The post-treatment process includes passivation, pre-phosphating and oiling. Passivation treatment can improve the surface structure and gloss of the galvanized layer, enhance the corrosion resistance of the galvanized layer, prolong its service life, and enhance the bonding force between the coating and the base metal. The current passivation treatment mainly uses chromate passivation. Add some activators, such as fluoride, phosphoric acid or sulfuric acid, etc. to the passivation solution to get a thicker chromate film after passivation. When there is fluoride in the passivation solution, it can weaken the surface tension of the strip steel, accelerate the film-forming reaction, and can enhance the chemical polishing effect to make the passivation film fine and bright. For hot-dip galvanized steel sheets for construction, Japan Steel Pipe Company has developed a special chromate passivation coating with excellent corrosion resistance and black rust resistance.
Over the years, relevant researchers 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 low Cr passivation. A small amount of molybdate and phosphate are added to the non-toxic water-soluble acrylic resin to obtain a passivation solution instead of toxic chromate for passivation treatment. The results show that the hot-dip galvanized layer is passivated with the non-toxic passivation solution, which can delay the appearance of white rust on the galvanized layer, and its corrosion resistance is close to that of chromate passivation water. Although various chromium-free passivation processes have been reported in the literature, there is no chromium-free passivation process that can completely replace the chromate passivation process. Some chromium-free passivation processes are equivalent to chromate passivation in some aspects, but its market prospects, application scope and environmental protection effects need to be further studied. However, the replacement of chromate passivation by chromium-free passivation is a general trend.
At present, there are few hot-dip galvanizing units with pre-phosphating capacity. The pre-phosphorized film can not only solve the problem of rust prevention during storage and transportation of hot-dip galvanized sheets that are relatively easy to rust before, but also play a role as a solid lubricating film in the stamping and forming process of automobile sheets. Reduce the surface friction coefficient of the hot-dip galvanized sheet, and effectively reduce the defects such as zinc layer shedding and powdering that are easy to produce after the hot-dip galvanized sheet is rubbed with the mold. Therefore, hot-dip galvanized pre-phosphorized sheets have been widely promoted in some steel plants in Europe, the United States, Japan, etc., and have become an effective technology to solve the cracking problem of difficult stamping parts. Therefore, it is foreseeable that the unit online pre-phosphating has great development potential.