Study Of Electroless Plating On AZ31D Magnesium Alloys And Renewability Of The Bath

Magnesium alloys have a number of desirable features, including low density, high strength-to-weight ratio, excellent castability, good machinability and damping characteristics, and its use in a variety of applications, particularly in aerospace, automobile, mechanical and electronics components, is now growing rapidly. However, poor corrosion resistance has often prevented it from further exploitation. Electroless-nickel-plating （EN）, being one of the surface treatments of magnesium alloys, has attracted much attention due to the possession of a combination of properties, such as good corrosion and wear resistance, deposit uniformity, electrical and thermal conductivity, and solder-ability etc. However, this process is now far from completeness, particularly in corrosion resistance aspects. Therefore, this dissertation is aimed at optimization procedure by orthogonal design ways. In this paper, sodium hypophosphite, citric acid, lactic acid, temperature and pH value was researched to understand that the factors influencing direct-electroless-plating on AZ31D magnesium alloys, and the optimization parameters were obtained. "Two-step" electroless nickel-plating and the stability of the bath and the regeneration of the bath.after activation on AZ31Dmagnesium alloy was also investigated.Using the scan electronic microscope equipped with energy dispersive, and X-ray diffraction, micro-hard tester,scratch tester, salt spray tester etc, the surface morphology and microstructure, micro-hardness, adherence between the plating and matrix, and the corrosion resistance of the plating were observed. The result showed that the optimization parameters were fit to direct electroless plating on AZ31D magnesium alloys. The morphology of the plating was very nice and the adherence between the plating and matrix was choiceness, and the microhardness of the plating was much higher than that of AZ31Dmagnesium alloys and the corrosion resistance of the plating was outstanding.The results of "Two-step" electroless nickel-plating showed that it could using NiSO₄ as the main salt in Electroless plating after 8 minutes direct alkaline nickel-plating on AZ31D magnesium alloy, The morphology of the plating was very nice and the adherence between the plating and matrix was choiceness, and the microhardness of the plating was much higher than that of AZ31D magnesium alloys. The coating is high phosphorus amorphous with higher corrosion resistance. Studying on the reproducibility of the bath showed that 5 hours after plating, plating liquid has serious aging. Through the titration experiment, the addition of main salt and reducing agent are determined, so the reproducibility of the bath is achieved.