CORROSION & CORROSION PREVENTION

Basic Principles

Dr. T. K. G. NamboodhiriConsultant-Metallurgy & Corrosion, Tiruvalla, Kerala

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

1. INTRODUCTION1.1 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.

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

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

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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 joulesn is the number of electrons involved in the reactionE is the electrode potential in voltsF 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 (E0). 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 corrosionWhile 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• PittingDealloying• 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• MechanismFig.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• Corrosion Fatigue

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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, N f)

Stre

ss A

mplitu

de

Log (Stress Intensity Factor Range, K

log

(Cra

ck G

row

th R

ate,

da/

dN

)

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

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

High Temperature or Dry Corrosion

Oxidation under dry conditions, high temperature corrosion reactions in gaseous atmospheres, and hot corrosion come under this classification.

OXIDATIONOxidation 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.----------------------

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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

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

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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 ENVIRONMENTSSulfur 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.

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 CorrosionHot 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 tests2. Pilot-plant tests3. Plant or actual service tests4. 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.

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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.

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 RateThe 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/ATDWhere, K is a constant for a specific unit of RW is weight lost in gmA is the surface area in sq.cmT is time of exposure in hoursD is density in g/cu.cmThe 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

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• 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• Pb- Dil. Sulphuric acid, Sn- Distilled water• Al- Atmosphere, Ti- hot oxidizers• Ta- Ultimate resistance

Improvement of materials

1) Purification of metals- Al , Zr2) 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

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• 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

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

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• 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

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