Study of the formation and growth of tannic acid based conversion coating on AZ91D magnesium alloy

A tannic acid based conversion coating was formed on AZ91D magnesium alloy through a solution containing C76H52O46 (tannic acid), NH4VO3, K2ZrF6 and H3PO4. The growth process and corrosion resistance of the conversion coating were investigated by SEM, FESEM, XPS, IR, potentiodynamic polarization and EIS test. The results showed that the main components of the coatings were penta-hydroxy benzamide–magnesium complex, Al2O3 and MgF2. Zr(HPO4)2·H2O(α-ZrP) was first formed in the time interval of 70 s–120 s, which acted as catalyst to activate the main reaction of gallic acid being oxidized into the penta-hydroxy benzamide–magnesium complex, as well as the formation of MgF2 and Al2O3, which took place in the time interval of 120 s to 600 s. The thickness of coating treated for 300 s and 600 s was 1.1 μm and 1.6 μm, respectively. Polarization measurement and EIS test in 3 wt.% NaCl solution indicated that the conversion coating treated for about 300–600 s shows good corrosion resistance while an excess time treatment (720 s) induces the coating surface coarse and uneven, leading to the reduction of corrosion resistance of the conversion coating.