Electroless Ni–P Deposition on AZ91 D Magnesium Alloy Prepared by Molybdate Chemical Conversion Coatings

In this study, electroless nickel-phosphorus plating with good adhesion strength was deposited on AZ91 D magnesium alloy prepared with environmentally acceptable molybdate chemical conversion coatings (Mo coatings). Energy dispersive X-ray spectroscopy of Mo coatings revealed that the conversion films were mainly composed of Mg 2 Mo 3 O 11 particles. The Mo coating showed a rough surface reflected by the nonuniform dissolution of α and β phases of the AZ91 D Mg alloy during the conversion process. Such a rough surface provided excellent adhesion strength of the plating layer to the Mo undercoating layer, reaching 18 MPa as measured by the pull-off method due to the anchor effect. Scanning electron microscopy results showed that the plating layer over Mo coatings was also rough but compact and almost pore free. Corrosion tests using potentiodynamic polarization in 3.5 wt % NaCl aqueous solution indicated that the electroless Ni–P-plated AZ91 D Mg alloy showed a significant improvement in corrosion resistance compared to the corrosion resistance of uncoated specimens.