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1、CORROSIVEDAMAGEINMATERIALS&ITSPREVENTIONDr.T.K.G.NAMBOODHIRIProfessorofMetallurgy(Retired)INTRODUCTIONDefinition:Corrosionisthedegenerationofmaterialsbyreactionwithenvironment.Examples:Rustingofautomobiles,buildingsandbridges,Foggingofsilverware,Patinaformationoncopper.UNIVERSALITYOFCORROSIONNot
2、onlymetals,butnon-metalslikeplastics,rubber,ceramicsarealsosubjecttoenvironmentaldegradationEvenlivingtissuesinthehumanbodyarepronetoenvironmentaldamagebyfreeradicals-Oxidativestress-leadingtodegenerativediseaseslikecancer,cardio-vasculardiseaseanddiabetes.CORROSIONDAMAGEDisfigurationorlossofapp
3、earanceLossofmaterialMaintenancecostExtractivemetallurgyinreverse-Lossofpreciousminerals,power,waterandman-powerLossinreliability&safetyPlantshutdown,contaminationofproductetcCOSTOFCORROSIONAnnuallossduetocorrosionisestimatedtobe3to5%ofGNP,aboutRs.700000croresDirect&IndirectlossesDirectloss:Mate
4、rialcost,maintenancecost,over-design,useofcostlymaterialIndirectlosses:Plantshutdown&lossofproduction,contaminationofproducts,lossofvaluableproductsduetoleakageetc,liabilityinaccidentsWHYDOMETALSCORRODE?Anyspontaneousreactionintheuniverseisassociatedwithaloweringinthefreeenergyofthesystem.i.e.an
5、egativefreeenergychangeAllmetalsexceptthenoblemetalshavefreeenergiesgreaterthantheircompounds.SotheytendtobecometheircompoundsthroughtheprocessofcorrosionELECTROCHEMICALNATUREAllmetalliccorrosionareelectrochemicalreactionsi.e.metalisconvertedtoitscompoundwithatransferofelectronsTheoverallreactio
6、nmaybesplitintooxidation(anodic)andreduction(cathodic)partialreactionsNextslideshowstheelectrochemicalreactionsinthecorrosionofZninhydrochloricacidELECTROCHEMICALREACTIONSINCORROSIONELECTROCHEMICALTHEORYTheanodic&cathodicreactionsoccursimultaneouslyatdifferentpartsofthemetal.Theelectrodepotentia
7、lsofthetworeactionsconvergetothecorrosionpotentialbypolarizationPASSIVATIONManymetalslikeCr,Ti,Al,NiandFeexhibitareductionintheircorrosionrateabovecertaincriticalpotential.Formationofaprotective,thinoxidefilm.Passivationisth
