Electroless nickel-plating on AZ91D magnesium alloy: effect of substrate microstructure and plating parameters

Electroless nickel-plating on AZ91D magnesium alloy has been investigated to understand the effect of substrate microstructure and plating parameters. The initial stage of the deposition was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis on substrates plated for a very short interval of time. The early stage of growth was strongly influenced by the substrate microstructure. Plating was initiated on β-phase grains probably due to the galvanic coupling of β and eutectic α-phase. Once the β-phase was covered with the coating, it then spread onto eutectic α and primary α-phase. The coating produced with the optimised bath showed 7 wt.% phosphorus with a hardness of approximately 600–700 VHN. The optimum ligand to metal ion ratio was found to be 1:1.5, while the safe domain for thiourea (TU) was in the range of 0.5–1 mg/l. Fluoride was found to be an essential component of the bath to plate AZ91D alloy with an optimum value of 7.5 g/l. The presence of 0.25–0.5 mg/l mercapto-benzo-thiosole (MBT) found to accelerate the plating process.