Long-Term Atmospheric Corrosion of 304 Stainless Steel Used in Spent Dry Nuclear Fuel Storage Containers

Long-term Atmospheric Corrosion of 304 Stainless Steel Used in Spent Dry Nuclear Fuel Storage Containers J. Srinivasan1, J. S. Locke1, T. Weirich1, J. Taylor2, C. Bryan2, E. J. Schindelholz2 1: Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, 105 W Woodruff Ave, Columbus, OH 43210 2: Sandia National Laboratories, Materials Reliability Department and Department of Storage and Transportation Technologies, Albuquerque, NM 87185 Containers made from welded austenitic stainless steels (types 304 and 316) are typically used to store dry spent nuclear fuel at several facilities across the US. Exposure to the ambient environment can lead to salt deposition on the metal that can result in the onset of pitting and possible risk for stress corrosion cracking. In this study, the effect of atmospheric sea salt environments on pitting and SCC in 304H stainless steel coupons was investigated with the aim of replicating near-marine environments. Key experimental variables examined include salt loading density, relative humidity (RH), and exposure time (ranging from 1 week to 2 years). Pit initiation was observed to be particularly dependent on the surface finish of the exposed metal. Pitting was observed only on the ground coupons and within 1 week of exposure, while those with a mirror finish were unaffected even after a year of exposure. Relative humidity played an influential role in determining pit density and morphology, with up to 7.5 times more pits growing at 40% RH than at 76% RH. Further, at 76% RH, pits were confined to specific areas on the surface in comparison to a more uniform distribution at 40% RH. This occurrence can be attributed to the restricted cathodic area available in the latter case, in broad accordance with model predictions outlined in the literature.1 Morphologically, pits grown at 76% RH displayed an ellipsoidal surface geometry with smooth facets at the pit base whereas those grown at 40% showed evidence of attack along distinct microstructural features that appear to be slip bands. Moreover, SCC initiation in the absence of an externally applied load was observed at the edges of the pits grown at 40% RH. This occurrence of SCC appeared to be associated with residual stresses from surface grinding. A possible mechanistic hypothesis for the observed pit morphology and consequent SCC is proposed based on synergistic electrochemical and microstructural causes. At low RH conditions, high [Cl−], mass transport restriction due to precipitated salts2, and preferential dissolution along slip bands3 result in a fissure-like pit geometry which serves as a stress concentrator. In concert, residual stresses from surface grinding may form strain-induced martensite, which could undergo hydrogen environment embrittlement4 due to hydrogen generation from metal ion hydrolysis in pits, resulting in favorable conditions for SCC initiation. Current studies are focused on examining this hypothesis further to understand the effects of RH and mixed salt electrolyte chemistry on pitting and to characterize the susceptible microstructure for evidence of strain-induced martensite. The SCC response of the alloy to externally applied loads to determine the effects of pit morphology, RH, and possibly sensitization on crack propagation will also be investigated. References Z. Y. Chen, F. Cui, and R. G. Kelly, J. Electrochem. Soc., 155, C360–C368 (2008). L. Guo et al., J. Electrochem. Soc., 166, 3010–3014 (2019). P. Sadler, N. C. Pruitt, T. S. Sudarshan, and M. R. Louthan, J. Mater. Eng, 9, 151–156 (1987). Y. Mine, Z. Horita, and Y. Murakami, Acta Mater., 57, 2993–3002 (2009). This work is supported at The Ohio State University by Sandia National Laboratories and The Center for Performance and Design of Nuclear Waste Forms and Containers (WastePD), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. The work conducted under WastePD is all stress corrosion cracking analysis and corrosion morphology investigations utilizing optical profilometry. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.