Passivation of iron by oxidation in H2O and O2/H2O mixtures

In order to obtain insight in the oxidation mechanism of Fe(1 0 0) in oxygen and water and determine the rate limiting step in the oxidation, we studied the initial oxidation of Fe(1 0 0) in water vapor and water vapor/oxygen gas mixtures using ERD, XPS and ellipsometry. The hydrogen content of the oxide layer grown at room temperature was determined. The H/O ratio in the oxide layer amounted to 0.28±0.02. The formation of a H and Fe3+ containing oxide significantly lowered the oxidation rate in O2 compared with an equally thick oxide without H or Fe3+. We discuss possible origins of the differences and show that cation diffusion properties of the oxide layer formed influences the oxidation rate. The oxidation rate in pure water vapor is determined by surface processes involving the adsorption/desorption/dissociation equilibrium. The oxidation rate in water/oxygen mixtures is limited by the rate of Fe cation diffusion through the grown oxide film.