Characterization of oxide layer and micro-crack initiation in alloy 316L stainless steel after 20,000h exposure to supercritical water at 500°C

Corrosion behavior of alloy 316L stainless steel capsule was studied by exposure to the supercritical water (SCW) at 500°C and 25MPa for 20,000h. The microstructural observations have been conducted on the cross section of the exposed surfaces to the SCW to perceive the internal oxidation of the grains and/or grain boundaries. Transmission electron microscopy (TEM) observations as well as elemental analyses such as energy dispersive spectroscopy (EDS) and Electron energy loss spectroscopy (EELS) were used to study the internal oxidation and micro-crack initiation on the surface. Elemental analyses indicated that long-term exposure to the SCW resulted in formation of scales identified as Fe3O4 (outer layer), Fe-Cr spinel/(Fe,Ni)Cr2O4/(Mn,Cr)2O3/SiO2 (inner layer) on the substrate, and Ni-enrichment (chrome depleted region) in the alloy 316L. Micro-crack initiation was observed ahead of the oxidized grain boundaries in which elemental enrichments happened ahead of the crack tip. The relevance of the observed oxidation phenomena on the crack susceptibility of Alloy 316L was discussed. Finally, prolonging the exposure time up to 20,000h has shown that the alloy 316L might be susceptible to micro-crack initiation in the supercritical water.