Suppression of pitting corrosion with passive film modification on type 304 stainless steel by ultra-violet light irradiation

Type 304 stainless steel was irradiated with ultraviolet light of wavelength 325 nm in 3.5%NaCl solution to suppress the pitting corrosion susceptibility. The uv irradiation during passivation at +100 mV(Ag/AgCl) for 600 s and during the potentiodynamic pitting corrosion test shifted the pitting potential in noble direction. The more potential shift in noble direction was observed for uv irradiation during passivation. The pitting corrosion induction period for potentiostatic pitting corrosion test was also analysed in terms of the birth and death stochastic probability process to obtain the pitting generating rate, lambda, and repassivation rate, mu. lambda was lowered by a factor of about 5 after uv light exposure during passivation prior to potentiostatic polarisation, whereas mu indicated almost no change. On the other hand, the uv exposure during pit generation period brought about only a small improvement on both lambda and mu. An X-ray photoelectron spectroscopy revealed the chromium enrichment in the passive film after the exposure of uv light during passivation, which leads to suppress the pit initiation rate. The uv irradiation on stainless steels in aqueous environments revealed to improve the corrosion resistance due to a modification of the passive film by the quantum effect of photo-excitation.