Electrochemical corrosion behaviors and corrosion protection properties of Ni–Co alloy coating prepared on sintered NdFeB permanent magnet

In this study, a protective Ni–Co alloy coating was prepared on sintered NdFeB magnet applying electrodeposition technique. A pure nickel coating was also studied for a comparison. The microstructure, surface morphologies, and chemical composition of coatings were investigated using X-ray diffraction, scanning electron microscope, and energy dispersive spectroscopy, respectively. The corrosion protection properties of coatings for NdFeB magnet in neutral 3.5 wt.% NaCl solutions were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The microstructure and surface morphologies analysis showed that the addition of cobalt element into matrix metal Ni altered the preferential orientation of pure nickel coating from (2 0 0) crystal face for pure nickel coating to (1 1 1) crystal face for Ni–Co alloy coating, and made the surface morphologies more compact and uniform due to the grain-refining. The results of potentiodynamic polarization test showed that compared with pure nickel coating, Ni–Co alloy coating exhibited much nobler corrosion potential (E corr) and lower corrosion current density (j corr), indicating better anticorrosive properties. The long-term immersion test by dint of EIS indicated that the Ni–Co alloy coating still presented high impedance value of 1.9 × 105 Ω cm2 with the immersion time of 576 h indicating the excellent anticorrosive properties, and corrosion protection properties of nickel coating for NdFeB magnet practically disappeared with the immersion time of 144 h, which also indicated that the Ni–Co alloy coating provided better corrosion protection properties for the NdFeB magnet compared with nickel coating.