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New architectured hybrid sol-gel coatings for wear and
corrosion protection of low-carbon steel
Claire Lavollée, Marie Gressier, Julien Garcia, Jean-Michel Sobrino, Jean
Reby, Marie-Joëlle Menu, Stefano Rossi, Michele Fedel
To cite this version:
Claire Lavollée, Marie Gressier, Julien Garcia, Jean-Michel Sobrino, Jean Reby, et al.. New
archi-tectured hybrid sol-gel coatings for wear and corrosion protection of low-carbon steel. Progress in
Organic Coatings, Elsevier, 2016, 99, pp.337-345. �10.1016/j.porgcoat.2016.06.015�. �hal-01473628�
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Eprints ID : 16642
To link to this article : DOI:10.1016/j.porgcoat.2016.06.015
URL :
http://dx.doi.org/10.1016/j.porgcoat.2016.06.015
To cite this version : Lavollée, Claire and Gressier, Marie and Garcia,
Julien and Sobrino, Jean-Michel and Reby, Jean and Menu,
Marie-Joëlle and Rossi, Stefano and Fedel, Michele New architectured
hybrid sol-gel coatings for wear and corrosion protection of
low-carbon steel. (2016) Progress in Organic Coatings, vol. 99. pp.
337-345. ISSN 0300-9440
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New
architectured
hybrid
sol-gel
coatings
for
wear
and
corrosion
protection
of
low-carbon
steel
Lavollée
Claire
a,b,∗,
Gressier
Marie
a,
Garcia
Julien
b,
Sobrino
Jean-Michel
b,
Reby
Jean
c,
Menu
Marie-Joëlle
a,
Rossi
Stefano
d,
Fedel
Michele
daUniversitédeToulouse,UniversitéPaulSabatier,InstitutCarnotCIRIMAT,118RoutedeNarbonne,31062ToulouseCedex09,France bCETIM,Pôle“MatériauxMétalliquesetSurfaces”,52AvenueFélixLouat,CS80067,60304SenlisCedex,France
cCETIM,Pôle“MatériauxMétalliquesetSurfaces”,74RoutedelaJonelière,CS50814,44308NantesCedex3,France dDepartmentofIndustrialEngineering,UniversityofTrento,viaSommarive9,38123Trento,Italy
Keywords: Mildsteel Architecturedcoating Sol-gel Corrosionprotection Wear Abrasion
a
b
s
t
r
a
c
t
Thereplacementofexpensivestainlesssteelinvarioussocio-economicsectorssuchasmechanicalor alimentaryisanissuethatwouldbepossibletosolvebydevelopingaprotectivecoatingonlow-carbon steel.Intheseapplications,complexpiecesareincontactwithdifferentkindsoffluids,withorwithout particleswhenfunctioning.Consequently,theexpectedcoatingfunctionistoeffectivelyprotectthe equipmentfromcorrosion,abrasionanderosion.
Inthisworkthinhybridcoatingsobtainedbythesol-gelprocesshavebeendevelopedforcorrosion andwearprotectiononlow-carbonsteel.Thisinnovativesystemisconstitutedofalumino-silicateepoxy basedsol-gelcoatingsactingasbarrierlayerswhich,whenloadedwithzirconiaparticles,improvethe mechanicalproperties.Takingintoaccountthespecificityofthecarbonsteel,wedevelopedtwo architec-turedcoatingsdisplayingcorrosionandwearprotection.Theyarebuiltbysuperpositionofabi-layered hybridprimercoatingandamono-layeredzirconialoadedhybridcoating.Usingtwodifferent zirco-niacontents,30wt.%and40wt.%,thincoatingsof5and10mareachieved.Whatisofinterestisthat thecombinationofantiweartestsandEIStoevaluatetheinfluenceofabrasivewearonanticorrosion propertieshas,forthefirsttime,beendemonstratedonsuchthinhybridsol-gelcoatings.Thelossof corrosionprotectionofthelowerzirconialoadedcoatingwasattributedtotheformationoflocalized defectsafterremovalofmaterial.Onthecontrary,thehigherzirconialoadedcoatingdemonstratedan interestingcorrosionandwearbehaviorwiththeformationofacompactedlayeratthetopoftheouter layerprovidingabarriereffectagainstwaterandionpermeation.Tofurthercharacterizetheprotective systems,themorphologyandthemicrostructureofthecoatingswereinvestigatedbyscanningelectron microscopy.
1. Introduction
Variousprotectionsystemsareusedforcorrosionprotection ofmildsteel.Theuseoforganiccoatingsisoneofthemost com-monmethodsforcorrosionprotectionofindustrialcomponents. Accordingto the way theyare applied, paints can cause envi-ronmentalproblems.Thisisthecaseofliquidpaintscontaining volatileorganiccompoundsthethicknessofwhichrangefrom140 to600mdependingonthecorrosionrequirements.Water-based paintsarealsodevelopedbut stillcontainaresidualofsolvent.
∗ Correspondingauthorat:UniversitédeToulouse,UniversitéPaulSabatier, Insti-tutCarnotCIRIMAT,118RoutedeNarbonne,31062,ToulouseCedex09,France.
E-mailaddress:lavollee@chimie.ups-tlse.fr(L.Claire).
Toovercomeenvironmentalconcerns,apowdercoatingprocess is usedin theindustry tocoat and protect metalsurfaces.The mostcommonlythermosettingresinpowdersusedarepolyesters, epoxy,epoxypolyesters[1–3].Thesecoatingsareappliedinone layer(60–80m)ortwolayers(160–200m)dependingonthe exposureconditionsandtherequireddurability.Recently, Monte-mor[4]described,inareview,differentfunctionalcoatingsand especially siloxane-modified epoxy coatingsdeposited onsteel [5–7] which significantly improve conventional epoxy coatings properties.Anotherprocessusedinindustrialsectorsisa protec-tivethinlayerelectrodeposited[8](15–25m),butdespitegood corrosionresistancetosaltspraytest,additivespresentinthebaths suchasco-solvents and heavymetals cancause environmental problems.
Theseprotectivesystemspresenthighthicknessesorevenshow limitationssuchasthepresenceofco-solvents,heavymetalsor volatileorganiccompounds.Asuitabletechniquetocombinehard andpliantmaterialsisthesol-gelprocess,wherethehydrolysisand condensationreactionsofinorganicalkoxidesprovideaninorganic networkafterappropriatethermaltreatment.Whenorganosilanes areinvolvedintheprocess,ahighlycross-linkednetwork contain-ingorganicandinorganicpartsisformed.Thesematerialsareof interestbecausetheycombinebothpropertiesoftheorganic poly-merandtheinorganicmaterials.Thehardnessandtheflexibility ofthecoatingcanbeadjustedbytheamountofinorganic com-pounds(alumina,silica...)aswellaspolymerizablegroups(e.g. epoxy,vinyl,methacrylate)fortheformationofadensenetwork. Moreoverthechemicalcompatibilitywithorganicpaintsfacilitates theadhesionofthecoatingtothesubstrateandtheintroduction of corrosion inhibitors suchas cerium(III) salts improves coat-ingperformancesagainst corrosion[9,10]. Finally,thehardness andmechanicalresistanceoftheorganic-inorganichybrid coat-ingscanbeincreasedbynanoparticlereinforcement.Amongthe well-describedcoatingscontainingoxidenanoparticleswhichare essentiallydepositedonaluminumalloys,somepapersdealwith coatedsteelrelatingtodifferentnanoparticlese.g.Zn[11], Mont-morillonite[12],Al2O3 [13,14],ZrO2 [15,16], SiO2 [14,15,17,18]
distributedinthehybridmatrix.Theintroductionofthe nanoparti-cleswithinthehybridsolmaybedoneeitherasnanoscalepowder [15],synthesizedin-situ[16]orcolloidalsuspension[18].
Theperformanceofacoatinginanabrasiontestdependson theintrinsicpropertiesofthecoatingmaterialaswellasthefilm thicknessandtheadhesionofthesol–gelcoatingtothesubstrate. Forexample,theoptimalamountofparticlesincorporatedinthe coatingsclearlydependsonitssol-gelmatrix.Thisishighlighted byGrundwürmeretal.[19]whodemonstratedthattheaddition ofzirconiananoparticlescouldeitherhaveapositiveornegative effectontheerosionperformances.Wilkesetal.[20].explainedthat sol-gelcoatingsappliedonvarioussubstrates,e.g.copper,brass andstainlesssteelareeffectiveinwearcontrolbuthaveapoorer performanceontheplainsteel,duetoalowerlevelofadhesionto thesubstrate.
Theaimedprotectivecoatingsrequiregoodresistanceto corro-sionandabrasion.Indeed,mechanicaldamagecanlocallyresultin thereductionoftheprotectivepropertiesofacoatingand,thus,a corrosionattackofthesubstratemightoccur.Hence,thenecessity toevaluatetheresistancetotheabrasionofthecoatingsystemsis achievedbyabrasiveweartests.Thesetestshavelimitationsinso farastheydonotdetermineaprecisevalueoftheprotective capac-ity,buttheydoallowtheperformanceofcomparativetestsamong thedifferentcoatingsystems.Themostinterestingabrasivewear testoncoatingistheTabertest[21],initiallydevelopedto evalu-atetheresistanceofthickorganiclayers[1–3],andlessappliedto othersol-gelcoatedsystems[16,20].
Wear and corrosion properties are usually independently investigatedsothattheinfluenceofmechanicaldegradationon corrosion coatings performance is not well-correlated. During mechanicaldamageaction,andtakingthicknessdecreaseandmass lossintoconsideration,asindicatedinthestandard,insufficient informationcanbeextrapolatedaboutthedamageofthecoating anditsresidualprotectiveproperties.Inthissense,atestprocedure hasbeendevelopedtoevaluatethedecreaseofprotective prop-ertiesbymeansofelectrochemicalimpedancespectroscopy(EIS), afterTaberAbraserstandardtest[1].
Todate,particle-reinforcedhybridsol-gelcoatingson corrod-ingmaterialssuchascarbonsteelshavebeenpoorlydeveloped. Oneofthestrategicissuesofthistechnologyishowthesol-gelcan beformulatedtoobtainbothhighcorrosionandabrasionresistant coatingsonmildsteel.Thissystemshouldcombinethebarrier
pro-Table1
Chemicalcompositionoflow-carbonsteel(Febalance).
Element C Mn Ni Cr Mo
Mass% 0.047 0.26 0.010 0.016 ≤0.005
tectioneffectofhybridsilicatedandceriumdopedcoatingswith thereinforcingactionofnanoparticles.
Theaimofthisworkwastodesigntwoinnovativearchitectured systemscombiningbi-andmonolayercoatings,theouterlayerof whichcontainszirconiananoparticles.Abrasiondegradationofthe architecturedorganic-inorganicsol-gelcoatingscausedbyTaber Abraserwasevaluated.Electrochemicalimpedancemeasurements andmicrostructuralanalysiswereundertakentoevaluatethe per-formanceoftheprotectingsystems.Thetwoarchitecturedsystems havebeencomparedandthebestanticorrosiveperformancesof suchsol-gelcoatingshavebeenhighlighted.
2. Experimentalprocedure
2.1. Samplepreparation
The metal substrates are panels of low-carbon steel 100mm×100mm×1mm the composition of which is given inTable1.Themetalsubstrates,aftersurfacecleaningwith ace-tone,werepre-treatedfollowingthesetwosteps:5minimmersion in NaOH,1.125molL−1 alkaline degreasing solutionmaintained at60◦Ctoremovealltracesofoilfollowingby5minimmersion inhydrochloricacid(5.5molL−1)at roomtemperature inorder toremoveoxidesorcompoundsonthesurfaceofthesampleand improvewettability.Samplesarefinallywashed inethanol and driedinair.
2.2. Solandcoatingpreparation
Glycidoxypropyltrimethoxysilane (GPTMS), aluminum iso-propoxide (AIP, ≥98%) and cerium nitrate hexahydrate (Ce (NO3)3,6H2O)areobtainedfromAldrich,polyethyleneglycolPEG
35000 is obtained from Fluka, zirconia nanoparticles (TOSOH, 40nm) were purchased from IMCD. The size of the TOSOH nanoparticleshasbeencontrolledbyFEG-SEMobservation. Zirco-niananoparticlesweresuspendedina1:25v:vwater:isopropanol mixture.Allothersreagentswereusedasreceived.
S0solformulationoftheprimerlayerwaspreparedbymixing GPTMS(5.94g,0.0251mol)andAIP(3.6g,0.0176mol)inamolar ratioof1.7,inisopropanol11.25mL(0.1479mol).Ceriumnitrate hexahydrate(1.00g,0.0023mol)wasdissolvedindistilledwater (25.8g,1.43mol)andthenaddedtothepreviousmixture.Thesol waspreparedforafinalvolumeof46.1mL.Itwasmaturatedduring 24hatroomtemperature.Aftermaturation,theviscosityoftheS0 formulationis7mPa.s.
FortheS1andS2zirconiananoparticlesloadedsols,firstGPTMS (9.52g,0.0403mol)andAIP(2.4g,0.0118mol))inamolarratioof 3.4,aremixedinisopropanol7.5mL(0.0986mol).Aqueoussolution ofceriumnitratehexahydrate(0.67g,0.0015molin6g,0.33mol ofwater,0.05molL−1)wasaddedtothesolwhichisvigorously stirred.ThenPEG35000(0.61g)dilutedindistilledwater 6mL) atafinalconcentrationof20gL−1wasaddedtothesol.Thesol wasmaturatedduring24hatroomtemperatureandthenXmLof solwasintroducedinthezirconiasuspensionscontaining13.6or 20.7gin25.9mLof1:25v:vwater:isopropanoltogiveS1andS2 solscontaining30wt.%and40wt.%ofZrO2nanoparticles,
respec-tively.Aftermaturation,theviscosityoftheS1andS2formulations reaches12and18mPa.srespectively.
Twoarchitecturedcoatingswereproducedbyadip-coating pro-cedureatacontrolledwithdrawalrateof20cmmin−1.Theywere
Fig.1.C1coatingafter1000cyclesabrasiontest.
composedofabi-layerscoatingusingS0sol,andazirconialoaded mono-layercoatinginvolvingS1(30wt.%)and S2(40wt.%)sols respectively,toprovideC1andC2architecturedcoatings.To elab-oratethesemulti-layeredsystems,thecoatingwasdriedaftereach dip1hat50◦Cthenthefinalheattreatmentwasprocessed50◦C for30minand150◦Cfor4h.
2.3. Characterizationofthesolsandthecoatings
TheviscosityofthesolsweremeasuredwithaRheomatRM100 rheometerforshearingratesrangingbetween644and966s−1.The thermalanalysisofthesolsandthexerogelswereperformedusing thedeviceSETARAM(TG-DTA92).Microstructuralpropertiesand coatingsthicknesswereobservedbyscanningelectronmicroscopy (SEM)withaJEOLJSM-6400SEMinstrument,byusingan environ-mentalscanningelectronmicroscopeTMPESEMFEI(ESEMPhilips XL30)andbyaJEOLJSM-6700F-EDSFEG-SEM.Theabrasiontests werecarriedoutwithaTaberAbraser5135usingabrasivewheels calledCS10wheels(smallabrasiveparticles,whichsimulatesthe effectofpolishingorcleaning),anappliedloadof250g;theentire testconsistedof1000cycles[18].Duringtheabrasiontesting,the wheelswererefacedaftereverystep abrasioncycles. The elec-trochemicalset-upexploitedtocarryouttheEISmeasurements consistedin a“mobile cell”[3] allowedtoinvestigatethe elec-trochemicalresponseoftheprotectionsystemaftereachabrasion step.
The“mobilecell”consistsintwoconcentricPVCcylinderswhich aredesignedtodelimitatetheTabertrack.Thecylindersarenot gluedtothesurfaceofthemetalbutarepressedtoonitbymeans offourscrewsconnectedtoanexternalframework.Theedgeofthe cylindersincontactwiththemetalarecoatedwithasilicongasket topreventtheelectrolytespread.AftereachEISmeasurement,the cellwasremovedandthesampleunderwentanewabrasionstep. Thetestedarea(about37.5cm2)includedthewholeabradedarea
whichconsistsofanapproximately1cmthickring,indicatedFig.1 bythedottedline.
Toavoidtheinfluenceofpossiblewateruptake,aftereveryEIS testthesamplewastreatedfor10minat45◦Ctoinducewater evaporationandcooled10minatroomtemperature.EIS measure-mentswerecarriedoutattheinitialstep(0cycle)andafter100and 1000cyclesusingaPAR273potentiostatandaSolartron1255 fre-quencyresponseanalyzerinthefrequencyrangebetween100kHz and10mHz,witha20mVamplitudesignal,atthefreecorrosion potential.Acounterelectrode ofplatinumanda reference elec-trodeofAg/AgCl(+207mVvsSHE)wereused.Theelectrolytewas
a0.3wt.%Na2SO4solution.Suchamildmediumhasbeenemployed
astheaimistoinvestigatetheabrasionresistanceofthecoatings. Forthispurpose,EIShasbeenexploitedinordertomonitorthe decreaseofprotectionpropertiesofthecoatingspromotedonly bythemechanicalactionoftheabrasivewheels.Inthissense,the useofaggressivesodiumchloridecontainmediawouldlead to an“additional”extentofdegradationnotrelatedtowearprocess. ZSimpwinsoftwarewasemployedforthenumericalfittingofthe experimentalspectra.
Eachexperimentwasrealizedintriplicateandwasrepeatable.
3. Resultsanddiscussion
3.1. Architecturedcoatings
Architecturedsystemsinvestigatedin thisstudyare builtby superpositionof a bi-layeredhybridprimer coatingand mono-layered zirconia loaded hybrid coating. As recently reported, anticorrosion protections of steel substrates are obtained from anorganic-inorganichybrid,mostofteninvolving tetraethoxysi-lane(TEOS)associatedwith3-methacryloxypropyltriethoxysilane (MAP) or methyltriethoxysilane (MTES) and rarely with 3-glycidoxypropyltrimethoxysilane(GPTMS)[22].Mechanical prop-erties are improvedwhen aluminium alkoxide [23] or zirconia nanoparticles[15]areintroducedintheformulation.Inourwork thesolformulationswerebasedon glycidoxypropyltrimethoxysi-lane(GPTMS)andaluminumiso-propoxide(AIP)inisopropanol. Ceriumnitrateinaqueoussolutionwasintroducedasaninorganic corrosioninhibitor[24,25].Asstudiedinourpreviouswork[27] involvingGPTMS-AIPformulations,improvedcorrosionproperties wereobservedwhentheSi/Alratiowasintherange1and2so aSi/Alratioequalto1.7hasbeenusedforthebi-layeredprimer coating.S0solformulatedfromthesethreecompoundswas mat-urated24hatroomtemperature.Dip-coatingprocedureoftwo S0soldepositsonmildsteelsubstratewasfollowedbyathermal treatmentgivingabi-layeredhybridprimercoating.
Inordertoobtainthickerdeposits,S0formulationwas investi-gatedgivingS1andS2solswhichwillconstitute,afterdeposition, theoutermono-layer.TheSi/Alratiowaschosenequalto3.4to increasetheorganicpart,PEG35000wasincorporatedasa plasti-cizertoincreasetheviscosity[26]andzirconiaparticleswereadded toprovideareinforcementofcorrosionandmechanical proper-ties.S1andS2formulationscontain30wt.%and40wt.%ofZrO2
nanoparticlesrespecttothetotal sol,respectively.Before depo-sition solswerematurated24hat roomtemperature.Theheat treatmenthasbeendeterminedbythermalanalysisofthe xero-gelswhichshowstwomaintrends(seeSupplementarymaterial, Fig.1).Thepresenceofastableanddensifiedmaterialupto250◦C andtheslowdegradationoforganicmatterfrom250◦Cto600◦C. Therefore,inordertopreservetheorganicpartofthearchitectured coating,itwasdriedat50◦Cfor30minandthentreatedat150◦C for4h.
Consideringthesurfaceroughnessofthemildsteelintherange 0.8–1maftercleaningandthelowsolviscosityofS0 formula-tionaround7mPas,theprimerwasdepositedasabi-layeredthe thicknessofwhichreachesamaximumof4m,dependingonthe surfaceroughness.Thisprimercoatingishomogeneous,crack-free, coveringandlevelingthesubstrate.Thethicknessoftheouterlayer variesfrom2to3mto8minagreementwiththedifferent con-tentsofzirconiananoparticles,thehighertheamountofparticles inthesol,thethickerthecoating.Thisiswellillustratedbyelectron micrographsofthecross-sectionofthetwoundamagedsystems, reportedinFig.2,wherethethicknessofthetotalarchitectured systemsreachs5mand10mforC1andC2respectively.Both samplesexhibitananocompositeouterlayerconstitutedof
zirco-Fig.2. (a)SEMimageoftheC1architecturedsystemand(b)ESEMimageoftheC2architecturedsystemsbeforeabrasioncycles.
niananoparticlesembeddedinalumino-silicatedmatrixwithsome aggregatesofnanoparticles(Fig.2aandb).
ArchitectureoftheC1andC2coatingsiswellhighlightedin theseimageswiththedifferenceincontrastbetweenthemostly hybridpartoftheprimercoatingandthezirconiapartoftheloaded layer.ForbothC1andC2coatings,anextremelygoodcohesionof bothlayers,hybridandloaded,isdemonstratedheretakinginto considerationthattheseimagesareobtainedbyliquidnitrogen brittlefracturemethodforthepreparationofthesamplesforSEM andESEMobservations.
3.2. InfluenceoftheZrO2contentonthemorphologydamage
Inthissection,theTabertestsresultsaredescribedforthetwo zirconiacontentsloadedcoatingswithanimposedweightof250g andCS10abrasivewheels.Thethicknesswasmeasuredin corre-spondencetotheabrasiontrackafter100and1000cyclestogether withtheobservationofthemorphologyofthedamagedareas.Fig.3 showstheelectronmicrographsofthecross-sectionofthecoatings asliquidnitrogenbrittlefracture.Comparingtheevolutionofthe thicknessofbothcoatings,C1andC2coatingsdidnotexhibitthe samebehaviortowardtheappliedTabertest.ConcerningtheC1 coating,onecanseeaveryslightdecreaseofthecoatingthickness after100abrasioncycles(Fig.3a)andevenafter1000abrasion cycles(Fig.3b) wherethe thicknessstill remainsaround 5m. TheC2coatingbehaviorisdifferentsincethethicknessdecreases from10mto8mafter100cyclesuntil5mafter1000abrasion cycles(Fig.3dande)dependingontheareapositioninthetrack.
Surprisingly,theseabrasion conditionsare severeenoughto induceseriousmechanicaldamagebutnotenoughtoremovethe coating.However,advancedmicroscopicinvestigationsofthecross sectionoftheabradedsurfacesafter1000abrasioncyclesreveal thatbothcoatingsaredamaged,butdeeperdefectsaredetectedin thelowerzirconialoadedcoating.Atotalremovaloftheouterlayer isobservedinsomeareasforC1coatingasillustratedinrightside ofthismicrograph(Fig.3c)whereonlythenon-damagedprimer coatingispreservedwith2m thickness.Onthecontraryonly compactionoftheoutermono-layerisobservedfor thethicker C2coating(Fig.3e).Tocompletemicroscopiccharacterization,the surfaceofthedamagedcoatingwasinvestigated.Fig.4showsSEM imagesofthedamagedsurfacesaftertheabrasiontest.Decreasing thezirconiacontentleadstotheformationoflocalizeddefectsin theshapeofpunctualdefects(Fig.4a).Ahighermagnificationofthe imagecorrespondingtotheC1coatingabradedsurfaceshowsthe removaloftheouterlayer(Fig.4b)providingbareprimercoating. Evidenceofthepreservationofthehybridprimeronthemetallic substrateisdemonstratedbytheabsenceofthesubstrate topog-raphy.The C2coatingpresents anothermorphologyof damage
(Fig.4d)withanheterogeneoussurfacehighlightedbythedifferent contrastareaswhenobservinginsecondaryelectronmode.High relief,whichappearsclearerinthemicrograph,isattributedtoa highercoatingthickness,whereaslowrelief,whichappearsdarker inthemicrograph,isattributedtoalowercoatingthickness.To bet-tercharacterizethesetwocontrastedareas,FEG-SEMobservations havebeenundertakeninbothrelevantareasinthetrack,indicated bytheredsquare(Fig.4e).Asalreadyobservedinthemicrographof thecrosssection,thepresenceofzirconiananoparticlesinthelower thicknessareawasconfirmedathighermagnification(Fig.4f).The sameconclusionwasreachedafterobservationoftheclearestarea (seeSupplementarymaterial,Fig.3).
It isworth notingthat during abrasiontesting, theC2 coat-ingiscompressedundertheactionoftheloadandacompacted layer is formed. Fig. 5a clearly shows the presence of a com-pactedlayerattheextremesurfacefollowingtheabrasioncycles asevidenceofthemechanicalactionoftheabrasivewheels.SEM microscopyobtainedbyback-scatteredelectrondetectionmode (Fig.5b)allowedustoestimatethecompactionthicknessofabout 2moftheouterlayer.
The effectof zirconia loadingshows a cleardependency on damagelevelsofthecoatingsduringabrasioncycles.Giventhe experimentaldata,itispossibletoconcludethatdependingonthe anti-wearprotectivesystem,abrasiveparticlesofthewheelwill havedifferentactions.Theywilleitherproduceaseriesoflocalized defectswiththinnercoatings,orallowtheformationofacompact andprotectiveouterlayerbymechanicalabrasion.
3.3. InfluenceoftheZrO2contentonthecorrosionbehavior
Theelectrochemicalpropertiesofthecoatingwereevaluated fromthebeginning ofthetest and atintermediate stages.This wasdoneinordertofollowtheprogressivemechanicaldamage tothesamesampleandtoavoidtheinitialdifferences.Because theaimofthisexperimentisnottosimulatearealcorrosive envi-ronmentbuttoevaluatethelossofprotectionpropertiesdueto theabrasionactionoftheabrasivewheels,amoderatelycorrosive electrolyte(i.e.0.3wt%Na2SO4)waschosen[28].Aremovable
elec-trochemicalcellwasusedinordertoallow,oncethemeasurement hadbeenmade,tocontinuewiththeabrasiontest.Measurements wererepeatedthreetimesandtherepeatabilitywasconsidered satisfactory.Toobtainabetterdescriptionofthedamageprocess, EISspectrawereacquiredattheinitialstep,after100and1000 cycles.Fig.6showstheimpedancediagramsofthenon-abraded andabradedsamplesafter100and1000cyclesofabrasionusing CS10wheelsandanimposedloadof250g.Abaresteelplate(same compositionofthecoatedsamples)wasinvestigatedfor compari-son,inordertoprovideareferenceofthebehaviorofthemetalin
Fig.3.ESEMandSEMimagesofthecoatingthicknessinthetrackafterabrasiontest(a)after100cycles;(b)and(c)after1000cyclesfortheC1coating;(d)after100cycles; (e)after1000cyclesfortheC2coating.
theemployedelectrolyte.Noticethattheohmicdropdueto elec-trolyteresistivityisaround1k×cm2.Consideringtherelatively
highimpedanceofthecoatings,theohmicdropisexpectednot toremarkablyaffecttheexperimentalresults.However,asfaras thebaremetalisconcerned,theelectrolyteresistivityaffectsthe impedancespectrumwhichshowsanalmostresistivebehaviorin thehighandmiddlefrequencyrange.
The EIS spectra of the investigated samples, C1(green dot) andC2(redtriangle),looksimilarbeforeabrasiontesting(Fig.6). Thevaluesoftheimpedancemodulusinthelowfrequencyrange (0.01Hz)areabout106cm2 and107cm2 forthe40ZrO
2 and
30ZrO2 monolayer,respectively.Noticethatthetotalimpedance
forthethinnercoating(C1)isaboutoneorderofmagnitudehigher comparedtothethickerone(C2).Thisfactcanberelatedtoahigher amountofdefectsinthe10mthickcoatingcomparedtothe5m thickone.
Inanycase,thelowfrequencyimpedancevaluesforboth coat-ingsaresignificantlyhigherthanthebaresteelsubstrate(black
square),whoseimpedancemodulusreachesaboutof103cm2.At
0cycle,thelow-frequencyimpedanceisrelatedtotheglobal pro-tectionpropertiesofthesystem.Itisbelievedthat,afterthevery fewminutesofimmersionthelowfrequencyimpedancevaluesof 106cm2and107cm2dependmainlyonthecoatingpore
resis-tance.ThemeasuredimpedancevaluesforsamplesC1andC2are inagreementwiththosereportedinliterature[29]andshowthat thecoatingshaveagoodbarrierpropertytowardtheelectrolyte penetration.
Withtheexpectationthattheimpedancemodulusvalueswould increasewiththeglobalthickness,theinitialimpedancemodulus valuesseemtobemoredependentonthemicrostructureofthe coatedsystemsandtothepresenceandextentofdefects.
Evenifthecomparisonoftheporeresistancetothoseobserved inliteratureisdifficult,sinceitdependsonthefilmthickness,the electrolyteusedandtheimmersiontimeinthecorrosivesolution, theinitialvaluesofbothcoatingscanbeconsideredindicativeof theirprotectiveproperties.
Fig.4.SEMimagesofthecoatingdamagedsurfaceoftheC1(a–c)andC2(d–f)coatingafter1000cyclesabrasiontest,(a)×500(b)×2500(c)×10,000(d)×500(e)×150(f) ×50,000inthedarkerarea.
However,tobetterinvestigatethepropertiesonthecoatings beforetheTabertest,theimpedancespectracollectedoverC1and C2sampleswereinvestigatedbynumericalfitting.FromFig.7it ispossibletoobservethatthreerelaxationprocessesarepresent forbothsamples.Thefirst,locatedinthe105÷102Hzfrequency
range,ispartiallyoverlappedtothesecondone,observedinthe 102÷100Hzfrequencyrange.Thelastrelaxationprocessispresent
inthelowfrequencydomain,below10−1Hz.
Accordingtoliterature[30],thecircuitdepictedinFig.8has beenexploitedtofittheexperimentaldata.Thefirstresistance (namelyRe)stands for theelectrolyte resistance. The high
fre-quencytime constant (modelled using a resistance, RHF,and a
constantphaseelement,QHF)hasbeenattributedtothesol-gel
coating.Inparticular,RHFisbelievedtorepresentthepore
resis-tance attribute to the hybrid coating, while QHF its dielectric
response.Themiddleandlowfrequencytimeconstantshavebeen modelledusingresistanceandconstantphaseelementsinparallel (namelyRMFQMFandRLFQLF,respectively).Thelowfrequencytime
constantisexpectedtoberelatedtothefaradicprocessoccurring atthemetalsubstrate.Therefore,RLFcanbeconsideredasacharge
transferresistance,whileQLFaconstantphaseelementrelatedto
thedoublelayercapacitance.Ontheotherhandthephysical mean-ingoftherelaxationprocessoccurringinmiddlefrequencyrange andmodelledusingRMFandQMFconnectedinparallel,isnotclear.
Itcanberelatedtoprimercoatingand/ortothecorrosionproducts accumulatingatthemetal/solutioninterface.
Table2showstheresultsofthenumericalfittingdatafor sam-plesC1andC2(electrolyteresistance,RE,equalto1.04kcm2in
bothcases).Noticethat,accordingtotheappearanceofthespectra inFig.7,theporeresistance(RHF)attributedtothesol-gel
coat-Fig.5. SEMmicrographsoftheC2coatingafter1000abrasioncyclestest(a)fracturedsample,(b)polishedcrosssection. 10-3 10-2 10-1 100 101 102 103 104 105 106 102 103 104 105 106 107 10-3 10-2 10-1 100 101 102 103 104 105 106 102 103 104 105 106 107 10-3 10-2 10-1 100 101 102 103 104 105 106 102 103 104 105 106 107 10-3 10-2 10-1 100 101 102 103 104 105 106 0 20 40 60 80 10-3 10-2 10-1 100 101 102 103 104 105 106 0 20 40 60 80 10-3 10-2 10-1 100 101 102 103 104 105 106 0 20 40 60 80
0 Cycle
B are steel C 1 C 2 Im pedan ce m o du lu s / cm 2100
Cycles
B are steel C 1 C 21000 Cycles
B are steel C 1 C 2 B are steel C 1 C 2 - P h a s e ang le / Deg re e F req ue nc y / H z B are steel C 1 C 2 F req uen cy / H z B are steel C 1 C 2 F req uen cy / H zFig.6. Impedancemodulusandphaseplotsobtainedina0.3wt.%Na2SO4solutionforbaresteel(emptyblacksquare)andC1(greendot)andC2(redtriangle)coatedsteel,
beforeabrasiontest(0Cycles),after100abrasioncyclesandafter1000abrasioncycles.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferred tothewebversionofthisarticle.)
Table2
ResultsfromthenumericalfittingofEISspectraforsamplesC1andC2in0.3wt%Na2SO4beforeTabercyclesC2(electrolyteresistance,RE,equalto1.04kcm2inboth
cases). RHF(cm2) Pre-factorQHF (Ss␣cm−2) Exponent␣HF RMF(cm2) Pre-factorQMF (Ss␣cm−2) Exponent␣MF RLF(cm2) Pre-factorQLF (Ss␣cm−2) Exponent␣LF C1 4.64105 4.5310−9 0.94 1.97106 2.8210−8 0.56 1.43×109 4.0310−6 0.74 C2 1.51105 18.00×10−9 0.88 4.5×105 8.7710−8 0.65 3.12×109 8.3710−6 0.70
Fig.7. Impedanceandphaseangleplotscollectedin0.3wt.%Na2SO4solutionfor
samplesC1andC2beforetheabrasioncycles.
Fig.8. Electricalequivalentcircuitemployedfornumericalfittingofthe
experi-mentalspectra.
ingishigherforthethinnersample,comparedtothethickerone. Aspreviouslyanticipated,thisfactmightrelyonthepresenceof preferentialpathways(cracks/pores)inC2coatinginwhichthe electrolytecanpermeate.AsfarasthethicknessofC1andC2 coat-ingsisconcerned,(around5and10m,respectively)itisexpected tofindalowercapacitancevalueforthethickersol-gellayer. How-ever,itshouldbenotedthattheconstantphaseelement(QHF)for
sampleC2isaboutfourtimeshighercomparedtoC1.Evenifthe highfrequencyCPEexploitedforthenumericalfittingdonot cor-respondtoapurecapacitance(exponent“␣”isaround0.88–0.94), theincreaseinpre-factorQHFcanberelatedtoahigheramountof
waterinthecoating,inaccordancewiththesimultaneousdecrease inRHF.
Consideringthemiddleandlowfrequencyrelaxationprocesses, theexponent“␣”values(equalto0.56–0.65and0.70–0.74, respec-tively)relatedtotheCPEsexploitedforthenumericalfittingdo not permit any clear interpretation of the corresponding pre-factors.ThevaluesoftheRLFsuggestthattheelectrolytereached
themetalsubstrateandcorrosionprocessesareoccurringatthe metal/solutioninterface.The higher values obtainedfor theC1 coating(3.12×109cm2)comparedtoC2(1.43×109cm2)
fur-thersuggestthatbeforetheTabercyclesthethinnercoatingismore protectivecomparedtothethickerone.
Considering the effect of the abrasion wheelsduring Taber cycles,noticethatafter100cyclesofabrasion(Fig.6),the mechan-ical degradation and subsequent penetration of the electrolyte throughthecoatingsleadtothereductionoftheprotective proper-ties.Indeed,impedancemodulusvaluesofthesystemsdecreased from aboutone decade to 105ohmcm2. Despite differences in
thecoatingthickness,lowfrequencyimpedancemodulusvalues remaincomparableupto1000abrasioncycles(Fig.6).However after1000cycles,veryslightdifferencesoftheanticorrosion
prop-ertiesofthetwocoatingsareobserved,whichcanbeexplainedby themicrostructuralobservations.Fig.3aanddshowsthe differ-encesinthicknessbetweenbothcoatings30ZrO2and40ZrO2with
valueslessthan2mand3–4m,respectively.Itisworth not-ingthatthecoatingcontaining40%ofzirconiaparticlesmaintains a higherimpedance moduluscompared tothecoating contain-ing30%ofzirconiaparticles.Inthecaseofthelatter,thethinnest layerconstitutesafasterpathwayfortheelectrolytetopenetrate tothesubstrate.Afterincreasingthenumberofcycles(Fig.6),the totalimpedanceremainedconstantatarelativelylowfrequency, showingtheprotectivepropertiesofthecoatingswhichlimit cor-rosionpropagation.Betterwearresistantpropertiesaretherefore expectedforthesamplecontainingthehighernumberofparticles. Theeffectofmechanicaldegradationandabsorptionofliquid ontothecoatingisevenmoreremarkableconsideringthephase anglediagram(Fig.6).Forthesamplewith40%ofzirconia parti-clesthehighfrequencytimeconstantdecreasesinintensityandthe secondtimeinthemiddlefrequencyrangebecomesmore promi-nentafter100and1000cycles.Therelaxationprocessinthelow frequencyrangeisstillobservable.Howevertheexperimentaldata scattersbelow100Hzandaclearinterpretationistherefore
com-plicated.
Infact,astheabrasioncreatedlocalizeddefectsonthecoating, itproveddifficulttoseparatethecontributionofbothdamagedand undamagedareasintheEISresponse.Theauthorsattemptedtofit theexperimentalspectraofthedamagedcoatingsbymeansofthe previouslydescribedcircuit.However,aconsistentmathematical modellingoftheexperimentalcurveswasnotpossibleduetothe scatteringdatainthelowfrequencyrange.Thisisprobablyrelated totheeffectofmechanicaldamagewhichproducesanotuniform surfacewithhighroughness,scratches,thicknessdifferencesonthe Tabertrackspromotedbytheabrasionactionofthewheels.Forthis reasonitwasnotpossibletoobtainasatisfactorynumericalfitting oftheEISspectracollectedoverthedamagedsurfaces.
Generallyspeaking,theeffectoftheabrasionpromotesthe for-mationoflocalizeddefectsofthecoatings,athicknessreduction andconsequentincreaseofwaterandionspermeabilitywiththe numberofabrasioncycles.Figs.3cand4aillustratethepresence andthemorphologyofthedefectsandexplainthedifferent corro-sionprotectivebehaviorsofthecoatings.Moreover,inFig.4c,the SEMmicrographshowsthattheoutercoatingistotallyremoved anddoesnotplaytheroleofanti-wearcoatinganymore.Onthe otherhand,inagreementwithFig.5,thecompactedlayerofthe 40wt.% zirconia loadedcoating permitshigher electrochemical valuestobeobtainedandthusincreasethedurabilityofthe pro-tectivehybridcoating.Indeed,after1000abrasioncycles,despite theappearanceofasecondtimeconstantinthemiddlefrequency, the40wt.%zirconiaparticlescoatingseemstomaintainacertain barriereffectagainstwaterandionspermeation.
4. Conclusions
Inthisworkwehavedemonstratedthatthesol-gel technol-ogyoffersthepossibility tosynthesizeinnovativearchitectured systems for anticorrosion and wear properties. Promising anti-corrosion coatings have been deposited on a highly corrosion sensitiveandunevenmildsteelsubstrate.Thearchitectureisbased onalumino-silicatedhybridcoatingscontainingzirconia nanopar-ticles in theouterlayer,well-adapted to theaimedproperties. Abrasiontestconditionsundertakeninthisworkappeared well-adjustedtosuchthincoatingswithathicknesslowerthan10m. Asthedamageproducedbytheabrasivewheelswasrathera thin-ningofthecoating,estimationofthedamagelevelofthesamples wasthus carried out through microstructural observations and electrochemicalmeasurements.Thelossofcorrosionprotectionof
thelowerzirconialoadedcoatingwasattributedtotheformationof localizeddefectsaftertheremovalofmaterial.Onthecontrary,the higherzirconialoadedcoatingdemonstratedaninteresting corro-sionandwearbehaviorwiththeformationofacompactedlayerat thetopoftheouterlayerprovidingabarriereffectagainstwater andionspermeation.Furthermechanicalcharacterizationscould provideadditionalinformationaboutthehardnessofthisouter layer.
Whatisofinterestisthesuccessfulcombinationofweartests andEIStoevaluatetheinfluenceofabrasivewearonanticorrosion propertiesonsuchthinhybridsol-gelcoatings.
Acknowledgments
TheauthorswouldliketothankStephaneLEBLONDDUPLOUY (UMSCastaing)forSEMandEDScharacterizations.Thisworkwas performedintheframeworkofajointlaboratory,calledCETIMAT, whereCIRIMATandCETIMcollaborateforsomeaspectsoftheir research.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.porgcoat.2016. 06.015.
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