Characterization of Electroless Ni-P Deposits on Surface of AZ91D Magnesium Alloy

摘要

Characterization of electroless nickel deposits on magnesium alloys was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electrochemical potentiodynamic polarization analysis. The result of XRD showed that the electroless nickel deposits on magnesium alloys were composed mostly of amorphous Ni-P and partially of microcrystalline nickel. The phosphorus content in the Ni-P deposits was 10.8% by the calculation of EDS. SEM micrographs showed that morphology of electroless nickel deposits on magnesium alloys was a typical cellular structure, and the growth of cellular structure was influenced by surface condition of substrate. Zinc layer between substrate and electroless nickel deposits was not found by the results of SEM and EDS, which indicated that the zinc layer on the surface of magnesium alloy was very thin, and the zinc would dissolved while being immersed in electroless nickel plating solution. The thin zinc layer was almost dissolved completely before Ni-P plating. The result of electrochemical potentiodynamic polarization analysis showed that the corrosion potential Ecort of electroless nickel deposits in 3.5%NaCl solution was about -0.53 V, which was 1 V higher than that of magnesium alloys substrate. And corrosion current density of Ni-P deposits in 3.5%NaCl solution was much smaller than that of magnesium alloys substrate at the same potential value, which indicated that corrosion resistance of electroless Ni-P deposits was excellent. Discoloration time of concentrated nitric acid test for electroless nickel deposits on magnesium alloys was 445 s, which also indicated excellent corrosion resistance.