A simple model for the photoelectrochemical behavior of corrosion layers with variable hydration degree

With the goal of quantitatively applying photocurrent spectroscopy to corrosion studies, the photoelectrochemical behavior of layered passive films having a decreasing hydration/hydroxylation degree, from the solution interface toward the metal interface, has been modeled. The model makes use of the correlations between optical gap and electronegativity of the constituents previously proposed for surface oxides and hydroxides. In this paper, the model was tested successfully against the experimental data pertaining to three high band gap insulating oxide-forming metals, like Zr Al, and Y. In the first case, the fitting of the experimental spectra was compatible with the occurrence of a duplex film, with a Zr(OH)4 layer adjacent to the solution and an inner ZrO2 layer. For the other two metals. sometimes a triple-layered structure of the passive film was inferred. with an intermediate oxyhydroxide region placed between the hydroxide and the anhydrous oxide. The application of the present model to these systems allowed more detailed information to be gathered on the film structure as a function of the metal surface preparation.