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Relative Humidity and the Susceptibility of Austenitic Stainless Steel to Stress Corrosion Cracking in an impure Plutonium Oxide Environment [electronic resource].

Published:
Washington, D.C. : United States. Dept. of Energy, 2010.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
Additional Creators:
United States. Department of Energy. Savannah River Site, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information
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Summary:
Laboratory tests to investigate the corrosivity of moist plutonium oxide/chloride salt mixtures on 304L and 316L stainless steel coupons showed that corrosion occurred in selected samples. The tests exposed flat coupons for pitting evaluation and 'teardrop' stressed coupons for stress corrosion cracking (SCC) evaluation at room temperature to various mixtures of PuO₂ and chloride-bearing salts for periods up to 500 days. The exposures were conducted in sealed containers in which the oxide-salt mixtures were loaded with about 0.6 wt % water from a humidified helium atmosphere. Observations of corrosion ranged from superficial staining to pitting and SCC. The extent of corrosion depended on the total salt concentration, the composition of the salt and the moisture present in the test environment. The most significant corrosion was found in coupons that were exposed to 98 wt % PuO₂, 2 wt % chloride salt mixtures that contained calcium chloride and 0.6 wt% water. SCC was observed in two 304L stainless steel teardrop coupons exposed in solid contact to a mixture of 98 wt % PuO₂, 0.9 wt % NaCl, 0.9 wt % KCl, and 0.2 wt % CaCl₂. The cracking was associated with the heat-affected zone of an autogenous weld that ran across the center of the coupon. Cracking was not observed in coupons exposed to the headspace gas above the solid mixture, or in coupons exposed to other mixtures with either no CaCl₂ or 0.92 wt% CaCl₂. SCC was present where the 0.6 wt % water content exceeded the value needed to fully hydrate the available CaCl₂, but was absent where the water content was insufficient. These results reveal the significance of the relative humidity in the austenitic stainless steels environment to their susceptibility to corrosion. The relative humidity in the test environment was controlled by the water loading and the concentration of the hydrating salts such as CaCl₂. For each salt or salt mixture there is a threshold relative humidity below which the necessary liquid electrolyte cannot exist, and therefore below which the SCC risk is very low. This threshold is a thermodynamic quantity known as the deliquescence relative humidity that is dependent on the identity of the salt but is independent of the quantity of salt. Below the deliquescence RH there should be low corrosion risk, and above it the corrosion risk increases rapidly as a liquid phase, which is initially saturated with salt, grows and becomes more widespread in the container.
Report Numbers:
E 1.99:srnl-sti-2010-00050
srnl-sti-2010-00050
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Note:
Published through SciTech Connect.
05/05/2010.
"srnl-sti-2010-00050"
Journal of Nuclear Materials Management FT
Dunn, K.; Zapp, P.; Lam, P.; Duffey, J.
Funding Information:
DE-AC09-08SR22470
View MARC record | catkey: 14445444