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

摘要

It has been established that mild steels, whichundergo the general corrosion in acidic to neutral environments,attain passivity in alkaline environments, thereby becomingliable to localized corrosion such as pitting corrosion and crevice corrosion.In the view of the lack of quantitative data on the transition fromgeneral corrosion-to-passivity of mild steel in natural waterenvironments of weak alkalinity, the present authers havedetermined the empirical E-pH diagram for a mild steel in thesimulated ground water of 1 mMol/L(HCO_3~-) +10 ppm (Cl~-)solutions at 20 deg C. It has been shown that (1) the transitionpH from general corrosion-to passivity (PH_d) was determinedto be 9.4, and the mild steel was shown to be liable to localizedcorrosion over large portion of the passivity domain, (2) whenbentonite is mixed with the simulated ground water, the pHbecome as high as 9.6 to 10.3 under low bentonite/solution ratiocondition, which is higher than PH_d of 9.4, and (3) thelocalized corrosion domain in bentonite suspension is wider thanthat in the non-bentonite-containing solutions.