METASTABLE PITTING AND SEMICONDUCTIVE PROPERTIES OF PASSIVE FILMS

Passive films on stainless steels, which are known to exhibit a significantly different susceptibility to pitting corrosion (DIN 1.4301, 1.4439, 1.4529), were examined in situ by photoelectrochemistry. The results show that the photoresponse is determined by the defects in the electronic structure of the films leading to localized states in the bandgap region. By applying a simple model it was found that the three different steels exhibit a different tailing of the density of states function into the bandgap region. The fewer deep localized states are present, the higher is the resistance of the materials against pitting corrosion. It is therefore concluded that a correlation between the distribution of localized states in the passive film, i.e., its defect structure, and the stability of the film exists.