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 at the Lawrence Livermore National Laboratory is being carried out for Yucca Mountain to determine the susceptibility of Alloy 22 to these two forms of corrosion using long-term immersion tests. Metallic coupons were exposed to several electrolyte solutions simulating concentrated ground water from pH 3 to 10 at 60°C and 90°C. This paper summarizes results on the characteristics of the surface deposits as well as the corrosion rate of 122 coupons of Alloy 22 obtained after more than a five-year exposure. The surface deposits consisted primarily of salt components in the respective solutions. Results showed little general corrosion and the absence of localized (crevice) corrosion.

INTRODUCTION

The proposed engineering barriers that will limit the release of radioactive material in the Yucca Mountain repository will consist of a sealed container and a detached drip shield. The container will be double walled with an internal barrier of type 316L stainless steel (S31603) and an external barrier of Alloy 22 (N06022). The drip shield will be made with Titanium Grade 7 (R52400). The internal barrier of the container will serve to shield radiation and also provide mechanical integrity. The primary purpose of the outer wall of the container is to provide protection against corrosion. The presence of the drip shield will guard the containers against water seepage and rock fall from the drift walls. Alloy 22 (N06022) was selected for the corrosion resistant barrier of the containers because it is well known commercially for its excellent corrosion behavior in aggressive environments. 1-5 It is nickel-based (Ni) and its nominal composition (weight percent) is ~57% 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.

In the absence of the drip shield, waters that contact the waste container are expected to be in the form of a multi-ionic solution. These solutions may form through two different mechanisms: (1) Dripping from the drift wall and concentrating on the container surface or (2) Deliquescence of salts that may accumulate on top of the container during dry periods. In both cases, the aqueous solution is expected to be concentrated and contain multiple components. The ground waters that are associated with the Yucca Mountain region have been well characterized. 6,7 Table 1 shows the composition of a saturated zone water (from a well designated, J-13) from near the repository site. The well water, J-13, 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 (SAW), simulated concentrated water (SCW) and simulated dilute water (SDW). Farmer et al.5 reported that after a 2-year immersion of Alloy 22 coupons in concentrated aqueous electrolytes from pH 2.8 to 10, at both 60°C and 90°C, the average corrosion rate by mass loss was approximately 20 nm/yr (~8 x 10-4 mpy). Wong et al.8 determined the corrosion rates after a 5-year immersion in the same concentrated aqueous electrolytes to be higher for crevice coupons th

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