Study on Preparing Technology and Corrosion Resistance of Ti/Zr Based Chromium-free Chemical Conversion Coating on Zinc Electroplating Layer

In this paper,we obtained environmental-friendly chromium-free chemical conversion coating on the non-cyanide alkaline zinc electroplating layer by employing the optimized Ti/Zr based conversion technique.At the same time,the performances of the Ti/Zr based conversion coating were compared with chromate colour passivation coating.The corrosion resistances of the conversion coatings were investigated by using neutral salt spray(NSS) test,potentiodynamic polarization and electrochemical impedance spectroscopy(EIS).The results showed that the white rust appeared after 96 hours when the NSS test was performed on the Ti/Zr based conversion coating,which was close to the corrosion resistance rating of chromate colour passivation coating.Compared with the chromate passivation coating,the Ti/Zr based conversion coating showed lower corrosion current and higher polarization resistance.However,the passivation characteristic of the former was less evident than the latter.