Assessment of crevice corrosion and hydrogen induced stress corrosion cracks of Ti-Pd alloys for HLW overpack in deep underground water environments

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Assessment of crevice corrosion and hydrogen induced stress corrosion cracks of Ti-Pd alloys for HLW overpack in deep underground water environments

Nakayama, G. / Murakami, K. / Akashi, M.

Crevice corrosion and hydrogen-induced stress corrosion cracking tendencies of titanium (Ti) alloys in deep underground water environments are investigated. The crevice corrosion repassavation potential, E_R,CREV of Ti-palladium (pd) alloys was measured and compared with the free corrosion potential, E_SP = +0.32 V vs. SHE. The minimum Pd content of the alloys for suppressing crevice corrosion in the envisaged most corrosive high-level radioactive waste (HLW) disposal environment, corresponding to the seawater originated underground water of 0.6 mol/L (NaCl) at 100 deg C, is 0.05%, Ti-Gr.17 alloy. The Pd content of 0.01% is found to be sufficient for realizing satisfactory crevice corrosion resistance in transuranium (TRU) disposal environment of 0.6 mol/L (NaCl) at 80 deg C. The hydrogen-induced stress corrosion cracking tendencies of the Ti alloys caused by the formation and cracking of hydride layers on alloy surfaces were assessed by galvanostatic and constant-load tests conducted in completely reducing environments. Test results indicate that Ti-Pd alloys are immune to stress-corrosion cracking in HLW underground disposal condition for 1000 years. Total electrical charge density in TRU waste overpacks for 60000 years, estimated from analyses of thermal history in waste emplacement drifts, was found to be 7.5 MC/m², suggesting the hydride layers formed were 30 microns thick and the crack depth was 15 microns on Ti-Gr.17. Thus, the depth of the stress-corrosion cracks is too small to be taken into engineering consideration. The above results suggest that Ti-Pd alloys with Pd content over 0.05% for HLW and 0.01% for TRU waste can be utilized as corrosion-resistant layers of waste-disposal containers of HLW and TRU waste.