PRINCIPLES OF PRINCIPLES OF CORROSIONCORROSION

Dr. T. K. G. NamboodhiriDr. 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, KeralaTiruvalla, Kerala

INTRODUCTIONINTRODUCTION

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.

ELECTROCHEMICAL CELLELECTROCHEMICAL CELL

ANODE REACTIONSANODE 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.

CATHODE REACTIONSCATHODE 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:

OTHER CATHODE REACTIONSOTHER CATHODE REACTIONS

Hydrogen ion reduction or hydrogen Hydrogen ion reduction or hydrogen evolution: 2 Hevolution: 2 H++

+ 2 e → H+ 2 e → H22

Oxygen reduction in acid solutions: Oxygen reduction in acid solutions:

OO22 + 4 H + 4 H++ + 4 e → 2 H + 4 e → 2 H22 O O

Metal ion reduction:Metal ion reduction:

MM+n+n + e→ M + e→ M+(n-1)+(n-1)

Metal deposition: MMetal deposition: M+n+n + ne→ M + ne→ M

ELECTROCHEMISTRY OF ELECTROCHEMISTRY OF CORROSIONCORROSION

DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,

222 HZnClHClZn -------------------- -(1) Written in ionic form as,

22 222 HClZnClHZn ----------------------(2)

The net reaction being,

222 HZnHZn ------------------------- (3)

Equation (3) is the summation of two partial reactions,

eZnZn 2*2 -----------------------------------------(4) and 222 HeH ------------------------------------------(5)

Equation (4) is the oxidation / anodic reaction and Equation (5) is the reduction / cathodic reaction

CORROSION OF ZINC IN HClCORROSION OF ZINC IN HCl

THERMODYNAMICS IN THERMODYNAMICS IN CORROSIONCORROSION

THERMODYNAMICS OF THERMODYNAMICS OF CORROSIONCORROSION

Like any reaction, electrode reactions involved in Like any reaction, electrode reactions involved in corrosion are associated with a decrease in free energycorrosion 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 joulesG 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

reactionreaction E is the electrode potential in voltsE is the electrode potential in voltsThe 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.

STANDARD ELECTRODE STANDARD ELECTRODE POTENTIAL & POLARIZATIONPOTENTIAL & 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 the will be a net oxidation or reduction and the potential shifts away from its equilibrium potential shifts away from its equilibrium value. This process is polarization.value. This process is polarization.

POURBAIX DIAGRAM OF IRONPOURBAIX DIAGRAM OF IRON

Pourbaix diagram is Pourbaix diagram is a potential-PH plot a potential-PH plot showing the showing the stability of various stability of various metal compoundsmetal 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 etcpassivation etc

KINETICS OF CORROSION-KINETICS OF CORROSION-MIXED POTENTIAL THEORYMIXED 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 accumulation of electrical charge during electrical charge during an electrochemical an electrochemical reaction reaction

Kinetics- Corrosion rate & PotentialKinetics- 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 of Electrode potentials of the two reactions the two reactions converge to the converge to the corrosion potential by corrosion potential by polarizationpolarization

The current density at The current density at this potential gives the this potential gives the rate of corrosionrate of corrosion

PASSIVATIONPASSIVATION 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.