In-Situ Microscopy Observations of Oxide Phases Formation During High-Temperature Oxidation of End-of-Life Ni/Cu/Ni-Coated-NdFeB Permanent Magnets

An understanding of the reaction phases formed during the high-temperature oxidation of rare earth permanent magnets is vital for developing appropriate strategies for their pyrometallurgy-based recycling processes aimed at the concentration and potential recovery of the rare earth elements. The current study uses in-situ high-temperature optical microscopy combined with ex-situ scanning electron microscopy to analyze oxide growth on the surface of an end-of-life Ni/Cu/Ni-coated NdFeB permanent magnet oxidized at 900 and 1100 °C for up to 4 h under ambient air conditions. Distinct oxide morphologies were observed at the two oxidation temperatures over time ranging from blister- and spike-like structures. At the longest oxidation times, visible cracking of the surface of the magnet and protrusion of FeO phase were observed. The presence of a Ni/Cu/Ni coating on the magnets was found to significantly affect the oxidation growth; a nickel ferrite phase was formed at 1100 °C after 4 h while at 900 °C, there was still discrete NiO and FeO.