Studies of the oxidation of iron by water vapour using X-ray photoelectron spectroscopy and QUASES™

The oxidation of Fe by water (H2O) vapour was studied using X-ray photoelectron spectroscopy (XPS) and QUASES™. The results indicated that the oxide formed at 25 °C and 150 °C was thinner than that formed when oxygen (O2) gas was present. The kinetics of the reaction were studied and found to follow a direct logarithmic relationship at both 25 °C and 150 °C. At 150 °C, a change in mechanism occurred after an oxide layer of approximately 0.7 nm had formed. It was determined the mechanism changed from a temperature-independent place-exchange to a temperature-dependent field-driven mechanism. The decreased rate of oxide growth in water vapour compared to that in O2 is proposed to result from the presence of H within the oxide as well as adsorbed to the surface. Its location within the oxide acts to restrict the amount of diffusion that can occur, while its presence on the surface restricts the number of surface states available for further adsorption of H2O. The slow conversion of hydroxide to oxide, as well as differences in transport properties between hydroxides and oxides, were also felt to decrease the thickness of the layer formed compared to equivalent reactions with oxygen.