Corrosion electron-hole pairs

Furthermore, as we saw in a foregoing section, photoexcitation produces in a semiconductor electrode electron-hole pairs and introduces a photo-potential, which reduces the space charge potential in the semiconductor. With an n-type semiconductor in contact with a corroding metal, photoexcitation raises the Fermi level up to the flat band level of the semiconductor, thus shifting the corrosion potential in the less positive direction toward the flat band potential of the n-type oxide as shown in Figure 22.35c. Photoexcitation therefore will shift the corrosion potential in the less positive (more cathodic) direction and the corrosion will then be suppressed. With some n-type oxides such as titanium oxide, photoexcitation brings the interfacial quasi-Fermi level, peF, down to a level lower than the Fermi level, F(redox> of the oxygen electrode reaction ... 

Photosensitivity Many corrosion products possess semiconducting properties. When interacting with photons (see Chapter 1, Volume 6) of higher energy than the band gap of the semiconductor, electron-hole pairs are generated, which change the conditions for chemical reactions in different ways. Yet, there is not ample evidence of photosensitivity in atmospheric corrosion processes. One... 

In the opposite case, hv > g, photons produce electron-hole pairs. Accumulation of holes at the oxide surface increases the local potential drop which may cause a fast photocorrosion. Ion migration is enhanced in the thin film, corrosion is enhanced, and altogether a fast dissolution of metal takes place by a photoelectro-chemical process in the passive film. An example is given for Ti [160]. This technique can be used for microstructuring of Ti- or Al surfaces [104]. On the other hand, anodic metal ion dissolution competes with the opposite anodic film forming ITR of oxygen ions. Therefore, in dependence on the special conditions, laser induced oxide growth may overcome pit formation [160]. 

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