Microstructure And Properties Of Magnesium Alloy Coating Prepared By Different Microarc Oxidation Systems

Magnesium alloys have attracted great attention for applications in automotive, electronic and aerospace industries because of their low density, high specific strength and high specific stiffness, etc. However, magnesium has a very low normal electrode potential, which makes it become one of the most active structural metals. Microarc oxidation (MAO) is an advancement of the conventional electrochemical anodizing. With this method, the wear protective and/or corrosion protective ceramic coatings can be formed on Mg.Magnesium alloy AZ91D was processed in silicate system and aluminate systems. One electrolyte with the best corrosion resistance was found out from each electrolyte systems. The morphology, thickness, corrosion resistance, friction and wear properties and adhesion force with the matrix of microarc coatings was observed and measured by scanning electron microscopy, electrochemical testing methods and surface performance apparatus.The results show that with the increasing of time and current density, the amount of micropores in the microarc oxidation coating decreases, but the size of micropore and the roughness of coating become greater. The microarc oxidation coating under long processing time and high current density has a greater thickness and a lower adhesion force than that at short time and low current density. The corrosion and wear resistance of microarc oxidation coating rise first and then decline with the processing time and current density.The coating is mainly composed of MgO (periclase, syn) and MgAl₂O₄ (spinel) phases and the optimum processing parameters for magnesium alloy in silicate system are in a processing time of 40 min and a current density of 0.20 A/cm². The coating contains MgO (periclase, syn), Mg₂SiO₄ (forsterite, syn) and SiO₂ and the optimum processing parameters for magnesium alloy in aluminate system are in a processing time of 40 min and a current density of 0.20 A/cm². The thickness, corrosion resistance and friction properties of the coating formed in silicate system is better and the wear properties and adhesion force with the matrix is worse than the coating formed in aluminate system.