Grain-boundary chemistry and intergranular corrosion in alloy 825
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I. INTRODUCTION
THE thermal stability of nuclear waste container materials is an important issue in the geological disposal of highlevel radioactive waste. Among the materials for containers in the proposed Yucca Mountain (YM) repository, Ni-CrFe-Mo and Ni-Cr-Mo alloys have been the main candidates. One of the former candidate materials, Alloy 825, is the focus of this study. Currently, Alloy 22 (Ni-22 pct Cr-13 pct Mo-3 pct W-4 pct Fe) is the main candidate material due to its superior corrosion resistance. Containers will be made by welding plates both longitudinally and circumferentially. Even in stabilized Ni-Cr-Fe-Mo alloys such as Alloy 825, welding produces a heat-affected zone in which grainboundary carbide precipitates and sensitization in a narrow region close to the fusion boundary may occur. Changes in material microstructure and microchemistry, particularly at grain boundaries, can affect mechanical properties due to thermal embrittlement or a decrease in corrosion resistance as a result of sensitization. Intergranular corrosion has been known to occur for NiCr-Fe-Mo alloys in specific environments and can promote stress corrosion cracking and even disintegration at the grain boundary. It is generally accepted that the principal feature responsible for this phenomenon in stainless steels and Crbearing nickel alloys is the existence of a narrow chromiumdepleted zone adjacent to carbide precipitates in the grainboundary region. Extensive research has been conducted to quantify the grain-boundary chemistry of stainless steels[1,2] and other Ni-Cr-Fe alloys such as Alloy 600.[3–6] However, very few studies exist in the literature on the development of chromium depletion during the sensitization of Alloy Y.-M. PAN and D.S. DUNN, Senior Research Engineers, G.A. CRAGNOLINO, Staff Scientist, and N. SRIDHAR, Manager, are with the Corrosion Science and Process Engineering Element, Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted March 5, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
825. Raymond[7] has conducted one of these studies on the mechanisms of sensitization and stabilization of Alloy 825. He found that the sensitivity to intergranular corrosion of Alloy 825 can be attributed to the precipitation of Cr-rich carbides and the formation of a Cr-depleted area adjacent to the grain boundary, but no Cr concentration profile was experimentally measured. Thus, determination of the chemistry in the vicinity of grain boundaries is important in understanding the thermal stability of these alloys. Energy-dispersive X-ray spectroscopy with an analytical electron microscope (AEM) is one of the few techniques available for analysis of grain-boundary chemistry. The development of the AEM, with a finely focused electron probe, has enabled sampling of very small volumes. Hall and Briant[1] have systematically determined the Cr distribution in the vicinity of carbides as a function of sensitization temperature and time in type 316LN austenitic sta
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