Effect of cerium concentration on corrosion resistance and polymerization of hybrid sol–gel coating on martensitic stainless steel

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Cambon, Jean-Baptiste and Ansart, Florence and Bonino, Jean-Pierre and Turq,

Viviane

Effect of cerium concentration on corrosion resistance and

polymerization of hybrid sol–gel coating on martensitic stainless steel. (2012)

Progress in Organic Coatings, pp. 75 (n° 4). pp. 486-493. ISSN 0300-9440

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Effect

of

cerium

concentration

on

corrosion

resistance

and

polymerization

of

hybrid

sol–gel

coating

on

martensitic

stainless

steel

Jean-Baptiste

Cambon

_,

_Florence

_Ansart,

_Jean-Pierre

_Bonino,

_Viviane

_Turq

InstitutCarnotCIRIMAT,UniversitédeToulouse,UMRCNRS5085,118RoutedeNarbonne,31062ToulouseCedex9,France

Keywords: Stainlesssteel Sol–gelcoating Corrosioninhibitor EIS NMRspectroscopy

a

b

s

t

r

a

c

t

Stainlesssteelsareincreasinglyusedintheaeronauticsfieldforthemanufactureofstructuralparts.One ofthem,theX13VDmartensiticstainlesssteel(X12CrNiMoV12-3),knownforitsgoodmechanical prop-erties,hasapoorcorrosionresistanceinconfinedorsevereenvironments.Inthepastyears,Cr(VI)based pre-treatmentshavebeencurrentlyusedforcorrosionprotectionofdifferentmetals,however,theyare toxicandduetoenvironmentalregulations,theywillbedefinitelybannedinanearfuture.Alternatives toreplaceCr(VI)showadvantagesanddrawbacksconsideringkeypropertiessuchas:corrosion resis-tance,adhesionofcoatings,fatigueresistance,durabilityandreliability.However,someoftheirpossible alternativesshowhighpotential.

Inthispaper,aprocesswasdevelopedtoimprovethecorrosionresistanceofthemartensitic stain-lesssteel.Organic–inorganichybridcoatingswithdifferentceriumconcentrationsweredepositedonto stainlesssteelbysol–gelprocess.Corrosionresistanceofthecoatingswasevaluatedby electrochemi-calimpedancemeasurementsandithasbeenprovedthatceriumconcentrationof0.01Mintohybrid coatingwasanoptimalcontent.Adhesiontestswerealsocarriedoutby“nanoscratchtest”to character-izethecoatingsmechanicalpropertiesasafunctionofceriumconcentrationbutresultsdonotclearly showtheinfluenceofceriumforthecoatingadhesiontowardthesubstrate.Totrytocorrelatewiththe electrochemicalproperties,liquid29_{SiNMRspectroscopywasthenperformedtoinvestigatehydrolysis}

andcondensationreactionsofsol–gelprocess,andbythismethod,itwasdemonstratedthatforhigher ceriumconcentration(>0.01M)thereisamodificationofthechemicalstructureofthesol–gelnetwork.

1. Introduction

Metallic corrosion occurs as a result of chemical reactions betweenthemetalsurfaceandtheenvironment,whichconverts themetalintoitsoriginalore.Inthecaseofsteelandaluminum, Cl−_,O

2andH2Ospecies,inadditiontoelectrontransport,playkey rolesinthecorrosionprocess[1,2].Generally,corrosion preven-tiontechnologyusesoneormoreofthefollowingmethods.The firstoneistheadditionofelements(Cr,Ni,etc.)inthemetalsto enrich thesurface witha corrosion-resistantcomponentduring thecorrosionprocess. Thiscan beassociatedtotheadditionof aqueousinhibitors,whichcanbestronglyadsorbedbychemical conversiontreatmentonthemetalsurfacetopreventthereaction withtheoxidizingagent.Chromates(Cr(VI))areusedasthemost commoncompoundsduetotheirefficiencyinsevereatmosphere andtheirlowcost.Butchromiumbasedcompoundsareextremely toxicadditives,carcinogen,mutagenandreprotoxic.Sotheuseof chromateswillbestrictlyprohibitedinthenextyears[3].

∗ Correspondingauthor.

E-mailaddress:cambon@chimie.ups-tlse.fr(J.-B.Cambon).

Sol–gelcoatingsareoneofthemostpromisingalternative pre-treatments,completelychrome-freewithotheradvantagessuchas cost-effectiveness,lowlife-cycleenvironmentalimpactandsimple applicationprocedures.Thepotentialapplicationofsol–gel coat-ingsas a corrosion protectionsystemfor metalsubstrates was highlighted15yearsagobyGuglielmi[4].Thenseveralworkshave beenundertakentomake varioussol–gelbasedprotective coat-ings,asdescribedinnumerousrecentreviews.Submicronmetal oxidefilmsinhibitcorrosionbyprovidingachemicallyinertbarrier tothediffusionofcorrosivespecies,andtheprotective character-isticsofsol–gelfilmscontainingSiO2[5],TiO2 [6],Al2O3[7],and ZrO2[8],havebeenreportedinvariousstudies.Despiteadvantages ofsol–gelprocessing,ceramiccoatingssufferfromseveral draw-backs:(a)high-temperaturetreatments(>600◦_{C)arerequiredto} decreaseporosityandstabilizethefilms;(b)cracksoccurduring temperaturefluctuationsduetothebrittlenatureofceramicfilms andincompatibilitybetweenthethermalexpansioncoefficientsof metalsandprotectivecoatings;(c)thick(>1mm)crack-freesol–gel coatingsaredifficulttoobtain.Theselimitationscanbeavoidedby thedevelopmentofhybridorganic–inorganicprotectivecoatings, whichcombinethehardness,scratchresistanceandthermal sta-bilityoftheceramiccomponentwiththeflexibility,transparency andtunableadhesionoftheorganicsubstances[9–12].

Table1

ChemicalcompositionofCX13VDstainlesssteel(Febalance).

Element C Cr Ni Mo Va

mol% 0.12 11.5 2.50 1.60 0.30

Fig.1. Semi-developedformulaofGPTMS(glycidoxyproil-trimethoxysilane)(a) andAl(Os_Bu)

3(aluminumsec-butoxide)(b).

Corrosioninhibitorsareincorporatedintothecoatinginorderto improveitscorrosionresistance.Rareearthelementsand particu-larlyceriumareamongthosecommonlyusedforthatpurposeand ceriumhasbeenfoundasanefficientcathodicinhibitor[13–16]. Theobjectiveofthisworkwastostudytheinfluenceofcerium con-centrationwithinasol–gelhybridcoatingappliedonmartensitic stainlesssteelanddevelopedfromthesystem: glycidoxypropyl-trimethoxysilane(GPTMS)andaluminumsec-butoxideAl(Os_Bu)

3. Aninvestigationofthesolformulationandchemicalmechanisms wasmade,kineticofhydrolysisandcondensationofalkoxysilanes hasbeenfollowedwithdifferentceriumconcentrationsbyliquid 29_{SiNMRspectroscopytoidentifysol–gelmechanisms.} Hydropho-bic and adhesion properties of different coatingswere studied respectivelybycontactanglemeasurementand“nanoscratchtest” devicetoverifyceriuminfluence.Finally,theanti-corrosion prop-ertiesofceriumdopedorundopedhybridorganic–inorganicwere investigatedbyelectrochemicalconventionaltechniquesandan optimalceriumconcentrationwasfound:0.01Mandacorrelation with29_{SiNMRresultshasbeentakeninevidence.}

2. Experimentalprocedure

2.1. Material

Metallicsubstrates,CX13VDmartensiticstainlesssteelSS (com-positiongiveninTable1)50mm×25mm×1mmsizessamples are cleaned and pre-treated using several steps, after alkaline degreasing,chemicalcleaningcorrespondsto:1minimmersionin fluonitricacid(pH=1)maintainedat60◦_{C.Sampleisfinallywashed} inethanolanddriedinair.

2.2. Solandcoatingpreparation

Solsfromwhich coatingswereprocessed, werepreparedby mixingglycidoxypropyl-trimethoxysilane(GPTMS)andaluminum

tri-sec-butoxideAl(Os_Bu)

3(formulasinFig.1)anddistilledwater andpropanolinmolarratio5:1:1:10.ThenCe(NO3)3·6H2Owas addedto reach0.001,0.005,0.01,0.05 and 0.1M Ce3+_{.The sol} wasstirredduring2hatroomtemperatureandmaturedfor24h beforedepositiononthesubstratetoinitiatethesol–gelreactions. Theycouldschematicallybedividedintotwosteps,thefirstisa hydrolysisreaction(1)whichaimstopromotereactivehydroxyl groupsreactiveM-OH.Thesecondstepisthecondensation reac-tioninvolvingthealkoxide(2)oronlythehydrolyzedspecies(3).

M(OR)n+nH₂O→ M(OH)n+nROH

M(OH)n+M(OR)n→(HO)n−1M-O-M(OR)n−1+ROH M(OH)n→(HO)n−1M-O-M(OH)n−1+H2O

Thesol–gelfilmswereobtainedbydip-coatingprocedure,using animmersion followedby a withdrawalat a controlledrateof 20cmmin−1_{.Thiswithdrawalspeedhasbeenchosenintherange} [1–50cmmin−1_{] becauseitcorrespondstothemoreconvenient} coveringintermofthickness(LandauandLevichlaw)and homo-geneityofthefilm.Afterdeposition,coatedsamplesweredriedin anovenat100◦_{Cduring24htocompletepolymerizationofthe} hybridfilm.

2.3. Characterization

Theviscosityofeachsolwasdeterminedbyaviscometer Rheo-matRM100of typeTaylor–Couette flowat ashear in therange [600–966s−1_].

Kineticsofhydrolysisandcondensationofalkoxysilaneswere studiedbyliquid29_{SiNMRspectroscopy.Themeasurementswere} performedonaBrucker400MHzspectrometer.NMRspectrawere recordedwithbenzenelikeinternalchemicalshiftreference.The chemicalshiftsweregiveninppmrelativetotheinternalreference. MacrostructuralpropertiesofcoatingswereevaluatedbySEM microscopy,observationswereperformedonsampleswithaJEOL JSM-6510LVapparatus.

Thickness measurements were performed with a fisher dualscopeFMP20,thistechniqueiscommonlyusedforthe deter-minationofcoatingthicknessessuchaspaints.

Coatingroughnessisevaluatedbywhite-lightinterferometry (ZygoInstruments).

Contactanglemeasurementswereperformedwithwaterdrop methodusingcamera.Thentheimagewasprocessedbyasoftware package(windrop).

ScratchtestswereperformedusingaNanoScratchTesterCSM withadiamondindenterwithconicalgeometry.Thescratchtest wascarriedoutwithprogressiveloadingfrom0to60mN.After thescratchtest,thechangesinthemechanismofscratchandthe criticalloadswerevisualizedbyopticalmicroscopy.

The electrochemical behavior of coating was evaluated by electrochemical impedance spectrometry (EIS) in NaCl (0.1M)+Na2SO4(0.04M)solution.AKClsatelectrode,connectedto theworkingsolution,wasusedasreferenceelectrodeanda plat-inumtapeasauxiliaryelectrode.Allelectrochemicalexperiments wereperformedafterstabilization(1h)offreeopencircuit poten-tial.Electrochemicalimpedancespectra(EIS)rangingfrom65kHz to10mHzwerecarriedwitha80mVsinusoidalperturbation sig-naltothecorrosionpotentialwithSolartronInstrumentSI1286 ElectrochemicalInterfacepotentiostatandaSolartronInstrument SI1260Impedance/Gain-Phase-analyser.Alltheelectrochemical experimentswerecarriedouttoroomtemperatureand theEIS diagramswererecordedforimmersiontimesupto24h.

3. Resultsanddiscussion

Thescanningelectronmicrographsofsubstratebeforeandafter cleaningareshowninFig.2anditwasrevealedaftersurface prepa-rationofmetallicsubstrate,themartensiticmicrostructurewith thepresenceoflamellas.

Theviscosityofasolundergoinghydrolysisand polyconden-sationisstronglydependentintimeandisrelatedtothesizeof particles.Thelargertheformedmolecules,thehighertheviscosity [17].Fig.3showstheeffectofceriumconcentrationonthe viscos-ityofthesolagedfor24h(justbeforedeposition).Anincreaseof viscositycanbeobservedwiththeincreasingofcerium concen-tration.Thisisassociatedwiththehighhygroscopiccharacterof ceriumwhichabsorbsthelargeamountofwatertrappedinthesol network[18].

Fig.2. SEMimagesofsurfacemorphologiesofX13VDstainlesssteelbefore(a)andafter(b)cleaning.

Fig.3.Viscosityofsolswithdifferentconcentrationsofcerium.

Forabetterunderstandingofsol–gelmechanismsintothesols theworkwasonthebondinvolvedinthereactionsversuscerium concentration.

Liquid29_{SiNMRspectroscopywasusedtoinvestigatethe} chem-icalstructureoftheinorganicnetworkjustafterthesynthesisof sols(0h)andafter(24h).Duringhydrolysisandcondensation reac-tions,thesiliconenvironmentchangesfromSi ORtoSi OHthento Si O Si.Then,eachcondensationreaction,withtheformationofa Si O Sioxobridgeinducesachemicalshiftof8–10ppminthehigh field[19].29_{SiNMRwascarriedouttofollowtheinorganicphase} polymerizationofeachsol.The29_{Sipresentstheresonancesignals} at−39,−49,−59and−67ppm,denotedT0_,T1_,T2_andT3 respec-tively[20,21].TheTspeciescorrespondtothesilanescontaining0, 1,2or3hydrolysedgroupsasshowninFig.4.

Fig.5showsthe29_{SiNMR ofdifferentsampleswithvarious} cerium concentrations atthe initialstate(0h). Atthis reaction stage,T0_,T1 _andT2_{,werethepresentspeciesinallsamples,no} studiedsamplesshowedanysignalofT3_{.Eachpeakwhich} corre-spondedtoT0_,T1 _{and T}2 _{respectivelyhadthesameareaforall} samples,whichmeansthattheconcentrationofceriumhad no realinfluenceonthecondensationofsol–gelnetworkand each speciewaspresentinthesameconcentration.Howeveraslight

Fig.5. Liquid29_{SiNMRspectraofsolwithdifferentceriumconcentrations(0h).}

displacementofsignalsinthelowfieldwasobservedforsolswith higherceriumconcentration(0.05and0.1M).Fig.6showsthe29_Si NMRofsamesamplesafterasol–gelreactionat24hjustbeforethe depositionstep.Nostudiedsampleshowedsignalofthefree tri-alkoxysilaneprecursorT0_{andpeakareacorrespondingtospecieT}2 increasedforallsamples.Itmeansthatthecondensationincreased inthesamewayforthesolswithdifferentceriumconcentrations. Thisphenomenonisstillobservedforsolswhichcontainedcerium concentrationof0.05and0.1M.Forexample,thefocusontheshift ofT2_{specie(Fig.7),thiswasclearlyobservedforcerium} concentra-tionhigherthan0.01M.Furthermore,onthiszoom,thepresence ofsatellitepeakscanbeunderlined.TheyalsocorrespondtoT2 bondsbutfordifferentspeciesinthesecondsphereof coordina-tion.Inallcases(meanpeakandsatellitepeaks)theshiftfrom0.01 to0.1ispreserved.Intheliterature[22,23],forothersilanebased matrixwithTEOSitcanbeduetotheelectronegativeeffectofCe(III) whichcanreducethelocaldiamagneticshieldinginthevicinityof

Fig.6. Liquid29_{SiNMRspectraofsolwithdifferentceriumconcentrations(24h).}

Fig.7.Liquid29_{SiNMRspectraofsolwithdifferentconcentrationsofcerium(24h)}

focuson10mm.

theattachedprotons.Ceriumhadnoinfluenceoncondensationof networksol–gelbutalargequantityofthisdopant(>0.01M)can changethechemicalstructureofsol–gelnetwork.

Nowitisinterestingtocharacterizethecoatingselaboratedby dip-coatingwiththesolsstudiedpreviously.Fig.8presentsaSEM micrographofasurfaceandcrosssectionofsol–gelcoating with-outceriumdepositedX13VDsubstrate.Atthisscale,thecoatingis homogeneous,covering,crack-freeandadherenttothesubstrate. Theaveragethicknessofcoating,estimatedbythecrosssection analysis(Fig.8b)isabout5–6mm.Thelevelingcharacterofthis filmcanbeunderlinedbywhitelightinterferometry(Fig.9)andthe surfaceroughnessRadecreases(0.42→0.17mm).

Thickness measurements performed by fisher dualscope for hybrid coatings with different cerium concentrations and are reportedinTable2.Thevaluesandtheiraccuracycannotshowa realinfluenceonthelayerthicknessalthoughtheceriumpresence hasincreasedthesolviscosity,parameterdirectlycorrelatedtothe layerthickness[24].

Thehydrophobicitywasevaluatedfromcontactangle measure-mentofawaterdroponhybridcoating.Theobtainedvaluesversus theceriumconcentrationaregiveninFig.10.Thisgraphshowsa slighttendencyofthecontactangleincreasewiththeincreaseof ceriumconcentration,from75◦_{foracoatingwithoutceriumuntil} about80◦_{forcoatingswithhigherceriumconcentration.Thisvalue} underlinesthehydrophobiccharacterofcoatingandimprovesthe tightnesstoward waterwhichisanimportantcharacteristic for anticorrosionproperties.

Anticorrosionpropertiesdependonintrinsicpropertiesofthe coatingsbutalsoonbondsestablishedwiththesubstrateandas a consequenceontheadhesion.To investigatetheinfluence of ceriumonthecoatingsadhesiontostainlesssteelsubstratesscratch testswithincreasingloadwereperformedonthevarious sam-ples.Differentcriticalloads,indicatingchangesinthemechanism ofscratchweredefined[25–27]asitcanbeseeninFig.11: - CL0:Plasticdeformation

-CL1:Brittlefracture -CL2:Delamination

Thedifferentcriticalloadvalues(mN)ofthehybridsol–gel coat-ingwhichcontaindifferentinhibitorconcentrationswereshowed inFig.12.Itwasobservedthatthisconcentrationhadno significa-tiveinfluenceontheadhesionpropertiesofcoatings.Thisgraph showsthatforallceriumconcentrationsinthecoating,thetwo criticalloadsCL0,CL1stayedalmostunchangedconsideringthe accuracyofthemeasurementsbecausethecurveswhich repre-senteachcriticalloadarequitestable.ForthecriticalloadCL2,a slightdecreaseofthisloadcanbeunderlined,probablyduetoa lowerdelaminationresistanceforhighceriumcontent.Thentotry

Fig.9.3Dtopographicanalysisofthesurfacesof(a)X13VDstainlesssteeland(b)hybridcoatingonX13VD,fromawhitelightinterferometer.

Table2

Hybridcoatingthicknessobtainedfordifferentconcentrationsofcerium.

Ceriumconcentration(M) 0 0.001 0.005 0.01 0.05 0.1

Thickness(mm) 5.73±0.6 5.35±0.7 6.01±0.8 5.61±0.5 5.77±0.7 5.89±0.9

Fig.10.ContactanglevaluesofawaterdroponhybridcoatingonX13VDwith differentceriumconcentrations.

tounderstandwhyceriumcontentplaysaroleonsolproperties, itwasplannedtodeterminetheceriumcontentinfluenceonthe corrosionresistanceofcoatings.

Theelectrochemicalimpedancespectroscopy(EIS)isoneofthe mostcommonlytechniqueusedforinvestigationandpredictionof theanticorrosionprotection[28,29].Thispowerfulmethodallows notonlyacomparisonbetweenperformancesofdifferentsystems butalsocangivekeyinformationonkineticsmechanismsofthe coatingdamageandthecorrosionactivityduringimmersioninthe corrosionmedia.InthisworkEISwasusedtoinvestigatetheeffect ofcerium concentrationin solonthecorrosionbehaviorofthe

0 10 20 30 40 50 60 0,12 0,1 0,08 0,06 0,04 0,02 0 Ce(NO3)3 (M) Load (mN) CL 0 CL1 CL2

Fig.12.CriticalloadsforhybridcoatingonX13VDstainlesssteelwithdifferent ceriumconcentrations.

X13VDsubstratescoatedincorrosivesolution(NaCl0.1M+Na2SO4 0.04M).EISdiagramswithX13VDstainlesssteelandhybridcoated sampleswereobtainedafterstabilizationofthecorrosionpotential andfordifferentimmersiontimes.

Fig.13showstheEISplotsinBoderepresentationobtainedfrom thesol–gel hybridcoating withoutceriumcompared toX13VD substrateafter respectively1hand 24himmersion. ForX13VD substrate,theelectrochemicalimpedancediagramsobtainedafter differentimmersiontimesarecharacterizedbytwotimeconstants

Fig.13.Bodephase(a)andmodulusimpedance(b)plotsforX13VDstainlesssteeluncoatedandhybridcoatingwithoutceriumonX13VDstainlesssteelinNaCl (0.1M)+Na2SO4(0.04M)1hand24himmersion.

Fig.14.NyquistplotsforhybridcoatingonX13VDstainlesswithdifferent concen-trationsofceriuminNaCl(0.1M)+Na2SO4(0.04M)1himmersion.

(Fig.13a).Thefirsttimeconstant[0–1000Hz]isindependentofthe immersiontimeandthesecond[0.01–1Hz]oneisbetterdefined forincreasingimmersiontimes.Accordingtoliterature[30,31]if thebarsteelisconsidered,thetimeconstantathighfrequency isattributedtochargetransferprocessandthetimeconstantat thelowfrequencyiscorrelated withtheredoxprocessestaking placeintothepassivefilm.Theincrease ofimpedancemodulus in thelow frequency range (Fig.13b)when immersion time is longercanbeinterpretedbytheimprovementoftheprotection ofthepassivefilm.Forthehybridcoatingdepositedonthe sub-strate(Fig.14a)after1himmersion,twotimeconstantscanbethen observed,oneforthehighfrequencyarea(HF)[1000–65000Hz] whichcanbeattributedtothehybridcoatingproperties:barrier effectwhichlimitschargetransferprocessandanothertime con-stantatmediumfrequency(MF)[1–1000Hz]whereaninflexionof thecurveisobservedwhichcanbeassociatedtothehybridfilm permeability.On hybridcoatingafter24himmersion (10mHz), theappearanceofathirdtimeconstantwasobservedatlow fre-quency(LF).Itisprobablyduetotheformationofapassivation layerattheinterfacebetweenthehybridcoatingandthesubstrate. Thislayer can resultfrom the reactionbetween stainless steel andtheelectrolyte,whichdiffusesintothecoating.Itisthesame phenomenonthatforuncoatedsubstratebuthoweverthis contri-butionremainedrelativelylowduetothehybridbarriereffect.The

highmodulusimpedance(Fig.14b)ofcoatedstainlesssteelslightly decreasedafter24hofimmersionandindicatesthebeginningof thefilmdamage;howevertheperformancesarestillgreatly supe-riortotheseofthepassivatedsubstrate,sothisprovesthegood barriereffectofthesol–gelcoating.

Fig.14showsEISmeasurementswithNyquistplotforhybrid sol–gelcoatingscontainingdifferentconcentrationsofceriumafter 1himmersion.Whenceriumwasaddedincoatingto0.01M,itwas clearlyobservedanincreaseofthecapacitiveloop,itmeansthatthe barriereffectwasimprovedwithlowceriumcontentintherange [0–0.01M].Whenceriumcontentishigherthan0.01M,another behaviorwastakeninevidence:animportantdropofthebarrier effectwithadecreaseofcapacitiveloops,butthistimeitisnot duetoadecreaseofthecoatingthicknessbecausethesolswith ceriumcontentof0.05and0.1Mhaveaviscosityhigher(Fig.3). Thedeteriorationofbarrierpropertiesofthefilmcanbeattributed tothemodificationin thehybridfilmsnetworkby theCeions incorporation,asitwasmentionedbeforebyotherothers[32].

Fig.15 shows EISmeasurements in theBode representation (phaseandmodulus)inordertocomparecorrosionperformance forhybridsol–gelcoatingscontainingdifferentcerium concentra-tions after1himmersion.Whenceriumwasadded insol until aconcentrationreaching0.01M,itwasclearlyobserved onthe curves (Fig. 15a), an improvement of the barrier effect and a decreaseofthepermeabilityofhybridcoatingbecausethe inflec-tionofthecurveatmediumfrequency(MF)wasshiftedtolower frequency.Forhigherceriumconcentrations(0.05and0.1M),the corrosionperformancesofthesecoatingsclearlycollapsedandthe inflectionof thecurveatMFwasshiftedtohigher frequencies. These coatings are more permeable and the electrolyte pene-tratesthecoatings moreeasilyto reactwiththesubstrate and toformthepassivationlayerbecausetheconstanttime at low frequency,characteristicofthispassivationlayer,ismore significa-tive.Theimpedancemodulus(Fig.15b)increasesforlowcerium content(<0.01M)and sharplydecreasesforthesesame cerium concentrationsin thesol–gel coating.Fig.16 shows impedance modulusatlowfrequency(10MHz)versusceriumconcentration. Thismodulusischaracteristicofcorrosionprotectionofthewhole system:hybridcoatingand reactivitywiththesubstrate.Itwas alsoobservedthatthismodulusincreasedwhencerium concen-trationreachedavaluebetween0.005and0.01M.Then,forhigher inhibitorconcentrations, themodulus highlydecreased andthe coatingbecamelessprotectorthancoatingwithoutcerium.The sameremark maybemade for coatingwithdifferent inhibitor concentrationsafteranimmersionof24hinthecorrosive solu-tion. Bode representation withphase (Fig. 17) and impedance

Fig.15.Bodephase(a)andimpedancemodulus(b)plotsforhybridcoatingonX13VDstainlesswithdifferentconcentrationsofceriuminNaCl(0.1M)+Na2SO4(0.04M)1h

immersion.

Fig.16.Impedancemodulus(f=10mHz)withdifferentconcentrationsofceriumin NaCl(0.1M)+Na2SO4(0.04M)1himmersion.

modulus(Fig.18), showsthattheoptimalcerium concentration inthecoatingcorrespondstoavalueof0.01M.

Takenintoaccounttheseresults,onekeypointisconfirmed: theexistenceofathresholdconcentrationofcerium correspond-ingto0.01M.So,itispossibletotrytocorrelatethesecorrosion resultswithchemicaldisplacementsobtainedbyliquid29_SiNMR spectroscopyforsolswithahighceriumconcentration.Theseshifts correspondtotheimportantcollapseinanti-corrosionproperties ofsol–gelhybridcoating.Ceriumhadnotanyinfluenceon conden-sationnetworksilanebutalargequantityofthisdopant(>0.01M) canchangethechemicalstructureofhybridnetwork.Thereisan

Fig.18.Impedancemodulus(f=10mHz)forhybridcoatingonX13VDstainlesswith differentceriumconcentrationsinNaCl(0.1M)+Na2SO4(0.04M)24himmersion.

abilitytocreatenewconnection(Ce–Al)withaluminumfromthe ASBofthematrixwhichcoulddestabilizethehybridnetworkand alsoreducetheresistancetowardcorrosionofcoatings.Itwillthen beinterestingtostudythechemicalstructureofsol–gelhybridsat solidstatewithtechniquessuchassolid29_Siand13_CNMR charac-terizationandRamanspectrometryinordertobetterunderstand nowtheinfluenceofceriumdirectlyoncoatingproperties(these experimentsarenowinprogress).

Fig.17. Bodephase(a)andimpedancemodulus(b)plotsforhybridcoatingonX13VDstainlesswithdifferentceriumconcentrationsinNaCl(0.1M)+Na2SO4(0.04M)24h

4. Conclusion

The influence of cerium concentration on behavior against corrosionofhybridsol–gel coatingonmartensiticX13VD stain-lesssteelhasbeeninvestigated.Theincrease ofceriumcontent improved hydrophobic character of coating and its tightness towardwater.Electrochemicalimpedancespectroscopyshowedan optimalceriumconcentrationof0.01Minthehybridlayer.Besides, theincreaseofceriumconcentrationabove0.01Mdecreasedthe barriereffectofsol–gelcoating.Thecoatingswereperformedby scratchtests,andnosignificativeceriuminfluencehasbeenproved onthe adhesion properties of coatingsto themartensitic sub-strate;soadirectcorrelationbetweenanticorrosionandadhesion performancesofsol–gelcoatingisnotpossible.Liquid29_SiNMR spectroscopywasthencarriedouttodeterminatethecerium influ-enceonsolcondensation.Itwasshownthatanexcessofcerium (>0.01M)leadtoachemicalshiftofTspeciesandcanchangethe chemicalstructure of sol–gelnetwork. Thereforea reduction of corrosionresistanceofcoatingsupwasnotedtothesamecerium concentration(0.01M).

Acknowledgments

Thiswork wascarried out in theframework of the ARCAM project,withthefinancial supportof DGCIS,Regions Aquitaine, AuvergneandMidi-Pyrénées.Theauthorsthankfullyacknowledge thepartnersoftheproject:Ratier-Figeac,Aubert&Duval,Olympus, theMechanicalandEngineeringInstituteofBordeaux,the Materi-alsDepartmentofICAMandtheInstituteCARNOTCIRIMAT. References

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