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Electrochimica Acta 102 (2013) 328–335 Contents lis ts ava ila ble at SciV erse ScienceDirect ElectrochimicaActa  j o u r na l h ome p a g e: www.elsevier.com/locate/electacta Theactivation/depassivationof nickel–chromium–molybdenum alloys:AnoxyanionorapHeffect—PartII A.K.Mishra a ,D.W.Shoesmith a,b,a Depart ment of Chemis try , Uni ver sit y of Wes ter n Ont ari o, London , ON N6A5B7, Can ada b Sur fac e Sci enc e Wes ter n, 999 Col lipCircl e, London , ON N6G 0J3, Canada articleinfo  Article history: Rece ived 14 Febr uary 2013 Recei ved in rev ise d for m 28 March 201 3 Acc epted 28 March2013 Available online 8 April 2013 Keywords: Nickel allo ys Bicarbonate Carbonate pH XPS AES abstract Thedepletionof Cran d Mointhepassivelmsgrownon comme rcial Ni–Cr–Mo(W) alloysincarbon- ate/bicarbonate solutionshasbeenstudiedpotentiodynamically andbyX-rayphotoelectron andAuger spectroscopy. Depletioninthesetwokey alloy ingelementsoccursatlowerpotentialsthan expec tedon al l thealloysstudied. It ha s be enshownthatthekey fea tureleadingtothisbehavioristhebuffering of surfacepHto>8.6,whenthesurfacedeposition(orretention) of insolubleprotectiveMo(VI)species doesnotoccuras woul dbeexpectedundertheacidiclocalconditionsgeneratedbyCr(VI)andMo(VI) dissolution. Thepresenceof a smal lsolutionconcentrationof Mg 2+ ionsleadstotheaccumulationof a Mg(OH) 2  depositwhenthesurfacepHismaintainedsufcientlyalkaline. © 2013 Elsevier Ltd. All rights reserved. 1. Intr oducti on Ni –Cr– Mo al loys havi ng an op ti mum concentr at io n of Cr and Mo de mo nstrate excell ent corros io n resi stance in bo th ox id iz- ing and red uci ng environme nts [1–4]. A Ni–Cr–Mo– W alloy, C-2 2 (Ni–22 Cr– 13Mo–3W),has bee n studied as a can did ate mat erial for nucle ar wastecontainer s in the propo sed YuccaMountain (Neva da, USA) repos itor y [5]. The expos ure condi tionsunder which thisalloy maybe suscep tible to local ized corro sion (espec ially crevic e corro- sion) have been studied extensivel y [6–12], with a pr imary focus on the ac idic condit ions likely to pr evail within an active crevice. In the last ten year s ma ny studies have been perf or med to under- st and the inuence of vari ous pa rameters, such as, pH, po tent ial and temper atu re, on the passive lmbehavior of Ni– Cr–Mo all oys [2,3,10,13–16] . However, the eff ects of solution composition have not been st udied in detail. In Part I of this study [17] it wa s de mo ns t ra te d tha t, in the pr esence of bi carbonat e ions, the passive l m on the Hybr id-BC1 alloy und ergoes bar rier lay er degradation at an unexpectedly low pot ent ial . Ele ctr och emi cal studies show tha t when the pot ent ial is increased above this apparent br eakdown value, an anodic peak, Corr es ponding au thor at : Depa rtment of Chemis try, Univer si ty of West er n Ontar io,Londo n, ON N6A 5B7, Canada. Tel.: +1 519 661 2111x86 366. E-ma il addres ses: [email protected] , [email protected](D.W. Shoesmith). foll owed by a secondar y passive region is observed. It was shown that this l m degradation di d not result in locali zed corrosion. X- ray phot oelect ron spectr oscopy (XPS) anal yses of the oxide l m demonstrat ed that , in the passive regi on pr ior to apparent br eak- down potential, the lm possesses the expected bi layer structure with a Cr( III)-dominat ed barrier lay er containing mixed oxi dat ion stat es of Mo and an outer domi nantly hydr oxide layer. However, at pote nt ials mo re po si ti ve than the ap pa rent br ea kd own value the Cr/Mo content of the l m decreases si gni cantly and the al loy becomes covered by a thick (>100 nm), and only partiall y pr ot ec- tive, Ni(OH) 2  layer in the sec ond ary pas sivation region . Recent ly, Zadorozne et al . [18] sho wed that this anod ic ac ti - va tio n in bic ar bo nat e so lu ti ons wa s no t li mi te d to Ni– C r–Mo alloys, but also observed for alloy 800H (Ni–21Cr–45Fe) and a llo y 600 ( Ni –16 Cr– 9F e) . Ho we ve r, su ch an anodic act i vati on w as no t ob se rv ed fo r all oy 2 01 ( Ni – 0. 1 Fe ), w h ic h does not con- tain any Cr. These results suggested that the presence of an anodic peak and a secondary passivation region are generic for Cr- contai ning Ni-based all oys in bic arb ona te/ car bonate solu- tion. Th is st ud y w as per fo rmed to ga in a be tter un der s tand ing of  thi s app are nt bre akd own pro ces s sin ce other aut hor s [19–21] have noted that this ac ti vation could render these alloys suscepti bl e to SCC. The ef fect s of parameters, such as al loy compos ition, pH and temper atu re wer e studied using several electr ochemical and sur- face chara cteri zati on techni ques. 0013 -468 6/$ – see front matt er © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org /10.1016/j. electacta.2013 .03.177

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