The-corrosion-of-silver-by-atmospheric-sulfurous-g

CorrosionScience,Vol.25,No.2,pp.133-143,19850010-938X/85$3.00+0.(~JPrintedinGreatBritain©1985PergamonPressLtd.THECORROSIONOFSILVERBYATMOSPHERICSULFUROUSGASESJ.P.FRANEY,G.W.KAMMLOTTandT.E.GRAEDELAT&TBellLaboratories,MurrayHill,NJ07974,USAAbstraet--PolycrystallinesilverhasbeenexposedtotheatmosphericgasesH2S,OCS,CS2andSO2inhumidifiedairundercarefullycontrolledlaboratoryconditions.OCSisshowntobeanactivecorrodantwhileCS2isquiteinactive.Atroomtemperature,theratesofsulfidationbyH2SandOCSarecomparable,andaremorethananorderofmagnitudegreaterthanthoseofCS2andSO2.ItappearsthatOCSistheprincipalcauseofatmosphericsulfidationofsilverexceptnearsourcesofH2Swherehighconcentrationsmayrenderthelattergasimportant.Atconstantabsolutehumidity,thesulfidationrateofsilverbybothHzSandOCSdecreasesfrom20to40°Candthenincreasesto40to80°C.Thisbehaviorisinterpretedasindicatingadecreaseinwater-enhancedsulfidationastherelativehumidityisreduced,followedbydominanceofsulfidationbyawater-independentprocesswithstrongpositivetemperaturedependence.Theinitialsulfidationofsilverby3.9+0.2ppmH2Sinhumidifiedairat22°Chasbeenstudiedindetail.ThedataareconsistentwithaninitialstageofsulfidationinvolvingrapidattackbyHeSatsurfacedefectsites.Asthesecorrosiveproductsspreadandmerge,diffusionofsilvertothesurfacewillbeimpeded.Inagreementwiththispicture,theresultsshowthatthefractionofH2Smoleculesstrikingthesurfacethatbecomeincorporatedintothesulfidefilmdropssharplyfrom~3×10-~(att=5min)to~1×10-8(att=50Oh).INTRODUCTIONTHEATMOSPHERICsulfidationofsilverhasbeenrecognizedforalongtimeandremainsaproblemtoday,beingdemonstratedtolaymanandscientistalikebythetarnishingofelectricalequipment,jewelryandhouseholdsilverware.Thesulfidefilmthatformsonsilvermayhaveasufficientlylowconductivitythatuseasanelectricalcontactisunsatisfactory.1-3Thusthereiswideinterestinunderstandingtheprincipalagentsofsilversulfidation,andthemechanismsbywhichtheyoperate.Webeginbysummarizingthecurrentstateofknowledge.Silversulfidizesuponexposuretoawidevarietyofgaseoussulfur-containingcompounds.Perhapsthemostextensiveinvestigationsofsulfidationhaveutilizedhydrogensulfide(HES)asthereactiveagent.1,3-15IthasbeendemonstratedthatHESconcentrationsaslowas0.2partsperbillion(ppb)aresufficienttocausesulfidation,11andthathighrelativehumiditiesmarkedlyacceleratetheprocess.1'4'5A°'11'15Exten-siveexperimentshavealsobeenperformedwithflowersofsulfur(Ss).3,s'16-1sConcentrationsaslowas30ppbhavebeenshowntosulfidizesilver.17Silveralsoreactsreadilywithavarietyofmercaptansandotherorganicsulfur-containingspecies.19Incontrasttoitsreactivitywithreducedsulfurcompounds,thesulfidationofsilverbysulfurdioxide(SO2)isveryslow,l'seitherdryorinthepresenceofwatervapor.8Manuscriptreceived28July1984.133134J.P.FRANEY,G.W.KAMMLOTTandT.E.GRAEDELUnlikesomeothermetals,silverdoesnotnaturallyformasurfaceoxide.11'20'21Exposuretoreducedsulfurcompoundsortoflowersofsulfurproducesafilmofsilversulfide(Ag2S).8'12Longexposuresproducesmallamountsofasecondcomponent,whichmaybeeithersilveroxide(Ag20)orsilversulfate(AgESO4).3'8'12Thesecompoundsarealsoseenonsilverthathasbeenexposedinfieldenvironments.22Severalresearchershaveexaminedthekineticsofsilversulfidation.ForHEStherateatmoderateexposureisparabolic,13becominglinearforsevereexposures.3'6Forflowersofsulfurtherateislinearthroughoutaverywiderangeofex-posures.3'12'17'18ThesedatahavebeeninterpretedtomeanthatgasphasediffusionisthecontrollingprocessinsulfidationexceptformoderateH2Sexposures,wheretherateofthesurfacereactioncontrolsthesulfidationrate.13Inthelightofthesulfidationresearchdiscussedabove,itisofinteresttoconsidertheatmosphericconcentrationsofsulfurcompounds.Onaglobalbasis,themostabundantiscarbonylsulfide(OCS),23'24acompoundwhoseeffectsonsilverhavenotyetbeenassessed,althoughitisknowntocorrodecopperreadily.24ThenextmostabundantgasisSO2,whichisproducedprincipallybythecombustionofsulfur-containingfossilfuels.SO2thustendstobeconcentrateddownwindofpowerplantsandindustrialoperations.H2Sislessabundant,beingemittedbywidelydispersedsourcesoflowerintensitysuchasmarshes,volcanoes,crudeoilprocessing,etc.Mercaptansandsulfidesareonlyoccasionallypresentandthenonlyinverylowconcentrations.25'26FreesulfurisrarelypresentintheatmosphereYAstheseremarksillustrate,silversulfidationisasignificantandubiquitousproblem.Sulfidationisknowntobecausedbyavarietyofreducedsulfurcompoundsandbyfreesulfur.Noneofthesespeciesareabundantintheatmosphere.Sulfurdioxide,amorecommonatmosphericconstituent,hasanegligibleeffectonsilversulfidation.Noinvestigationshavethusfarbeenperformedtostudytheeffectsonsilverofcarbonylsulfide,theatmosphere'smostabundantsulfurspecies.Thekineticsofsulfidefilmgrowthhavebeendeterminedonlyatrathersevereexposures,exceptforfreesulfur(knowntoberareintheatmosphere).Thecontrollingprocessesatmildexposuresarevirtuallyuninvestigated.Intheworkreportedinthispaper,weattempttoremedysomeoftheseuncertainties.Wehaveconductedthefirstlaboratoryexposuresofsilvertocarbonylsulfide,derivingkineticdataintheprocess.Wehaveexposedsilvertohydrogensulfid