The document discusses various topics related to corrosion including theories of corrosion, types of corrosion, and factors affecting corrosion. It covers two main theories of corrosion - chemical (dry) corrosion which occurs without a liquid electrolyte and electrochemical (wet) corrosion which is an electrochemical process requiring an electrolyte. Some key types of corrosion mentioned are galvanic, pitting, crevice, and waterline corrosion. Factors discussed include the nature of the metal, environment, and potential differences in alloys or surfaces.

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2. Corrosion
Topics
Theories of corrosion:
Chemical corrosion (dry corrosion)
Electrochemical corrosion (wet corrosion)
Types of corrosion:
Galvanic corrosion (bimetallic corrosion)
Concentration cell corrosion)
Pitting corrosion
Water-line corrosion
Factors affecting corrosion :
Prevention of corrosion:
Methods based on treatment of metals and
Methods based on treatment of medium.
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7. CORROSION
The loss of materials(metal and alloys) or its useful
properties, by chemical or electrochemical interaction
with its environment is called corrosion.
Example:
1. Rusting of iron
2. formation of green layer on copper surface.
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8. Corrosion Engineering
The branch of Engineering that deals with
the study of corrosion mechanisms and
to prevent or control it economically and
safely.
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9. FACTS ABOUT CORROSION
• a natural process.
• an electrochemical process.
• Corrosion leads to tremendous loss. It can’t be
eliminated completely. However, its extent can be
minimized.
• Corrosion is exactly the reverse of extraction of
metals and also known as weeping of metals.
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10. CAUSE OF CORROSION
Metal Metal salt
(higher energy state) (lower energy state)
It’s a natural process
Exothermic process
Entropy increases
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11. THEORIES OF CORROSION
1. Direct chemical attack theory
(Chemical corrosion or dry corrosion)
By direct attack of gas in absence of a liquid or
electrolyte
• Insoluble corrosion product – further corrosion is
checked. eg. Attack of Cl2 or I2 on Ag
• soluble or liquid corrosion product – further
corrosion is continued. eg. in the de-tinning of
metal cans
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12. Types of chemical corrosion
i) Oxidation Corrosion
2M 2Mn+ + 2ne- (oxidation)
n/2O2 + 2ne- nO2- (reduction)
2M + n/2O2 2Mn+ + nO2-
Metal oxide
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13. Take examples of Fe, Mg and Al
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14. Case I: When metal ion diffuses faster outward:
In this case oxide layer is formed at the metal oxide –
gas or scale – gas interface. eg. In FeO, CoO, NiO, Cu2O
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15. Case II: When oxygen diffuses inward:
In this case oxide layer is formed at the metal- scale
interface or metal – metal oxide interface. eg in ZnO,
CdO, TiO2 etc
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16. Nature of metal oxide layer
a) Stable film –An impervious layer is formed,
which checks further oxidation corrosion.
e.g. oxide films on Al, Sn, Pb, Cu, Pt etc.
b) Unstable film-
Metal oxide Metal + O2
O2
e.g. in Au and Ag
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17. c) Volatile film: oxide layer volatilizes leaving the
underlying metal surface for further attack.
E.g. molybdenum oxide (MoO3) is volatile.
O2
d) Porous film: atmospheric O2 have access to the
underlying surface of metal.
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18. Pilling Bedworth rule
If the volume of metal oxide ≥ volume of metal,
Oxide layer is protective or non-porous
e.g. Al
If Volume of metal oxide < volume of metal,
Oxide layer is porous
e.g. Alkali and alkaline earth metals
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19. ii) Corrosion by other gases:
by CO2, SO2,Cl2,H2S,
Extent of corrosion depends upon
• the chemical affinity between metal and the gas
• the nature of the film formed on the surface.
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20. Factors affecting chemical corrosion
1. Nature of the metal
i) Position in the Galvanic series
ii) Relative areas of anode and cathode
iii) Purity of metal
iv) Solubility of corrosion products
vi) Volatility of corrosion products
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21. protective or non-porous.
eg. AgCl layer formed by the attack of
Cl2 on Ag( this is a protective layer)
Film
non-protective or porous.
eg. i) formation of volatile SnCl4 by the
attack of dry Cl2 on Sn.(de-tinning)
ii) in petroleum industry, H2S at high T
attacks steel forming porous FeS scale
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22. iii) Liquid metal corrosion- occurs due to the
chemical action of flowing liquid metal at high T on
solid metal or alloy.
Possible processes are:
• Dissolution of a solid metal by a liquid metal
• Internal penetration of the liquid metal into the solid
metal
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23. Wet or electrochemical corrosion
(Electrochemical theory)
Two essential requirements are:
i) Formation of anodic and cathodic areas
ii) Electrical contact between the cathodic and anodic
parts to enable the conduction of electrons
Mechanism
i)Anodic reactions
M(s) Mn+
(aq) + ne- (oxidation)
Fe(s) Fe2+
(aq) + 2e- (oxidation)
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24. ii) Cathodic reactions
a) In acidic solution in the absence of O2
2H+ + 2e- H2
b) In acidic solution in the presence of O2
O2 + 4H+ + 4e- 2H2O
c) In neutral or alkaline medium in the absence of O2
2H2O + 2e- H2 + 2OH-
d) In neutral or alkaline medium in the presence of O2
2H2O + O2 + 4e- 4OH-
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25. (e)In neutral or alkaline medium in the presence of O2
O2 + 2H2O + 4e- 4OH-
(such type of corrosion involving O2 is called oxygen
type corrosion)
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26. e.g. Rusting of iron occurs by O2 in the presence of
aqueous solution
At anode Fe Fe2+ 2e-
At cathode 1/2O2 + H2O + 2e- 2OH-
Overall reaction Fe + 1/2O2 + H2O Fe2+ + 2OH- or Fe(OH)2
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27. (i)In excess supply of oxygen: In excess supply of
oxygen, ferrous hydroxide is easily oxidized to ferric
hydroxide.
2Fe(OH)2 + H2O + 1/2O2 2Fe(OH)3
Fe2O3.xH2O
Yellow rust
(ii) In limited supply of oxygen: In limited supply of
oxygen, black magnetite Fe3O4 or ferroferric oxide is
formed.
Fe(OH)2 Fe2O3.FeO.6H2O
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28. Wet corrosion Dry corrosion
Occursin presence of water occurs in absence of liquid or
or an electrolyte. electrolyte.
It is an electrochemical attack. It is a chemical attack.
Occurs at low temperature. occurs e at high temperature.
low temperature corrosion high temperature corrosion.
.
It is generally fast. It is generally slow.
Eg. Rusting of iron in water. Eg. Attack of steel furnace by gases at
high temperature.
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29. 3) The Acid Theory – applicable particularly to
rusting of iron
2Fe + O2 + 4CO2 + 2H2O 2Fe(HCO3)2
2Fe(HCO3)2 + H2O + [O] 2Fe(OH)CO3+2CO2+ 2H2O
2Fe(OH)CO3 + 2H2O 2Fe(OH)3 + 2CO2
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30. TYPES OF WET CORROSION
1. Galvanic Corrosion /Bimetallic corrosion/
Differential metal corrosion
E.g. Zinc and copper couple
More reactive Zn Zn2+ + 2e- At anode
(Corrodes)
Less reactive Cu + 2e- Cu At Cathode
(protected) no reaction takes place
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32. Factors affecting galvanic corrosion:
(i) Potential difference between the two metals
coupled
(ii) Relative area of cathode and anode
(iii) contact resistance
(iv)Electric resistance of the electrolyte
(v) Presence of a passive film
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33. This type of corrosion is observed in
a) Steel pipe connected to copper plumbing.
b) Steel screw in brass marine hardware
c) zinc coating on mild steel
d) Tin coating on copper
e) lead – antimony solder around copper wires
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34. Electrochemical series Galvanic series
The arrangement of metals and non-metals in increasing order The arrangement of metals and alloys in
their standard reduction potential is known as decreasing order of their corroding
electrochemical series tendency in an unpolluted
sea water is known as galvanic series.
It contains metals and non-metals It contains metals and alloys.
It is an ideal series It is a practical series
ECS is based upon the electrode potential which This series is based on actual
is determined by using Nernst equation corrosion rate
Position of metals is fixed in ECS Position of a given metal in
Galvanic
series may change
It gives no idea about the position of alloys It gives clear idea about the position of
alloys
It gives information about the It gives information about the
displacement tendencies relative corrosion tendencies
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35. [II] Erosion Corrosion:
Due to abrading action of flow of gases or
mechanical rubbing action of solids over the
metal surface.
[III] Crevice Corrosion:
Due to cracks in paint coating
[IV] Pitting Corrosion:
Most dangerous form of corrosion as it leads to
sudden failure of material due to formation of
holes.
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36. Facts about pitting corrosion
• Pitting corrosion is autocatalytic, self
stimulating and self propagating.
• It takes place exclusively in chloride and
chloride containing environment.
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39. [IV] Differential aeration Corrosion
(Oxygen Concentration Cell Corrosion)
• One part of the metal is exposed to a different
air/O2 concentration from the rest of the part.
• Portion with lesser O2 = Anode
• Portion with more O2 = Cathode
• e.g. A iron nail inside the wood undergoes
corrosion easily
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40. [V] Waterline Corrosion
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41. [VI] Micro-Biological Corrosion :
Due to metabolic activity of various micro-
organisms
[VII] Stress-Corrosion Cracking
•Metal under stress becomes more anodic and
tend to increase the rate of corrosion.
•The stress can be due to non-uniform
deformation by unequal cooling from high
temperature as in welding
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42. Stress corrosion
Caustic embrittlement:
Occurs in steam boilers at crevices
Due to usage of Na2CO3 during softening
Na2CO3 + H2O → 2NaOH + CO
This NaOH accumulates in the crevices and reacts with Fe
to form sodium ferrate.
2Na2FeO2 +4H2O → 6NaOH + Fe3O4 + H2
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43. Concentration cell formation
Fe | Conc.NaOH || dil.NaOH | Fe
anode cathode
• At anode iron undergoes oxidation
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44. Factors affecting corrosion
1. Metallic factors:
a) anodic and cathodic areas: smaller anodes
corrode faster
b) electrode potentials: more difference, more
corrosion
c) hydrogen over voltage and the presence of
impurities
d) Physical state of the metal
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45. 2. Nature of environment
i) Temperature: Rate of chemical reaction and rate of
diffusion increases with temperature
ii) Presence of moisture:
Critical humidity is the relative humidity above which
the atmospheric corrosion rate of metal increases
sharply
(iii)Presence of impurities in atmosphere: due to
CO2, H2S, SO2 etc in the vicinity of industrial area
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46. (iii)Effect of pH: corrosion is faster in acidic
medium rather than in basic medium
(iv)Nature of ions present: Chloride ions present in
the medium destroy the passive film, while silicate
in the medium leads to the formation of insoluble
film over the metal surface.
(v) Concentration of O2
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47. Protection
• cathode protection
• Corrosion
• Cathode protection
• Stress current protection
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48. PROTECTION FROM CORROSION
[I] Design and Material Selection
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51. When contact of dissimilar metals is unavoidable,
suitable insulator should be inserted between them
to reduce current flow and attack on the anode.
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52. [II] Cathodic Protection
(i) By appropriate galvanic coupling:
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53. (ii) By impressed current
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54. (vii) Nature of surface film :
Specific volume ratio = Volume of metal oxide
volume of metal
(viii) Passive character of metal: Tl, Al, Cr, Mg etc form
thin layer of passive metal oxide.
e.g. Cr present in stainless steel
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55. advantages over sacrificial
anodicprotection
• It is controlled from outside.
•No anode has to be replaced.
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56. [III] Modifying the Environment
i) Deaeration
ii) Deactivation : addition of chemicals, capable of
combining rapidly with O2 in aqueous solution
iii) Dehumidification: by using alumina or silica gel
iv) Alkaline neutralization
v) Use of inhibitors
a. By forming a layer in between which acts as a
barrier between the material and environment.
b. Or by retarding the anodic or cathodic or both
processes
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57. 4. Metallic coatings
i. Electroplating
ii. Hot dipping
iii. Vaporising
iv. Metal spraying
v. Cementation: The base metal articles are packed
in the powdered coating metal and is heated to a
temperature just below the m.p. of more fusible
metal, so that an alloy layer is formed over the
surface.
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58. Metallic coating are of two types:
Sacrificial coating
Noble coating
Sacrificial coating Noble coating
Base metal is coated with a metal which is Base metal is coated with a
more reactive than the base metal. metal which is more noble than the
base metal.
Protects the underlying base metal sacrificially. Protects the underlying base metal
due to its noble character and
higher corrosion resistance.
This is known as anodic coating as the reduction This is known as cathodic coating as
potential of coating metal is less than that the reduction potential of coating
the base metal. Metal is more than that of the base
metal
Zn, Cd, Al are generally used as sacrificial coating Ni, Ag, Cr, Pb, Au etc. are generally
used as noble coating
E.g. Galvanised iron i.e. coating of Zn E.g. coating of Sn on Fe
on Fe.
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59. 5. Inorganic non-metallic coatings
i. Chemical dip coating or surface conversion
ii. Anodized oxide coating
6. Organic coatings
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