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

The corrosion behaviour of Nd–Fe–B nanocrystalline magnets made from differently processed powders such as melt spun, intensively milled and HDDR (Hydrogenation–Disproportionation–Desorption–Recombination) powders has been investigated in sulphuric acid solution and in air at 25°C. It is observed that the corrosion resistance of the investigated magnets can be correlated with the grain size of the hardmagnetic phase and the distribution and composition of the Nd-rich intergranular phases. The magnet made from HDDR 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 Aresistance of the investigated magnets by replacing the high corrosion sensitive intergranular Nd-rich phases with more noble phases. The surface layer formed during anodic polarization of the HDDR magnet was analysed by Auger electron spectroscopy indicating the formation of (Nd,Fe)-oxide with small amounts of Co and Al. The measured electrostatic surface potential differences of magnets containing Co, Al and Ga were lower than those of magnets without alloying additives.