Abstract
Light induced effects on passivity and the initiation of pitting corrosion on pure iron in borate buffer pH 8.4 were studied. By illuminating the samples with monochromatic (325 nm) light, a very strong increase in the pitting corrosion resistance, apparent from pitting potential and incubation time measurements in the chloride containing electrolyte, was observed. This effect strongly depends on the light intensity. The decrease of pitting susceptibility is not caused by a change in the field distribution within the passive film, but it is much more associated with light induced changes in passive film properties during passivation. These changes cannot be attributed to a film thickening caused by illumination. The electronic properties of the passive films were studied by Mott-Schottky analysis, photocurrent transient, and photoimpedance measurements. The results indicated that illumination during passivation leads to modifications of the film, which are reflected in their electronic properties, specifically to an increase of the surface state density. Since surface states of passive films on iron can be associated with the adsorption of hydroxyl ions, a possible reason for the improved pitting resistance might be, in terms of the competitive Cl exp - /OH exp - adsorption model, a suppression of Cl exp - adsorption on the oxide surface.