Effect of niobium doping on the atmospheric oxidation of Fe-Si-Al magnetic alloys

Abstract

Magnetic alloys having a composition around Fe-10Si-5Al are called “Sendust”. Doping with high valence metal elements, such as titanium, vanadium, zirconium and niobium, is considered to be effective in improving the corrosion resistance of Sendust, but systematic data have not been reported. In the present work, the effect of niobium doping on the atmospheric oxidation of Sendust was studied with XPS and ellipsometry. In air with a relative humidity (RH) of 42.8–90% at 80°C, oxide films grew rapidly on both doped and undoped Sendust in the initial 2 days. After 20 days, the thickness reached 5–6 nm with mirror finished specimens (by ellipsometry), thinner than on pure iron ( ca 7 nm). The films were thicker on specimens polished with No. 600 emery paper (by XPS). The films were estimated to be a mixture of silica-alumina composite oxides and Fe(III)-oxyhydroxides. With RH= 42.8% (no condensed water on the surface), the proportions of Si(IV) and Al(III) in the film were larger with the niobium doped Sendust than with the undoped Sendust. This suggests that a small amount of niobium doping may improve the corrosion resistance of Sendust in a dry atmosphere. With RH= 90% (with condensed water on the surface) the film composition did not show a significant difference with doping, in agreement with the results of anodic polarization experiments, indicating that under wet conditions no doping effect may be expected.