The electronic properties of disordered passive films

The understanding of passivity and its role in corrosion research is closely connected with the electronic properties of passive films. The approach of describing passive films in terms of their semiconducting properties can be improved by using models developed for amorphous semiconductors where localized electronic states play an important role. It is shown that localized states in passive films are of importance for the photo-electrochemical behavior where it expresses itself in sub-band gap photocurrents and a potential dependence of the photocurrent according to the Poole-Frenkel effect. Non-linear Schottky-Mott plots resulting from capacity data can be attributed to bulk localized states in the band gap and can thus be used to characterize highly disordered and/or non-stoichiometric passive films. The use of a photo-electrochemical laser scanning technique for the in-situ imaging of passivated surfaces and of corroding surfaces is also shown.