Effects of Si on the repassivation kinetics and SCC susceptibility of stainless steels

Effects of Si on the repassivation kinetics and stress corrosion cracking (SCC) of alloy 304Si were examined by a scratch electrode technique and slow strain rate test in chloride solutions and compared with those of other stainless steels (SSs), 304 and 316L. Repassivation kinetics of these alloys and Fe-20Cr-xSi (x=0-2) alloy were analyzed in terms of the current density, i (t), flowing from the scratch as a function of the charge density, q (t), that has flowed from the scratch. Passive film initially nucleated and grew according to the place exchange model, and then grew according to a high-field ion-conduction model in which log i (t) has a linear relationship with 1/ q (t) with a slope of cBV. The repassivation rate of Fe-20Cr-xSi (x=0-2) alloys, evaluated by the cBV value, increased with an increase in Si content. Furthermore, repassivation rate decreased on the order of 304Si, 316L, and 304. The resistance to SCC of the SSs (304Si, 304, and 316L) measured in 35 wt % MgCl₂ solution at 120°C was in good agreement with those predicted based on the repassivation kinetics. The beneficial effects of Si on SCC of 304Si SS appears to be associated with the enrichment of Si in the passive film.