Photoelectrochemical techniques can be applied for the characterization of passive layers on metals. Useful information about nature, composition and solid-state properties of very thin surface films can be gathered in-situ, together with an energetic picture of the metal/passive layer/electrolyte junction. Principles and advantages of the technique are briefly outlined. However, due to the fact that thin passive films and corrosion layers on metals have often a strongly disordered structure, complications arise in the interpretation of the experimental results, the paper evidences the need of new theoretical models, which take into account the physical properties of the films. Mathematical expressions fro the measured photocurrent are presented and they are tested against the experimental results obtained in our laboratory on different systems. Selected examples are discussed concerning with both model systems, which form stable surface films with constant composition in large ranges of electrode potential and film thickness, and corrosion product layers on base metals of commercial interest.
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