CorrosionScience, Vol.19,pp.3to14 PergamonPressLtd. 1979.PrintedinGreatBritain ANX- RAYPHOTO- ELECTRONSPECTROSCOPI CS T UDYON THEROLEOFMOL Y B D E N U MI NI NCREAS I NGTHE CORROS I ONRESI STANCEOFFERRI TI CSTAI NLESS STEELSI NHC1 K.HASHIMOTO, K.ASAMI andK.TERAMOTO TheResearch InstituteforIron,Steel andOther Metals, TohokuUniversity, Sendai, Japan Abstract--X-rayphoto-electron spectroscopy hasbeen used toinvestigate thecorrelationbetween composition of surface films and the beneficial effects of molybdenum addition to high purity, 30Cr ferritic stainless steels in improving the corrosion resistance properties in HCI. It has been found that the passive films formed consist mainly of hydrated chromium oxy-hydroxide and the composition of the films on30Cr and30Cr-2Mo stainless steels is essentially the same, except forthe existence of asmall amountof hexavalent molybdenum onthelattersteel. Thesurface film formed inthe active region contains alarge amountof hexavalent molybdenum. Thebeneficial effects of moly- bdenum have been interpreted asfollows: molybdenum eliminates the active surface sitesthrough theformation of molybdenum oxy-hydroxide or molybdate on these site, on which itis difficult to form the stable passive film. This leads to the appearance of a homogeneous steel surface and to the formation of a homogeneous passive film. I NTRODUCTI ON HIGHPURITY, highchromiumferriticstainlesssteelswithmolybdenumb_ave been developedascorrosion-resistantalloys,resistantinparticular,tohalidesolutions. Since ferritic30Crstainlesssteelswith high hot-workability haveeasily been prepared byvacuum-calciumrefininginsteadofelectron-beamrefining, x thecommercialuse of theferriticstainlesssteelsisfurtherstimulated.Theimprovedcorrosion resistance isderivedmainlyfromtheadditionofmolybdenum.Thebeneficialeffectof molyb- denumisfurtherincreasedbyincreasingthechromiumcontentinthesteels, z HorvathandUhlig 3haveexplainedthebeneficialeffectsofmolybdenumand chromiuminchloridesolutionsintermsofcompetitiveadsorptionofoxygenand chlorideonthesteel.Ithasbeenreportedbymanyinvestigatorsthatmolybdenum increasesthethicknessofpassivefilmsonthesteels 4,5andchangestheprotective propertiesofthepassivefilmsbyincludingmolybdenum. 5-sHowever,Lumsden e t a l . S , 1inreportingtheAugerelectronspectroscopicstudyhavestatedthatthe improvedcorrosionresistanceobtainedbytheadditionofmolybdenumcannotbe explainedbyitsenrichmentintheprotectivefilm.Ashasbeenreportedpreviously, n thecomposition andthicknessof passive films formedon 30Cr and30Cr-2Moferritic stainlesssteelsin0.5MH2SO4areessentiallythesameaseachother. The presentwork herein has been carriedout to determine the role of molybdenum inimprovingthecorrosionresistanceofhighpurity30Crferriticstainlesssteelsin HCI.X-rayphoto-electronspectroscopy hasbeenappliedtodeterminethecomposi- tionandthickness of thesurface film formed on the steelsas a function of polarization potential. Manuscript received 5 October 1977; in revised form 15 March1978. 4K.HASHIMOTO,K. ASAMIand K. TERAMOTO EXPERI MENTALMETHOD Highpurity Fe-30Cr-0-2Mo ferriticstainlesssteels were preparedbyvacuum-calciumrefining. The steels were rolled and then cut to specimens of 20x20x2 mm for the electrochemical polariza- tionmeasurementand16x6x0.4mmforX-rayphoto-electronspectroscopy.Thechemical compositionsaregiveninTableI.Thespecimenswere solution-treatedfor1800 sat1143K in vacuum andsubsequentlywater-quenched.Priortopolarization, thespecimenswere mechanically polishedwithemerypapertoNo.2000, rinsedultrasonically in carbon tetrachloride,cleanedwith methyl alcoholand finally driedby air blasting. TABLE 1.CHEMICALCOMPOSITIONOF FERRITICSTAINLEssSTEELS(wt ~/~ SpecimenCrMoMnSiCNO 30Cr30.8- - 0.010.010.0030.0070.0039 30Cr-lMo31.21.000.010.010.0100.0070.015 30Cr-2Mo29.61.710.0230.130.0040.00970.0012 Polarization was carriedout in 1MHCIwhichwasopen totheair.Anodic polarization curves were measuredafterholding thepotential for60 s ateachsetting,whichwassuccessively increased in stepsof 20 mVfromthecorrosion potential.Inordertoclarifytheeffect ofthemolybdenum addition ontherateofpassivation, thechangeincurrentdensitywithtimewasmeasuredafter abrading thespecimensurfaceduring anodic polarization atvariousconstant potentials.Theab- rasion of the specimen surface was performedby a rotating abrasive rod made of a resin solidified with No. 2000 silicon-c,arbide powderwhich was attached as abit to abench drilling machine.A constant weight was appliedto the arm of the drilling machine.The steel specimen was embeddedin aresin, so as to expose the area of specimen smaller than the cross-section of the abrasive rod in the electro- lyte.Immediatelybeforethe cessationof abrasion themeasurement of current wascommencedby adigitalmemory withaclock rateof 1 ms/wordthrough alogarithmic amplifier. Surfacefilmswere formedpotentiostaticallybypolarization for3600satvariouspotentials. Afterremovalfromtheelectrolyte,thespecimenswererinsedwithde-ionizedwaterandplaced immediatelyinthevacuumsystem.X-rayphoto-electronspectraweremeasuredbyAEI-ES200 electronspectrometerwithMgKctl.2 excitation(1253.62 eV). Binding energiesofelectronswere determinedbyacalibration methoddescribedelsewhere.12,~3 Aquantitative determination ofthe composition and thicknessof the passive films and substrate steels by X-ray photo-electron spectro- scopy was carried out by the same method as reportedin the previous papers 1~,x5 by using integrated intensitiesof Fe 2 p s t 2 , Cr 2pair, Me3d, Ols, CI 2p and Cls spectrascanned in the narrow binding- energyranges.Photo-electron-cross-section-ratiosusedforthequantitativedeterminationhave been reportedpreviously, nexceptforCI 2pelectrons.Theratioofphoto-electroncross-sectionof the latter to the Ols electronswas estimatedby using integratedintensitiesof X-ray photo-electron spectraof sodium chloride,sodium chromatesand sodium tungstate and was1.017. EXPERI MENTALRESULTS Fi gure1showsanodi cpol ar i zat i oncurvesof ferritic30Crstainlesssteelscon- t ai ni ng0- 2wt %Moi n1MHCIat293K. Becauseofthehighchr omi umcont ent ,anabr upt risei nthecur r ent causedbypi t t i ng doesnot appearonthesesteelsupt o t hepot ent i al i nt het ranspassi veregion.5 Theaddi t i onofmol ybdenum decreasesthe cur r ent densi t yi nbot htheactiveandpassiveregions. Thecurrent densi t yfort he 30Crsteeli nthepassiveregi onisfairlyhigh,andhenceel ect rochemi calpassi vat i on oft hesteel wi t hout mol ybdenum alsot akespl acebut isi ncompl et e i n t heaggressive hydrochl ori cacid. The passi vat i on behavi our for t he30Cr - 2Mosteel hasbeen compar edwi t h t hat for the30Crsteel,t hr oughthechangei n t hecur r entdensi t y wi t htime, i mmedi at el y after abr adi ng t hesteelsurfacedur i nganodi c pol ar i zat i on i n1MHCIasshowni nFig.2. Thecur r ent densi t yi nthepassiveregi onforbot hsteelsdecreasesasympt ot i cal l y duetofilm f or mat i on. Asharpdecreasei n t hecur r entdensi t y wi t ht i mecorresponds Spectroscopicstudyontheroleof mol ybdenum5 FIG.1. lO3 ~ 1 02 I 0 I i lO -t IG I 11~HCl , 20*C ~ 3 0 Crf -IMo YOCr / : r - 2Mo - 0 3 00.51.01.5 Po t e n t i a l , V( 5 CE )Anodi cpol ari zat i oncurvesof Fc--30Cr-2Moferriticstainlesssteelsin 1MHCI .104 l o3 E5 ~ I 0,~ I 0 0 . l 0.204040.50.60.70.80.9 Ti me I 02030405060708090100 S eCFIG.2.Changeincurrentdensitywithtimeafterabradi ngt hesteelsurfaceduring pol ari zat i onin1 MHCI atvarious potentials relative to the saturatedcalomel electrode whichareindicatedinthefigure. torapiddecreaseinthereactivityof baresurfacei.e.rapidformationofaprotective film.Immediately after removal ofthe rotatingabrasive rod from the steel surface,the filmformationtakesplacesimultaneouslywiththewettingofthesurfacewiththe electrolyte.Accordingly,thecurrentdensityobservedimmediatelyaftercessation oftheabradingdoesnotcorrespondtothereactivityofthesteelitself withoutthe surfacefilm.Itisassumedfromtheextrapolationofthecurrentdensityvstime 6K, HASmMOTO, K. ASAMI andK. TEKAMOTO curvet ot het i mezerot hat t hereactivityoft hebaresurfacef or t he30Cr - 2Mosteel isnot greatlydifferent f r omt hat f or t he30Crsteelorevenhighert hant hat f or t he 30Crsteel.Thecur r ent densityf or t he30Cr - 2Mostainlesssteeldecreasesrapi dl y wi t h time,and hence t he addi t i on ofmol ybdenum t o t he ferritic stainless steel increases t herat eof prot ect i vefilmf or mat i onin1 MHCI.Inparticular, at- - 0.3V(SCE) whi chiscloset ot heFl adepot ent i al int hepassiveregion, t hecur r ent densityoft he 30Cr - 2Mostainlesssteeldecreasescont i nuousl yandfinallyshowsnegativecur r entat 8s.Ont heot her hand, at t hesamepot ent i al , t he30Crstainlesssteelexhibitsa cur r ent densi t yofc a . 4l 0 s A/ m a but suddenlydecreasesaft er25st ot heor der of3102 A/ re.2 X-ray phot o- el ect r on spectra were measuredf or the30Cr - 2Moand30Crstainless steelsaft erpol ar i zat i onf or 3600sin1MHCI.TheFe2p8/~,Cr2p31~andMo3d spectraconsistedof t wopeakscorrespondi ngt ot heoxidizedstateinthesurface film andt he metallic statein t hesubstratesteel.TheFe 2p~/2spectrumfor theoxidized statewasf ur t her composedof spectraf or ferrousandferricstates} 6-~sChr omi um int hepassivefilmwasinat ri val ent state. 13 Themaj ori t yof oxidizedmol ybdenum wasint hehexaval ent state,but asmallamount oft et raval ent mol ybdenumwasalso i ncl udedint hepasivefilms. 19IntheOIsspect rum, alowbi ndi ngenergypeak assignedt ooxygeninme t a l - O- met al bond(OMoxygen) wassuperi mposedupona highbi ndi ngenergypeakf or oxygeninmet al - OHbondand/ or boundwat er( OH oxygen)YTMThel at t ert wokindsof oxygencoul dnot beseparat edont heOl s spect rum. Asmallamount ofchl ori deionswasalsof oundint hesurfacefilm.Aft er t hei nt egrat edintensitieswereseparatelyobt ai nedf or oxidizedandmetallicstatesofFe, Cr andMoandf orOMandOHoxygen,aquant i t at i ve, si mul t aneousdet er mi na-t i onof t hecomposi t i onandthicknessof asurfacefilmandt hecomposi t i onof t he subst rat esteel i mmedi at el yunder t hesurfacefilmwasper f or med.Fi gure3 showst he thicknessof surface film as afunct i onofpol ari zat i onpotential.4- oo 0 ?3 1 Q. = .4 tJ -0. 6-0.4I.O 1O~ Q 030Or 30Cr - 2Mo i nI MHCIt - 0 2 00.20.40.60.~ P o t e n t i a l ,V( S CE )FIG.3.Changei n t he t hi cknessof surfacefilm f or medon30Crand30Cr - 2Moferri t i c st ai nl esssteelsbypol ar i zat i onat var i ouspot ent i al sfor3600sinIMHCI. Spectroscopic study on the role of molybdenum7 Thethicknessof passivefilmincreasesgradual l ywi t hanincreaseinpol ari zat i on pot ent i al int heregionhi ghertitan0.2V(SCE).Ther eisnodifference int hethick- nessofpassivefilmsont he30Cr - 2Moand30Crstainlesssteels.Consequent l y, t he addi t i onofmol ybdenumt otheferritic stainlesssteel does not affect ont hegr owt hoft hepassivefilm. Theweightfract i onof cat i onswhi chconst i t ut et hesurfacefilmsont he30Cr -2Moand30Crstainlesssteels is given asafunct i on ofpol ari zat i on pot ent i al in Fig.4. Int hepassiveregion, aremarkabl eenri chment of chr omi umoccursint hepassive filmsonbot hstainlesssteelst hr oughselective dissolutionof iron. Oxidizedmol yb- denumisalsof oundint hepassivefilmont he30Cr - 2Mostainlesssteel,but t he rat i oof mol ybdenumt ochr omi umint hepassivefilm is smaller t hant hat in t hebulk steel.Therefore, mol ybdenumisdeficientint hepassivefilm.Therat i oof ferrous speciestot ot al i ronspeciesdecreasedwi t hanincreaseinpol ari zat i on pot ent i alin t he passiveregionbut wasof t heor der of 0.4t hr oughout t hepassiveregion.Because t he cont ent ofoxidized mol ybdenum is small,t he cat i oni c composi t i ons oft he passive films f or medonbot h30Cr - 2Moand30Crstainlesssteelsatagiven pot ent i al in1M HCIarenot essentiallydifferentf r omeachot her. Accordi ngl y, t headdi t i onofmol ybdenumt otheferritic stainlesssteeldoesnotchange t he composi t i on of passive filmf or medinhydrochl ori cacid. The most interesting feat ureoffilmcomposi t i onint heactive stateist he existence of alargeamount of hexaval ent mol ybdenuminthesurfacefilmont he30Cr - 2Mo stainlesssteelasseeninFig.4.Theamount of t hehexaval ent mol ybdenumspecies l.o o . 8~ o.e .~ o.4 ~o. e - 0 I ..r -' \ ! ! -i,"' - - 0 - - 0 - - 0 - - e - - 0 - -- 0 . 4 - 0 . 2 0 0 . 2 0 . 4 0 0 . 8 1.0 Pot ent i al ,Y( 5CE)FI~.4.Weight fractionsof cationsinsurface films formed on30Cr and30Cr-2Mo stainlesssteels inI MHCI. Asterisksindicateresults forthespecimenwhich was polarizedat -- 0.4 C(SCE) for 3600 s and subsequently passivatexl at+0.2 V(SCE) for 3600 s. 8K.HASmMOTO,K.As~a~nand K. TERAMOTO int heactive regi on changessimilarly t ot hecurrent densitywithpot ent i al . Therat i os of chr omi umt oi ronint hesurfacefilmsf or medat agivenpot ent i al int heactive regi ononbot h30Cr - 2Moand30Crstainlesssteelsarenearl yequal.Ashasbeen r epor t edpreviously, ~sthisrat i oint heactive region wasalsonearl y t hesameast hatint hebul ksteel.Consequent l y, t hehexaval ent mol ybdenumpreferent i al l yremains,wi t hout dissolving,int hesurfacefilmint heactiveregi onont hemol ybdenum-beat i ngferriticstainlesssteelasisalsot hecasewi t ht ri val ent chr omi umint he passiveregion. AnEPMAexami nat i onrevealst hat mol ybdenumwasnot localized but di st ri but eduni forml y, regardlessof mi crost ruct ure, inthesurfacefilmf or med ont he30Cr - 2Mostainlesssteel pol ari zedint heactivepot ent i al region.Immedi at el yaft erpolarizingt he30Cr - 2Mostainlesssteelf or 3600sat - - 0.4V (SCE)t of or malargeamount ofhexaval ent mol ybdenumont hespecimensurface,t he pol ar i zat i on pot ent i alwasswitched t o+0.2V(SCE)andpol ar i zat i on wasf ur t hercont i nuedf or 3600st of or mapassivefilm.Theresultsofanalysisofthisspecimen arealsoi ncl uded in Figs.3-5as i ndi cat edby asterisks.The thicknessandcomposi t i on oft hepassivefilmthusf or medcoincideexact l ywitht hoseof t hefilmsonstainless steelspol ari zedonl yat +0.2V(SCE).It canbesaidt hat , whent hestainlesssteel dissolvesrapi dl yintheactive region, t herat eofdi ssol ut i onofmol ybdenumisslower t hant hoseof i ronandchr omi um, wi t haconsequent enri chment of hexaval entmol ybdenumspeciesont hesteelsurface.However, whent heactivedi ssol ut i onis st oppedbyswitchingt hepot ent i al t ot hepassiveregion, t hemaj ori t yof t hemol yb- de numspecies f or med is dissolvedi nt ot heelectrolyte pr i or t ot hepassive film f or ma-t i on. Accordi ngl y, t heprot ect i vequal i t yof t hehexaval ent mol ybdenumspeciesis not hi ghascompar edwi t ht hat oft hepassivefilm. Fi gure5showst hecomposi t i onof subst rat esteeli mmedi at el yunder t hesurface o30Cr~3 0 Cr - 2 Mo0. 8~inI MHCle o -'O - - - | ~ - - - - ' - 8 = ! . I 8 go.4 ' - - - - i ' ~ - i - - - - - I ~ - - - ' % - t - - |- t - A - ;0~___~___4L. - - - - ' - - - - - _ e _ e _ e _ e _ e _ ' - -- 0-0.4- 0200.20.40.60.8I,0 Pot ent i al ,V( $C E )FzG.5.Compositionofsubstratesteel immediatelyundersurfacefilm. Included for comparison is composition of bulk steels which is indicated by dottedlines. Spect roscopi cst udyont herol eofmol ybdenum9 film.Thedot t edlinesindicatet heresultof chemicalanalysisof bul ksteels.Inspite of significantenri chment of chr omi umint hepassivefilm,t hecomposi t i onsofsubst rat esteelsi mmedi at el yunder t hepassivefilmarenearlyt hesameast hosef orbul kstainlesssteels.Thi sfactalsoindicatest hat theenri chment of chr omi umin thepassivefilmoccurst hr oughselective dissolutionofi ronandmol ybdenum.Number sof OMandOHoxygenpercat i oninthepassivefilmaregivenasa funct i onof pol ari zat i onpot ent i al inFig.6.Theamount of OMoxygendoesnot~ 2! _ _ [ ._ _ L . _ !l. . . . . . . . .0 u ~! " Tzo!m 0 . 0. 6-0.4-0.2 ~ m0~20.4~6~6~0 Po t e n t i a l ,V( $ CE ) FIG.6.Number s of OM(i nmet al - O- met al bond) andOH(i nmet al - OHbondand boundwat er)oxygenper cat i oni nt hesurfacefilmf or medon30Crand30Cr - 2Mo stainless steelsin!MHCI. significantlychangewi t hpol ari zat i onpotential, whiletheamount of OHoxygenin the passive filmonthe30Cr - 2Mosteelt endstodecrease withanincrease inpol ari za- t i onpot ent i al . TheOHoxygenconsistsof oxygeninmet al - OHbondandbound wat erwhi chareindistinguishableintheOl sspectrum. Sincet henumber of OM oxygen per cat i on is al mosti ndependentofpol ari zat i on pot ent i alandsince t he average valenceofcat i onsdoesnot becomelower withincreaseinpol ari zat i onpot ent i al , the decreaseint henumber of OHoxygenarisesf r omt hedecreaseint heconcent r at i on of boundwat erint hepassivefilmwitht heincreaseinpol ari zat i onpotential. The concent rat i onofboundwat erint hesurfacefilm isgivenasafunct i onofpol ari zat i on pot ent i al inFig.7.Theconcent r at i onof boundwat erint hesurfacefilmsf or medin theactive regiononthe30Cr - 2Mostainlesssteel islow.Wi t hanincreaseinpot ent i al10K. HASHIMOTO, K.ASAMIandK. TERAMOTO int hepassiveregion,t heconcent r at i onof boundwat erincreasesonceandt hen decreasest hr ought hemaxi mum. Theconcent rat i onof boundwat erint hepassive filmont he30Crstainlesssteelvarieswidelyf r ompot ent i al topot ent i al andshowed poor reproduci bi l i t y., .Q FIG. 7. H ~30Cr-2MoinI MHCl2O - - (~. . . . I - -e l l A_o -0-0.4-0.20020.40.60.8 Pot ent i al ,V( $CE)Numberof boundwaterpercationinthesurface film formed on30Cr and 30Cr-2Mo stainless steels in1 M HCI. Smallamount sofchl ori deionsweref oundinthepassivefilm.Theconcent r at i on of chl ori deionswasnot differentinbot hpassivefilmson30Crand30Cr - 2Mo stainlesssteels andtendst odecrease slightly withtheincrease in pol ar i zat i on pot ent i alint hepassiveregion.Fr omt heaboveanalyticalresultstheaveragecomposi t i onofthepassivefilmcan beestimated, f or exampl eonthe30Cr - 2Mostainlesssteelas (.Felol.!1 and II (Fro.J3 Feo.12ui ",-'0.75c'IIIMoVl2). C10.14 Oi.02 (OH)0.77.1.09H20 at0.2V(SCE), HIC. I I I VI Feo.16~'0.70Mo.ol)C1o.12 Ol.13(OH)0.52" 0. 4H20 at0.8V(SCE). Consequent l y, thepassivefilmsformedonbot h30Cr - 2Moand30Crstainlesssteels in1 MHCl consi st mainlyofhydr at ed chr omi umoxy-hydroxi de,CrOx(OH)a_2.~'nH20. Anexampl eoft heaverage composi t i onofthesurface filmformedintheactive region is: 11I~-I11c - I l l Vl (Feo.22 1~0.37 -~0.27 M0o.14) C1o.02 O1.3o (OH)0.46"0. 61H20 at- - 0.55V(SCE). Ingeneral,wi t hanincreaseint hecont ent of t ot al ironinsurfacefilms,thecont entSpectroscopic study on the role of molybdenum11 ofOMoxygenandt herat i oofferricspeciestot ot al i ronint hefilm t endt oincrease andbot hOHgroupandboundwat erdecrease. DI SCUSSI ON Scat t erappearsintheconcent r at i onof boundwat erint hepassivefilmon30Cr stainlesssteel.Someof theboundwat eranal yzedmayexistont hefilmsurfaceas adsorbedwater.I f alargeamount ofwat erisadsorbedont hefilmsurface,t hefilm thicknessanal yzedmust beaffected andvary widelysimilarlyt ot heconcent r at i onofboundwat eranalyzed.The filmthicknessandt heconcent r at i onof boundwat erin thesurfacefilmonthe30Crstainlesssteelint heactiveregionshowsomescatter. Thi sscatterarisesmainlyf r omt headsorbedwater, suggestingnon- uni f or mi t yofthe surfaceonthe30Crstainlesssteelint heactiveregion. Conversely, t hemaj or pro- por t i onof boundwat erinthepassivefilmsonbot hstainlesssteelsisnot adsor bed wat erbut t rueboundwat erint hepassivefilms,i nasmuchasthethicknessofpassive filmsonbot hsteelscoincide wi t heachot herandchangecont i nuousl ywi t hpol ari za- t i onpotential. Theconcent r at i onof boundwat erint hepassivefilmf or medonthe 30Cr - 2Mostainlesssteelt endstodecreasewithanincreaseinpol ari zat i onpot ent i al .Thist endencyisinagreementwi t ht heresultofanalysisofboundwat erwi t ht heaid of t ri t i at edwat erasat racerbyOkamot oe t al .2,21Theyhaver epor t edt hat t he cont ent of boundwat erinpassivefilmsdependsupont hedegreeofdevel opment oft hef i l ms- - t hat is,t heconcent rat i onof boundwat erislowinthewell-developed passivefilmf or medbypol ari zat i onathi gherpot ent i al and/ or f or longertime.Int he present work, thecont i nuousdecreaseint heconcent r at i onof boundwat erint he passive filmformedont he30Cr - 2Mostainlesssteel witht heriseofpot ent i al suggests t hat t hepassivefilmisstableandhencepossessest hegoodprot ect i vequality.Okamot o 21 hasalsosuggestedt hefollowingdual funct i onof theboundwat er: t he boundwat er mayreact togive vari ousspeciesorbereplacedby t heot her ions comi ng f r omsurroundi ngs, resultingindegr adat i onofpassivefilm.Ont heot her hand, t he boundwat eractsast hebeneficialspeciest ocapt uret hedissolvingmet al ionswi t h consequentf or mat i on oft henewfilm resisting againstf ur t herat t ackby surroundi ngs.Theboundwat erdet ect edont he30Crstainlesssteelisdi st ri but edt hr oughout the entirepassivefilm,i nasmuchast hethicknessof passivefilmont hestainlesssteel doesnot varywidelyf r ompot ent i al t opot ent i al . I t cannot be said,however, t hat all oft heboundwat erdet ect edont he30Crstainlesssteeldoes cor r espondt ot hebound wat erpr oposedbyOkamot o. Someofitisnot beneficial inmai nt ai ni ngaprot ect i ve pr oper t yofthepassive filmandareresponsible f ori nhomogenei t y ofthe passive film, resultinginaleakageofcur r ent t hr ought hepassivefilm. Thepassivefilmsonbot h30Crand30Cr - 2Mostainlesssteelsincludeasmall amount of chl ori deionswhi cht endst odecreasewitht heincreaseinpol ari zat i on potential. Theconcent r at i onof chl ori deionsinthepassivefilmmaydependont he degreeof devel opment of thefilm,similart ot heconcent r at i onof boundwater.In general,theaddi t i onof mol ybdenumt ostainlesssteelsi mprovesresistanceagainst chl ori deat t ack. Nevertheless,t heconcent r at i onof chl ori deionsisnot differentin t hepassivefilmson30Crand30Cr - 2Mostainlesssteels. Accordi ngl y,cont ami nat i on of passivefilmsbychl ori deionsmaynot benecessaryorsufficientt obr i ngabout12K.HASmMOTO, K. ASAMIand K. TERAMOTOpittingbutinhomogeneity ofthepassivefilmsmustalsobepresumedtoplayan important partfor pitting. Thefactthatthemainconstituentofthepassivefilmsonthe30Cr-2Moand 30Crstainlesssteelsishydrated chromium oxy-hydroxide agreeswith theresultsfor these steels inH2SO4,11 aseriesof Fe-Cralloys inH~SO4,15 amorphousFe-Cr-P-C alloy in HC122 andamorphous Ni-Cr-P-Balloys in HC1 andH2SO4.28 Thequestion is raised why corrosion resistance propertiesare improved by theaddition of molyb- denum despite the fact that the main constituents of the passivefilms formed onthe 30Cr-2Moand30Crstainlesssteelsare not different from each other.There are two recentpapersinterpretingtheroleofmolybdenumadditiontostainlesssteelsin improving corrosionresistance propertiesintermsof compositionsof passivefilms onstainlesssteels.Yanive t al .1havestatedthattheimproved corrosion resistance propertiesobtainedbytheadditionofmolybdenumcannotbeexplainedbyits endctunentintheprotectivefilmandhavesuggestedthatthebeneficialeffectsof molybdenumarisefromimprovingthequalityofthebondingatthemetal-oxide interface creating a barrier layer. Sugimoto and Sawada 5 have stated that the thickness of passive film increaseswith anincreasein the molybdenum content and hexavalent molybdenum oxide, which existsinthepassivefilm,improvestheprotectivequality ofpassivechromiumhydroxidefilm,duetoitshighstabilityinacidicchloride solutions. In the present work, molybdenum is deficient in the passivefilm and the composi- tionof passivefilmsonboth30Cr-2Moand30Crstainlesssteelsisessentially the same except for scatter of the content of bound water in the film on the 30Crstainless steel. The thickness of passivefilm is also unchanged by the addition of molybdenum. Nevertheless,the addition of 1.7 ~oMo to the 30Crstainlesssteel decreasesthe current density in the passiveregion two ordersof magnitude in1 MHCI. Ingeneral,once pitting and crevicecorrosion initiate concentrations of hydrogen and chloride ions within the pit and crevice increase and the potential at the dissolving front within thepitandcreviceisnolongerinthepassiveregion.24'25Accordingly, tostudy theeffect of alloying elementsonpittingandcrevicecorrosion,attention 'must begiven tothebehaviourof thealloys intheactive regioninacidic solutions containingconcentratedchlorideions.Theratioofchromiumtoironinsurface films formed in theactive region isalmost thesameasthatinthebulksteels.How- ever,markedenrichment of hexavalent molybdenumoccursinthesurfacefilmson the30Cr-2Mostainlesssteelintheactiveregionandtheamountof molybdenum varieswithpolarizationpotential.ThisdependenceconfirmsthatXPSanalysisis representative of the filmformed during polarization in the active region andis thus not an artifact of the sampling process.As seen in Fig.1, the addition of molybdenum totheferriticstainlesssteeldecreasessignificantlythecurrentdensityintheactive region,although thereactivity of baresurface withoutsurface filmisnotdecreased by the addition of molybdenum asseeninFig. 2.It can be said from the above con- sideration thatMo(VI)oxy-hydroxide orironorchromium molybdate is formed by polarization in the active region on the steelsurface,including the active surface sites, with consequent inhibition of active dissolution. Ingeneral,passivationoccursthroughactivedissolutionof metalsattheinitial timeperiod36"~7 SatoandOkamoto 26 havestatedthat,evenif thepotentialisset Spectroscopic study on the role of molybdenum13 inthepassiveregion,anincreaseintheconcentraion of solublemetalsorhydroxyl ionsinthevicinityoftheelectrodesurface,i.e.activedissolution,isanecessary precursor for passivefilm formation. Becausethe composition of passive films formed onthe30Crand30Cr-2Moferriticstainlesssteelsbypolarizationfor1hisnot differentfromeachother,theaccumulatingspeciesinthevicinityofthesurface must be thesame for both stainlesssteels.However, the time difference for the onset of passivityof30Crand30Cr-2Mostainlesssteelsistoobigtoassumethesame passivation processfor bothstainlesssteels,in particular,at--0.3V(SCE).There- fore,inhibition of active dissolution during polarization in thepassiveregion occurs on the30Cr-2Moferritic stainlesssteelbefore the accumulation of speciesnecessary forthefilmformation.Itcanbesaidthatsomeof molybdenumoxy-hydroxide or molybdate remain, without dissolving,onthe surface of the 30Cr-2Mostainlesssteel duringinitialactivedissolutionbypolarizationinthepassiveregion.Although themolybdenumspeciesthemselvesdonothaveahighprotectivequalityas comparedwiththepassivehydratedchromiumoxy-hydroxidefilm,themolyb- denumspeciesmaydecreasetheactivityoftheactivesurfacesitespriortothe passivefilmformation.Thiswouldbeconsistentwiththesharpdecreaseinthe current density with time for the30Cr-2Moferritic stainlesssteel as compared to the 30Crstainlesssteelimmediately after abrading thesteelsurface during anodic polari- zationinthepassiveregion.Fromthepointofviewoftheformationofpassive hydratedchromiumoxy-hydroxide film,thedecreaseintheactivity ofactivesites, onwhichitisdifficult toformthestablepassivefilm,wouldcorrespondtothe appearanceofahomogeneous steelsurfaceandhenceleadtotheformation ofa uniform passive film. The importance of alloy homogeneity has been illustrated by the extremely corrosion-resistant amorphous alloys, whicharechemically homogeneous, singlephasealloys without crystal defects thatact asactive sitesfor corrosion.2s CONCLUSION FromXPSstudyofthepassivefilmsformedon30Crand30Cr-2Moferritic stainlesssteelsbypolarization for1 hin1 MHClthefollowing conclusions canbe drawn: The thicknesses of the passivefilm are not different from each other. Molybdenum is deficient in the passivefilm on tbe 30Cr-2Moferritic stainlesssteeland the passive filmsonboth30Crand30Cr-2Moferritic stainlesssteelsconsist mainly of hydrated chromium oxy-hydroxide. The composition of passivefilms onboth stainlesssteels is essentially the same except for scatter of the content of bound water in the film on the 30Crferriticstainlesssteel.Thisscatterresultsfromnon-uniformity of thepassive film containing weak pointsfilled with water,which areresponsible for aleakage of current through the passivefilm. 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