The Effect of Environmental Chemistry on the Pb Assisted Stress Corrosion Cracking Susceptibility of Mill-Annealed Alloy
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The Effect of Environmental Chemistry on the Pb Assisted Stress Corrosion Cracking Susceptibility of Mill-Annealed Alloy 22 and GTAW Weldments Aladar A. Csontos1, Yi-Ming Pan2, Darrell S. Dunn2, Leitai Yang2, and Gustavo A. Cragnolino2 1 U.S. Nuclear Regulatory Commission, Mailstop: T7-F3, Washington, DC 20555-0001 USA 2 Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166, USA ABSTRACT The susceptibility of mill-annealed (MA) and gas tungsten arc welded (GTAW) Alloy 22 to Pb-assisted stress corrosion cracking (PbSCC) was evaluated in supersaturated PbCl2 and Pb(NO3)2 solutions. The solution chemistries and pH were varied to evaluate the effects of specific Pb-bearing species on the SCC resistance of GTAW Alloy 22 double U-bend samples. Anodic polarization tests in acidic and basic supersaturated Pb(NO3)2 solutions indicated that the passive current densities generally increased with applied potential. These results were then compared to previous anodic polarizations results for supersaturated acidic PbCl2 solutions. Constant deformation SCC tests indicate that MA and GTAW Alloy 22 were resistant to PbSCC in supersaturated PbCl2 (16,000ppm Pb, 95°C, pH 0.5, and Eapplied = –100, -50, 0, and 50mVSCE) and Pb(NO3)2 (16,000ppm Pb, 95°C, pH 3.9 and 10.7 and Eapplied = 50 and 140mVSCE, respectively). The overall results from this PbSCC investigation suggest that MA and GTAW Alloy 22 are resistant to SCC in supersaturated PbCl2 and Pb(NO3)2 solutions at 95°C. Provided that these high Pb concentrations are not attainable in the anticipated repository environments, Alloy 22 is unlikely to be susceptible to SCC by the presence of Pb. INTRODUCTION Alloy 22 (UNS N06022) is a Ni-Cr-Mo alloy with additions of Fe and W that is currently the material preferred by the U.S. 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 stress corrosion cracking over a wide variety of environmental conditions. Recent studies by Pulvirenti, et al. [1] though reported that PbSCC of MA Alloy 22 occurred at 250°C in a 1,000× J–13 water containing 5,000ppm Pb (as Pb acetate) and acidified to a pH of 0.5 with HCl. Nevertheless, these investigators could not reproduce their preliminary PbSCC result at 250°C. It should be noted that this temperature-pressure regime for aqueous electrochemical processes to occur is considered unlikely in the anticipated repository environment due to the continuous ventilation and elevated temperatures that will act to dry out the emplacement tunnels. Estill et al. [2] and Andresen et al. [3] have also investigated PbSCC of MA and 20% cold worked Alloy 22 samples in PbCl2 solutions and Basic Saturated Water with PbNO3 additions. They report that no SCC was observed with a slow strain rate test in water containing 1 percent PbCl2 (7,450ppm Pb) at 95°C [2] and with compac
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