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This paper reviews the current understanding of hydrogen-induced cracking (HIC) of Ti Grade 7 and other relevant titanium alloys within the context of the current waste package design for the repository environmental conditions anticipated within the Yucca Mountain repository.The review concentrates on corrosion processes possible in the aqueous environments expected within this site. A brief background discussion of the relevant properties of titanium alloys, the hydrogen absorption process, and the properties of passive film on titanium alloys is presented as the basis for the subsequent discussion of model developments. The key corrosion processes that could occur are addressed individually. Subsequently, the expected corrosion performance of these alloys under the specific environmental conditions anticipated at Yucca Mountain is considered. It can be concluded that, based on the conservative modeling approaches adopted, hydrogen-induced cracking of titanium alloys will not OCCUT under nuclear waste repository conditions since there will not be sufficient hydrogen in the alloy after 10,000 years of emplacement.

Artificially creviced Alloy 22 (N06022) is susceptible to crevice corrosion in presence of high chloride aqueous solution when high temperatures and high anodic potentials are applied. The presence of oxyanions in the electrolyte, especially nitrate, inhibits the nucleation and growth of crevice corrosion. Crevice corrosion may initiate when a constant potential above the crevice repassivation potential is applied. The occurrence of crevice corrosion can be divided into three characteristic domains: (1) nucleation, (2) growth and (3) stifling and arrest. That is, crevice corrosion reaches a critical stage after which growth stops and the specimens start to regain the passive behavior displayed before the nucleation of localized attack.

IKoster [94] conducted SCC tests in aggressive chloride brines (some containing Mg and Ca) at 90, 170, and 200°C in the presence and absence of a gamma radiation field (lo5 rad/hr). They concluded that Ti Grade 7 was resistant to both localized corrosion and SCC in those environments.In test media relevant to the repository environments, SCC tests were conducted using U-bend (constant deflection) specimens of Ti Grades 7 and 12 [95]. The specimens were exposed for up to 4 years in SCW, SAW, and SDW solutions (Table 3) (Table 3) at 110°C and subjected to very low cyclic loading (-0.001 Hz).Crack length versus time was monitored in-situ using a reversing DC potential drop technique. A typical result is shown in Figure 15. The loading characteristics were modified at several times through the addition of long hold times at the maximum stress intensity factor (ISmax) culminating with a very long hold time (-2000 hours) about 4100 hours into the test. The crack growth rate to 800 pg/g (Ti Grade 2) and 400 to 600 pg/g (Ti Grade 12) have been determined [98,102].Ikeda and Quinn reported that the Hc value for Ti Grade 16 is between 1000 and 2000 pg/g [39,40]. . Since both Ti Grade 7 and Ti Grade 16 are virtually identical except for palladium content (Table 1) and are a-alloys containing minimal amounts of &phase, it is reasonable to expect that both alloys will exhibit very similar responses to applied stresses in acidic environments. Thus, the controlling factor in determining the Hc will be the solubility of the hydrogen in the alloy which increases with the Pd-content. This clearly suggests that the HIC behavior of Ti Grade 7 should be at least as good as that of Ti Grade 16 (Le. -1000 pg/g). Ikeda et al. studied the HIC behavior of Ti Grade 16 in comparison with TiGrades 2 and 12 [103, 1041 and concluded that the much higher Hc for Ti Grade 16 is due to prevention of hydride formation by the higher solubility of hydrogen in Pd intermetallic particles. The authors stated that intermetallic particles do not act as hydrogen absorption windows but may promote proton discharge favoring hydrogen gas production. 29An experimentally obtained critical hydrogen concentration for Ti Grade 24 is not available at this time. However, an approximate Hc value of this alloy can be inferred from the corrosion behavior of Ti Grade 5. Testing of Ti Grade 5 and its Pd-modified version, Ti Grade 24, showed that the addition of Pd improves the alloy's corrosion resistance in a manner similar to that observed when Pd is added to the Ti Grade 2 alloy to produce Ti Grade 16 [20, 27, 1051. For Ti Grade 5, Hardie and Ouyang showed that the fracture toughness was not significantly altered until the hydrogen level in the alloy exceeded 200 pg/g [106]. For smooth tensile specimens, the authors showed that the reduction in area and elongation of Ti Grade 5 did not decrease until the hydrogen concentration reached about 1,500 pg/g [ 1061. Addition of Pd to Ti Grade 5 (to produce Ti Grade 24) should lead to a higher value of Hc as it did for ad...

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