Analysis of passive films formed on SUS 304 and SUS 447J1 stainless steel exposed in an urban-industrial atmosphere

Abstract

The 2B-finished SUS 304 and SUS 447J1 stainless steels were atmospherically exposed to different seasons for 7 and 61 days in the Chiba urban-industrial site. The pitting and corrosion potentials of the samples were determined by an electrochemical technique. AES sputtering was applied to evaluate passive film thickness, and chemical compositions in passive film were investigated by XPS analysis. The pitting and corrosion potentials of SUS 304 and SUS 447J1 stainless steels increase after exposure. With the same exposure period, the pitting and corrosion potentials of SUS 304 stainless steel are lower than those of SUS 447J1 stainless steels. The passive film thickness of SUS 304 stainless steels after exposure decrease. The passive film of specimens exposed for 7 days is thinner than that of specimens exposed for 61 days. Cr enrichment in passive film increases after exposure, but the speciment exposed for 7 days has less Cr enrichment than that exposed for 61 days. By comparing with the unexposed specimen, after the 7-day exposure period the photoelectron intensity ratio of H[subscript 2]O and OH[superscript -] per O[superscript 2-] in passive film decreases, but it is nearly the same ratio after the 61-day exposure period. It is purposed that H[subscript 2]O and OH[superscript -] film of SUS 304 stainless steel is removed after exposure for 7 and 61 days, and hydroxide was formed in passive film after the 61-day exposure period. By assuming that the 7-days exposure period represents an early stage of passive film change, the former H[subscript 2]O and OH[superscript -] film of SUS 304 stainless steel may be removed after the 7-days exposure, and the new H[subscript 2]O and OH[superscript -] film is continuously developed during the exposure for 7 and 61 days. This novel film has resistance to be removed by the environment that is superior to the H[subscript 2]O and OH[superscript -] film of the unexposed specimen. For SUS 304 stainless steels, the Cr enrichment and the formation of hydroxide in passive film after exposure may relate to the positive shift of pitting and corrosion potentials. For SUS 447J1 stainless steels, the changes of Cr enrichment and passive film thickness cannot be considerably observed after the exposure periods of 7 and 61 days. However, the increase inthe photoelectron intensities of OH[superscript -] per H[subscript 2]O, OH[superscript -] and O[superscript 2-] can be observed. The hydroxide film formed after exposures may relate to the positive shift of pitting and corrosion potentials.