The Improvement in Corrosion Resistance of Electroless Ni-P Coating on Micro-arc Oxidized AZ31 Alloy Prepared with Sodium Dodecyl Sulfate Additive and Post-supercritical CO₂ Mixed Electrolyte

Abstract

In this study, AZ31 Mg alloy was employed as the substrate and the widely used micro-arc-oxidation (MAO) coating was first prepared on its surface. For enhancing the corrosion and wear resistance, the electroless nickel plating was further prepared on the MAO surface. Since the porous nature of the MAO coating, the effect of using sodium dodecyl sulfate (SDS) as the surfactant to reduce the surface tension and enhance surface wettability in the electroless process was investigated. Moreover, the post-supercritical CO2 mixing with the electrolyte had similar function as reported in literature. Therefore, the effects different concentrations of SDS used in the conventional and the post-supercritical CO2 processes, respectively, were compared. The experimental results showed the slightly better improvement on the surface roughness, hardness and anti-corrosion characteristics from the post-supercritical CO2 mixing than the use of SDS additive. Those improvements from their conventional electroless process without SDS are indebted to the higher phosphorus (P) content, finer grain texture, and better filling in the porous cavity of the prepared coating. However, the combined use of post-supercritical CO2 mixing and high SDS concentration of 1.5 g/L had a downturn effect on the anti-corrosion performance due to the occurrence of intensive cracking caused by the high internal stress in the prepared coating. The transmission electron microscopy (TEM) examination revealed the mechanical interlocking between the MAO and Ni-P coating helped to enhance their adherence.