您的当前位置：首页 Analysis and interpretation of refractory microstructures in studies of corrosion

Analysis and interpretation of refractory microstructures in studies of corrosion

来源：小侦探旅游网

Available online at www.sciencedirect.comJournaloftheEuropeanCeramicSociety28(2008)1557–1568

Analysisandinterpretationofrefractorymicrostructuresin

studiesofcorrosionmechanismsbyliquidoxides

J.Poiriera,∗,F.Qafssaouib,J.P.Ildefonsec,M.L.Bouchetoua

CRMHT,1D,avenuedelaRechercheScientiﬁque45071Orl´eanscedex2,FranceMoulayIsma¨ıUniversity,FacultyofScience,B.P.4010Zitoune,50000Meknes,Morocco

cPolytech’Orl´eans,8rueL´eonarddeVinci,45072Orl´eanscedex2,France

Received1August2007;receivedinrevisedform29September2007;accepted7October2007

Availableonline4January2008

Abstract

Therefractoriesmustnotonlyresisthightemperaturesbutalsocorrosionbyliquidoxides.Thiscorrosioninvolvesphenomenaofdissolutionandprecipitationofnewcrystallinephases.Thestudyofthemicrostructuresofcorrodedrefractoriesprovidesessentialinformation.However,theinterpretationofthemicroscopicobservationsisdifﬁcult.Indeed,becauseofthecrystallizationofliquidglassesduringcooling,themineralphasesobservedatroomtemperaturearenotrepresentativeofthoseobservedathightemperature.Theconceptoflocalthermodynamicequilibriumandtheuseofthephaserulemakesitpossibletointerpretthemicrostructuresofcorrodedrefractories,toexplaintheobservedmineralzonationandtoquantifythecompositionoftheliquidphaseathightemperaturefromchemicalproﬁlesestablishedbySEM.Experimentaldatafromcorrosionofhighaluminarefractorieswillillustrateandvalidatethistheoreticalapproach.©2007ElsevierLtd.Allrightsreserved.

Keywords:Refractories;Corrosion;Microstructure;Thermodynamicequilibrium;Electronmicroscopy

1.Introduction

Therefractoriesareceramicsusedtolinemanyindustrialhightemperaturefurnaces.Thesematerialsaresubjectedtocom-plexdegradationssuchasthermalshock,erosionorchemicalcorrosionwhichcanoccurseparatelyortogether.

Corrosionbyliquidoxidesisoneofthemostseveremodesofdegradationswhichlimitthelifetimeoftherefractorylinings.Theexaminationofthemicrostructures1ofrefractoriesafteruseisextremelyusefultoevaluatethecorrosionresistanceofvariousrefractoriesandtodeterminethemechanismsofchem-icalattackthusmakingitpossibletoproposenewwaysofimprovementsfortheformulationofrefractories.

However,themicrostructuresofcorrodedrefractoriesareverydifﬁculttointerpret.Thereasonsareasfollows:

•Therefractoriesaremulti-componentandheterogeneousceramics.Consequently,theyhavecomplexmicrostructures.

•Themicroscopicobservationsandtheanalysesarecarriedoutatroomtemperature.Theyarenotrepresentativeofthemineralandvitreousphasesexistingathightemperature.•Duringcooling,newsolidphasesappearbycrystallizationofliquidoxides.Thecompositionofthevitreousphasesalsoevolveswiththetemperature(seeFig.1).Consequently,theinformationobtainedisoftenlimitedandgivesalreadyknownconclusions.

Inthispaper,wewillpresentamethodtoanalyseandinterpretthemicrostructuresofrefractoriesafteruse.Typicalexamplesofcorrosionofhighalumina-basedrefractorieswillbepre-sented.

Theexperimentalresults,whicharenotwellexplained,willbere-analysedandclariﬁed.

Byusingthermodynamicdataandphasediagrams,thestudyofthemicrostructureswillenableustodeduce:

•Theproportionandthecompositionofthehightemperatureintergranularliquidphase.

•Thegradientsofthechemicalpropertiesbetweenthehotfaceandthebackoftherefractorylining:thechemicalproﬁleof

∗

Correspondingauthor.Tel.:+33238255514;fax:+33238638103.E-mailaddress:Jacques.Poirier@univ-orleans.fr(J.Poirier).

1558J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

Fig.1.SEMmicrographsofandalusitebasedrefractoryin(a)naturallycooledand(b)quenchedcrucibles(laboratorytest:T=1600◦C,50wt%Al2O3–50wt%CaOslagandcruciblemethod).

compositionofglasses,theevolutionoftheviscosityoftheliquids.

2.Experimentalprocedureandexaminationofmicrostructures

Twohighaluminarefractorymaterialsbasedonandalusiteandbauxitehavebeenselectedforthepurposeofthisresearch.TheirformulationsandprincipalcharacteristicsareindicatedinTables1and2.ThetypicalmicrostructuresoftheoriginalbricksareshowninFigs.2and3.Fortheandalusiterefractory,aﬁnematrixcomposedofmulliteneedlesinasilica-richglassbindsthecoarsegrainsofmullitizedandalusiteconvertedintoamullite/glasscomposite.2Thebindingglasshasacomposi-tionsimilartothatresultingfromthemullitizationofandalusite(≈70%SiO2,≈25%Al2O3,≈2%Fe2O3,≈1%TiO2and≈1%K2O).

Table1

Formulationsofandalusiteandbauxitebricks(manufacturerdata)RawmaterialRandalusiteKerphaliteKAKerphaliteKFKerphaliteKFChinesebauxiteChinesebauxiteChinesebauxiteCalcinedaluminaClayRR40PlastiﬁerTotal

Size(mm)1–40.3–1.60–0.160–0.0551–40–10–0.16

Andalusitebrick(wt%)3035125

323391493100Bauxitebrick(wt%)

144

100

J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

Table2

Chemicalcomposition,densityandapparentporosityofandalusiteandbauxitebricksRefractories

Composition(wt%)Al2O3

AndalusitebrickBauxitebrick

.178.6

SiO233.411.2

CaO0.10.4

MgO0.10.4

Fe2O30.71.7

TiO20.23.3

Na2O0.10.2

K2O0.20.3

M***,A*

A***,M**,TiO2*,(Al,Fe)2TiO5*

2.653.24

11.0916.17

Mineralphases

Density(g/cm3)

1559

Apparentporosity(%)

Mineralphases:A→corundum,M→mullite(***:major;**:mean;*:minor).

Fig.2.Backscatteredelectronimagesofthetypicalmicrostructureofandalusitebrickshowingthemullite–glasscompositeandthebondingmatrixmadefromsilica-richglass:(a)lowmagniﬁcation(50×)and(b)highmagniﬁcation(1000×).M:mullite,G:glassandP:pore.

Forthebauxiterefractory,coarsegrainsarecomposedofcorundumwithsometialite(titaniumaluminatecontainingsomeiron).Theﬁnematrixcontainscorundum,mulliteandaglassyphase.Theintergranularglassisveryﬁneandhasacomplexchemicalcomposition(≈45%SiO2,≈20%Al2O3,≈8%Fe2O3,≈8%P2O5,≈8%TiO2,≈5%CaO,≈2%MgO,≈2%K2O,≈1%Na2O).

Corrosiontests,usingthestaticcruciblemethod,havebeencarriedoutat1600◦C,inanelectricfurnace,incorporatinganelevatinghearth,allowingfortheloadingandunloadingofthecrucible.Cruciblesof100mm×100mm×60mminsizewerecut-outfromthebricks,previouslyﬁredat1550◦Cfor12h.After6hofcorrosiontests,thecrucibleswererapidlyquenchedinwaterinordertoavoidpartialcrystallizationoftheliquidphaseduringcooling.

TheselaboratorytestswereperformedwithanAl2O3(50wt%)–CaO(50wt%)slag(labelledAC)whichisclosetothatofaladleslagallowingproductionofAlkilledcarbonsteels.Thissyntheticslagwaspreparedbymixing,inappropriatepropor-tions,powderscomprisingcalciumcarbonate(CaCO3>98%,AlfaAesar)andreactivealumina(CT300SG,Al2O3>99.8%).

Thepost-mortemanalysisofthecruciblesaftertesting(Figs.4and5)permittedtheidentiﬁcationoffourzones3:•theslagzone,

•theprecipitationzone,

•thepenetrationzone,

•theunaffectedrefractoryzone.2.1.Theslagzone

Aftercooling,aglassyslaglayerisobservedatthebottomofthecrucible.ItscompositionchangeswithadecreaseinCaOandanenrichmentinSiO2.TheAl2O3contentremainsnearlyconstantwithandalusitebricksandshowsasmallincreaseinthecaseofbauxitebricks.2.2.Theprecipitationzone

Theprecipitationzoneisformedbyasuccessionofmonominerallayersofcrystalssurroundedbyglass.Theglasscontentmaybehighandislocallyvariable(30–80%).

Thesamemineralsuccessioncanbeobservedinbauxiteandinandalusitebricks.Fromthepenetratedzonetotheslagzone,threesuccessivemonominerallayersareobserved:•corundumAl2O3,

•calciumhexaaluminateCaAl12O19(CA6),•calciumdialuminateCaAl4O7(CA2).

Thetextureandtheshapeofthelayersclearlyindicatesthatthecrystalswereprecipitatedslowlyfromaliquid4,5(theydonotresultfromacrystallizationduringcooling).Corundumfrom

1560J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

Fig.3.Backscatteredelectronimagesofthetypicalmicrostructureofthebauxitebrickshowingtheporousnatureofbauxiteandabondmatrixmadefromamixofcorundum(A),mullite(M),aluminumtitanate(AT),titania(T)andglass(G):(a)lowmagniﬁcation(25×)and(b):highmagniﬁcation(500×)→microstructuresofagrainofbauxitetransformedintocorundum,(c)highmagniﬁcation(1000×)→thebondingmatrix(c;1000×).AT:(Al,Fe)2TiO5,T:TiO2andP:pore.

theﬁrstlayershowswell-formedcrystalsthatdifferclearlyfromthoseofthetransformedbauxite.TheCA6phasesarestillpresent,butinthecaseofanimportantcorrosionoftherefractory,theCA2phasesareabsent.2.3.Thepenetrationzone

Theviscousslaginvadesthematrixbycapillarypenetrationandmixeswiththepre-existingintergranularliquidbydiffusion.Thepenetrationofthecorrosiveliquidphasesleadstothesuper-ﬁcialdissolutionoftheaggregates,sochangesaffectmainlythematrixwithaprogressiveincreaseintheglassphase.Fortheandalusitebrick,thelimitbetweenthepenetratedzoneandtheprecipitationzoneismoreregularthanforthebauxitebrick.Atthatzonelimit,dissolutionofmullitizedandalusitegrainsseemstobecompletedbythecorrosiveliquid,butlargebauxitegrainsconvertedtocorundummayremainbeyondintheprecipitationzone.

2.4.Therefractoryzone

Thiszoneiscomposedofunaffectedbauxiteorandalusiterefractories

3.Interpretationofcorrodedmicrostructures3.1.Equilibriumaspect

Inthezoneofcorrosion,theheterogeneoussystemrefrac-tory/slagisopenandexchangesmatterwiththesurrounding.Theconceptofthermodynamicequilibriumappliestoaclosedsystemwhichexchangesonlyheatandenergy.Itrequiresthechemicalpotentialofeachcomponentinallphasestobeequal.Whenasystemisopen,equilibriumisnottotal,butitcanbeappliedonthelocalscale.Theconceptof“localequilibrium”iswellestablishedintheliterature6,7andisafundamentalassumptioninmodelsofdiffusion-controlledreaction.

Inasolid–liquidrefractorysystem,chemicalexchangesoccurthroughtheliquidphase(diffusion,inﬁltrationandpercola-tion).Whensolid–liquidreactions(dissolution,precipitation)arefasterthanchemicaltransport,equilibriummotionmaybelocallyapplied:

•thesolidphasesarelocallyinequilibriumwiththeliquidwhichsurroundsthem;

•theGibbs’phaserulemaybeused;

J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–15681561

Fig.4.MicrostructureofcorrodedandalusiterefractorybyAl2O3–CaOslagattack.

•thechemicalmobilityentailsconcentrationgradients,whicharethedrivingforceofcorrosionandwilltendtobringthesystembacktowardsglobalequilibrium.Dependingonitslocalcomposition,theliquiddissolvesthesolidphaseswhicharenotinequilibriumwithitandprecipitatesnewphasesafteritbecomessaturated.3.2.Useofthephaserule

ThedegreeoffreedomF(variance)ofthesystemisgivenbythephaseruleofWillardGibbs.8Thefollowingequationgivestheusualmathematicalformofthephaserule:F=C+2−P

(1)

C:numberofcomponentsofthesystem;P:numberofphasespresentatequilibrium;2:numberofenvironmentalfactors(tem-peratureandpressure).

Whenthetemperatureandthepressureareﬁxed,theGibbs’phaserulereducestothefollowingequation:F=C−P

(2)

Thephaseruleappliesonlytoequilibriumstatesofasystem,whichrequirebothhomogeneousequilibriumwithineachphaseandheterogeneousequilibriumbetweenco-existingphases.Thephaseruledoesnotdependonthenatureandamountsofthephasespresent,butonlyontheirnumbers;nordoesitgiveinformationconcerningratesofreactions.Non-conformitywiththephaseruleistheproofthatequilibriumconditionsdonotexist.

Datafrommicrostructuralexaminations,describedabove,haveshowedthatasuccessionofzonesisobserved:theinitialrefractory,thepenetrationzone,theprecipitationzoneandtheremnantslag.Furthermore,theprecipitationzonemaycontainoneorseveralmonominerallayers.

Considerasanexamplethecorrosionofa3:2mullitizedandalusite-basedrefractorybyanAl2O3(50wt%)–CaO(50wt%)modelslag,at1600◦C.

3.2.1.Theinitialrefractory

Theinitialrefractoryiscomposedofasolidphase(3Al2O3–2SiO2)andaliquidphase(asilica-richglass)whichissaturatedwithmullite(P=2).ItcontainsonlyAl2O3andSiO2(C=2).ThedegreeoffreedomF(variance)isequalto0.

1562J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

Fig.5.MicrostructureofcorrodedbauxiterefractorybyAl2O3–CaOslagattack.

Consequently,thecompositionoftheliquidphaseisconstantandisgivenbythesaturationsolubilityofthemulliteintheliquidphase.

3.2.2.Thepenetrationzone

Therefractoryispartiallypermeatedbytheslag.Whenthelimediffusesthroughtheinterstitialliquid,thenumberofcom-ponentsCincreasesto3whereasthenumberofphasesPremainsequalto2.Thedegreeoffreedombecomesequalto1,andthecompositionoftheliquidisnotconstantanymore.Theconcentrationsofalumina,limeandsilicavaryintheliquid.3.2.3.Theprecipitationzone

Inthiszone,locatedbetweentheslagandthezoneofimpreg-nation,newlycrystallinephasesprecipitateduringthereactionsrefractory/slag.TheliquidcontainsthreeoxidesAl2O3,CaO,SiO2;thenumberofcomponentsCisequalto3.ThedegreeoffreedomFcanneverbelessthan0underinvariantconditions.Consequently,thenumberofphasesPcannotexceed3:twosolidsandoneliquid.Whentwosolidphasescoexist,thevari-anceisequalto0andthecompositionoftheliquidisconstant.However,becauseofdiffusionandofthechemicalgradients,theconcentrationofanelementintheliquidvariesregularlydependingonthedistanceandthetimeperiod.Attheendofatimet,theconditionsofequilibriumbetweentwosolidphases

canexistonlyatagivendistancefromtheinitialinterface.Lastly,whenonlyonesolidphaseispresent,thevarianceisequalto1,andthecompositionoftheliquidcanvary.

Thenewphaseswhichcrystallineduringdissolution–precipitationprocesses9consistofdifferentsuccessivemonominerallayersseparatedbysharpboundaries.3.3.Predictionofreactionproductsatrefractory/slaginterface

Attheinitialstageofthereaction,onbothsidesoftheinitialinterface,thereisliquidslag(50%CaOand50%Al2O3)andasiliceousliquidwhichimpregnatethemulliteaggregatesoftheandalusite-basedrefractory.ThisinitialsituationisdescribedinFig.6.Formoresimplicity,considertheexampleofaslagcontainingtheelementAandarefractorycontainingtheelementB.Theslagandtherefractoryarecoupledaccordingtoaplaneinitialinterface.

Intheslag,allareliquid,theconcentrationofelementAis

A),andthatoftheelementBisequalto0.Intherefrac-high(CSlag

tory,composedofthephaseB,thereisalittleliquidwithahigh

B=solubilityofBintheliquid),whiletheconcentrationofB(CB

concentrationofAisequalto0.

Onbothsidesoftheinitialinterface,thereisa“step”ofchemicalconcentration.Thissituationisunstable,andthestep

J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–15681563

Fig.6.Initialstageofcorrosionbetweenarefractoryandaliquidslag.

willbetransformedintoagradientofconcentration:elementAoftheslagwillmigratetowardstheliquidoftherefractory,andelementBoftherefractorywillmigratetowardstheslag.Fig.7showstheevolutionofthereaction.

Considerthat,inthesystemA–B,existthedifferentphasesA,AB,AB2,andB.Theconcentrationsatequilibrium,intheliquid,areasfollows:

••solubilityconcentrations,oftherespectivelyphaseBintheofAliquidandBslag:intheCBB

equilibriumwiththeassemblyB+ABAliquidphaseBin

•concentrations,respectivelyofAand2:CBinABthe2/BandCliquidABphase

2/B

inequilibriumwiththeassemblyAB2+AB:CACBAB/AB2

andAB/AB2

.AslongastheconcentrationofAintheliquidremainsthanCAlower

AB(inequilibriumwiththephasesA+B),itwillbelocalized2in/B

thezoneofinﬁltration.Theincreaseintheconcen-trationofAintheliquidandthedecreaseinBwhichdiffusestowardstheslag,involvesadissolutionofphaseB.

WhentheconcentrationinAexceedsCAbecomesstableandprecipitateswhilephaseAB2/B

,thephaseAB2Bdisappearsbydissolution.PhaseAB2remainstheonlystablesolidphaseaslongastheconcentrationofAintheliquidremainslowerthanCBAB/AB2

.Fig.7.Evolutionofthecorrosionofasolidrefractorybyliquidslag.Formationofazoneofimpregnation,reactionalmonomineralzonesanddisplacementoftheinterfaces.

Fig.8.Evolutionofthecorrosionofasolidrefractorybyliquidslag.Disappear-anceofthemonomineralzoneofABwhentheconcentrationofAisdecreasing

intheslag.

Thus,thereistheformationofamonomineralzoneofAB2.

Beyondthislimit,thephaseABprecipitatesandformsasec-ondmonomineralzone.Thelatterisgraduallydissolvedintheslag.

Variousmonomineralzonesseparatedbyboundariesareobtainedwhichmovegraduallywhileadvancingonrefractorymaterial.

Theoretically,inasemi-inﬁnitesystem,thenumberofmonomineralzoneswhichareformedisequaltothenumberofintermediatecompoundslikelytobeformedbetweenthephasesAandB.Inpractice,inthelaboratorytests,thequantityofslagislimitedanditscompositionchanges.ThemigrationofcalciumtowardsrefractoryinvolvesadecreaseintheconcentrationofthiselementtionCBintheslagwhichcanbelowerthantheconcentra-AB/ABinequilibriumwiththeassemblyAB+AB2.PhaseABisnotstable2

anymore,andthecorrespondingmonomineralezonedisappearsasFig.8shows.

TheAl2O3–CaO–SiO2phasediagram9makesitpossibletodeterminewhicharethephaseslikelytoappearduringthecor-rosionofhighaluminabyAl2O3–CaO.Italsomakesitpossibletopredictthesuccessionofthereactionalzonationsforeachkindofrefractories:

•mullitizedandalusite-basedrefractory:slag/dialuminateofcalcium/hexaaluminateofcalcium/corundum/mullite

•bauxite-basedrefractory:slag/dialuminateofcalcium/hexaaluminateofcalcium/corundum

•alumina-basedrefractory:slag/dialuminateofcalcium/hexaaluminateofcalcium/corundumForexample,Figs.9and10showmicrostructuresofthepre-cipitationzonesofbauxiteandandalusiterefractoriesaftercorrosion.Thealterationoftherefractorymicrostructurescanbeexplainedbydissolution–precipitationprocessesinsidealiquidphase.5Severalminerallayerscanbeobserved.

ThesamesuccessionwillbeobservedforallthetypesofslagaslongastheirinitialcompositiondoesnotdiffertoomuchfromaratioC/A+Scloseto1.Thiszonationcorrespondstothemax-imumnumberofobservablemonomineralzones,knowingthat

15J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

Fig.9.CorrosionofbauxitebrickbyAl2O3–CaOslag;backscatteredelectronsSEMmicrographs.(A)Transitionbetweenthethreemonominerallayersoftheprecipitationzoneand(B)neogeniclayermadeoflargewell-formedcorundumcrystalsdevelopedattheexpenseofsmallbadlyformedcorundumissuedfromthetransformationofabauxitegrain.C:corundum;CA2:calciumdialuminate;CA6:calciumhexaaluminate.Thelight-greybackgroundrepresentsthehigh-temperatureliquidphasepartiallyrecrystallizedduringcooling.

Fig.10.CorrosionofandalusitebrickbyAl2O3–CaOslag;backscatteredelectronsSEMmicrographs.(A)Transitionbetweenthecalciumhexaaluminate(CA6)andthecorundumlayersand(B)transitionbetweenthecorundumlayeroftheprecipitationzoneandtherefractory(mullite+glass)C:corundum;CA2:calciumdialuminate;CA6:calciumhexaaluminate;M:mullite.Thelight-greybackgroundwiththeneedle-likecrystalslocalizedinthecorundumlayerin(A)representsthehigh-temperatureliquidphasepartiallyrecrystallizedduringcoolingandconvertedtoanorthite(CAS2)andresidualglass.

theevolutionofthecompositionoftheslagduringthereactioncanmakethemostexternalzonestodisappear.4.Determinationofthecompositionproﬁlesoftheliquidphaseathightemperature4.1.Method

Thegradientsofconcentration,whichexistintheliquidphaseathightemperaturearethedrivingforcebehindcorrosionpro-cessesathightemperature.Unfortunately,twophenomenamakeitdifﬁculttodeterminethecompositionofthisliquid.•Ontheonehand,thepartialcrystallizationoftheliquidduringcoolingdoesnotmakeitpossibletoassimilatethecompo-sitionofresidualglass(whichcanbedeterminedbyEDSanalysis)withthatoftheliquidwhichexistedathightem-perature.TheonlyrepresentativeanalysiswouldbeanEDSmappingofazoneincludingresidualglassandthecrystalsformedduringcooling.

•Inaddition,theexistenceofmonomineralzoneswithnewlyformedsolidphasesintheliquidmaketheanalysesbymap-pingnon-representative,since,theyintegratenotonlythecrystalsformedduringcoolingandresidualglass,butalsothecrystalsnewlyformed.Oneofthepossibilitiesconsistsincarryingoutafastquenchattheendofthecorrosiontesttolimitthecrystallizationoftheliquidduringcooling.Undertheseconditions,theonlycrys-tallizedphasesareduetocorrosionandthemicroanalysesofglassbetweenthecrystalsarerepresentativeofglassathightemperature.

Concurrentlywiththisdirectmethod,anindirectmethodcanbeimplemented.Itconsistsinusingtheanalyses(obtainedbymapping)whicharecarriedoutonthetotalityofthecorrodedzone.Theresultsareinterpretedusingthephasesdiagramsandtheconceptof“localequilibrium”.

Fig.11showstheprincipleofthemethod.Withintheframe-workofthe“localequilibriumconcept”,theinformationwhichisprovidedbythephasediagramsislimited:

J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–15681565

Fig.11.Determinationofthecompositionoftheliquidat1600◦CforananalysedzonerepresentedbypointM(thickline:1600◦Cisothermline).

•thepointLisrepresentativeofthecompositionoftheliquidat1600◦C;

•thecompositionrepresentedbythepointMcorrespondstoanassemblageofaliquidandasolidphaseat1600◦C.ThediagramAl2O3–CaO–SiO2givesnodirectinformationonthecomposition,neitheroftheliquid,norofthesolidcor-respondingtothepointM.Itcannotbeusedlikeadiagramofthermodynamicequilibrium.However,itsuseasaternarydiagramofcompositionremainsvalid.Asurfaceanalysedbymappingwiththescanningelectronmicroscopy(SEM)consistsofliquidandnewlyformedsolidsduringcorrosionat1600◦C.Thosecanbedistinguishedfromthephasesformedbythecrys-tallizationofglassduringcooling.

Forananalysedzone,representedbythepointM,inwhichthesolidphaseisthecorundum(pointS),theliquidat1600◦CisrepresentedbythepointLobtainedbytheintersectionofthesegmentSMwiththeisotherm1600◦Cofthediagram.TheproportionofliquidisgivenbytheratioMS/LS,andthecom-positionofthisliquidisdeducedfromthepositionofthepointLinthetriangleofcomposition.

Inacertainnumberofcases,theanalysedsurfaceissitu-atedonseveralzonesofreactionandcontainsseveralphases:forexample,mullite+corundum,orcorundum+CaO.Instrictlyspeaking,itwouldbenecessarytoconsidertherelativepropor-tionofthetwophasesandtoplacethepointrepresentativeofthesolidinproportiononthesegmentwhichjoinsthesetwosolids.

4.2.Validationofthemethodandresults

Compositionproﬁles(chemistryasafunctionofdis-tancefromtheinitialinterface),etablishedforthemullitizedandalusitebasedrefractoryincontactwithaAl2O3–CaOslag,bythedirectmethod(quenchoftheliquidphaseattheendofthecorrosiontest)andtheindirectmethod(conceptoflocalequi-libriumappliedtophasediagram)aregiveninFig.12.Thedataobtainedbythesetwomethodsareinagreement.

Acrossthemodiﬁedarea(zoneIIandIII),theliquidcom-positionpresentsgradientsofconcentrationconnectingthecompositionoftheinterstitialliquidintherefractorytotheslag.Thechemicalexchangesoccurthroughtheliquidphase.Dependingonitslocalcomposition,theliquiddissolvesthesolidphaseswhicharenotinequilibriumwithitandprecipitatesnewphasesafteritbecomessaturated.10

Fig.13showsanothercompositionalproﬁleoftheliq-uidphaseassociatedwiththezonationofmineralsolidphasespresentatatemperatureof1600◦C(mullitizedandalusiterefrac-tory,Al2O3–CaOslagandquench).

Foreachzone,theequilibriumconstants,thenumberofphasesinequilibriumandthevarianceofthesystemaredeter-mined.

4.2.1.Zoneoftheinitialrefractory

Atthisdepth,CaOdoesnotinﬁltrate.Theliquidiscom-posedofAl2O3andSiO2.As[Al2O3]+[SiO2]=1,aluminaand

1566J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

Fig.12.Compositionalproﬁleoftheintergranular-liquidphaseintheandalusiterefractory(a:experimentalvalues,b:obtainedwiththeAl2O3–CaO–SiO2phasediagram).

silicacontentsareﬁxed.Theliquidissaturatedwithmullite,thesaturationisdeﬁnedbytheequilibriumconstantKMofthereaction:

󰀋Al6Si2O13󰀌=3[Al2O3]+2[SiO2]KM=[Al2O3]3·[SiO2]2

(I)

graduallydissolvesuntilittotallydisappearsattheboundaryseparatingthezonesofpenetrationandprecipitation.Thecom-positionoftheliquidphaseisnotconstantanymoreandthedegreeoffreedombecomesequalto1.

4.2.3.Boundarybetweentheimpregnationzoneandtheprecipitationzone

Twomineralphasescoexist:residualmulliteandneoformedcorundum.Theliquidphaseissimultaneouslyinequilibriumwithcorundumandmullite(C/Mequilibrium).Thecorundumsaturationisexpressedbythereactionconstant:󰀋Al2O3󰀌=[Al2O3]

KC=[Al2O3]

(II)

󰀋·󰀌means“inthesolidstate”and[·]“dissolvedinaliquidphase”Theliquidcompositionisthenconstantandthedegreeoffreedomis0.

4.2.2.Impregnationzone

CaOinﬁltratestheinterstitialliquid.Asaconsequence,theliquidcompositionwillnotbeinequilibriumwithmullite,which

Al2O3andSiO2contentsarebothconstant:

󰀁󰀂1/2KM−1

[Al2O3]C/M=KCand[SiO2]C/M=KC

Astheliquidcontainsonlythreespecies,theconcentrationofCaOcanbedeterminedbymeansofthefollowingequation:[Cao]C/M=1−([Al2O3]C/M+[SiO2]C/M)

Itthusappearsclearlythatthecontentsofthethreecon-stituentsoftheliquidareﬁxedattheboundarybetweenthezonesofprecipitationandpenetration.Thedegreeoffreedomisequalto0.

Thisresultisconsistentwithdataobtainedfromcompositionproﬁlespreviouslyestablished(eitherbydirectanalysisofglassorusingaCaO–Al2O3–SiO2diagram)whichrevealedtheexis-tenceofaconstantcompositionatthisinterfacewhateverthenatureoftheslag.Intheprecipitationzone,aﬁrstmonominerallayercontainingcorundumformedduringcorrosionat1600◦Cisobserved.Equilibriumofliquidwithcorundumisdeﬁnedbyreaction(II)mentionedabove.ItfollowsthatAl2O3contentisﬁxedinthiszone([Al2O3]C=KC),andconcentrationsofCaOandSiO2mustinverselyvarybecausetheirsumhastoremainconstant.Itmustbenotedthatcorundumisreallystableonlynearthesurfaceoftherefractorywhereitprecipitates,while

Fig.13.Anothercompositionalproﬁleoftheliquidphaseassociatedwiththezonationofthemineralsolidphasespresentat1600◦C(mullitizedandalusiterefractory,Al2O3–CaOslag,quench).

J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–15681567

Fig.14.Viscosityproﬁlesoftheliquidphaseinthetestedandalusite(a)andbauxite(b)refractories,accordingtoUrbain’smodel.(I)slagzone,(II)precipitationzone,(III)penetrationzone,and(IV)unaffectedrefractory.Initialslag–refractoryinterfaceislocatedatd=0mm.NotethatzoneIVin(b)islesswidethanin(a)becauseofdeeppenetrationofslaginbrickB.

farfromthecorrosionfront,itdissolvestomaintainaconstantAl2O3contentintheliquidandthustomakeupfortheincreaseintheCaOcontent.4.2.4.Precipitationzone

•Monominerallayerofcorundum

AlongtheCA6layer,concentrationsofAl2O3,CaOandSiO2arevariable.Thedegreeoffreedomisequalto1.

•Boundarybetweenthecorundumandthehexaaluminateofcalciumlayers

AttheinterfacebetweenlayersofcorundumandCA6,theliquidissaturatedwiththesetwophases,thuscorrespondingtoequilibriumbetweenreaction(II)and(III):=6[Al2O3]+[CaO]KH

=[Al2O3]6·[CaO]

(III)

TheconcentrationsofCaOandAl2O3aredeﬁnedasfol-lowsatthisinterface:[Al2O3]C/H=KC

and

[CaO]C/H=

KH6KC

As[Al2O3]+[CaO]+[SiO2]=1,theSiO2concentration

isalsodeterminedattheinterface.Thedegreeoffreedomisequalto0.

•Monominerallayerofhexaaluminateofcalcium

AlongtheCA6layer,concentrationsofAl2O3,CaOandSiO2arevariable.Thedegreeoffreedombecomesequalto1.

•Boundarybetweentheprecipitationzoneandtheslagzone

CA6disappearsattheboundaryinterface.Beyondthisinterface,thereisnomoreliquidphase.4.3.Viscosityoftheliquidphases

Thegradientofthechemicalcompositionoftheintergranularliquidphaseplaysafundamentalrole.Theviscosityoftheliquidphaseisthesecondparameter,whichinﬂuencesthemigrationofthespeciesinthisliquid.

ViscosityηwascalculatedaccordingtoUrbain’smodel11bythefollowingequation:ATexp(103B/T),AandBaretwoparam-etersdependingonthecompositionoftheglass.TheviscosityoftheintergranularliquidphaseisshowninFig.14for(a)and(b)andalusiteandbauxiterefractories.

EDSanalysisoftheglassinthetworefractoriesintegratesalltheelementsoftheslag–refractorysystemincludingtheimpurities.Forbauxiterefractory,inwhichthereisnoresid-ualslagzone(absenceofzoneIcorrespondingtothepartofrefractoryreplacedbyslag),alowviscosityoftheliquidphase,duetothepresenceinahighcontentofminorelements(amounting∼20–30%)suchasFe2O3,P2O5,TiO2,alkalineandearth–alkalineoxides,isobservedalongthemodiﬁedpartoftherefractory(areaextendingfromtheunaffectedzonetotheslagzone).Thisfactexplainswhythepenetrationzoneiswiderinbauxitethaninandalusiterefractory.Furthermore,thefractionofimpuritiescomingfromrawmaterialsmaybenegligibleintheandalusitebrick,sothattheliquidphaseisconsideredtobepredominantlyCaO+Al2O3+SiO2.

Forandalusiterefractory,theliquidviscosityisconstantalongtheslagzone(zoneI;μ≈1Pas)aswellasintheunaf-fectedrefractory(zoneIV;μ≈1570Pas).Fromtheunaffectedzone,aconsiderabledecreaseintheliquidviscosityisobserveduptotheprecipitationzone.Theliquidbecomesmoreﬂuidnearthecorrosionfrontandeasilyinﬁltratesthegrainboundariesleadingtothedissolutionofmostpresentﬁnephases.Asaresult,thedissolvedphasesbecomepartofthelocalliquid,thusincreas-ingtheliquidcontent.However,anexplanationforthelackofdeepslagpenetrationinandalusiterefractoryliesinthefactthatahighamountofviscousliquid(silica-richwithalowercontentofsecondaryoxides)isformedinthepartoftherefractoryincontactwiththeslag,whichsloweddownthepenetrationoftheslag.

5.Conclusion

Thecorrosionmechanismsofrefractoriesbyliquidoxidesarefarfrombeingcompletelyknown.Thestudyofmicrostructures,whichimprovesourknowledgeofcorrosion,isextremelyuse-fultodeterminethemechanismsofchemicalattack.However,

1568J.Poirieretal./JournaloftheEuropeanCeramicSociety28(2008)1557–1568

themicrostructuresofcorrodedrefractoriesareverydifﬁculttointerpret.

Inthispaper,anapproachbasedontheconceptoflocalther-modynamicequilibriumandtheuseofthephaseruleisproposedtoanalysethemicrostructures.

Anexperimentalcorrosionstudyofhigh-aluminarefracto-riesbyanalumina–limemodelslagiscarriedouttoillustratethisapproach.Thepost-mortemanalysisofmicrostructuresaftertesting,revealedthedevelopmentofapenetrationzone,followedbyaprecipitationzoneformedofasuccessionofmonominerallayersandshowtheexistenceofcompositiongra-dientsintheinterstitialliquid.Thelocalchemicalequilibriumconceptmakesitpossibletoexplaintheobservedmineralzona-tion,andtoquantifytheproportionandcompositionoftheliquidathightemperaturefromchemicalproﬁles,establishedbyanalyzingsuccessivezonesbymeansofscanningelectronmicroscopy.

Thecompositionproﬁlesarecomparedtothoseobtainedbydirectanalysisoftheinterstitialglassinrefractoriesafterquenching.

Theviscosityproﬁles,estimatedbymeansofUrbain’smodelfromthecompositionproﬁlesoftheliquidphase,showedthattheviscosityoftheinterstitialliquiddeterminestheextensionofthepenetrationzoneinagivenrefractory.

Theseresultsclearlyvalidatethisthermodynamicapproachwhichoffersnewprospectstodevelopmorecorrosion-resistantrefractories.12

References

1.Lee,W.E.,Characterisationofcorrosionmechanismsinrefractoriesbypost-mortemmicrostructuralanalysis.Br.Ceram.Proc.,1997,57,7–15.2.Ildefonse,J.P.,Gabis,V.andCesbron,F.,Mullitizationofandalusiteinrefractorybricks.KeyEng.Mater.,1997,132–136,1798–1801.

3.Qafssaoui,F.,Rˆoleeteffetdel’andalousitesurlecomportement`a

lacor-rosiondesc´eramiquesr´efractaires`a

hauteteneurenalumineparleslaitierssid´erurgiques,PhDThesis.UniversityofOrleans,France,2004.

4.Zhang,S.,Rezaie,H.R.,Sarpoolaky,H.andLee,W.E.,Aluminadissolutionintosilicateslag.J.Am.Ceram.Soc.,2000,83(4),7–903.

5.Guha,J.P.,Reactionchemistryindissolutionofpolycrystallinealuminainlime–alumina–silicaslag.Br.Ceram.Trans.,1997,96(6),231–236.

6.ThompsonJr.,J.B.,Localequilibriuminmetasomaticprocesses.InResearchesinGeochemistry,Vol1,ed.P.H.Abelson.JohnWiley&Sons,NewYork,19.

7.Mueller,R.F.,Mobilityofelementsduringmetamorphism.J.Geol.,1967,75,565–582.

8.Gibbs,J.W.,Equilibriumofheterogeneoussubstances.Trans.Conn.Acad.Sci.,1874,3,108–248;

Gibbs,J.W.,Equilibriumofheterogeneoussubstances.Trans.Conn.Acad.Sci.,1878,3,343–524.

9.Gentile,A.L.andFoster,W.R.,PhaseDiagramsforCeramists.TheAmer-icanCeramicSociety,Columbus,OH,USA,1963,p.220.

10.Qafssaoui,F.,Poirier,J.,Ildefonse,J.P.,Hubert,P.andBenyaich,F.,

Microstructuralandphysicochemicalstudiesofcorrodedhigh-aluminarefractories.SilicatesInd.Ceram.Sci.Technol.,2005,70(7–8),109–117.11.Urbain,G.,Viscosityestimationofslags.SteelRes.,1987,58(3),111–

116.

12.Poirier,J.,Bouchetou,M.L.,Qafssaoui,F.andHubert,P.,Corrosionofhigh

aluminarefractoriesbyAl2O3/CaOslagunderthermalcyclingconditions.Interceram.Int.(Part1andPart2),2006,55(4),270–272,and348–351.

因篇幅问题不能全部显示，请点此查看更多更全内容

查看全文