Surface Analysis of Alloy 22 under Conditions that Promote Stress Corrosion Cracking

The U.S. Department of Energy (DOE) is currently preparing a license application for the permanent disposal of high-level waste at Yucca Mountain, Nevada. The waste package design for the disposal of high-level waste could consist of an outer container made of Alloy 22, a corrosion resistant Ni-Cr-Mo-W-Fe alloy, surrounding an inner container made of Type 31 6 nuclear grade stainless steel. Alloy 22 is resistant to stress corrosion cracking in chloride solutions; however, under a limited range of conditions, stress corrosion cracking of Alloy 22 has been observed in environments containing chloride and bicarbonate, which may be present in water entering the emplacement drifts. The initiation of stress corrosion cracking may significantly decrease waste package lifetimes. Analyses of the oxide films on Alloy 22 produced under conditions where stress corrosion cracking has and has not been observed are presented in this paper. The composition and thickness of the oxide films were found to be dependent on test conditions. Under conditions where stress corrosion cracking is possible, the chromium concentration in the oxide film was significantly lower compared to oxides produced under benign conditions.