The intermediate stages of the corrosion of Nd–Fe–B powders in ambient air

Abstract

Nd–Fe–B alloys powdered by hydrogenation and mechanical grinding were oxidised at room temperature in ambient air. Changes in the X-ray spectra of the powder at the subsequent stages of the oxidation are explained by the progressive absorption of water molecules from ambient air to form structurally imperfect Nd(OH) 3 and the subsequent improvement of the hydroxide structure. The structural imperfectness of Nd(OH) 3 results in a very low intensity of the hydroxide spectra even at advanced stages of oxidation. This means either that the hydroxide is composed of very fine grains and/or that its structure is amorphous. We observed the similar improvement of the structure with time for hydroxides produced by oxidation of the hydrogenated Nd–Fe–B and NdH x . It is well established that the oxidation proceeds much slower in the virgin Nd–Fe–B powder than in the hydrogenated one. We present evidence that the rate of the oxidation process can be controlled by the amount of hydrogen absorbed by the alloy. We also show that the oxidation curves of the partially hydrogenated samples can be decomposed into the oxidation curves of fully hydrogenated and not hydrogenated samples.