Technical Note: Relationship between Chromium Depletion in Sensitized Alloy 600 and U-Bend Cracking in Sodium Thiosulfate Solution

Abstract

In order to further investigate the stress corrosion cracking (SCC) of Alloy 600 steam generator tube failures as in the Three Mile Island nuclear power plant unit 1 (TMI-1), an Alloy 600 tube was directly sensitized from the mill annealed condition (MAS) by a thermal treatment at 621C for 18 h prior to corrosion cracking studies. Reverse U-bends of the MAS material did not suffer SCC from exposure to sodium thiosulfate solutions up to 0.01M concentration and 80C. Results of past experiments have indicated that the U-bends cracked readily in a solution containing a few ppM sodium tetrathionate leading to the conclusion that some reduced species of sulfur could be the cause for SCC of the TMI-1 tubes. Tests were conducted on various specimens of MAS material, and new results indicate that SCC was very rapid and occurred in very dilute levels of thiosulfate. It is also indicated that SCC initiated by thiosulfate probably occurs only after a critical or threshold level of chromium depletion is achieved. The results also most pointedly indicate that processing variations from mill to mill in MAS material cause wide variations in structure and related properties.