Corrosion behavior of dual-main-phased (Nd_0.8Ce_0.2)₂Fe₁₄B magnets with and without annealing process

In this work, the corrosion behavior of the sintered Nd₂Fe₁₄B in which Nd was substituted by Ce to form dual-main-phased magnet with/without annealing process was studied. The mass loss of as-sintered magnets (SM) is larger than that of as-annealed magnets (AM) in the accelerated corrosion tests. In particular, the mass loss of SM increases sharply when the corrosion time increases to 48 h. The free corrosion current of the SM samples is larger than that of AM in 0.1 mol·L⁻¹ HCl; this is due to that the content of RE-rich phase at the intergranular triple junctions (TJs) of the SM samples is larger than that of AM samples and the corrosion rate of the RE-rich phase at the TJs is higher than that at grain boundaries. But the free corrosion current of the SM samples is smaller than that of AM samples in 3.5 wt% NaCl as the electron transfer resistance (R _ct) of SM is larger than that of AM. Inductive loop in the Nyquist diagrams at low frequency of SM is observed, which originated from the pitting. It shows that pitting of SM occurs more easily than that of AM. Corrosion morphology of samples indicates that the corrosion area of SM is bigger and deeper than that of AM after accelerated corrosion for 60 h. Through the annealing process, the distribution of rare-earth-rich phase becomes more uniform, which changes the free corrosion current and effectively suppresses the occurrence and propagation of pitting.