1Forms_of_Corrosion[1]

Chapter1FORMSOFCORROSION1.1Thisintroductorysectionincludesbasicdefinitionsrelatedtochemicalandelec-trochemicalreactionsintheforward(f)andreverse(r)directions.ThewordCorrosionstandsformaterialormetaldeteriorationorsurfacedamageinanaggressiveenvironment.Corrosionisachemicalorelectrochemicaloxidationprocess,inwhichthemetaltransferselectronstotheenvironmentandunder-goesavalencechangefromzerotoapositivevalueTheenvironmentmaybealiquid,gasorhybridsoil-liquid.Theseenvironmentsarecalledelectrolytessincetheyhavetheirownconductivityforelectrontransfer.Anelectrolyteisanalogoustoaconductivesolution,whichcontainsposi-tivelyandnegativelychargedionscalledcationsandanions,respectively.Anionisanatomthathaslostorgainedoneormoreouterelectron(s)andcarriesanelectricalcharge.Thus,thecorrosionprocesswhichcanbechemicalinna-tureorelectrochemicalduetoacurrentflow,requiresatleasttworeactionsthatmustoccurinaparticularcorrosiveenvironment.ThesereactionsareclassifiedasanodicandcathodicreactionsandaredefinedbelowforametalMimmersedinsulfuricacidsolutionasanexample.Hence,metaloxidationoc-cursthroughananodicreactionandreductionisthroughacathodicreactionasshownbelowwhereM=Metal=HydrogencationZ=Valenceoroxidationstate=Metalcation=SulfateanionINTRODUCTION2CHAPTER1.FORMSOFCORROSIONTheinterpretationoftheaboveequationsindicatethatananodicreaction,whichisequivalenttowhatisknownasoxidation,losesmetalelectronsandthecathodicreactionacceptsorgainselectronsforreducingpertinentions.Consequently,bothanodicandcathodicreactionsarecoupledinacorrosionprocess.Addingeqs.(1.1a)and(1.1b)yieldseq.(1.1c).Thus,REDOX(RED=reductionandOX=oxidation)istheresultantreactionequation,eq.(1.1c),andrepresentstheoverallreactionatequilibriumwheretheanodicandcathodicreactionratesareequal.Observethattheanodicreactionisalsoreferredtoasanoxidationreactionsinceithaslostelectrons,whichhasbeengainedbythecathodicreactionforproducingsulfuricacidThus,acathodicreactionisequivalenttoareductionreaction.Furthermore,Thearrowsineq.(1.1)indicatethereactiondirectionsaswrittenandtheyrepresentirreversiblereactions.Ontheotherhand,areversiblereactionisrepresentedwithanequalsign.Thus,themetalreactioncanproceedtotherightforoxidationortotheleftforreductionasindicatedbyeq.(1.2)Thisexpressionmeansthatthereactionproceedsfromlefttorightorviceversaunderspecificchemicalorelectrochemicalconditions.TheconceptsofmetaloxidationandmetalreductionorelectrodepositionareschematicallyshowninFigure1.1.Thedarkthicklineonthemetalelectrodeisarepresentationofmetaldepositionasaresultofmetalionreductionandmetaloxidationisshownontheleft-handsideoftheelectrode.1.2.CLASSIFICATIONOFCORROSION31.2CLASSIFICATIONOFCORROSIONThereisnotauniqueclassificationofthetypesofcorrosion,butthefollowingclassificationisadaptedhereafter.1.2.1GENERALCORROSIONThisisthecasewhentheexposedmetal/alloysurfaceareaisentirelycorrodedinanenvironmentsuchasaliquidelectrolyte(chemicalsolution,liquidmetal),gaseouselectrolyte(air,etc.),orahybridelectrolyte(solidandwater,biologicalorganisms,etc.).Sometypesofgeneralcorrosionandtheirdescriptionaregivenbelow[8].AtmosphericCorrosiononsteeltanks,steelcontainers,parts,Alplates,etc..GalvanicCorrosionbetweendissimilarmetal/alloysormicrostructuralphases(pearliticsteels,copperalloys,leadalloys).High-TemperatureCorrosiononcarburizedsteelsthatformsaporousscaleofseveralironoxidephases.Liquid-MetalCorrosiononstainlesssteelexposedtoasodiumchlorideenvironment.Molten-SaltCorrosiononstainlesssteelsduetomoltenfluoridesetc.).BiologicalCorrosiononsteel,–alloys,–alloysinseawater.Stray-CurrentCorrosiononapipelineneararailroad.1.2.2LOCALIZEDCORROSIONThistermimpliesthatspecificpartsofanexposedsurfaceareacorrodesinasuitableelectrolyte.Thisformofcorrosionismoredifficulttocontrolthangeneralcorrosion.Localizedcorrosioncanbeclassifiedas[9]CreviceCorrosionwhichisassociatedwithastagnantelectrolytesuchasdirt,corrosionproduct,sand,etc.Itoccursonametal/alloysurfaceholes,underneathagasket,lapjointsunderbolts,underrivetheads.FiliformCorrosionisbasicallyaspecialtypeofcrevicecorrosion,whichoccursunderaprotectivefilm.Itiscommononfoodandbeveragecansbeingexposedtotheatmosphere.PittingCorrosionisanextremelylocalizedcorrosionmechanismthatcausesdestructivepits.OralCorrosionoccursondentalalloysexposedtosaliva.BiologicalCorrosionduetofoulingorganismsnon-uniformlyadheredonsteelinmarineenvironments.SelectiveLeachingCorrosionisametalremovalprocessfromthebasealloymatrix,suchasdezincification(isremoved)inalloysandgraphitization(Feisremoved)incastirons.41.3ATMOSPHERICCORROSIONThisisauniformandgeneralattack,inwhichtheentiremetalsurfaceareaexposedtothecorrosiveenvironmentisconvertedintoitsoxideform,providedthatthemetallicmaterialhasauniformmicrostructure.Aqueouscorrosionofiron(Fe)insolutionandofindilutedsolutionareexamplesofuniformattacksinceFeandcandissolve(oxidize)atauniformrateaccordingtothefollowinganodicandcathodicre-actions,respectively.whereishydrogengas.Thecathodicreactionisthecommonhydrogenevolutionprocess.Infact,theaggressivenessofasolutiontocauseametaltooxid