Degradation of Alloy 690 After Relatively Short Times

The purpose of this work is to understand quantitative processes which underlie the initiation of stress corrosion cracking (SCC) of cold-worked, thermally treated Alloy 690 after long-term exposures in high-temperature water. Long-term stress corrosion cracking initiation tests up to 34,484 h have been performed on cold-worked, thermally treated Alloy 690 under static load conditions using 0.5T compact tension specimens with a shallow depth of the precrack (0.1 mm to approximately 0.3 mm) and blunt notch compact tension type specimens. These specimens were exposed in the primary coolant environments of pressurized water reactors under static load conditions both at 320°C and 360°C. Three important patterns were observed. First, intergranular cracking observed from the shallow precracks after 21,838 h at 320°C and 20,653 h at 360°C, respectively, in 20% cold-worked Alloy TT690 in primary water. Second, clear evidence of cavities were identified ahead of the SCC-tip after the tests both at 360°C and 320°C. Cavities seem to result from condensations of vacancies induced by cold work, which were driven by stress gradients judging from the distributions of the population of cavities. Third, oxidation occurred inside the cavities near the SCC-tips before the advance of SCC. Bonding strength at grain boundaries is assumed to weaken as a result of the formation of cavities and oxidation inside cavities during the incubation of long-term SCC. A model is proposed for the initiation of SCC after long-term operation with cold-worked, thermally treated Alloy 690 in high-temperature water, involving the combination of local corrosion and the formation of cavities. These result from the collapses of vacancies, which seem to arise from the initiation of SCC after long times in high-temperature water.