Effect of ambient air pressure on the oxidation kinetics of Fe–6 at.% Si alloy

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 °C. Chemical and phase compositions of the samples were analyzed using X-ray photoelectron spectroscopy, conversion electron Mössbauer spectroscopy (CEMS), transmission Mössbauer 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 α-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.