Abstract
Surface oxidation of Fe-6 at.% Si alloy was investigated during annealing in ambient air of various pressures with simultaneous isothermal resistivity registrations. Measurements have been done in the temperature range 500-540 degrees C. Chemical and phase compositions of the samples were analyzed using X-ray photoelectron spectroscopy, conversion electron Mossbauer spectroscopy (CEMS), transmission Mossbauer spectroscopy (TMS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Phase analysis showed that during isothermal resistivity measurement in a low pressure air 100 mbar a protective film of hematite alpha-Fe2O3 was formed on the surface of FeSi substrate. By decreasing pressure to 10(-2) mbar the time dependence of the resistivity exhibits an increase due to the transformation of hematite to magnetite Fe3O4. The activation energy for this transformation is 115 +/- 5 kj/mol. By regressive increasing the pressure back from 10(-2) to 100 mbar a non-protective oxide scale of hematite + magnetite was formed. The results were interpreted in the light of the iron-oxygen phase diagram. (c) 2008 Elsevier Ltd. All rights reserved.