3.1 Passivation (STM measurements)

The averaged tunneling barrier height measured on the oxide-free iron surface approaches ® ~ 0.4 eV, but with the values in the range 0.05 < V< 1.1 eV (Fig. 4, E = -750 mV). This is in good agreement with the published results for a gold surface immersed in perchloric acid [10]. Compared to measurements in vacuum this result is substantially lower, which can be explained by electronic interactions of water molecules with the metal surface [11, 12]. The formation of an oxide layer leads to a decreased tunneling barrier (Fig. 4). Already the very thin, so-called pre-passive layer at E = -600 mV lowers the barrier height by a factor of 10. Nevertheless the measured values recorded at such a potential scatter widely and can be attributed to both the low thickness and the inhomogeneity of the pre-passive film. When the film grows thicker and becomes more structured the barrier height is further lowered by another order of magnitude.

Fig. 4. Mean tunneling barrier height vs. substrate potential on iron in a boron buffer solution. The formation of an oxide layer leads to lower tunneling barrier height.

This lowering may be explained by induction of localized electronic states with their energy levels lying within the tunneling barrier. They therefore act as resonating states for indirect tunneling processes. A comparison with photoelectrochemical results [1, 13] leads to the hypothesis that it is the Fe2+ ion which induces such localized states, since it disturbs the physical structure of the oxide layer, leading to dangling bonds.

The same tendency of the oxide layer to lower the tunneling barrier height can be seen for stainless steel. As seen in Fig. 5, the formation of a passive film on DIN-1.4301 also leads to a decrease of the measured tunneling barrier height. Two regions with different tunneling barrier height values can be distinguished. The pre-passive layer, which is not strictly chemically bound to the surface, is formed at potentials in the range -300 mV < E < -200 mV. At potentials higher than E = -200 mV the film becomes more structured and chemically bound to the surface. The influence of Fe2+ misfits is enhanced and diminishes the tunneling barrier by another order of magnitude.

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