Corrosion behavior of textured and isotropic nanocrystalline NdFeB-based magnets

Textured nanocrystalline Nd₁₄Fe₈₀B₆ and Nd₁₂Dy₂Fe_73.2Co_6.6Ga_0.6Br_5.6 magnets were produced from melt-spun materials by hot pressing and die-upsetting. Isotropic hot-pressed precursors with various grain sizes obtained by additional annealing were also prepared for comparative studies. The corrosion behavior of these NdFeB-based magnets was studied in 0.1 M H₂SO₄ electrolyte (pH = 1) by in situ inductively coupled plasma MS solution analysis (ICPMS), gravimetric measurements and electrochemical polarization techniques. The sorption behavior of hydrogen formed during the corrosion process has been investigated by means of hot-extraction and thermal-desorption analysis. Coarsening of the microstructure of hot-pressed magnets results in a better corrosion performance. The sensitivity to abnormal cathodic dissolution is lowered by additions. The introduction of texture to NdFeB-based magnets via die-upsetting improves their corrosion resistance. Textured magnets with Dy, Co, and Ga additions exhibit the best corrosion performance which is due to the reduction in the strength of the galvanic coupling effect between the magnet phases and the lower effective diffusivity of corrosion hydrogen inside the bulk magnet. The corrosion behavior of the magnets in relation to phase composition and phase distribution is discussed in terms of materials dissolution, hydrogenation and mechanical degradation.