Illumination effects on the stability of the passive film on iron

The effect of illumination on passivity and the initiation of pitting corrosion on pure iron in borate buffer pH 8.4 is studied. It is observed that a monochromatic (325 nm) illumination of the sample leads to a very strong increase in its pitting corrosion resistance—apparent from pitting potential and incubation time measurements in chloride containing electrolytes. This effect is strongly dependent on the light intensity. The cause for the decreased pitting susceptibility is not based on a change in the field distribution within the passive film but is much more associated with light induced changes in the passive film properties during passivation. These changes can neither be attributed to a film thickening nor to an altered temperature caused by illumination. The electronic properties of the passive films were studied by Mott-Schottky analysis and photocurrent transient measurements. The results indicate that illumination during passivation leads to modifications in the electronic properties of the passive films—specifically to an increase of the bulk doping—and the surface state density. Further it can be concluded that the electronic properties of the passive film on iron can rather be described in terms of a semiconductor model than by a dielectric approach.