09409 Corrosion of Austenitic Stainless Steels Exposed to Mixtures of Plutonium Oxide and Chloride Salts

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Product Number: 51300-09409-SG

ISBN: 09409 2009 CP

Author: Philip E. Zapp, Ronald R. Livingston, D. Zane Nelson, Kerry A. Dunn and Jonathan M. Duffey

Publication Date: 2009

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Laboratory corrosion tests were conducted to investigate the corrosivity of moist plutonium oxide/chloride (PuO2/Cl-) salt mixtures on 304L and 316L stainless steel coupons. 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 two flat coupons were placed so that the solid oxide/salt mixture contacted about one half of the coupon surface. One teardrop coupon was placed in contact with solid mixture; the second teardrop was in contact with the headspace gas only. The mixtures were loaded with nominally 0.5 wt % water under a helium atmosphere. Observations of corrosion ranged from superficial staining to pitting and SCC. The extent of corrosion depended on the total salt concentration and on the composition of the salt. The most significant corrosion was found in coupons that were exposed to 98 wt % PuO2, 2 wt % chloride salt mixtures that contained calcium chloride. 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, nor in coupons exposed to other mixtures with either 0.92 wt% CaCl2 or no CaCl2. The corrosion results point to the significance of the interaction between water loading and the concentration of the hydrating salt CaCl2 in the susceptibility of austenitic stainless steels to corrosion.

Keywords: stress corrosion cracking, pitting, radiation