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

The electrode functions (anode or cathode) of coupled single phases (ferromagnetic phase: Nd₂Fe₁₄B; boron rich phase: Nd_1+εFe₄B₄; neodymium rich phase: Nd₄Fe) of Nd‐Fe‐B permanent magnets have been investigated. The electrochemical potential differences and current densities between the three phases were determined. Furthermore, the corrosion current density of the magnetic materials in 0.1 M H₂SO₄ was calculated from measurements of current density‐potential curves. The neodymium rich phase works as the anode in contact with both the ferromagnetic and the boron rich phase. The calculation of corrosion current densities for systems of three coupled phases from the current density‐potential curves of single phases measurements demonstrates very high values for the 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 presented results do not demonstrate an important influence of the alloying elements gallium or dysprosium on the corrosion rate of the Nd‐Fe‐B magnets. Therefore, new concepts have to be developed to increase the corrosion resistance of these magnets.