Corrosion and Stress Corrosion Cracking of Alloy 22 in Lead-Containing Solutions
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Corrosion and Stress Corrosion Cracking of Alloy 22 in Lead-Containing Solutions Yi-Ming Pan, Darrell S. Dunn, Leitai Yang, and Gustavo A. Cragnolino Center for Nuclear Waste Regulatory Analyses (CNWRA) Southwest Research Institute 6220 Culebra Road, San Antonio, TX 78238-5166, USA ABSTRACT The susceptibility of Alloy 22 to localized corrosion and stress corrosion cracking was evaluated in lead-containing solutions. Speciation calculations indicate that aqueous dissolution of PbCl2 resulted in the formation of Pb2+ and Pb-Cl complexes. PbCl2 was selected among several lead compounds for its high solubility and dissociation rate. The total dissolved lead species concentration was measured by inductively coupled plasma-atomic emission spectrometry. Comparative anodic polarization tests were conducted in a range of deaerated, acidified PbCl2 solutions. The presence of lead species in a super-saturated PbCl2 solution with a pH of 0.5 promotes the occurrence of a pronounced anodic peak and significantly increases the passive current density. Enhanced dissolution of Alloy 22 resulted, as characterized by the presence of randomly distributed etch pits. At an applied potential of !100 mVSCE, which is the potential corresponding to the anodic peak, Alloy 22 was found to be resistant to stress corrosion cracking when tested in super-saturated PbCl2 solutions (pH 0.5) at 95 °C [203 °F]. Moreover, no deleterious effect of lead species on localized corrosion was observed in crevice specimens over a wide range of lead concentrations with the exception of acidified, super-saturated PbCl2 solutions. Provided that these high lead concentrations are not attainable in the anticipated repository environments, Alloy 22 is unlikely to be susceptible to stress corrosion cracking or localized corrosion. INTRODUCTION Small concentrations of lead ranging from 2.5 to 6 ppm were found to cause 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 [572 °F] [1]. The presence of lead in pressurized water reactor environments has also been reported to cause SCC of Alloy 600 steam generator tubes in the 1990s [2,3]. The lead concentration measured in the sludge deposits of most steam generators was in a range of 100 to 500 ppm; however, higher concentrations of lead up to 10,000 ppm were detected on the surface scale of the Alloy 600 tubes [2]. Helie, et al. [3] attributed the lead assisted SCC of nickel base alloys (Alloys 600, 690, and 800) to enhanced dissolution and/or oxidation of the alloys. It should be noted that an activation energy of 125 kJ/mol [30 kcal/mol] has been measured [3], indicating that SCC susceptibility decreases substantially with decreasing temperature. Rock-groundwater interactions over extended periods can lead to leaching of specific detrimental species that may become concentrated in the water dripping on the drip shield and waste packages. Carlos, et al. [4] observed the presence of Mn-oxide minerals, in which l
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