Corrosion behavior and strengthening mechanism of Ni-Cu alloy coating on NdFeB magnets

In this paper, the pulse-reverse current electroplating technique was utilized to deposit nickel–copper (Ni-Cu) alloy coatings on the surface of neodymium magnets (NdFeB). Compared with the nickel (Ni) coating, the corrosion resistance of the nickel–copper alloy coating has been significantly improved. Additionally, the results show that alloying can effectively prolong the incubation period of pitting nucleation and improve the self-healing ability of coating. The structure and microstructure of the coating show that the surface of the nickel–copper coating is flat and the grains preferentially grow along the (111) close-packed surface, which also makes the coating to have higher densification and significantly reduces the number of self-corrosion sites and corrosion tendency of the coating. The lower binding energy copper(I) oxide (Cu₂O) produced by nickel–copper coatings at the initial corrosion stage can reduce the formation of metal cation holes and prolong the incubation period of pitting corrosion. After pitting formation, the corrosion products copper(I) oxide and dicopper chloride trihydroxide (Cu₂(OH)₃Cl) of copper (Cu) in the pitting hole have a certain hindrance to corrosion and are conducive to promoting passive reconstruction, which is an important reason for higher self-healing ability and higher corrosion resistance of the nickel–copper alloy coating.