涂装基本知识之Zinccoatings

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