An organic chromium-free conversion coating on AZ91D magnesium alloy

Traditional conversion coatings on magnesium alloys are usually immersed in a solution containing hexavalent chromium compounds. However, the replacement treatments have been proposed by the present environmental driving to eliminate hexavalent chromium. In this work, a tannic acid based conversion coating on AZ91D magnesium alloy was obtained by treatment in a solution containing tannic acid and ammonium metavanadate. SEM, XPS and IR were used to determine the morphology and structure of the conversion coatings. Continuous and uniform conversion coating was deposited on AZ91D alloy and the main components of the coatings were Al2O3, MgF2 and penta-hydroxy benzamide–magnesium complex. The formation mechanism of the coating was discussed. Polarization measurement and salt spray test showed that the corrosion resistance of the conversion coating was much higher than that of traditional chromate conversion coating.