Anti-corrosion properties of electroless Ni-Cu-P ternary deposits on sintered NdFeB magnets

Electroless plating of a Ni-Cu-P ternary alloy was carried out on sintered NdFeB magnets to improve their corrosion resistance. The effects of complexing agent concentration, metallic-ion ratio[Cu²⁺]/[Ni²⁺], pH value, and temperature on the deposition rate and coatings' composition were studied. Surface morphologies of the coatings were observed through SEM (Model FEI SIRION), and the element composition of the coatings was analyzed through EDAX. The corrosion resistance of the Ni-Cu-P ternary alloy was investigated through polarization curves in a 3.5% NaCl solution and neutral salt spray tests. The results show that good adhesion of the Ni-Cu-P deposits to the magnets can be achieved after ultrasonic cleaned with alkaline solution and activated by acid; with increasing[Cu²⁺]/[Ni²⁺], the deposits transformed from amorphous to crystalline and became more smooth and compact, the phosphorus content of the deposits firstly increased then decreased. It is indicated that the electroless deposits on sintered NdFeB surface exhibited a superior corrosion resistance than electroless Ni-P deposits; the corrosion resistance of the Ni-Cu-P ternary alloy obtained from the solution with a metallic-ion ratio of 0.02 was the best.