Study On Technology And Performance Of Green Passivation On Zinc Coating

Hexavalent chromium plating has been widely used for decoration and anti-corrosion on zinc coating owning to the great advantages. However, Cr(VI) is categorized as a high toxic substance, with serious adverse effects on human health and environment. Trivalent Cr plating is considered to be a promising replacement technology for hexavalent Cr plating due to the similar performance but very low toxicity. Although there are several commercially available trivalent Cr passivators in China, the coating can not meet the requirement of high corrosion resistant and excellent decoration in automobile and electronics industries. It is significant to develop new trivalent Cr passivation process with high corrosion resistant and excellent decoration.Shiny bright blue-white coatings were deposited from trivalent chromium blue-white bath, and the optimum operating conditions are as follow: bath temperature 25°C, bath pH 1.9, immersion time 20 s and dilution ratio 8%. The Cr(Ⅲ) blue-white samples could last 250 hours in NSS (Neutral Salt Spray) test. Shiny bright light-iridescent coatings were obtained from trivalent chromium iridescent bath, and the optimum operating conditions are: bath temperature 40°C, bath pH 1.3, immersion time 20 s and dilution ratio 15%. The NSS test duration of iridescent coatings was more than 250 hours. Dark black coatings were obtained from trivalent chromium black bath, and the optimum operating conditions are: bath temperature 25°C, bath pH 1.3, immersion time 20 s and dilution ratio 6%. Surface treatment process also affected the appearance and corrosion resistance of the black coatings besides the operating conditions. Passivating, passivating-sealing and passivating -sealing-painting significantly affected the appearance and the corrosion resistance,and thus improve the decoration of the coatings. NSS test duration of passivating-sealing-painting black coatings was over 168 hours. The performance of the three green chromium passivating agents can meet or exceed that of imports.Three types of green chromium passivator were successfully developed, which are trivalent chromium blue-white passivator, trivalent chromium iridescent passivator and trivalent chromium black passivator, respectively. In this paper, we studied the technological condition, performance and corrosion resistance of the passivation trivalent chromium coatings.Trace hexavalent chromium would appear on the passivation trivalent chromium coatings after a few days in the air, especially the iridescent coatings. And the Cr(Ⅵ) content increased with increasing storage time. which was a great green barrier and should be urgently solved during the application of trivalent chromium passivation technology. In this paper, the mechanism of Cr(Ⅵ) appearance in the Cr(Ⅲ) coatings was discussed from thermodynamics view. The Cr(Ⅵ) content was found to be dependent on various parameters (bath temperature, bath pH, immersion time, dilution ratio, etc.). Results showed that the time that Cr(Ⅵ) appeared in the test could be effectively prolonged by conditioning the operation process.