Investigations of the corrosion behaviour of nanocrystalline Nd-Fe-B hot pressed magnets

The corrosion behaviour of Nd-Fe-B nanocrystalline magnets made from differ ently processed powders such as melt spun, intensively milled and HDDR (Hyd rogenation-Disproportionation-Desorption-Recombinatoin) powders has been in vestigated in sulphuric acid solution and in air at 25 degrees C. It is obs erved that the corrosion resistance of the investigated magnets can be corr elated with the main size of the hardmagnetic phase and the distribution an d composition of the Nd-rich intergranular phases. The magnet made from HDD R powder exhibits a comparable, if not better, corrosion resistance in acid solution than magnets made from melt spun and intensively milled powders. Small additions of Co, Al and Ga improved the corrosion resistance of the i nvestigated magnets by replacing the high corrosion sensitive intergranular Nd-rich phases with more noble phases. The surface layer formed during ano dic polarization of the HDDR magnet was analysed by Auger electron spectros copy indicating the formation of (Nd,Fe)-oxide with small amounts of Co and Al. The measured electrostatic surface potential differences of magnets co ntaining Co, Al and Ga were lower than those of magnets without alloying ad ditives. (C) 2000 Elsevier Science S.A. All rights reserved.