Investigation of the Sulfur and Lithium to Sulfur Ratio Threshold in Stress Corrosion Cracking of Sensitized Alloy 600 in Borated Thiosulfate Solution.

摘要

The stress corrosion cracking (SCC) of sensitized Alloy 600 was investigated in aerated solutions of sodium thiosulfate containing 1.3% boric acid, using U-bends, constant load, and slow strain rate tests. The aim of the investigation, among others, was to determine the existence, if any, of a threshold level of sulfur, and Li to S ratio governing the SCC. For U-bends, 5 ppm Li as LiOH in the presence of 7 ppm S as thiosulfate prevented occurrence of SCC. However, in slow strain rate tests, significant SCC occurred at a S level of 30 ppb in the presence of 0.7 ppm of Li. For a specimen held under constant load, a propagating crack continued to grow until fracture during controlled progressive dilution of the bulk solution, leading to final Li concentration of 1.5 ppm and S concentration of 9.6 ppb respectively. The implications of the results to initiation and propagation of SCC in aerated thiosulfate solutions, and their relevance to future operation of the steam generators at Three Mile Island Unit 1 (TMI-1) are discussed.