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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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Stress corrosion cracking (SCC) behavior of Alloy 617 was studied to evaluate its suitability for Generation IV supercritical water reactor concept. A series of slow strain rate tensile (SSRT) tests at a constant strain rate of 5 × 10−7 s−1 were carried out in superheated steam and supercritical water over a pressure range of 0.1–25 MPa at 650 °C. SSRT test in dry N2 gas was also performed to compare the results in non‐corrosive environment with corrosive medium environment. Intergranular cracks were observed for all specimens regardless of the test environment. Alloy 617 showed susceptibility to SCC at the tested experimental conditions. The dominance of intergranular SCC was observed on the gage surface and fracture surface by using scanning electron microscope analysis. The effect of varying environmental conditions on SCC susceptibility is further discussed.

The oxidation tests of Ferritic Steel T22 exposed to supercritical water (SCW) at 540–620°C and 25 MPa was performed for up to 1000 h. The oxidation rate increased with increasing exposure temperature and time. Oxide films formed on T22 have a double-layered structure with an outer layer consisting of iron oxide and an inner layer consisting of spinel oxide. Numerous pores on the surface can be observed at the initial oxidation stage while they seemed to heal with increasing exposure time at 620°C. Cracks occurred along grain boundaries in the oxide scale when T22 exposed for 200 h at 620°C. The influence of time and temperature on the oxidation of Ferritic Steel T22 was discussed.