The Oxidation of Iron as Studied by Conversion Electron Mössbauer Spectroscopy

Back scattered, conversion electron, Mössbauer spectroscopy was used to study oxide films formed by the thermal oxidation of iron at 225, 350, and 450 C (437, 662, and 842 F). The technique was found to be useful in the study of oxide films from approximately 50 to several hundred Angstroms in thickness without removal from the substrate. Nonstoichiometric Fe₃O₄, corresponding to Fe_2.91O₄ or a mixture of stoichiometric Fe₃O₄ and Ce-Fe₂O₃ was observed after oxidation at 225 C. Oxidation at 350 C for short times produced a duplex film consisting of Fe₃O₄ and α-Fe₂O₃, however, after oxidation at 450 C nearly stoichiometric Fe₃O₄ was the only oxide phase found. The absence of α-Fe₂O₃ in short time oxidation at 450 C was attributed to an increased cation flux at the higher temperature. Oxide growth at 225 C was studied as a function of oxidation time, and the increase in film thickness showed a logarithmic time dependence.