综述sol-gel coating on metal for corrosion protection

ProgressinOrganicCoatings64(2009)327–338

ContentslistsavailableatScienceDirect

ProgressinOrganic

Coatings

journalhomepage:http://wendang.chazidian.com/locate/porgcoa

t

Review

Sol–gelcoatingsonmetalsforcorrosionprotection

DuhuaWang,Gordon.P.Bierwagen?

DepartmentofCoatingsandPolymericMaterials,NorthDakotaStateUniversity,Fargo,ND58105,USA

articleinfoabstract

Sol–gelprotectivecoatingshaveshownexcellentchemicalstability,oxidationcontrolandenhancedcorro-sionresistanceformetalsubstrates.Further,thesol–gelmethodisanenvironmentallyfriendlytechniqueofsurfaceprotectionandhadshowedthepotentialforthereplacementoftoxicpretreatmentsandcoat-ingswhichhavetraditionallybeenusedforincreasingcorrosionresistanceofmetals.Thisreviewcoverstherecentdevelopmentsandapplicationsofsol–gelprotectivecoatingsondifferentmetalsubstrates,suchassteel,aluminum,copper,magnesiumandtheiralloys.Thechallengesforindustrialproductionsandfutureresearchonsol–gelcorrosionprotectivecoatingsarealsobrie?ydiscussed.

©2008PublishedbyElsevierB.V.

Articlehistory:

Received31October2007

Receivedinrevisedform12August2008Accepted12August2008Keywords:Sol–gel

CorrosionresistanceProtectivecoatings

Contents1.2.

Introduction.........................................................................................................................................Generalbackgroundofsol–gelcoatings............................................................................................................2.1.Briefhistoryofsol–gelchemistry...........................................................................................................2.2.Preparationofsol–gelcoatings..............................................................................................................Corrosionprotectivesol–gelcoatings..............................................................................................................3.1.Steelsubstrates..............................................................................................................................

http://wendang.chazidian.comanic–inorganichybridsol–gelcoatings........................................................................................3.1.3.Inhibitordopedsol–gelcoatings...................................................................................................3.1.4.Inorganiczinc-richcoatings.......................................................................................................

3.2.Aluminumsubstrates........................................................................................................................

http://wendang.chazidian.comanic–inorganichybridsol–gelcoatings.......................................................................................3.2.3.Hybridsol–gelmagnesium-richcoatings.........................................................................................

3.3.Copperandmagnesiumsubstrates.........................................................................................................Challengesandfuturestudiesofsol–gelcorrosionprotectivecoatings............................................................................4.1.Basictheorystudiesofsol–gelcoatings.....................................................................................................4.2.Optimizationandnewsynthesisroutesofsol–gelcoatings................................................................................4.3.Newrawmaterialsandmultiplecomponentsystems......................................................................................Conclusions.........................................................................................................................................Acknowledgments..................................................................................................................................References...........................................................................................................................................

327328328329329329329331332332333333334335336337337337337337337337

3.

4.

5.

1.Introduction

Metals,suchasiron,aluminum,copperandmagnesiumandtheiralloysareusedinamyriadofstructural,marine,aircraftappli-

?Correspondingauthor.

E-mailaddress:Gordon.Bierwagen@ndsu.edu(Gordon.P.Bierwagen).0300-9440/$–seefrontmatter©2008PublishedbyElsevierB.V.doi:10.1016/j.porgcoat.2008.08.010

cationsandculturalheritage,etc.Whilethesemetalsareusefulbecauseoftheirphysicalcharacteristics,suchasstiffnessandhighstrengthtoweightratios,theyarehighlysusceptibletocorrosioninaggressiveenvironments.Corrosionisalwaysthemajorreasonofenergyandmaterialloss.Itwasreportedthat1/5ofenergygloballyandaverage4.2%ofgrossnationalproduct(GNP)islosteachyearduetocorrosion[1]andtheeconomicimpactofcorrosionisesti-matedtobegreaterthan$100,000,000,000peryearintheUnited

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Statesalone[2].Thiscostincludestheapplicationofprotectivecoatings(paint,surfacetreatment,etc.),inspectionandrepairofcorrodedsurfacesandstructures,anddisposalofhazardouswastematerials.Agenericwaytoprotectmetalsfromcorrosionistoapplyprotective?lmsorcoatings,whichalsopermitthedesiredpropertiesofthesubstratetobecoatedthroughthechemicalmod-i?cationofthecoatings[3,4],suchasmechanicalstrength,opticalappearance,bioactivity,etc.

Thereareseveraltechniquesforthedepositionofcoatingsonmetals,includingphysicalvapordeposition(PVD),chemicalvapordeposition(CVD),electrochemicaldeposition,plasmaspray-ingandsol–gelprocess.Therearemanyadvantagesusingsol–gelcoatings,severalmostimportantfeaturesarelistedasfollows[5,6]:

(A)Sol–gelprocessingtemperaturegenerallyislow,frequently

closetoroomtemperature.Thusthermalvolatilizationanddegradationofentrappedspecies,suchasorganicinhibitors,isminimized.

(B)Sinceliquidprecursorsareuseditispossibletocastcoatingsin

complexshapesandtoproducethin?lmswithouttheneedformachiningormelting.

(C)Thesol–gel?lmsareformedby“green”coatingtechnologies:

Itusescompoundsthatdonotintroduceimpuritiesintotheendproductasinitialsubstances,thismethodiswaste-freeandexcludesthestageofwashing.

Tenyearago,Guglielmi[7]hasalreadydiscussedthepotentialofsol–gelcoatingsasacorrosioninhibitingsystemformetalsub-strates.Sincethen,agreatdealofworkhasbeendonetomakevarioussol–gelbasedprotectivecoatings.Thisreviewwillintro-ducethebasicchemistryinvolvedinsol–gelprocesses,thentheprogressanddevelopmentofsol–gelprotectivecoatingsonmetalsubstrate,suchassteel,aluminum,etc.Finallysomeproblemsandfutureworkonsol–gelcoatingswillbesummarizedbrie?y.2.Generalbackgroundofsol–gelcoatings2.1.Briefhistoryofsol–gelchemistry

Thesol–gelprocessisachemicalsynthesismethodinitiallyusedforthepreparationofinorganicmaterialssuchasglassesandceramics[8].Andthisprocesscanbetracedbackto1842,whenFrenchchemist,J.J.Ebelmenreportedthesynthesisofuraniumoxidebyheatingthehydroxide,buttheagingandheatingprocesslastalmostayeartoavoidcrackingwhichmadeitdif?cultforwiderapplicationanddidnotcatchmanyeyesthattime[9].Itwasnotuntil1950s,whenR.Royandhiscolleaguechangedthetraditionalsol–gelprocessintothesynthesisofnewceramicoxides,makingthesol–gelsilicatepowdersquitepopularinthemarket[10–12].In1971,theproductionprocessofso-calledlow-bulkdensitysilicainvolvingthehydrolysisoftetraethoxysilane(TEOS)inthepresenceofcationicsurfactantswaspatented[13].Inthe

middle

Fig.1.Hydrolysisandcondensationinvolvedinmakingsol–gelderivedsilicamaterials.

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1980s,manymaterialscientistsandchemists,representedbyH.SchmidtandG.L.Wilkesstartedtosynthesisorganic–inorganichybridmaterials(OIHMs)bysol–gelprocessandpublishedaseriesofpioneeringresearcharticles[14–17].Sincethen,sol–geltechnologyhasattractedagreatdealofattention,especiallyinthe?eldsofceramics,polymerchemistry,organicandinorganicchemistry,physicsandplayedanindispensableroleinpreparingnovelOIHMs[5,18,19].

2.2.Preparationofsol–gelcoatings

Thesol–gelprocesscanbedescribedasthecreationofanoxidenetworkbyprogressivecondensationreactionsofmolecularpre-cursorsinaliquidmedium[18].Basically,therearetwowaystopreparesol–gelcoatings:theinorganicmethodandtheorganicmethod.Theinorganicmethodinvolvestheevolutionofnetworksthroughtheformationofacolloidalsuspension(usuallyoxides)andgelationofthesol(colloidalsuspensionofverysmallparticles,1–100nm)toformanetworkincontinuousliquidphase.Butthemostwidelyusedmethodistheorganicapproach,whichgenerallystartswithasolutionofmonomericmetalormetalloidalkoxideprecursorsM(OR)ninanalcoholorotherlow-molecularweightorganicsolvent.Here,Mrepresentsanetwork-formingelement,suchasSi,Ti,Zr,Al,Fe,B,etc.;andRistypicallyanalkylgroup(CxH2x+1).

Generally,thesol–gelformationoccursinfourstages:(a)hydrolysis,(b)condensationandpolymerizationofmonomerstoformchainsandparticles,(c)growthoftheparticles,(d)agglom-erationofthepolymerstructuresfollowedbytheformationofnetworksthatextendthroughouttheliquidmediumresultinginthickening,whichformsagel.Infact,boththehydrolysisandcondensationreactionsoccursimultaneouslyoncethehydrolysisreactionhasbeeninitiated.AsseeninFig.1,boththehydrolysisandcondensationstepsgeneratelow-molecularweightby-productssuchasalcoholandwater.Upondrying,thesesmallmoleculesaredrivenoffandthenetworkshrinksasfurthercondensationmayoccur.Theseprocessesarebasicallyaffectedbytheinitialreac-tionconditions,suchaspH,temperature,molarratiosofreactants,solventcomposition,etc.Readersmayrefertootherstudiesandreviewsforamorecompleteunderstandingoftheentiresol–gelprocess[6–8,18,19].

Asol–gelcoatingcanbeappliedtoametalsubstratethroughvarioustechniques,suchasdip-coatingandspin-coating,whicharethetwomostcommonlyusedcoatingmethods.Spraying[20,21]andelectrodeposition[22–24]alsoemergedrecentlyandcouldbethemajorsol–gelcoatingapplicationmethodsinthefuture.Butwhatevertechniqueisused,afterthecoatingdeposition,thereisasubstantialvolumecontractionandinternalstressaccumula-tionduetothelargeamountofevaporationofsolventsandwater.Cracksareeasytoformduetothisinternalstressifthe?http://wendang.chazidian.comuallythecuringandheattreatmentofsol–gelcoatingsvarysubstantiallydepend-ingondifferentmicrostructures,qualityrequirementandpracticalapplication.

Theformationofsilicasol–gelsalsoholdstruefornon-silicateinorganicalkoxides.Infact,metalalkoxidesoftitanium,zirco-nium,tinoraluminumaremuchmorereactivetowardswaterthanalkoxysilanesduetothelowerelectronegativityandhigherLewisacidity[8,25].Butitisthatthereactionisquitegentleandmildmakesthealkoxysilanesstudiedmostextensivelyintheformationofsol–gelmaterials,especiallyOIHMs.Alkoxysilanes,includingtetraoxysilicate(Si(OR)4)andorganicallymodi?edsil-icates(Ormosils,R’nSi(OR)4?nor(RO)3SiR’Si(OR)3)havebeenthemostwidelyusedmetal-organicprecursorsforpreparationofhybridmaterialsbysol–gelprocessing.Table1andFig.2listssome

Table1

Abbreviation,chemicalnameandfunctionalgroupofsomecommonlyusedalkoxysilaneprecursorsforsol–gelprotectivecoatingAbbreviationChemicalname

Functionalgroup

TEOSTetraethylorthosilicateTMOSTetramethylorthosilicateMTESMethyltriethoxysilaneMethyl-MTMSMethyltrimethoxysilaneMethyl-VTMSVinyltrimethoxysilaneVinyl-PTMSPhenyltrimethoxysilanePhenyl-PHSDiethylphosphonatoethylPhosphonato-triethoxysilaneAPS3-AminopropylAmino-trimethoxysilane

AEAPS3-(2-Aminoethyl)aminopropylAmino-trimethoxysilaneGPTMS3-GlycidoxypropylGlycido-trimethoxysilane

MAPTS?-MethacryloxypropylMethacryloxy-trimethoxysilaneMPTMS?-MercaptopropylMercapto-trimethoxysilane

BTSTS

Bis-[3-(triethoxysilyl)-Sul?de-

propyl]tetrasul?de

ofthemostcommonlyusedalkoxysilanesinsol–gelprotectivecoatingsarea.

3.Corrosionprotectivesol–gelcoatings3.1.Steelsubstrates

Steelandstainlesssteelarewidelyusedindifferentindus-trial?eldsbecauseoftheirmechanicalandcorrosionproperties.However,theystilltendtocorrodeinthepresenceofhalideions.Thecorrosionresistancebehaviorofsol–gelcoatingsorthin?lmsdepositedontosteelsubstratehasbeenextensivelystudied[26–45],assummarizedinTable2followingthetimeofpublication.3.1.1.Metaloxidecoatings

SiO2,ZrO2,Al2O3,TiO2andCeO2,etc.allhaveverygoodchemicalstabilityandcanprovideeffectiveprotectiontometalsubstrate.

SiO2canimprovetheoxidationandacidiccorrosionresistanceofmetalsunderdifferenttemperaturesduetoitshighheatresis-tanceandchemicalresistance[29,34].Vasconcelosetal.madeSiO2coatingonAISI304stainlesssteelusingtetraethylorthosilicate(TEOS)aschemicalprecursor[34].ItwasfoundthatthecoatingcontainedSi,OandFeelementsandformedatransitionlayerbetweensteelsubstrateandSiO2layer.Theobtainedsol–gelsilicacoatingswerehomogeneous,freeofcracks.Samplesweretestedin1mol/LH2SO4solutionand3.5%NaClsolution,bothcorrosionpotentialincreasedandcorrosioncurrentdensitydecreased,indi-catingthis100nmthinSiO2layerimprovedtheanti-corrosionperformanceofstainlesssteelsubstrate.

ZrO2hasahighexpansioncoef?cientveryclosetomanybulkmetals,whichcanreducetheformationofcracksduringhightem-peraturecuringprocess[26,36].ZrO2alsoshowsgoodchemicalstabilityandhighhardness[35]whichmakesitagoodprotectivematerials.Perdomoetal.[31]madeZrO2coatingson304stainlesssteelbysol–gelmethodusingzirconiumpropoxideasprecursoranddensi?edinairandinoxygen-free(argonornitrogen)atmo-spheres.Thecorrosionbehaviorofthestainlesssteelsubstratewasstudiedbypotentiodynamicpolarizationcurves.ItwasfoundthattheZrO2coatingsextendedthelifetimeofthematerialbyafactorofalmosteightinaveryaggressiveenvironment,independentlyofthepreparationprocedure.Inordertoimprovetheadhesionbetweenprotectiveorganiccoatingandmetalsubstrate,Fedrizzi

330D.Wang,Gordon.P.Bierwagen/ProgressinOrganicCoatings64(2009)

327–338

Fig.2.Chemicalstructureofsomecommonlyusedalkoxysilaneprecursorsforsol–gelprotectivecoating.

etal.[35]preparedZrO2sol–gelcoatingonlowcarbonsteelsheets,thenappliedpolyesterorganiccoatingontotheZrO2layer.Accord-ingtoadhesiontesting,thesamplespretreatedwithZrO2layershowedpromisingperformance,incomparisonwithcommercialchemicaltreatments,suchastricationicphosphateandironphos-phatepretreatment.Lietal.[36]alsoreportedonthinZrO2sol–gel?lmonmildsteelsheets,andfoundthatZrO2layersheat-treatedat400?Cand800?Cwerehomogeneous,crack-freeandincreasedthecorrosionresistanceofthemildsteelbyafactorof6.3and2.3,respectively.

Al2O3isawell-knowninsulatorandhasverylowconduc-tivityfortransmittingelectrons,whichisidealforprotectivecoatings.Masalskietal.[33]preparedtwo-,four-andsix-layerAl2O3coatingsonAISI316stainlesssteelinordertoimproveitslocalanti-corrosionability.Itwasfoundthatthecathodecur-rentdensityvariedwithsinteringtemperature:highersinteringtemperature(withintherange500–850?C),thelowercathodecur-rentdensityvalues,butalsothelowerbreakdownpotentials.Theauthorbelievedthatathighertemperaturesconversionof?-Al2O3(lessresistanttoaggressiveagents)intothe?-Al2O3modi?cation(corundum,moreresistanttoaggressiveagents)proceedsmore

readily.However,ontheotherhand,anincreaseinthesinteringtemperatureresultedinamarkedincreasedontheanodicbranchofthepolarizationcurveandthusincreasedthenumberofdefectsinthecoating.

TiO2hasexcellentchemicalstability,heatresistanceandlowelectronconductivity,makingitanexcellentanti-corrosionmate-rial.ButpureTiO2?lmismostlyusedincatalystchemistry.VeryfewTiO2?lmshavebeenreportedasprotectivecoatingsonsteelsubstrate[28].CeO2isinthesimilarsituation,althoughwidelyusedinoptics,catalystchemistry,pigments,superconductorsandsen-sors,ceriumismorepopularinhybridsol–gelcoatingsascorrosioninhibitors[41,44],whichwillbediscussedlater.

Twoandmultiple-componentoxidecoatingscanovercomethelimitationofsingle-componentoxidelayers,broadentheirappli-cationareasandimprovethecomprehensiveprotectiveabilityofsteelsubstrates.Earlyworks,suchasAtiketal.[26]reported70SiO2-30TiO2and75SiO2-25A12O3actingveryef?cientlyascorrosionprotectorsof316Lstainlesssteelsubstratesinaque-ousNaClandacidmediaatroomtemperature.The?lmscouldincreasethelifetimeofthesubstratebyafactorofupto10in3%NaCland5in15%H2SO4solutions.Inordertoimprove

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Table2

Corrosionprotectivesol–gelcoatingsonsteelsubstratesCompositionandprecursorsZrO2

TiO2-SiO2Al2O3-SiO2ZrO2-PMMACeO2TiO2

SiO2

SiO2-CaO-P2O5CH3-SiO2B2O3-SiO2MgO-SiO2ZrO2

ZrO2-PMMAAl2O3SiO2ZrO2ZrO2

TEOS-MAPTSTEOS-MAPTSSiO2-Na2OAPSAEAPSGPTMSMAPTSSiO2-PMMASiO2-PVB

Cerium-APSTEOS-MAPTSTEOS-MTES

Cerium-TEOS-MTESCaO-P2O5

Steelsubstrate316LSS316LSS304SS316LSS

CoatingmethodDip-coatingDip-coatingDip-coatingDip-coating

Thickness(?m)0.4–0.60.20.50.4–1.4

Referenceandyear[26]1995[27]1997[28]1997[29]1998

331

304SS,430SS304SS316LSS316LSS304SS

CarbonsteelMildsteel304SS304SS316LSS

Zinc-platedsteel

Dip-coatingDip-coatingDip-coatingDip-coatingDip-coatingDip-coatingDip-coatingDip-coatingDip-coatingElectrodepositing

0.2–20.70.2–1.02.0–3.00.150.3–0.60.20.21.0

[30]1998[31]1998[32]1999[33]1999[34]2000[35]2001[36]2001[37]2001[38]2003[22]2003

IronplateDip-coating10–12[39]2003

304SS,

Zinc-platedsteelCarbonsteelCarbonsteelGalvanizedsteel304SS316LSS

Dip-coatingDip-coatingBrushingDip-coatingSpin-coatingSpin-coating

1.02.1–2.5N/A4.01.9–2.01.0

[40]2004[41]2005[42]2006[43]2006[44]2006[45]2007

thebioactivityandcorrosionresistanceofanimplantmaterial,VijayalakshmiandRajeswari[45]recentlyreportedtheprepa-rationofCaO-P2O5coatingon316Lstainlesssteel.Thesol–gel?lmhadcombinedeffectsofgoodadherencewithhighercor-rosionresistanceactingasadiffusionbarrierandcouldbeusedasapotentialmaterialforimplantationpurposes.SimilarSiO2-CaO-P2O5coatingwasalsostudiedtoimprovethecorro-sionresistanceandbioactivityofstainlesssteelimplantmaterial[29].

http://wendang.chazidian.comanic–inorganichybridsol–gelcoatings

Fromthestudiesabove,theinorganicoxidecoatingscanprovidegoodprotectiononmetalsubstrates.Buttherearestillsomemajordrawbacksofthesecoatings,fromthestandpointofcorrosionresis-tantlayers:(1)oxide?lmsarebrittleandthickercoatings(>1?m)aredif?culttoachievewithoutcracking;(2)relativelyhightemper-atures(400–800?C)areoftenrequiredtoachievegoodproperties[8].

Toovercomethelimitationofpureinorganicsol–gelcoatings,suchasbrittlenessandhightemperaturetreatment,muchworkhasbeendonetointroduceorganiccomponentintotheinor-ganicsol–geltoformtheorganic–inorganichybridsol–gelcoatings.Thesematerialsturnedouttobeamongthemostinterestingareasofcoatingsscienceinlastdecade[27,32,39–44].

Thoughmanyorganic(polymeric/oligomeric)specieshavebeensuccessfullyincorporatedwithininorganicnetworksbydif-ferentsyntheticmethods,theyareclassi?edintothreemajorapproachesaccordingtothechemicalbondbetweeninorganicandorganicphases:(1)mixorganiccomponentdirectlyintotheinorganicsol–gelsystem,theproductisasimplemix-ture,andthereisnochemicalbondingbetweenorganicandinorganiccomponents;(2)utilizealreadyexistingfunctionalgroupswithinthepolymeric/oligomericspeciestoreactwiththehydrolizedofinorganicprecursors,thusintroducingchemi-calbondingbetweenthem;(3)usealkoxysilanesR’nSi(OR)4?nasthesoleoroneoftheprecursorsofthesol–gelprocesswithR’beingasecond-stagepolymerizableorganicgroupoftencar-riedoutbyeitheraphotochemicalorthermalcuringfollowingthesol–gelreaction,e.g.methacryloxygroupinMAPTS(seeTable1andFig.2).

Atiketal.[27]madehybridcoatingsofpolymethylmethacrylate(PMMA)andZrO2onto316Lstainlesssteel.Coatings’anticorrosionbehaviorwasanalyzedin0.5MH2SO4solutionthroughpotentio-dynamicpolarizationcurvesatroomtemperature.Thecoatingsactasgeometricblockinglayersagainstthecorrosivemediaandincreasethelifetimeofthesubstrateuptoafactor30.Messaddeqetal.[32]analyzedthemicrostructureofZrO2-PMMAcoatingbyscanningelectron(SEM)andatomicforcemicroscopy(AFM)andfoundthatzirconiumconcentrateddomainsweresurroundedbycontinuousPMMAsecondaryphasedomains.Maximumcorrosionresistanceofthesubstratewasobservedforthecoatingcontain-ing17vol.%PMMA.HigherPMMAvolumemadethickercoatingsbuttendedtoformasingle-phasestructureatthemicrometerscaleandtheiradhesiontothesubstratewasworseresultinginthebreakdownandthepeelingofthecoatingduringtheelectro-chemicaltesting.Similarly,aSiO2-PVB(polyvinylbutyral)hybrid