The mechanism of high temperature oxidation of a SmCo-based magnetic alloy

Highlights: ► Sm appears not to be externally oxidized at 500–700 °C. ► Sm is internally oxidized to form fine Sm2O3 particles. ► Deep internal oxidation zone occurs at each temperature. ► Higher temperatures increase the susceptibility to breakaway oxidation. ► Oxidation is greatly accelerated with the temperature increase. - Abstract: The oxidation of a Sm(CobalFe0.22Cu0.08Zr0.02)7.5 magnetic alloy in air at 500, 600 and 700 °C has been studied. The temperature increase greatly promotes the external and internal oxidation. The external scale consists of a thin outer layer of CuO and a thicker inner layer of (Cox, Fe1−x)Fe2O4 with distribution of α-Fe2O3. Samarium oxide is not seen in the scale but it is mainly internal oxidation product. Breakaway oxidation is observed at higher temperatures. The mechanisms of external and internal oxidation as well as breakaway oxidation are presented, based on detailed compositional and microstructural characterization of the oxidized zone of the alloy.