Effect of Cr and Ni on Stress Corrosion Cracking Susceptibility in Ni-Cr-Fe Alloys Under Simulated Pressurized Water Reactor Primary Conditions

The primary water stress corrosion cracking (PWSCC) susceptibility of Ni-based alloys, such as Alloy 600 (UNS N06600), has been much higher than that of Fe-based alloys, such as Type 316 stainless steel (UNS S31600). This phenomenon is generally recognized; however, the effects of Fe, Ni, and Cr are not fully understood. In this study, the effect of Cr and Ni in Ni-Cr-Fe alloys on PWSCC susceptibility was investigated using alloys with a wide range of Ni content. The role of these elements is discussed from the standpoint of grain boundary oxidation. According to the stress corrosion cracking (SCC) test results, high-Ni alloys with low Cr contents had high SCC susceptibility, and the SCC susceptibility in high Ni-content alloys was improved by increasing Cr content. On the other hand, low Ni-content alloys had low SCC susceptibility, irrespective of Cr content. These features agree with the field experience in pressurized water reactor plants. In comparison with the results of SCC tests and immersion tests, the effect of Cr and Ni on SCC susceptibility was consistent with that of these elements on grain boundary oxidation. This suggests that grain boundary oxidation plays an important role in PWSCC.