Review - Photocurrent spectroscopy in corrosion and passivity studies: A critical assessment of the use of band gap value to estimate the oxide film composition

A critical assessment of the Photocurrent Spectroscopy (PCS) Technique for the semi-quantitative characterization of passive film and corrosion layers composition is carried out. We take into account more than three decades of PCS usage as "in-situ" analytical technique and related results as well as the criticism of the underlying semi-empirical correlation relating the measured optical bandgap (Eg) to the passive film composition. The discrepancies between the experimental data, gathered by PCS measurements, and Egestimates originating from recently developed Density Functional Theory based modeling of solid state properties are stressed with particular emphasis on the case of anodic passive film grown on technologically important alloys (Fe-Cr and stainless steels). The extension of this correlation to mixed oxides and its use for relating the oxide composition to the bandgap values is critically reviewed by comparing the predicted Egof mixed oxides with the experimental values. Suggestions on how to account for different bandgap values of oxide polymorphs and how to correlate the Egvalues to the composition of mixed s,p-d-metal oxides are presented and discussed on the basis of experimental results reported in the literature. On the basis of this assessment, the ability of PCS in providing quantitative information on the composition of passive film and corrosion layer is generally confirmed.