Abstract
Effects of Co addition on improving the corrosion resistance of Nd-Fe-B ternary alloy have been investigated through the measurements of potentiodynamic polarization curves. Nd-Fe-B ternary magnets are easily corroded with contact of wet air because the natural electrode potential (Ecorr) of Nd-rich phase shows almost half level compared with that of 2-14-1 phase. In order to improve reliability of Nd-Fe-B magnets, it is necessary to control the component phases of the magnets as follows; 1) Improvement of the Ecorr, level of main phase to be noble potential, 2) Improvement of the difference of Ecorr between main phase and grain boundary phase to be minimum. Our research shows that Ecorr of Nd11.8Fe82.3-xCoxB5.9(x=0, 5, 10, 20, 50) alloys varies with Co content of the alloy because standard redox potential of Co is higher than that of Fe. In addition, the relation between weight changes by oxidation under 80°C×90%R.H. test and Ecorr of Nd11.8Fe82.3-xCoxB5.9(x=0, 5, 10, 20, 50) alloys shows a linear correlation. This result indicates the corrosion resistance of Nd-Fe-B magnets is estimated by the polarization evaluation.
