Oxidation behavior of iron-based alloy HCM12A exposed in supercritical water

Supercritical water (SCW) is recommended for applications in contemporary and future power plants due to its high thermal efficiency and subsequent low emissions of deleterious combustion gases. One of the properties of most concern for materials selection in the SCW environment is oxidation resistance. The oxidation behavior of ferritic/martensitic alloy HCM12A exposed in SCW at 500 °C, 25 MPa, and two levels of oxygen partial pressure were investigated in this study by means of gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD). In addition to the spinel and magnetite formed on all of the samples, a thin layer of hematite containing a small amount of Cr was only observed on high-oxygen exposed samples. Oxygen partial pressure effects on oxide structure and porosity in oxide scale are rationalized in terms of thermodynamics and diffusion mechanism. This study indicates a desirable continuous protective oxide layer structure with limited pores may be obtained by controlling the oxygen partial pressure.