Electrodeposition of high corrosion resistance Cu/Ni–P coating on AZ91D magnesium alloy

High corrosion resistance Cu/Ni–P coatings were electrodeposited on AZ91D magnesium alloy via suitable pretreatments, such as one-step acid pickling-activation, once zinc immersion and environment-friendly electroplated copper as the protective under-layer, which made Ni–P deposit on AZ91D Mg alloy in acid plating baths successfully. The pH value and current density for Ni–P electrodeposition were optimized to obtain high corrosion resistance. With increasing the phosphorous content of the Ni–P coatings, the deposits were found to gradually transform to amorphous structure and the corrosion resistance increased synchronously. The anticorrosion ability of AZ91D Mg alloy was greatly improved by the amorphous Ni–P deposits, which was investigated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The corrosion current density (Icorr) of the coated Mg alloy substrate is about two orders of magnitude less than that of the uncoated.