Microstructural examination of oxide layers formed on an oxide dispersion strengthened ferritic steel exposed to supercritical water

A transmission electron microscopy study was carried out on the oxide layers formed on a 9Cr oxide dispersion strengthened (ODS) ferritic steel after exposure to supercritical water at 500 °C with a 25 ppb dissolved oxygen concentration. Compared with the oxide layers formed on a conventional 9Cr steel NF616 ferritic steel with a similar major element composition and exposed to similar test conditions, the 9Cr ODS steel exhibited a different oxide structural morphology and growth mechanism. The most pronounced difference was the development of a substantially thick internal oxidation layer for the ODS steel that is associated with its finer grain size. For the oxidized ODS steel, yttrium and chromium rich oxide ribbons formed at the steel grain boundaries in the internal oxidation layer. The improved overall corrosion resistance of 9Cr ODS steel at elevated temperatures, as measured by weight gain data, may be a result of yttrium acting as a barrier for cation diffusion along oxide/metal grain boundaries. The gradual decrease in scale density with increasing exposure times appears to be caused by depletion of iron, which resulted in more vacancies and pores in the inner (FeCr)3O4 spinel layer.