Characterization of the Corrosion Behavior of Alloy 22 after Five Years Immersion in Multi-ionic Solutions
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Characterization of the Corrosion Behavior of Alloy 22 after Five Years Immersion in Multi-ionic Solutions Lana L. Wong, David V. Fix, John C. Estill, R. Daniel McCright and Raúl B. Rebak Lawrence Livermore National Laboratory 7000 East Ave, L-631, Livermore, CA 94550, USA
ABSTRACT Alloy 22 (N06022) is the candidate material for the corrosion resistant, outer barrier of the nuclear waste container. Two of the potential corrosion degradation modes of the container are uniform corrosion and localized corrosion. A testing program is under way at the Lawrence Livermore National Laboratory to determine the susceptibility of Alloy 22 to these two forms of corrosion using immersion tests. Metallic coupons are being exposed to several electrolyte solutions simulating concentrated underground water from pH 3 to 10 at 60°C and 90°C. This paper describes the results obtained after more than a five-year exposure of 122 specimens to the testing electrolyte solutions. Results show little general corrosion and the absence of localized corrosion. The maximum general corrosion rate was 23 nm/yr.
INTRODUCTION The current waste container (or waste package) design for the Yucca Mountain Project consists of two concentric metal containers. The design specifies an internal barrier to be made with type 316L stainless steel (S31603), which will serve to shield radiation and also provide mechanical integrity. Alloy 22 is the candidate material for the external barrier of the high-level nuclear waste containers. The primary purpose of the outer container is to provide protection against corrosion. Alloy 22 (N06022) was selected for this application because it is well known commercially for its excellent corrosion resistance in aggressive environments [1-5]. This alloy is nickel-based (Ni) and its nominal composition (weight percent) is ~56% nickel (Ni), 22% chromium (Cr), 13% molybdenum (Mo), 3% tungsten (W) and 3% iron (Fe). Because of its high Cr content, Alloy 22 remains passive in most industrial environments and thus, has an exceptionally low general corrosion rate. Waters that contact the waste container are expected to be in the form of a multi-ionic solution. This solution may form through two different mechanisms: (1) Dripping from the drift wall and concentrating on the container surface and (2) Deliquescence of salts that may accumulate on top of the container during dry periods. In both cases, the aqueous solution would be concentrated. The ground waters associated with the Yucca Mountain region have been well characterized [6,7]. Table 1 shows the composition of the saturated zone water, from well J-13, at near the repository site. The well water is near-neutral and bicarbonate-rich with significant concentrations of sulfate, nitrate, chloride, alkalis and alkaline earths ions. Table 1 also shows the composition of various laboratory-prepared, aqueous, concentrated electrolyte solutions in which testing was performed. These electrolyte solutions range from pH ~3 to 10 and are designated as simulated acidified water (S
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