Characterisation of electrochemical interactions between single phases of Nd-Fe-B permanent magnets

The electrode functions (anode or cathode) of coupled single phases (ferrom agnetic phase: Nd2Fe14B; boron rich phase: Nd1+epsilonFe4B4; neodymium rich phase: Nd4Fe) of Nd-Fe-B permanent magnets have been investigated. The ele ctrochemical potential differences and current densities between the three phases were determined. Furthermore, the corrosion current density of the m agnetic materials in 0.1 M H2SO4 was calculated from measurements of curren t density-potential curves. The neodymium rich phase works as the anode in contact with both the ferrom agnetic and the boron rich phase. The calculation of corrosion current dens ities for systems of three coupled phases from the current density-potentia l curves of single phases measurements demonstrates very high values for th e small area of the corroding neodymium rich phase at the grain boundaries. An increase of corrosion resistance of Nd-Fe-B-magnets is possible only by an increase of the corrosion resistance of neodymium rich phase. The presen ted results do not demonstrate an important influence of the alloying eleme nts gallium or dysprosium on the corrosion rate of the Nd-Fe-B magnets. The refore, new concepts have to be developed to increase the corrosion resista nce of these magnets.