Understanding the surface oxide evolution of T91 ferritic-martensitic steel in supercritical water through advanced characterization

Abstract

The surface oxide scale formed on T91 steel after exposure to deaerated supercritical water (SCW) at 600 °C for 100–1500 h are characterized in detail to reveal the oxide evolution over time. A triplex structure with outer, inner and internal oxide layers has been confirmed. The thickness of each oxide layer increases with the exposure time following the power law kinetics. The different oxide phases in each oxide layer are the results of the decreasing partial pressure of oxygen inwards the depth of oxide scale. The inner and internal oxide layers are not uniform in Fe and Cr contents. The internal oxide layer consists of Cr-rich oxide precipitates surrounded by metal matrix in the grain interior, which could be explained by the internal oxidation mechanism. After a further long-time exposure, a continuous thin Cr-rich spinel oxide layer is formed at the internal oxide layer-metal matrix interface, which is the result of transition from internal to external oxidation when the outward diffusion of Cr exceeds the inward diffusion of oxygen. The inner oxide layer consists of Cr-rich spinel oxide precipitates surrounded by Fe-rich spinel oxide. The formation of the inner oxide layer is the result of further oxidation of the internal oxide layer where the metal matrix in the grain interior is oxidized into Fe-rich spinel.