Microstructure and corrosion resistance of a chromium-free multi-elements complex coating on AZ91D magnesium alloy

A chromium-free multi-elements complex coating (MECC) for AZ91D magnesium alloy has been obtained using a new kind of chemical conversion process without acid pickling step. The coating layer could be formed in less than 10 min by immersing the specimen in a bath at 55 °C. The composition and microstructure of the coating were analyzed and determined by SEM, EDX and XRD. The conversion coating was mainly consisted of Mg, Al, P, K, Ca, Mn, Zn and O, and it was a continuous coating with good adhesion to the matrix. It is discovered that the coating layer contains the following compounds: Ca0.965Mg2Al16O27, Mn5.64P3, ZnAl2O4 and (Mg0.66Al0.34)(Al0.83Mg0.17)2O4. Besides, salt spray test results show that the specimen with this coating layer has good corrosion resistance. Electrochemical potentiodynamic polarization experiment was also conducted in a 5% NaCl solution to evaluate corrosion rate of the coatings. The results of corrosion rates of this coating and Dow No. 1 coating were 2.1348 and 13.2033 mm per year, respectively, which indicated it could provide effective protection better than that of Dow 1 coating.