The Long-Term Prediction of Corrosion of Stainless Steel Nuclear Waste Canisters

Abstract

In this paper, we describe the preliminary stages of the development of a mathematical model of the evolution of the solution chemistry within a corroding crevice on passive stainless steel. It is based on a formulation by Oldfield and Sutton [1], but models the physical and chemical processes which determine the crevice solution in a more rigorous manner. The model will eventually be used to assess whether a ‘critical solution composition’, which results in the depassivation of the crevice and the onset of localised corrosion, is attainable for a range of repository conditions, steel types and canister designs etc. We also describe experiments that provide input data in the form of passive currents for this model. Preliminary sensitivity tests with the model have indicated a need for accurate thermodynamic data for the chemical equilibria constants (particularly those for the chromium reactions). These tests also suggest that there are certain critical relationships between various parameters in the system (such as crevice dimensions, the composition of solution outside the crevice and the passive current) that mark different behaviour in the evolution of the solution composition. Further experiments will be performed, as part of this work, both to validate the predictions of the model and to determine whether the predicted compositions of the crevice solutions are sufficiently aggressive to initiate crevice corrosion.

Yucca Mountain project canister material corrosion studies as applied to the electrometallurgical treatment metallic waste form

“…Microbiological organisms that produce acid-forming metabolism products would also result in more aggressive environmental conditions. Sharland and Newton [48] have tried to determine a "critical" solution composition which results in the breakdown of the passive frlm within the crevice, and whether such a condition is achievable for a range of repository conditions, types of steels and canister designs.…”

“…During handling and emplacement of containers, it is important to limit the amount of scratches and other surface defects so as to lessen the number of sites on the container surface that would be prone to localized corrosion. It has been suggested that crevice attack initiates more easily than pitting corrosion on stainless steel [48]. The crevice corrosion rate for low carbon structural steels can be as high as 15 times the general corrosion rate in wet steam conditions [33].…”

“…Crevice corrosion models of canister materials have generally been constructed to predict the evolution of the solution composition within the crevice [48]. They have tried to ascertain either a limiting composition resulting from mass-transfer considerations or a "critical solution composition" (defined in terms of pH and chloride concentration) that causes the f h to breakdown and initiate rapid corrosion.…”