Research On Chrome-free And Colorless Passivation Process On Hot Dip Galvanized Layer

The toxic carcinogenic of hexavalent chromium ions severely limits the application of traditional chromate passivation technology on hot-dip galvanizing layer.For the need of environmental protection and human health,the newly green and pollution-free passivation process must be developed.This paper identified suitable passivation recipe for hot-dip galvanizing layer and used two chrome-free passivation process in the manufacture of the protective film on hot-dip galvanizing layer.The effect of passivation process conditions on the corrosion resistance of the film was studied.The final passivation solution formulation and process condition are:(1)Silicate/titanium salt composite passivation: Sodium silicate 35 g/L,sodium tungstate 25 g/L,cobalt sulfate 6 g/L,hexafluoro titanic acid 6 g/L,hydrogen peroxide 60 g/L,magnesium sulfate 25 g/L,nitrate 6 ml/L,phosphate 6 ml/L,phytic acid 5 ml/L,additive 8 g/L,passive temperature is 30 degrees,passivation time is 30 seconds,passivation pH is 2.7;(2)Rare earth/molybdate salt composite passivation: molybdenum 30 g/L,magnesium 20 g/L,sodium phosphate 15 g/L,hydrogen peroxide 20 g/L,boric acid 2 g/L,ceric nitrate 2.5 g/L,phosphate 6 ml/L and passive temperature is 40 degrees,the deactivation time is 40 seconds,passivation pH is 2.5.Both processes are obtained bright,uniform,dense protective film.The durable time to salt spray corrosion time was up to 81.5 hours and 67 hours.The passive film is sealed by vinyltrimethoxysilane and acrylic resin,the corrosion resistance was increased more than 30 hours.The corrosion resistance are good enough to approach the chromate passivation.The film phase composition,microstructure morphology and chemical composition were analyzed.Silicate/titanium salts passivation layer is containing silicon oxide,titanium oxide,zinc silicate coatings,rare earth/molybdate salt passivation layer is containing molybdenum oxide,cerium oxide.And discussed the formation mechanism,The silicate / titanium salt compound passivation film is composed by the compound deposition of silicon,titanium,zinc compounds;rare earth salt / molybdate sal compound passivation film is composed by the compound deposition of molybdenum,phosphorus,cerium compounds.