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.