The effect of substrate on the electroless nickel plating of Mg and Mg alloys

Magnesium (Mg) and its alloys are being used as structural components in industry because of their high strength-to-weight ratio and relatively high stiffness. A shortcoming of Mg based alloys is their high electrochemical activity and poor corrosion resistance. Therefore, coatings or surface treatment are needed for protection purpose. This paper reports our work on electroless plating of Ni on Mg alloys. Pure Mg, AZ31 and AZ91 Mg alloys were used as the substrates to investigate the deposition rate, nucleation process, chemical composition and microstructure of the electroless Ni coatings. It was found that the deposition processes are strongly affected by the alloy substrates. Ni plating on AZ31 alloy has higher deposition rates and lower surface roughness than that of other substrates. SEM investigations showed that nodular morphology of Ni coatings formed on the substrates. Chemical pre-treatment before plating increases the substrate surface roughness significantly. The plating process was strongly influenced by the substrate microstructure at the early stage. Deposition was initiated on the phase and grain boundaries due to the inter-grain galvanic coupling. Fine substrate microstructure such as that of AZ31 alloy provided a large amount of nucleation sites, benefiting the nucleation and growth processes of the electroless nickel plating. The adhesion strengths of the coatings on AZ31 and AZ91 alloys are higher than that on pure Mg. The critical load (a measure of adhesion strength) of AZ31 reached 13.1 N.