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This presentation recalls the peculiarities of anodic polarization valve metals namely aluminium and Tantalum. Both metals are very unnoble and react immediately with oxygen or water. They form an anodic oxide that hinders further transport of anions and cations through this film. Upon higher polarization the electrical field in the oxide exceeds the critical value of oxide formation field strength enabling metal cations and oxygen anions to migrate within the high field gradient in a thermally activated and field supported hopping process. This mechanism that is known as the high field mechanism of oxide formation nicely explains the most important aspects of oxide formation but some of the kinetic observations require a more sophisticated description that is called the extended high field model. As a result of the ions injected into the adjacent phases through the interface, a space charge is formed that drastically changes the local field strength. Since the potential drop over the entire film thickness remains constant, other regions of the film must compensate the reduced interface potential drop. This leads to a temporal imbalance of the driving forces within the oxide. For a limited time the charge carrier concentration can become extremely high, resulting in an experimentally observed maximum in current density named overshoot. This overshoot is not only observed in potentiostatic experiments but also in potentiodynamic scans. Both types of experiments can be satisfyingly simulated on the basis of this model. Under specific conditions however, oxide formation is not the only occurring process, meaning that the current efficiency in terms of oxide formation is below 100%. There are at least two different charge transport mechanism leading to oxygen evolution. The first one is the more obvious one in which the oxide breakdown field strength is exceeded resulting in a, at least temporal and/or local, destruction of the oxide film, that allows charge flow into this side reaction. The second one is less obvious since direct elastic tunnelling of electrons is not possible over a range of a few nm. A large number of defects in the insulating oxide film which can also be described as states in the band gap may allow charge transport in an unexpected way. The importance of these findings, the model behind, and the industrial applications will be discussed. |
