Electroless Ni–Sn–P coating on AZ91D magnesium alloy and its corrosion resistance

An electroless Ni–Sn–P coating was deposited on AZ91D magnesium alloy in an alkaline-citrate-based bath where nickel sulphate and sodium stannate were used as metal ion sources and sodium hypophosphite was used as a reducing agent. The phase structure of the coating was amorphous. SEM and attached EDS observation revealed the presence of dense and uniform nodules in the ternary coating and the content of tin was 2.48wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution proved that the ternary Ni–Sn–P coating exhibited better corrosion resistance than the Ni–P coating in protecting the magnesium alloy substrate.