Transmission electron microscopy of oxide development on 9Cr ODS steel in supercritical water

Oxide layers formed on 9Cr oxide dispersion strengthened ferritic steel alloys during exposure to 600 °C supercritical water for 2- and 4-weeks were examined using cross-sectional transmission electron microscopy. A focused ion beam in situ lift-out technique was used to produce site-specific samples with electron transparent areas up to 8 μm by 10 μm. The oxide layers consist of several sub-layers: an Fe-rich outer oxide, a Cr-rich inner oxide, and a diffusion layer, extending beyond the oxide front into the metal. An evolution of the oxide layer structure is seen between 2 and 4 weeks, resulting in the development of a band of Cr₂O₃ at the diffusion layer/metal interface from the previously existing continuous mixture of FeCr₂O₄ 'fingers' and bcc metal. It is believed that transport in this Cr₂O₃ layer at the diffusion layer/metal interface becomes the rate-limiting step for oxide advancement, since this change in oxide structure also corresponds to a decrease in corrosion rate.