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 SCCresistant 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 hightemperature 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.
Austenitic stainless steels are widely used in primary circuits of PWR plants owing to their satisfactory corrosion performance at high temperature. However, both field experience and laboratory testing have shown that these materials are not immune to degradation issues such stress corrosion cracking (SCC). Cold-work was also found to be a contributory factor to SCC susceptibility.1
