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 berate buffer pH 8.4 is studied. It is obser ved that a monochromatic (325 nm) illumination of the sample leads to a very strong increase in its pitting corrosion resistance-apparent fr om pitting potential and incubation time measurements in chloride cont aining electrolytes. This effect is strongly dependent on the light in tensity. The cause for the decreased pitting susceptibility is not bas ed on a change in the field distribution within the passive film but i s 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 illumina tion. The electronic properties of the passive films were studied by M ott-Schottky analysis and photocurrent transient measurements. The res ults indicate that illumination during passivation leads to modificati ons 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 tha n by a dielectric approach.