The corrosion behavior of a ferritic/martensitic steel P92 exposed to supercritical water (SCW) at 500-600 degrees C and 25 MPa was investigated by means of gravimetry, scanning electron microscope/energy dispersive X-ray spectroscopy and X-ray diffraction. A dual-layered oxide scale, which was mainly composed of an outer magnetite layer and an inner magnetite/spinel-mixed layer, formed on P92. The initial oxide scale was rather porous, while the porosity decreased with an increase of exposure time. Oxidation rates at three different temperatures followed the parabolic law. The oxidation at 600 degrees C was so severe that cracks occurred along grain boundaries in the oxide scale. A probable corrosion mechanism for P92 exposed in SCW was proposed based on the above observations, focusing on oxide formation by oxygen absorption without any metallic dissolution. (C) 2009 Elsevier B.V. All rights reserved.