Naiqiang Zhang

Oxidation of low-alloy steel in high temperature steam and supercritical water

Abstract The oxidation behaviour of a low-alloy steel T22 was investigated by exposure to ordinary pressure steam (0.1 MPa), high-pressure superheated steam (8 MPa), and supercritical water (SCW) (25 MPa) at 600 °C for 1000 h. The results show the exposure pressure has a significant influence on weight gains and oxide scale morphologies. All the samples approximately obey near-parabolic oxidation kinetics. Some pores are found on the samples exposed to steam while severe cracks are formed in SCW. Further, the influence of steam pressure on the oxidation process is discussed in detail.

Oxidation and chromia evaporation of austenitic steel TP347HFG in supercritical water

ABSTRACT The evaporation of protective chromia scales is expected to be a primary oxidation mechanism. An oxidation test of austenitic steel TP347HFG in supercritical water was carried out at 550 and 600°C under 25 MPa with a dissolved oxygen content of 2000 ppb. The results show that the protective chromia scale forms at the earlier stage and then disappears gradually with the increasing exposure time due to chromium evaporation in the presence of both dissolved oxygen and supercritical water. The effect of chromia evaporation on oxidation resistance of austenitic steels in supercritical water is discussed.

Oxidation behaviour of Nimonic 263 in high-temperature supercritical water

ABSTRACT 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 TiO2 and an inner layer of a mixture of NiCr2O4 and Cr2O3 while the innermost layer is made up of Cr2O3. The MoO3 can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.