Oxidation of NdFeB-type magnets modified with additions of Co, Dy, Zr and V

The oxidation characteristics of NdFeB alloys, modified by additions of Co up to 10 at.% and with minor additions of Dy, Zr and V, have been examined in this article over the temperature range 250–500 C. The surface oxide consisted of an outer surface layer of polycrystalline haematite, regardless of Co content, and an inner layer of Co-modified magnetite. Underlying this two-layer surface oxide was an extensive internal oxidation zone (IOZ) formed as a result of preferential reaction with oxygen of the magnetic phase, Nd2(Fe, Co)14B. The IOZ consisted of a bcc solid solution of Fe and Co and contained a fine dispersion of Nd-rich oxide particles. The zone grew by inward oxygen diffusion, principally along high-angle grain boundaries within the Fe-rich matrix. The thickening of the IOZ occurred with parabolic kinetics but at a rate which decreased with increasing alloy Co content by a consistent factor of 2–3 in parabolic rate constant at all test temperatures. The activation energy for the growth of the IOZ lay in the approximate range 91–104 kJ/mol for the various alloys but there did not appear to be a systematic variation with alloy composition.