Abstract
The research focused on developing a solid-state repair welding technique based on friction stir welding (FSW) to heal stress corrosion cracks (SCC) on stainless steel (SS) dry storage canisters (DSCs) used for storage of spent nuclear fuel (SNF). In real service conditions, the welded and stressed sections of the DSCs become prone to SCC when exposed to a chloride-containing environment. The FSW was performed at the state-of-the-art friction stir facility available at the Pacific Northwest National Laboratory. Simulated cracks were machined using electric-discharge machining on the 304L SS plates, and repaired welded by the FSW process at three different isothermal conditions (725, 825, and 900 ºC). Microstructural and corrosion resistance of the FSW specimens were evaluated using different electrochemical techniques. Since pitting is considered a precursor for SCC initiation, extensive investigations were carried out to evaluate the pitting resistance of the friction stirred materials in neutral and acidified chloride solutions at different temperatures in the range of 20 – 80 ºC. The SCC behavior of the friction stirred specimens were evaluated using U-bend specimens in 3.5% NaCl in neutral and acidified conditions, and in boiling MgCl2 solution. The outcomes of this research are: a) correlation between grain boundary character and localized corrosion resistance; b) understanding the role of low angle grain boundaries on the SCC initiation and propagation; c) passive film breakdown potentials as a function of FSW parameter; d) development of a solid-state repair welding technique for the stainless steel dry storage canisters, and e) proposal of a mechanistic model for SCC initiation based on the morphological instability of the passivated surface due to competing effects of a reduction in the surface energy by chloride adsorption and increased strain energy due to passive film growth stresses. The brittle nature of the SCC growth is modeled based on the Cottrell equation by considering the interaction between metal vacancies and dislocations piled up at the low-energy crystallographic planes. Based on this study, an optimized FSW condition was developed and successfully applied to repair the stress corrosion cracks of a fusion-welded 304L SS plate.