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The oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600-700°C under 25 MPa were carried out for up to 1000 h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni-Fe/Ni-Cr spinel oxide, Ni/Co hydroxide and TiO 2 and an inner layer of a mixture of NiCr 2 O 4 and Cr 2 O 3 while the innermost layer is made up of Cr 2 O 3 . The MoO 3 can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.

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Oxidation tests of austenitic steel Sanicro25 and TP347HFG were implemented at 600–700 °C in supercritical water under 25 MPa without dissolved oxygen. With an increase in temperature, the weight change of TP347HFG steel increases significantly while the weight change of Sanicro25 increases relatively slow. A double layer oxide scale formed on TP347HFG and Sanicro25 steel, which was composed of Fe‐rich oxide layer and a Cr‐rich inner oxide layer. Small amount of Cr2O3 was observed for TP347HFG but a continuous Cr2O3 formed at interface of oxide and matrix for Sanicro25 at 600 and 700 °C. Furthermore, Fe2O3 is detected at 600–700 °C for Sanicro25. The effect mechanism of temperature and the Cr content on oxidation rate and oxide composition were discussed.

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