Preparation and performance of MAO coatings obtained on AZ91D Mg alloy under unipolar and bipolar modes in a novel dual electrolyte

Ceramic coatings were obtained on AZ91D magnesium alloy by microarc oxidation under unipolar and bipolar modes in a silicate–aluminate dual electrolyte. The surface and cross-sectional morphology, coating thickness and phase composition of coatings were characterized by scanning electron microscopy (SEM), layer thickness meter and X-ray diffraction (XRD), respectively. The corrosion resistance of coatings in a 3.5% NaCl neutral solution was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The results showed that the microarc oxidation (MAO) process in the unipolar mode was very similar to that in the bipolar mode. The average thickness of the coating obtained in the unipolar mode was thinner than that in the bipolar mode. Both coatings were mainly composed of MgO, Mg2SiO4 and MgAl2O4. Introduction of a cathodic component during the MAO process was helpful to obtain a uniform and compact coating. The electrochemical study indicated that the coating obtained in the bipolar mode was in better corrosion resistance due to its compact microstructure and thickness.