Corrosion behavior of Sm-Co-based permanent magnets in oxidizing environments

Investigations of the oxidation behavior of Sm(Co, Fe, Cu, Zr)_z-type high temperature permanent magnets are of particular importance for the assessment of their stability under application conditions in the temperature range up to 500°C. Sintered magnets of the Sm₂Co₁₇-type were studied regarding the dependence of surface on the exposure temperature and time in air. For the investigation of the oxide layer growth processes at elevated temperatures, time-dependent mass changes were determined using thermo-gravimetrical analysis. Oxidized magnet surfaces were characterized by means of X-ray diffraction, optical microscopy and scanning electron microscopy (incl. energy dispersive analysis). It was found from time-dependent mass changes that a parabolic growth law controls the oxidation on the Sm₂Co₁₇-type magnet surfaces. An oxide layer thickness of 135 μm was measured after 312 h exposure time at a temperature of 500°C. The outermost oxidic surface layer is mainly formed by oxidized Fe and Co species. The Sm concentration at the surface is depleted due to vaporization. The oxide layer is separated in Co-rich and Sm-rich regions as well as Cu-rich segregations near microcracks.