Tests were conducted to determine the passive corrosion rate, and the susceptibility of Alloy 22 to localized corrosion and stress corrosion cracking. The passive corrosion rate was found to be on the range of 10 -9 tO 10 -7 A]cm 2. The passive corrosion rate was not strongly dependent on either the solution pH in the range of 2.7 to 8.0 or chloride concentration from 0.028 to 4.0 M. Increasing the temperature from 25 to 95 °C resulted in an increase in the passive current density from 2 × 10 -9 tO 4 X 10 -s A/cm 2. Results from repassivation potential measurements indicate that Alloy 22 was resistant to localized corrosion especially in solutions where the chloride concentration was less than 0.5 M. Similar results were obtained in stress corrosion cracking tests using fatigue precracked wedge loaded double cantilever beam specimens with an initial stress intensity of 34.8 MPa.m 1~2. No stress corrosion cracking was observed after 8 months in 5 percent NaCI at 90 °C. Minor grain boundary attack and limited secondary cracking were observed after an 8 month exposure to 40 percent MgClz at 1 i0 °C.
The proposed high-level nuclear waste (HLW) repository at Yucca Mountain (YM) Nevada is intended to provide near-complete containment of radionuclides within the waste packages (WPs) for thousands of years and an acceptably low annual dose to the public living near the site. The WP is the primary engineered barrier to the release of radionuclides to the biosphere and the performance of WPs for the initial several thousand years after radioactive waste emplacement is extremely important to protecting public health and safety. Two attributes, slow corrosion of WP materials and engineered enhancements designed to extend WP lifetimes, are identified in the Total System Performance Assessment (TSPA) for the viability assessment (VA) of the YM site ~ as vital to the overall repository performance. As a result, assessment of WP degradation modes and the determination corrosion rates are necessary to evaluate the overall repository performance.
The design of the WP has undergone several iterations and many of the WP designs have included corrosion resistant Ni-Cr-Mo alloys. Recently, five enhanced design alternatives (EDA) were considered as a means to prolong WP lifetimes beyond those estimated in the VA 2. In the EDA II design, which is the design of choice, Alloy 22 is selected as the outer container material for the WPs that will be fabricated by shrink fitting the outer container to a 5-cm thick inner container fabricated of types 316 nuclear grade (NG) or 316L stainless steel (SS). The WPs will be enclosed by a self-supported, 2-cm thick Ti-grade 7 mailbox-shaped drip shield with overlapping sections that will extend over the length of the horizontal emplacement drifts. It is assumed that the slow uniform passive corrosion of the 2-cm thick Alloy 22 container will lead to WP lifetimes well beyond 10,000 yr. The inner container is designed to provide sufficient structural strength during the lifetime of the WP to avoid mechanical failure as a result of rock fall, but no performance allocation in terms of corrosion resistance is assigned to this container. It is assumed that Alloy 22 will be immune to localized (crevice) corrosion because a RH greater than 80 percent will be attained only at WP surface temperatures lower than 80 °C due to a combination of low thermal loading and high rates of ventilation during preclosure 3.
The objective of this paper is to present the results of experimental investigations on passive dissolution, crevice corrosion, and SCC of Alloy 22 in chloride solutions under a range of environmental conditions including temperature, chloride concentration
