This paper presents a review hydrogen-induced cracking (HIC) of Ti Grade 7 and other relevant titanium alloys under nuclear waste repository environmental conditions, with a primary emphasis on the corrosion processes possible in aqueous environments at Yucca Mountain. The current understanding of hydrogen absorption and the role of passive film on titanium alloys is presented. The key corrosion processes that could occur are addressed individually. Finally, the models developed to assess the hydrogen concentration in the drip shield due to passive general corrosion and galvanic coupling to less noble metals under repository conditions is described. To facilitate the discussion, Ti Grades 2, 5, 7, 9, 11, 12, 16, 17, 18, and 24 are included in this review. It can be concluded that under repository conditions, HIC of titanium alloys will not occur because there will not be sufficient hydrogen in the metal even after 10,000 years of emplacement. Based on the many assumptions adopted this assessment can be considered very conservative.
The Nuclear Waste Policy Act of 1982 (as amended in 1987) designated Yucca Mountain in Nevada as the potential site to be characterized for high-level nuclear waste (HLW) disposal.[1] Longterm containment of waste and subsequent slow release of radionuclides into the geosphere will rely on a system of natural and engineered barriers including a robust waste containment design. The waste package (WP) design consists of a highly corrosion resistant Ni-based Alloy 22 cylindrical barrier surrounding a 316 stainless steel inner structural vessel.[2] The waste package is covered by a mailboxshaped drip shield (DS) composed predominantly of Ti Grade 7 with Ti Grade 24 structural support members.[2] The Ti Grade 7 drip shield provides defense in depth, because the WP and DS do not have common failure modes, lending a further safety margin to the repository design.
The minimum target lifetime for containment of HLW, without exceeding a regulatory specified individual dose rate at the site boundary, is 10,000 years.[2] Over the years, numerous studies have been performed to evaluate the susceptibility to stress corrosion cracking, general, localized, galvanic and microbially influenced corrosion for Alloy 22 [3, 4, 5, 6, 7, 8, 9, 10, 11] and Ti Grade 7.[4, 5, 11, 12, 13]
The purpose of this paper is to review hydrogen induced cracking (HIC) of Ti Grade 7 and other relevant titanium alloys under repository environmental conditions. The review will concentrate on the corrosion processes possible in aqueous environments at Yucca Mountain. A brief review of hydrogen absorption and the properties of the passive film on titanium alloys is presented. The key corrosion processes that could occur will be addressed individually. Finally, the models developed to predict the rate of hydrogen absorption into the alloy due to passive general corrosion and galvanic coupling to less noble metal under repository conditions are described.
