Corrosion resistance of Nd-Fe-B permanent magnetic alloys - Part 1: Role of alloying

The electrochemical behaviour of Nd-Fe-B magnetic alloys was investigated in acid and neutral solutions. The differences in the chemical composition of these materials have distinct influence on the corrosion rate and polarization behaviour. Small additions of cobalt, aluminium and gallium increase the corrosion resistance of the magnetic alloys at high cathodic potential. It was also observed that the increase of hydrogen evolution rate on the surface of the magnetic materials raises the rate of dissolution of these magnets. The electrostatic surface potential was examined by scanning probe microscopy. A relation between the electrostatic surface potential and electrochemical behaviour of these alloys was found. The high values of electrostatic surface potential of the intergranular phases reflect higher corrosion attack. Auger electron spectroscopy was used to analyse the surface layer which formed during anodic polarization of the magnet containing alloying additives. The result indicates the formation of (Nd,Fe)-oxide with small amounts of cobalt and aluminium.