Determination of Key Marine Environment Effects on Bounding Pit Size Predictions

Abstract

Stainless steels (SS) have been used extensively in coastal marine environments due to their enhanced corrosion resistance. One such example is the interim storage and the eventual transport of spent nuclear fuels (SNF) in SS casks. These SS casks may be subjected to corrosive exposure conditions, thus are susceptible to degradation through localized corrosion and subsequent stress corrosion cracking (SCC). Assessing the current damage state on the surface of these canisters would be ideal for informing future transport decisions, however, this is difficult due to confinement imposed by the concrete overpack. Due to the spatial limitation, prediction of the surface pitting corrosion damage is crucial to determining SCC susceptibility. Recent work in which the maximum attainable pit size can be calculated for given exposure conditions could form the foundation for predictions [1]–[3]. A review of relevant literature to SS’s in marine environments shows that potential surface conditions on the cask surface could be a thin electrolyte film containing high chloride concentration at elevated temperatures [4], [5]. Therefore, cathodic and anodic kinetics were evaluated in seawater and seawater simulated solutions (NaCl and MgCl2) at various temperatures. The reaction mechanisms for each solution and the impact on maximum pit size predictions will be discussed. Furthermore, certain bulk cations in seawater solutions (typically Mg2+ and Ca2+) have a strong thermodynamic driving force to form stable precipitates (such as hydroxides). Such precipitates have the propensity to decrease the cathodic current available and can decrease the maximum pit size. Calculations for max pit size predictions are carried out in order to account for environmental phenomena happening on the surface of a SS304L alloy. Overall, influences from precipitates and reaction mechanisms will lead to a decrease in the calculated bounding maximum pit size. Acknowledgements SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This document is SAND2020-5134 A.