Photo-inhibition of pitting corrosion on types 304 and 316 stainless steels in chloride-containing solutions

The influence of incident monochromatic uv light on the susceptibility of types 304 and 316 stainless steels to pitting corrosion in chloride-containing solutions is described. Illumination of the immersed surfaces, in neutral and acidic solutions, gave rise to an enhanced resistance to the onset of pitting corrosion that persisted for some 220 h after the irradiation had been removed. This was evident from an increase in the breakdown potential, longer induction periods, and notable changes in the current noise at constant potentials for specimens polarized under illumination. It was found that the degree of inhibition to pitting attack depended on the photon energy, the illumination period and the nature of the passive film. Higher breakdown potentials and longer induction periods were observed with high energy photons (λ < 375 nm); these photons having energies in excess of the bandgap energy. Also, the photo-inhibition effect increased with the period of illumination; the maximum effect being observed following a 5-h illumination period. A much reduced photo-inhibition effect was observed for specimens polarized in alkaline solutions. This was attributed to the formation of a precipitate layer at the film-solution interface, which hindered the interaction of the incident photons with the barrier layer