The proposed waste packages for the disposal of high-level radioactive waste at the potential repository at Yucca Mountain, Nevada, consist of an outer container and inner container. The outer container is made of alloy 22, a corrosion-resistant Ni-Cr-Mo-W alloy, while the inner container is made of type 316 nuclear-grade stainless steel. Fabrication processes such as welding and postweld heat treatments can induce changes to the microstructure of alloy 22. Such microstructural changes can reduce ductility and impact strength and promote localized corrosion. Environmental conditions within the emplacement drifts, such as composition of solutions contacting the waste packages, can also affect localized corrosion susceptibility. In this study, the effects of fabrication processes on impact strength, fracture toughness, and crevice-corrosion resistance of alloy 22 were investigated.

The temperature dependence of crevice corrosion initiation on titanium grade-2 has been investigated using a galvanic coupling technique. The film breakdown/repassivation transients indicate a temperature threshold, around 65°C, for the initiation of crevice corrosion. The number, size, frequency, and background current of these events increased when the temperature was increased from 65 to 80°C. Analysis of single transients suggests that the temperature also has an effect on the repassivation mechanism. Image analysis of corroded coupons reveals that crevice initiation and deeper penetration occur around the edge of the creviced area. Surface studies using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy show oxide film flaws accompanied by water incorporation at temperatures above the 65°C threshold. Under creviced conditions, the occurrence of film flaws is the key process leading to the development of acidity and the initiation of crevice corrosion.

The effects of iron content on the microstructure and crevice corrosion of Grade-2 titanium (Ti-2) were studied using a galvanic coupling technique combined with optical microscopy and secondary ion mass spectrometry (SIMS) imaging. This study reveals that iron content has a significant effect on the microstructure and crevice corrosion behavior of Ti-2. The grain size decreased significantly with increasing iron content. For a Ti-2 material with medium iron content, crevice corrosion readily initiated and the metal exhibited extensive intergranular attack that could be associated with the more reactive iron-stabilized β-phase within the α-phase matrix, as revealed by SIMS imaging. By contrast, Ti-2 materials with low and high iron contents showed suppressed crevice attack. The smaller surface area of available grain boundaries in Ti-2 of low iron content could account for this limited attack. For the material with high iron content, SIMS imaging suggests that some Ti x Fe intermetallic particles were formed. These particles may act as proton reduction catalysts and enhance crevice corrosion resistance via cathodic modification.

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The proposed waste package design for the disposal of high-level radioactive waste at the potential repository at Yucca Mountain, Nevada, consists of an outer container made of Alloy 22, a corrosion resistant Ni-Cr-Mo-W alloy, surrounding an inner container made of Type 316 nuclear grade stainless steel. Models to assess the influence of waste package degradation modes on the overall system performanceconsider uniform and localized corrosion processes. Based on measurements of passive anodic current density, the uniform aqueous corrosion rate of Alloy 22 is estimated to be slow, and long waste package lifetimes are projected in the absence of conditions that promote accelerated degradation processes. The initiation of localized corrosion is possible in chloride-containing waters with low concentrations of inhibiting oxyanions such as nitrate. Although propagation rates for localized corrosion are typically orders of magnitude greater than the passive uniform corrosion rates, the maximum penetration depth of localized attack may be limited to depths significantly less than the container thickness as a result of stifling andarrest of localized corrosion.

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