Effects of post-sinter annealing on the electrochemical corrosion resistance of Nd–Fe–B sintered magnets

• Annealing increased corrosion resistance of NdFeB magnets.
• Improved resistance depends on uniform distribution of small-sized Nd-rich phases.
• Less active reaction channels occurred in modified grain boundaries.

Effects of the annealing temperature on the corrosion resistance of (Pr, Nd)32.5FebalAl0.3Cu0.1Nb0.1B1.0 magnets were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). These magnets were annealed at 420, 520 and 620 °C. It was found that the corrosion resistance of magnets in hydrochloric acid solution is related to the annealing temperature, hence, to their grain boundary microstructure. Annealing at 620 °C, the small-sized (Pr, Nd)-rich phase grains harmoniously distributed at the intergranular triple junctions, in which less active reaction channels occurred. It therefore led to much better electrochemical corrosion resistance for the magnet annealed at 620 °C in comparison with other magnets annealed at lower temperatures and especially with the as-sintered magnet.