Analysis of the Causes of Differences between the Upper and Lower Surfaces of Electroless Ni–P Coating on LZ91 Magnesium–Lithium Alloy

Abstract

To address the issue of poor corrosion resistance of the Mg–Li alloy, electroless Ni–P plating was used to create a protective coating. However, there were significant differences between the upper and lower surface coatings, which were summarized as follows: (1) compared with the lower surface, the longitudinal differences between different areas of the upper surface coating were larger; and (2) the denseness of the upper surface coating was insufficient in areas where the insoluble phase was concentrated, resulting in significantly lower corrosion resistance of the upper surface coating than the lower surface. Resolving these differences could compensate for the defects of the upper surface coating so as to improve the overall corrosion resistance of the material. Therefore, in this paper, the deposition process of Ni–P was observed and speculated, and the reasons for these differences were analyzed in combination with experimental phenomena. Based on these, two optimization measures were proposed. The SEM observation results showed that the differences between the upper and lower surface coatings were significantly reduced after optimization. The results of potentiodynamic polarization tests and EIS tests showed that the optimized upper surface coating had good corrosion resistance similar to the lower surface coating.