The initiation and repassivation of pitting and crevice corrosion of two candidate high-level nuclear waste container materials, type 316L stainless steel and alloy 825, were examined under a range of environmental conditions possible in a repository. The initiation and repassivation potentials for localized corrosion in short-term tests were determined to be a function of temperature, chloride concentration, and potential. Under long-term potentiostatic conditions, pitting and crevice corrosion were initiated at potentials much lower than those measured in short-term tests. Crevice corrosion occurred preferential y over pitting corrosion. Similar results were obtained when creviced specimens were connected through a zero resistance ammeter to a boldly exposed cathode of the same material. Localized corrosion was only observed when the potential of the creviced specimen was higher than the repassivation potential.
The U.S. Department of Energy has proposed a waste package with multiple barriers which would be expected to contain radioactive waste for hundreds to thousands of years. According to the Advanced Conceptual Design (ACD), 1 the spent fuel will be emplaced in a multipurpose canister (MPC)2 fabricated from type 316L stainless steel (SS). For disposal, the MPC will then be placed inside an alloy 825 inner overpack surrounded by a thick carbon steel outer overpack. Fusion welding is proposed as the method to close both the MPC and the disposal overpacks.
After emplacement in the repository, the radioactive decay of the spent nuclear fiel will elevate the temperature of the waste packages. Depending on the thermal loading strategy adopted for the repository, the surface of the waste packages may be at temperatures greater than 100 ?C for an extended period. The combination of high thermal loading and emplacement of the waste packages in an unsaturated zone has been proposed as a means to keep the containers dry for several hundred years. Under these conditions, the thick carbon steel layer will oxidize at a slow and predictable rate.
However, it has been observed that groundwater contact with the waste packages is possible even in the unsaturated zone by flow of water through fractures in the rock. 4-6 Computer modeling has suggested that evaporation of the groundwater may increase the concentration of the anionic species on the container surface and create a locally aggressive environment.7 In the presence of groundwater, uniform corrosion of the carbon steel overpack is anticipated. However, if the pH of the groundwater is altered to alkaline values as a result of reactions with concrete in the repository, then the carbon steel outer surface can be passivated. Localized corrosion of the carbon steel overpack can then occur in SIightl y alkaline chloride containing groundwater.8 If the carbon steel disposal overpack is perforated, localized corrosion of the alloy 825 inner overpack and the type 316L SS MPC may be minimized by galvanic coupling to the ruptured carbon steel overpack. However, in an unsaturated environment, partial wetting of the container may limit the ability of the carbon steel outer barrier to act as a sacrificial anode to the inner barriers. In this paper, the localized corrosion behavior of alloy 825 and type 316L SS in a repository environment is investigated under both applied potentials and open-circuit potentials using redox couples. The effect of environmental variables such as redox potential and chloride concentration on the localized corrosion initiation and propagation rate of these proposed container materials is examined. Limitations to the use of the repassivation potential, ET, measured in short-term laboratory tests a
