The Critical Condition for Initiation of Localized Corrosion of Mild Steel in an Alkaline Water Environment in Contact with Bentonite

摘要

Mild steels undergo general corrosion in acidic to neutral environments, but in alkaline environments they attain passivity, thereby becoming liable to localized attack, such as pitting corrosion and crevice corrosion.In view of the lack of quantitative data on the transition of mild steel from general corrosion to passivity in weakly alkaline natural water environments, the authors have compiled an empirical E-pH diagram for a mild steel in simulated groundwater of 1 mMol/L (HCO_3~-) + 10 ppm (Cl~-) solution at 20 deg C. It shows that (1) the transition pH from general corrosion to passivity (PH_d) was 9.4, and the mild steel was liable to localized corrosion over a large portion of the passivity domain, (2) when bentonite is mixed with the simulated groundwater, the pH becomes as high as 9.6 to 10.3 under a low bentonite/solution ratio condition, which is higher than PH_d of 9.4, and (3) the localized corrosion domain is wider in bentonite suspension than in non-bentonite-containing solutions.