涂装基本知识之Zinc-coatings

JotunProtectiveCorrosionresistancezinccoatingsJotunKoreaCoatingsCo.,Ltd.Coatingadvisor:Danny(DanGuo-qiang)2015-07JotunProtectiveContents1.Hotdipgalvanizing2.Organiczinc-richcoatings3.Inorganiczinccoatings:(1)Post-curedinorganiczinccoatings(Waterbased)(2)Self-cureinorganiczinccoatings(Waterbased)(3)Organicsilicatezinccoatings(Zincethylsilicatecoatings)JotunProtectiveHotdipgalvanizingGalvanizingistheprocessofcleaningsteelfreeofallmillscaleorothercontaminationandthendippingthesteelintomoltenzinc.Technologicalprocess:DegreasingRinseRinseAcidpicklingDryingCoolingDippingplatingassistantdippingJotunProtectiveHotdipgalvanizing1.Degreasing(10-15min):1)Alkalidegreasing：～10％NaOH(orNa2CO3/Na3PO4etc.)，above60℃2)Acidicdegreasing：DiluteH3PO42.Acidicpickling(30-40min):1)H2SO4－Needheating，pollution、harmful2)HCl(200g/L)－Noneedheatingorlittle，widelyused3.platingassistantdipping(1-2min):NH4Cl:ZnCl2(moleratio)＝1～4，200～400g/L,pH4~5,60℃Effect：FormZn(NH3)2Cl2toprotectsteelfromrustingeliminatetheoxideonthesteel,purifythebathJotunProtectiveHotdipgalvanizing4.Galvanizing:Thedippingprocessmaytakefrom30secondsto8hoursdependingonthecompositionofsteelandtheconfigurationandmassofthecoatedarticles.Generally,theworkingpiecemustattainthetemperatureofzincbath,whichmayvaryfrom450to475℃.5.Cooling:bywatercangainaverysmoothandglossysurfaceJotunProtectiveHotdipgalvanizingMicroscopiccrosssectionofhotdipcoatingJotunProtectiveHotdipgalvanizingJotunProtectiveHotdipgalvanizingThecalculationofDFTJotunProtectiveHotdipgalvanizingConditionCauseGroundsforrejectionDullGrayappearanceCompositionoforiginalsteel(highsilicon,phosphorousorCarbon)；ServercoldworkingSlowcooling；Normally,acceptedRoughsurfaceCompositionoforiginalsteel；Overpickling；LongimmersiontimeHighgalvanizingtemperatureNormally,acceptedBarespotsGreaseoroilresidue；Scaleorrustresidue;Rollingdefects；Excessaluminum，weldslag…RejectedifrepairingistoomuchRuststainWeepingofacidfromseamsAcceptedBlisteringDefectsorhydrogenoutofsteelAcceptedJotunProtectiveHotdipgalvanizingConditionCauseGroundsforrejectionDrossprotrusionDrossAcceptedunlessdrossIsheavyLumpandrunHighwithdrawspeed；Coldbath；Delayrunofffromseams；ContactbetweenwithdrawedDependsontheprioragreementFluxInclusionFluxresidue；Residueonsteelsurface；FluxpickedontopofbathRejectedAshinclusionAshburnedonduringdipping；AshontopofbathAcceptedunlessgrosslumpsWhiterustStorageorpackingunderDampconditionAcceptedifnotseriousJotunProtectiveHotdipgalvanizing低氧浓度区（阳极）镀锌层水珠镀锌层高氧浓度区（阴极）Whiterust:Anode：Zn→Zn2++2e；Cathode：O2+4e-+2H2O→4OH-；Reaction：2Zn+O2+2H2O→2Zn(OH)2（white、loose:3-5timelarger）ZnCO3cantbeformedduetolackofCO2JotunProtectiveHotdipgalvanizingWhiterust:Mediumorheavywhiteshouldberemovedbyhardbrush(nowirebrush!!!-damage)WhiterustwillslowtheformationofprotectiveZnCO3StorageiskeyimportantifthegalvanizedarticlesarenotusedinashortperiodJotunProtectiveHotdipgalvanizingWhiterust:StorageStorageJotunProtectiveOrganiczinc-richcoatingsOrganiczinccoatingsareamixtureofzincdustormetalliczincpigment(typically50to95%byweightinthedriedfilm)inanorganicvehicle.JotunProtectiveOrganiczinc-richcoatings•Goodcompatibilitywithbothsteelandorganic•Cathodicprotection(zincpowderparticletoparticlecontact)•Easyforapplication(RHlimitation)•Touchupandrepairofgalvanizedarticles•Binder’srole(Differentbinder,differentrole)•LesssubjecttocriticalsurfacepreparationJotunProtectiveInorganiczinccoatingsJotunProtectiveInorganiczinccoatingsComparationofphysicalpropertiesJotunProtectiveInorganiczinccoatingsType2–PostcuredinorganiczincreactionStep1:Dehydration-basicsolventwater,leavingthezincandthealkalinesilicate(sodiumsilicate)onthesurfaceasacoating(hardandmetalicbutsensitivetowater)Step2:Amine+H3PO4areappliedonthesurfacebybrushorsprayingStep3:Silicicacidisformed,andreactwithFeandZinctoformmatrixtoholdzinc(Silicicaciddoesn’treactcompletely);atthesametime,Sodiumphosphatesarealsoformed.Step4:ZinccontinuesreactwithH2OandCO2intozincionizeswhichreactwithsilicatepolymeruntilitbecomesessentiallyasaturatedzincsillicatepolymer.JotunProtectiveInorganiczinccoatingsType3–Self-curedinorganiczinccoatingStep1:Dehydration-basicsolventwater,leavingthezincandthealkalinesilicate(sodiumsilicate,lithiumsilicate,etc.)onthesurface(thealkalinesilicateismuchlessalkalinethantype2duetomoreformofpoly-silicateacid,thisallowsfasterreactionofzincintocoating)Rewettingbywaterorrainisdetrimental!!!(binderleaching,powderycoating)Step2:SilicicacidreactswithFeandZinctoformmatrixtoholdzinc(Silicicaciddoesn’treactcompletely)Step3:ZinccontinuesreactwithH2OandCO2intozincionizeswhichreactwithsilicatepolymeruntilitbecomesessentiallyasaturatedzincsillicatepolymer.JotunProtectiveInorganiczinccoatingsType4–Organicsilicatezinccoating(Ethylsilicatezinc)Step1:Evaporationofthesolventintheorganicsilicatevehicle,leavingtheorganicsilicatezincmixtureonthemetalsurface.Step2:moisturefromtheaircontinuestohydrolyzetheethylsilicatetosilicicacid;SilicicacidreactswithFeandZinctoformmatrixtoholdzinc(Silicicaciddoesn’treactcompletely)Step3:ZinccontinuesreactwithH2Oand