Monte-Carlo Simulation of Pitting Corrosion with a Deterministic Model for Repassivation

Abstract

A theory is developed for calculating the delayed repassivation (“stifling”) rate constant for stable pits on a metal surface with attention being paid to pitting of the carbon steel overpack in the Belgian supercontainer concept of the disposal of high-level nuclear waste. It is shown that, under the conditions that are expected to exist at the overpack surface, substantial concentration and potential drops cannot exist between the pit internal surface and the external surface far away from the pit mouth. This conclusion is valid for both open and semi-closed pits, if there is no porous, high resistance, corrosion product layer (salt film and/or remnants of the ruptured passive film) between the active metal surface inside the pit and the external surface, as supported by recent artificial pit experiments. It is shown that the existence of a low conductivity, corrosion product layer between the active pit base and the external surface can protect pits from the repassivation. The condition of repassivation is based upon the concept of the existence a critical pit propagation rate, Vcr, corresponding to a critical coupling current. If the rate of pit propagation becomes less than Vcr the pit passivates, because the coupling current is too low to maintain the necessary aggressive conditions within the pit cavity. A model and computer code have been developed for estimating the probability of failure in the Belgian high-level nuclear waste (HLNW) disposal repository.