Development of In-Situ Monitoring Technique on a Crack of Steam Generator Tubing: The Effect of Thermal Treatment on the SCC of Alloy 600

TT (Thermal Treatment) for mill annealed Alloy 600 highly improves the resistance against IGSCC (Intergranular Stress Corrosion Cracking) in caustic solution, but its treatment changes the microstructure. The change of microstructure can affect the SCC resistance in various corrosion environments. This kind of corrosion can be monitored using an electrochemical noise measurement. Therefore, in this study, the effects of heat treatment on SCC resistance in Alloy 600 have been evaluated 0.1M Na2S4O6 at 25 °C from electrochemical noise measurement, and quantification has been tried for resistance against crack formation. While there was no generation of the stress corrosion crack of Alloy 600MA by reverse U-bend method in 0.1M Na2S4O6, the stress corrosion crack was generated in sensitized Alloy 600SE, SE+, and TT materials. The cracking could be detected using an electrochemical noise monitoring. Also, if the aging heat treatment is performed, the stress corrosion crack resistance is reduced. In the case of this behavior, it is analyzed that the stress corrosion crack resistance is closely related with the grain boundary corrosion rate based on the results of the micro-structural analysis for the chromium carbide distribution, the DL-EPR method, and the measuring of the grain boundary corrosion rate through the modified Huey test.