Examination of the oxidation of iron by oxygen using X-ray photoelectron spectroscopy and QUASESTM

Oxidation of iron (Fe) by oxygen (O2) at low pressures was investigated using XP spectra and QUASESTM Analyze and Generate software programs. The results indicated that the oxide films produced after short oxygen exposure times contained a mixture of magnetite (Fe3O4) and maghemite (γ-Fe2O3) without distinct interfaces between the two being seen. Further investigation suggested that a reaction mechanism involving cationic diffusion was mostly responsible for the formation of Fe3O4, while a mechanism involving anionic diffusion was responsible for γ-Fe2O3 growth. The kinetics of the overall reaction were also investigated and found to follow a direct logarithmic relationship with increasing time. The rates of oxide growth were calculated for different pressures studied at the same temperature and found to be the same. This observation was used to conclude that the rate-limiting step was the formation of a thin oxide film by place-exchange and not anionic or cationic diffusion.