissolution of a
solid metal by a liquid metal or internal penetration of the liquid
metal into the solid metal.
Eg. Coolant (sodium metal) leads to corrosion of cadmium in
nuclear reactors.
(ii) If the volume of the corrosion film formed is less than the
underlying metal, it forms pores/cracks and allow the penetration
of corrosive gases leading to corrosion of the underlying metal.
Ex. In petroleum industry, H2S gas at high temperature reacts with
steel forming a FeS scale. Fe (steel) + H2S FeS (porous)

 The electrochemical corrosion occurs when
i) a conducting liquid is in contact with a metal.
ii) when two dissimilar metals or alloys are immersed
partially in the solution.
 Corrosion occurs due to the presence of anodic and
cathodic areas.
 At anode oxidation reactions takes place
 At cathode reduction reactions takes place

 Occurs when aqueous solution or liquid electrolytes are
present
 Wet corrosion takes place in environments where the relative
humidity exceeds 60 %.
 Wet corrosion is most efficient in waters containing salts,
such as NaCl (e.g. marine conditions), due to the high
conductivity of the solution.

Mechanism Of Electrochemical Corrosion
Anodic Reaction:
Dissolution of metal takes place.
As result metal ions are formed with the liberation of free
electrons.
M ↔ M+n + e-
Metal Ion

Cathodic Reaction
(i) Hydrogen Evolution :- Occurs usually in acidic medium
2H+ + 2e- ↔ H2 (g)
(ii) Oxygen Absorption :- occurs when solution is aerated
sufficiently.
O2+ 4H+ + 4e- ↔ 2H2O (In acidic medium)
O2+ 4H+ + 4e- ↔ 4OH- (In basic medium)

 Hydrogen gas evolution
 Oxygen gas absorption

This type of corrosion occurs in acidic medium.
E.g. Considering the metal Fe, anodic reaction is dissolution of
iron as ferrous ions with liberation of electrons.
Anode: Fe---- Fe2
+ + 2e- (Oxidation)
 The electrons released flow through the metal from anode to
cathode, whereas H+ ions of acidic solution are eliminated as
hydrogen gas.
Cathode: 2H+ + 2e- ----H2 (Reduction)
 The overall reaction is: Fe + 2H+ -----Fe2
+ +H2
 All metals above hydrogen in electrochemical series have a
tendency to get dissolved in acidic solution with simultaneous
evolution of H2 gas.

Absorption of oxygen: For example, rusting of
iron in neutral aqueous solution of electrolytes
in presence of atmospheric oxygen.
 Usually the surface of iron is coated with a
thin film of iron oxide.
If the film develops cracks, anodic areas are
created on the surface.
While the metal parts act as cathodes. It
shows that anodes are small areas, while the
rest metallic part forms large cathodes.
The released electrons flow from anode to
cathode through iron metal.
.

At anode: Fe ----Fe2
+ + 2e- (Oxidation)
At cathode: ½ O2 + H2O + 2e- ---2OH- (Reduction)
Overall reaction: Fe2
+ + 2OH- ----- Fe (OH)2
The product called yellow rust corresponds to
Fe2O3. 3H2O

 The factors that effect corrosion are
i) Nature of the metal
ii) Nature of the environment

 The rate and the extent of corrosion depends on the
following factors
A ) Purity of a metal
B ) Position in galvanic series
C ) Over voltage
D ) Nature of oxide film
E ) Nature of corrosion product

1. Purity of the metal: Heterogeneity of the metal is due to the
presence of impurities which form tiny electrochemical
cells at the exposed parts. The anodic parts get corroded.
2.Electrode potentials: metals with higher reduction potentials
do not corrode easily. They are noble metals like gold,
platinum and silver. Whereas the metals with lower
reduction potentials readily undergo corrosion (e.g.. Zn,
Mg, Al etc.).
3.Position of metal in Galvanic series: Metals which possess
low reduction potentials and occupy higher end of galvanic
series undergo corrosion easily.

When two metals are in electrical contact in presence of an
electrolyte, then the metal which is more active undergoes
corrosion.
The rate of corrosion depends on the difference in their
position in Galvanic series. Greater the difference more will be
the extent of corrosion at anode.
4.Relative areas of anodic and cathodic cells: When two
dissimilar metals or alloys are in contact, the corrosion of the
anodic part is directly proportional to the areas of the cathodic
and anodic parts. i.e, the relative areas o of corrosion is
influenced by cathodic to anodic cells.
5. Physical state of metal: Metals with small grain size have
more tendencies to undergo corrosion. Metal with more
stress/strain also undergoes corrosion easily.

6. Hydrogen over voltage: when a cathode reaction is hydrogen
evolution type, the metal with lower hydrogen over voltage on
its surface is more susceptible for corrosion, since the liberation
of hydrogen gas is easy at this condition. Hence the cathodic
reaction is very fast which in turn makes anodic reaction fast.
Hence the rate of corrosion increases. Higher the over voltage,
lesser is the corrosion.
7. Nature of surface film: If the corrosion product formed is more
stable, insoluble and non porous, it acts as protective layer and
prevents further corrosion (E.g.. Ti, Al and Cr). If the corrosion
product is porous, volatile and soluble, it further enhances the
corrosion (Fe, Zn and Mg).

1. Temperature: the rate of corrosion reactions increases with
increase in temperature.
2. Humidity in air: the moisture or humidity present in atmosphere
furnishes water to the electrolyte which is essential for setting up
of an electrochemical cell. The oxide film formed has the tendency
to absorb moisture which creates another electrochemical cell.
3. Presence of impurities: Atmosphere is contaminated with gases
like CO2, SO2, H2S; fumes of H2SO4, HCl etc. and other
suspended particles in the vicinity of industrial areas. They are
responsible for electrical conductivity, thereby increasing
corrosion.

4. pH value: pH value of the medium has the greater effect on
corrosion. Acidic pH increases the rate of corrosion.
5. Amount of oxygen in atmosphere: As the percentage of
oxygen in atmosphere increases, the rate of corrosion also
increases due to the formation of oxygen concentration cell.
The decay of metal occurs at the anodic part and the cathodic
part of the metal is protected.
6.Velocity of ions which flow in the medium: As the velocity of
the diffusion of the ions in the medium increases, the rate of
corrosion increases .

The various measures taken for corrosion protection are
I) CATHODIC PROTECTION
II) SURFACE COATINGS
CORROSION CONTROL METHODS

 The method of protecting metals and alloys from corrosion
making them completely cathode.
 To achieve this auxilary anode is provided in the corroding
medium which is connected to the structure.
 Cathodic protection is of two types
i) Sacrificial anodic method
ii) Impressed current cathodic method

Force the metal to be protected to behave like cathode.
(i) Sacrificial anodic protection:
 Metal to be protected from corrosion connected to more
anodic metal
 Commonly used metals Mg, Zn, Al and their alloys
Eg. A ship-hull which is made up of steel is connected to
sacrificial anode (Zn-blocks) which undergoes corrosion leaving
the base metal protected.
Eg. The underground water pipelines and water tanks are also
protected by sacrificial anode method. By referring to the
electrochemical series, the metal with low reduction potential is
connected to the base metal which acts as anode.

 Direct current is applied in opposite direction to nullify the
corrosion current
 Converts the corroding metal from anode to cathode.

 The metal surface is covered with a coating to protect it from
corrosion.
 These may be
i) Metallic
ii) Organic

The metals used for coatings may be placed under two
categories
i) metals which are anodic to the metal i.e., metals above it in
galvanic series.
ii) metals which are cathodic to the metal i.e., metals below it in
galvanic series.

 Hot dipping: This technique is most widely used to control
corrosion. Hot dipping is used for producing a coating for
low melting metals such as Zn, Sn, pb ,Al.
 on Iron, steel, copper which are having high melting point
usually underwent into corrosion due to their oxidising
property.
 The process in general consisting of immersed a metal in a
bath of its molten coating& covered by a molten layer.
 The flux cleans the base metal& prevent oxidation of metal
coating with molten solution.
 Hot dipping is widely applied either by
(1)GALVANIZING (2)TINNING

 It is the process in which iron or steel is protected from
corrosion by coating with a thin layer of zinc.
 Iron or steel is first cleaned by pickling with dil. H2SO4
solution for 15-20 mts at 600-900c. The steel is then washed
well and dried .
 Then the metal sheet is dipped in bath of molten Zinc
maintained at 4250-4300c.
The surface of bath is kept covered with a Zncl2 flux to
prevent oxide formation.
 Then the metal sheet is passed through a pair of hot rollers
which removes excess of Zinc and maintain uniform
thickness of Zinc on metal sheet.

Uses of Galvanization: It is used to protect Iron used for
roofing sheets, wires, pipes, nails, bolts, screws, buckets,
and tubes.
Defects :Galvanised utensils are not used for cooking
because of solubility of Zn.
The sheet is subjected to annealing process at 650c and
cooled slowly.

Tinning is the process of coating of tin over the Iron sheet or
steel articles
 A cleaned iron sheet is passed through a bath of molten flux,
then passes through a tank of molten tin and finally through a
layer of palm oil which protect hot tin coated surface against
oxidation.
Uses: (1)It is widely used for coating of steel, copper, brass &
bronze etc . It is used to store food stuff, ghee oils, kerosene&
pickels and used for refrigerators equipment.
TINNING

ELECTROPLATING
Deposition of coating metal on the base metal by passing
direct current through an electrolytic solution which contain
the soluble salts of the coating material.
Pure metal is made as cathode and base metal as anode.
Electrochemically coat metal is deposited on base metal.
This metal gives smooth, fine and uniform coating
It depends on
(i) Temperature
(ii) Current density
(iii) Electrolyte Concentration
(iv) Nature of base metal (v) Time

 It is most important & frequently used technique in industries
to produce metallic coating
 Both metals & non metals can be electroplated.
 In metals the electroplating increases resistance to corrosion,
chemical attack, hardness, wear resistance and surface
properties.
 In non metals electroplating increases strength and decorates
the surface of non metal like plastics, wood, glass.

 Pre treatment of surface:
 (1) CO, Ni, steel and Al can be directly plated.
 (2)Pb, Cd and Sn are first electroplated with Ni prior to
electroless plating.
 (3)Non conductors are activated in Sncl2 & HCl
followed by dipping in Pdcl2,Hcl.on drying thin pd layer
is formed.

 Organic coatings protect the surface as they act as
physical barriers between the metal to be protected and
the corroding environment and are decorative.
 The extent of protection given by the organic coatings
depends on
A) Impermeability to gases ,salts and water
B) Chemical inertness to the corrosive environment
C) Good surface adhesion

 Paints form a protective layer over the surface of the metal to
prevent corrosion.
The main constituents of the paints are
 Pigment
 Vehicle
 Thinner
 Driers
 Fillers
 Plasticizers
 Anti skinning agents

 It is a solid constituent that obscures the surface and provides
a decorative color.
 A liquid binds the pigment to the surface and protects it from
decay.
 It usually comprises of 60 to 80% of the weight of the paint
film.
 Increases the life of paint film since it prevents the entry of
ultra violet rays.
 E.g.; lead oxide, red lead, lead chromate etc.,

 Film forming constituent of the paint.
 They hold the pigment on the metal surface.
 They give metal adhesion to the metal surface.
 They impart durability and toughness to the film.
 They isomerize , polymerize and condense to form
characteristic tough, elastic ,coherent, highly cross linked
structured macromolecular film.
 E.g.: mustard and sunflower oil etc.,

 Viscosity is reduced by the addition of thinners.
 It is a volatile solvent helps to adjust the consistency of the paint.
 Thinner being volatile evaporates.
 They remain permanently in paints and varnishes.
 They improve elasticity of the film.
 They suspend the pigments in the paints.
E.g. Turpentine, mineral spirits, xylol, kerosene.

 They are inert materials.
 They are used to improve the properties of the paints.
 They reduce the cost of paints.
 They serve to fill the voids in the film.
 Increases the random arrangement of pigment particles.
 Improves the durability of the film.
 E.g.: talc, chalk, silica etc.,

 Drying of the oil film is accelerated or catalyzed by
driers.
 To reduce the drying time a drier is added.
 They are oxygen carrier catalyst.
 To improve the drying quality of oil film.
 This is done by condensation , oxidation and
polymerization.
E.g. linoleates , borates, resinates etc.,

 They remain permanently in paints and varnishes.
 They improve elasticity of the film.
 To prevent cracking of the film.
 Anti skinning agents prevent the gelling and skinning of
the paint film.
E.g.: tricresyl phosphate, tri phenyl phosphate,
di butyl tartarate etc.,

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