Evaluation of Corrosion Processes Affecting the Performance of Alloy 22 as a Proposed Waste Package Material
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Gustavo A. Cragnolino, Darrell S. Dunn, and Yi-Ming Pan Center for Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Institute San Antonio, TX 78238-5166, U.S.A. $%675$&7 This paper presents recent work on evaluating localized corrosion and stress corrosion cracking, two corrosion processes that are important to the long-term performance of Alloy 22 (58Ni-22Cr-13Mo-3W-4Fe). This alloy is the material preferred by the U.S. Department of Energy (DOE) for the outer container of the waste package to be used in the proposed high-level radioactive waste repository at Yucca Mountain, Nevada. It was found that both welded and thermally aged materials are more susceptible to localized corrosion in chloride solutions at temperatures above 60 ΕC than the mill-annealed material. This observation suggests that welding and certain post-welding operations may decrease the estimated life of the waste packages. However, no stress corrosion crack growth was observed in concentrated chloride solutions and simulated, concentrated groundwater at 95 ΕC when precracked compact tension specimens were tested under both constant and cycling loading. ,1752'8&7,21 The reference waste package design that the U.S. Department of Energy (DOE) is considering for a potential license application for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, consists of an outer container made of Alloy 22 (58Ni22Cr-13Mo-3W-4Fe) surrounding an inner container made of Type 316 nuclear grade stainless steel [1]. The purpose of this inner container is to provide structural integrity to the waste package. For undisturbed repository conditions, uniform corrosion, which in this class of Ni-CrMo corrosion-resistant alloys is a very slow dissolution process through a protective oxide film, is anticipated to be the primary process limiting the life of the waste package [2]. However, crevice corrosion, which is the predominant form of localized corrosion in these alloys, and environmentally assisted cracking in the form of stress corrosion cracking may also occur in the aqueous environments that could prevail under certain thermo-hydrological conditions in the emplacement drifts [3]. In previous investigations [4,5], the combined ranges of potentials, chloride concentrations, and temperatures within which mill-annealed Alloy 22 could be susceptible to crevice corrosion were determined. This was accomplished, on the basis of results verified for another Ni-Cr-Mo alloy [6], by conservatively adopting the repassivation potential for crevice corrosion ((rcrev) as the lowest potential at which this form of localized corrosion can occur in long-term exposures under natural corroding conditions. Figure 1 is a plot of (rcrev as a function of the Cl- concentration at 95 ΕC where data points are shown for mill-annealed Alloy 22 in comparison with previously reported results (data points are not shown for simplicity) for other c
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