Oxidation of 9Cr oxide dispersion strengthened steel exposed in supercritical water

The oxidation behavior of a 9 at.% Cr oxide dispersion strengthened (ODS) ferritic/martensitic (F/M) steel exposed to supercritical water at different application temperatures was examined. For comparison, two non-ODS F/M steels HCM12A (∼12 at.% Cr) and NF616 (∼9 at.% Cr) were also examined. The oxidized samples were characterized using gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction, electron back-scatter diffraction and transmission electron microscopy/selected area diffraction. A lower weight gain was consistently observed in 9Cr ODS steel at 500 °C and 600 °C. During exposure, the formation of an internal oxidation layer in the 9Cr ODS steel played a key role in establishing the oxidation behavior. In the 600 °C exposure, grain boundary diffusion of cations may no longer proceed dominantly as it did in the 500 °C exposure. Volume diffusion was likely accelerated and the bulk grains became a more important path for element migration, and the benefit from a small quantity of yttrium (0.28 wt%) in the steel became limited. However, because of the fine grain size of the 9Cr ODS steel, Cr quickly segregated to the ferritic grain boundary region in the internal oxidation layer and especially to the internal oxidation layer/base steel interface, resulting in the formation of Cr-enriched spinel ribbons and a Cr-enriched continuous spinel layer. The microstructure that developed slows down the further diffusion of both cations and anions at 600 °C.