Corrosion behaviour of Nd–Fe–B permanent magnetic alloys

The corrosion behaviour of Nd–Fe–B permanent magnetic alloys as well as of single phases of these alloys has been investigated in sulphuric acid at room temperature and humid air at 150°C using mass loss and electrochemical techniques. Scanning electron microscopy and scanning probe microscopy were used to study the surface topography. The electrostatic surface potential was examined by scanning probe microscopy using tapping mode. A correlation between the electrostatic surface potential and the corrosion rate of these alloys was found. The intergranular phases with higher electrostatic surface potential shows higher corrosion rate in both environments. The corrosion rate of the phases in the sintered permanent magnetic alloys increases as follows: φ-phase<η-phase<n-phase. The differences in the chemical composition as well as in the preparation methods of these magnetic alloys have a distinct influence on the corrosion rates. Small additions of Co, Al and Ga increase the corrosion resistance of the magnetic Nd–Fe–B alloys.