FusionBonded Epoxy Coatings a Technology for Rebar Corrosion Prevention.docx

FusionBonded Epoxy Coatings a Technology for Rebar Corrosion Prevention.docx

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FusionBondedEpoxyCoatingsaTechnologyforRebarCorrosionPrevention

Fusion-BondedEpoxyCoatings:

aTechnologyforRebarCorrosionPrevention

J.AlanKehr,3MCorrosionProtectionDepartment,USA

FikryF.Barouky,ConsultingServicesDept.,SaudiAramco,SaudiArabia

Introduction

CorrosionofsteelinconcretehasbecomeacostlyproblemintheUnitedStatesoverthelasttwenty-fiveyears.ApproximatelyhalfofthenearlysixhundredthousandbridgesintheUSFederalAidHighwaysystemhavestructuraldeficienciesorarefunctionallyoutmoded.AccordingtoUSFederalHighwayAdministration（FHWA）estimates,aquarterofUSbridgedecksarebadlydeteriorated.Sincethebeginningofroad-saltapplication,expensiverepairsareoftenrequiredwithinfivetotenyears.

It’saworldwideproblem.ResearchindicatesthattheservicelifeofbuildingsintheArabianGulfmaybefivetofifteenyears.ReinforcedconcretebridgesneartheseashoreinJapanshowrapiddeteriorationwithintenyearsofconstruction.

Theproblemiscausedprimarilybyinorganic-saltinducedcorrosionofsteelinconcrete.Thesalt,primarilychloride,penetratestheconcretefromsourcessuchasroad-deicingsaltsorseaexposure.Itcanalsobebuiltinthroughtheuseofsalt-contaminatedaggregate,seawaterintheconcrete,orchloride-basedadmixtures.

Thechlorideioninitiatesandcatalyzesthecorrosionreaction.Theironcorrosionproductsresultingfromthereactionoccupyamuchgreatervolumethanironandcausetremendouspressureontheconcrete.Thepressurecausestheconcretetocrackandspall,allowingevengreateraccessofcorrodentstothesteelandaccelerateddeteriorationofthestructure.2

Whilemostattentionispaidtorebar,allsteelcomponentsareaffectedaswellbridgedecks,piers,pilings,andguardrails.1Thispaperaddressestheprotectionofthoseelementsbydescribingavailablealternativesandprovidinganoverviewoffusion-bondedepoxycoatingmaterials,casehistories,trendsintheindustry,andspecificconcernsabouttheuseoffusion-bonded-epoxycoatedrebar（FBECR）.

MechanismofReinforcingSteelCorrosioninConcrete

Thetraditionalviewofthereinforcedconcretestructureisthattheconcreteisprotectivetothereinforcingsteelbarsthroughthecombinedeffectsofthechemicalreactionsbetweenthesteelandthecementhydrationproductsandtheenvironmentalbarrierprovidedbytheconcretecover.Iftheseconditionsaremaintainedwithintheconcretemass,thesteelbarsdonotcorrodeandthestructureshouldhavetheexpectedtrouble-freelifespan.

Poorqualityreinforcedconcretestructurecontributestoafasterdeteriorationofthesteelreinforcingbars.Lowdegreeofcompaction,excesswaterintheconcretemix,andthehydrationprocessareconsideredthemainfactorstocreatevoidswithintheconcreteandmaketheconcretestructureporous.

Porosityofconcreteallowspenetrationandingressofaggressiveelements（e.g.,chloride,oxygen,carbondioxide,andothermaterialsthatvaryfromonelocationtoanother）totheembeddedsteelrebarandtoinitiatecorrosion.

Theprimaryfactorscontrollingtheinitiationofthesteelcorrosionanditsmechanisminconcretearesummarizedinthefollowingpoints:

∙Therateofsteeldepassivation

∙Theinitiationofthemacrocellsduetothedifferentialaerationandchlorideabsorption

∙Thelowresistivityattributedbytheconcreteporewater

∙Thepresenceofoxygentoacceleratethecorrosionprocess

Thecorrosionofsteelinconcreteisanelectrochemicalprocess,whichresultsintheformationofacorrosioncell.Thefollowingcorrosionmechanismisthemostlikelyforsteelrebarembeddedintheconcretewhensignificantvariationsexistinthesurfacecharacteristicsofthesteel.Thesteelsurfaceinitiatescathodesandanodeselectricallyconnectedthroughthebodyofthesteelbar.The“halfcellreaction”takesplace,byinducinganelectromotiveforceknownasstandardredoxpotentialwhenthemetalisconnectedtoahydrogenelectrode–seeEquation1.

Equation1

Foriron:

Fe--Fe+2+2e-（Anode）

TheelectronsliberatedattheanodemigratetothecathodeandreactinvariouswaysdependantuponthepHvalueandtheavailabilityofoxygen.SeeEquation2,Equation3,andEquation4.

Equation2

2e+2H+½O2------H2O

Equation3

2e+H2O+½O2-----H2O

Equation4

2e+2H----------------H2

Theanodicandcathodicreactionsareautocatalyticandresultinthetransformationofmetalliciron（Fe）torust.Therustformationisaccompaniedbyasignificantincreaseinthevolume,suggestedaslargeasseventimesthatoftheoriginalFevolume.Thevolumeincreasecausesconcretecrackingandspalling.

EffectofChlorideIons

Whenthesteelisplacedinahighlyalkalinesolution（pH>11.5）,eveninthepresenceofoxygen,corrosionwillnotbeinitiated.Infact,slightlyrustedbarswillbedissipatedwhenplacedinstrongalkali.Thatisthereasonwhy,duringconstruction,slightlyrustedsteelbarsdonotcreateaconcern.

ThechlorideionsingressdoesnotlowerthepHintheconcrete.However,itdestroysthepassivelayeronthesteelbars.Thedepassivatedsteelbarsdonotcorrodeinthepresenceofthechlorideionsonly.ThecorrosionoccursafterthepresenceofthecarbondioxidelowersthepHbelow11,thuscontributingtocorrosioninitiation.

Sourcesofchlorideareeitherintheconcretemix,mainlyfromthesand,aggregates,orthewaterused,oraschlorideingressfromtheenvironment,suchasinthemarineatmosphericenvironment.

EffectofCarbonation

Carbonationisthealkalinitylossintheconcretemass.Theproductofthereactionbetweencarbondioxideinthenormaloutsideairandthealkalineproducts,mainlythecalciumhydroxides,iscalciumcarbonate.Incaseofhighwater/concreteratio,carbonationcontinuestothedepthwherethereinforcingsteelbarisembedded.

Whencarbondioxidepenetratesthroughtheconcretecoverinthepresenceofwaterinthepores,itdrivesthepHtolowervalueswhichdepassivatesthesteel

Otherhydrationproductsinthecementcangothroughthesamereactionwithcarbondioxidecausingasignificantqualitylossofthecementandfasterdeteriorationoftheconcretemix.

EffectofotherElements

Sulfidecanbefoundinthecementasacontaminant（morethan0.2%）.Thesulfideionhasbeenfoundmoredestructivetothesteelrebarembeddedintheconcreteifitgoeshigherthantheregulatedpercentageshown.Regardlessofthesulfideionsource,ithasbeenthecauseofseveralcasesofhydrogenembrittlement–particularlyinpre-stressedrebar.

MechanismofFBEcoatedsteelcorrosioninconcrete

InthewakeoftheprematurefailureofFBEcoatedsteelrebarintheFloridaKeysbridgesubstructures,manyresearchworksbyusersandacademiainvestigatedtheperformanceofFBEcoatedsteelinvariousenvironmentsandserviceconditions.MostofthelaboratorytestresultsconfirmedthattheFBEmaterial,appliedundercontrolledconditions,passedsuccessfullyallqualificationandservicesimulatedtests.However,inafewcases,fieldsamplesshowedpooradhesiontotheextentofdelaminationanddisbondingoftheFBErebar.Oftentimes,thatdelaminationwasusedasthedefinitionfora‘coatingfailure,’ratherthancorrosionorconcretedistress.InordertounderstandwhatwentwrongtocauseprematurefailureoftheFBErebar,numerousdatawerecollectedfromvariousfieldsforinvestigationandassessment.

CorrosioncontroloftheFBEcoatingisafunctionofthecoating’sabilitytoprovideabarrieragainstwater,oxygen,chloride,andotheraggressiveelementsthatpreventspermeationthroughthecoatingfilmtoattackthemetalsubstrate.TherearecriticalpropertiesrequiredforcorrosionprotectionFBEcoatingsthatincludeadhesionandwettingabilitytotherebar.

Reductioninadhesivestrengthwillincreasethedelaminationprocessrate.,.AninvestigationintodelaminationofFBEcoatingsinasimulatedporesolutionenvironmentsuggestedthefollowingdelaminationmechanism:

∙Delaytimebeforeinitiationofobservabledelaminationprocessesmaybeafunctionofwaterpenetrationthroughthecoatingtotheinterfacialorinterphasialcoating/substrateregion.

∙DelaminationofFBEcoatingsfromsteelsubstratesispredominantlycausedbyhydroxylions.

∙RateofFBEdelaminationiscontrolledbytransportprocessesfromaporeinthecoatingandalongthedelaminatedcoating/substrateinterfacetothedisbondmentfront.

∙Thelocalityoffailureofcoatingadhesionisintheinterfacialorinterphasialcoating/substrateregion.

∙TherateofFBEdelaminationinnear-passiveconditionsiscontrolledbyhydroxylionmigrationfromthebulkexternalsolutiontothecoating/substratedisbondmentfront.

∙TherateofFBEdelaminationintheconditionofunderfilmcorrosioniscontrolledbyhydratedcationmovementtothecathodesite.

DesignofFBEcoatingpowderforsteelrebarcoating

NewtechnologiesareundercontinualdevelopmenttooptimizethepropertiesoftheFBEcoatingtoimprovecoatingutility.Thestoiciometricratiomustbecontrolledbytheequilibriumbetweenthecuringgroupandtheepoxygroup.Forexample,increasingthelevelofcuringagentmayreducethecross-linkdensityandincreaseflexibility,whiledecreasingchemicalresistance.

Impactresistanceorhardnessisafunctionofthecross-linkdensity.Higherdensitiescanbeachievedusinglowmolecularweightcuringagentsthatshowtightlycross-linkedstructures.Addingnon-reactivediluentscaninterferewiththisstructure,providingtheendproductwithmoreflexibilitybutlesstoughness.

Mechanicaladhesionisthegrippingforcethatresultsfromthero