Photoelectrochemical investigation of passive layers formed on Fe in different electrolytic solutions

Abstract The passive films formed on Fe in different electrolytic solutions, spanning almost the overall pH range, and with different growth procedures were investigated systematically by photocurrent spectroscopy. The potentiodynamic growth curves are compared both in aerated and de-aerated electrolytes and a potentiostatic growth procedure is also employed. For high positive formation potentials, similar anodic spectra are recorded in all solutions giving an optical gap very close to that expected for crystalline Fe2O3. The origin of photocurrent spikes is also investigated and the effect of the formation potential upon the measured absorption threshold is discussed taking into account the ordered or disordered structure of the films and its Fe(II) species content. Films formed at quite negative potentials display different photocurrent spectra, characterized by cathodic stationary photocurrent, absence of spikes and lower absorption in the visible range, indicating the formation of Fe(II) passive layers. The same behaviour is observed with film formed at positive potentials, after a long negative biasing.