CORROSION

Prepared by: Patel Dipsa

 Contents
• Introduction
• Types of corrosion
• Factors influencing corrosion
• Corrosion control
 Introduction
• Most metals exists in nature in combined forms Like
– metal oxides, sulphides, chlorides, carbonates, hydroxy
carbonates ,etc
• They are extracted out from their ores in pure form but
energy level is high i.e. excited state
• They have a natural tendency to come back to combined state
(low energy state)
• When the metal is put into use , the metal surface is exposed
to environment , exposed metal surface begin to decay.
 Introduction of corrosion
• The destructive electrochemical or chemical
attack of a material
• Any process of destruction and consequent
loss of a solid metallic material through an
unwanted or unintentional chemical or
electrochemical attack by its environment,
starting at its surface is called corrosion
 CORROSION
• “It is the process of deterioration
• ( decay) of a metal as a result of its
reaction with air and water.”
• In case of Iron, it is called Rusting and in
case of silver, it is called Tarnishing.

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 Examples of corrosion
(1)Rusting of iron
• 4Fe + 6H2O + 3O2  4Fe(OH)3
– gives ferric hydroxide
• 2Fe(OH)3  Fe2O3  3H2O
– gives iron oxide (rust) and water

(2) Formation of green film of carbonates on

copper
– On the surface of copper , when exposed to
moist air containing CO2 forms green film of
carbonates of Cu
 Basic corrosion requirements

1. The metal is oxidized at the anode of an

electrolytic cell
2. Some ions are reduced at the cathode
3. There is a potential or voltage difference
between the anode and cathode
4. An electrolyte (fluid) must be present
5. The electrical path must be completed
 Gravity of corrosion problem
• Process of corrosion is slow but losses due to
corrosion are very high
• e.g. destruction of machines, equipments and
different types of metallic products
• Losses due to corrosion all over the world is 2
to 2.5 billion dollars per annum
• The engineer must understand the
mechanism of corrosion
 Thermodynamics of corrosion
• Magnitude of Gibbs free energy change ∆G
predicts the tendency of corrosion reaction to
occur
• For Mg corrosion ∆G = -596.6 kJ mol-1
• For Cu corrosion ∆G = -119.7 kJ mol-1
• For Au corrosion ∆G = 66.68 kJ mol-1
 Kinetics of corrosion
• Rate of corrosion depends on:
– Nature of metal
– Concentration of the corroding medium
– Temperature
– pH of the corroding medium
– Purity of the metal
– Porosity and volatility of corrosion product
 Mechanism of corrosion
• The process of corrosion involves oxidation by
the metal
• M Mn+ + ne-
• oxidation and reduction reaction occurs
simultaneously. The corroding medium gains
electrons and get reduced
• 2H+ + 2e- H2 (acid medium pH<7)
• 2H2O + O2 + 4e- 4OH-
(neutral or basic medium pH>= 7)
 GALVANIC SERIES
• Ranks the reactivity of metals/alloys in seawater

Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
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 STANDARD EMF SERIES
• EMF series
o
Vmetal
metal
Au +1.420 V
Cu +0.340
Pb - 0.126
Sn - 0.136
Ni - 0.250 o
Co - 0.277 DV =
Cd - 0.403 0.153V
Fe - 0.440
Cr - 0.744
Zn - 0.763
Al - 1.662
Mg - 2.262
Na - 2.714
K - 2.924
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Metal with smaller
V °metal corrodes.
• Ex: Cd-Ni cell
 Types of corrosion
• Dry or chemical corrosion
• Wet or electrochemical corrosion
• Galvanic or bimetallic corrosion
• Concentration cell corrosion
• Pitting corrosion
• Waterline corrosion
• Stress corrosion
Dry corrosion
 DRY CORROSION
This type of corrosion occurs by direct
chemical attack of atmospheric gases like O2 ,
hydrogen sulphide etc. on the metal surface. it
is further of following types:
• (a) Corrosion by O2 or Oxidation corrosion.
• (b) Corrosion by other gases.

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 OXIDATION CORROSION
• It is due to chemical attack of O2 on the
metal resulting the formation of metal
oxide layer as:
• 2M + nO2-------- 2MOn
(metal oxide)
• This theory is called Wagner’s Theory.

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Nature of Metal oxide layer :
(1) Stable: It acts as a barrier between metal
and O2, So prevents further corrosion.
(2) Unstable: It decomposes back to metal and
O2. Ag, Au, Pt are protected by this manner.

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(3)Volatile: Layer volatalises and metal
exposed for further corrosion.
(4) Porous: Corrosion occur through pores
and continues till entire metal gets
corroded.

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Pilling Bedworth Rule:
If the volume of oxide layer formed is equal
or greater than the volume of the metal, the
oxide layer is protective. However if the
oxide layer formed has volume lesser than
that of the metal, then the oxide layer is
non-protective.

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 CORROSION BY OTHER GASES

• Corrosion is also caused by some other

gases like Cl2 ,H2S, SO2 etc. This type of
corrosion depends on chemical affinity
between metal and gas.

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Wet corrosion
 WET CORROSION
• This type of corrosion occurs when a
liquid medium is involved. One part
behaves as anode and undergoes
oxidation and other part acts as cathode
and undergoes reduction.

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Liquid medium involved acts as
electrolyte. It is further of two types:

(a) Evolution of H2 type corrosion

(b) Absorption of O2 type corrosion

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 Evolution of H2 type corrosion
• It occurs in the acidic environment.
At anode:
M-------- M n+ + ne- (oxidation)
At cathode:
2H + + 2e- ----- H2 (reduction)

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 Absorption of O2 type corrosion
• E.g. Rusting of iron

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If the layer of iron develops some
cracks, this damaged surface acts as
anode while the other intact metal
behaves as cathode. Presence of
moisture acts as electrolyte.

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The reactions occuring are:
 Fe -------> Fe+2 + 2 e- (oxidation)
1/2O2 (g) + H2O + 2e- ------> 2OH-
(reduction)
Fe+2 + 2OH- --------Fe(OH)2

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In the presence of excess oxygen, Fe(OH)2 oxidises
to Fe(OH)3 as:
4Fe(OH)2 + O2 + 2H2O---- 4Fe(OH)3
This ferric hydroxide formed decomposes to
hydrated ferric oxide as:
Fe(OH)3 --------- Fe2O3.xH2O
(Rust)

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Galvanic corrosion
Cont…

11/8/2022
Concentration cell corrosion
Pitting corrosion
Waterline corrosion
Stress corrosion
 Introduction to SCC
• Stress-corrosion cracking (SCC) is a cracking
process that requires the simultaneous action
of a corrodent and sustained tensile stress.
• This excludes corrosion-reduced sections that
fail by fast fracture.
• It also excludes inter crystalline or trans
crystalline corrosion, which can disintegrate an
alloy without applied or residual stress.

11/8/2022
 Mechanism of SCC
Stress corrosion cracking results from the
conjoint action of three components:
(1) a susceptible material;
(2) a specific chemical species (environment) and
(3) tensile stress.
For example, copper and its alloys are susceptible
to ammonia compounds, mild steels are
susceptible to alkalis and stainless steels are
susceptible to chlorides.
11/8/2022
 Examples of SCC
• Season cracking
• Caustic embrittlement

11/8/2022
 Factors influencing corrosion
1. Nature of metal
2. Nature of corrosive environment.

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1.Nature of Metal:
(1)Purity of metal: Impure the metal, more is
the rate of corrosion. This is because impurities
act as “minute electrochemical cells.”
(2) Physical state of metal: Physical state of
metal like grain size, stress affects the corrosion
rate. Stressed areas undergo more corrosion.

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(3) Position in Galvanic series:
More the oxidation potential i.e if metal is
higher in the galvanic series, it is more anodic
and hence more is the rate of corrosion.
(4) Nature of Oxide layer: Corrosion rate
depends upon the nature of oxide layer
whether it is stable, unstable, porous etc.

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(5) Passive nature of metal: Some metals
like Ti, Al, Cr show more corrosion resistance
than expected from their position in the
electrochemical series. These are called
passive metals. This is due to formation of
thin protective layer on metal surface.

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(6) Nature of corrosive products: If the
corrosion products are soluble or volatile in
nature, then corrosion increases.
(7) Relative area of anode and cathode:
smaller the anodic area, more is the
corrosion rate because oxidation of anode
occurs at faster rate.

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2. Nature of corroding environment:
(1)Temperature: Rate of corrosion
increases with increase in temperature.
(2) Moisture: More the moisture, more is
the rate of corrosion.

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(3) pH Value: Lesser the pH i.e. acidic
conditions, more is corrosion.
(4) Nature of electrolyte: Presence of
salts in the electrolyte increases the rate
of corrosion.

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(5) Presence of impurities in atmosphere:
Impurities increase the rate of corrosion.
Corrosion is more in industrial areas and
sea. This is because of presence of gases
like CO2, H2S, SO2

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 Corrosion control methods
(protection against corrosion)
• Cathodic protection
– sacrificial anodic protection method
– impressed current cathodic protection
• Application of protective coatings
– anodic coating
– cathodic coating
• Use of inhibitor
– Anodic inhibitor
– Cathodic inhibitor
sacrificial anodic protection method impressed current cathodic protection
 Methods of application of metal
coatings
• Hot dipping
– Galvanizing
– Tinning
Hot dipping
Galvanizing
Tinning