Corrosion behavior of polymer-bonded NdFeB-based nanocrystalline magnets

The corrosion resistance of polymer bonded nanocrystalline NdFeB magnets with Nd-rich, stoichiometric, and Fe-rich compositions and that of their main constituent single phases is assessed in aqueous environments. For the evaluation of corrosion properties, gravimetric tests as well as potentiodynamic polarization measurements were performed in different nitrogen-purged electrolytes: 0.1 M sulfuric acid (pH=1), 0.5 M phthalate buffer (pH=5), and 0.1 M sodium hydroxide (pH=13). The single phases show anodic dissolution in strong acidic environment and stable passivation in alkaline solution. In weakly acidic phthalate buffer Nd-rich and Fe phases exhibit limited passive regions, whereas Fe₃B and Nd₂Fe₁₄B do not show stable passivation. Polymer-bonded magnets of different compositions have a significantly improved resistance toward strongly acidic environments as compared to hot pressed magnets. Enrichment in Fe content and additions like Co and Nb improve their corrosion behavior.