Effect of Na4P2O7·10H2O as complexing agent on deposition behavior and property of electroless Ni-P coating on ZK61M magnesium alloy

Abstract

The electroless Ni-P coating was prepared on ZK61M magnesium alloy by using Na4P2O7·10H2O as main complexing agent and NiSO4·6H2O as main salt in the electroless nickel plating solution. The effects of Na4P2O7·10H2O on deposition rate, growth progress and corrosion resistance of the Ni-P coating were investigated systematically in this study. The properties of the plating were characterized by the measurements including, scanning electronic microscope (SEM), energy dispersive spectrometer (EDS), x-ray diffraction (XRD), electrochemical polarization and immersion tests in 5% NaCl solution. The results showed that the initial uniform deposition of the plating on the HF-activated substrate was achieved by using Na4P2O7·10H2O as the main complexing agent. The deposition rate showed an obvious upward trend with the increment on the concentration of Na4P2O7·10H2O. It was worth noting that the content of phosphorus reached more than 7 wt% in Ni-P coating while the concentration of Na4P2O7·10H2O was higher than 40 g L−1. Most importantly, the plating with an obvious microcrystal structure had excellent corrosion resistance. Moreover, no corrosion on the surface was observed while the sample was immersed into 5% NaCl solution for 12 h. The self-corrosion potential of the Ni-P coating exceeded −0.5 V. The self-corrosion current could be reduced about 2 orders of magnitude comparing to magnesium alloy substrate.