Passivation mechanisms in the initial oxidation of iron by oxygen and water vapor

Abstract

163164 Summary trapolation of the fitted curves to slightly higher electron energies provided us with a value for the AL of N(1s) photoelectrons needed in our marker layer experiments in chapters 5 and 7. Chapters 4 and 5 describe the oxidation of Fe(100) and Fe(110) in O2. The most important observation in chapter 4 is the simultaneous change in Fe oxidation state (in the oxide layer, from mainly Fe 2+ to both Fe 2+ and Fe 3+ ), refractive index and oxidation mechanism at NO = 4 X 10 15 atoms/cm 2 . This was explained with the nucleation and growth of ox- ide clusters for NO < 4 X 10 15 atoms/cm 2 . Typical nucleation and growth kinetics was not observed, because the oxidation rate is limited by the ar- rival of O atoms from the O2 gas. Both processes are not included in the FC theory. For NO > 4 X 10 15 atoms/cm 2 , where the oxidation rate starts to decrease, an oxide layer containing both Fe 2+ and Fe 3+ (stoichiometry Fe0:77O) is formed on top. Here, the conditions for applying the FC theory are met: N marker layer experiments reveal that Fe is the mobile species and the reaction between O and Fe takes place at the surface. Still, the observed saturation behavior differs from the prediction of the FC theory. This temperature dependent saturation thickness is explained in chapter 5 by the (temperature dependent) presence of Fe 3+ . By anneal- ing in vacuum at 473K (200 degrees C), all Fe 3+ present in the layer is reduced to Fe 2+ and the room temperature growth of a homogeneous FeO layer can be compared with the FC theory. Indeed, we find that the presence of Fe 3+ leads to the \early" saturation: after reduction of Fe 3+ , agreement was obtained with the FC theory. Using a single parameter set, we were able to fit the oxidation rates at room temperature, 435K (162 degrees C) and 473K (200 degrees C). For the room temperature oxidation, the transport of Fe cations through the oxide layer determines the rate for oxygen coverages from 4 X 10 15 atoms/cm 2 to 12 X 10 15 atoms/cm 2 . The presence of Fe 3+ in uences the maximum cation diffusion current. (A change in electron conductivity due to Fe 3+ could be ruled out within the framework of the FC theory, because of non-physical parameter values needed to describe the observed saturation.) The decrease of the oxidation rate enhances the formation of Fe 3+ , which in a self-amplifying combination of processes re- duces the oxidation rate even further. In conclusion, the deviations from the FC theory at T < 423K (150 degrees C) were due to the inhomogeneity of the oxide films formed at these temperatures: the system is not a model system for the theory. Summary 165 Neither is the oxidation of Fe(100) in H2O vapor. The measurements presented in chapters 6 (room temperature) and 7 (temperature depen- dence) revealed that during the oxidation in H2O at room temperature, hydrogen is taken up, probably as OH groups in the oxide layer. The oxi- dation rate in H2O which is 3 orders of magnitude lower than the oxidation rate in O2, is entirely determined by the earlier mentioned surface processes: the oxidation could be completely described with a nucleation and growth model containing rate equations for the surface reactions. The oxidation rate reaches a maximum at T = 340K (67 degrees C). Below this temperature, H2 desorption (from H atoms at the surface) determines the oxidation rate. Above 340K, the decrease of the oxidation rate is due to the recombina- tion of OH and H at the surface and subsequent desorption of H2O. The rate limiting step is the dissociation of OH at step edges. Further evidence for this model is formed by the oxidation rate of Fe(100) covered with N marker atoms, which were found to facilitate the nucleation and growth of the oxide islands. Experiments where H2O was replaced with D2O revealed a distinct isotope effect, which is due to a dierent energy barrier for one of the surface processes, but possibly also due to a lower exponential prefac- tor (in an Arrhenius description of rate constants) corresponding to a lower molecular vibration frequency in the D2O molecule or OD surface species (compared with H2O of OH, respectively). Again, the differences could be explained completely by surface processes. The transport properties of the oxide layers formed in H2O at room temperature, as probed by subsequent oxidation in O2, could be described in the FC model. The lower oxidation rates (compared with oxide layers formed in O2, to the same O coverage) could be partly attributed to the effect of Fe 3+ present in the layer. In addition, the incorporation of H in the layer was shown to lead to a lower oxidation rate as well: both H and Fe 3+ affect the cation diffusion properties of the oxide layer. Similar effects were observed for the oxidation of Fe(100) in H2O/O2 mixtures. After an initial stage in which the oxidation rate is completely determined by the arrival rate of O2 molecules, the oxidation decreases and a saturation coverage is reached, which decreases with increasing H2O vapor partial pressure. However, in this case it was shown that the effect was entirely due to the amount of Fe 3+ present in the oxide.