Low Temperature Stress Corrosion Cracking of Sensitized Inconel 600 in Tetrathionate and Thiosulfate Solutions Available to Purchase

This paper addresses some aspects of low temperature stress corrosion cracking (SCC) in Inconel 600 steam generator tubing, a recent instance of which apparently resulted from the contamination of the primary coolant by sulfur compounds. The low temperature SCC of sensitized Inconel 600 was studied in concentrated and dilute solutions of tetrathionate and thiosulfate. The material tested was a tubing directly sensitized from the mill-annealed condition. Tests were performed using U-bends, slow strain rate, and constant extension. The results show that the propensity to SCC, especially its initiation, in thiosulfate is low and is much lower than in tetrathionate. However, when slow strain rate was applied in thiosulfate, or when the pH of the thiosulfate solution was lowered, the specimens cracked readily. Constant extension tests, where the extension was applied using dynamic straining with the solution present, caused cracking much faster than in U-bends, suggesting that U-bend cracking is initiation controlled. Temperature accelerated the crack propagation significantly and seemed to reduce the threshold stress for cracking. Flat coupons of the sensitized metal developed pits in a short period of time in tetrathionate and low pH thiosulfate, but not in neutral thiosulfate; pits sometimes act as precursors to SCC in this system.