A STRUCTURAL AND KINETIC-STUDY OF THE OXIDATION OF THE INTERMETALLIC SM-2(FE,CO)(17) COMPOUND FOR PERMANENT-MAGNETS

The oxidation of a Sm-2(Fe0.5Co0.5)(17) alloy in air leads to a comple te destruction of the initial intermetallic phases in the superficial layers. Three successive oxide scales are formed with parabolic growth kinetics. The activation energies obtained for the three oxide scales are, respectively, 69, 136 and 62 kJ mol(-1) from the surface to the core of the samples. Samarium sublimation occurs in the external scale , constituted of alpha-Fe2O3 haematite and CoFe2O4 spinel. The first s ubscale, composed of alpha-(Fe,Co) and Sm(Fe,Co)O-3 perovskite, is evi denced only above 630 degrees C. The second subscale microstructure is made up of alternate parallel platelets of alpha-(Fe,Co) ferrite and Sm2O3 sesquioxide. From the parabolic rate constants, the oxygen diffu sion mechanism is investigated in these subscales.