A Nano-CeO2/Zn–Mn Composite Conversion Coatings on AZ91D Magnesium Alloy Surface of Corrosion Resistance Research

Abstract

The modified nano-CeO2/Zn–Mn phosphate composite coating was deposited on AZ91D magnesium alloy by chemical conversion to enhance its densification and corrosion resistance. The growth mechanism and corrosion resistance of the composite coating is clarified by adding different concentrations of ZnO and a certain amount of nano-CeO2 into the phosphate-plating solution. XRD and EDS show that the composite membrane is mainly composed of MgO, Mg(OH)2, Mn3(PO4)2·5H20, Zn, Zn3(PO4)2·4H2O and CeO2. Among them, AZ91D magnesium alloy matrix presents dispersed granule, clustered and petal-shaped under the action of different concentrations of ZnO. Under the optimum ZnO concentration, after adding nano-CeO2, dense grains appear, and cracks and pores in the riverbed are obviously reduced. Compared with single-layer phosphate coating, the performance of composite coating is significantly improved. The results show that the obvious double-layer structure is observed by SEM, and the thickness of the coating is about 48 μm. The LCSM shows that the surface roughness of composite coating is moderate. EIS shows that when the fitting impedance is 8050.43 Ω and PH = 3, the dropping time of copper sulfate in the composite coating is 58.6 s, which is better than that in the single-layer coating. The Tafel polarization fitting curve shows that the corrosion current density of the composite coating is obviously lower than that of the single coating, the corrosion current density is decreased from 1.86 × 10−6 A/cm2 to 9.538 × 10−7 A/cm2, and the corrosion resistance is obviously improved.