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

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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

To cite this version:

Jean-Baptiste Cambon, Florence Ansart, Jean-Pierre Bonino, Viviane Turq.

Effect of cerium

concentration on corrosion resistance and polymerization of hybrid sol-gel coating on

marten-sitic stainless steel.

Progress in Organic Coatings, Elsevier, 2012, pp.

75, pp.

486-493.

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To link to this article:

DOI:10.1016/j.porgcoat.2012.06.005

Official URL:

http://dx.doi.org/10.1016/j.porgcoat.2012.06.005

This is an author-deposited version published in:

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

Eprints ID: 8743

To cite this version:

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_Si_NMR_spectroscopy_was_then_performed_to_investigate_hydrolysis

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)_are_required_to

decreaseporosityandstabilizethefilms;(b)cracksoccurduring temperaturefluctuationsduetothebrittlenatureofceramicfilms andincompatibilitybetweenthethermalexpansioncoefficientsof metalsandprotectivecoatings;(c)thick(>1mm)crack-freesol–gel coatingsaredifficulttoobtain.Theselimitationscanbeavoidedby thedevelopmentofhybridorganic–inorganicprotectivecoatings, whichcombinethehardness,scratchresistanceandthermal sta-bilityoftheceramiccomponentwiththeflexibility,transparency andtunableadhesionoftheorganicsubstances[9–12].

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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_Si_NMR_spectroscopy_to_identify_sol–gel_mechanisms.

Hydropho-bic and adhesion properties of different coatingswere studied respectivelybycontactanglemeasurementand“nanoscratchtest” devicetoverifyceriuminﬂuence.Finally,theanti-corrosion prop-ertiesofceriumdopedorundopedhybridorganic–inorganicwere investigatedbyelectrochemicalconventionaltechniquesandan optimalceriumconcentrationwasfound:0.01Mandacorrelation with29_Si_NMR_results_has_been_taken_in_evidence.

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._Sample_is_ﬁnally_washed

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_._This_withdrawal_speed_has_been_chosen_in_the_range

[1–50cmmin−1_] _because_it_corresponds_to_the_more_convenient

coveringintermofthickness(LandauandLevichlaw)and homo-geneityoftheﬁlm.Afterdeposition,coatedsamplesweredriedin anovenat100◦_C_during₂₄_h_to_complete_{polymerization}_of_the

hybridﬁlm.

2.3. Characterization

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

Kineticsofhydrolysisandcondensationofalkoxysilaneswere studiedbyliquid29_Si_NMR_{spectroscopy.}_The_measurements_were

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].

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

Fig.3.Viscosityofsolswithdifferentconcentrationsofcerium.

Forabetterunderstandingofsol–gelmechanismsintothesols theworkwasonthebondinvolvedinthereactionsversuscerium concentration.

Liquid29_Si_NMR_spectroscopy_was_used_to_investigate_the

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_Si_NMR_was_carried_out_to_follow_the_inorganic_phase

polymerizationofeachsol.The29_Si_presents_the_resonance_signals

at−39,−49,−59and−67ppm,denotedT0_,_T1_,_T2_and_T3

respec-tively[20,21].TheTspeciescorrespondtothesilanescontaining0, 1,2or3hydrolysedgroupsasshowninFig.4.

Fig.5showsthe29_Si_NMR _of_different_samples_with_various

cerium concentrations atthe initialstate(0h). Atthis reaction stage,T0_,_T1 _and_T2_,_were_the_present_species_in_all_samples,_no

studiedsamplesshowedanysignalofT3_._Each_peak_which

corre-spondedtoT0_,_T1 _and _T2 _respectively_had_the_same_area_for_all

samples,whichmeansthattheconcentrationofceriumhad no realinﬂuenceonthecondensationofsol–gelnetworkand each speciewaspresentinthesameconcentration.Howeveraslight

Fig.5. Liquid29_Si_NMR_spectra_of_sol_with_different_cerium_{concentrations}₍₀_h).

displacementofsignalsinthelowﬁeldwasobservedforsolswith higherceriumconcentration(0.05and0.1M).Fig.6showsthe29_Si

NMRofsamesamplesafterasol–gelreactionat24hjustbeforethe depositionstep.Nostudiedsampleshowedsignalofthefree tri-alkoxysilaneprecursorT0_and_peak_area_{corresponding}_to_specie_T2

increasedforallsamples.Itmeansthatthecondensationincreased inthesamewayforthesolswithdifferentceriumconcentrations. Thisphenomenonisstillobservedforsolswhichcontainedcerium concentrationof0.05and0.1M.Forexample,thefocusontheshift ofT2_specie_(Fig._7),_this_was_clearly_observed_for_cerium

concentra-tionhigherthan0.01M.Furthermore,onthiszoom,thepresence ofsatellitepeakscanbeunderlined.TheyalsocorrespondtoT2

bondsbutfordifferentspeciesinthesecondsphereof coordina-tion.Inallcases(meanpeakandsatellitepeaks)theshiftfrom0.01 to0.1ispreserved.Intheliterature[22,23],forothersilanebased matrixwithTEOSitcanbeduetotheelectronegativeeffectofCe(III) whichcanreducethelocaldiamagneticshieldinginthevicinityof

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Fig.6. Liquid29_Si_NMR_spectra_of_sol_with_different_cerium_{concentrations}₍₂₄_h).

Fig.7.Liquid29_Si_NMR_spectra_of_sol_with_different_{concentrations}_of_cerium₍₂₄_h) 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◦_for_a_coating_without_cerium_until

about80◦_for_coatings_with_higher_cerium_{concentration.}_This_value

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

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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

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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

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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_Si_NMR

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_Si_and13_C_NMR

charac-terizationandRamanspectrometryinordertobetterunderstand nowtheinﬂuenceofceriumdirectlyoncoatingproperties(these experimentsarenowinprogress).

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

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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_Si_NMR

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

[1]M.R.Ryan,D.E.Williams,R.J.Chater,B.M.Hutton,D.S.McPhail,Nature415 (2002)770.

[2]I.Betova,M.Bojinov,T.Laitinen,K.Makela,P.Pohjanne,T.Saario,Corros.Sci. 44(2002)2675.

[3]ToxicologicalProﬁleforChromium,ATDSR/Tp-88/10,AgencyforToxic Sub-stances,USPublicService,Washington,DC,1989.

[4]M.Guglielmi,J.Sol–GelSci.Technol.8(1997)443.

[5]D.C.L.Vasconcelos,J.A.N.Carvalho,M.Mantel,W.L.Vasconcelos,J.Non-Cryst. Solids273(2000)135.

[6]A.Nazeri,P.P.TrzaskomaPaulette,D.Bauer,J.Sol–GelSci.Technol.10(1997) 317.

[7]J.Masalski,J.Gluszek,J.Zabrzeski,K.Nitsch,P.Gluszek,ThinSolidFilms349 (1999)186.

[8] F.Perdomo,P.D.Lima,M.A.Aegerter,L.A.Avaca,J.Sol–GelSci.Technol.15 (1999)87.

[9] V.H.V.Sarmento,M.G.Schiavetto,P.Hammer,A.V.Benedetti,C.S.Fugivara, P.H.Suegama,S.H.Pulcinelli,C.V.Santilli,Surf.Coat.Technol.204 (2010) 2689–3270.

[10]E.Roussi,A.Tsetsekou,D.Tsiourvas,A.Karantonis,Surf.Coat.Technol.205 (2011)3235–3244.

[11]R.T.Sakai,F.M.Di,L.daCruz,H.G.deMelo,A.V.Benedetti,C.V.Santilli,P.H. Suegama,Prog.Org.Coat.74(2012)288–301.

[12]T.P.Chou,C.Chandrasekaran,G.Z.Cao,J.Sol–GelSci.Technol.26(2003)321. [13] A.Pepe,M.Aparicio,S.Ceré,A.J.Durán,J.Non-Cryst.Solids348(2004)162–171. [14]M.A.Arenas,J.J.deDamborena,Electrochem.Acta48(2003)3693–3698. [15] F.Andreatta,L.Paussa,A.Lanzutti,N.C.RoseroNavarro,M.Aparicio,Y.Castro,

A.Duran,D.Ondratschek,L.Fedrizzi,Prog.Org.Coat.72(2011)3–14. [16]C.Motte,M.Poelman,A.Roobroeck,M.Fedel,F.Deﬂorian,M.-G.Olivier,Prog.

Org.Coat.74(2012)326–333.

[17]L.L.Hench,J.K.West,Chem.Rev.90(1990)33–72.

[18]X.Zhong,Q.Li,J.Hu,X.Yang,Prog.Org.Coat.69(2010)52–56.

[19]M.Feuillade,C.Croutxé-Barghorn,C.Carré,J.Non-Cryst.Solids352(2006) 34–341.

[20]L.B.Toung,W.S.Trahanovsky,J.Am.Chem.Soc.91(1969)5060. [21]F.L.S.Purgato,J.R.Romero,J.Catal.209(2000)394.

[22]D.L.Pavia,G.R.Lampman,G.S.Kriz,IntroductiontoSpectroscopy,2nded., Har-courtBrace,London,1996.

[23] S.Aubonnet,C.C.Perry,J.AlloysCompd.300–3001(2000)224–229. [24]L.D.Landau,B.G.Levich,ActaPhysiochim.U.R.S.S.17(1942)42–54. [25]A.Kupicka,Prog.Org.Coat.46(2003)32.

[26]F.Bondioli,R.Taurino,A.M.Ferrari,J.ColloidInterfaceSci.334(2009)195–201. [27] S.M.Noh,J.W.Lee,J.H.Nam,J.M.Park,H.W.Jung,Prog.Org.Coat.74(2012)

192–203.

[28]S.V.Lamaka,M.L.Zheludkevich,K.A.Yasakau,R.Serra,S.K.Poznyak,M.G.S. Ferreira,Prog.Org.Coat.58(2007)127–135.

[29] M.L.Zheludkevich,K.A.Yasakau,A.C.Bastos,O.V.Karavai,M.G.S.Ferreira, Elec-trochem.Commun.9(2007)2622–2628.

[30]L.Freire,M.J.Carmezin,M.G.S.Ferreira,M.F.Montemor,Electrochim.Acta55 (2010)6174–6181.

[31] C.M.Abreu,M.J.Cristóbal,R.Losada,X.R.Nóvoa,G.Pena,M.C.Pérez, Elec-trochim.Acta51(2006)1881–1890.

[32]C.F.Malfatti,T.L.Menezes,C.Radtke,J.Esteban,F.Ansart,J.P.Bonino,Mater. Corros.62(2011)9999.