Corrosion And Its Control：腐蝕及其控制

CorrosionAndItsControlUNITIICorrosionItisthedegradationofamaterialduetoareactionwithitsenvironment.

ORProcessofDistructionofthematerialthroughchemicalorelectrochemicalattackbyitsenvironment.SlowprocessMeasuredinweightlossperunittime.Classification:DryorChemicalCorrosionWetorElectrochemicalcorrosion1.DryorChemicalCorrosionOccursduetochemicalattackofbytheenvironmentsuchasdrygas.Occursduetohightemperatureandwithoutliquidphase.Itisoftwotypes:a)Oxidationcorrosionb)Corrosionbygases(a)OxidationCorrosion:Itisduetodirectattackofoxygenonmetals.OxygenmoleculesareattractedtothesurfacebyVanderWallForceMechanism:-1.Whentempincreasesthemetalundergoesoxidationandlossese-2M→

2M+n

+2ne-

MetalIon2.Electronaregainedbytheoxygenmoleculesformsoxideions

nO2+4ne-→2nO2-

OxideIon3.Scaleofmetaloxideformed

2M+nO2→2M+2nO2-

MetalOxideStableCorrosion:-Aluminium,Tin,Lead,CopperNon-stablecorrosion:-Silver,Gold,PlatinumPilling–BedWorthRatioRatioofvolumeofoxideformedtothevolumeofmetalconsumed.(b)CorrosionbyGasesCarbondi-oxide,Chlorine,HydrogenSulphide,Sulphurdi-oxide,FlourineDependsonchemicalaffinityb/wmetalandthegas.2.WetorElectrochemicalCorrosionOccurswhenaqueoussolutionorliquidelectrolytesarepresentWetcorrosiontakesplaceinenvironmentswheretherelativehumidityexceeds60%.Wetcorrosionismostefficientinwaterscontainingsalts,suchasNaCl(e.g.marineconditions),duetothehighconductivityofthesolution.MechanismOfElectrochemicalCorrosionAnodicReaction:Dissolutionofmetaltakesplace.Asresultmetalionsareformedwiththeliberationoffreeelectrons.M?

M+n

+e-

MetalIonCathodicReactionHydrogenEvolution:-Occursusuallyinacidicmedium2H++2e-?H2(g)(ii)OxygenAbsorption:-occurswhensolutionisaeratedsufficiently.O2+4H++4e-?2H2O(Inacidicmedium)O2+4H++4e-?4OH-(Inbasicmedium)FormsofCorrosion:GalvanicCorrosion:-Whentwodifferentmetalsarepresentincontactwitheachotherinconductingmediume.g.Electrolyte(b)ConcentrationCellCorrosion:-SameasGalvaniccorrosionOccurswhentwodifferentmetalsareexposedtodifferentairconc.(c)PittingCorrosion:-FormedasaresultofpitandcavitiesLocalizedattackandformedbycrackingprotectivecoating(d)StressCorrosion:OccursinthepresenceoftensilestressandcorrosiveenvironmentE.g.brassgetcorrodeintracesofammonia.ForClassTestIFactorsAffectingCorrosionNatureoftheMetal2.Natureoftheenvironment.

NatureofMetalPositioninGalvanicSeries:Iftwometalsarepresentininelectrolyte,themetalwithlessreductionpotentialundergoescorrosion.Greaterthedifferencefasterthecorrosion.(ii)OverVoltage:Duetohighevolutionofhydrogen,therateisslow.(iii)AreaandDistance:Whenanodicmetalareaissmallerthancathodicarea,rateofcorrosionatanodeishigherbecauseofdemandofelectronbycathodicarea.(iv)PhysicalandMechanicalpropertiesofMetal:Puremetalsaremorecorrosionresistant.Smallergrainsizemetalhavehighsolubilityandcorrosion.Uniformdistributionofstressonmetalreducesrateofcorrosion.Passivemetalsshowshighercorrosionresistancebecauseofformationofprotectiveoxidefilmontheirsurface.Polycrystallineformsaremoresensitive.2.NatureofEnvironmentTemperature:directlyproportionalHumidity:fasterinhumidconditionspH:Iflessthan7rateishigh.Al,Zn,Sn,Pb,andFeareaffectedbybothacidandbases.ImpuritiesandSuspendedParticles:Whenthesewillgetdissolvedinmoisture,provideselectrolyteforconductivityandhencecorrosionincreases.CorrosionControl:Selectionofmetalandalloy:UsingpureandnoblemetalsPracticallynotpossiblebecauseoflowstrengthofpuremetalUseofmetalalloyswhicharehomogeneous2.Properdesignofmetal:MinimalcontactwithmediumPreventionfrommoistureAdequateventilationanddrainageWeldingAvoidcervicesb/wadjacentpartsBendshouldbesmoothBimetalliccontactsshouldbeavoidedPaintcathodicportionPreventunevenstress3.CathodicProtection:Forcethemetaltobeprotectedtobehavelikecathode.Sacrificialanodicprotection:MetaltobeprotectedfromcorrosionconnectedtomoreanodicmetalCommonlyusedmetalsMg,Zn,AlandtheiralloysImpressedcurrentmethod:DirectcurrentisappliedinoppositedirectiontonullifythecorrosioncurrentConvertsthecorrodingmetalfromanodetocathode.4.ModifyingEnvironmentEliminatingdissolvedoxygen:De-aerationByusingchemicalsubstanceslikesodiumsulphiteandhydrazine.AlsocalledDeactivation.ReducingMoisture:DehumidificationbyusingsilicagelsReducingAcidity:Neutralizingtheacidicenvironmentbyaddinglime,NaOH,AmmoniaCommonlyusedinrefineries5.

Protectivecoating:ApplicationofcoatingCoatingmaterialshouldbechemicallyinertunderparticulartempandpressure.6.UseofcorrosionInhibitorAnodicInhibitor:Theseareoxygenandoxidizingagent.TheycombinetheanodicmetalforminganoxidefilmwhichreducecorrosionCathodicProtection:Organicinhibitorslikeamines,mercaptans,ureaandthioureareducestheHiondiffusionbyadsorptionMercury,arsenicandantimonydepositsfilmsatcathodicareawhichraisethehydrogenovervolume.EliminatingOxygenfromthemediumbyaddingsodiumsulphateandhydrazine.ProtectiveCoatingSurfacepreparationforCoating:Cleaning:ToprepareforsuitableconditionRemovingcontaminantstopreventdetrimentalreactionproductE.g.de-greasing,sandblasting,vapourdegreasing,picklingandalkalinecleaning.SolventCleaning:Mustbenon-inflammableandnontoxicTrichlorotrifluoroethanewhichhaslowtoxicityarecostlierVapourde-greasingiseconomicalandadvantageousbecauseofcontinuouscleaningwithsmallquantitiesofsolvent.ElectrolytePickling:ProvidesbetterandrapidcleaningbyincreasinghydrogenevolutionresultinginagitationandblastingactionSandblastingismechanicalcleaning.4.AlkalineCleaning:CheaperandlesshazardousUsedinconjunctionwithsurfaceactive(wetting)agentAbilitydependsonpH,rapidlydecreasesbelow8.5Otherabilitiesarerinsability,detergentproperties,sequestering,wettingetc.AcidCleaningAcidsuchasHCl,H2SO4,H3PO4isveryeffective.5-10%H2SO4andHClusedtoremoveinorganiccontaminants.Picklingareperformedathightemp.(60?C)Iseffectiveforremovalofgrease,oil,dirtandrust.MethodsofApplicationofMetallicCoatingHotDipping:Metaliskeptinmoltenstateandbasemetalisdippedintoit.UsedforproducingacoatingoflowM.PE.G.Tinning(TincoatingonIron)Processisfollowedbycoolingthecoatingthroughapalmoiltopreventoxidationoftinplatetoitsoxide.Palmoillayerisremovedbyalkalinecleansingagent.MetalCladding:Thesurfacetobeprotectedissandwichedbetweentwolayersofthecoatingmetalsandpressedbetweenrollers.E.g.AlcladSheeting–Plateofduraluminissandwichedbetween99.5%purealuminum3.ElectroPlating:Puremetalismadeascathodeandbasemetalasanode.Electrochemicallycoatmetalisdepositedonbasemetal.Thismetalgivessmooth,fineanduniformcoatingItdependson(i)Temperature(ii)Currentdensity(iii)ElectrolyteConcentrationNatureofbasemetal(v)Time4.ElectrolessPlating:Nobelmetalisdepositedcatalyticallyonlessnoblemetalbyusingreducingagentwithoutusingelectricalenergy.AdvantageoverElectroplatingMoreeconomicalsincenoelectricityrequiredIrregularshapecanbeplateduniformlyPlatingonplasticscanalsobedone5.MetalSpraying:CoatingisappliedbymeansofsprayingdeviceE.g.AluminumisplatedinthiswayonAircrafts.ChemicalConversionCoatingTheseareformedonmetalsurfacebychemicalreactionb/wmetalsurfaceandinorganicsaltsolutionCoatingbasemetalisconvertedintooneoft