The Critical Condition for the Initiation of Localized Corrosion of Mild Steels in Contact with Bentonite Used For Nuclear Waste Package

Abstract

ABSTRACTIt has been established that the mild steels which undergo the general corrosion in the acidic to neutral environments, attain the passivity status in alkaline environments, thereby becoming liable to the localized corrosion, such as pitting corrosion and crevice corrosion. Now, for the case of using bentonite as a buffer to stand between the hostrock and the geological disposal packages of high level nuclear waste, localized corrosion behaviors of mild steel as a candidate for such a package has been studied quantitatively for environments where the the otherwise neutral ground water would be turned slightly alkaline with pH = 9.5 - 10.0.In view of the lack of quantitative data on the passivity-to-localized corrosion of mild steel in natural water environments of weak alkalinity, the present authors have previously determined an empirical E-pH diagram for mild steel with a 20 °C, 1 m mol/L [HC03-], 10 ppm [CI -] solution simulating the natural water environment concerned; it has been shown that the general corrosion-to-passivity transition condition was determined to be pHd =9.4, and the mild steel was shown to be liable to localized corrosion over a large portion of the passivity domain. The present paper discusses behaviors, mechanisms, and critical conditions for initiation of localized corrosion in mild steel placed in bentonitesuspending natural water environment, in terms of the critical potentials for pitting (Vc), and crevice corrosion (ER,CREV). Bentonite was addid to the solution in varying amounts to give bentonite-to-solution ratios up to 0.1, while the pHvalue was adjusted appropriately with sodium carbonate, always keeping the bentonite particles in suspension.It is demonstrated that bentonite particles suspended in water will deposit upon the steel on receipt of Fe2 + ions, thereby promoting pitting corrosion by preventing repassivation and promoting crevice corrosion by acting as an effective crevice, once the environment conditions become favorable for localized corrosion.We conclude therefore that disposal package made of mild steel and placed in an underground water environment with bentonite as buffer will be liable to localized corrosion.