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

Abstract

It has been established that mild steels, which undergo the general corrosion in acidic to neutral environments, attain passivity in alkaline environments, thereby becoming liable to localized corrosion such as pitting corrosion and crevice corrosion.
In the view of the lack of quantitative data on the transition from general corrosion-to-passivity of mild steel in natural water environments of weak alkalinity, the present authers have determined the empirical E-pH diagram for a mild steel in the simulated ground water of 1mMol/L [HCO₃^-]+10ppm [Cl^-] solutions at 20°C. It has been shown that (1) the transition pH from general corrosion-to passivity (pH_d) was determined to be 9.4, and the mild steel was shown to be liable to localized corrosion over large portion of the passivity domain, (2) when bentonite is mixed with the simulated ground water, the pH become as high as 9.6 to 10.3 under low bentonite/solution ratio condition, which is higher than pH_d of 9.4, and (3) the localized corrosion domain in bentonite suspension is wider than that in the non-Bentonite-containing solutions.