Under the primary conditions of a pressurized water reactor, the intergranular stress corrosion cracking susceptibility of a Ni based alloy is improved by chromium carbide precipitation in grain boundaries. The effects of chromium carbide precipitation treatments are explained by the grain boundary strengthening mechanism, the intergranular corrosion prevention mechanism, and so on. However, few studies have demonstrated these mechanisms, and the effects on intergranular stress corrosion cracking susceptibility are not completely understood. Therefore, for the purpose of demonstrating the change in the internal stress distribution with or without grain boundary carbide precipitation treatment in the 600 alloy, in situ measurements were performed in this study using the X-ray diffraction technique with the BL-28B2 beamline at the SPring-8 synchrotron radiation facility under tensile stress. The results confirmed that there is a difference in the strain distribution in grain boundaries and trans-grains. Intergranular stress corrosion cracking is suppressed by preventing stress concentration in grain boundaries owing to the carbides precipitated at the grain boundaries.
The effect of Cr and Ni on Stress Corrosion Cracking (SCC) susceptibility was studied in non-sensitized materials under simulated BWR condition. SCC tests were conducted by creviced 4 bent beam test as static strain method, and Slow Strain Rate Test (SSRT) as dynamic strain method. Cr affected markedly to prevent SCC in both SCC tests. Ni was effective to reduce SCC susceptibility under the dynamic strain test, however, not clear to affect SCC susceptibility under the static strain test. The effect of Cr was discussed for corrosion resistant film by examining strain electrode behavior and corrosion products. With increasing the Cr content in material, the repassivation behavior was improved and Cr concentration in inner side of corrosion product increased. These results suggest that Cr affects to suppress SCC due to improve corrosion resistance by Cr concentrated corrosion film.
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