A Comparison of Intergranular Attack in Inconel Alloy 600 Observed in the Laboratory and in Operating Steam Generators

Intergranular attack (IGA) has been observed in Inconel 600 tubing on the hot leg of some operating steam generators. IGA has also been reproduced in the laboratory in both high temperature (316-343 C) sodium hydroxide solutions and environments containing sulfur compounds such as polythionic acid and sodium tetrathionate. The degradation morphology and associated chemical species were compared in tubing from two steam generators and tubing exposed to the above laboratory environments. The tubing was subjected to a detailed microstructural analysis. There was a mixture of IGA and SCC in tubing from the steam generators and that exposed to laboratory caustic environments, whereas only IGA was present in material exposed to sodium tetrathionate. Grain boundary carbide precipitation and an associated chromium depleted layer were necessary for IGA in sodium tetrathionate. Such a microstructure was not present in the tubing removed from the steam generators. It was concluded that, of the two environments evaluated, caustic was more likely to have instigated IGA in the steam generators.