The Corrosion Resistance Modification Of V/Zr Conversion Coating On AZ31B Mg Alloy With Phytic Acid, Cerium

Mg alloys are widely used in aerospace and aviation, 3C electronics, automotive and other fields due to their excellent performances, such as high strength-to-weight ratio, good heat dissipation effect and electromagnetic shielding. However, Mg alloys have high chemical activity and are easily corroded, which largely limited their extensive use. More and more researchers are dedicating to develop environmentally friendly chromium-free treatment technology of Mg alloys.In order to further improve the corrosion resistance of V/Zr conversion coating on Mg alloys developed by our research group, this paper researched the corrosion resistance of V/Zr conversion coating on Mg alloys modified by organic and inorganic compounds.The effect of six different organic anticorrosion modifiers on V/Zr conversion coating was investigated by salt spray test. The results showed that phytic acid could improve the corrosion resistance of the coating most effectively. The optimal preparation conditions were as follows: phytic 1.0mL/L, NaVO3 3.0g/L, K2ZrF6 2.0g/L, pH 2.0, 50?and 15 minutes. Cerium nitrate was the best among four different inorganic anticorrosion modifiers, the optimal preparation conditions were: Ce(NO3)3 4.0g/L, NaVO3 2.5g/L, K2ZrF6 2.0g/L, pH 3.0, 50?and 20 minutes.The analysis results of scanning electron microscopy(SEM),energy dispersive spectrometer(EDS), fourier transform infrared spectrum(FTIR), the coating thickness technology and electrochemical test showed that the conversion coating modified by phytic was the amorphous structure, mainly composed of C, O, P, F, Mg, Al, V, Zr, and contained the phytic acid functional groups. The conversion coating consisted of the inorganic conversion coating of the effective anti-corrosion components V2O5, ZrO2, and the organic film of the complex of phytic, and the coating thickness increased from 3.52?m before modification to 6.25?m after modification. It made salt spray time of V/Zr conversion coating on AZ31 B Mg alloy modified by phytic increase from 80 h before modification to 140 h after modification, the corrosion potential of the sample increase by 91 mV, the corrosion current density reduce by about one order, and the capacitive reactance increase. Therefore, the corrosion resistance of AZ31 B Mg alloy was greatly improved.The analysis result showed that the conversion coating modified by cerium nitrate was mainly composed of Ce, V, Zr, C, O, F, Mg, Al, Zn. The formation of the effective anti-corrosion components V2O5, ZrO2, CeO2, Ce(OH)3, and the coating thickness increasing(from 3.52?m before modification to 5.05?m after modification) made the salt spray time of V/Zr conversion coating on AZ31 B Mg alloy modified by cerium nitrate increase from 80 h before modification to 90 h after modification, the corrosion potential of the sample increase by 21 mV, the corrosion current density reduce by about one order, the capacitive reactance become larger. Therefore, the corrosion resistance of AZ31 B Mg alloy was improved.