Processes and properties of electroless Ni-P-ZrO_2 composite coating on AZ91D magnesium alloy

Traditional process of electroless Ni-P plating on AZ91D magnesium alloy was improved,and the poisonous substances of hydrofluoric acid and Cr~(6+) were not used.The mechanism of pretreatment and nickel initial deposition for new process was investigated.Meantime,the process of Ni-P-ZrO_2 composite plating was developed,and the microstructure and properties of the composite coating were studied.The results show that the Ni-P coating prepared with new process is more compact and uniform,and the corrosion resistance is superior to that of traditional process Ni-P coating.The potentiodynamic polarization curves in 3.5%NaCl solution indicate that the Ni-P-ZrO_2 composite coating could provide adequate protection to magnesium alloy substrate.The wear mass loss of Ni-P coating is nearly twice more than that of Ni-P-ZrO_2 coating,implying that the wear resistance of the composite coating is improved due to the co-deposition of ZrO_2 nanometer particles.