Effect of alloy microstructure on electroless NiP deposition behavior on Alloy AZ91

Various magnesium alloys, which were assumed as reference to the individual behavior of each microstructural constituents of Alloy AZ91 in the coating bath, were designed to analyze the hypophosphite-reduced electroless nickel coating characteristics on them in both uncoupled and coupled conditions. Galvanic coupling of β phase to the matrix phase in the coating bath was observed to decelerate the deposition rate on the matrix phase. This behavior was attributed to the effective cathodic action of β phase in the coating bath as a result of the catalytic cathodic reactions on it. The variation in the Al content of the matrix phase was also estimated to affect the NiP deposition behavior due to the small scale galvanic interactions. The previously Ni nucleated sites on the alloy surface behaved as the most preferential sites for further Ni nucleation and growth. Low temperature heat treatment (230 °C for 2 h) caused a considerable increase in the surface hardness with negligible decrease in the corrosion resistance. High temperature heat treatment (400 °C for 1 h) improved the surface hardness further with significant deterioration in the corrosion resistance.