ANISOTROPIC HYDROGEN DECREPITATION AND CORROSION BEHAVIOR IN NDFEB MAGNETS

A number of Nd176Fe76B8 sintered magnets were produced in isotropic (u naligned) and anisotropic (aligned) forms. The magnets were prepared i n the conventional way via isostatic pressing and finished with a cent reless grinder, no coatings were applied and the samples were left unm agnetised. These magnets were identical in every way apart from the or ientation of the Nd2Fe14B grains. Each magnet was exposed to hydrogen and the decrepitation behaviour observed. The anisotropic samples were found to decrepitate exclusively from the ends of the rods whereas th e isotropic magnets were attacked by the hydrogen at all points on the ir surface. Bulk corrosion studies in steam gave comparable results, t he oriented sample suffered severe corrosion at its poles and the isot ropic sample was attacked over the whole surface. X-ray diffraction st udies on the corrosion product indicated a greatly expanded lattice wi th the Nd2Fe14B structure. Both the hydrogen decrepitation and the cor rosion which take place in these materials can be attributed to the fo rmation of hydrides.