Electroless Ni-P plating on Mg-10Gd-4.8Y-0.6Zr magnesium alloy with a new pretreatment process

A new pretreatment process for electroless Ni-P plating on Mg-10Gd-4.8Y-0.6Zr was investigated in this paper. The morphology, component, chemical composition and structure of the pretreatment layers and Ni-P coating were analyzed by scanning electronic microscopy, energy dispersive spectroscopy and X-ray diffraction spectroscopy. The structure of Ni-P coating was also detected by transmission electron microscopy (TEM). Potentiodynamic polarization analysis and salt spray test were used to test the corrosion resistance of the Ni-P coating. Experimental results indicate that: metal Cr generated in the new pretreatment process, which provided active points for later zinc immersion process, was beneficial to nickel deposition. The subsequent Ni-P coating was amorphous, in which the content of P was 9.43 wt.%. It was uniform and its thickness reached about 50 μm at 2 h deposition. Compared to Mg-10Gd-4.8Y-0.6Zr alloy substrate, the corrosion potential of the coated alloy shifted by 1090 mV positively and the corrosion current density decreased one order of magnitude in 3.5 wt.% NaCl solution. The salt spray test time of Ni-P coating was 210 h. All of these results suggest that the electroless nickel plating procedure researched in this paper is suitable for Mg-10Gd-4.8Y-0.6Zr alloy.