General Corrosion Studies of Candidate Container Materials in Environments Relevant to Nuclear Waste Repository

Abstract

Abstract Alloy 22 (UNS N06022) and Ti Grade 7 (UNS R52400) have been proposed as the corrosion resistant materials for fabricating the waste package outer barrier and the drip shield, respectively, for the proposed nuclear waste repository at the Yucca Mountain Project. In this work, the general corrosion behavior of welded and annealed Alloy 22 and Ti Grade 7 in BSW-12 environments from 60° to 105°C was studied by simple immersion test for four weeks and eight weeks. The corrosion rate for Alloy 22 was found to be 0.003 mpy at 60°C and 0.012 mpy at 105°C (0.075 and 0.30 microns/year) and 0.008 mpy at 60°C to 0.026 mpy at 105°C (0.2 and 0.65 microns/year) for Ti Grade 7. The corrosion of Ti Grade 7 and Alloy 22 is discussed in terms of the corrosion rates as functions of temperature and exposure time and the correlation between the corrosion behavior and surface morphology. For Ti Grade 7, the imperfections (precipitates, inclusions, stringers, pores etc.) inherited from materials processing may serve as the sites of preferential dissolution or “traps” for corrosion products. In case of Alloy 22, the existence of graphite particles (or carbides) did not seem to deteriorate the excellent corrosion resistance of both annealed and welded materials. The apparent activation energies for Ti Grade 7 and Alloy 22 in BSW-12 environment were found to be approximately 22.4 - 23.5 KJmol-1 and 24.9 - 26.8 KJ mol-1, respectively. There was no obvious difference between the annealed and welded materials.