Current knowledge and future challenges on SCC in Alloy 690 TT

Stress corrosion cracking (SCC) of Alloy 690 TT in the primary PWR system has been evaluated by reviewing past INSS experiments along with literature data. Although Alloy 690 TT has excellent PWSCC resistance, the degree of cold work and temperature contribute as accelerating factors for SCC propagation. Since Alloy 690 TT has a high chromium content, SCC caused by grain boundary oxidation which is often observed in Alloy 600 is hard to initiate in a short period. Only the condition with dynamic strain reproduces the SCC, which is assumed to be due to the protective oxide film being mechanically fractured and not repaired in time. On the other hand, a high-temperature atmosphere above 410degC reproduced the cracking of Alloy 690 TT, which was accompanied by the formation of cavities. However, extra acceleration factors such as oxidation and hydrogen are necessary for cracking caused by cavity formation during the lifetime of power plants in the service temperature range. The typical influences of corrosion are assumed to be the acceleration of vacancy diffusion by involvement of hydrogen generated as a result of the cathodic reaction. Cavity formation caused by vacancy diffusion during long-time usage is considered to be the major contributor to the SCC initiation mechanism of Alloy 690 TT. To clarify the possibility of crack initiation, quantitative evaluations based on interaction with various influencing factors such as cold work, temperature, stress, carbide, grain size and corrosion are necessary. (author)