Passivation behavior of a ferritic stainless steel in concentrated alkaline solutions

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Availableonlineatwww.sciencedirect.com

Original

Article

Passivation

behavior

of

a

ferritic

stainless

steel

in

concentrated

alkaline

solutions

Arash

Fattah-alhosseini

,

Saeed

Vafaeian

DepartmentofMaterialsEngineering,Bu-AliSinaUniversity,Hamedan,Iran

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Received18June2014 Accepted4February2015 Availableonline12March2015

Keywords: Alkalinesolution Ferriticstainlesssteel Mott–Schottky Donordensity

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ThepassivationbehaviorofAISI430ferriticstainlesssteelwasinvestigatedinconcentrated alkalinesolutionsinrelationtoseveraltestparameters,usingelectrochemicaltechniques. IncreasingsolutionpH(varyingfrom11.5to14.0)leadstoanincreaseinthecorrosionrate ofthealloy.Mott–SchottkyanalysisrevealedthatpassivefilmsformedonAISI430ferritic stainlesssteelbehaveasn-typesemiconductorandthedonordensitiesincreasedwithpH. Electrochemicalimpedancespectroscopy(EIS)resultsshowedthatthereciprocal capaci-tanceofthepassivefilmisdirectlyproportionaltoitsthickness,whichdecreaseswithpH increase.Theresultsrevealedthatforthisferriticstainlesssteelinconcentratedalkaline solutions,decreasingthesolutionpHoffersbetterconditionsforformingpassivefilmswith higherprotectionbehavior,duetothegrowthofamuchthickerandlessdefectivefilm.

©2015BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevier EditoraLtda.Allrightsreserved.

1.

Introduction

Proton exchange membrane fuel cell (PEMFC) has widely attractedanattention.Bipolarplatesareimportant compo-nentsinPEMFCandstainlesssteelispopularlyusedasbipolar plate material.Inthe recentyears,many studies on stain-lesssteelbipolarplatematerialhavebeenonausteniticsteels [1–5].Also,thereweresomeworksontheeffectsofvarious factorssuchasacidity,fluorideions,temperatureand polar-izationpotentialonthecorrosionbehavioroftheaustenitic stainlesssteelbipolarplates[6–10].

∗ _{Correspondingauthor.}

E-mail:[email protected](A.Fattah-alhosseini).

Former investigations showed that the effectiveness of the austenitic stainlesssteels determined by the quantity ofCr,withhigherCrleading tobetterbehavior[6–9]. How-ever,austeniticstainlesssteelshaveahighNicontent,which increasestheircost.Generally,formerworksonthenatureof thepassivefilmformedonausteniticstainlesssteelsrevealed thatNiisnotamajorcomponentofit.ThisindicatesthatNi couldbeeliminatedwithoutchangingthepassivation effec-tiveness[5].Also,Windetal.[11]reportedthatNifromAISI 316Laustenitic stainlesssteelcan beamajorcontaminant inthemembrane,reducingthetotalconductivity.Thusfar, therehasbeenverylittleresearchonferriticstainlesssteels,

http://dx.doi.org/10.1016/j.jmrt.2015.02.003

whichhavelowornoNibutsimilarCrcontents.Therefore, theselectionofaferriticstainlesssteelwaschosenforthis work.

Inthepolymerelectrolytemembranefuelcell,the interfa-cialcontactresistanceisinfluencedbythematerialproperty, surfacetopology,operationconditionandthepropertyofthe passivefilmsuchasthecompositionandthickness[12].Of late,there havebeen severalmethods tomodifythe prop-ertyofthepassivefilm(suchastheelectrochemicalsurface treatment[13]andtheadditionofthealloyingelement[14]). Moreover,the immersioninthe alkalinesolution isoneof theusefulmethodstoimprovethepropertiesofthepassive film.InNaOHsolutions,thepassivefilmtransformsfromiron oxide-richfilmtopassivefilmenrichedwithchromiumoxide anddepletedinironoxide[12,15].

Generally, increasing research on the electronic proper-tiesofpassivefilmsformedonstainlesssteelshasgivenan importantcontributiontotheunderstandingofthecorrosion behaviorofthesealloys.Inpractice,thepassivefilm compo-sitionvarieswiththesolutionpHusedforfilmformationand thisisexpectedtoaffectthesemiconductingpropertiesofthe passivefilm[16,17].Accordingtothepointdefectmodel(PDM) [18–20],thegrowthofthepassivefilminvolvesthemigration ofthesepointdefectsundertheinfluenceoftheelectrostatic field inthe film. Thus, the key parametersin determining thetransportofpointdefectsandhencethekineticsoffilm growthisthedensityandthediffusivityofthedefectsinfilm. Inthelastdecade,byemployingtheMott–Schottkyanalysis inconjunctionwiththePDM,thepointdefectsdensityand diffusivityforsomemetalsandalloyshavebeendetermined [18–20].

Inthiswork,the EISandMott–SchottkyanalysisofAISI 430ferriticstainlesssteelinNaOHsolutionsatopencircuit potential(OCP)wasperformedandthepassivation parame-tersanddefectsconcentrationswerecalculatedasafunction ofpHsolution.Also,therelationshipbetweenthedonor den-sityandpHsolutionarediscussedinordertounderstandthe passivationcharacteristicsofAISI430ferriticstainlesssteel.

2.

Experimental

procedures

ThechemicalcompositionofAISI430ferriticstainlesssteel isshowninTable1.Allsamplesweregroundto2000gritand cleanedwithdistilled waterpriortotests. Aeratedalkaline solutions(withoutpurgingoxygenoranygas)withNaOHand distilledwaterwerepreparedatdifferentpH(11.5,12.0,12.5, 13.0,13.5,and14.0).

Theelectrochemicalmeasurementswereperformedinthe followingsequence:

(a) Potentiodynamic polarization curves were measured potentiodynamically at ascan rate of1mVs−1 _starting

from−0.25V(vs.OCP).

(b) EIStestatOCPDCpotentialwithACpotential perturba-tionamplitudeof10mVandfrequencyrangeof100kHz to10mHz.

(c) Mott–Schottky analysis was carried out on the passive filmsatafrequencyof1kHzusinga10mVacsignalanda steppotentialof25mV,inthecathodicdirection.

Log i (i/A cm–2₎ Log i (i/A cm–2₎ E / V Ag/AgCl E / V Ag/AgCl –0.7 –0.5 –0.3 –0.1 0.1 0.3 0.5 –3 –4 –5 –6 –7 –8 –9 AISI 430 - pH=11.5 AISI 430 - pH=12.0 AISI 430 - pH=12.5 –0.8 –0.6 –0.4 –0.2 0.0 0.2 0.4 –3 –4 –5 –6 –7 –8 –9 AISI 430 - pH=13.0 AISI 430 - pH=13.5 AISI 430 - pH=14.0

Fig.1–PotentiodynamicpolarizationcurvesofAISI430 stainlesssteelinNaOHsolutionswithpHvaryingfrom11.5 to14.0.

Before all electrochemical tests, the working electrodes wereimmersedintheinvestigatedsolutionsatOCPtoform a steady-state passive film. All electrochemical measure-mentswereperformedinaconventionalthree-electrodeflat cell. The counter electrode was a Pt plate, and all poten-tials were measured againstAg/AgCl in saturated KCl. All electrochemicalmeasurementswereobtainedusingAutolab potentiostat/galvanostatcontrolledbyapersonalcomputer. FortheEISdatamodelingandcurve-fittingmethod,theNOVA impedancesoftwarewasused.

3.

Results

and

discussion

3.1. Potentiodynamicpolarizationmeasurements

Fig.1showsthepotentiodynamicpolarizationcurvesofAISI 430ferriticstainlesssteelinNaOHsolutionswithpH vary-ing from11.5to14.0.By comparingthepolarizationcurves forthisstainlesssteelindifferentpHsolutions,thecurrent densitywasfoundtoincreasewithpotentialduringthelow polarizationrangeandnoobviouscurrentpeakwasobserved. Also,allcurvesexhibitsimilarfeatures,withabroadpassive rangeuptotheonsetoftranspassivity.

Tafelextrapolation methodis widelyused forthe mea-surementofthecorrosionrate.Bythismethod,thecorrosion currentdensity(icorr)wascalculatedofthelinearpartforthe

Table1–ChemicalcompositionofAISI430ferriticstainlesssteel. Cr Ni Mo Mn Si C P Cu N Co Fe AISI430/wt% 16.50 0.13 0.02 0.53 0.50 0.05 0.025 0.07 0.06 0.02 Bal 0.1 0.2 0.3 0.4 0.5 0.6 0.7 14.0 13.5 13.0 12.5 12.0 11.5 pH icorr / μ A cm –2

Fig.2–EffectofsolutionpHonthecorrosioncurrent

densityofAISI430stainlesssteel.

cathodicbranchbacktothemixedpotentialofzeronetcurrent (Ecorr)withaccuracyofmorethan95%forthepointsmore neg-ativetoEcorrby50mV[21–23].Fig.2showstheeffectofsolution pHonthecorrosioncurrentdensityofAISI430stainlesssteel. Itisevidentfromthisfigurethatthecorrosioncurrentdensity increasedlinearlywithpH.

3.2. Mott–Schottkyanalysis

Fig.3showstheMott–SchottkyplotsofAISI430ferritic stain-lesssteelinNaOHsolutionswithpHvaryingfrom11.5to14.0. Itshould benoted that forthis stainlesssteel, C−2 _clearly

decreasewithincreasingpH.Inthisfigure,thepositiveslopes inthemainpassiveregionareattributedton-typebehavior. AccordingtoEq.(1),thedonordensityhasbeendetermined fromthepositiveslopesinthemainpassiveregion[24–26]:

1 C2 = 2 εε0eND

forn-typesemiconductor (1)

whereeistheelectroncharge,NDisthedonordensityfor n-typesemiconductor(cm−3_{),εisthedielectricconstantofthe}

C –2/ 10 10 F-2 cm 4 E / V_Ag/AgCl 0.0 0.4 0.8 1.2 1.6 –0.6 –0.4 –0.2 0.0 0.2 0.4 0.6 AISI 430 - pH=11.5 AISI 430 - pH=12.0 AISI 430 - pH=12.5 AISI 430 - pH=13.0 AISI 430 - pH=13.5 AISI 430 - pH=14.0

Fig.3–Mott–SchottkyplotsofAISI430stainlesssteelin NaOHsolutionswithpHvaryingfrom11.5to14.0.

passivefilm(usuallytakenas15.6),ε0isthevacuum

permittiv-ity,kistheBoltzmannconstant,Tistheabsolutetemperature andEFBistheflatbandpotential.Fig.4showsthecalculated

donordensitiesforthepassivefilmsformedonAISI430ferritic stainlesssteelinNaOHsolutions. Theorders ofmagnitude arearound1021_cm−3 _{andarecomparabletothosereported}

inother studies[27].AccordingtoFig.4,the donordensity increases with pH. Based on PDM [19], the flux of oxygen vacancyand/orcationinterstitialsthroughthepassivefilmis essentialtothefilmgrowthprocess.Inthisconcept,the dom-inantpointdefectsinthepassivefilmareconsidered tobe oxygenvacanciesand/orcationinterstitialsactingaselectron donors.

3.3. EISmeasurements

TheEISresponseofAISI430ferriticstainlesssteelinNaOH solutionswithpHvaryingfrom11.5 to14.0 wasperformed and theresultsare presentedasNyquistandBodeplotsin Fig. 5. For this stainless steel, the Nyquist and Bode plots showaresistivebehaviorathighfrequencies,butinthe mid-dle tolow frequencyrange there wasa marked capacitive response. The Bode-phasecurves show one time constant (onlyonemaximumphaselagatthemiddlefrequencyrange). Thephaseanglevaluesremainedverycloseto80◦.This evo-lutionrevealedthe formationand growthofapassivefilm. Also,therewasadecreaseinlowfrequencyimpedancewith increasingpH.

Based on theseresults, the equivalent circuit shown in Fig. 6was usedtosimulate the measuredimpedance data ofAISI430ferriticstainlesssteelinNaOHsolutionswithpH varyingfrom11.5to14.0.Thisequivalentcircuitiscomposed of:Rs–solutionresistance;Qpf–constantphaseelement

cor-respondingtothecapacitanceofthepassivefilm;andRpf–

resistanceofthepassivefilm.Thisequivalentcircuithas pro-videdbestfittingfortheimpedancedataasshowninFig.5. This equivalent circuit composed byone time constant as

0.615 0.709 0.859 1.267 1.785 2.401 0.5 1.0 1.5 2.0 2.5 pH=14.0 pH=13.5 pH=13.0 pH=12.5 pH=12.0 pH=11.5 ND / 10 21 cm–3

Fig.4–EffectofsolutionpHonthedonordensityofthe passivefilmsformedonAISI430stainlesssteel.

0 10 000 20 000 30 000 40 000 50 000 100 000 80 000 60 000 40 000 20 000 0 AISI 430 - pH=11.5 AISI 430 - pH=12.0 AISI 430 - pH=12.5 AISI 430 - pH=13.0 AISI 430 - pH=13.5 AISI 430 - pH=14.0 Simulated - pH=11.5 Simulated - pH=12.0 Simulated - pH=12.5 Simulated - pH=13.0 Simulated - pH=13.5 Simulated - pH=14.0 1 10 100 1000 10 000 100 000 100 000 10 000 1000 100 10 1 0.1 0.01 AISI 430 - pH=11.5 AISI 430 - pH=12.0 AISI 430 - pH=12.5 AISI 430 - pH=13.0 AISI 430 - pH=13.5 AISI 430 - pH=14.0 Simulated - pH=11.5 Simulated - pH=12.0 Simulated - pH=12.5 Simulated - pH=13.0 Simulated - pH=13.5 Simulated - pH=14.0 0 10 20 30 40 50 60 70 80 100 000 10 000 1000 100 10 1 0.1 0.01 AISI 430 - pH=11.5 AISI 430 - pH=12.0 AISI 430 - pH=12.5 AISI 430 - pH=13.0 AISI 430 - pH=13.5 AISI 430 - pH=14.0 Simulated - pH=11.5 Simulated - pH=12.0 Simulated - pH=12.5 Simulated - pH=13.0 Simulated - pH=13.5 Simulated - pH=14.0 -Z imag / Ω cm 2 Z / Ω cm 2 Zreal / Ωcm 2 Frequency / Hz

Phase shift / deg

Frequency / Hz

a

b

c

Fig.5–(a)Nyquist,(b)Bodeand(c)Bode-phaseplotsofAISI 430stainlesssteelsinNaOHsolutionswithpHvarying from11.5to14.0.

proposedbyFengetal.[28,29]todescribethepassive behav-ior of AISI 316L stainless steel in alkaline solutions. The impedanceoftheconstantphaseelementispresentedusing Eq.(2):

ZQ=[C(jω)n]−1 (2)

R

R

C

Q

Fig.6–Thebestequivalentcircuittestedtomodelthe experimentalEISdatawithonetimeconstant.

111.65 89.61 50.15 44.12 32.16 21.56 20 40 60 80 100 120 pH=14.0 pH=13.5 pH=13.0 Rpf / kΩ cm2 Cpf / μF cm–2 pH=12.5 pH=12.0 pH=11.5 30 40 50 60 70 pH=14.0 pH=13.5 pH=13.0 pH=12.5 pH=12.0 pH=11.5

Fig.7–EffectofsolutionpHonthepassivefilmresistance andpassivefilmcapacitanceofAISI430stainlesssteelin NaOHsolutions.

where nis associatedwith the roughness ofthe electrode surface[28,29].Fig.7showstheeffectofsolutionpHonthe passivefilmresistanceandcapacitanceofAISI430stainless steel.AscanbeseeninFig.7,forthisstainlesssteel,passive film resistancedecreases withincreasingpH whilepassive filmcapacitanceincreases.Accordingtotheequivalentcircuit showninFig.6,thepassivefilmthickness(d)canbecalculated usingEq.(3)[30]:

d= εε0

C (3)

whereCisthetotalcapacitanceofthepassivefilm. Gener-ally,achangeinthetotalcapacitanceofthepassivefilmcan beusedasanindicatorforchangeinthepassivefilm thick-ness.Therefore,thereciprocalcapacitanceofthepassivefilm (1/C)isproportional toits thickness, whichdecreases with increasingpH.

Fig.8showsalinearrelationshipbetweenthepassivefilm thicknessandthesolutionpH.AscanbeseeninFig.8,the calculatedthicknessrangesfromabout0.132nmatpH=11.5 to0.081nmatpH=14.0.Thesevaluesofthethicknessare con-sideredtobeeminentlyrealistic[31].Itisclearthatdecreasing thesolutionpHgivebetterconditionsforformingthepassive

0.132 0.121 0.109 0.098 0.088 0.081 0.08 0.10 0.12 0.14 pH=14.0 pH=13.5 pH=13.0 pH=12.5 pH=12.0 pH=11.5 d / nm

Fig.8–EffectofsolutionpHonthepassivefilmthickness ofAISI430stainlesssteelinNaOHsolutions.

filmswithhigherprotectionbehavior,duetothegrowthofa muchthickerandlessdefectivepassivefilms.

4.

Conclusions

TheeffectsofsolutionpHonthepassivationbehaviorofthe passivefilmformedonAISI430ferriticstainlesssteelatOCP inconcentrated alkalinesolutionswere investigatedinthe presentwork.Conclusionsdrawnfromthestudyareas fol-lows:

1. Potentiodynamicpolarizationcurvesshowedthatthe cor-rosioncurrentdensityofAISI430stainlesssteelincreased withincreasingpH.

2. Mott–Schottkyanalysisrevealedthatpassivefilmsformed onAISI430stainlesssteelbehaveasn-typesemiconductor andthedonordensityincreasedwithincreasingpH. 3. NyquistandBodeplotsshowaresistivebehaviorathigh

frequencies,butinthemiddletolowfrequencyrange,there wasamarkedcapacitiveresponse.

4. Bode-phasecurvesshowonetimeconstant(onlyone max-imumphaselagatthemiddlefrequencyrange).Thephase anglesvaluesremainedverycloseto80◦. Thisevolution revealedtheformationandgrowthofapassivefilm. 5. EISresultsshowedthatthereciprocalcapacitanceofthe

passivefilmdecreasedwithincreasingpH.

6. Also,EISresultsshowedthatdecreasingthesolutionpH offered betterconditions forforming passive films with higherprotectionbehavior,duetothegrowthofamuch thickerandlessdefectivefilm.

Conflict

of

interest

Theauthorsdeclarenoconflictsofinterest.

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