Corrosion behavior and strengthening mechanism of Ni-Cu alloy coating on Nd-Fe-B magnets

In this paper, the Pulse-Reverse Current (PRC) electroplating technique was utilized to deposit Ni-Cu alloy coatings on the surface of Nd-Fe-B magnets. Compared with the Ni coating, the corrosion resistance of the Ni-Cu alloy coating has been significantly improved, with the corrosion potential Ecorr and the corrosion current density Icorr of -246 mV and 0.9 μA·cm-2, respectively. 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 Ni-Cu coating is flat and the grains preferentially grow along the (111) close-packed surface, which also makes the coating have higher densification and significantly reduces the number of self-corrosion sites and corrosion tendency of the coating. The lower binding energy Cu2O produced by Ni-Cu 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 Cu2O and Cu2(OH)3Cl of Cu in the pitting hole have a certain hindrance to corrosion and are conducive to promoting passive reconstruction, which is an important reason for the Ni-Cu alloy coating shows higher self-healing ability and higher corrosion resistance.