Internal oxidation and crack susceptibility of alloy 310S stainless steel after long term exposure to supercritical water at 500°C

We study oxidation and crack susceptibility of alloy 310S stainless steel upon exposure to supercritical water (SCW) at 500°C and 25MPa for 20000h. Electron microscopy observations and elemental analyses are conducted on the static capsule sample to investigate morphological features of the oxidation, micro-crack initiation as well as chemical composition along the micro-crack surface oxides and corrosion products around the micro-crack tip area. The elemental and phase analyses indicate that long term exposure to the SCW resulted in the formation of scales identified as Fe3O4 (magnetite, outer layer), Fe-Cr spinel (FeCr2O4, inner layer), [Mn,Cr]2O3+SiO2 (Cr-rich spinel and silicon dioxide, transition layer) on the substrate, and Ni-enrichment (chrome depleted region) in the alloy 310S. The presence of carbides along with chromium depletion region most probably result in grain boundary embrittlement around the micro-crack tip. Additionally, crack region is filled Cr-rich oxide, SiO2 and lower amount of carbides. We believe that the intergranular crack initiation occurred through grain boundary oxidation, leading to stress concentration. The oxidation mechanism and scales grown on the alloy 310S stainless steel exposed to SCW and micro-crack initiation as the results of long term exposure at high temperature SCW are discussed.