Fundamental Research for Evaluation of IASCC Susceptibility
Yoshimasa Iwabuchi, Takahiro Fujimoto, Yutaka Watanabe, Tetsuo Shoji
In order to ensure a safety margin for crack initiation and growth of LWR internal components, irradiation assisted stress corrosion cracking (IASCC) of austenitic stainless steel has been receiving a particular concern. IASCC is believed to be attributable to radiation induced segregation and/or depletion (RIS) of elements at grain boundaries. From this view point, materials characterization, namely RIS detection and quantitative evaluation of them, can be considered as an essential issue both for an understanding of IASCC mechanism and for evaluation of IASCC susceptibility of internal components. In this study, electrochemical methods for RIS evaluation have been investigated by use of simulated materials in which segregation and/or very narrow chromium depletion was reproduced by controlled thermal treatments. Slow strain rate tests at 288°C were also performed in the simulated LWR environments to evaluate IGSCC (intergranular stress corrosion cracking) susceptibility of those materials directly. A Modified EPR (electrochemical potentiodynamic reactivation) method that can evaluate very narrow chromium depletion with high sensitivity and a potentiostatic electrolysis method that can detect impurities segregation at grain boundaries have been developed. IGSCC susceptibility of the materials seemed to depend both on chromium depletion and on impurities segregation and was able to be evaluated by the above electrochemical methods.