The effect of potential and aging on the Pb-assisted stress corrosion cracking susceptibility of alloy 22 gas tungsten a
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. INTRODUCTION
COPSON and Dean first reported that Pb concentrations ranging from 2.5 to 6 ppm caused rapid stress corrosion cracking (SCC) of alloy 600 (Ni-18Cr-5Fe) in deionized water adjusted to pH 10 with the addition of ammonia at temperatures above 300 °C.[1] Subsequent experimental investigations have shown that the presence of Pb at levels as low as 0.1 ppm caused SCC in high Ni-Cr alloys such as alloys 600, 690, and 800 at temperatures around 300 °C.[2] High Pb concentrations in sludge deposits have been implicated in the rapid SCC failures and accelerated degradation of several commercial pressurized water reactor (PWR) steam generator tubes made from alloy 600 in the 1990s.[2,3,4] The Pb concentrations measured in the sludge deposits of typical steam generators were in the range of 100 to 1,000 ppm; however, higher concentrations of Pb up to 10,000 ppm were ALADAR A. CSONTOS, Materials Engineer and Team Lead, is with the United States Nuclear Regulatory Commission, Washington, DC 205550001. Contact e-mail: [email protected] YI-MING PAN, Senior Research Engineer, DARRELL S. DUNN, Principal Engineer, LEITAI YANG, Senior Research Engineer, and GUSTAVO A. CRAGNOLINO, Institute Scientist, are with the Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, TX 78238-5166. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
detected on the surface scale of the alloy 600 and 690 tubes.[3,5,6] This high Pb content on the surface of high NiCr alloys has been attributed to enhanced dissolution or oxidation of the alloys and a cause for accelerated SCC.[2,4] Temperature effects have also been examined and an activation energy of 125 kJ/mol was measured, which indicates that Pb-assisted stress corrosion cracking (PbSCC) susceptibility decreases substantially with decreasing temperature.[4] Alloy 22 (UNS N06022) is a high Ni-Cr alloy with significant additions of Mo and W, and is the material currently preferred by the United States Department of Energy (DOE) for the outer barrier of the proposed waste package for the potential high-level nuclear waste repository at Yucca Mountain, Nevada. Alloy 22 was selected due to its excellent resistance to corrosion and SCC over a wide variety of environmental conditions. Recent studies sponsored by the State of Nevada,[7] though, reported that localized corrosion and PbSCC of millannealed (MA) alloy 22 occurred at 250 °C in a 1000 J-13 water-containing 5000 ppm Pb (as Pb acetate) and acidified to a pH of 0.5 with HCl. Nevertheless, the State of Nevada investigators could not reproduce their preliminary PbSCC result at this 250 °C temperature. It should be noted that this temperature-pressure regime for aqueous electrochemical processes to occur is co
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