Influence of sulphate-reducing bacteria on environmental parameters and marine corrosion behavior of

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ElectrochimicaActa

journalhomepage:www.elsevier.com/locate/electacta

Influenceofsulphate-reducingbacteriaonenvironmentalparametersandmarinecorrosionbehaviorofQ235steelinaerobicconditions

YiWana,b,DunZhanga,∗,HuaiqunLiua,b,YongjuanLia,b,BaorongHoua

ab

KeyLaboratoryofCorrosionScience,ShandongProvince,InstituteofOceanology,ChineseAcademyofSciences,7NanhaiRoad,Qingdao266071,ChinaGraduateSchooloftheChineseAcademyofSciences,19(Jia)YuquanRoad,Beijing100039,China

articleinfoabstract

Thegrowthcycleofsulphate-reducingbacteria(SRB),Desulfovibriocaledoniensis,andtheeffectofSRBontheenvironmentalparametersandcorrosionbehaviorofQ235steelduringagrowthcycleinaero-bic(air-andO2-saturatedculturesolutions)andanaerobic(N2−saturatedculturesolutions)conditionswereinvestigated.OxygendissolvedintheculturesolutionsinducedslowgrowthandfastdecayofSRB.ThegrowthprocessofSRBunderanaerobicandaerobicconditionsinfluencedsulphideanionconcentra-tion(Cs2−),pH,andconductivity(Ä).ThevaluesofCs2−andÄunderaerobicconditionswerelowerthanthoseunderanaerobicconditions,andthepHvaluesincreasedfromO2-toair-toN2-saturatedculturesolutions.Aerobicconditionsinducedtheopencircuitpotential(EOC)toshiftinthepositivedirectionafterthestationaryphaseofSRBgrowth.Thechargetransferresistance(Rct)increasedquicklyduringtheexponentialgrowthphase,almostmaintainedstabilityduringthestationaryphase,anddecreasedafterthestationaryphaseinallthreeconditions,andtheimpedancemagnitudedecreasedfromO2-toair-toN2-saturatedculturesolutions.ThebiofilmsinducedbySRBwereobservedbyscanningelectronmicroscopy(SEM)underaerobicandanaerobicconditions,andenergydispersivespectroscopy(EDS)wasperformedinabioticandSRB-containingsystemstodistinguishthecorrosionproducts.ThereasonsfortheeffectsofSRBontheenvironmentalparametersandcorrosionbehaviorofcarbonsteelarediscussed.

Crown Copyright © 2009 Published by Elsevier Ltd. All rights reserved.

Articlehistory:

Received23July2009

Receivedinrevisedform4October2009Accepted5October2009

Available online 13 October 2009Keywords:

Sulphate-reducingbacteriaAerobicconditionMarinecorrosionQ235carbonsteel

Environmentalparameters

1.Introduction

Sulphate-reducingbacteria(SRB)areubiquitousinthemarineenvironment,wheretheyusesulphateasaterminalelectronaccep-torfordegradationoforganisms,resultingintheproductionofsulphide[1].TherelevantactivitiesofSRBinducetherock-ribbedproblembecauseoneofmetabolicproducts,sulphide,ishighlycorrosive,toxic,andreactive.Inaccordancewiththis,SRBcausecorrosionofsteel,whichhasledtoseriousenvironmentalproblemsandenormouseconomiclosses[2].

SRBgenerallyarerecognizedasanaerobicmicroorganisms,butitwasrecentlyreportedthattheycanbearoxygenconcentrationsofupto1.5mMusingseveraldefensestrategies[3].SRBpos-sessvariousself-protectionenzymesthatfacilitatesurvivalduringperiodsofoxygenexposure.SomeSRBcangeneratetherelevantsuperoxidedismutaseandcatalaseenzymesnecessaryforscaveng-ingthetoxicanddeleteriousreduceddioxgenspecieshydrogenperoxideandsuperoxide[4–8].AnotherimportantpointforthesurvivalofSRBintheoxicsystemsisthatcytochromebdoxi-daseandcytochromecoxidasearetheterminalmembraneoxygenreductasescapableofconsumingoxygen[9–11].Inaddition,somespeciesofSRBinthemarineenvironmentcoexistwiththeaerobicnativemicrobialcommunity[12,13].

Sincethe1960s,considerableeffortshavebeenmadetoinvesti-gatethemechanismsofmicrobiallyinfluencedcorrosion(MIC)bySRB.SeveralmechanismsofMICofmetalmaterialsunderanaerobicconditionarewidelyaccepted,suchasthecathodicdepolariza-tiontheory[14],thelocalcorrosionmechanism[15],andcorrosionbehaviorofmetabolicproducts[16,17].Moreover,inourprevi-ousstudywereportedtheeffectofthegrowthprocessofSRBonthecorrosionbehaviorofcarbonsteelinanaerobicconditions[18].Currently,theinfluenceofbiofilmsontheMICofcarbonsteelinartificialseawaterconditionshasgainedincreasingattentionamongresearchers[13,19–21].TheelectrochemicalreactionsthatoccurduringmetalcorrosioninthepresenceofaSRBbiofilmareasfollows[14]:Theanodicreaction:

∗Correspondingauthor.Tel.:+865328260;fax:+865328260.E-mailaddress:zhangdun@ms.qdio.ac.cn(D.Zhang).

Fe→Fe2++2e−

(1)

0013-4686/$–seefrontmatter.Crown Copyright © 2009 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2009.10.009

Y.Wanetal./ElectrochimicaActa55 (2010) 1528–15341529

Thecathodicreactions:2H2O+2e−→2Had+2OH−(2)SO42−+8Had→S2−+4H2O

(3)

Reactionsforthecorrosionproduct:Fe2++S2−→FeS

(4)3Fe2++6OH−→3Fe(OH)2

(5)SRBconsumetheatomichydrogenaccumulatedatthecathodeandusesulphateastheelectronacceptorandreduceittothesul-phide.Finally,theproductionofsulphidefromSRBmetabolismandferrousionfromanodicdissolutionleadtotheformationofthecorrosionproductsFeSandFe(OH)2.

ToexplorethegrowthofSRBandtheeffectofSRBontheenvi-ronmentalparametersandcorrosionbehaviorsofcarbonsteelinagrowthcycleunderaerobicconditions,wemeasuredthesulphideanionconcentration(Cs2−),pH,andconductivity(Ä)oftheresearchsystem.Wealsousedscanningelectronmicroscopy(SEM),energydispersivespectroscopy(EDS),theopencircuitpotential(Eoc),andelectrochemicalimpedancespectroscopy(EIS)toinvestigatetheeffectsofSRBoncarbonsteel.2.Experiments

2.1.Materialsandreagents

Q235steel(ASTMA284GradeD)withamasspercentofelementsof0.18C,0.22Si,0.60Mn,0.02S,and0.016Pwascutintocylinders(5mmdiameterand5mmheight)foruseasworkingelectrodes.Thecylinderswereembeddedinamoldofnon-conductingepoxyresin.Electricalconnectionwasviaacopperwiresolderedontothetipofthesample.

Aseriesofchemicals,includingmagnesiumsulphate(Shang-haiMeixingChemicalCo.,Ltd.),ammoniumchloride(ShanghaiYunlingChemicalPlants),sodiumsulphate(SinopharmChemi-calReagentCo.,Ltd.),calciumchloride(TianjinBASFAuxiliaryChemicalsCo.,Ltd.),dipotassiumhydrogenphosphate(TianjinGuangchengChemicalReagentCo.,Ltd.),sodiumhydroxide(Tian-jinRgentChemicalsCo.,Ltd.),sodiumlactate(TianjinFuchenChemicalReagentFactory),andyeastextract(BeijingAoboxingBiotechCo.,Ltd.),wereusedtopreparethemodifiedPostgate’sculturemedium.Ultra-highpuritynitrogenandoxygen(>99.999%,QingdaoHeliGasCo.,China)wereusedforthepreparationoftheN2-andO2-saturatedculturesolutions,respectively.2.2.ThecultureofSRB

TheseedbacteriawereisolatedfrommarinesludgecollectedfromtheBohaiSeaofChina.ThepurifiedseawaterusedinthisworkwascollectedfromHuiquanBayinQingdao,China.

ThemodifiedPostgate’sculturesolutioncontained2.0gmag-nesiumsulphate,0.5gdipotassiumhydrogenphosphate,1.0gammoniumchloride,0.5gsodiumsulphate,0.1gcalciumchlo-ride,1.0gyeastextract,and2.0mlsodiumlactatein1lseawater.ThepHvaluewasadjustedto7.0usingtheappropriateamountofsodiumhydroxidebeforethesolutionwasautoclavedat121◦Cfor20min.Aftercooling,theSRBculturewasincubatedinsterilizedglassbottlesaeratedusingN2,air,andO2gasesfor1hat30◦Cinatemperatureincubator.2.3.SEMandEDSanalysis

SEMandEDSanalysiswereconductedatthesurfaceofthecarbonsteelafter11daysofexposuretoartificialseawatersup-

plementedwithnutrients,underbothbioticandabioticconditions.ForthecarbonsteelsoakinginSRB-containingmedium,SRB-coatedcarbonsteelhadtobefixedin5%glutaraldehydefor3handthenwashedwithPBSandMili-Qwater.Thesamplewasthenimmersedinagradientseriesofethanol(50%,75%,and99%)solutionsfordehydration.TheseexperimentswereperformedusingaJeolJSM5900LVscanelectronmicroscope(Tokyo,Japan)atanaccelerationof25kV.

2.4.MeasurementsofthenumberofSRBandofenvironmentalparameters

TheexperimentswereperformedtoillustratethegrowthcurveofSRBandtodeterminethechangesinenvironmentalparameters(e.g.,Cs2−,pH,andÄvalues)thatoccurredduringtheSRBgrowthprocessinN2-,air-,andO2-saturatedculturesolutions.A5mlsam-pleofSRBseedswasfirsttransferredintoa250mlsterilizedbottlecontaining200mlofculturesolution.TheculturesolutionsweresubsequentlyaeratedwithN2,air,orO2for1heachdaytoensurethattheculturesystemwaskeptsaturated.Followingtheaeration,thebottlesweresealedimmediatelywitholefinandincubatedinaconstanttemperatureincubatorat30◦C.Meanwhile,thenum-berofactiveSRB(NSRB)intheN2-,air-,andO2-saturatedculturesolutionswasestimatedusingthemostprobablenumber(MPN)methodaccordingtotheAmericanSocietyofTestingMaterials(ASTM)standardD4412-84.

TheCs2−valuesweredetectedusingasulphide-selectiveelec-trode(ShanghaiKangyiInstrumentCo.,Ltd.)accordingtotheASTMstandardD4658-03.ThevaluesofpHandÄinthethreesystemsweremeasuredusinganOrion5starmeterpurchasedfromThermoFisherCompany.

2.5.Electrochemicalexperiments

TheEocandEISexperimentswereconductedinacellwiththreeelectrodesusingaCHI760C(CHInstruments,Inc.)controlsystemintheN2-,air-,andO2-saturatedculturesolutionsduringtheSRBgrowthprocess.Theelectrodesusedwereasfollows:aQ235steel(5mmdiameter)workingelectrode;aPtwirecounterelectrode;andasilver/silverchloride(Ag/AgCl,3MKCl)referenceelectrode(CHInstruments,Inc.).TheresultsofEISwereanalyzedbyfittingthedatausingZsimpwinsoftware.

Beforetheexperiments,theQ235steelelectrodewaspolishedwith3000#and5000#siliconcarbidepaperand1␮mand0.05␮maluminapowder.TheelectrodethenwasultrasonicallycleanedinMili-Qwaterfor10min.Allelectrochemicalexperimentswereperformedat25±2◦C.3.Resultsanddiscussion3.1.ThegrowthcurvesofSRB

Fig.1showsthegrowthcurvesofSRBinN2-,air-,andO2-saturatedculturesolutions.ThegrowthcurvesofSRBinallthreeconditionsdisplayedatypicalthree-stagegrowthcycle,withanexponentialgrowthphase,astationaryphase,andadeathphase.Comparedtotheanaerobiccondition(N2-saturated,curvea),SRBintheaerobicconditions(curvesbandc)exhibitedaslowgrowthrateandafastdecayrateduringthegrowthcycle.Theexperimen-talresultsagreewiththeenumerationofSRBinanO2-saturatedconditionreportedbyKrekeleretal.[22].

TheseresultsillustratethetoxicityofoxygentoSRB.TheeffectofoxygenusuallyisrelatedtothesensitivityofseveralproteinsinSRB,suchashydrogenases[23],aldehydedehydrogenase[24],andNAD-dependentalcoholdehydrogenase[25].SRBuseseveralstrategies

1530Y.Wanetal./ElectrochimicaActa55 (2010) 1528–1534

Fig.1.ThegrowthcycleofSRBinN2-saturated(a),air-saturated(b),andO2-saturated(c)culturesolutions.

todealwithexposuretooxygen,thesestrategiesaccordingtoCyp-ionka’sreport[26]canbedividedintobehavioralstrategiesandmolecularmechanisms.Theformerincludesaggregateformationandaerotaxis,bywhichSRBareabletoprotectthemselvesfromfulloxygenexposurebyformingco-culturesoraggregates;thelatterincludesmechanismssuchasoxygenreductionandreactiveoxy-genspeciesdetoxification,whichallowSRBtoscavengeoxygenmoleculesandpreventthemselvesfromexperiencingdetrimentaleffects.

3.2.SEMandEDSanalysis

Fig.2showsthemicrographsoftheSRBbiofilmsthatdevel-opedonQ235steelthatwasimmersedinSRBmediuminN2-,air-,andO2-saturatedculturesolutionsduringagrowthcycle.ThebiofilmthatformedintheN2-saturatedculturesolutioniscom-pactandrelativelyporous,withanet-likestructure(Fig.2A).Incontrast,thebiofilmsthatformedunderaerobicconditionsexhibitahomogeneousandloosestructure(Fig.2BandC)becausethepro-liferationandmetabolicactivitiesofSRBunderaerobicconditionswereinhibited,resultinginchangesinthethicknessanddensityofbiofilm.

Fig.3AandBshowtheresultsoftheSEMandEDSanalysisofthelayerthatformedoncarbonsteelmaintainedinSRB-free(abiotic)mediuminanair-saturatedculturesolutionfor12days.SEManalysisrevealedthepresenceofsomefloccules(ferricoxide)withaheterogeneousdistributionattachedtothecarbonsteel.Thepresenceofsulphur-basedcompoundswasnegligibleintheabioticsystem(Fig.3B).

SEMandEDSanalysisalsoillustratedthemorphologicalandchemicalcompositionofthelayerthatformedoncarbonsteelexposedtoSRB-containingmediuminanair-saturatedculturesolutionfor12days.IntheSEMimage(Fig.3C),ahighconcentrationofcellsisvisible,alongwiththeinitialformationofpolysaccha-ridesthathaveadefinedstructureandasmoothsurfaceandareinteractingwiththemicroorganisms.Inaddition,alayerofthecorrosionproduct(ironsulphide),withvisiblesphericalgranules,wasgenerated.EDSanalysis(Fig.3D)furtherindicatedthattheregionwascomposedofahighamountofphosphates,carbohy-drates,andsulphide-basedcompoundsderivedfrombacterialcellsaswellaspolysaccharidesandcorrosionproducts(ironsulphide).Theseresultsprovidedapriorinformationthatwereusedtocreateathreetimeconstantelectrochemicalmodel.3.3.Variationsoftheenvironmentparameters

Fig.4illustratesthevariationofCs2−valueswithSRBgrowthinN2-,air-,andO2-saturatedculturesolutions.TheCs2−valuesinthe

N2-saturatedculturesolutionwerehigherthanthoseintheO2-andair-saturatedculturesolutionsduringthegrowthcycle.Moreover,theshapeofthecurvesfortheCs2−variationwasanalogoustotheshapeoftheSRBgrowthcurves.TheseresultsdemonstratethattheCs2−valuesareinfluencedbytheactivityofSRBviavariationintheSRBmetabolicproduct.

Fig.5showsthechangesthatoccurredinpHvaluesinN2-,air-,andO2-saturatedculturesolutionsduringSRBgrowth.pHvaluedecreasedabruptlyintheN2-saturatedculturesolutionwithbac-terialgrowthandthenremainedconstantatabout6.1.AlthoughthevariationsinpHfortheotherconditionsalsoexhibitedasimilarpattern,thepHvaluesinair-andO2-saturatedculturesolutionsremainedatabout6.3and6.5,respectively,andwerehigherthanthatintheN2-saturatedcondition.ThediscrepancyinpHvaluesbetweenanaerobicandaerobicconditionsmaybeduetothedifferenceintheaccumulationoforganicacidsbasedonthedifferentnumberofactiveSRBunderthethreedifferentconditions.

ThevaluesofÄduringaSRBgrowthcycleforthethreecul-tureconditionsareshowninFig.6.Inallthreecases,theÄvalueincreasedwithincreaseintheconcentrationofSRBduringtheexponentialgrowthphase,decreasedwiththedecreaseofSRBinthedeathphase,andaresteadyavaluethathigherthantheoriginoftheSRBgrowth.ThispatternoccurredbecauseduringthegrowthcycletheSRBpopulationconsumesyeastandsodiumlaurateandproducescopiousinorganicions.

Fig.2.SEMimagesofanSRBbiofilmobtainedinamediuminN2-saturated(A),air-saturated(B),andO2-saturated(C)culturesolutionsduringagrowthcycle.

Y.Wanetal./ElectrochimicaActa55 (2010) 1528–15341531

Fig.3.SEMimagesofcarbonsteelexposedtoanabiotic(A)andSRB-containing(C)systeminanair-saturatedcondition.Energydispersivespectroscopy(EDS)analy-siscorrespondingtotheSEMregionundertheabiotic(B)andSRB-containing(D)system.

Fig.4.Thechangesinthesulphideanionconcentration(Cs2−)thatoccurredinN2-saturated(a),air-saturated(b),andO2-saturated(c)culturesolutionsduringSRBgrowth.Fig.5.ThechangesinpHvaluesthatoccurredinN2-saturated(a),air-saturated(b),andO2-saturated(c)culturesolutionsduringSRBgrowth.

Fig.6.Thevariationsinconductivity(Ä)thatoccurredinN2-saturated(a),air-saturated(b),andO2-saturated(c)culturesolutionsduringSRBgrowth.

3.4.CorrosionbehaviorofQ235steel

3.4.1.Opencircuitpotential(EOC)

Fig.7showstheEOCobtainedforQ235steelversusimmersiontimeinN2-,air-,andO2-saturatedculturesolutionsbothwithandwithoutSRBduringagrowthcycle.Intheabioticenvironment,theEOCvaluesexhibitedaminornegativeshiftandquicklyreachedastationarystateinallthreecultureconditions.Incontrast,inallthreecultureconditionswithSRB,theEOCvaluesshiftedrapidlyinthenegativedirectionfromthefirsttothesixthday.Afterthat,theEOCmaintainedaconstantvalueofc.a.−0.65VintheN2-saturatedculturesolution.Intheair-saturatedsolution,theEOCremainataminimumvalueofabout−0.Vfromthesixthtothe10thday,

Fig.7.TheEOCvaluesobtainedfromQ235steelinN2-saturated(a0,a1),air-saturated(b0,b1),andO2-saturated(c0,c1)culturesolutionswithout(a0,b0,c0)andwith(a1,b1,c1)SRBfor12days.

1532Y.Wanetal./ElectrochimicaActa55 (2010) 1528–1534

thenbouncedbackto−0.60Vfromthe10thtothe14thday.IntheO2-saturatedculturesolution,theEOCshiftedquicklyinthepositivedirectionandalmoststabilizedatc.a.−0.58Vfromthesixthtothe14thday.

Theshifttowardsamorenegativepotentialwithin6daysofexposureinthethreeaeratedculturesolutionswassimilartothetrendinEOCvariationspreviouslyreportedforsteelmaterialinSRBculture[27,28].TheinitialdropinpotentialcanbeattributedtothedecreaseinpHvalues(Fig.4)thatwascausedbythegenerationoforganicacidgeneratedbytheactiveSRB.

ThepatternsofvariationinEOCvaluesafterthesixthday(thestationaryphaseofSRBgrowth)differedamongthethreecondi-tions.IntheN2-saturatedculturesolution,theSRBbiofilmonthesurfaceofQ235steelprovidedasteadymicroenvironmentthatwasnotinfluencedbyexternalenvironmentalfactors;thusstableEOCvalueswererecorded.However,inthetwoaerobicconditions,theEOCshiftedinapositivedirectionbyc.a.0.04V(air-saturated)and0.05V(O2-saturated).TheresultsareparalleltothosereportedbyWashizuetal.[29].TheyalsoobservedthatEOCvaluesshiftedinapositivedirectionbyc.a.0.05Vat31◦C,andtheyreportedthattheenrichmentofH2O2inthebiofilmproducedbySRBisanessentialfactoroftheennoblementofEOCforsteelinseawater.Inaddition,thecatalyticeffectofSRBenzymesoncathodicreactionkineticsmayalsoinducetheennoblementofEOC[30,31].

3.4.2.Electrochemicalimpedancespectroscopy

Fig.8AshowsNyquistplotsobtainedforQ235steelintheair-saturatedculturesolutionwithoutSRB;itillustratesthatthemagnitudeofimpedancedecreasedwithtime.Comparedtotheimpedancemagnitudesobtainedintheair-saturatedculturesolu-tionwithSRB(Fig.8B),theimpedancemagnitudeinair-saturatedculturesolutionwithoutSRBwasfarlower.Thisresultindicatesthatferricoxidesserveasstrongcathodestoacceleratetheoxida-tionofiron[32].

Fig.8BshowsNyquistplotsobtainedforQ235steelintheair-saturatedculturesolutionduringaSRBgrowthcycle.Themag-nitudeofimpedancevariedwiththeSRBgrowthprocess.Clearly,theimpedancerepresentedbythediameterofthesemicircleincreasedduringtheexponentialgrowthphase,almostmaintainedstabilityduringthestationaryphase,anddecreasedafterthesta-tionaryphase.Theincreaseofimpedanceduringtheexponentialgrowthphasederivedfromtheincreasingthicknessofthebiofilmandthelayerofcorrosionproducts(suchasferroussulphidepro-ducedfromSRBandferricoxideunderaerobicconditionsthatgeneratesahigherchargetransferresistanceandinhibitsthecor-rosionoftheQ235steelatshorttimes[18]).Thestableimpedance

duringthestationaryphasemayhavebeencausedbythestableSRBbiofilmonthesurfaceofthesteelandthemicroenvironment.Afterthestationaryphase,thedecreaseinimpedancewasinducedbychangesinthemicroenvironmentandthebiofilmandbythecor-rosionofQ235steelthatresultedfromthedecayofSRBandtheSRBmetabolism.Additionally,themagnitudeofthecorrosioncurrentshowedareversetrendcomparedtothevariationinimpedanceduringthegrowthcycle.Thisoccurredbecausethematurebiofilmandthecorrosionproductlayerintheexponentialphasehinderedthechargetransferprocess;thatis,itdecreasedthecorrosioncur-rentandincreasedthemagnitudeofimpedance.ThevariationofimpedancespectrashowninFig.8BisverysimilartothatobtainedforstainlesssteelAISI304inanaturalseawaterhopperinsterilemediawithSRB[33].

3.4.3.Equivalentcircuitandchargetransferresistance

FortheSRB-freemedium,Fig.9AshowsatwotimeconstantmodelcontainingapassivefilmandadoublelayertofittheEISdatafortheabioticenvironment.TheEISdatafortheSRB-containingmediumcanbeinterpretedintermsofequivalentcircuitofthethreetimeconstantmodelshowninFig.9Bbecausethesystemcontainsacorrosionproductfilm,abiofilm,andadoublelayer(Fig.3).ThethreetimeconstantmodelforcarbonsteelorstainlesssteelforthecorrosioninfluencedbySRBwaspreviouslypresentedbyPérezetal.[20]andShengetal.[34].Oncethecarbonsteelwassoakedintheculturesolution,thebiofilmwouldquicklyform.Asimmersiontimecontinued,thecorrosionproductfilmwouldbecomematureontheelectrode.Finally,thedoublelayerwouldformasrelativeionsfromtheseawaterculturediffusedontothebiofilmsurface.Intheequivalentcircuit,theQbfasaconstantphaseelement(CPE)isemployedduetodonotbehaveideallyforbioflimcapacitorintheEISspectrum.TheappearanceoftheCPEcanbeexplainedbydispersioneffectsthatarecausedbysurfacerough-nessorbiofilmmalformationonthesamplesurface[35].Moreover,ithasbeenreportedthatsomeindissolublematter,suchasCaCO3andMgCO3,alsoaffectthedielectricpropertiesofthebiofilmcapac-itor[36,37].Thechargetransferresistance,Rct,isaparameterusedtocharacterizethecorrosionrate:Rct=Rct(1)−Rct(2)

(6)

whereRctisthechargetransferresistance,Rct(1)isthechargetrans-ferresistanceintheSRBculture,andRct(2)isthechargetransferresistancewithoutSRB.

Fig.10showsthetime-dependentchangeintheRctcalculatedfromNyquistplots.ThechangeinRctvaluesagreeswiththechangeinNSRBshowninFig.1forallthreeconditions.ThevaluesofRct

Fig.8.NyquistplotsobtainedforQ235steelintheair-saturatedculturesolutionwithout(A)andwithSRB(B).

Y.Wanetal./ElectrochimicaActa55 (2010) 1528–15341533

Fig.9.EquivalentcircuitmodelsproposedtofitexperimentalimpedancediagramsforN2-,air-,andO2-saturatedculturesolutionswithout(A)andwith(B)SRB.Rs:solutionresistantce;Cdl:capacitanceofdoublelayer;Rct(1):chargetransferresistancewithoutSRB;Rct(2):chargetransferresistancewithSRB;Qcp:capacitanceofcorrosionproductlayer;Rcp:resistanceofcorrosionproductlayer;Qbf:capacitanceofbiofilm;Rbf:resistanceofbiofilm.

EOCvaluesunderaerobicconditionsshiftedinthepositivedirec-tion.

Theimpedanceincreasedduringtheexponentialgrowthphase,almostmaintainedstabilityduringthestationaryphase,anddecreasedafterthestationaryphaseinallthreeculturecondi-tions.TheimpedancemagnitudedecreasedfromO2-toair-toN2-saturatedculturesolutions.Acknowledgements

ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(GrantNo.40876041),theNationalKeyTech-nologyR&DProgramofChina(GrantNo.2007BAB27B01),ScienceandTechnologyBasicResearchProgramofQingdao(GrantNo.09-1-3-16-jch)andtheChineseAcademyofSciences(GrantNo.KZCXZ-YW-210).References

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Fig.10.ThevariationinRctvaluesthatoccurredinN2-saturated(a),air-saturated(b),andO2-saturated(c)culturesolutionsduringSRBgrowth.

intheanaerobicconditionwerehigherthanthoseunderaerobicconditionsbecauseunderaerobicconditionsacompactandintactSRBbiofilmcannotform;thisisbecauseinaerobicconditionsthehighconcentrationofoxygencannotbecompletelyscavengedbytheactivityofSRB,whichthusinhibitsthegrowthofthemicrobes.Inaddition,thecorrosionproductlayer,ironsulphide,alsoprotectsthecarbonsteelsurfacefromcorrosionandanincreaseinRct[18].However,themetabolicactivitiesofSRBinhibitedunderaerobicconditionsrestraintheformationofironsulphide.4.Conclusions

ThepresentresultssuggestthatSRBpossesssomeabilitytotol-erateexposuretooxygen.TheSRBgrowthcurvesshowatypicalgrowthcyclethatcontainsthreestages(exponentialgrowth,asta-tionaryphase,andadeathphase)intheN2-,air-,andO2-saturatedculturesolutions.OxygendissolvedintheculturesolutionsinducedslowgrowthandfastdecayofSRB.

ThemetabolicactivitiesofSRBintheanaerobicandaerobiccon-ditionsinfluencedtheenvironmentparametersCs2−,pH,andÄ.ThevaluesofCs2−andÄunderaerobicconditionswerelowerthanthoseunderanaerobiccondition,andthepHvaluesincreasedfromO2-toair-toN2-saturatedculturesolutions.

Inallthreecultureconditions,theEOCvaluesshiftedrapidlyinthenegativedirectionduringtheexponentialphaseandwerestableduringthestationaryphase.Afterthestationaryphase,the

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