Modeling Corrosion Processes for Alloy 22 Waste Packages

Abstract

The proposed waste package design for the disposal of high-level radioactive waste at the potential repository at Yucca Mountain, Nevada, consists of an outer container made of Alloy 22, a corrosion resistant Ni-Cr-Mo-W alloy, surrounding an inner container made of Type 316 nuclear grade stainless steel. Models to assess the influence of waste package degradation modes on the overall system performanceconsider uniform and localized corrosion processes. Based on measurements of passive anodic current density, the uniform aqueous corrosion rate of Alloy 22 is estimated to be slow, and long waste package lifetimes are projected in the absence of conditions that promote accelerated degradation processes. The initiation of localized corrosion is possible in chloride-containing waters with low concentrations of inhibiting oxyanions such as nitrate. Although propagation rates for localized corrosion are typically orders of magnitude greater than the passive uniform corrosion rates, the maximum penetration depth of localized attack may be limited to depths significantly less than the container thickness as a result of stifling andarrest of localized corrosion.

Localised corrosion of heat-treated alloys Part II – Predicting grain boundary microchemistry and its effect on repassivation potential

“…The modelling studies reported here rely on the previous experimental results of Tormoen et al 18 and Pan et al 14 for the repassivation potential of heat-treated alloys 600 and 825 respectively. In addition, the authors use the repassivation potential data reported by Dunn et al [15][16][17] for welded samples of alloy 22. The nominal compositions of the alloys studied here are summarised in Table 1.…”

“…alloy 825 14 and alloy 22. [15][16][17] In the first part of this study, 18 the authors investigated the localised corrosion of alloy 600. However, no generalised treatment has been proposed so far to rationalise and predict the effects of heat treatment on the susceptibility of Fe-Ni-Cr-Mo alloys to pitting or crevice corrosion.…”

Localized corrosion of alloy 22 in the potential Yucca Mountain repository environment

“…Figure 7 plots the maximum penetration depths (d max ) as a function of time from all the tests of Alloy 22 coupled to titanium grade 7 along with the penetration depths reported previously by coupling Alloy 22 to Alloy 22. 8,9 A fit of the experimental data to the exponential relationship yielded: d max = 41.3t 0.524 (4) where t = time in days and d max = maximum penetration depth in m. The equation obtained previously by coupling Alloy 22 to Alloy 22 8,9 is listed for comparison: d max = 91.2t 0.233 (5) The greater time exponent (0.524) suggests that crevice corrosion propagation of Alloy 22 is more efficient by coupling to Titanium Grade 7 in the same solution at the same temperature than by coupling to Alloy 22.…”

“…The results suggest that molybdenum and tungsten play a common role. Additionally, MoO 4 2in the outer region of the oxide may lead to a cation-selective character and discourage the incorporation of Clinto the passive film.…”

“…Numerous short-term tests have been conducted at NRC and CNWRA to measure the E corr and the E rcrev as a func-tion of metallurgical and environmental conditions to assess the susceptibility of Alloy 22 to crevice corrosion using metal-to-polytetrafluoroethylene crevices. [4][5][6][7] Limited tests were conducted in NaCl solutions to understand the localized corrosion propagation behavior and the effects of similar and dissimilar metal crevices on crevice corrosion initiation and propagation. This paper summarizes the effects of environmental conditions (temperature, chloride concentration, and inhibitor concentration), metallurgical conditions (e.g., mill-annealed and welded plus solution annealed), and similar and dissimilar metal crevices on the crevice corrosion susceptibility of Alloy 22.…”