Modification of Passive Film Formed on an Fe-18Cr Alloy in Sulphuric Acid Solution by Ultra-violet Light Irradiation

Ultra-violet (uv) light of wavelength 300 nm was irradiated on Fe-18Cr alloy during passivation in a deaerated 0.1 kmol·m−3 H2SO4 solution to modify the property of passive film. XPS analysis of the passive film revealed that uv light irradiation during passivation enhanced Cr enrichment in the oxide layer of the passive film, assuming that passive film consists of three layers; hydrocarbon contaminant covering a hydroxide layer of Cr and an underlying oxide layer of Cr and Fe oxides. The increasing Cr content in the film correlated with an improved pitting corrosion resistance in solution. As usually observed, the Cr content in the passive film increases with passivation time without uv irradiation. The uv irradiation accelerated the Cr enrichment in the passive film to reach a steady Cr content in a shorter time. The steady Cr content, however, was not necessarily increased by uv irradiation. Therefore, uv irradiation enhanced Cr enrichment mainly at the initial stage of passivation. The photoelectrochemical current response test revealed that the semiconductor property of the passive film changes from n to p type with time in the initial stage of passivation. The accelerated Cr enrichment was observed mainly during the period of the n-type semiconductor property in which field-assisted cation outward migration is possible to be accelerated by electric field in the film. The experimental results in the present work suggest that the photo excitation in the passive film by uv irradiation changes the energy band slope to accelerate outward migration of Fe ions resulting in the enrichment of Cr in the film.