Critical pit solution concentration for repassivation of stainless steel

摘要

The critical pit solution concentration for repassivation ( Ccrit) is a key parameter that determines the pit repassivation process [1]. Unfortunately, there is no such a method that can determine the Ccrit accurately, thus the Ccrit reported in previous work varies greatly. Generally, the Ccrit is considered as a fixed value located between 50%–80% of the saturation pit solution concentration ( Csat)[2]. However, this consideration ignores the effect of potential and temperature. To gain a deep insight into pit repassivation process, it is necessary to develop an approach for accurate determination of Ccrit, and to reveal its dependence on potential and temperature. In this work, the downward potential scan was performed on growing one - dimensional (1D) artificial pit to determine the critical point for repassivation. To locate the critical point accurately, the variation of potential at pit surface ( Esurf) during the growth in charge-transfer-control stage must be obtained, thus the potential drop of pit solution (Φsol) needs to be estimated accurately. Based on this consideration, we have developed a calculation model for Φsol, which takes the effect of concentration gradient inside the pit into account. Further analysis shows that the calculated Esurf in the charge-transfer-controlled growth stage decreases with pit current density, but to a critical point it increases. Further verification experiment shows that this lowest point corresponds to the critical point for pit repassivation. Clearly, the concentration of pit solution at this critical point is the Ccrit, and it can be calculated by considering both diffusion and electro-migration. Based on this approach, the effect of surface potential (Esurf) and temperature as well as the alloy composition on Ccrit was investigated. The Ccrit was found to range from 75% to 89%, and it increase with the increase of Esurf and decrease with the increase of temperature. Additionally, the Mo addition was found to increase the Ccrit.