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

The stress corrosion cracking (SCC) of sensitized Alloy 600 was investigated recently in borated sodium thiosulfate solution.¹  The study was undertaken to investigate the mechanisms underlying the SCC of Alloy 600 steam generator tube failures in the Three Mile Island nuclear power plant unit 1 (TMI-1).²  In this investigation, an Alloy 600 tube was directly sensitized from the mill annealed condition (MAS) by a thermal treatment at 621 C for 18 h. Reverse U-bends prepared from this MAS material did not suffer SCC in sodium thiosulfate solution up to 0.1 M concentration and 80 C (with or without boric acid). However, these U-bends cracked readily in a solution containing only a few ppm sodium tetrathionate.¹  The tubes in the TMI-1 steam generators became sensitized as a result of the postweld stress relieving heat treatment given the steam generator vessel with the tubes in place. Therefore, it was...