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Chromate Treatment of Galvanized Sheet
Chromate treatment should be passivation treatment. Post-treatment of hot-dip galvanizing includes passivation, pre-phosphating, and oiling. Passivation treatment can improve the surface structure and gloss of the galvanized layer, improve the corrosion resistance and service life of the galvanized layer, and improve the adhesion between the coating and the base metal.
At present, passivation treatment mainly adopts chromate passivation. Add some activators, such as fluoride, phosphoric acid or sulfuric acid, to the passivation solution to make a thick chromate film after passivation. When there is fluoride in the passivation liquid, it can reduce the surface tension of the steel strip, accelerate the film-forming reaction, and can increase the chemical polishing effect, making the passivation film fine and bright. For China supply galvanized steel roofing sheet for construction, NKK Japan has developed a special chromate passivation coating with excellent corrosion resistance and black rust resistance.
Over the years, scholars have conducted a lot of research on non-toxic or low-toxic inorganic corrosion inhibitors as passivating agents. Molybdate is one of them, its toxicity is lower than that of chromate, but the corrosion resistance after passivation is only equivalent to that of low Cr passivation. A small amount of molybdate and phosphate (abbreviated as M) is added to the non-toxic water-soluble acrylic resin (abbreviated as AC) to obtain a passivation solution (abbreviated as ACM), which replaces the toxic chromate for passivation treatment. The results show that The hot-dip galvanized layer is passivated with this non-toxic passivation solution, which can delay the time of white rust on the galvanized layer, and its corrosion resistance is close to the level of chromate passivation.
Although various chromium-free passivation processes have been reported in the literature, there is currently no chromium-free passivation process that can completely replace the chromate passivation process. Some chromium-free passivation processes are already equivalent to chromate passivation methods in some respects, but their market prospects, the scope of application, and user environmental protection effects require further study. However, it is a general trend to replace chromate passivation with chromium-free passivation.