Relative Humidity and the Susceptibility of Austenitic Stainless Steel to 4 Stress-corrosion Cracking in an Impure Plutonium Oxide Environment

Laboratory tests to investigate the corrosivity of moist plutoniumoxide/chloride salt mixtures on 304L and 316L stainless steelcoupons showed that corrosion occurred in selected samples. Thetests exposed flat coupons for pitting evaluation and “teardrop”stressed coupons for stress-corrosion cracking (SCC) evaluationat room temperature to various mixtures of PuO2 and chloridebearingsalts for periods up to 500 days. The exposures wereconducted in sealed containers in which the oxide-salt mixtureswere loaded with about 0.6 wt percent water from a humidifiedhelium atmosphere. Observations of corrosion ranged from superficialstaining to pitting and SCC. The extent of corrosiondepended on the total salt concentration, the composition of thesalt and the moisture present in the test environment. The mostsignificant corrosion was found in coupons that were exposedto 98 wt percent PuO2, 2 wt percent chloride salt mixtures thatcontained calcium chloride and 0.6 wt percent water. SCC wasobserved in two 304L stainless steel teardrop coupons exposed insolid contact to a mixture of 98 wt percent PuO2, 0.9 wt percentNaCl, 0.9 wt percent KCl, and 0.2 wt percent CaCl2. The crackingwas associated with the heat-affected zone of an autogenousweld that ran across the center of the coupon. Cracking was notobserved in coupons exposed to the headspace gas above the solidmixture, or in coupons exposed to other mixtures with either noCaCl2 or 0.92 wt percent CaCl2. SCC was present where the 0.6wt percent water content exceeded the value needed to fully hydratethe available CaCl2, but was absent where the water contentwas insufficient.These results reveal the significance of the relative humidityin the austenitic stainless steels’ environment to their susceptibilityto corrosion. The relative humidity in the test environmentwas controlled by the water loading and the concentration of thehydrating salts such as CaCl2. For each salt or salt mixture thereis a threshold relative humidity below which the necessary liquidelectrolyte cannot exist, and therefore below which the SCC riskis very low. This threshold is a thermodynamic quantity knownas the deliquescence relative humidity that is dependent on theidentity 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 morewidespread in the container.