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Adsorption

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AdsorptionAdsorp

    Adsorption

2236

    ;物理化学学报(WuliHuaxueXuebao)

    ;ActaPhys.Chim.Sin.,2008,24(12):2236-2242December

    ;[Articlewww.whxb.pku.edu.cn

    ;AdsorptionBehaviorandInhibitionCorrosionEffectofSodium

    ;CarboxymethylCelluloseonMildSteelinAcidicMedium

    ;BAYOLE.GORTENA.A.DURSUNM.KAYAKiRiLMAZK.

    ;(DepartmentofChemistry,FacultyofScienceandArt,NigdeUniversity,51200Nigde,Turkey)

    ;Abstract:Theeffectofsodiumcarboxymethylcellulose(Na-CMC)onthecorrosionbehaviorofmildsteelin1.0

    ;mol.LHCIsolutionhasbeeninvestigatedbyusingweightloss(WL)measurement,potentiodynamicpolarization,

    ;linearpolarizationresistance(LPR),andelectrochemicalimpedancespectroscopy(EIS)methods.Theseresultsshowed

    ;thattheinhibitionefficiencyofNaCMCincreasedwithincreasingtheinhibitorconcentr

    ation.Potentiodynamic

    ;polarizationstudiesrevealedthattheNaCMCwasamixedtypeinhibitorin1.0molL

    HC1.Theadsorptionofthe

    ;inhibitoronmildsteelsurfacehasbeenfoundtoobeytheLangmuirisotherm.Theeffectoftemperatureonthe

    ;corrosionbehaviorofmildsteelin1.0mol?LHClwithadditi0nofO.04%ofNa.CMChasbeenstudiedinthe

    ;temperaturerangeof298-328K.Theassoci

    dapparentactivationenergy:)ofcorrosionreactionhasbeendetermined. ;Scanningelectronmicroscopy(SEM)hasbeenappliedtoinvestigatethesurfacemorphologyofmildsteelinthe

    ;absenceandpresenceoftheinhibitormolecules.

    ;KeyWords:Corrosion;Mildsteel;Adsorption;Sodiumcarboxymethylcellulose;Electrochemical

    ;impedancespectroscopy

    ;Ironanditsalloysfindutilityinawidespreadspectrumof

    ;manyindustrialunitsbecauseofitslowcostandexcellentme-?

    ;chanicalproperties.Forthisreason,thecorrosionbehaviorof

    ;thesematerialshasattractedtheattentionofseveralinvestiga-

    ;tions.Steelisthemostcorrosionvulnerablemeta1.Thus,much

    ;attentionisgivenforitsprotectionfromthehostileenviron-

    ;merits.Acidsolutionsarewidelyusedinindustry.Themostim

    ;portantareasofapplicationareacidpickling,industrialacid

;cleaning,aciddescaling,andoilwellacidizingI-6].Polymersare

    ;usedascorrosioninhibitors.whentheyareusedinsomeparticu

    ;larfunctionalgroups.Theycanoftenformcomplexeswithmetal ;ions.Thesecomplexesoccupyalargesurfaceareaonthemetal ;surface,therebyblockingthesurfaceandprotectingthemetal ;fromcorrosiveagentspresentinthesolutiont7-11].Furthermore, ;somelowcostpolymericcompoundsaregoodcorrosionin

    ;hitIitorsformetallicmaterialsinanacidicmedium]. ;Sodiumcarboxymethylcellulose(NaCMC)isananionicwa

    ;tersolublepolymerderivedfromcellulose.Duetoitsinnocu- ;ousness.itisusedasastabilizer.binder,thickener,forsuspen- ;sionandaswaterretainingagentinfoodindustry,pharmaceuti ;cal,cosmetic,paper,andotherindustrialareas?3l.However, ;mostofthecorrosioninhibitorsusedinaqueousheatingand ;coolingsystemsarehazardousforhealth.Theirtoxicproperties ;limittheirapplicationareast161.ThestudyofNa.CMCasacidin. ;hibitorisparticularlyimportantbecauseofitscheapness,water ;solubility,nontoxicity,andasanenvironmentallyacceptable ;polymer.

    ;Inthisstudy,mildsteelcorrosionwithvariousconcentrations ;ofNa-CMCin1.0mol’LHCIusingweightlosstestandelec?

    ;trochemicaltechniquessuchaspotentiodynamic,linearpolariza? ;]ionresistance(LPR),andimpedancemeas~ementshavebeen ;investigated.Temperatureeffectonthedissolutionofmildsteel ;in1.0mol?LHC1containingO.04%Na-CMCwasalsostudied

    ;andactivationenergyofthecorrosionreactionwascomputed ;fromivaluesobtainedfromtheTafelextrapolationmethod. ;1Experimental

    ;1.1Weightlossmeasurement

    ;Themildsteelcouponsof4.0cmx2.0cmx0.07cmwithan ;exposedtotalareaof16.84cm2werecleanedusing20%NaOH ;solutioncontaining200g?Lofzincdustfor12h.Theywere

    ;washedwithdistilledwater,driedinacetone,weighed,and ;storedinamoisturefreedesiccatorpriortouse[71.Theprecleaned ;andweighedcouponsweredippedinbeakerscontaining1.O ;mo1.LHClsolutionanddifferentmassfractionsofNa.CMC ;containing0,0.001%,0.005%,0.O1%,0.02%,0.03%,and ;0.O4%.respectively(for250mLsolutionconsistof1.0mo1.L ;HC1andNaCMC),forthevimemcexperimentsinwhich

    ;Received:June12,2008;Revised:September3,2008;Publishedoi1Web:October9,2008.

    ;EnglisheditionavailableonlineatWWW.sciencedirect.coin ;Correspondingauthor.Email:emelbayol@nigde.edu.tr;Tel:+~3882252094;Fax:+90

    388-2250180

    ;?EditorialofficeofActaPhysicoChimicaSinica

;No.12BAYOLE.eta1.:AdsorptionBehaviorandInhibitionCorrosionEffectofSodiumC

    arboxymethylCellulose2237

    ;immersiontimeforweight1OSSwas24hat298K.A1ltestswere ;performedinaeratedsolutionsandwererunintriplicate.Atthe ;endofthetest.thespecimenswerecarefullywashedwithdis. ;tilledwater,dried,andweighed.Theweightlosswascalculated ;fromthedifferencebetweenbeforeandendoftheexperiment. ;Thisallowedcalculationofthemeancorrosionrateexpressedin ;mg?cm-2~h’..

    ;1.2Electrochemicalmeasurement

    ;Mildsteelwithmassfractionof0.097%C.0.00321%Pb. ;0.488%Cu,0.117%Cr,0.032%P,0.099%Si,0:012%V,

    ;0.004%Nb,0.054%Mo,0.07%S,O.018%Sn,0.0l%W,

    ;0.0042%Co.0.137%Ni.and0.459%Mnandtheremaining ;ironwasusedfortheelectrochemica1measurements.Thespeci

    ;menswereembeddedinpolyester;0.5cmsurfaceareawasin ;contactwiththecorrosivemediaandtheelectricalconductivity ;wasprovidedbyacopperwire.Priortoeachexperiment,the ;mildsteelsurfacesweremechanicallypolishedwimdifferent ;gradesofemerypaper(150,600,and1200),degreasedwithace

    ;tone.rinsedwithdistilledwater.andplacedinthecel1.A11the ;reagentsusedwereofanalyticalgradepurchasedfromSigma

    ;Aldrich(NaCMC,CatNo:419311)andMerck(HC1).The

    ;molecularstructureofNaCMCisshowninFig.1.

    ;Electrochemicalexperimentswerecarriedoutinaconven. ;tionalthree.electrodecel1.Theworkingelectrodewithashape ;ofadiscwascutfromthemildsteelsheet.Aplatinumelectrode ;andanAg/AgC1electrodewereusedascounterandreference ;electrodes.respectively.Thetemperatureconditionswerether- ;mostaticallycontrolledbyusingwear-jacketedcel1.Electro

    ;chemica1measurementswerecarriedoutusingaCHI660B ;electrochemica1analyzerundercomputercontro1.Themildstee1 ;electrodewasimmersedinthesolutionfor30minandthenthe ;freecorrosionpotential(‰)wasrecorded.Foreachtest,fresh-

    ;lypreparedsolutionswereused.

    ;Electrochemicalimpedancemeasurementswereobtainedat ;thecorrosionpotentialwhensinusoida1potentia1waveof5mV ;ofamplitudewasappliedatfrequenciesrangingfrom10to10 ;Hz.TheimpedancediagramsaregivenintheNyquistrepresen

    ;tation.

    ;Inthelinearpolarizationresistancemeasurements.themild ;steelwaspolarizedto?10mVofthecorrosionpotentialata ;scanrateof0.1mV?s.Themildsteelwaspolarizedfromthe

    ;negativetothepositivesideofthecorrosionpotentialstoasin

;O

    ;R--CH2CONa

    ;Fig.1MolecularstructureofNa-CMC

    ;glecycleateachmeasurement.Theresultingcurrentversuspo’

    ;tentialwasplotted.Polarizationresistance(Rp)valueswereob

    ;minedfromthecurrentpotentialplot.

    ;Potentiodynamicpolarizationwascarriedoutat-170mVca-

     ;thodicpotentialandat+170mVanodicpotentialoftheCOITO

    ;sionpotentialat1mV.ssweeprateinordertoobservethecor- ;rosioninhibitionectofNaCMC.Thecorrosioncurrentden-

    ;sities(beforeandafteraddingtheNaCMCweredetermined

    ;usingtheTafe1extrapolationmethod.

    ;1.3Scanningelectronmicroscopy

    ;Themicrographsofpolished,corroded,andinhibitedmild ;steelsurfacesweretakenusingscanningelectronmicroscope ;(SEM)(Leon440).Theenergyoftheaccelerationbeamem- ;ployedwas2OkV.Thousandfoldofmagnificationwasapplied ;foralImicrographs.

    ;2Resultsanddiscussion

    ;2.1Gravimeicmeasurement

    ;TheeriectofdifferentconcentrationsofNaCMConthemild

    ;stee1corrosionin1.0moLHC1wasstudiedbyweightlOSSat

    ;298Kafter24hofimmersionperiod.Thecorrosionratew)of ;mildsteelwasdeterminedbyusingthefollowingrelation: ;W=Am

    ;St

    ;whereAm,S,andtaremasslOSS,surfaceareaoftheelectrode ;(herel6.84cm2),andimmersionperiod(11ere24h),respectively. ;TheinhibitionefficiencyfIE1ofcorrosioninhibitorisdefined ;bythefollowingexpression:

    ;IE_()x100%

    ;whereW0andWarethecorrosionratesintheabsenceandpres- ;enceofinhibitor.respectively.

    ;Table1includesthecorrosionratevaluesofmildsteelandin. ;hibitionefficiencyofNaCMC.AccordingtoTable1.thedisVi

    ;butionofcorrosionratevariedfrom0.319to0.088mg?cm?h ;andtheinhibitioneciencyincreasedwithincreasingthein. ;hibitorconcentration.Inhibitioneciencyreachedavalueof

    ;72%at0.O4%NaCMC.Theelectrochemicalresultspartially ;showedsimilaritytotheinhibitioneciencyinthesensethat

    ;theyincreasedasinhibitorconcentrationincreased. ;2.2EISandLPR

    ;Thecorrosionbehaviorofmildsteelin1.0mo1.LHClsolu. ;Table1Inhibitioneicienciesforvariousconcentrations

    ;0fNa_CMCforthecorrosionofthemildsteelin1.0tool?L ;HCIobtainedfromweightlossmeasurement

    ;2238ActaPhys.Chim.Sin.,2008Vo1.24

    ;z/Q

    ;Fig.2Nyquistdiagramsformildsteelelectrodein

    ;1mol?LHClwithandwithoutNa.CMC

    CMCandwithoutNaCMCwas ;tionswith0.001%-0.04%Na

    ;investigatedbyelectrochemicalimpedancespectroscopy(EIS) ;at298K.Nyquistplotsofmildsteelinacidicsolutionswithand ;withoutinhibitordisplayedonlyonedepressedsemicircle,as

    ;seeninFig.2.

    ;Thepolarizationresistance(valueswerecalculatedfromthe ;differenceintheimpedanceatlowerandhigherfrequencies.

    ;Theelectrochemicalequivalentcircuitmodelemployedforthis ;systemispresentedinFig.3.Accordingtotheequivalentcircuit, ;therealimpedanceatlowerandhigherfrequenciesispermitted ;toobtainthepolarizationresistance(Rp).Thepolarizationresis

    ;tanceincludeschargetransferresistanced),whichcorresponds ;toresistancebetweenthemetal/outerHelmholtzplane,diffuse ;lairresistance(attributedtotheadsorbedinhibitormolecules, ;corrosionproducts,ions,andaccumulatedspeciesonthemetal ;surfaceofthesemiellipsemode1.Thishasbeenreportedbv

    ;ErbiltL9,20andSolmaz[leta1..TheRdandRdresistancevalues ;calculatedusingthesemiellipsemodelaregiveninTab1e2.

    ;TheimpedanceparametersdeterminedfromNyquistdiagram ;suchasR.,Rp,O,d,andIEaregiveninTab1e2.

    ;Inanacidicmedium.theimpedanceresponseofmildsteel ;significantlychangeswithNaCMCconcentrationandthesize

    ;ofsemicircle,whichcorrespondstothepolarizationresistances ;ofmildstee1.TheRvaluesincreasedfrom44to222nandthe ;capacitancevaluesdecreasedfrom114to49IxFbytheaddition ;ofNaCMC(Table2).Astheinhibitorconcentrationsincreased, ;theRnvaluesincreased,thecapacitancevaluestendedtode

    ;crease:thisisprobablyduetotheadsorptionoftheinhibitoron ;Fig.3Equivalentelectricalcircuitmodel

    ;Rp=R~+Ra,Ra=R~+R;Re:solutionresistance,Rd:chargetransferresistance,

    ;Rd:diffuselayerresistance,Rf:filmresistance,R:accumulatedresistance,

    ;Q?:differentialcapacitance

    ;Table2ImpedanceandLPRparametersforcorrosionof ;mildsteelin1tool?LHClatvariOUScontentsofNa.CMC ;themetalsurface[IJThepresenceofalowfrequencyinductive

    ;loopistypicalforironandmildsteelandcouldbeattributedto ;themoleculesthatarescatteredatthehighfrequencyregion. ;Thesemoleculesarereorientedandaccumulatedontheelec

;trodesurfaceinthelowfrequencyregion.

    ;Thecorrodedmetalrepresentsageneralbehaviorwherethe ;doublelayerontheinterfaceofmetalsolutiondoesnotbehave

    ;asarealcondenser.Thepotentialtrendisexponentialgoing ;frommetaltosolution.Ifthepotentialdropatthedistancedxis ;dE,thecapacitanceloweringwillbeequaltodC.Thecapaci

    ;tanceofthewholesystemistheintegralofdCvalues,calleddif- ;ferentialcapacitance(Q.Inmodellingcorrosionprocess,the ;termOm,aconstantphaseelement(CPE)thatcouldbesubstitut

    ;edbyinthetimeconstantsassociatedwiththecorrosionpro

    ;cess.iSrepresentedasanexperimentaldeviationfromasemi. ;circlet.Inordertoobtainthedifferentialcapacitance.thefre. ;quencyatwhichtheimaginarycomponentoftheimpedanceis ;maximum,(wasdeterminedandavalueswerealsocalcu

    ;latedfromthefollowingequation:

    ;)

    ;whereBisaconstantdependingonthespecificanalyzedsys

    Jistheangularfrequency, ;tern,Jistheimaginaryunit(#-1),c

    ;andisasurfaceinhomogeneitycoefficientrangingbetween0 ;and1.TheCPE(Qisrelatedtothecapacityofthedoublelayer ;andtheexponent()ofQd,relevanttothecapacitivesemicircle

    ;ofelectrode/electrolytesystem[22s41.

    ;Thedifferentialcapacitanceisconsideredastheelectricalca

    ;pacitorbetweenchargedmetalsurfaceandsolution.Itisgener

    ;allyassumedthatacidcorrosioninhibitorsadsorbonthemetal ;surfaceandthestructureofdoublelayerchangeswithreducing ;electrochemicalpartia1reactionrate.Inhibitionprocesstakes ;placebyadecreaseintheelectricalcapacityofthemildsteel ;surfaceinthepresenceoftheinhibitorandthiscouldberelated ;withtlledecreaseinthecorrosiveareaonthemildsteelsurface ;owingtotheincreaseoftheinhibitorcoveredarea.Thedecrease ;ofcapacitancevaluesmaybeduetotheadsorptionofNaCMC

    ;onmetalsurfacethusleadingtoafilmf0rmationonthemild ;steelsurfacethathas1edtoanincreaseinpercentageinhibition ;efficiencyrIE)’.Thecapacitancevaluesdecreaseduetoan

    ;increaseinthethicknessoftheelectrica1double1ayerand/ora ;decreaseinlocaldielectricconstantthatarecausedbythead. ;

    ;2240ActaPs.-Chim.Sin..2o08VO1.24

    ;(Na-CMCl1%)

    ;Fig.6Langmuiradsorptionplotforthemildsteelin1.0 ;tool?LHCIcontainingdifferentconcentrationsofNa-CMC ;muirisothermwithcorrelationcoefficientof0.9967.According ;tothisisotherm,0isrelatedtotheinhibitorconcentrationG:

;0+

    ;whereistheadsorptionequilibriumconstantoftheadsorp

    ;tionprocess(Fig.6).

    ;Theadsorptionequilibriumconstantisrelatedtothefreeen

    ;ergyofadsorption?Gaasshownintheequationbelow

    ) ;exp

    ;whereRisthegasconstant(8.314J.K.mol),Tistheabso

    ;lutetemperature(K),andthevalue55.5istheconcentrationof ;waterinsolutionexpressedinmol?L.Inordertocalculatethe

    ;freeadsorptionenergy(?G,itisnecessarytoknowtheaverage ;molecularweightofNaCMC.SinceNaCMCisapolymerwith

    ;anaveragemolecularweight(M~--250000),itwasusedinthede

    ;terminationofthemolarconcentrationsofthestudiedpolymeric ;solution.Obviously,theadsorptiveequilibriumconstanthas ;aunitofL’mol.Therefore,adsorptiveequilibriumconstant

    ;unitL?mgqshouldbeconvertedintoL?molqas45L?molq.

    ;Thefleeenergyofadsorption(AGcanbeobtainedfromthe

    =2O.232.481riM..Thevaluesofequilibrium ;equationof?G.

    ;constantandfreeenergyofadsorptionofthemildsteelare1.1× ;l0L?moland-51.0kJ?mol.respectively.Thenegativeval

    ;ueofAG~suggeststhattheadsorptionofNaCMCmoleculeon

    ;themildsteelsurfaceisaspontaneousprocess.Avalueof-40 ;

    ;p

    ;

    ;0

    ;bD

    ;E/V(vsAg/AgCI)

    ;Fig.7Effectoftemperatureonthecathodicandanodic ;responsesformildsteelin1.0tool?LHC1

    ;

    ;

    ;064-0.600.56-0.52-0.48O.44O.400.360.32O.28

    ;E/V(vsAg/AgCI1

    ;Fig.8Effectoftemperatureonthecathodicandanodic ;responsesformildsteelin1.0tool?L-HCI+0.04%Na-CMC ;kJ?molisusuallyadoptedasathresholdvaluebetween ;chemisorptionandphysisorption.Generally.thevalueof

    ;?Gforchemisorptionismorenegativethan—40kJ’mol-1.

    ;Suchavalueimplieseithertransferofelectronsorsharingwith ;inhibitormoleculesonthemetalsurface,whichformsacoordi

    ;natetypeofbondthatexplainsthestrongadsorptionofNa

    ;CMConthemildsteelsurface.Similarinterpretationsaboutthe ;adsorptionofwatersolublepolymeronthemetalsurfacehave

;beenreportedbyotherresearcherst34aT~.

    ;2.5Effectoftemperature

    ;Temperaturecouldaffecttheinteractionbetweenthemild ;steelelectrodeandtheacidicmediumintheabsenceandpres

    ;enceoftheinhibitor.Polarizationcurvesforthemildsteeljn1.0

    CMCat ;mol?LHC1and1.0mol?L-HC1containing0.04%Na

    ;thetemperataarerange298-328KaregiveninFigs.7and8and ;thecorrespondingdataaregiveninTable4.

    ;Table4showsthatthecorrosioncurrentdensityincreases ;withincreasingtemperature,whereasinhibitorefficiencyde- ;creasesastemperatureincreases.Thedecreaseininhibitioneffi. ;ciencyshowsthatthefilmformedonthemetalsurfaceisless ;protectiveathighertemperatures,sincedesorptionrateofthein- ;hibitorisgreaterathighertemperatures.?

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