Pb-Assisted Stress Corrosion Cracking Susceptibility of Alloy 22

Abstract The susceptibility of mill-annealed (MA), solution annealed (SA), and short-term thermally aged (STA) Alloy 22 and gas tungsten arc welded (GTAW) Alloy 22 to Pb-assisted stress corrosion cracking (PbSCC) is evaluated in supersaturated PbCl2 and Pb(NO3)2 solutions. Parameters varied include potential, Pb concentration, solution chemistry, and pH to evaluate the effects of Pb-bearing species on the SCC resistance of Alloy 22. Chemical speciation calculations determined the type and concentration of Pb-bearing species that may evolve during the evaporation-condensation cycles of J-13 groundwater and the applicable test solutions (PbCl2 and Pb(NO3)2) to evaluate the effect of these species on the SCC behavior of Alloy 22. Anodic polarization tests in acidic PbCl2 solutions showed that 16,000ppm of Pb produced a strong anodic peak and an order of magnitude greater passive current density for both MA and GTAW Alloy 22 as compared to pure NaCl solutions. Anodic polarization tests in acidic and basic supersaturated Pb(NO3)2 solutions indicated that the passive current densities generally increased with applied potential. Double U-bend constant deformation SCC tests indicated that MA, SA, STA, and GTAW Alloy 22 were resistant to PbSCC in supersaturated PbCl2 solutions at 95°C, pH 0.5, and applied potentials near the anodic peak ranging from –100-50mVSCE. Constant deformation SCC tests also indicated that MA and GTAW Alloy 22 were resistant to PbSCC in the Pb(NO3)2 solutions (16,000ppm Pb, 95°C, pH 3.9 and 10.7 and Eapplied = 50 and 140mVSCE, respectively). Enhanced dissolution of Alloy 22 was observed in the crevice region of the double U-bend samples tested in the 16,000ppm PbCl2 solutions. Note that these Pb concentration are ~7 orders of magnitude greater than that found in the anticipated repository environments and that additional chemical speciation calculations showed that Pb2+ is strongly immobilized in J-13 groundwater through the precipitation of PbCO3 solids. Therefore, although enhanced dissolution of the inner U-bend did occur in certain tests, the overall results from this PbSCC investigation suggest that MA, SA, STA, and GTAW Alloy 22 are resistant to SCC in extremely aggressive, acidic, and 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, localized corrosion, and enhanced dissolution by the presence of Pb.