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Corrosionresistancezinccoatings
JotunKoreaCoatingsCo.,Ltd.Coatingadvisor:Danny(DanGuo-qiang)2015-07
Contents
1.Hotdipgalvanizing2.Organiczinc-richcoatings3.Inorganiczinccoatings:
(1)Post-curedinorganiczinccoatings
(Waterbased)(2)Self-cureinorganiczinccoatings
(Waterbased)(3)Organicsilicatezinccoatings(Zincethylsilicatecoatings)Hotdipgalvanizing
Galvanizingistheprocessofcleaningsteelfreeofallmillscaleorothercontaminationandthendippingthesteelintomoltenzinc.Technologicalprocess:DegreasingRinseRinseAcidpicklingDryingCoolingDippingplating
assistant
dippingHotdipgalvanizing
1.Degreasing(10-15min):
1)Alkalidegreasing:~10%NaOH(orNa2CO3/Na3PO4
etc.),above60℃
2)Acidicdegreasing:DiluteH3PO4
2.Acidicpickling(30-40min):
1)H2SO4-Needheating,pollution、harmful
2)HCl(200g/L)-Noneedheatingorlittle,widelyused
3.plating
assistant
dipping(1-2min):
NH4Cl:ZnCl2(moleratio)=1~4,200~400g/L,pH4~5,60℃Effect:Form
Zn(NH3)2Cl2toprotectsteelfromrusting
eliminatetheoxideonthesteel,purifythebath
Hotdipgalvanizing4.Galvanizing:
Thedippingprocessmaytakefrom30secondsto8hoursdependingonthecompositionofsteelandtheconfigurationandmassofthecoatedarticles.Generally,theworkingpiecemustattainthetemperatureofzincbath,whichmayvaryfrom450to475℃.5.Cooling:bywatercangainaverysmoothandglossysurfaceHotdipgalvanizingMicroscopiccrosssectionofhotdipcoatingHotdipgalvanizingHotdipgalvanizingThecalculationofDFTHotdipgalvanizingConditionCauseGroundsforrejectionDullGrayappearanceCompositionoforiginalsteel(highsilicon,phosphorousorCarbon);ServercoldworkingSlowcooling;Normally,acceptedRoughsurfaceCompositionoforiginalsteel;Overpickling;LongimmersiontimeHighgalvanizingtemperatureNormally,acceptedBarespotsGreaseoroilresidue;Scaleorrustresidue;Rollingdefects;Excessaluminum,weldslag…RejectedifrepairingistoomuchRuststainWeepingofacidfromseamsAcceptedBlisteringDefectsorhydrogenoutofsteelAcceptedHotdipgalvanizingConditionCauseGroundsforrejectionDrossprotrusionDrossAcceptedunlessdrossIsheavyLumpandrunHighwithdrawspeed;Coldbath;Delayrunofffromseams;ContactbetweenwithdrawedDependsontheprioragreementFluxInclusionFluxresidue;Residueonsteelsurface;FluxpickedontopofbathRejectedAshinclusionAshburnedonduringdipping;AshontopofbathAcceptedunlessgrosslumpsWhiterustStorageorpackingunderDampconditionAcceptedifnotseriousHotdipgalvanizingWhiterust:Anode:Zn→Zn2++2e;Cathode:O2+4e-+2H2O→4OH-;Reaction:2Zn+O2+2H2O→2Zn(OH)2
(white、loose:3-5timelarger)ZnCO3cantbeformedduetolackofCO2Hotdipgalvanizing
Whiterust:
Mediumorheavywhiteshouldberemovedbyhardbrush(nowirebrush!!!-damage)WhiterustwillslowtheformationofprotectiveZnCO3
StorageiskeyimportantifthegalvanizedarticlesarenotusedinashortperiodHotdipgalvanizing
Whiterust:StorageStorageOrganiczinc-richcoatings
Organiczinccoatingsareamixtureofzincdustormetalliczincpigment(typically50to95%byweightinthedriedfilm)inanorganicvehicle.
Organiczinc-richcoatingsGoodcompatibility
withbothsteelandorganicCathodicprotection(zincpowderparticletoparticlecontact)Easyforapplication(RHlimitation)TouchupandrepairofgalvanizedarticlesBinder’srole(Differentbinder,differentrole)LesssubjecttocriticalsurfacepreparationInorganiczinccoatingsInorganiczinccoatingsComparationofphysicalpropertiesInorganiczinccoatingsType2–Postcuredinorganiczincreaction
Step1:Dehydration-basicsolventwater,leavingthezincandthealkalinesilicate(sodiumsilicate)onthesurfaceasacoating(hardandmetalicbutsensitivetowater)Step2:Amine+H3PO4areappliedonthesurfacebybrushorspraying
Step3:Silicicacidisformed,andreactwithFeandZinctoformmatrixtoholdzinc(Silicicaciddoesn’treactcompletely);atthesametime,Sodiumphosphatesarealsoformed.
Step4:ZinccontinuesreactwithH2OandCO2intozincionizeswhichreactwithsilicatepolymeruntilitbecomesessentiallyasaturatedzincsillicatepolymer.InorganiczinccoatingsType3–Self-curedinorganiczinccoating
Step1:Dehydration-basicsolventwater,leavingthezincandthealkalinesilicate(sodiumsilicate,lithiumsilicate,etc.)onthesurface(thealkalinesilicateismuchlessalkalinethantype2duetomoreformofpoly-silicateacid,thisallowsfasterreactionofzincintocoating)
Rewettingbywaterorrainisdetrimental!!!(binderleaching,powderycoating)
Step2:SilicicacidreactswithFeandZinctoformmatrixtoholdzinc(Silicicaciddoesn’treactcompletely)
Step3:ZinccontinuesreactwithH2OandCO2intozincionizeswhichreactwithsilicatepolymeruntilitbecomesessentiallyasaturatedzincsillicatepolymer.
InorganiczinccoatingsType4–Organicsilicatezinccoating(Ethylsilicatezinc)
Step1:Evaporationofthesolventintheorganicsilicatevehicle,leavingtheorganicsilicatezincmixtureonthemetalsurface.Step2:moisturefromtheaircontinuestohydrolyzetheethylsilicatetosilicicacid;SilicicacidreactswithFeandZinctoformmatrixtoholdzinc(Silicicaciddoesn’treactcompletely)
Step3:ZinccontinuesreactwithH2OandCO2intozincionizeswhichreactwithsil
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