腐蚀学-热浸锌

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1、HotDipGalvanizingHowGalvanisingpreventscorrosionofsteel:- Hotdipgalvanizingisasurfacetreatmentofsteelthatuseszincascoating material.Zinccoatingbeingelectricallymoreactivethansteel,actsasa sacrificialanodelayer.(SeeFigure2and3)Thisinhibitscorrosionof steel,evenwhen

2、thecoatingisscratched.Figure1:Similartocorrosionofsteel(seeCorrosionofSteel),Zincbeingmore electrolytic-allyactivecorrodesinplaceofthesteellayer.Figure2:Zinclooses2 electronstosteelbecausesteelisthecathode,andiselectricallyattractive.Figure3:Zincformszincoxideandw

3、ater.Hydrogengasisreleasedintheprocess.Figure4:Ineventofascratch,steelmaybeexposed.Thezinccoatingbecomesananode andprotectsthebasemetalbycorrodingfirst.Thisinhibitstheexposedsteel fromcorroding.(seeFigure4).PicturesfromZALAShandbook WeoftenseeSteel(Iron)assimplyru

4、stedbutitisactually3different chemicalreactionshappeningsimultaneously.Ironisanelectropositivemetalandformscathode-anodecellsthatreactwiththeelectrolyte,inmostcases moistureorwater. Corrosionstartsatthesurfaceofiron,wheredifferencesinpotentialexist, becauseofimpur

5、ities.Thisnon-uniformitycausesIrontolosses2electronsto thecathodethatiselectricallypositive:- Fe à Fe++ + 2e-  ElectronsmigratefromsurfaceofIrontothecathodepartofthemetal.The depletedironcombineswith2unitsofOH-becomingironoxide(rust)and water:- Fe++ + 2OH- àFeO + 

6、H20 2unitsofH+ionscombinewithfreeelectronsfromIrontobecomehydrogen gas:- 2H+ + 2e- àH2Gas ProcessofHotDipGalvanising: DuringHotDipGalvanising,thesteelsurfaceisdippedintomoltenzinc,anda protectivelayerofzinc-ironalloyandpurezincformquicklytosealitfrom corrosion.Gal

7、vanisingisdividedinto3mainsteps.SurfacePreparation, FluxingandGalvanizing. SurfacePreparation:-HotCaustic(alkaline)cleanerisusedtoremoveoil, greaseandsolublepaints.Subsequently,acidcleaningremovessurfacerustand contaminants.Steelcastingsareabrasiveblastcleaned,fol

8、lowedbybriefacid cleaningorelectrolyticcleaning.Thentheworkisrinsedwithwatertoget ridofacidspriortofluxing. Fluxingofsteel:-completelyimmersethesteelina