Atmospheric Stress Corrosion Cracking Susceptibility of Welded and Unwelded 304, 304L, and 316L Austenitic Stainless Steels Commonly Used for Dry Cask Storage Containers Exposed to Marine Environments (NUREG/CR-7030)
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Publication Information
Manuscript Completed: June 2010
Date Published: October 2010
Prepared by:
L. Caseres and T.S. Mintz
Southwest Research Institute
6220 Culebra Road
San Antonio, TX 78228
M.M. Bayssie, NRC Project Manager
NRC Job Code N6195
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
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Abstract
This report documents information on the susceptibility to atmospheric stress corrosion cracking of welded and unwelded 304, 304L, and 316L austenitic stainless steels commonly used for dry storage containers exposed to marine environments. Standard U-bend specimens were exposed to both salt spray and salt fog conditions at temperatures ranging from 25 to 176 °C [77 to 350 °F] and varying relative humidity. The first test conducted was a salt spray test to scope out material and residual stress effects. This deliberately conservative test resulted in extensive cracking and corrosion as was observed in all the 93 and 176 °C [200 and 350 °F] U-bends in as early as 1 month after exposure. Salt spray tests are non-prototypical since deposition of salt on dry casks is in the form of a dry aerosol; hence, the first test was not suitable for evaluation of stress corrosion cracking susceptibility of austenitic stainless steels in coastal atmospheres. The second test conducted, the salt fog test, was expected to closely simulate field conditions around dry cask containers, and resulted in stress corrosion cracking and pitting corrosion in all the 43 °C [109 °F] specimens. Corrosion developed after 4 weeks in the 304 and 304L specimens, and after 32 weeks in the 316L specimen. This suggests that the alloy composition plays a role in stress corrosion cracking susceptibility. Cracking was mainly transgranular with sections of intergranular branching. Cracks were concentrated within the arch region in all unwelded U-bends and at the heat-affected zone of the welded specimens, as previously reported in the literature. None of the 85 and 120 °C [185 and 248 °F] specimens exposed to the salt fog test exhibited stress corrosion cracking; the data are consistent with the inability of salt deposits to deliquesce at high temperatures.
The tests indicate that chloride-induced stress corrosion cracking is highly dependent on the concentration of deposited sea salt, residual stress, cask temperature and the relative humidity of the surrounding environment. The results of the salt fog test may be conservative because the high absolute humidity used for the test was chosen to bound natural conditions expected in the dry storage cask environment. However, the results are still pertinent because they demonstrate that the deliquescence of dry deposited sea salt can lead to stress corrosion cracking of austenitic stainless steels at temperatures that are only slightly greater than ambient temperatures. The SCC acceleration factor used in laboratory testing and its implication on SCC initiation in actual dry storage cask units in the field are difficult to compute. Under the assumptions presented in this report, which simulate ideal conditions for the onset of SCC, initiation would be expected between 32 and 128 weeks. However, this is still a rough estimation because it does not take into account the operating history of the dry storage cask and the local environmental properties at each cask location.
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