Effects of Thermal Demagnetization in Air on the Microstructure and Organic Contamination of NdFeB Magnets

Demagnetization is an essential step for the demounting and safe handling of end-of-life NdFeB. Thermal demagnetization in air is a straightforward option to demount adhesive-fixed or segmented magnets. However, this process is suspected to increase the uptake of contaminants like O, C and Zn from coatings and adhesives, potentially degrading the recyclate quality. This study tests the effects of thermal demagnetization in air at 400 °C for 15 to 240 min on variously coated samples with different initial oxidation levels. Furthermore, the possible reversal of the contaminant uptake is explored. Samples with low previous oxidation levels showed significant uptake in oxygen with a minimal diffusion depth, while the uptake depended on the used coating. The best protectiveness was achieved with NiCuNi with an increase in oxygen of only around 30%. Epoxy (up to ~130% O uptake) and Zn coatings (up to ~80% O uptake) disintegrated during the treatment and offered less protection but still made a difference compared to uncoated samples (up to ~220% O uptake). Samples with high initial oxidation levels show no clear tendency towards further oxygen uptake and the carbon uptake is generally low, likely due to contemporary epoxy coatings featuring a passivation underneath as a barrier layer. Zn infiltration, which carried organic debris, was observed. Short demagnetization times proved to be favorable for limiting the depth of the diffusing contaminants. Mechanical coating removal after thermal demagnetization in air can mitigate the contaminant uptake, producing clean, recyclable end-of-life material.