The Effects of Sulfate on the Parameters for Maximum Pit Size Calculations on Stainless Steel 316L

Abstract

Dry storage canisters that contain radioactive waste containers in the United States are typically made of stainless steel such as 316L. The longevity of such canisters put into service around forty years ago is a primary concern to the United States Department of Energy. Due to their location, many of these canisters undergo fluctuations in relative humidity and temperature, as well as an accumulation of deliquescent salts on their surface. The nature of the anions present in these salts establishes whether or not proper conditions for pit propagation exist. In order to predict these canisters’ maximum pit size, computational models that employ input parameters on the pit kinetics can be utilized. Such kinetic parameters are the critical pit stability product (i·x)crit and the repassivation potential (Erp), as they are key to the anodic current demand in a pit and the cathodic current supply outside a pit, respectively. Previous models that focused on the effect of chloride concentrations on the maximum observed pit size have shown to be overly conservative. Therefore, to increase the accuracy of the model, a more complex solution that contains inhibiting ions, such as sulfates, should be considered. In this study, artificial 1-D pit electrochemical experiments were carried out in a 50.8μm diameter SS316L wire exposed to solutions containing different concentrations of magnesium chloride (MgCl2) and magnesium sulfate (MgSO4). Maximum pit size calculations were performed utilizing the kinetic parameters obtained from these experiments. A theoretical model that seeks to describe the formation and growth of a complex salt film precipitate at the bottom of the pit is presented as well.