Electrochemical characterisation of a martensitic stainless steel in a neutral chloride solution

(1)

O

pen

A

rchive

T

OULOUSE

A

rchive

O

uverte (

OATAO

)

OATAO is an open access repository that collects the work of Toulouse researchers and

makes it freely available over the web where possible.

This is an author-deposited version published in :

http://oatao.univ-toulouse.fr/

Eprints ID : 14078

To link to this article : doi:

10.1016/j.electacta.2012.09.011

URL :

http://dx.doi.org/10.1016/j.electacta.2012.09.011

To cite this version : Marcelin, Sabrina and Pébère, Nadine and

Régnier, Sophie Electrochemical characterisation of a martensitic

stainless steel in a neutral chloride solution. (2013) Electrochimica

Acta, vol. 87. pp. 32-40. ISSN 0013-4686

Any correspondance concerning this service should be sent to the repository

administrator:

staff-oatao@listes-diff.inp-toulouse.fr

(2)

Electrochemical

characterisation

of

a

martensitic

stainless

steel

in

a

neutral

chloride

solution

Sabrina

Marcelin

a,1

_,

_Nadine

_Pébère

a,∗,2

_,

_Sophie

_Régnier

b

a_Université_de_Toulouse,_CIRIMAT,_{UPS/INPT/CNRS,}_ENSIACET,_4,_Allée_Emile_Monso_–_BP_44362,₃₁₀₃₀_Toulouse_cedex_4,_France b_Institut_Catholique_d’Arts_et_Métiers,_75,_Avenue_de_{Grande-Bretagne,}₃₁₃₀₀_Toulouse,_France

Keywords: Stainlesssteel Passivefilm Surfacestate SIE

a

b

s

t

r

a

c

t

Thispaperfocusesonthecharacterisationoftheelectrochemicalbehaviourofamartensiticstainlesssteel

in0.1MNaCl+0.04MNa2SO4solutionandisapartofastudydevotedtocrevicecorrosionresistance

ofstainlesssteels.Polarisationcurvesandelectrochemicalimpedancemeasurementswereobtainedfor

differentexperimentalconditionsinbulkelectrolyte.X-rayphotoelectronspectroscopy(XPS)wasused

toanalysethepassivefilms.Atthecorrosionpotential,thestainlesssteelwasinthepassivestateandthe

corrosionprocesswascontrolledbythepropertiesofthepassivefilmformedduringairexposure.During

immersioninthedeaeratedsolution,thepassivefilmwasonlyslightlymodified,whereasitwasaltered

bothincompositionandthicknessduringimmersionintheaeratedsolution.Aftercathodicpolarisationof

thestainlesssteelelectrodesurface,theoxidefilmwasalmosttotallyremovedandthesurfaceappeared

tobeuniformlyactiveforoxygenreduction.Thenewpassivefilm,formedatthecorrosionpotential,was

enrichedwithironspeciesandlessprotective.Impedancediagramsallowedthecharacterisationofboth

theoxidefilm(high-frequencyrange)andthechargetransferprocess(low-frequencyrange).

1. Introduction

Martensitic stainless steels are mainlyused for applications wherehighmechanical performanceisrequired[1,2].However, duetotheirlowchromiumcontent,theyarerelativelysensitive tolocalisedcorrosion,particularlycrevicecorrosionencountered inconfinedenvironments.Intheseareas,oxygenisprogressively consumedandcannotberenewedbydiffusionorconvection. Out-sidetheconfinedarea,thecathodicreactionofoxygenreduction stilloccursonthemetalsurface,whereas,intheconfinedarea,the metalsurfacebecomestheanode.Thecorrosionprocessleadstoa modificationofthechemicalcompositionoftheelectrolyteinthe crevicewithsimultaneousacidificationandincreaseofchlorideion concentration[3].Thesenewlocalconditionsinducedepassivation ofthestainlesssteelandleadtoseverecorrosion.Thecrevice corro-sionresistanceofthestainlesssteelsisstronglydependentonthe passivefilmproperties[4].Thesubstrate(natureandcontentof alloyingelements),theenvironment(aerated,deaerated,neutral, acidicoralkaline)andalsospecificexperimentalconditions(e.g. anodicpolarisation)affectboththechemicalcompositionandthe structureofthepassivelayer[4–8].Toobtainabetterknowledge

∗ Correspondingauthor.Tel.:+33534323423. E-mailaddress:nadine.pebere@ensiacet.fr(N.Pébère).

1 _ISE_student_member. 2 _ISE_active_member.

ofthepassivefilmsformedonstainlesssteels,manystudieshave beendevotedtochemicalanalysisbyX-rayphotoelectron spec-troscopy(XPS)[9–14].Inneutraloralkalineenvironments,passive filmsareoftendescribedasduplexlayerswithaninnerpartwhich isenrichedinchromiumoxideandanouterpart,incontactwith theelectrolyte,enrichedinelementaliron(oxidesandhydroxides)

[12,14].

Toreducetheair-formedoxidefilm,thestainlesssteelssurface wasoftencathodicallypolarised,particularlytoinvestigate passiv-itybyMott–Schottkyanalysis[15–20].It wasalsoreportedthat thereproducibilityofelectrochemicalmeasurementsperformed onpassivefilmswasimprovedaftercathodicpolarisation[21,22]. However,theinfluenceofthisprocedureontheformationofthe newpassivefilminanelectrolyticsolutionwasnotclearly indi-cated.

Thecorrosion behaviourof thestainless steelsis commonly assessedbyusingelectrochemicalimpedancespectroscopy(EIS) because,coveringawidefrequencyrange,EISyieldsinformation ondifferent processes occurring onthe metal/oxide/electrolyte interface.Theimpedancedataobtainedonstainlesssteelswere generallyanalysedbyusingvariousequivalentcircuitstoextract quantitativeparameters.However,thedifferentelementsusedin thecircuitswerenotattributedtothesamephenomena.Andrade etal.[23] studiedtheelectrochemicalbehaviourofsteelrebars in concrete and the influence of environmental factors (CaCl2

or NaNO2). The impedance diagrams presented only one time

constant, but the experimentaldata were modelled using two

(3)

hierarchicalparallelRCcircuitstoobtainbetteragreementwiththe experimentaldata.Thehigh-frequencytimeconstantwas associ-atedtothecorrosionprocess(doublelayercapacitanceinparallel withthechargetransferresistance)andthelow-frequencytime constantwascorrelatedtoredoxprocessestakingplaceinthe pas-sivelayer.Theseprocessesinvolvetheoxidationofthecorrosion productsFe(OH)2toformmagnetite(Fe3O4)[23].Themagnetite

canalsobepartiallyoxidisedtog-FeOOH[14,23]andchemically reduced tog-Fe2O3 [24].Abreuet al. [21,24]studied the

elec-trochemicalbehaviourofAISI304Lstainlesssteelinadeaerated 0.1MNaOHsolution.Theyusedahierarchicalarrangementofthree stackedRCpairs.Thehigh-frequencytimeconstantwascorrelated tothemetal/filminterface.Themediumandlow-frequencytime constantswereassociatedtotheredoxprocesses.Freireetal. stud-iedthepassivebehaviourof AISI 316stainlesssteel inalkaline aeratedmedia[14].Accordingtotheauthors,thehigh-frequency timeconstantcanbecorrelatedwiththechargetransferprocessor tothebarrierpropertiesofthepassivefilm.Thelow-frequencytime constantwasassociatedtotheredoxortotheinterfacialprocesses. ForAISI304inalkalinesolutionswithdifferentpHandinthe pres-enceofchlorides,thehigh-frequencytimeconstantwasassigned totheareascoveredwiththepassivefilm(protectiveoxide);the low-frequencytimeconstantwascorrelatedwiththeactivesurface area(filmdefectsorporesintheabsenceofchloridesandactivepits inthepresenceofchlorides)[25].Recently,amodelwhichinvolved thepresenceoflow-conductivity(chromiumrichregionsof insu-latingcharacter)and high-conductivity (ironrich regionsbased onmagnetiteofconductingcharacter)oxideareasoverthemetal substratewasconsideredtoanalysetheimpedancedataobtained onAISI316Linsodiumhypochloriteandperaceticacidsolutions

[26].Fromrecentliterature,itappearsthattheimpedanceanalysis onpassivefilmsstillremainsasubjectopentodiscussion[25].It canbementionedthatthemainreferencescitedabove,concerned impedanceresultsforstainless steelsinalkalinemediawithor withoutchlorides.Theimpedancediagramswererelativelysimilar tothoseobtainedinthepresentstudyinneutralchloridesolutions andthus,ourdatacanbecomparedwiththeresultsobtainedin alkalinemedia.

Thepresentworkwasdesignedtoobtainabetterknowledge oftheelectrochemicalbehaviourofa martensiticstainlesssteel in a neutral chloride electrolyte. To ourknowledge, nostudies have been reported in the literature concerning the corrosion behaviourofmartensiticstainlesssteel.Polarisationcurveswere plottedandimpedancemeasurementscarriedoutusingarotating diskelectrode(500rpm).Impedancediagramswereobtainedboth atthecorrosionpotentialandintheanodicandcathodicranges. Sincethemartensiticstainlesssteelsaresensitivetocrevice corro-sion,particularattentionwaspaidtotheinfluenceofoxygenand experimentswereperformedinaeratedordeaeratedmedia.The influenceofthesurfacestate(cathodicpolarisation)onthe devel-opmentofthepassivefilmwasalsoinvestigated.XPSanalyseswere carriedouttodeterminethechemicalcompositionofthefilms.

2. Experimental 2.1. Material

The composition in weightpercent of theX12CrNiMoV12-3 martensiticstainlesssteelwasC=0.12,Cr=11.5,Ni=2.5,Mo=1.6, V=0.3andFetobalance.

2.2. Electrochemicalmeasurements

The electrochemical measurementswere performed using a conventionalthree-electrodecell.Itcontainedaplatinumgrid,a

saturatedcalomelreferenceelectrode(SCE)andarodofthe stain-lesssteel,of1cm2_{cross-sectional}_area,_used_as_working_electrode

(rotating disk). The body of the rodwas covered with a heat-shrinkablesheathleavingonlythetipofthecylinderincontactwith thesolution.Priortoanyexperiment,theelectrode wasground withsuccessiveSiCpapersanddiamondpaste(grade1000–3mm), rinsedandsonicatedwithethanolandfinallydriedinwarmair. Theelectroderotationratewasfixedat500rpm.

Thecorrosivemediumwaspreparedfromdistilled waterby adding0.1MNaCl+0.04MNa2SO4(reagentgrade).Theaddition

ofsulphateionsenabledlimitationoftheincreaseoftheanodic currentdensitywhenpitsstartedtoformatanodicpotentials.The inhibitiveeffectofSO42−aniononpittingcorrosioniswellknown [27,28].Fordeaeratedexperiments,theelectrolyticsolutionwas initiallypurgedwithnitrogenfor1handthen,theworking elec-trodewasintroducedintothecell.N2 bubblingwasmaintained

duringtheexperiment.

Polarisationcurveswereplottedunderpotentiodynamic reg-ulation using a Solartron 1287 electrochemical interface. The cathodicandanodicbrancheswereplottedeitherconsecutively, startingfromthecathodicpotentialof−1V/SCE,orindependently from the corrosion potential (Ecorr). In both cases, the curves

wereobtainedatapotentialsweeprateof0.6V/h. Electrochem-icalimpedancemeasurementswerecarriedoutusingaSolartron 1287electrochemicalinterfaceconnectedwithaSolartron1250 frequencyresponseanalyser.Impedancediagramswereobtained overafrequencyrangeof65kHztoafewmHzwitheightpoints perdecadeusinga15mVpeak-to-peaksinusoidalvoltage.The lin-earityofthesystemwascheckedbyvaryingtheamplitudeofthe acsignalappliedtothesample.Theelectrochemicalresultswere obtainedfromatleastthreeexperimentstoensurereproducibility. Theimpedancespectrawerefittedbyusingelectricalequivalent circuitswithZ-viewimpedancesoftware.

2.3. XPSanalysis

TheXPSmeasurementswerecarriedoutonaK-alphaThermo Scientificspectrometer.Thesampleswerepreparedbythesame procedureasfor theelectrochemicalexperiments.After17hof immersion,thestainlesssteelsamplewasremovedfromthe solu-tion,rinsedwithethanolthendriedinwarmairandplacedina vacuumchamber.ThespecimenswereirradiatedwiththeAlKa raysource(h=1486.6eV).TheX-raypowerwas300W. Angle-resolvedmeasurementsweremadeatatake-offangle=90◦_(e.g.

normaltothemetalsurface).Thesurfaceareaanalysedwasabout 1mm2_._The_experimental_resolution_of_the_binding_energy_was₁_eV.

TheShirleymethodwasusedforbackgroundsubtractionandpeak deconvolutionwasperformedusingThermoAvantagesoftware. Bindingenergieswerecorrectedforpossiblechargingeffectsby referencingtotheC1s(284.8eV)peak.Foreachsample,three dif-ferentanalyseswereperformedtoverifythehomogeneityofthe layercomposition.

3. Results

First,polarisationcurvesandimpedancemeasurementsatthe corrosion potential (Ecorr) were carried out for three different

experimentalconditions:aeratedmedium,deaeratedmediumand inaeratedmediumafteracathodicpolarisationfor1hat−1V/SCE. Thislastconditionwasappliedtopartlyortotallyreducetheoxide filmnaturallyformedduringairexposure.Then,impedance mea-surementswereperformedintheaeratedsolution,fordifferent anodicandcathodicoverpotentials.Tocharacterisetheinitialstate ofthenativeoxidefilmbeforeimmersionintheaggressive solu-tion,XPSanalysiswasperformedonasamplejustafterpolishing

(4)

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0 5 10 15 20 25

Corrosion potential vs SCE / V

Immersion time / h

C

B

A

Fig.1. CorrosionpotentialofX12CrNiMoV12-3martensiticstainlesssteelasa func-tionoftime,(A)indeaerated0.1MNaCl+0.04MNa2SO4solution,(B)inaerated

solutionand(C)afteracathodicpolarisation(−1V/SCE,1h)inaeratedsolution.

andcleaning.Then,XPSanalyseswerecarriedoutontwodifferent samplesafter17hofimmersionintheaeratedelectrolytewithand withoutthesurfacetreatment(polarisationat−1V/SCE).

3.1. DCmeasurements

Fig.1illustratesthevariationofthefreecorrosionpotential, Ecorr, of the stainless steel dipped into theaggressive solution

for the three experimental conditions. In all cases, the shape ofthecurves is similar.Duringthe first5hof immersion,Ecorr

increasessignificantlywithtimeandthenprogressivelystabilises after15–20hofimmersion.Thetimerequiredtoreachthe sta-tionarystateisrelatively long(oneday)which isinagreement withdifferentworksreportedintheliteraturefordifferenttypes ofstainlesssteel[14,29,30].ThevaluesofEcorr,after23hof

immer-sion,are−0.16V/SCEforthesampleindeaeratedsolution(curve A),−0.06V/SCEforthesampleinaeratedmedium(curveB)and 0.17V/SCEfor thesampleaftercathodic polarisationin aerated medium (curve C). The shift of Ecorr in thecathodic direction,

observedindeaeratedmedium,isattributedtothedecreaseofthe cathodiccurrentdensity.Inaeratedmedium,Ecorrstronglydepends

onthesurfacestate.After23hofimmersion,itwasshiftedbyabout 230mVintheanodicdirectionwhentheelectrodewaspolarised at−1V/SCE(curveC)bycomparisonwiththesamplewhichwas notpreviouslypolarised(sampleB).

Fig.2showsthepolarisationcurvesforthemartensitic stain-lesssteelobtainedafter2hofimmersioninaeratedmediumand plottedeitherfromEcorrorfrom−1V/SCEtoEpit.Thecurves

dif-feraccordingtothepotentialscan.Inthecathodicdomain,when thecurveisplotted fromEcorr,apseudoplateaudue tooxygen

reductionisobservedaround−0.7to−0.8V/SCE.Then,atabout −0.8V/SCE,anincreaseofthecathodiccurrentdensityisobserved. Thisbehaviourisexplained bythecontributionofthecathodic reductionoftheoxidefilmformedduringimmersion[31,32].This isconfirmedbythefactthatwhenthecurveisplottedfromthe cathodicpotential,thecurrentdensityissignificantlyhigherandits valueat−1V/SCE(100mAcm−2)correspondstothatobtainedona uniformlyactivesurfaceaccordingtotheLevichtheory[33].Thus, itcanbeconcludedthatinthepresentstudy,duringthecathodic plotfrom−1V/SCEtoEcorr orwhentheelectrodeispolarisedat

−1V/SCE,mostoftheoxidesarereducedandthesurfacebecomes totallyactiveforoxygenreduction.

Intheanodic domain,for both curves,a passivityplateauis observedbeforeanabruptincreaseofthecurrentdensitydueto thebreakdownofthepassivefilmandtothedevelopmentofpits. Theanodiccurrentdensityfor thecurveplotted from−1V/SCE

10-8 10-7 10-6 10-5 10-4 10-3 -1 -0.5 0 0.5 Current density / A cm -2 Potential vs SCE / V

E

pit

Fig.2. Polarisationcurvesobtainedforthemartensiticstainlesssteelafter2hof immersionintheaeratedsolution:(d)fromEcorr(thecathodicandanodicbranches

wereobtainedseparately)and( )from−1V/SCEtoEpit(thecathodicandanodic

brancheswereobtainedconsecutively).

(0.4mAcm−2)isofthesameorderofmagnitudeasthatmeasured forthecurveplottedfromEcorr(0.7mAcm−2).Inaddition,the dif-ferenceinthepittingpotential(Epit)isnotsignificantlymodifiedfor

thetwoexperimentalprocedures.Generally,thepittingpotential isnotareproducibleparameter[34].

Fig.3comparesthepolarisationcurvesforthemartensitic stain-lesssteelobtainedafter17hofimmersionatEcorrinaeratedand

deaeratedelectrolytes.Thecathodiccurrentdensityislowerand Ecorrisshiftedtowardscathodicvalueswhenthemediumis

deae-rated.Theanodiccurrentdensityontheplateauisapproximately thesameinbothmedia(0.5–0.6mAcm−2).Indeaeratedelectrolyte, thepassivityplateauislargerduetotheshiftofEcorrinthecathodic

direction.Inthesetwocurves,thedifferenceobservedinEpit

can-notbediscussedasmentioned earlier.However,it canbeseen, bycomparisonwiththeanodiccurveplottedafter2hof immer-sion(Fig.2),thatEpitisshiftedtowardsmoreanodicvaluesafter

17hofimmersionindicatingamodificationofthepassivefilmwith increasingimmersiontime.FromFig.3,itcanbeconcludedthat theanodicbranchesarerelativelysimilarinaeratedanddeaerated media.Theanodicbranchplottedafterapreliminarypolarisation at−1V/SCEand17hofimmersionatEcorrisalsoreportedinFig.3

andrevealsbothanincreaseoftheanodiccurrentdensitytoreach

10-8 10-7 10-6 10-5 10-4 10-3 -1 -0.5 0 0.5 C u rr e n t d e n s it y / A c m -2 Potential vs SCE / V

E

pit

Fig.3.Polarisationcurvesobtainedforthemartensiticstainlesssteelafter17hof immersion:( )aeratedmedium,(d)deaeratedmediumand(×)afteracathodic polarisationat−1V/SCEinaeratedmedium(inallcasesthecathodicandtheanodic brancheswereobtainedseparately).

(5)

101 102 103 104 105 106 10-3 ₁₀-2 ₁₀-1 ₁₀0 ₁₀1 ₁₀2 ₁₀3 ₁₀4 ₁₀5 2 h 6 h 17 h fit result

Impedance modulus /

Ω

cm

2 Frequency / Hz -80 -60 -40 -20 0 10-3 10-2 10-1 100 101 102 103 104 105 2 h 6 h 17 h fit result

Phase angle / degree

Frequency / Hz

Fig.4. ElectrochemicalimpedancediagramsobtainedatEcorrafterdifferenthold

timesintheaeratedsolution.

about2mAcm−2andashortpassivityplateaubycomparisonwith thetwoothercurves.

3.2. Electrochemicalimpedancemeasurements

Inthissection,impedancemeasurementswerefirstperformed atEcorrunderthesameexperimentalconditionsasthepolarisation

curves(aeratedanddeaeratedmedia,andwithapre-polarisation oftheelectrodeat−1V/SCE).Fig.4showstheimpedancediagrams plottedversusimmersiontimeintheaeratedsolutionaftersurface polishingonly.Independentlyoftheimmersiontime,thediagrams arecharacterisedbytwotimeconstants.Thetwotimeconstantsare betterdefinedandtheimpedancemodulusinthelow-frequency rangeincreaseswiththeimmersiontime.Forlongerimmersion times(>17h),theimpedancediagramsarerelativelysimilarand arenotreportedhere.Theimpedancediagramswerealsoobtained forthemartensiticstainlesssteelafterpolarisationat−1V/SCEand fordifferentimmersiontimesatEcorrintheaeratedsolution.Inthis

case,thediagramsarepoorlydependentontheimmersiontimeand onlythediagramobtainedafter17hofimmersionisshowninFig.5. BycomparisonwiththediagramsreportedinFig.4,itcanbeseen thatthetwotimeconstantsarebetterseparatedandthemodulusat lowfrequencyisofthesameorderofmagnitude(about106_cm2_).

Theimpedancediagramobtainedinthedeaeratedsolutionforthe stainlesssteelafter17hofimmersionatcorrosionpotential(Fig.6) showsthatthetimeconstantatlowfrequencydisappearsanditcan benotedthattheimpedancemodulusatlowfrequencyis approx-imatelythesameasinaeratedmedium.Thus,itcanbeconcluded

101 102 103 104 105 106 -80 -60 -40 -20 0 10-3 ₁₀-2 ₁₀-1 ₁₀0 ₁₀1 ₁₀2 ₁₀3 ₁₀4 ₁₀5

data

fit result

Impedance modulus /

Ω

cm

2

Phase angle / degree

Frequency / Hz

Fig.5. Electrochemicalimpedancediagramobtainedafterapreliminarycathodic polarisationat−1V/SCEfor1hofthestainlesssteelandthen17hofimmersionat Ecorr(aeratedsolution).

thatthelow-frequencytimeconstantislinkedtothepresenceof oxygenintheelectrolyticsolution.

El-EgamyandBadaway[27]haveproposedanequivalent cir-cuitcomposedofanR//CPEelementtodescribethebehaviourof 304stainlesssteelinasodiumsulphatesolution.Thetime con-stantwasassociatedtotheresponseofthepassivefilm.Thissimple modelwasusedtofittheimpedancedataobtainedinadeaerated medium(Fig.7a).Then,totakeintoaccountthepresenceofthe secondtime constantinthelow-frequencyrange,anadditional R//CPEcircuitwasintroduced(Fig.7b).Thetwopartsofthecircuit wereimbricatedtotakeintoaccounttheoxygencontributionand theprogressivemodificationofthepassivefilmasdescribedinthe literature[35–37].Thus,thetimeconstantinthehigh-frequency rangewasattributedtotheoxidefilm(Rox,˛ox andQox)andthe

timeconstant inthelow-frequencyrange wasattributedtothe chargetransferreaction:oxidationofthesubstrate/oxygen reduc-tion(Rt,˛dlandQdl).

Some parameters (˛ox, Qox, ˛dl and Qdl) were graphically

extracted[38].Thesamevalueswereobtainedfromthe equiva-lentcircuitsbythefittingprocedure.Theexperimentaldiagrams are well fitted with the equivalent circuits, as can be seen in

Figs.4,5and6.Thus,thevaluesoftheparameterscanbeextracted anddiscussed.TheyarereportedinTables1,2and3 correspond-ingtoFigs.4,5and6,respectively.Forthedifferentexperimental conditions,the˛ox values arerelativelysimilar(about 0.9)and

donotdependontheimmersiontime. TheQox valuesdecrease

withimmersiontimeforthethreeexperimentalconditions.This

101 102 103 104 105 106 -80 -60 -40 -20 0 10-3 ₁₀-2 ₁₀-1 ₁₀0 ₁₀1 ₁₀2 ₁₀3 ₁₀4 ₁₀5 data fit result

Im

p

e

d

a

n

c

e

m

o

d

u

lu

s

/

Ω

c

m

2 P h a s e a n g le / d e g re e Frequency / Hz

Fig.6.ElectrochemicalimpedancediagramobtainedatEcorrafter17hofimmersion

(6)

Table1

FittedparametersvaluesobtainedatEcorrforthemartensiticstainlesssteelasafunctionoftheexposuretimetotheaeratedsolution.

Immersiontime(h) ˛ox±0.3% Qox(M−1cm−2)sa±1% Rox(kcm2)±11% ˛dl±4% Qdl(M−1cm−2)sa±12% Rt(kcm2)±5% 2 0.90 90 50 0.82 90 260 6 0.90 87 62 0.84 64 400 10 0.90 86 76 0.88 61 510 17 0.90 76 110 0.89 42 1680 Table2

FittedparametersvaluesobtainedatEcorrforthemartensiticstainlesssteelasafunctionoftheexposuretimetotheaeratedsolution(acathodicpolarisationat−1V/SCEfor

1hwaspreviouslyappliedtothestainlesssteel).

Immersiontime(h) ˛ox±0.2% Qox(M−1cm−2)sa±1% Rox(kcm2)±5% ˛dl±1.3% Qdl(M−1cm−2)sa±1.3% Rt(kcm2)±3%

2 0.91 32 6.1 0.73 63 110

6 0.91 29 7.2 0.81 64 260

10 0.91 27 7.3 0.83 64 470

17 0.91 26 7.7 0.87 64 700

Fig.7.Equivalentcircuitsusedtofittheexperimentalimpedancedataobtainedin (a)deaeratedand(b)aeratedmedia.

indicatesa modificationof thepassivelayer(thickening and/or modificationof thecompositionof thefilm).The values ofQox

arerelativelyhighforoxidecapacity.However,theCPE param-eter,Q,cannotbeattributedtoacapacity.SimilarvaluesofQox

werealreadyreportedforaFe17Crstainlesssteel[39].Toexplain theCPEbehaviour,theauthorsproposedanormaldistributionof localresistivitythroughthepassivefilmwithstrongdifferencesin resistivitybetweenthemetal/filminterfaceandthefilm/electrolyte interface,duetothesemiconductorcharacterofthepassivelayer. ThisanalysispartlycorroboratestherecentworkofGuitiànetal.

[26].ThedecreaseofQoxisaccompaniedbyanincreaseofRoxwith

increasingimmersiontime.Indeaeratedmedium,thevariations ofRox and Qox withimmersion timearelimited bycomparison

Table3

FittedparametersvaluesobtainedatEcorrforthemartensiticstainlesssteelasa

functionoftheexposuretimetothedeaeratedsolution.

Immersiontime(h) ˛ox±0.1% Qox(M−1cm−2)sa ±0.6% Rox(kcm2) ±1.7% 6 0.90 30 330 10 0.91 26 388 17 0.92 23 443

withthoseobservedinaeratedmedium.Roxvaluesobtainedafter

thecathodicpre-treatmentaresmallcomparedtothoseobtained forthetwootherconditionsandtheydidnotsignificantlychange withimmersionwhereasthevaluesofQoxarecomparabletothose

obtainedinthedeaeratedmedium.Thisindicatesthatthepassive filmformedintheelectrolyte(aftercathodicpolarisation)is dif-ferentfromthefilmformedintheairand thenexposed tothe electrolyticsolution.ThevaluesofRt arealsodependentonthe

experimentalconditions.From2hto17h,Rtincreasedfrom260

to1700kcm2_when_the_electrode_was_not_submitted_to_cathodic

polarisationandfrom110to700kcm2_when_the_oxide_film_was

developedinthesolutionaftercathodicpolarisation.

Then,impedancemeasurementswereperformedintheanodic andcathodicdomainstovalidatetheimpedanceanalysisatthe corrosionpotential.Thesemeasurementswereonlyobtainedin aeratedmediumwithoutapreliminarypolarisationat−1V/SCE.

Fig.8showsimpedancediagramsobtainedonthestainlesssteel fortwoanodicpotentials:−0.2V/SCE(closetoEcorr)and+0.4V/SCE

(onthepassivityplateau).Thetimeconstantinthehigh-frequency rangewasshiftedtowardshigherfrequenciesandthetime con-stantinthelow-frequencyrangedisappearedwhenthepotential increased.Thevaluesobtainedfortheparametersassociatedtothe high-frequencylooparereportedinTable4.ItcanbeseenthatRox

increasedandQoxdecreasedwhenthepotentialbecame

increas-inglyanodic.Theseresultscanbeexplainedbyathickeningofthe oxidefilmbytheanodicpolarisationandconfirmtheattribution ofthehigh-frequencytimeconstanttothepassivefilmresponse. Twoimpedancediagramsobtainedinthecathodicrange(Fig.9) showthatthetwotimeconstantsaremodifiedandtheimpedance

101 102 103 104 105 106 -80 -60 -40 -20 0 10-3 ₁₀-2 ₁₀-1 ₁₀0 ₁₀1 ₁₀2 ₁₀3 ₁₀4 ₁₀5 -0.2 V/SCE +0.4 V/SCE fit result

Impedance modulus /

Ω

cm

2

Phase angle / degree

Frequency / Hz

Fig.8.Electrochemicalimpedancediagramsobtainedintheanodicdomain,after 17hofimmersionatEcorr(aeratedsolution).

(7)

Table4

Fittedparametersvaluesobtainedforthemartensiticstainlesssteelafter17hof immersionandfordifferentanodicpotentials(aeratedsolution).

Appliedpotential (V/SCE) ˛ox±0.3% Qox(M−1cm−2)sa ±1% Rox(kcm2) ±1.5% −0.2 0.89 84 270 0 0.90 70 400 +0.1 0.91 50 420 +0.2 0.92 39 483 +0.4 0.92 33 782 101 102 103 104 105 106 -80 -60 -40 -20 0 10-3 ₁₀-2 ₁₀-1 ₁₀0 ₁₀1 ₁₀2 ₁₀3 ₁₀4 ₁₀5 -0.3 V/SCE -0.8 V/SCE fit result

Im

p

e

d

a

n

c

e

m

o

d

u

lu

s

/

Ω

c

m

2

P

h

a

s

e

a

n

g

le

/

d

e

g

re

e

Frequency / Hz

Fig.9.Electrochemicalimpedancediagramsobtainedinthecathodicdomain,after 17hofimmersionatEcorr(aeratedsolution).

modulus strongly decreaseswhen thepotential becomes more cathodic.Thevaluesobtainedfortheparametersassociatedtothe high-frequencylooparereportedinTable5fordifferentcathodic potentials.ItcanbeseenthatthevaluesofQoxincreaseandRox

decreasewhenthepotentialdecreases.Thesignificantmodification ofthetwoparameterscanbeexplainedbytheprogressive reduc-tionoftheoxidefilminagreementwiththepolarisationcurves (Fig.2).

3.3. XPSanalyses

Fig.10showstheX-rayphotoelectronspectraofO1s,Fe2p3/2

andCr2p_3/2forthemartensiticstainlesssteelpassivefilmformed incontactwithair.Similarspectrawereobtainedfor the stain-lesssteelsurfaceafterimmersionintheaeratedsolutionwithor withoutpreviouscathodicpolarisationoftheelectrode.

Theparametersusedforthedeconvolutionofthe experimen-talXPSspectraandtheatomicpercentofthespeciesconstituting thepassive filmsarereported inTable 6.The filmsaremainly composedofoxides(FeO,Fe2O3andCr2O3),hydroxides(FeOOH

andCr(OH)3)andcontaminants(C O)(Fig.10).Itisknownthat

theFe3+_contribution_from_Fe

2O3andFe3O4isdifficulttoidentify

onXPSspectra[14,22]andaglobalvalueforthetwoironoxides wasconsidered.Itcanbenotedthatafterimmersioninthe elec-trolyte,ions fromthesolution(sodiumand sulphuronly)were

Table5

Fittedparametersvaluesobtainedforthemartensiticstainlesssteelafter17hof immersionandfordifferentcathodicpotentials(aeratedsolution).

Appliedpotential (V/SCE) ˛ox±0.2% Qox(M−1cm−2)sa ±1% Rox(kcm2) ±5% −0.3 0.85 110 38 −0.4 0.85 122 34 −0.5 0.84 127 27 −0.6 0.80 140 5 −0.8 0.81 140 3 0 100 5 103 1 104 1.5 104 2 104 2.5 104 528 529 530 531 532 533 534 535

Intensity / cps

Binding energy / eV

O

2-

OH

-

C=O

H

₂

O

O1s

0 100 1 104 2 104 3 104 4 104 5 104 706 708 710 712 714 716

Intensity / cps

Binding energy (eV)

Fe2p

_3/2

Fe(0)

FeO

Fe

₂

O

₃

FeOOH

satellite peak

3 103 5 103 6 103 8 103 1 104 572 574 576 578 580

Intensity / cps

Binding energy (eV)

Cr2p

_3/2

Cr(0)

Cr

2

O

3

Cr(OH)

₃

Fig.10.ExperimentalandfittedO1s,Fe2p3/2andCr2p3/2XPSspectraofmartensitic

stainlesssteelonlyincontactwithair.

detected.Thepresenceofmolybdenumandnickelisalsoobserved forthetwoimmersedsampleswithsimilaratomicpercent con-tent.Fe/CrratioswerecalculatedfromXPSdata(Table6).Forthe filmformedincontactwithair,theFe/Crratioisthehighest(7.2). Afterimmersionintheaeratedsolution,theratiodecreasesto1.5. Thus,thefilmisimpoverishedinironoxidesandhydroxideand enrichedinchromiumoxideandhydroxideafterimmersioninthe electrolyte.Thismodificationcanbeexplainedbythepartial dis-solutionoftheironspeciespresentinthefilmduringimmersion

(8)

Table6

ParametersusedfordeconvolutionofXPSspectraforthemainspeciespresentinthepassivefilms,atomicpercent(%)ofthefilmscompositionandFe/Crratioobtainedfor thethreesystems.

Components Filmformedincontactwithair FilmformedinaeratedsolutionatEcorr Filmformedaftercathodicpolarisationthen

immersedinaeratedsolutionatEcorr

Bindingenergy(eV) FWHMa_(eV) _(%) _Binding_energy_(eV) _FWHMa_(eV) _(%) _Binding_energy_(eV) _FWHMa_(eV) _(%)

Fe0 _706.5 _1.2 _4.0 _706.6 _1.2 _3.4 _706.5 _1.2 _2.2 FeO 709.5 2.0 13.6 709.6 2.0 5.4 709.6 2.0 8.5 Fe2O3 711.1 2.0 11.4 711.1 2.0 5.0 711.0 2.0 8.1 FeOOH 712.5 2.0 2.5 712.5 2.0 0.9 712.6 2.0 1.9 Satellitepeak 714.8 2.9 2.4 714.7 2.9 0.7 714.7 2.8 1.4 Cr0 _574.1 _1.4 _0.9 _573.9 _1.2 _0.7 _573.8 _0.4 _0.4 Cr2O3 576.1 1.8 2.4 576.1 4.5 4.5 576.1 1.9 2.3 Cr(OH)3 577.5 1.8 1.4 577.5 1.8 3.2 577.5 1.9 1.6 Mo3d5/2 232.7 4.5 0.7 232.7 4.5 1.1 232.7 4.5 1.0 Ni2p3/2 – – – 852.7 1.2 0.6 852.6 0.9 0.2 O2− _530.2 _1.3 _45.2 _530.2 _1.3 _32.9 _530.2 _1.3 _37.1 OH− _531.2 _1.4 _6.3 _531.4 _1.4 _6.9 _531.2 _1.4 _5.9 S2p3/2 – – – 169.2 1.9 4.4 169.8 1.9 2.4 Na1s – – – 1072.0 1.8 11.6 1072.6 1.9 6.2 Contaminants 9.2 18.7 20.8 Total(%) 100 100 100 Fe/Crratio 7.2 1.5 4.7

G/L:ratioofGaussianandLorentzianfunction(G/L=0.3fordeconvolutionofCr2p3/2XPSspectrum,0.5fordeconvolutionofFe2p3/2andO1sXPSspectrum). a_FWHM:_full_width_at_half_maximum.

polarisationisenrichedinironoxidesandhydroxideand impov-erished inchromium oxideand hydroxide (Fe/Cr ratio=4.7)by comparison withthesample immersed in theaerated solution (Fe/Crratio=1.5).Aspreviously mentioned,thecathodic polari-sationledtoasurfacetotallyactiveforthecathodicreactionthus facilitatingironoxidation(Fe2+_species)_when_the_electrode_was

heldatthecorrosionpotentialfor17h,explainingthesignificant quantityofironoxidesandhydroxideinthepassivefilmforthis experimentalprocedure.

Aproceduretoestimatethefilmthicknesseswasused,similar tothatreportedbyseveralauthors[13,17,40].Anaverage thick-nessof3nmwasobtainedforthethreeexperimentalconditions. Itmustbeunderlinethat,beforetheintroductionofthesamplesin theXPSchamber,thepassivefilmsformedintheelectrolytecanbe

modifiedduringairexposure,andtheestimatedthicknessforthe filmformedduringairexposurecannotbedirectlycomparedwith thethicknessofthefilmsdevelopedintheelectrolyteatthe cor-rosionpotential.ThisisconfirmedbytheXPSdata(Table6)which revealedthatthefilmsformedinthesolutionarecomposedof addi-tionalspeciescomingfromtheelectrolyteorofcontamination.As aconsequence,theXPSresultsmustbeconsideredcarefullytobe comparedwiththeelectrochemicalresults.

4. Discussion

The impedancediagrams obtainedon themartensitic stain-lesssteelinaeratedelectrolyte werecharacterisedbytwo time constants which were more or less distinct depending onthe

(9)

experimentalprocedure. Thehigh-frequencytime constantwas attributedtothepropertiesof theoxidefilm.Themagnitudeof theRoxvaluescouldbelinkedtothefilmcomposition,whereas

Qox couldbeindicativeofthefilmthickness.Thelow-frequency

timeconstantwasattributed,inaeratedconditions,tothecharge transferprocess[26,31].Indeaeratedmedium,thecharge trans-fer process was limited, due to the lack of oxygen, and the impedance diagramscharacterised thepassive filmonly. These differentpointsvalidatethetwoproposedequivalentcircuitsto analysetheimpedancedata(Fig.7).Indeaeratedmedium,Roxand

Qoxslowlyevolvedwithimmersiontime(Table3)andthevalues

ofRoxwerehigh(300–400kcm2)comparedtothosemeasured

fortheotherconditions.Thissuggeststhatthefilmformedinair wasonlyslightlymodifiedintheelectrolyteduetotheabsenceof oxygen.Indeaeratedcondition,theoxidefilmisthoughttoremain closetothatformedinair(compositionandthickness).Incontrast, inaeratedconditions,thepassivefilmformedinairwas progres-sivelymodifiedduringtheimmersion.ThevaluesofRoxwerelower

andQoxhigher(Table1)thanindeaeratedmedium(Table3).Thus,

thepassivefilmdevelopedinaeratedsolutionisassumedtobe thinnerthanthefilmformedindeaeratedmedium.Thefilmgrown insolutionafterthecathodicpolarisationappearedstrongly differ-entasunderlinedbythevalueofRox(Table2).XPSresultsrevealed

ahighironoxidesandhydroxidecontentduetohighercorrosion ofthestainlesssteel.Thisfilmislessprotectiveasunderlinedby thehighpassivitycurrentandthepittingpotentialwhichwasless anodicthanforthetwootherconditions(Fig.3).Thesignificant shiftofEcorr intheanodicdirection(Fig.1,curveC)canthusbe

explainedbythecathodiccurrentswhicharehigherbecausethe surfaceismoreactiveaftertheremovaloftheoxides(Fig.2).

Inaeratedcondition,withoutthepre-polarisationofthe sur-face,thevaluesofRt werehighandincreasedsignificantlywith

immersiontime(Table1).Thepassivelayer,enrichedinchromium oxideandhydroxide(Fe/Crratio=1.5),impedesthechargetransfer processandthusexplaintheincreaseofRtwithimmersiontime.

Incontrast,thelowervaluesofRtforthesamplewithpreliminary

cathodicpolarisation(Table2)corroboratesthehighercorrosion rateandthefactthatthelayerwasenrichedinironoxidesand hydroxideand impoverishedinchromiumoxideand hydroxide (Fe/Crratio=4.7).Fig.11presentsaschematicillustrationofthe passivefilmsformedunderthedifferentconditionstosummarise thedifferentpointsdiscussedabove.

Inconclusion,fromthedatareportedinTables1,2and3itcan beemphasisedthatindeaeratedmedium,thepassivefilmconfers highprotectiontothestainlesssteelandthefilmwasonlyslightly modifiedwithimmersiontime.Whenthefilmformedinairwas exposedtotheaggressivesolution,animprovementofits protec-tiveeffectwasobservedwithincreasingimmersiontimeandthe corrosionresistancewasalsohigh.Finally,aftercathodic polarisa-tion,thefilmgrownintheaggressivemediumwaslessprotective asshownbythelowRoxandRtvalues,andalsobythehigh

passiv-itycurrent(Fig.3).Itmustalsobeunderlinedthat,independently oftheexperimentalprocedure,thepassivecharacterofthefilms wasshownbythehighvaluesofRox(Tables1and3)inspiteofthe

lowCrcontent.

5. Conclusions

In this work, the behaviour of a martensitic stainless steel wasstudiedbyelectrochemicaltechniquesandXPSanalysesin a neutral sodium chloride solution. Dissolved oxygen plays an importantroleontheformationand/ormodificationofthepassive filmduringimmersionintheelectrolyticsolution,atthe corro-sionpotential.Thetwotimeconstants,observedintheimpedance diagramsinthehighandlow-frequencyranges,areattributedto

theoxidefilmandtothechargetransfer,respectively.Theresults obtainedintheanodicandcathodicrangescombinedwiththeXPS analysescorroboratetheimpedanceresultsatthecorrosion poten-tial.Theequivalentcircuitsusedtoanalysetheimpedancedata provided quantitativeinformation onthestainlesssteel/passive films/electrolyteinterfaces.However,theCPEbehaviourobserved intheimpedanceresponseofthepassivefilmsshouldbebetter interpretedtogiveaphysicalmeaningforthepassivefilmresponse. Followingstudieswillfocusoncrevicecorrosionusingathin-layer cell[41].

Acknowledgments

Thiswork wascarried out in the framework of theARCAM project, withthefinancial supportofDGCIS, RegionsAquitaine, AuvergneandMidi-Pyrénées.Theauthorsgratefullyacknowledge thepartnersoftheproject:Ratier-Figeac,Aubert&Duval,Olympus, theMechanicalandEngineeringInstituteofBordeaux,the Mate-rialsDepartmentofICAMandtheInstituteCarnotCIRIMAT.The authorsthankJérômeEsvan(CIRIMAT)forXPSanalyses.

References

[1]D.Thibault,P.Bocher,M.Thomas,Residualstressandmicrostructureinwelds of13%Cr-4%Nimartensiticstainlesssteel,JournalofMaterialsProcessing Tech-nology209(2009)2195.

[2] X.P.Ma,L.J.Wang,C.M.Liu,S.V.Subramanian,Microstructureandproperties of13Cr5Ni1Mo0.025Nb0.09V0.06Nsupermartensiticstainlesssteel,Materials

ScienceandEngineeringA539(2012)271.

[3] J.R.Oldfield,W.H.Sutton,Crevicecorrosionofstainlesssteels-J.A. Mathemat-icalmode,BritishCorrosionJournal13(1978)13.

[4]P.Schmuki,S.Virtanen,H.S.Isaacs,M.P.Ryan,A.J.Davenport,H.Böhni,T. Sten-berg,ElectrochemicalbehaviourofCr2O3/Fe2O3artificialpassivefilmsstudied

byinsituXANES,JournaloftheElectrochemicalSociety145(1998)791. [5] V.Maurice,W.P.Yang,P.Marcus,X-rayphotoelectronspectroscopyand

scan-ningtunnellingmicroscopystudyofpassivefilmsformedon(100)Fe-l8Cr-l3Ni single-crystalsurfaces,JournaloftheElectrochemicalSociety145(1998)909. [6] T.Ohtsuka,H.Yamada,Effectofferrousioninsolutionontheformationof

anodicoxidefilmoniron,CorrosionScience40(1998)1131.

[7]J.S.Kim,E.A.Cho,H.S.Kwan,Photoelectrochemicalstudyonthepassivefilm onFe,CorrosionScience43(2001)1403.

[8] N.LeBozec,C.Compère,M.L’Her,A.Laouenan,D.Costa,P.Marcus,Influenceof stainlesssteelsurfacetreatmentontheoxygenreductionreactioninseawater, CorrosionScience43(2001)765.

[9]A.Nishikata,Y.Ichihara,T.Tsuru,Electrochemicalimpedancespectroscopyof metalscoveredwithathinelectrolytelayer,ElectrochimicaActa41(1996) 1057.

[10]K.Asami,K.Hashimoto,AnX-rayphoto-electronspectroscopicstudyofsurface treatmentsofstainlesssteels,CorrosionScience19(1979)1007.

[11] M.Stratmann,H.Streckel,Ontheatmosphericcorrosionofmetalswhichare coveredwiththinelectrolytelayers-I.Verificationoftheexperimental tech-nique,CorrosionScience30(1990)681.

[12]G.Lorang,M.DaCuhnaBelo,A.M.P.Simoés,Chemicalcompositionofpassive filmsonAISI304stainlesssteel,JournaloftheElectrochemicalSociety141 (1994)3347.

[13]A.Ithurbide,I.Frateur,A.Galtayries,P.Marcus,XPSandflow-cellEQCMstudy ofalbuminadsorptiononpassivatedchromiumsurfaces:influenceofpotential andpH,ElectrochimicaActa53(2007)1336.

[14]L.Freire,M.J.Carmezim,M.G.S.Ferreira,M.F.Montemor,Thepassivebehaviour ofAISI316inalkalinemediaandtheeffectofpH:acombinedelectrochemical andanalyticalstudy,ElectrochimicaActa55(2010)6174.

[15]S.F.Yang,D.D.Macdonald,Theoreticalandexperimentalstudiesofthepitting oftype316Lstainlesssteelinboratebuffersolutioncontainingnitrateion, ElectrochimicaActa52(2007)1871.

[16]A.Fattah-Alhosseini,M.A.Golozar,A.Saatchi,K.Raeissi,Effectofsolution con-centrationonsemiconductingpropertiesofpassivefilmsformedonaustenitic stainlesssteels,CorrosionScience52(2010)205.

[17]R.-H.Jung,H.Tsuchiya,S.Fujimoto,XPScharacterizationofpassivefilms formedonType304stainlesssteelinhumidatmosphere,CorrosionScience 58(2012)62.

[18]Z.Feng,X.Cheng,C.Dong,L.Xu,X.Li,Passivityof316Lstainlesssteelinborate buffersolutionstudiedbyMott–Schottkyanalysis,atomicabsorption spec-trometryandX-rayphotoelectronspectroscopy,CorrosionScience52(2010) 3646.

[19] V.Vignal,O.Delrue,O.Heintz,J.Peultier,Influenceofthepassivefilmproperties andresidualstressesonthemicro-electrochemicalbehaviorofduplexstainless steels,ElectrochimicaActa55(2010)7118.

(10)

[20]H.Luo,C.F.Dong,K.Xiao,X.G.Li,Characterizationofpassivefilmon2205duplex stainlesssteelinsodiumthiosulphatesolution,AppliedSurfaceScience258 (2011)631.

[21]C.M.Abreu,M.J.Cristóbal,R.Losada,X.R.Nóvoa,G.Pena,M.C.Pérez, Compar-ativestudyofpassivefilmsofdifferentstainlesssteelsdevelopedonalkaline medium,ElectrochimicaActa49(2004)3049.

[22]L.Freire,X.R.Nóvoa,M.F.Montemor,M.J.Carmezim,Studyofpassivefilms formedonmildsteelinalkalinemediabytheapplicationofanodicpotentials, MaterialsChemistryandPhysics114(2009)962.

[23] C.Andrade,M.Keddam,X.R.Nóvoa,M.C.Pérez,C.M.Rangel,H.Takenouti, Electrochemical behaviour ofsteelrebarsin concrete:influence of envi-ronmental factors and cement chemistry, ElectrochimicaActa 46 (2001) 3905.

[24] C.M.Abreu,M.J.Cristóbal,R.Losada,X.R.Nóvoa,G.Pena,M.C.Pérez,Theeffect ofNiintheelectrochemicalpropertiesofoxidelayersgrownonstainlesssteels, ElectrochimicaActa51(2006)2991.

[25]L.Freire,M.J.Carmezim,M.G.S.Ferreira,M.F.Montemor,Theelectrochemical behaviourofstainlesssteelAISI304inalkalinesolutionswithdifferentpHin thepresenceofchlorides,ElectrochimicaActa56(2011)5280.

[26] B.Guitiàn,X.R.Nóvoa,B.Puga,ElectrochemicalImpedancespectroscopyasa toolformaterialsselection:waterforhaemodialysis,ElectrochimicaActa56 (2011)7772.

[27]S.S. El-Egamy, W.A.Badaway, Passivity and passivitybreakdown of 304 stainlesssteelinalkalinesodiumsulphatesolutions,JournalofApplied Elec-trochemistry24(2004)1153.

[28]J.-L.Trompette,L.Arurault,S.Fontorbes,L.Massot,Influenceoftheanion specificityontheelectrochemicalcorrosionofanodizedaluminumsubstrates, ElectrochimicaActa55(2010)2901.

[29]G.Blanco,A.Bautista,H.Takenouti,EISstudyofpassivationofausteniticand duplexstainlesssteelsreinforcementsinsimulatedporesolutions,Cement& ConcreteComposites28(2006)212.

[30]M.Sànchez-Moreno,H.Takenouti,J.J.García-Jare ˜no,F.Vicente,C.Alonso,A theoreticalapproachofimpedancespectroscopyduringthepassivationofsteel inalkalinemedia,ElectrochimicaActa54(2009)7222.

[31]C.M.Abreu,M.J.Cristóbal,R.Losada,X.R.Nóvoa,G.Pena,M.C.Pérez,High fre-quencyimpedancespectroscopystudyofpassivefilmsformedonAISI316 stainlesssteelinalkalinemedium,JournalofElectroanalyticalChemistry572 (2004)335.

[32] Y.P.Kim,M.Fregonèse,H.Mazille,D.Feron,G.Santarini,Studyofoxygen reductiononstainlesssteelsurfacesanditscontributiontoacousticemission recordedduringcorrosionprocesses,CorrosionScience48(2006)3945. [33]V.G.Levich,Thetheoryofconcentrationpolarization,ActaPhysicochimica

URSS17(1942)257.

[34]S.Frangini,N.DeCristofaro,Analysisofthegalvanostaticpolarizationmethod fordeterminingreliablepittingpotentialsonstainlesssteelsincrevice-free conditions,CorrosionScience45(2003)2769.

[35] G.Okamoto,Passivefilmof18-8stainlesssteelstructureanditsfunction, CorrosionScience13(1973)471.

[36] M.Sakashita,N.Sato,Theeffectofmolybdateanionontheion-selectivityof hydrousferricoxidefilmsinchloridesolutions,CorrosionScience17(1977) 473.

[37] H.H.Uhlig,Passivityinmetalsandalloys,CorrosionScience19(1979)777. [38]M.E.Orazem,N.Pébère,B.Tribollet,Anewlookatgraphicalrepresentationof

impedancedata,JournaloftheElectrochemicalSociety153(2006)B129. [39]B. Hirschorn, M.E.Orazem,B. Tribollet, V. Vivier,I. Frateur,M.Musiani,

Constant-phase-elementbehaviorcausedbyresistivitydistributionsinfilms: 2.Applications,JournaloftheElectrochemicalSociety157(2010)C458. [40]R.Kirchheim,B.Heine,H.Fischmeister,S.Hofmann,H.Knote,U.Stolz,The

passivityofiron-chromiumalloys,CorrosionScience29(1989)899. [41]E.Remita,E.Sutter,B.Tribollet,F.Ropital,X.Longaygue,C.Taravel-Condat,N.

Desamais,Athinlayercelladaptedforcorrosionstudiesinconfinedaqueous environments,ElectrochimicaActa52(2007)7715.