Corrosion Behavior of Sintered Nd-Fe-B Magnets

The corrosion behavior of sintered Nd-Fe-B magnets in 3.5 wt% NaCl solution for different time was studied by an immersion test. The morphology and composition of corrosion products were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The microstructure and corrosion behavior of specimens at different stages were investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The electrochemical equivalent circuits were used to fit the EIS data. Results show that pitting corrosion, optional intergranular corrosion and total corrosion occur successively during the immersion process. However, the corrosion product layer is a loose structure that can only temporarily slow down the corrosion rate. The Nd-rich phase at the grain boundary preferentially dissolves in the medium so that the main phase particles are exposed to the corrosive solution and quickly corroded, thereby causing damage to the magnets. The corrosion products are mainly iron and neodymium oxides or hydroxides, the type of which varies with the immersion time.