Preparation of molybdate conversion coating on magnesium alloy reinforced by micro-arc oxidation

The work aims to prepare the molybdate coating enhanced by micro-arc oxidation to improve the corrosion resistance of the molybdate conversion coating. Molybdate conversion coating, micro-arc oxidation coating and micro-arc oxidation enhanced molybdate coating were prepared on AZ91D magnesium alloys by chemical conversion and micro-arc oxidation. The electrochemical behavior and corrosion weight loss of the coatings were studied. SEM, EDS, XRD and laser confocal microscopy were used to analyze the surface morphology, element composition, phase composition and roughness of different conversion coatings. XRD analysis showed that after the micro-arc oxidation treatment of the molybdate coating, the obtained coating had a new phase MoSi2 compared with the micro-arc oxidation coating. After micro-arc oxidation treatment, compared to the micro-arc oxide coating, the surface of molybdate conversion coating became smooth, the pore particles became smaller, and the roughness reduced. After the molybdate conversion coating was subjected to micro-arc oxidation treatment, the obtained coating had the lowest weight loss when immersed in 3.5wt.% NaCl solution for 48 h. Through electrochemical tests, the corrosion potential of molybdate conversion coating enhanced by micro-arc oxidation was 0.643 V higher than that of molybdate conversion coating, and 0.419 V higher than that micro-arc oxidation coating. The corrosion current density decreased by three orders of magnitude compared with molybdate conversion coating and by one order of magnitude compared with micro-arc oxidation coating. The corrosion resistance of molybdate conversion coatings treated by micro-arc oxidation is better than that of molybdate conversion coatings and micro-arc oxidation coatings, thus improving the application prospects of magnesium alloys. © 2019, Chongqing Wujiu Periodicals Press. All rights reserved.