Corrosion kinetics of spark plasma sintering Nd-Fe-B magnets in different electrolytes

The chemical stability of spark plasma sintering (SPS) anisotropic Nd-Fe-B magnets as well as the conventional sintered magnets in different electrolytes was studied by means of anodic potentiodynamic polarization experiments. In an alkaline solution, both magnets exhibit evident passive behavior, indicating that they possess good corrosion resistance in such an environment. In saline and weak acid solutions, however, no passivation occurred and SPS Nd-Fe-B has displayed a more positive corrosion potential E_corr and a lower corrosion current density I_corr than that of the conventional sintered Nd-Fe-B with same composition. Then, electrochemical impedance spectroscopy (EIS) tests were carried out to investigate their corrosion kinetics in different solutions. According to Nyquist and Bode diagrams, a convenient equivalent circuit was modeled and the values of the proposed equivalent circuit components were calculated. It was found from the EIS plot formats that the electrochemical reaction of SPS Nd-Fe-B magnets was controlled by the step of surface charge transfer, i.e., activation polarization in alkaline solution, while controlled by the step of active substances adsorption on the magnet surface in saline and acidic solutions. Compared with conventional sintered Nd-Fe-B magnets, SPS Nd-Fe-B magnets possess better chemical stability in saline and weak acidic environment.