Electroless Ni-P Plating on AZ91D Magnesium Alloy with a New Homogenization Pretreatment

A new environment-friendly homogenization pretreatment process was adopted for the electroless nickel plating on AZ91D magnesium alloy. Then, the morphologies, chemical composition and microstructure of the pretreated layer and the formed Ni-P coating were studied using scanning electronic microscopy(SEM), energy-dispersive X-ray spectroscopy(EDS) and X-ray diffraction spectroscopy(XRD). It was indicated that a dense Zn film formed on the substrate surface after the homogenization pretreatment, due to the selective removal of β phase during the processing.As the result, the subsequently formed Ni-P coating was uniform, compact and defect-free with P content around 7.0%, which was characterized as an amorphous matter by XRD. Potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) tests revealed that the Ni-P coating could significantly improve the corrosion resistance of AZ91D magnesium alloy with an obvious positive shifting of corrosion potential by 1.136 V and a dramatic reduce of corrosion current density by two orders of magnitude. Furthermore the obtained Ni-P coating showed excellent adhesion to AZ91D substrate.