Abstract
To clarify the caustic Intergranular Stress Corrosion Cracking (IGSCC) mechanism of alloy 600, some metallurgical factors were extracted from the various IGSCC contributing factors, and several kinds of experimental studies were conducted. Caustic IGSCC susceptibilities of mill-annealed alloy 600 (600MA), sensitized one (600FS) and thermally treated one (600TT) which have different grain boundary characteristics were examined using U-bend, C-ring and SSRT tests. Grain boundary characteristics, such as carbide precipitation, impurity segregation, intergranular slipping behavior, and electrochemical dissolution, were examined using Auger Electron Spectroscopy (AES), Transmission Electron Microscopy equipped with Field Emission Gun, and electrochemical measurements, respectively. The C-ring study showed that the caustic IGSCC resistance were improved with an increase in aging time. From the AES and U-bend studies, it was revealed that the intergranular segregations were sufficient, but not necessary for the caustic IGSCC. The slips at the grain boundary were easy to detect in the alloy 600MA and alloy 600FS, while the formation of slips were suppressed in the 600TT material. Electrochemical study showed that the chromium dissolution was increased with the increases in temperature and pH under the caustic condition. From the above test results, it was concluded that the slipping behavior, the chromium dissolution of intergranular carbide and intergranular purification due to the incorporation of impurities into the intergranular carbide could work together as the contributing factor to caustic IGSCC.
