Preparation and corrosion resistance of MAO/Ni–P composite coat on Mg alloy

Microarc oxidation (MAO) coat was designed as an intermediate layer for the electroless plated Ni–P top coat, providing inert surface and necessary hardness for Mg alloy substrate. The composite coat was successfully prepared to improve the corrosion resistance of Mg alloy. The preparation and the characterization of the composite coat were investigated. The results show that the pre-treatment of MAO before electroless plating plays an important role in the deposition of compact composite coat. The activation (by HF solution) makes the MAO coat dense with uniform cracks which supply excellent bonding interface for Ni–P coat. Compared with monolithic MAO or Ni–P coat, the composite coat has excellent corrosion resistance and stable bonding interface. There is main pit corrosion at substrate after the corrosive medium penetrating through the whole coat. With the inert MAO interlayer, the electrochemical corrosion between the Ni–P and substrate is effectively inhibited.