Passivation of galvanized steel in a solution based on hexafluorozirconic acid

Due to the increasingly stringent environmental requirements, the development of methods for the creation of passivating coatings on zinc that do not contain toxic compounds of metals such as nickel, cobalt, or chromium is an urgent task. This article deals with the development of a technology for applying protective conversion coatings in a solution containing Zr4+ ions onto galvanized surfaces to replace highly toxic chromating processes. The compositions of the solutions have been developed and optimized, and the physicochemical and mechanical properties of the protective coatings formed in these solutions on zinc, which are alternatives to chromate coatings, have been studied. The addition of gallic acid to a solution of hexafluorozirconic acid was found to increase the protective ability of deposited coatings and enhance their homogeneity, uniformity, abrasion resistance, and frost and heat resistance. It was found that the coating contains zirconium compounds (zirconium dioxide, zirconium gallate), zinc compounds (zinc hydroxide, zinc oxide and zinc gallate), and a tungsten compound (tungsten trioxide). Moreover, it was found that the coatings obtained using zirconium-containing solutions with addition of gallic acid can endure the effects of high positive or negative temperatures without deterioration of their protective characteristics. The thickness of zirconium-containing coatings obtained in a solution with gallic acid was found to be similar to 190 +/- 5 nm, which is less than the chromate coating thickness that can be as large as 1000 nm.