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CORROSION & CORROSION PREVENTION

Basic Principles

Dr. T. K. G. Namboodhiri

Consultant-Metallurgy & Corrosion, Tiruvalla, Kerala

(Ex-Professor of Metallurgical Engineering, Banaras Hindu University)

1. INTRODUCTION

1.1 What is corrosion?

Corrosion may be defined as the destruction or deterioration in properties of materials by

interaction with their environments. It is a natural phenomenon. Engineers generally

consider corrosion when dealing with metallic materials. However, the process affects all

sorts of materials, for example, ceramics, plastics, rubber etc. Rusting of iron and steel is the

most common example of corrosion. Swelling in plastics, hardening of rubber, deterioration

of paint, and fluxing of the ceramic lining of a furnace are all incidences of corrosion in non

metallic materials. Metallurgists may think of corrosion as reverse extractive metallurgy.

Metals are extracted from their compounds occurring in nature through extractive metallurgy

processes involving considerable expenditure of energy, natural resources, time, and man

power. Corrosion works to convert the metal I back into the same compounds.

1.2. Why is corrosion important?

Corrosion is a destructive natural process. It causes huge losses of money, material, and

natural resources. Estimated annual loss due to corrosion ranges from 3.5 to 5 % of the GNP

of a nation. Any effort to reduce this huge loss will be of much advantage to the economy.

Hence, it is advisable that all, particularly those involved with engineering materials, are

aware of the basic principles of corrosion and corrosion prevention.

1.3. Cost of corrosion

Loss due to corrosion may be direct or indirect.

Direct costs: cost of material lost, cost of repair and replacement of corroded parts, cost of

painting & other protective measures, over-design to allow for corrosion, and inability to use

otherwise suitable & cheaper materials.

Indirect costs: May be economical or social in nature. These include, contamination of

products like food items or drugs, loss of valuable products from leaking tanks or pipes, loss

of production due to shut downs, loss in appearance, as in automobiles or homes, and loss in

safety reliability of structures, machines, pipelines, storage tanks etc.

As per 2004 estimates, the annual direct loss due to corrosion in India was Rs. 36,000 crores,

while in the USA the loss was $364 billion. If the indirect costs are also taken into account,

this figure will be several times more. The higher the state of industrialization of a country,

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the higher will be the loss due to corrosion. India with its hot and humid tropical climate will

experience a larger proportion of corrosion loss than a temperate country like the U.K. or

France. A developing country like India, where a considerable percentage of population lives

under the poverty line, can ill afford the loss of such huge amounts due to corrosion. Hence,

corrosion should be dealt with all the seriousness it deserves.

2. PRINCIPLES OF CORROSION

Why do metals corrode?

Every system in the universe tries to reduce its energy content so as to become stable. This is

true for all types of reactions we see in nature. A spontaneous chemical reaction will occur

only if it leads to a lowering of the total energy content of the system. In thermodynamics we

say that all spontaneous reactions are accompanied by a lowering of the free energy. Most

metals and alloys, except the few noble metals, have higher free energies than those of their

chemical compounds. This is the reason they are not seen in nature as native metals.

Metallurgists spend lot of energy to convert metal compounds into metals, which thus

become unstable. As soon as they are put to service, they tend to get converted into their

more stable compound form. This is the basis of metallic corrosion.

Corrosion is an interdisciplinary phenomenon. It involves principles of thermodynamics,

electrochemistry, metallurgy, and physics & chemistry.

2.1 Thermodynamics of corrosion

Thermodynamics deals with the energy content of metals. The free energy of formation of a

compound is a measure of its energy content and stability. A compound with a high negative

free energy is very stable and requires a high energy input to convert it to the metal. This

metal will have a high tendency to be converted back to the compound, so as to reduce its

energy content, and so a high tendency for corrosion. The free energy change associated with

the compound formation is thus indicative of the tendency for corrosion of the metal. As is

discussed in the next section, the free energy change is used to calculate the electrode

potentials of metals and their corrosivity. Potential – pH diagrams or Pourbaix diagrams are

used to predict the corrosion tendency of a metal electrolyte system at various conditions.

2.2 Electrochemical nature of corrosion

Metallic corrosion is essentially electrochemical in nature. An electrochemical reaction is a

chemical reaction where, in addition to mass transfer, electron transfer also takes place. In

order to understand electrochemistry, we must understand what an electrode means. A

metallic conductor in contact with an ionic conductor (electrolyte) is called an electrode. Any

electrode will have a stable electrode potential, which develops due to the electrochemical

reactions taking place at the interface, and is the difference in potential of the metal and

electrolyte surrounding it. The electrode potential developed under standard conditions, the

standard electrode potential, is a characteristic property of the electrode.

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Electrochemical reactions take place in electrolytic cells, which consist of two different

electrodes immersed in an electrolyte

and connected electronically outside

the cell, as in Fig. 1.

Let us consider the corrosion of Zn

in hydrochloric acid. This process

can be represented by the electrolytic

cell shown in Fig. 1. We have Zn

metal as one electrode, and the inert

pt as the second electrode. When Zn

comes in contact with HCl, Zn atoms

dissolve, as per the following

equations.

The Zn atoms get converted to Zn ions by the oxidation reaction (4) at the Zn electrode and

get into the electrolyte, leaving behind two electrons/atom, which travel through the external

circuit to the Pt electrode and reduce two hydrogen ions to a hydrogen molecule by the

reaction (5). Thus one Zn atom dissolves while one molecule of hydrogen is liberated at the

DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,

222 HZnClHClZn -------------------- -(1)

Written in ionic form as,

2

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

The net reaction being,

2

22 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

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Pt electrode. The metal continues to dissolve liberating hydrogen molecules continuously.

Such an electrochemical reaction leads not only to a mass transfer from the metal to the

electrolyte, but also electron transfer from one electrode to the other. He electrode on which

oxidation takes place is called the anode, while that on which reduction occurs is a cathode.

Every electrochemical reaction thus has two components, one oxidation, and the second

reduction. These two reactions occur simultaneously at the same rate so that there is a charge

balance. The corrosion of a piece of Zn metal in HCl is shown schematically in Fig.2.

Here both the

anodic and

cathodic

reactions take

place on the

same piece, at

different

locations. The

metal dissolves

as ions and the

electrons left

behind move to

another point on

the metal

surface where

they reduce

hydrogen ions

from the

electrolyte to

hydrogen

atoms..

Fig. 2 Corrosion of Zn in HCl

As mentioned before, each electrode has a characteristic electrode potential, called the single

electrode potential. This potential is related to the nature of the electrode reaction taking

place, and thus to the free energy change involved in the reaction. The relationship may be

given as,

∆G = -nFE --------------------------------------- (6)

Where,

∆G is the free energy change in joules

n is the number of electrons involved in the reaction

E is the electrode potential in volts

F is the Faraday constant, 96500 Coulombs/ g.equivalent.

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The single electrode potential of an electrode when all the reactants are at unit activity and at

250 C is called the standard electrode potential (E

0). These potentials are also referred to as

redox potentials because they represent the equilibrium between an oxidation and a reduction

reactions taking place at the electrode interface.

EMF series & Galvanic series

Standard electrode potentials of many common elements are tabulated as an EMF series

which is used by electrochemists to determine the possible direction of reactions. Corrosion

engineers use another series, the galvanic series where metals are listed in the order of their

electrode potentials measured under actual service conditions. Galvanic series predicts

corrosion reactions more accurately.

Polarization

When corrosion takes place on a metal, its electrode potential shifts away from the standard

electrode potential, according to the equation,

E = E0 +2,303 RT/nF (product of activities of reactants/ product of activities of products)

This shifting of the potential from the standard value is called polarization, which forms the

basis of the kinetics of corrosion reactions.

Kinetics of corrosion

While thermodynamics predict the possible direction of a reaction, it cannot predict the rate

at which the reaction will occur. For this we require kinetics. The single electrode potential

of an electrode gets polarized when an electric current is flowing, ie, when corrosion takes

place. The rate at which corrosion occurs is determined by the Mixed Potential Theory of

corrosion.

Mixed Potential Theory

The mixed potential theory of Wagner

and Traud helps us to determine the

kinetic parameters of electrochemical

corrosion. It consists of two simple

hypotheses, 1) any electrochemical

reaction can be split into two or more

partial oxidation and reduction reactions,

and 2) there can be no net accumulation

of electrical charge during an

electrochemical reaction. Accordingly, a

corroding metal cannot spontaneously

accumulate electrical charge. All the

Fig. 3 Mixed potential theory

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electrons released by the anodic oxidation reaction must be consumed simultaneously by one

or more cathodic reduction reactions. The potential of a corroding metal will be determined

by the partial oxidation and reduction reactions involved in the process. This is schematically

shown in Fig.3.

2.3 Metallurgy of corrosion

Metals and alloys are widely used in engineering applications and are the ones which

undergo corrosion in service. The nature and extent of corrosion strongly depend upon the

metallurgical characteristics of the material. The tendency for corrosion of a metal depends

primarily on its electrode potential and polarization behavior. The chemical composition of

the material determines the electrode potential. Crystal structure of the metal may influence

corrosion. Metals are made up of tiny crystals called grains and many properties of the

material depend upon the grain size. The grain boundary, the region between adjacent grains,

is a defective region where impurities accumulate. Besides grains, engineering materials

generally contain different phases in their structure. These phases will have different

chemical compositions and hence different corrosion tendencies. Metals may also contain

defects like dislocations, internal surfaces, inclusions and voids. All of these may affect the

corrosion behavior of the material. Mechanical properties like strength and ductility have

much influence on certain forms of corrosion.

2.4 Physics & Chemistry in corrosion

Materials undergo corrosion during service because they are exposed to corrosive

environments. Different environments have different corrosive properties. Hence, chemistry

of the environment is a very important factor in corrosion. Physical properties like density,

viscosity, melting point and boiling point, surface tension of the environment as well as the

corroding material may affect the corrosion behavior. Velocity of liquid media will have a

great influence on the extent of corrosion. Temperature and pressure are two other important

variables in corrosion.

3. FORMS OF CORROSION

The process of corrosion may be classified in different ways. Some of these classifications

are given below.

A) Wet corrosion and Dry corrosion: Wet corrosion is corrosion in presence of water or a

liquid corrodent. Dry corrosion occurs in presence of gaseous atmospheres or in

contact with solids.

B) Room Temperature corrosion and High Temperature corrosion: Generally all wet

corrosion takes place at or around room temperature. High temperature corrosion

occurs generally much above the boiling point of water or other aqueous corrodents,

and is a dry corrosion processes.

C) Electrochemical corrosion and Chemical corrosion: Metallic corrosion is always

electrochemical in nature. Dissolution of a metal in an acid was previously thought to

be a simple chemical reaction and was called chemical corrosion. But now it is also

seen as an electrochemical corrosion.

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the original Fontana classification of corrosion based on the appearance of the corroded

metal. . Here we have 8 forms of room temperature or aqueous corrosion, and oxidation

and corrosion under complex gaseous environments at high temperatures.

Room Temperature or Aqueous Corrosion

Based on the appearance of the corroded metal, wet corrosion may be classified as

• Uniform or General

• Galvanic or Two-metal

• Crevice

• Pitting

Dealloying

• For the purpose of this lecture, let us consider

• Intergranular

• Velocity-assisted

• Environment-assisted cracking

UNIFORM CORROSION

• Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric

corrosion.

• Maximum metal loss by this form.

• Not dangerous, rate can be measured in the laboratory

GALVANIC CORROSION

• When two dissimilar metals are joined together and exposed, the more active of

the two metals corrode faster and the nobler metal is protected. This excess

corrosion is due to the galvanic current generated at the junction

CREVICE CORROSION

• Intensive localized corrosion within crevices & shielded areas on metal surfaces

• Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits,

lap joints

PITTING

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• A form of extremely localized attack causing holes in the metal

• Most destructive form

• Autocatalytic nature

• Difficult to detect and measure

• Mechanism

Fig.4 shows the mechanism of pitting.

Fig. 4 Mechanism of Pitting.

DEALLOYING

• Alloys exposed to corrosives experience selective leaching out of the more active

constituent. e.g. Dezincification of brass.

• Loss of structural stability and mechanical strength

INTERGRANULAR CORROSION

• The grain boundaries in metals are more active than the grains because of

segregation of impurities and depletion of protective elements. So preferential

attack along grain boundaries occurs. e.g. weld decay in stainless steels

VELOCITY ASSISTED CORROSION

• Fast moving corrosives cause

• a) Erosion-Corrosion,

• b) Impingement attack , and

• c) Cavitation damage in metals

CAVITATION DAMAGE

• Cavitation is a special case of Erosion-corrosion.

• In high velocity systems, local pressure reductions create water vapour bubbles

which get attached to the metal surface and burst at increased pressure, causing

metal damage

ENVIRONMENT ASSISTED CRACKING

• When a metal is subjected to a tensile stress and a corrosive medium, it may

experience Environment Assisted Cracking. Four types:

• Stress Corrosion Cracking

• Hydrogen Embrittlement

• Liquid Metal Embrittlement

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• Corrosion Fatigue

STRESS CORROSION CRACKING

• Static tensile stress and specific environments produce cracking

• Examples:

• 1) Stainless steels in hot chloride

• 2) Ti alloys in nitrogen tetroxide

• 3) Brass in ammonia

HYDROGEN EMBRITTLEMENT

• High strength materials stressed in presence of hydrogen crack at reduced stress

levels.

• Hydrogen may be dissolved in the metal or present as a gas outside.

• Only ppm levels of H needed

LIQUID METAL EMBRITTLEMENT

• Certain metals like

Al and stainless

steels undergo

brittle failure

when stressed in

contact with liquid

metals like Hg, Zn,

Sn, Pb Cd etc.

• Molten metal

atoms penetrate

the grain

boundaries and

fracture the metal

•

•

Fig 5 a). Tensile behavior under LME

•

Fig. 5 b). Brittle IG

fracture in Al alloy

by Pb

CORROSION

FATIGUE:

S-N DIAGRAM

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AirAir

CorrosionCorrosion

log (cycles to failure, Nf)

Str

ess

Am

pli

tud

e

Log (Stress Intensity Factor Range, K

log

(C

rack

Gro

wth

Rat

e, d

a/d

N)

Fig. 6a) gives schematic S-N curves for fatigue and corrosion-fatigue.

Synergistic action of corrosion

& cyclic stress. Both crack

nucleation and propagation

are accelerated by corrodent

and the S-N diagram is

shifted to the left

Fig. 6a) S-N curves for fatigue and corrosion fatigue

CRACK PROPAGATION

Fig. 6b) shows schematic crack

propagation curves under fatigue as well

as corrosion-fatigue conditions.

Crack propagation rate is

increased by the corrosive action

Fig. 6b) Crack propagation rates for fatigue and corrosion-fatigue

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High Temperature or Dry Corrosion

Oxidation under dry conditions, high temperature corrosion reactions in gaseous

atmospheres, and hot corrosion come under this classification.

OXIDATION

Oxidation refers to the reaction between a metal and air or oxygen in the absence of water or

an aqueous phase. Scaling, tarnishing and dry corrosion are other names for this process.

Nearly all metallic materials react with oxygen at high temperatures. As the temperature

increases, the oxidation resistance of materials decreases. As there are many applications of

metals at high temperatures, like gas turbines, rocket engines, refineries, and furnaces, the

importance of high temperature oxidation is considerable.

The oxidation resistance of a material may be related to the relative volumes of the metal and

its oxide, through the Pilling-Bedworth ratio, R = Md / nmD, where, M is the molecular

weight of the scale, D is the density of the scale, m is the atomic weight of the metal, d is the

density of the metal, and n is the number of metal atoms in a molecular formula of the scale.

R gives the volume of oxide formed from a unit volume of the metal. At a ratio of less than 1,

the scale does not cover the metal completely, and the metal continues to get oxidized, while

a ratio much greater than one tends to produce too much oxide which introduces high

compressive stress and tendency for spalling of the scale. The ideal R value is close to one.

Oxidation, like aqueous corrosion is an electrochemical process, and consists of two partial

processes,

M → M +2

+ 2 e- ----------- Metal oxidation at metal-scale interface

½ O2 + 2 e- → O2 --------- Oxygen reduction at scale-gas interface.

----------------------

M + ½ O2 → MO --------------------Overall reaction

The oxide scale acts as

the electrolyte through

which ions and

electrons move to make

the above reactions

possible. The electronic

and ionic conductivities

of the scale thus

determine the rate of

oxidation of the metal.

Oxidation kinetics

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When a fresh metal is exposed to oxygen, a thin surface layer of the oxide forms on the

metal. As oxidation continues, the scale thickness increases and the reaction rate decreases

depending upon the scale characteristics. Many empirical rate equations have been developed

to fit experimental oxidation data. Some of these are linear, parabolic, logarithmic and cubic.

These are shown schematically in Fig. 7. Fig.7 Oxidation Rate Laws

Oxidation-resistant alloys

The oxide characteristics determine the oxidation resistance of an alloy. Most oxides are non-

stoichiometric compounds with structural defects. They may be n-type or p-type

semiconductors whose conductivities could be altered by alloy additions. This principle is

used in developing high temperature oxidation resistant alloys like Fe-Cr, Fe-Cr-Al, and Ni-

base alloys.

Catastrophic oxidation

Metals that follow linear oxidation kinetics at low temperatures may experience oxidation at

continuously increasing rates at high temperatures. Metals like Mo, W, Os, Rh, and V which

have volatile oxides may oxidize catastrophically. Alloys containing Mo and V may oxidize

catastrophically by the formation of low melting eutectic oxide mixtures. Combustion of fuel

oils with high V compounds produces vanadium oxides in the gas phase, and can lead to

catastrophic oxidation.

Internal Oxidation

In some alloys, one or more dilute components may form more stable oxides than the base

metal which get distributed below the metal-oxide interface. This is called internal oxidation

because the oxide precipitate forms within the metal matrix. Dilute copper and silver based

alloys containing Al, Zn, Cd, Be show such a behavior.

CORROSION IN OTHER GASEOUS ENVIRONMENTS

Sulfur compounds

High temperature degradation of metals occurs when exposed to sulfur compounds like H2S,

SO2 and vaporized sulfur. This process is referred to as sulfidation. In reducing gases

containing hydrogen, such as gasified coal, H2S is a major gaseous constituent. In oxidizing

gases such as fossil fuel combustion products, considerable SO2 may exist. These sulfur

bearing gaseous compounds can lead to rapid scaling and to internal precipitation of stable

sulfides. Mechanical properties of high temperature alloys are seriously affected by these

precipitates.

Decarburization and hydrogen attack.

When steels are exposed o hydrogen at high temperatures, the carbon present either in

dissolved form or s carbides, reacts with hydrogen to produce methane gas, as per the

following reaction.

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C (Fe) + 4 H→CH4

This phenomenon is called hydrogen attack. Decarburization leads to a decrease in the

strength of the steel. The methane formed inside the steel may lead to cracking. Cr and Mo

additions to steels improve their resistance to decarburization and cracking. A Nelson

diagram is used to predict safe working conditions of hydrogen partial pressure and

temperature for various steels.

Hot Corrosion

Hot corrosion refers to the accelerated high temperature corrosion of materials under sulfur

gaseous atmospheres and the presence fused sulphate compounds on the metal surface.

4) CORROSION TESTING.

Corrosion tests are of four types;

1. Laboratory tests

2. Pilot-plant tests

3. Plant or actual service tests

4. Field tests

Laboratory tests use small specimens and small volumes of corrodents and actual conditions

are simulated as far as possible. These are most useful as screening tests to determine which

material warrants further studies.

Pilot plant or semi-works tests are made in a small-scale plant that essentially duplicates the

intended large-scale operation. This type of tests generally gives the best results.

Actual plant tests are done when an operating plant is available. The purpose is in evaluating

better or more economical materials or in studying corrosion behavior of existing materials

as process conditions are changed.

Field tests are designed to obtain general corrosion information. Examples are atmospheric

exposure of a large number of specimens in racks at one or more geographical locations and

similar tests in soil or sea water.

Corrosion tests are essential for the following purposes;

1. Evaluation and selection of materials for a specific environment or a given

application.

2. Evaluation of materials as regards to their compatibility to various environments. The

information generated helps in the selection of materials for a specific application.

3. Control of corrosion resistance of the material or corrosiveness of the environment.

These are routine quality control tests.

4. Study of the mechanisms of corrosion or other research and development purposes.

These are specialized tests involving precise measurements and close control.

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Corrosion test standards have been developed by many organizations like ASTM, NACE in

USA and similar ones in other countries. Corrosion data exists for a large proportion of

materials used in several environments which could be used by material selectors.

Corrosion Rate The most basic corrosion property of a material is the rate at which material is lost due to

exposure to an environment. This is expressed as the corrosion rate, which is expressed in

two ways; 1) weight of material lost per unit surface area per unit time, and 2) the rate of

penetration or thinning down of a material. The common corrosion rate units are mdd

(mg/dm2/day) in the first category, and mpy (mils/year) in the second category.

Weight loss after immersion in the corrodent for a specific time is measured on a specimen of

known surface area and then the corrosion rate can be calculated as

R = KW/ATD

Where,

K is a constant for a specific unit of R

W is weight lost in gm

A is the surface area in sq.cm

T is time of exposure in hours

D is density in g/cu.cm

The constant K varies from unit to unit. For mdd, the value of K is 2.4 x 106D, and for mpy

the K value is 3.45x 106.

5) PROTECTION AGAINST CORROSION

Need for corrosion prevention

• The huge annual loss due to corrosion is a national waste and should be minimized

• Materials already exist which, if properly used, can eliminate 80 % of corrosion loss

• Proper understanding of the basics of corrosion and incorporation in the initial design

of metallic structures is essential

Methods

• Material selection

• Improvements in material

• Design of structures

• Alteration of environment

• Cathodic & Anodic protection

• Coatings

Material Selection

• Most important method – select the appropriate metal or alloy.

• “Natural” metal-corrosive combinations like

• S. S.- Nitric acid, Ni & Ni alloys- Caustic

• Monel- HF, Hastelloys- Hot HCl

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• Pb- Dil. Sulphuric acid, Sn- Distilled water

• Al- Atmosphere, Ti- hot oxidizers

• Ta- Ultimate resistance

Improvement of materials

1) Purification of metals- Al , Zr

2) Alloying with metals for:

• Making more noble, e.g. Pt in Ti

• Passivating, e.g. Cr in steel

• Inhibiting, e.g. As & Sb in brass

• Scavenging, e.g. Ti & Nb in S.S

• Improving other properties

Design of Structures

• Avoid sharp corners

• Complete draining of vessels

• No water retention

• Avoid sudden changes in section

• Avoid contact between dissimilar metals

• Weld rather than rivet

• Easy replacement of vulnerable parts

• Avoid excessive mechanical stress

Alteration of Environment

• Lower temperature and velocity

• Remove oxygen/oxidizers

• Change concentration

• Add Inhibitors

– Adsorption type, e.g. Organic amines, azoles

– H evolution poisons, e.g. As & Sb

– Scavengers, e.g. Sodium sulfite & hydrazine

– Oxidizers, e.g. Chromates, nitrates, ferric salts

Cathodic & Anodic Protection

• Cathodic protection: Make the structure more cathodic by

– Use of sacrificial anodes

– Impressed currents

Used extensively to protect marine structures, underground pipelines, water heaters

and reinforcement bars in concrete

• Anodic protection: Make Passivating metal structures more anodic by impressed

potential. e.g. 316 s.s. pipe in sulfuric acid plants

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Coatings

• Most popular method of corrosion protection

• Coatings are of various types:

– Metallic

– Inorganic like glass, porcelain and concrete

– Organic, paints, varnishes and lacquers

• Many methods of coating:

– Electro deposition

– Flame spraying

– Cladding

– Hot dipping

– Diffusion

– Vapour deposition

– Ion implantation

– Laser glazing

Surface Engineering

The process of altering the surface characteristics of materials is known as surface

engineering. Corrosion is a surface property and all the coating processes mentioned above

come under surface engineering. Besides corrosion, wear, fretting, fatigue etc are also

dependent on the surface characteristics of materials.

CONCLUSION

• Corrosion is a natural degenerative process affecting metals, nonmetals and even

biological systems like the human body

• Corrosion of engineering materials lead to significant losses

• An understanding of the basic principles of corrosion and their application in the

design and maintenance of engineering systems result in reducing losses considerably

REFERENCES

1. Corrosion Engineering, Mars G. Fontana, 3rd

Ed. McGraw-Hill International,

Singapore, 1987

2. Corrosion and Corrosion Control, Herbert H. Uhlig, 3rd

Ed. John Wiley & Sons,

New York, 1985

3. Metals Handbook, 9th

ed. Volume 13, Corrosion, ASM International, Metals Park,

Ohio, 1988