Principles of Corrosion

1. PRINCIPLES OF
PRINCIPLES OF
CORROSION
CORROSION
Dr. T. K. G. Namboodhiri
Dr. T. K. G. Namboodhiri
(Retd. Professor, Inst. Of Tech. , Banaras
(Retd. Professor, Inst. Of Tech. , Banaras
Hindu University)
Hindu University)
Consultant-Metallurgy & Corrosion,
Consultant-Metallurgy & Corrosion,
Tiruvalla, Kerala
Tiruvalla, Kerala

2. INTRODUCTION
INTRODUCTION
 Metallic corrosion reactions are
Metallic corrosion reactions are
electrochemical in nature.
electrochemical in nature.
 An electrochemical reaction is
An electrochemical reaction is
associated with charge transfer in
associated with charge transfer in
addition to mass transfer.
addition to mass transfer.
 All electrochemical reactions may be
All electrochemical reactions may be
split into two or more partial
split into two or more partial
oxidation and reduction reactions.
oxidation and reduction reactions.

3. ELECTROCHEMICAL CELL
ELECTROCHEMICAL CELL

4. ANODE REACTIONS
ANODE REACTIONS
 Anode metal corrodes and metal ions
Anode metal corrodes and metal ions
get into solution as:
get into solution as:
 If steel is corroding,
If steel is corroding,
 If Aluminum is corroding,
If Aluminum is corroding,
 These reactions are also called
These reactions are also called
Oxidation reactions.
Oxidation reactions.

5. CATHODE REACTIONS
CATHODE REACTIONS
 The cathodic reaction consumes the
The cathodic reaction consumes the
electrons generated by the anodic
electrons generated by the anodic
reaction.
reaction.
 In nature, and neutral or basic
In nature, and neutral or basic
environments containing oxygen, the
environments containing oxygen, the
cathodic reaction is:
cathodic reaction is:

6. OTHER CATHODE REACTIONS
OTHER CATHODE REACTIONS
Hydrogen ion reduction or hydrogen
Hydrogen ion reduction or hydrogen
evolution: 2 H
evolution: 2 H+
+
+ 2 e H
→
+ 2 e H
→ 2
2
Oxygen reduction in acid solutions:
Oxygen reduction in acid solutions:
O
O2
2 + 4 H
+ 4 H+
+
+ 4 e 2 H
→
+ 4 e 2 H
→ 2
2 O
O
Metal ion reduction:
Metal ion reduction:
M
M+n
+n
+ e M
→
+ e M
→ +(n-1)
+(n-1)
Metal deposition: M
Metal deposition: M+n
+n
+ ne M
→
+ ne M
→

7. ELECTROCHEMISTRY OF
ELECTROCHEMISTRY OF
CORROSION
CORROSION
DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,
2
2
2 H
ZnCl
HCl
Zn 

 -------------------- -(1)
Written in ionic form as,
2
2
2
2
2 H
Cl
Zn
Cl
H
Zn 



 



----------------------(2)
The net reaction being,
2
2
2 H
Zn
H
Zn 

 

------------------------- (3)
Equation (3) is the summation of two partial reactions,
e
Zn
Zn 2
*
2

 -----------------------------------------(4) and
2
2
2 H
e
H 


------------------------------------------(5)
Equation (4) is the oxidation / anodic reaction and
Equation (5) is the reduction / cathodic reaction

8. CORROSION OF ZINC IN HCl
CORROSION OF ZINC IN HCl

9. THERMODYNAMICS IN
THERMODYNAMICS IN
CORROSION
CORROSION

10. THERMODYNAMICS OF
THERMODYNAMICS OF
CORROSION
CORROSION
 Like any reaction, electrode reactions involved in
Like any reaction, electrode reactions involved in
corrosion are associated with a decrease in free energy
corrosion are associated with a decrease in free energy
 The free energy change is related to the electrode
The free energy change is related to the electrode
potential by the relationship
potential by the relationship
∆
∆G = -nFE
G = -nFE
Where,
Where,
∆
∆G is the free energy change in joules
G is the free energy change in joules
n is the number of electrons involved in the
n is the number of electrons involved in the
reaction
reaction
E is the electrode potential in volts
E is the electrode potential in volts
The single electrode potential is a measure of the
The single electrode potential is a measure of the
tendency for the reaction to take place. Negative
tendency for the reaction to take place. Negative
potentials indicate spontaneous reaction.
potentials indicate spontaneous reaction.

11. STANDARD ELECTRODE
STANDARD ELECTRODE
POTENTIAL & POLARIZATION
POTENTIAL & POLARIZATION
 Standard electrode potential is the
Standard electrode potential is the
potential developed when all the reactants
potential developed when all the reactants
have unit activity and the temperature is
have unit activity and the temperature is
25 degree C. Oxidation and reduction
25 degree C. Oxidation and reduction
reactions are of equal rate.
reactions are of equal rate.
 When these reactions are not equal, there
When these reactions are not equal, there
will be a net oxidation or reduction and
will be a net oxidation or reduction and
the potential shifts away from its
the potential shifts away from its
equilibrium value. This process is
equilibrium value. This process is
polarization.
polarization.

12. POURBAIX DIAGRAM OF IRON
POURBAIX DIAGRAM OF IRON
 Pourbaix diagram is
Pourbaix diagram is
a potential-PH plot
a potential-PH plot
showing the stability
showing the stability
of various metal
of various metal
compounds
compounds
 Based on the
Based on the
stability of the
stability of the
various phases, we
various phases, we
can predict regions
can predict regions
of corrosion,
of corrosion,
immunity,
immunity,
passivation etc
passivation etc

13. KINETICS OF CORROSION-
KINETICS OF CORROSION-
MIXED POTENTIAL THEORY
MIXED POTENTIAL THEORY
 The mixed potential
The mixed potential
theory of Wagner and
theory of Wagner and
Traud consists of two
Traud consists of two
simple hypotheses,
simple hypotheses,
 1) any electrochemical
1) any electrochemical
reaction can be split into
reaction can be split into
two or more partial
two or more partial
oxidation and reduction
oxidation and reduction
reactions, and
reactions, and
 2) there can be no net
2) there can be no net
accumulation of electrical
accumulation of electrical
charge during an
charge during an
electrochemical reaction
electrochemical reaction

14. Kinetics- Corrosion rate & Potential
Kinetics- Corrosion rate & Potential
 Anodic & cathodic
Anodic & cathodic
reactions occur
reactions occur
simultaneously at
simultaneously at
different parts of the
different parts of the
metal.
metal.
 Electrode potentials
Electrode potentials
of the two reactions
of the two reactions
converge to the
converge to the
corrosion potential by
corrosion potential by
polarization
polarization
 The current density at
The current density at
this potential gives
this potential gives
the rate of corrosion
the rate of corrosion

15. PASSIVATION
PASSIVATION
 Many metals like Cr,
Many metals like Cr,
Ti, Al, Ni and Fe
Ti, Al, Ni and Fe
exhibit a reduction in
exhibit a reduction in
their corrosion rate
their corrosion rate
above certain critical
above certain critical
potential because of
potential because of
the formation of a
the formation of a
protective, thin oxide
protective, thin oxide
film.
film.
 Passivation is the
Passivation is the
reason for the
reason for the
excellent corrosion
excellent corrosion
resistance of Al and
resistance of Al and
S.S.
S.S.