Evaluation of microstructural effects on corrosion behavior of AZ31B magnesium alloy with a MAO coating and electroless Ni-P plating

Abstract

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. (C) 2020 The Authors. Published by Elsevier B.V.