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

PREVENTIONPREVENTION

Dr. T. K. G. NAMBOODHIRIDr. T. K. G. NAMBOODHIRI

Professor of Metallurgy Professor of Metallurgy (Retired)(Retired)

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 buildings and bridges, Fogging of bridges, Fogging of silverware, Patina silverware, Patina formation on copper.formation on copper.

UNIVERSALITY OF UNIVERSALITY OF CORROSIONCORROSION

• 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.

CORROSION DAMAGECORROSION DAMAGE

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

Loss of precious minerals, power, Loss of precious minerals, power, water and man-powerwater and man-power

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

product etcproduct etc

COST OF CORROSIONCOST OF CORROSION

• Annual loss due to corrosion is estimated Annual loss due to corrosion is estimated to be 3 to 5 % of GNP, about Rs.700000 to be 3 to 5 % of GNP, about Rs.700000 crorescrores

• Direct & Indirect lossesDirect & Indirect losses• Direct loss: Material cost, maintenance Direct loss: Material cost, maintenance

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

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

WHY DO METALS WHY DO METALS CORRODE?CORRODE?

• Any spontaneous reaction in the Any spontaneous reaction in the universe is associated with a universe is associated with a lowering in the free energy of the lowering in the free energy of the system. i.e. a negative free energy system. i.e. a negative free energy changechange

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

ELECTROCHEMICAL ELECTROCHEMICAL NATURENATURE

• All metallic corrosion are All metallic corrosion are electrochemical reactions i.e. metal is electrochemical reactions i.e. metal is converted to its compound with a converted to its compound with a transfer of electronstransfer 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

ELECTROCHEMICAL REACTIONS ELECTROCHEMICAL REACTIONS IN CORROSIONIN 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

ELECTROCHEMICAL ELECTROCHEMICAL THEORYTHEORY

• The anodic & The anodic & cathodic reactions cathodic reactions occur simultaneously occur simultaneously at different parts of at different parts of the metal.the metal.

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

PASSIVATIONPASSIVATION

• Many metals like Cr, Many metals like Cr, Ti, Al, Ni and Fe exhibit Ti, Al, Ni and Fe exhibit a reduction in their a reduction in their corrosion rate above corrosion rate above certain critical certain critical potential. Formation of potential. Formation of a protective, thin oxide a protective, thin oxide film.film.

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

FORMS OF CORROSIONFORMS OF CORROSION

• Corrosion may Corrosion may be classified in be classified in different waysdifferent ways

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

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

CORROSION

WET CORROSION DRY CORROSION

CORROSION

ROOM TEMPERATURECORROSION

HIGH TEMPERATURECORROSION

WET & DRY CORROSIONWET & DRY CORROSION

• Wet / aqueous corrosionWet / aqueous corrosion is the is the major form of corrosion which major form of corrosion which occurs at or near room temperature occurs at or near room temperature and in the presence of waterand in the presence of water

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

WET / AQUEOUS WET / AQUEOUS CORROSIONCORROSION

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

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

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

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. metal is protected. This excess corrosion This excess corrosion is due to the galvanic is due to the galvanic current generated at current generated at the junctionthe junction

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

CREVICE CORROSIONCREVICE CORROSION

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

• 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

PITTINGPITTING

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

• Most destructive Most destructive formform

• Autocatalytic natureAutocatalytic nature• Difficult to detect Difficult to detect

and measureand measure• MechanismMechanism

DEALLOYINGDEALLOYING

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

INTERGRANULAR INTERGRANULAR CORROSIONCORROSION

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

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

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

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

STRESS CORROSION STRESS CORROSION CRACKINGCRACKING

• Static tensile Static tensile stress and specific stress and specific environments environments produce crackingproduce cracking

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

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

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

ammoniaammonia

HYDROGEN HYDROGEN EMBRITTLEMENTEMBRITTLEMENT

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

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

• Only ppm levels of Only ppm levels of H neededH needed

LIQUID METAL LIQUID METAL EMBRITTLEMENTEMBRITTLEMENT

• Certain metals like Certain metals like Al and stainless Al and stainless steels undergo steels undergo brittle failure when brittle failure when stressed in contact stressed in contact with liquid metals with liquid metals like Hg, Zn, Sn, Pb like Hg, Zn, Sn, Pb Cd etc.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 Fig. Shows brittle IG fracture in Al IG fracture in Al alloy by Pballoy by Pb

CORROSION FATIGUECORROSION FATIGUE

• 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 corrodentcorrodent

• Effect on S-N Effect on S-N diagramdiagram

• Increased crack Increased crack propagationpropagation

AirAir

CorrosionCorrosion

log (cycles to failure, N f)

Stre

ss A

mplit

ude

Log (Stress Intensity Factor Range, K

log

(Cra

ck G

row

th R

ate,

da/

dN

)

PREVENTION OF PREVENTION OF CORROSIONCORROSION

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

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

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

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 Pb- Dil. Sulphuric acid, Sn- Distilled

waterwater• Al- Atmosphere, Ti- hot oxidizersAl- Atmosphere, Ti- hot oxidizers• Ta- Ultimate resistanceTa- Ultimate resistance

IMPROVEMENTS OF IMPROVEMENTS OF MATERIALSMATERIALS

• 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

DESIGN OF DESIGN OF STRUCTURESSTRUCTURES

• 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 Avoid contact between dissimilar

metalsmetals• Weld rather than rivetWeld rather than rivet• Easy replacement of vulnerable partsEasy replacement of vulnerable parts• Avoid excessive mechanical stressAvoid excessive mechanical stress

ALTERATION OF ALTERATION OF ENVIRONMENTENVIRONMENT

• 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 Oxidizers, e.g. Chromates, nitrates, ferric

saltssalts

CATHODIC & ANODIC CATHODIC & ANODIC PROTECTIONPROTECTION

• Cathodic protection: Make the structure Cathodic protection: Make the structure more cathodic bymore cathodic by– Use of sacrificial anodesUse of sacrificial anodes– Impressed currentsImpressed currents

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

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

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

CONCLUSIONCONCLUSION

• Corrosion is a natural degenerative Corrosion is a natural degenerative process affecting metals, nonmetals and process affecting metals, nonmetals and even biological systems like the human even biological systems like the human bodybody

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

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