On the kinetics of the initial oxidation of iron and iron nitride

The initial oxidation of polycrystalline alpha-Fe and epsilon-Fe2N1-x was investigated with X-ray photoelectron spectroscopy, ellipsometry and high-resolution transmission electron microscopy. Oxidation was performed at temperatures ranging from 300 to 600 K. The oxidation kinetics were described quantitatively with the coupled currents model according to Fromhold and Cook, adopting time-dependent work functions at the metal/oxide and oxide/oxygen interfaces. The evolution of the work functions of a-Fe could be related to the change of the oxide film composition, which evolved with increasing film thickness from approximately FeO to a composition close to Fe3O4. Upon oxidation of epsilon-Fe2N1-x, the N atoms accumulated underneath the oxide film, which could lead to an N concentration larger than the maximum equilibrium solubility of nitrogen in epsilon-Fe2N1-x. This excess nitrogen hinders the oxidation and transport of Fe atoms and is associated with a negative charge, which led to a slower initial oxidation. Upon prolonged oxidation, the oxidation rate of epsilon-Fe2N1-x could be related to the presence of an Fe1-deltaO-like oxide or oxynitride in the oxide film.