Stress corrosion and hydrogen-induced cracking of carbon steel and stainless steel

This section presents the SCC behaviour of carbon steel disposal containers and stainless steel storage canisters. Carbon steel is mostly susceptible to hydrogen-induced cracking due to residual weld (and heat affected zone) stress or seismic-induced impact stress. Hydrogen is generated during general corrosion or gamma radiolysis of groundwater in a reducing envi­ronment (Ahn and Soo, 1995).

In a marine (coastal) environment, salt deposits may occur on the stain­less steel canister surface due to salt deposits in the humid air. The salt deposits on the canister when the canister surface temperature is above ambient. Aqueous conditions of high chloride concentration may form due to this salt deliquescence. With the residual tensile stress at welds (including

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7.5 Examples of size and distribution of pits. Pit diameter vs time for 18Cr-12Ni-2Mo-Ti stainless steel in 0.1 n H2SO4 + 0.1 n NaCl. Normal pit size observed is in the range of micrometers to millimeters and pit density is for 304 stainless steel after potentiostatic polarization in 1m NaCl solution. The term ‘d’ is pit diameter and ‘t’ is time (reprinted with permission) (Szklarsksa-Smialowska, 1986). Used with permission from NACE International.

heat affected zone), this high concentration of chlorides may induce SCC (Shirai et al., 2011; EPRI, 2005). SCC can be screened out based on stress mitigation techniques (or other remediation) such as applying compressive stress in the weld. In the absence of this mitigation, the opening surface area by SCC (or hydrogen-induced) cracks may be estimated, if a canister were susceptible to SCC. This quantitative estimate allows assessment of the radionuclide release due to waste form degradation inside the container or the canister.

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