The Influence of Cr Concentration near the Surface on Repassivation Potential for Crevice Corrosion in SUS 304L Steel
When stainless steels are oxidized in air, surface oxide scales with outer chromium enriched layer and with inner chromium depleted layer are formed due to the delay of chromium diffusion from matrix to surface, deteriorating wet corrosion resistance. In this work, the influence of chromium concentration near the surface on crevice corrosion of stainless steel was studied. SUS 304L specimens were oxidized in air-opened siliconit furnace at 1, 273K for 600-2, 400s, and then removing oxide scale by mechanical polishing to measure the chromium depth-profiles by electrochemical method and repassivation potential, ER for crevice corrosion. ER were measured in 3.5% NaCl (303K, deaerated by argon), and the shapes of crevice corrosion were observed by SEM in plane and cross-section. The concentration of chromium at the surface and the grain boundary were decreased due to the selective oxidation and the quick diffusion of chromium through grain boundary. These changes near the surface induced crevice corrosion with shallow and intergranular dissolution mode, and made ER less nobel from -0.13V to -0.23V (Ag/AgCl, sat. KCl) due to the formation of sub crevice, in which repassivation was restrained.
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Zairyo-to-Kankyo Vol.42(1993), No.4