An investigation of SCC susceptibility of virgin and ruthenium-modified austenitic stainless steels in high-temperature oxygenated water by SSRT testing

Stress corrosion cracking (SCC) of austenitic stainless steels is a significant degradation issue affecting components within light water reactors (LWRs) and therefore the deployment of SCC-resistant materials is highly desirable. However, whilst new alloys would have notable cost implications and require re-approval, minor changes would allow them to remain within specification and avoid such issues. In this programme the effect of minor alloying ruthenium additions on stress corrosion cracking (SCC) susceptibility of austenitic stainless steels has been investigated in high-temperature high pressure oxygenated water. Slow strain rate tensile (SSRT) tests were performed on virgin and ruthenium-modified (1 wt. % Ru) 304 stainless steels that were either only sensitised or sensitised and subsequently 20 % cold-worked. Both Ru-doped 304SS and the base alloy exhibited intergranular SCC (IGSCC), however, the ruthenium modified alloys were less susceptible as indicated by lower SCC indices. With respect to the cold-worked specimens, the mean crack growth rate for the ruthenium alloys was typically half the value recorded for 304SS and both alloys showed evidence of both IGSCC and transgranular SCC (TGSCC). The Ru-doped alloy displayed higher levels of TGSCC. There is some evidence that, in high temperature oxygenated water, the cracks undergo a transition from intergranular (IG) to transgranular (TG) prior to plastic failure and that the crack path is dependent on grain morphology. The results obtained demonstrate an improved performance of the Ru-doped 304 SS towards SCC susceptibility.