Electrochemical corrosion behaviors and protective properties of Ni-Co-TiO₂ composite coating prepared on sintered NdFeB magnet

In this paper, a protective Ni-Co-TiO₂ composite coating was prepared on the sintered NdFeB magnet by direct current electrodeposition. The surface morphologies, microstructure, and chemical composition of the composite coating were studied using scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS), respectively. The surface morphologies and microstructure analysis showed that the composite coating possessed cauliflower-like grain colonies, and formed face-centered cubic (fcc) solid solution. The electrochemical corrosion behaviors of the composite coating in 0.5 mol/L H₂SO₄, 0.6 mol/L NaOH, 0.6 mol/L Na ₂SO₄, and neutral 3.5wt% NaCl solutions were evaluated by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS), showing good protection for NdFeB magnet. In order to further investigate the protective properties of the composite coating for NdFeB magnet and the practicability of the composite coating, the long-term immersion test was carried out in neutral 3.5wt% NaCl solutions using EIS.The results of long-term corrosion test showed that the Ni-Co-TiO2 composite coating could provide long-term protection in neutral 3.5wt% NaCl solutions for NdFeB magnet.