A bi-layer coating is deposited on the surface of the AZ31B Mg alloy for corrosion protection of the Mg alloy. The bi-layer coating is composed of a micro-arc oxidation coating (MAO), and an electroless plated Ni-P coating. The micro-arc oxidation (MAO) treatment in the electrolyte with or without the addition of phosphate ions is carried out under unipolar power mode. The microstructure and composition of the MAO coatings are analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The roughness of the MAO coatings is obtained by using 3D white light interferometry. The corrosion resistances are evaluated by potentiodynamic polarization test as well as electrochemical impedance spectroscopy (EIS) in a 3.5 wt.% NaCl solution and salt spray test. The adhesion test results of the MAO/Ni-P composite coatings by drawing test machine. The results showed that the micro-pores size of the MAO coating treated in the phosphate-free bath is uniform and smaller than the phosphate-containing solution, which could decrease the roughness and enhance the corrosion resistance. The corrosion resistance of MAO coating with the phosphate-free solution for 2.8 times was better than that of phosphate-containing solution. Moreover, phosphate-free MAO/Ni-P composite coating showed the corrosion rate is more than 4 times lower than the phosphate-containing MAO/Ni-P composite coating. Furthermore, the MAO/Ni-P composite coatings prepared by the MAO coating treated in the phosphate-free bath could suppress the erosion of aggressive media during exposure after 120 h salt spray test (corrosion area is approximately 5%) and present excellent adhesion.

在AZ31B镁合金的表面上沉积了一层双层涂层，以保护镁合金。双层涂层由微弧氧化涂层（MAO）和化学镀Ni-P涂层组成。在单极性功率模式下，在有或没有添加磷酸根离子的情况下，在电解液中进行微弧氧化（MAO）处理。通过扫描电子显微镜（SEM）和能量色散光谱（EDS）分析MAO涂层的微观结构和组成。通过使用3D白光干涉仪获得MAO涂层的粗糙度。通过电位动力学极化测试以及在3.5 wt。％NaCl溶液中的电化学阻抗谱（EIS）和盐雾测试来评估耐腐蚀性。通过拉伸试验机对MAO / Ni-P复合涂层的附着力测试结果。结果表明，在无磷酸盐浴中处理的MAO涂层的微孔尺寸均匀且小于含磷酸盐的溶液，这可以降低粗糙度并增强耐蚀性。使用无磷酸盐溶液的MAO涂层的耐腐蚀性是含磷酸盐溶液的2.8倍。此外，无磷酸盐的MAO / Ni-P复合涂层的腐蚀速率比含磷酸盐的MAO / Ni-P复合涂层低4倍以上。此外，