Crevice Corrosion Penetration Rates of Alloy 22 in Chloride-Containing Waters

Abstract

A possible waste package design for the disposal of high-level radioactive waste at the potential repository at Yucca Mountain, Nevada, may consist of an outer container made of Alloy 22 (UNS N06022) and an inner container made of Type 316 (UNS S31600) nuclear-grade stainless steel. The crevice corrosion propagation behavior of Alloy 22 in 5 M sodium chloride (NaCl) solution at 95°C (203°F) has been investigated using single and multiple crevice assemblies. After the addition of cupric chloride (CuCl2) to the solution as an oxidant to raise the corrosion potential, crevice corrosion initiated rapidly. The subsequent active propagation of crevice corrosion resulted in significant decreases in corrosion potential. The current density decayed quickly with time as a result of the concurrent potential drop. At short times, the crevice corrosion penetration rates were several orders of magnitude greater than the passive uniform corrosion rates. For longer times, the penetration rates decreased significantly as a result of stifling and repassivation of crevice corrosion observed in several tests. Alloy 22 shows a strong tendency to repassivate. The results of the investigation on crevice corrosion propagation indicate that although crevice corrosion of Alloy 22 is possible under some environmental conditions, the maximum penetration depth of localized attack is limited to depths significantly less than the possible outer container thickness. The stifling and arrest of crevice corrosion significantly decrease the actual values of crevice corrosion propagation rates even under the aggressive environmental conditions used in these laboratory tests.