Microstructure and chemical composition of the oxide scale formed on the sanicro 25 steel tubes after fireside corrosion

•Cr2O3 scale having tens nanometer sized grains was formed on Sanicro 25 at 650 °C.•M23C6 carbides was thermodynamically unstable in Cr-depleted zone. Beneath the Cr2O3 a layer with short-range ordering typical for glass was formed.•Nanometer sized Cu-rich particles were precipitated from the austenitic matrix.

The Sanicro 25 austenitic steel is foreseen as a potential material for steam superheaters and reheaters in ultra-supercritical power plants. The aim of investigation was to characterize in detail an oxide scale grown on the Sanicro 25 tube after fireside oxidation at 650 °C. The morphology of the oxide scale and the microstructure of the bulk material were investigated using advanced electron microscopy methods. A multilayered and multicomponent oxide scale containing mainly Cr2O3 was found on the surface exposed to combustion gases. Directly under the oxide scale, a chromium-depleted zone with no M23C6 carbides was observed.