CORROSION—AUGUST 2004764

Alloy 22 (UNS N06022) and Ti Grade 7 (UNS R52400) are the current corrosion-resistant materials of choice for fabricating the waste package outer barrier and the drip shield, respec-tively, for the proposed high-level nuclear waste repository at Yucca Mountain. In this work, the general and crevice corro-sion behavior of annealed and welded Alloy 22 and Ti Grade 7 exposed in basic saturated water (BSW-12) for four and eight weeks at 60°C to 105°C were evaluated using the ASTM G78 method combined with surface analysis and sta-tistical analysis of corrosion rate. The general corrosion rates for Alloy 22 and Ti Grade 7 were found to increase linearly with temperature but decrease with the exposure time. The mean corrosion rate was found to be 0.003 mpy (0.075 µm/y) at 60°C and 0.010 mpy (0.25 µm/y) at 105°C for Alloy 22 and 0.008 mpy (0.20 µm/y) at 60°C and 0.022 mpy (0.56 µm/y) at 105°C for Ti Grade 7. No signifi cant difference in corrosion behavior between the annealed and welded materials was observed. For both materials the surface imperfections inher-ited from materials processing did not seem to deteriorate the excellent corrosion resistance of the materials but might serve as the “traps ” for corrosion products. The apparent activation energies for the temperature dependence of corrosion rates of Ti Grade 7 and Alloy 22 in BSW-12 environment were ob-tained as 25.3 (±5.5) KJ/mol and 23.7 (±4.5) KJ/mol, respec-tively. Although none of the materials was found susceptible to crevice corrosion under the test conditions, to conclude that these materials are immune to crevice corrosion in BSW-12 would require longer-term testing