Corrosion Behaviour of Injection- and Compression-Moulded Nd-Fe-B and Sm-Fe-N Magnets with Different Polymer Binders

The corrosion behaviour and environmental durability of injection- and compression-moulded Nd-Fe-B and Sm-Fe-N magnets were investigated. For injection-moulded magnets, the effects of magnetic powder type (Nd-Fe-B and Sm-Fe-N), magnetic powder particle size (100 µm and 400 µm), and polymer binder (PPS and PA12) on corrosion resistance were studied. For compression-moulded magnets with an epoxy binder, the effects of powder type and size were examined. Corrosion resistance was investigated using potentiodynamic polarisation in electrolytes of varying pH (1.8-12.8). The Sm-Fe-N magnets exhibited slightly better corrosion resistance than the Nd-Fe-B magnets, irrespective of the polymer binder. The finer magnetic powders (100 µm) showed lower corrosion resistance due to their larger specific surface area, with a more pronounced effect in the compression-moulded magnets. The type of polymer binder had only a minor effect. The hygrothermal corrosion resistance and thermal stability were evaluated using bulk corrosion (BCT) and thermal shock tests, respectively. Surface corrosion was observed in all magnets after the BCT, with the compression-moulded magnets exhibiting a greater irreversible loss of magnetic properties. The thermal shock test caused a temporary reduction in magnetic properties, with recovery after remagnetisation, demonstrating the good thermal stability of both magnet types.