Effect of Zinc Phosphating Pretreatment on NiP Electroless coating on AZ31B Magnesium Alloy Surface

AZ31B is a kind of deformed magnesium alloys with good mechanical property, which is mainly used in the field of aerospace, new energy automobile, electronic products and so on. The ZnP is firstly deposited on the surface of AZ31B used for new energy automobile by phosphating pretreatment. And then, the electroless NiP coating is obtained on the surface of ZnP on AZ31B magnesium alloy. In order to further improve the hardness and corrosion resistance, the electroless NiP coating prepared on the surface of magnesium alloy is passivated finally. It is found out that the surface morphology of ZnP shows many micro cracks, which could disperse and decrease corrosion current. The micro cracks and zinc elements provide favorable nucleation conditions for the electroless NiP coating with better hardness and great corrosion resistance performance. Furthermore, the electroless NiP coating after passivation treatment has the largest hardness and the smallest corrosion current density, which are 417.51 HV0.1 and 12.58 mu A/cm(2) respectively.