Effect of oxygen, carbon, and nitrogen contents on the corrosion resistance of Nd-Fe-B magnets

The corrosion rate of Nd-Fe-B magnets is found to vary with O, C, and N contents. The corrosion rate rapidly decreases with increasing oxygen content and reaches the minimum when the oxygen content is between 0.6 and 1.2%. The corrosion rate rapidly decreases as the carbon content increases up to about 0.06% and then gradually decreases as the carbon content increases up to about 0.1%. Further increase in the carbon content up to about 0.15% slowly increases the corrosion rate and then increases the rate substantially with further carbon increase (>0.15%). Therefore, the proper carbon content for a low corrosion rate is 0.10+or-.04%. The effect of carbon content on the corrosion rate becomes smaller when the oxygen content increases in the alloy. The effect of nitrogen content on the corrosion rate is less dramatic when it is less than 0.1%, i.e. the corrosion rate decreases with increasing nitrogen content up to 0.10% when oxygen and carbon contents are properly high, whereas the corrosion rate increases when the carbon and oxygen contents are low. If the nitrogen content exceeds 0.1% in a low C and O alloy, the corrosion rate increases dramatically. The results of this investigation indicate that the corrosion rate reaches the minimum when O>or=0.6% (preferably 0.6 to 1.2%), C=0.06 to 0.14%, and N=0.05 to 0.10%.<
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