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CHAPTER - I

INTRODUCTION

Most of the metals except noble metals which find extensive application in

industries and domestic uses exist in nature as ores in the form of oxides, silicates,

chlorides, etc. They are extracted from the ores by a series of metallurgical operations

involving expenditure of energy. The pure metal thus being at a higher energy state and

unstable tends to revert back to the compound form. The metals are converted into metallic

compounds at the surface and these compounds were known as corrosion products. This

phenomenon of deterioration of metals is known as corrosion. The greater the energy

expended in extraction more readily does the metal tends to corrode. Corrosion is a serious

problem in all fields of application of metals. The loss of metal resources whose abundance

is limited is cumulative and poses a danger to conservation and serious economic problem.

Trends in corrosion research changed rapidly over the years. It has started with simple

galvanic measurements and has now come to use of computer controlled potentiostat /

galvanostat. Sophisticated instrumentation and the revolutions in computer technology

make the possibility of online monitoring of corrosion. The understanding of mechanism

and kinetics of these reactions will help in devising ways and means to minimize the loss

due to corrosion.

1.1. DEFINITION AND IMPORTANCE OF CORROSION

Corrosion is defined as a deterioration of material due to the unavoidable reaction

with the environment. It is also defined as the destruction or deterioration of materials

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under chemical action of the surrounding environment [1]. Shreir [2] describes corrosion

as a reaction of solid with the environment.

Metal atoms in nature are present in chemical compounds, such as, minerals. The

same amount of energy is needed to extract metals from their minerals, they are emitted

during the chemical reactions that produce corrosion. Corrosion returns the metal to its

combined state i.e chemical compounds that are similar or even identical to the minerals

from which the metals were extracted. Thus, corrosion is defined as “Extractive metallurgy

in reverse”. Most iron ores contain oxides of iron and rusting of steel by water and oxygen

resulting in a hydrated iron oxide. Rusting is a term reserved for steel and iron corrosion,

although many other metals form their oxides when corrosion occurs [3].

1.2. COST OF CORROSION

It is justifiable that several crores of rupees are spent on research for controlling

corrosion. Losses caused by corrosion could be direct or indirect.

1.2.1. Direct Costs

Inability to use otherwise desirable materials.

Over design to allow for corrosion.

Cost of repair or replacement of the corroded component or equipment.

Cost of anti corrosive painting or other protection methods.

1.2.2. Indirect Costs

Contamination of the product.

Shutdown losses in the form of loss of production.

Loss of efficiency.

Safety due to possible explosions and loss of life.

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1.3. PRINCIPLES OF CORROSION

Principles of corrosion depends upon the following such as

1. Thermodynamic principles

2. Physical & Chemical principles

3. Metallurgical principles

4. Electrochemical principles

Thermodynamic and electrochemical principles play a major role in determining

the corrosion behaviour of materials. Thermodynamic principles can indicate the

spontaneous direction of a chemical reaction. They are used to determine whether the

corrosion is theoretically possible or not. Thermodynamic principles are applicable to free

energy, electrical double layer, Nernst equation for electrode potentials and Pourbaix

diagrams [4,5].

Electrochemical principles are extensively used to determine the corrosion

behaviour of materials. Here, the corrosion reaction can be represented by partial reactions

such as metal oxidation and reduction of some reducible species of the environment both

occurring simultaneously at equal rates, at the mixed potential [6] of the reaction.

Corrosion reaction mainly occurs at the metal-environment interface.

The electrochemical nature of corrosion can be illustrated by the attack of iron in

hydrochloric acid. When the iron is dipped in acid a vigorous action occurs as a result

hydrogen gas is evolved and iron gets dissolved.

Hence the reaction is

Fe + 2H+

Fe2+

+ H2

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The above reaction can be divided into two partial reactions,

Oxidation (anodic) reaction : Fe Fe2+

+ 2e-

Reduction (cathodic) reaction : 2H+

+ 2e H2

1.4. CLASSIFICATION OF CORROSION

Corrosion has been classified in different ways. One way of classification is high

temperature and low temperature corrosion, another way is wet and dry corrosion. The

more preferred classification is based on mechanisms, which fall into two types:

1. Electrochemical corrosion, involving an interface, in which anodic and cathodic

areas can be distinctly identified (or) such identification is not possible.

2. Chemical corrosion, which involves direct chemical reaction of the metal with the

environment.

1.4.1. Electrochemical Corrosion (or) Wet Corrosion

Electrochemical corrosion occurs due to the existence of separate anodic and

cathodic areas/peaks between which current flows through the conducting solution. This

type of corrosion occurs,

(i) where a conducting liquid is in contact with metal (or)

(ii) when two dissimilar metals or alloys are either immersed (or) dipped partially in

a solution.

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Electrochemical corrosion can be further classified into three types:

(a) Separable Anode / Cathode Type

In these cases certain areas of the metal can be experimentally identified as

predominantly anodic or cathodic. The distance of separation of these areas may be very

small, of the order of fractions of a millimeter. There is a macroscopic flow of charge

through the metal.

(b) Interfacial Anode / Cathode Type

Here, one entire interface will be cathode and the other will be anode. In this case,

charge is transported through a film of reaction product on the metal surface.

(c) Inseparable Anode / Cathode Type

Here, the anodes and cathodes cannot be distinguished by experimental methods,

though their presence is postulated by theory, e.g., the uniform dissolution of metal in

fused salt, non-aqueous solution, acid, alkaline or neutral solutions.

1.4.2. Direct Chemical Corrosion (or) Dry Corrosion

This type of corrosion involves direct chemical reaction of a metal with its

environment. There is no transport of electric charge and the metal remains film free. This

would include corrosion in gaseous environments such as oxygen, halogens, hydrogen

sulphide, nitrogen,, when the reaction product is volatile, corrosion in liquid metals, fused

halides and organic liquids. Dry corrosion is practically non-corrosive to ordinary steel, but

moist chlorine (or) chlorine dissolved in water, is extremely corrosive and attacks most of

the common metals and alloys. Dry corrosion is most often associated with high

temperatures [7]. A general scheme for the classification is presented below:

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Table 1.1

CLASSIFICATION OF CORROSION PROCESS

CORROSION

Metal- Liquid

(electroyle)

reaction

Immersed

Corrosion

Underground

Corrosion

Hydrogen

Evolation

Type

Oxygen

absorption

Type

Anaerobic

Aerobic

Meal Gas reaction

Direct

Oxidation

(tarnishing)

Exposed

to rain

Sheltered

Corrosion

Types of

Film

Formation

Above

Critical

Humidity

Below

Critical

Humidity

(a) Linear

(b) Parabolic

(c) Logritmic

(d) Asymtatic

Atmospheric

Corrosion

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The extent of corrosion depends upon the

(i) chemical affinity between the corrosive environment and solid metals and

(ii) ability of reaction product on metal surface to form a protective film.

There are three main types of chemical corrosion:

(a) Oxidation Corrosion

It is brought about by the direct action of oxygen at low (or) high temperatures on

metals usually in the absence of moisture.

(b) Corrosion by Hydrogen

Gases like hydrogen also attack metals at ordinary temperature. It is known as

„hydrogen embrittlement‟.

(c) Liquid Metal Corrosion

It is due to chemical action of flowing liquid metal at high temperature on solid

metal (or) alloy.

1.5. FACTORS INFLUENCING THE RATE OF CORROSION

The extent of corrosion and its nature largely depend upon the metal and the

environment surrounding it. The important factors that influence the corrosion phenomenon

are as follows:

a) Nature of the metal

b) Environment

c) Temperature

d) Concentration of the electrolyte

e) Nature of the corrosion product

f) Electrode potential

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g) Aeration

h) Agitation

i) Hydrogen over voltage

j) pH of the electrolyte

1.6. FORMS OF CORROSION

Due to the different causes to corrosion, it takes various forms. It is desirable to

know the form of corrosion, since it will help in identification of the cause of corrosion and

in the choice of the most effective method of preventing it.

(a) General and Local Electrochemical Corrosion

When separate corrosion cells can be distinguished by variation of the electrode

potential over the metal surface, by the appearance of corrosion currents (or) of separate

anodic and cathodic corrosion products, local electrochemical corrosion is said to occur.

General electrochemical corrosion occurs when separate anodic and cathodic surfaces do

not appear (or) they are of small dimensions (sub micro cells) (or) fluctuate over the

surfaces, the attack will be more uniform. Hence there are possibilities for the corrosion

products to form a continuous film and retard continuous attack. Therefore general

electrochemical corrosion leads to uniform attack where as local electrochemical corrosion

results in localized attacks.

(b) Galvanic Corrosion

Galvanic corrosion occurs when two or more dissimilar metals in electrical contact

are placed in an electrolyte. This causes a potential difference between the metals, which

results in the flow of current between them. Several investigations [8,9] have shown that

galvanic corrosion is directly proportional to the area ratio of the cathodic metal to the

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anodic metal. Galvanic corrosion is maximum at the junction of the two metals. The attack

decreases with increasing distance from the junction.

(c) Crevice Corrosion [10,11]

This occurs in certain metal-environment combinations. Only metals and alloys,

which depend upon the oxide film for corrosion resistance, are susceptible to crevice

corrosion. It is an intense localized corrosive attack, which occurs within the confined

space (or) crevices created by certain mechanical configurations. Surface deposits of

corrosion products, scratches in paint films etc.create them.

(d) Filiform Corrosion

This is a special type of crevice corrosion, which results in irregularly developed

hair-fine lines (or) filaments of corrosion products below the coatings of paints, tin, silver,

etc. It does not destroy the component but affects the surface appearance.

(e) Intergranular Corrosion

Grain boundaries are usually more reactive than the grain matrix. Hence localized

attack occurs at and adjacent to grain boundaries with relatively little corrosion of the

matrix. This attack is usually rapid and penetrates deep into the metal resulting in loss of

strength and causes catastrophic failures. Most metal alloys are susceptible to intergranular

corrosion (IGC), when exposed to specific corrodents. Mostly corrosion of iron-nickel-

chromium alloy is considered because of its commercial importance. Many reviews have

appeared on this subject.

IGC is caused by (i) impurities at the grain boundaries (ii) enrichment of one of the

element in the alloy (iii) depletion of one of the elements in the grain boundary area. The

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grain structure has a similar effect in both exfoliation and SCC, thus creating a link

between these two forms of corrosion [12].

(f) Pitting Corrosion

This is also a form of intensive localized attack, the rate of attack being non-

uniform. It is the most destructive form of corrosion and results in sudden failure of the

equipment due to the formation of pits (or) holes. It is reported to occur in the presence of

chloride ions, depending upon the concentration of chloride ions and the applied potential

[13, 14]. Pitting corrosion usually occurs under stagnant conditions such as crevices liquid

trapped in the lower part of a pipe or liquid stored in a container. Depth of pitting is

expressed by the term “pitting factor”. This is the ratio of the deepest metal penetration to

the average metal penetration. The most susceptible metal which undergoes pitting

corrosion is aluminium and its alloys.

(g) Exfoliation

The loss of metals as layers (or) leaves from a solid metal (or) alloy is called

exfoliation. This type is observed mostly in wrought products. Further, Al-Mg, Al-Cu,

Al-Zn and Al-Mg-Si alloys undergo exfoliation.

(h) Stress Corrosion Cracking

The cracking of metal (or) alloy by the conjoint action of a tensile stress and a

corrosive environment is known as stress corrosion cracking (SCC). The susceptibility to

stress corrosion cracking is due to certain metallurgical factors such as chemical

composition, preferential orientation of grains, composition and distribution of

precipitates, dislocation structure and environmental factors such as temperature, stress

level and structure of metal [15].

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(i) Corrosion Fatigue

Reduction in the fatigue strength due to the presence of a corrosive environment is

known as corrosion fatigue cracking. It occurs due to the combined action of tensile and

compressive stress alternatively. Fatigue occurs above a critical value of cyclic stress

known as the fatigue limit. Fatigue occurs at lower stress [16] in corrosive environment.

(j) Erosion Corrosion

Erosion corrosion is defined as increased corrosion rate due to relative motion

between metal surface and the environments usually liquid (or) gas. This type of corrosion

is also known as impingement corrosion. Such corrosion occurs in agitators, copper pipes,

centrifuge etc. Localized attack due to erosion corrosion usually has bright surface free

from corrosion products. e.g., pits, grooves, rounded holes and valleys.

(k) Cavitation Corrosion

This is a special type of erosion corrosion caused due to the formation of vapour

bubbles in a corrosive environment near a metal surface and when the bubbles collapse

attack arises. e.g., hydraulic turbulence, ship propellers, etc.

(l) Fretting Corrosion

This type of corrosion occurs between two surfaces in contact with each other in dry

(or) humid air when subjected to slight relative motion of small amplitude. Various

alternate terms such as friction oxidation, wear oxidation, chafting, false brinelling are

used to describe this phenomenon.

1.7. THEORIES OF CORROSION

Corrosion theories may be classified into:

1) Homogeneous theory and

2) Heterogeneous theory

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1.7.1. Homogeneous Theory

Surface heterogeneous of the metal cause corrosion process. Wagner and Traud

emphasize that the necessary condition for corrosion is the dissolution of the metal and

electronation reaction that takes place at the metal-environment interface. Metals become

unstable due to the charge transfer reaction taking place at the interface. Therefore it is

necessary that the potential difference across the interface should be more negative than

the equilibrium potential for the metal dissolution (anodic) reaction or more positive than

the equilibrium potential for the electronation (cathodic) reaction. This theory is helpful in

explaining the corrosion of pure metals and uniform corrosion, as the cathodic and anodic

sites interchange their characters quite frequently.

1.7.2. Heterogeneous Theory

According to this theory, the presence of impurities on the metal surface is

necessary and thus a local cell is setup between cathodic and anodic areas. Due to the

electrochemical reactions taking place at the interface between the metal and ionically

conducting films or actual electrolyte, the metal becomes unstable and hence corrodes.

Thus a corroding metal consists of

(a) an electron sink area where de-electronation reaction occurs.

(b) an electron source area where electronation reaction occurs.

(c) an ionic conductor to keep the ion current flowing.

This model is the basis for the local cell theory of corrosion or heterogeneous

corrosion.

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1.8. THERMODYNAMICS OF CORROSION

One can use thermodynamics, e.g. Pourbaix or E-pH diagrams, to evaluate the

theoretical activity of a given metal or alloy in a corrosion situation provided the chemical

make-up of the environment is known. But for practical situations it is important to realize

that the environment is a variable that can change with time and conditions. It is also

important to realize that the environment actually affects a metal corresponding to the

micro-environmental conditions i.e. the local environment at the surface of the metal. The

simplified potential-pH diagram is shown in the Figure (Fig. 1.1).

When a metal is exposed to an aqueous environment in the presence of air corrosion

can occur, the cathodic reaction being either hydrogen evolution or oxygen reduction. By

thermodynamical calculations Pourbaix has been worked out in the well known pH

potential diagrams. When used with care, these diagrams can be helpful in gauging

whether a metal is immune to corrosion or would get corroded in any particular

environment.

The diagram can also be helpful in gauging the onset of passivity. The passivity is

caused by a more or less contiguous oxide or salt film with low ionic conductivity. The

result would be that the metal ion would not be able to move out. If the film has a high

electronic conductivity, dissolution would be prevented even at high anodic potentials,

since the low field caused by the highly conducting film, would largely prevent the

migration of metal ions.

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Fig.1.1 Pourbaix diagram for Fe-H2O System

The diagram (Fig.1.1) can be extremely useful often in predicting in general way

the corrosiveness of environment as well as about the potable nature of the products of

corrosion. The pourbaix diagrams are now being extended to cover a variety of metals,

alloys, environments and even special types of corrosion such as pitting.

1.9. KINETICS OF CORROSION

Thermodynamic principles give only the corrosion tendency in terms of potentials,

when the chemical or electrochemical reactions are at equilibrium. But corroding systems

are not equilibrium and therefore thermodynamic calculations cannot be applied from the

engineering point of view. The major interest is in the kinetics or rate of corrosion.

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The rate of a chemical reaction is determined by

(i) activities of the reactants

(ii) height of the energy barrier

(iii) frequency of the activated complex that crosses the energy barrier.

Considering the metal-solution interface, the electrode field across the interface is

also to be taken into account. The relationship between the current and the potential is

given by Butler-Volmer equation. For a single electron transfer, the reaction can be written

as [17].

A+ + e

- ↔ A

i = i 0 [ e (1-β)Fη/RT

- e -βFη/RT

] ... (1)

Where i0 is the exchange current density, η is the over voltage and β is the

symmetry factor. At high positive overvoltage i.e η >> RT/F, the above equation reduces

to

i = i 0 e (1-β)Fη/RT

… (2)

At high negative overvoltage, i.e. η >> RT/F, the equation (1) reduces to

i = i 0 e- βFη/RT

… (3)

Equations (2) and (3) give the Tafel equation as shown below:

η = a + b log i … (4)

For anodic process,

- 2.303 RT 2.303 RT

a = ------------- log i 0 and b = ---------------

(1- β) F (1- β) F

For cathodic process,

2.303 RT - 2.303 RT

a = ------------- log i 0 and b = ---------------

βF βF

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For reaction involving more than one electron, the equation (1) is modified as

i = i 0 [ e αFη/RT

- e-α Fη/RT

] ... (5)

where, α is the charge transfer coefficient which is given as

n- γ

α = ----- - γβ and

V

n- γ

α = ----- + γβ

V

where, n = Total number of electrons taking part in the reaction,

γ = Number of individual electron steps before the rate determining step (rds)

γ = Stoichiometric number

V = Number of electrons taking part in the rds for its one time occurrence.

1.10 MECHANISM OF CORROSION PROCESSES

In majority of corrosion processes, the reduction reaction is confined to hydrogen

evolution or oxygen reduction and the anodic reaction is always the metal dissolution. The

metals are divided into three groups based on the differences in the mechanism of

hydrogen evolution, exchange current densities and high over voltages. It is not always

easy to predict corrosion rate based on the hydrogen evolution reaction, little is known

about the mechanism of oxygen reduction. This reaction takes place in many steps and also

on the oxide covered surface, which is a poor electron carrier. The anodic reactions in

corrosion processes are equally complicated. The process involves the movement of the

metal ions from metal phase to the solution phase which takes place through several steps

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like diffusion of ions on the surface, entry into the solution phase as an intermediate step

and the final step being the solvation of ions. Only dissolution and deposition of iron has

been studied in greater detail by Bockris and his co-workers [18]. They have given five

possible mechanisms for the deposition and desolution of iron.

Mechanism-1

Fe + OH-

+ FeOH (FeOH)2 + e

(FeOH)2 RDS 2FeOH

(FeOH)2 FeOH+ + e

FeOH+

Fe2+

+ OH-

Mechansim-2

Fe + H2O FeOH + H+ + e

FeOH FeOH+ + e

FeOH+

+ Fe RDS Fe2OH+

Fe2OH+ Fe

2+ + FeOH + e

FeOH + H+ Fe

2+ + H2O + e

Mechanism-3

Fe + OH- RDS Fe (OH)

+ + 2e

Fe (OH)

+ Fe

2+ + OH

-

Mechanism-4

Fe + OH- FeOH + e

FeOH + OH- RDS FeO + H2O + e

FeO + OH- HFeO2

-

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HFeO2- + H2O Fe (OH) 2 + OH

-

Fe (OH) 2 Fe2+

+ 2 OH-

Mechanism-5

Fe + H2O FeOH + H+ + e

FeOH+ + H

+ Fe

2+ + H2O

Kabanov and Frumkin [19,20] gave the first indication that OH- ions are taking part

in dissolution although these are present in traces in acidic solutions.

1.11. CORROSION MONITORING TECHNIQUES

Corrosion monitoring techniques have been divided into

1. Physicochemical methods and

2. Electrochemical methods.

1.11.1. PHYSICOCHEMICAL METHODS

These methods include techniques, such as,

(a) weight loss method

(b) gasometric method

(a) Weight Loss Method

In this method, the loss of metal due to corrosion is measured by exposing the

metal specimen of known area to the environment for a known period of time and the

difference in the weight before and after the exposure is calculated. The corroded

specimens are cleaned in the cleaning solution meant for each metal to remove any

corrosion product and the rate is calculated as follows:

KW

Corrosion rate (mmpy) =

ATD

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Where, K = 8.76 X 10 4 (constant), W = weight loss in g, A = area in sq.cm and

D = 7.86 gm / cu.cm (density).

(b) Gasometric Method

This method is reliable and accurate. The volume of gas (in acid corrosion) is

measured directly at a constant temperature and atmosphere pressure and metal loss is

calculated. The authors [ 21 - 23] have used this technique and designed gasometric unit

which operated under controlled conditions of temperature and pressure. The main

disadvantage is that this cannot be applied to strong oxidizing medium. When inhibitors

used undergo reduction with hydrogen gas evolved and in situations when the

stoichiometry of the corrosion reaction is not clearly defined.

1.11.2 ELECTROCHEMICAL METHODS

Corrosion process occurring in aqueous media are generally electro-chemical in

nature. The overall reaction is the combination of anodic and cathodic reactions. The

anodic reaction is the dissolution of metal and the cathodic reaction is the evolution of

hydrogen or reduction of oxygen. The main advantage is that they require short duration

and highly accurate results are obtained.

These include direct current and alternating current (AC and DC) techniques shown

by Mansfield and Lorenz [24] for the determination of corrosion rates in the system having

simple kinetics.

DC Methods

There are number of DC methods and some of which are described below:

(a) Tafel Extrapolation Method

The current and potential of the corroding electrode is related by

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I = icorr [(exp (2.3 /ba) –exp (-2.3 /bc) ]

Where, icorr is the corrosion current, is the over voltage and ba, bc are the anodic and

cathodic Tafel slopes.

When >> RT/F,

I = icorr exp(2.3 /ba)

Therefore, log I = log icorr + /ba

A plot of vs log i gives a straight line and slope gives ba or bc and the intercept

gives the corrosion current. The corrosion rate is obtained with the following equation.

Rate in mpy = 0.1288 icorr (w/ )

icorr is the corrosion current, is the density of the metal and w is the equivalent weight of

the metal.

The advantages of this method are

1. Many numbers of measurements are made to obtain the data for each curve.

2. The Tafel region gets distorted for systems having more than one cahtodic reaction.

3. This method requires a good conducting medium to operate.

4. This method is applicable for those reactions which are under activation controlled.

(b) Linear Polarization Method

Stern and Geary [25] have shown that there is a linear relationship between current

and potential when <<20mV. Measuring the slope of / I, the corrosion current icorr can

be calculated as follows:

Icorr = ba bc/2.3(ba+bc) i/

The term, i/ has the dimension of resistance and hence this method is known as

polarization resistance method.

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Icorr = ba bc/2.3(ba+bc) 1/Rp [ Where, Rp= / I ]

Riggs [26] has introduced an expression for certain corrosive systems. The

correlation coefficient factor introduced by him is expressed as

Qc = K mV/ln t

Where, K is the function for the process hindering the normal dissolution of the

metal electrode. mV/ln t is the slope for the potential difference ( mV) vs natural log of

time plot. For freely corroding system K=1.

IR drop has an influence on Rp [27]. It has been shown by Williams and Tailor, a

method for obtaining polarization curves free of IR drop.

The main advantages of this method are

1. Rapid corrosion rate is measured.

2. Perturbation of corroding specimen is small.

3. Even low corrosion rate is measured accurately.

The main disadvantages are

1. The media should be conducting.

2. Hysterisis effects are observed in the polarization curves affecting the Rp values

[28].

3. This technique can be applied only to systems exhibiting uniform corrosion.

(c) A.C. Impedance Method

Among the various AC techniques, impedance method is widely used due to the

main advantage that the double layer capacitance and charge transfer resistance can be

determined. The term resistance and impedance both imply a restriction to the current

flow. When dealing with DC only resistors produce this effect but in the case of AC both

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inductors and capacitors influence electron flow. The circuit diagram for stimulated

metal/solution interface [29] is presented below:

Fig. 1.2 The circuit diagram for stimulated metal/solution interface

Where,Cdl = Double layer capacity, Rs = Solutions resistance, Rt = Charge transfer

resistance.

The cell impedance consists of a real part (Z‟) and an imaginary part (Z”) of

impedance. The charge transfer resistance is related to icorr as

Rt = ba bc/2.3 (ba+bc) icorr

Where, ba and bc are Tafel slopes, Rt is obtained from the impedance plots. The impedance

Z is the total resistance to the alternating current and is given by

Z = Rsol + 1/[j Cdl+1/ Rt]

= Rsol + Rt/[1+j Cdl Rt]

= Rsol + Rt[1-j Cdl Rt]/1+2 Cdl

2 Rt

2

= Rsol + Rt/ 1+2 Cdl

2 Rt

2 –j[ Cdl

Rt

2/1+

2 Cdl

2 Rt

2]

= Z‟-JZ”

Where, Z‟ is Rsol + Rt/ 1+2 Cdl

2 Rt

2

Z” is Cdl Rt

2/1+

2 Cdl

2 Rt

2

Cdl

Rs

Rt

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Hence, total cell impedance is resolved in to real part {Z‟} and imaginary part

{Z”}. A plot of Z‟ and Z” for various frequencies gives a semicircle. At high frequency,

( ) and Z = Rsol. At low frequency, = 0 and Z= Rsol + Rt.

The difference between these two gives Rt and from the values corrosion rate can

be calculated. The double layer capacity is obtained from the frequency at which Z” is

maximum.

[Z” max.] =1/ Cdl Rt

Advantages of AC Impedance Method

1. AC impedance involves measurement of both capacitance and charge transfer

resistance and hence this technique is more valuable.

2. This technique uses amplitudes, which in the range 5-10 mV peak to peak.

Hence the perturbation is minimum and reduces the errors caused by the

measurement.

3. This method does not involve potential scan and therefore can be applied to low

conductivity media. The impedance of the corroding system at various

frequencies can be measured using lock in amplifiers for high frequencies and

fast Fourier transform [30] technique for low frequencies.

(d) Faradiac Rectification Method [31,32]

In this method, due to non symmetrical nature [ba bc] of the electrode -solution

interface, flow of direct component of the current results when a voltage of frequency is

applied to the electrode. By measuring the rectified current and potential, the corrosion rate

of the metal can be calculated. This effect is known as Faradaic rectification and was

discovered by Doss and Agarwal [33].

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In this method, the open circuit potential is steady. The current passing, through the

cell is reduced to zero by adjusting the back off voltage. The potential now corresponds to

the corrosion potential (Ecorr). Now an alternating voltage Em (30-40 mV) is applied across

the cell and the direction of the current and magnitude are noted. The potential of the

electrode is E1, now by adjusting the back off voltage, the current i is reduced to zero,

keeping Em as constant. Now the potential of the electrode is E2.. From the above values

icorr can be calculated.

Icorr = 2 icorr (E2-E1) 2.3 / bc-2(E2-Ecorr) Em2

The important features of methods are:

(a) Measurements of single AC frequency.

(b) Absence of errors from double layer charging.

(c) Built-in correction for the exchange current of the metal/metal ion reaction.

1.12. Corrosion Control Methods

Corrosion is destructive and silent, operating all the time, at all levels and in all

establishments. It poses problems to big as well as small industries. Since corrosion is

impracticable to eliminate, the effective Corrosion Science and Engineering lies in

controlling rather than preventing it.

Corrosion of metals occur when they come in contact with a corrosive environment.

Therefore, metallic corrosion can be prevented by either changing the metal (or) altering

the environment or by separating the metal from the environment (or) by changing the

electrode potential of the metal. A large number of corrosion failures are due to improper

design of equipment and the corrosion control can be therefore warranted at the design

stage itself. The various corrosion control methods are summarised in Table 1.2

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1.12.1 Design Improvement

The usual procedure followed at the design stage is to

(a) Establish basic requirements.

(b) Selecting the most suitable protective method and carrying the final design

work.

Some of the most general rules for design which should be observed are described

below:

1. Structures should have simplified forms. A complicated shape having more angles,

edges and internal surface will be easily corroded.

2. Avoid crevices, to avoid trapping of moisture and dirt which results in increased

corrosion.

3. Avoid residual moisture by having proper drainage holes and ventilation.

4. Avoid contact with absorbent materials.

5. Avoid galvanic corrosion by using suitable electrical insulators. Cheap and easily

exchangeable corroding pieces (or) paints where the contact of two different metals

is unavoidable and

6. Avoid mechanical stress.

1.12.2. Change of Metal

Mostly corrosion protection involves bulk alloying (or) surface coatings. Surface

coatings may pose problems related to adhesion, thermal expansion compatibility etc.

Surface processing of metals has been improved by iron implantation technique [34] and

laser treatment [35, 36] which results in a homogeneous and single phase surface layer.

26

Recently, electron beam surface area glazing has been found to increase the clear

life of iron base tool materials [37].

1.12.3. Change of Electrode Potential of the Metal

Corrosion can be prevented by changing the electrode potential by taking the metal

to the immune region (or) passive region. According to Pourbaix diagram this can be

accomplished by making the potential of the cathode equal to the open circuit potential of

anode.

(i) Cathodic Protection [38]

Cathodic protection is defined as the reduction (or) prevention of corrosion by

making it a cathode in the electrolytic cell. There are two methods of applying cathodic

protection to metallic structures, such as, galvanic (or) sacrificial anode and impressed

current method.

In each method, a direct current supply is more available for the protection of metal

structures. The choice of the method to be used depends upon a number of economic and

technical considerations.

27

Table 1.2

CLASSIFICATION OF CORROSION CONTROL METHODS

CORROSION PREVENTION BY

Design

Improvement

Change of

Metal

Change of

enviroment

Change of

metal electrode

Potential

Use of coatings

for separating

metal from the

environment

Change of

Composition

Change

of Micro

Structure

Elimination

of Tensile

Stress

Introduction

of surface

compressive

stresses

Remvoval of

corrosive

constituents

Oxygen

removal

Mositive

removal

Use of

inhibitrors

Chemcial

passivators

Adsorption

inhibitors

Film

forming

inhibitors

Vapours

phase

inhibitors

Change of

Operating

variables

Temperat

ure

Velocity

pH

Cathodic

Protection

Sacrificial

anodes

Impresed

current

Anodic

Protection

Organic

(Paints

etc)

Inorganic

Metallic

coatings

Cathodic

Anodic

Non Metallic

coatings

Vitreous

Cement

28

It is possible to protect ship hulls from corrosion. An active metal, generally, zinc is used

as a sacrificial anode in contact with the corroding material. The two metals in contact

form a galvanic cell, the terminals of which have been short circuited. In the impressed

current method of cathodic protection, an external source of direct current is connected to

the structure to be protected and the anode, the negative terminal of the power source is

connected to the structure to be protected. Some important impressed current anodes are

graphite, scrap iron, platinum and lead-silver alloys. Power sources used in these systems

are rectifiers, batteries etc. Sacrificial anode system and impressed current anode system

are complimentary to each other.

Cathodic protection can be applied to buried pipelines, underground cables,

equipments for handling and storage of chemicals, steel structures in marine atmosphere,

hulls of ships and oil-cargo-ballast tanks. Some of the limitations of cathodic protection

are:

(a) If polarization is too weak, materials remain exposed to corrosive

environment and remain unprotected.

(b) Results in stray-current corrosion in a neighboring unprotected buried

structure.

(c) An application of cathodic current may lead to destruction of passivity in

certain passive alloys, such as stainless steel.

(d) If polarization is too high, certain metals such as lead and tin are attached by

gasification, with the formation of gaseous hydrides, which can lead to

weakening and consequently disintegration of articles.

29

(ii) Anodic Protection [39-42]

Anodic protection is defined as the protection of a metal by maintaining it in the

passive condition. This technique is based on the phenomenon of passivity. The metal to

be protected is given a fixed potential so as to produce a passive film on it and the structure

is protected from corrosive environment.

This method is applicable to metals which can obey the following conditions:

1. The metal (or) alloy should have a active passive transition. (eg. Fe, Ni, Cr, Ti etc.)

2. It must require only a small current to maintain passivity.

3. The passive range for it must be wide

4. Sufficient electrical conductivity of the aggressive medium to which metal (or)

alloy is exposed.

The cathode is connected to the negative poles of the power source and completes

the electrical circuit. Some of the cathodes used are platinum, clad brass, chromium,

nickel, steel etc.,To measure the potential of the structures to be protected, a reference

electrode is needed. The reference electrodes used are calomel, Ag/AgCl, Hg/HgSO4 and

Pt/PtO. These should be insoluble in corrosive fluid and have potential that is stable with

respect to time. A potentiostat is necessary to maintain the potential at required level.

Advantages

1. In storage of acids.

2. In fertilizer industries and some other chemical industries.

Limitations

1. This method is applicable only for few metals which can be passive under certain

environments only.

30

2. This require costly instruments like potentiostat.

3. The metal cannot be protected above waterline.

4. This method cannot be used for metals exposed to environment containing

aggressive anions such as chloride.

1.12.4. Use of Coating

Corrosion can be prevented by separating the metal from the corrosive environment

by using protective coatings. Metallic and Non-metallic coating are the two types. The

characteristics of these are:

(a) Good resistance to corrosion

(b) Perfect adherence to the underlying metal

(c) Continuity so as to cover the metal surface completely.

Cathodic and anodic metallic coating provides a physical barrier between the

environment and discontinuity in the coating will result in localized attack. e.g., brass,

chromium (or) gold coating on steel as cathodic coating. Zinc and aluminium coating on

steel as anodic coating.

Non-metallic coating such as vitreous enamels, portland cement coating, chemical

conversion coatings such as varnishes, paints, enamels and lacquers can also be used.

1.12.5. Modification of the Corrosive Enviroment

Use of inhibitors is one of the predominant methods of this category. A detailed

discussion of inhibitors is presented in the next chapter (Chapter - II).

31

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