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CORROSIVE DAMAGE IN CORROSIVE DAMAGE IN MATERIALS & ITS MATERIALS & ITS PREVENTION PREVENTION Dr. T. K. G. NAMBOODHIRI Dr. T. K. G. NAMBOODHIRI Professor of Metallurgy Professor of Metallurgy (Retired) (Retired)

CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

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AN INTRODUCTORY LECTURE ON CORROSION OF METALS, ITS PRINCIPLES AND FORMS, & METHODS OF CORROSION PREVENTION

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Page 1: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

CORROSIVE DAMAGE IN CORROSIVE DAMAGE IN MATERIALS & ITS MATERIALS & ITS

PREVENTIONPREVENTION

Dr. T. K. G. NAMBOODHIRIDr. T. K. G. NAMBOODHIRIProfessor of Metallurgy Professor of Metallurgy

(Retired)(Retired)

Page 2: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

INTRODUCTIONINTRODUCTION• Definition: Corrosion Definition: Corrosion

is the degeneration of is the degeneration of materials by reaction materials by reaction with environment. with environment. Examples: Rusting of Examples: Rusting of automobiles, automobiles, buildings and bridges, buildings and bridges, Fogging of silverware, Fogging of silverware, Patina formation on Patina formation on copper.copper.

Page 3: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

UNIVERSALITY OF CORROSIONUNIVERSALITY OF CORROSION

• Not only metals, but non-metals like Not only metals, but non-metals like plastics, rubber, ceramics are also plastics, rubber, ceramics are also subject to environmental degradationsubject to environmental degradation

• Even living tissues in the human body Even living tissues in the human body are prone to environmental damage by are prone to environmental damage by free radicals-Oxidative stress- leading free radicals-Oxidative stress- leading to degenerative diseases like cancer, to degenerative diseases like cancer, cardio-vascular disease and diabetes.cardio-vascular disease and diabetes.

Page 4: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

CORROSION DAMAGECORROSION DAMAGE• Disfiguration or loss of appearanceDisfiguration or loss of appearance• Loss of materialLoss of material• Maintenance costMaintenance cost• Extractive metallurgy in reverse- Loss of Extractive metallurgy in reverse- Loss of

precious minerals, power, water and precious minerals, power, water and man-powerman-power

• Loss in reliability & safetyLoss in reliability & safety• Plant shutdown, contamination of product Plant shutdown, contamination of product

etcetc

Page 5: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

COST OF CORROSIONCOST OF CORROSION• Annual loss due to corrosion is estimated to Annual loss due to corrosion is estimated to

be 3 to 5 % of GNP, about Rs.700000 croresbe 3 to 5 % of GNP, about Rs.700000 crores• Direct & Indirect lossesDirect & Indirect losses• Direct loss: Material cost, maintenance cost, Direct loss: Material cost, maintenance cost,

over-design, use of costly materialover-design, use of costly material• Indirect losses: Plant shutdown & loss of Indirect losses: Plant shutdown & loss of

production, contamination of products, loss production, contamination of products, loss of valuable products due to leakage etc, of valuable products due to leakage etc, liability in accidentsliability in accidents

Page 6: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

WHY DO METALS WHY DO METALS CORRODE?CORRODE?• Any spontaneous reaction in the Any spontaneous reaction in the

universe is associated with a lowering universe is associated with a lowering in the free energy of the system. i.e. a in the free energy of the system. i.e. a negative free energy changenegative free energy change

• All metals except the noble metals All metals except the noble metals have free energies greater than their have free energies greater than their compounds. So they tend to become compounds. So they tend to become their compounds through the process their compounds through the process of corrosionof corrosion

Page 7: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

ELECTROCHEMICAL NATUREELECTROCHEMICAL NATURE• All metallic corrosion are electrochemical All metallic corrosion are electrochemical

reactions i.e. metal is converted to its reactions i.e. metal is converted to its compound with a transfer of electronscompound with a transfer of electrons

• The overall reaction may be split into The overall reaction may be split into oxidation (anodic) and reduction oxidation (anodic) and reduction (cathodic) partial reactions(cathodic) partial reactions

• Next slide shows the electrochemical Next slide shows the electrochemical reactions in the corrosion of Zn in reactions in the corrosion of Zn in hydrochloric acidhydrochloric acid

Page 8: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

ELECTROCHEMICAL ELECTROCHEMICAL REACTIONS IN CORROSIONREACTIONS IN CORROSION

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

Page 9: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

ELECTROCHEMICAL THEORYELECTROCHEMICAL THEORY• The anodic & cathodic The anodic & cathodic

reactions occur reactions occur simultaneously at simultaneously at different parts of the different parts of the metal.metal.

• The electrode The electrode potentials of the two potentials of the two reactions converge to reactions converge to the corrosion potential the corrosion potential by polarizationby polarization

Page 10: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

PASSIVATIONPASSIVATION• Many metals like Cr, Ti, Many metals like Cr, Ti,

Al, Ni and Fe exhibit a Al, Ni and Fe exhibit a reduction in their reduction in their corrosion rate above corrosion rate above certain critical potential. certain critical potential. Formation of a Formation of a protective, thin oxide protective, thin oxide film.film.

• Passivation is the reason Passivation is the reason for the excellent for the excellent corrosion resistance of Al corrosion resistance of Al and S.S. and S.S.

Page 11: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

FORMS OF CORROSIONFORMS OF CORROSION• Corrosion may be Corrosion may be

classified in classified in different waysdifferent ways

• Wet / Aqueous Wet / Aqueous corrosion & Dry corrosion & Dry CorrosionCorrosion

• Room Room Temperature/ Temperature/ High Temperature High Temperature CorrosionCorrosion

CORROSION

WET CORROSION DRY CORROSION

CORROSION

ROOM TEMPERATURECORROSION

HIGH TEMPERATURECORROSION

Page 12: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

WET & DRY CORROSIONWET & DRY CORROSION• Wet / aqueous corrosionWet / aqueous corrosion is the is the

major form of corrosion which occurs major form of corrosion which occurs at or near room temperature and in at or near room temperature and in the presence of waterthe presence of water

• Dry / gaseous corrosionDry / gaseous corrosion is is significant mainly at high significant mainly at high temperaturestemperatures

Page 13: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

WET / AQUEOUS CORROSIONWET / AQUEOUS CORROSION Based on the appearance of the corroded Based on the appearance of the corroded

metal, wet corrosion may be classified asmetal, wet corrosion may be classified as• Uniform or GeneralUniform or General• Galvanic or Two-metalGalvanic or Two-metal• CreviceCrevice• PittingPitting• DealloyingDealloying• IntergranularIntergranular• Velocity-assistedVelocity-assisted• Environment-assisted crackingEnvironment-assisted cracking

Page 14: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

UNIFORM CORROSIONUNIFORM CORROSION• Corrosion over the Corrosion over the

entire exposed entire exposed surface at a uniform surface at a uniform rate. e.g.. rate. e.g.. Atmospheric Atmospheric corrosion.corrosion.

• Maximum metal loss Maximum metal loss by this form.by this form.

• Not dangerous, rate Not dangerous, rate can be measured in can be measured in the laboratory.the laboratory.

Page 15: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

GALVANIC CORROSIONGALVANIC CORROSION• When two dissimilar When two dissimilar

metals are joined metals are joined together and exposed, together and exposed, the more active of the the more active of the two metals corrode two metals corrode faster and the nobler faster and the nobler metal is protected. This metal is protected. This excess corrosion is due excess corrosion is due to the galvanic current to the galvanic current generated at the generated at the junctionjunction

• Fig. Al sheets covering Fig. Al sheets covering underground Cu cablesunderground Cu cables

Page 16: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

CREVICE CORROSIONCREVICE CORROSION• Intensive localized Intensive localized

corrosion within corrosion within crevices & shielded crevices & shielded areas on metal areas on metal surfacessurfaces

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

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PITTINGPITTING• A form of extremely A form of extremely

localized attack localized attack causing holes in the causing holes in the metalmetal

• Most destructive formMost destructive form• Autocatalytic natureAutocatalytic nature• Difficult to detect and Difficult to detect and

measuremeasure• MechanismMechanism

Page 18: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

DEALLOYINGDEALLOYING• Alloys exposed to Alloys exposed to

corrosives experience corrosives experience selective leaching out selective leaching out of the more active of the more active constituent. e.g. constituent. e.g. Dezincification of Dezincification of brass.brass.

• Loss of structural Loss of structural stability and stability and mechanical strengthmechanical strength

Page 19: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

INTERGRANULAR CORROSIONINTERGRANULAR CORROSION• The grain boundaries in The grain boundaries in

metals are more active metals are more active than the grains because than the grains because of segregation of of segregation of impurities and depletion impurities and depletion of protective elements. of protective elements. So preferential attack So preferential attack along grain boundaries along grain boundaries occurs. e.g. weld decay occurs. e.g. weld decay in stainless steelsin stainless steels

Page 20: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

VELOCITY ASSISTED VELOCITY ASSISTED CORROSIONCORROSION• Fast moving Fast moving

corrosives cause corrosives cause • a) Erosion-a) Erosion-

Corrosion, Corrosion, • b) Impingement b) Impingement

attack , and attack , and • c) Cavitation c) Cavitation

damage in metalsdamage in metals

Page 21: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

CAVITATION DAMAGECAVITATION DAMAGE• Cavitation is a special Cavitation is a special

case of Erosion-case of Erosion-corrosion.corrosion.

• In high velocity In high velocity systems, local pressure systems, local pressure reductions create water reductions create water vapour bubbles which vapour bubbles which get attached to the get attached to the metal surface and burst metal surface and burst at increased pressure, at increased pressure, causing metal damagecausing metal damage

Page 22: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

ENVIRONMENT ASSISTED ENVIRONMENT ASSISTED CRACKINGCRACKING• When a metal is subjected to a tensile When a metal is subjected to a tensile

stress and a corrosive medium, it may stress and a corrosive medium, it may experience Environment Assisted experience Environment Assisted Cracking. Four types:Cracking. Four types:

• Stress Corrosion CrackingStress Corrosion Cracking• Hydrogen EmbrittlementHydrogen Embrittlement• Liquid Metal EmbrittlementLiquid Metal Embrittlement• Corrosion FatigueCorrosion Fatigue

Page 23: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

STRESS CORROSION CRACKINGSTRESS CORROSION CRACKING• Static tensile stress Static tensile stress

and specific and specific environments environments produce crackingproduce cracking

• Examples:Examples:• 1) Stainless steels in 1) Stainless steels in

hot chloridehot chloride• 2) Ti alloys in 2) Ti alloys in

nitrogen tetroxidenitrogen tetroxide• 3) Brass in ammonia3) Brass in ammonia

Page 24: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

HYDROGEN EMBRITTLEMENTHYDROGEN EMBRITTLEMENT• High strength High strength

materials stressed in materials stressed in presence of hydrogen presence of hydrogen crack at reduced crack at reduced stress levels.stress levels.

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

• Only ppm levels of H Only ppm levels of H neededneeded

Page 25: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

LIQUID METAL EMBRITTLEMENTLIQUID METAL EMBRITTLEMENT• Certain metals like Al Certain metals like Al

and stainless steels and stainless steels undergo brittle failure undergo brittle failure when stressed in when stressed in contact with liquid contact with liquid metals like Hg, Zn, metals like Hg, Zn, Sn, Pb Cd etc.Sn, Pb Cd etc.

• Molten metal atoms Molten metal atoms penetrate the grain penetrate the grain boundaries and boundaries and fracture the metalfracture the metal

• Fig. Shows brittle IG Fig. Shows brittle IG fracture in Al alloy by fracture in Al alloy by PbPb

Page 26: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

CORROSION FATIGUE CORROSION FATIGUE S-N S-N DIAGRAMDIAGRAM

Synergistic action Synergistic action of corrosion & of corrosion & cyclic stress. Both cyclic stress. Both crack nucleation crack nucleation and propagation and propagation are accelerated by are accelerated by corrodent and the corrodent and the S-N diagram is S-N diagram is shifted to the leftshifted to the left

AirAir

CorrosionCorrosion

log (cycles to failure, N f)

Stres

s Am

plitu

de

Page 27: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

CORROSION FATIGUE, CORROSION FATIGUE, CRACK PROPAGATIONCRACK PROPAGATION

Crack propagation Crack propagation rate is increased by rate is increased by the corrosive actionthe corrosive action

Log (Stress Intensity Factor Range, K

log (C

rack

Gro

wth R

ate, d

a/dN

)

Page 28: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

PREVENTION OF CORROSIONPREVENTION OF CORROSION

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

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

• Proper understanding of the basics of Proper understanding of the basics of corrosion and incorporation in the initial corrosion and incorporation in the initial design of metallic structures is essentialdesign of metallic structures is essential

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METHODSMETHODS• Material selectionMaterial selection• Improvements in materialImprovements in material• Design of structuresDesign of structures• Alteration of environmentAlteration of environment• Cathodic & Anodic protectionCathodic & Anodic protection• CoatingsCoatings

Page 30: CORROSIVE DAMAGE IN METALS AND ITS PREVENTION

MATERIAL SELECTIONMATERIAL SELECTION• Most important method – select the Most important method – select the

appropriate metal or alloy .appropriate metal or alloy .• ““Natural” metal-corrosive combinations Natural” metal-corrosive combinations

likelike• S. S.- Nitric acid, Ni & Ni alloys- CausticS. S.- Nitric acid, Ni & Ni alloys- Caustic• Monel- HF, Hastelloys- Hot HClMonel- HF, Hastelloys- Hot HCl• Pb- Dil. Sulphuric acid, Sn- Distilled waterPb- Dil. Sulphuric acid, Sn- Distilled water• Al- Atmosphere, Ti- hot oxidizersAl- Atmosphere, Ti- hot oxidizers• Ta- Ultimate resistanceTa- Ultimate resistance

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IMPROVEMENTS OF MATERIALSIMPROVEMENTS OF MATERIALS

• Purification of metals- Al , ZrPurification of metals- Al , Zr• Alloying with metals for:Alloying with metals for:• Making more noble, e.g. Pt in TiMaking more noble, e.g. Pt in Ti• Passivating, e.g. Cr in steelPassivating, e.g. Cr in steel• Inhibiting, e.g. As & Sb in brassInhibiting, e.g. As & Sb in brass• Scavenging, e.g. Ti & Nb in S.SScavenging, e.g. Ti & Nb in S.S• Improving other propertiesImproving other properties

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DESIGN OF STRUCTURESDESIGN OF STRUCTURES• Avoid sharp cornersAvoid sharp corners• Complete draining of vesselsComplete draining of vessels• No water retentionNo water retention• Avoid sudden changes in sectionAvoid sudden changes in section• Avoid contact between dissimilar metalsAvoid contact between dissimilar metals• Weld rather than rivetWeld rather than rivet• Easy replacement of vulnerable partsEasy replacement of vulnerable parts• Avoid excessive mechanical stressAvoid excessive mechanical stress

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ALTERATION OF ENVIRONMENTALTERATION OF ENVIRONMENT

• Lower temperature and velocityLower temperature and velocity• Remove oxygen/oxidizersRemove oxygen/oxidizers• Change concentrationChange concentration• Add InhibitorsAdd Inhibitors

– Adsorption type, e.g. Organic amines, azolesAdsorption type, e.g. Organic amines, azoles– H evolution poisons, e.g. As & SbH evolution poisons, e.g. As & Sb– Scavengers, e.g. Sodium sulfite & hydrazineScavengers, e.g. Sodium sulfite & hydrazine– Oxidizers, e.g. Chromates, nitrates, ferric saltsOxidizers, e.g. Chromates, nitrates, ferric salts

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CATHODIC & ANODIC CATHODIC & ANODIC PROTECTIONPROTECTION• Cathodic protection: Make the structure more Cathodic protection: Make the structure more

cathodic bycathodic by– Use of sacrificial anodesUse of sacrificial anodes– Impressed currentsImpressed currents

Used extensively to protect marine structures, Used extensively to protect marine structures, underground pipelines, water heaters and underground pipelines, water heaters and reinforcement bars in concretereinforcement bars in concrete

• Anodic protection: Make passivating metal Anodic protection: Make passivating metal structures more anodic by impressed potential. structures more anodic by impressed potential. e.g. 316 s.s. pipe in sulfuric acid plantse.g. 316 s.s. pipe in sulfuric acid plants

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COATINGSCOATINGS• Most popular method of corrosion protectionMost popular method of corrosion protection• Coatings are of various types:Coatings are of various types:

– MetallicMetallic– Inorganic like glass, porcelain and concreteInorganic like glass, porcelain and concrete– Organic, paints, varnishes and lacquersOrganic, paints, varnishes and lacquers

• Many methods of coating:Many methods of coating:– ElectrodepositionElectrodeposition– Flame sprayingFlame spraying– CladdingCladding– Hot dippingHot dipping– DiffusionDiffusion– Vapour depositionVapour deposition– Ion implantationIon implantation– Laser glazingLaser glazing

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CONCLUSIONCONCLUSION• Corrosion is a natural degenerative process Corrosion is a natural degenerative process

affecting metals, nonmetals and even affecting metals, nonmetals and even biological systems like the human bodybiological systems like the human body

• Corrosion of engineering materials lead to Corrosion of engineering materials lead to significant lossessignificant losses

• An understanding of the basic principles of An understanding of the basic principles of corrosion and their application in the design corrosion and their application in the design and maintenance of engineering systems and maintenance of engineering systems result in reducing losses considerably result in reducing losses considerably