Investigation of SCC behavior in 309 SS clad DMWs under seawater ingress using novel high-temperature autoclave

In this study, pitting and stress corrosion cracking (SCC) behaviour of 309 stainless steel (SS) cladding on SQV2A low-alloy steel (LAS) was investigated under a simulated sea water intake transient water environment such as high-chloride, high-temperature, and high-pressure (HTHP) conditions, replicating a seawater leak scenario similar to that of the Japanese BWR incidence. A novel autoclave system was developed, employing a loading system by hydraulic pressure difference between 309 SS clad side and LAS side which are separated by O-ring located at the peripheral surface of the disc specimen prepared from DMW. This system can decouple loading system from corrosive environment. The system successfully reproduced the service-like conditions with chloride concentrations reaching beyond 400 mg/L at temperatures up to 250 °C. Pressure difference of 7 MPa caused by 8 MPa pressure of clad side and 15 MPa of LAS side can generate biaxial stresses in the cladding of disc specimens. Post-test analysis revealed significant plastic deformation and extensive interphase cracking within 309 SS cladding. Optical and SEM observations showed cracks predominantly followed ferrite–austenite phase boundaries. Crack depths of 1.17 ± 0.45 mm were statistically uncorrelated with stresses of crack locations across the disc. EDS and DL-EPR analysis indicated minimal chromium depletion, suggesting that electrochemical heterogeneity and phase-boundary dissolution, rather than sensitisation, might be responsible for SCC initiation, even though more detailed sensitisation measurements are required.