Factors Impacting Atmospheric Pitting Morphologies

Abstract

Stainless steels exposed to marine and near-marine environments can suffer from localized corrosion due to the appreciable amount of chloride aerosols present. In some cases, this localized corrosion, in the presence of sufficient tensile stress, can lead to stress corrosion cracking (SCC). Spent nuclear fuel (SNF) is currently stored in stainless steel canisters at independent spent fuel storage installation sites (ISFSI), many of which are located in near-marine environments. The possibility of localized corrosion features transitioning to SCC is a potential concern, particularly as the use of the SNF canisters at ISFSIs has been extended beyond their original licensing period.There is evidence that the likelihood of this transition occurring is dependent on the morphology of corrosion damage. Therefore, it is important to examine factors impacting the corrosion morphology. This work focuses on the effects of environment, surface finish, and stress state.304L stainless steel u-bends of various surface finishes were deposited with fine droplets of magnesium chloride solution or artificial sea water. They were divided among several exposure environments, cyclic or static in nature. Flat coupons of 304H, 304L, or 316L were finished to the same surface roughnesses and exposed in the cyclic environment. Static exposures were 75% or 40% RH at an elevated temperature while the cyclic environment varied both temperature and relative humidity. The cyclic environment is designed to mimic diurnal cycles experienced on a warm SNF canister after an initial storage (or cooling) period. In the flat coupons, the roughest surface finish developed elongated pits and microcracks formed along the edges of the pits, largely running perpendicular to the grind lines. When viewed in cross-section, pits on the roughest surface finish also appeared to have more severe under-cutting and to be more irregularly shaped. The results of the flat coupons will be compared to the u-bends exposed with various salt types and stress states. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2022-4056 A. PNNL-SA-171827