Oxidation behavior of ferritic-martensitic and ODS steels in supercritical water

Ferritic-martensitic and ODS alloys are primary candidates for application as cladding and structural materials in the Generation IV Supercritical Water Reactor. One of the main in-service degradation mechanisms for these alloys is uniform corrosion. This article analyzes the oxide microstructure formed on these alloys to better understand their oxidation behavior. Corrosion tests were performed in both steam and supercritical water (SCW) at 500 and 600°C. The oxide microstructure was analyzed using microbeam synchrotron radiation diffraction and fluorescence associated with electron microscopy. The oxide forms a three-layer structure with an outer layer containing only Fe ₃O₄, an inner layer containing a non-uniform (Fe,Cr) ₃O₄ spinel structure, and a diffusion layer containing a mixture of metal grains and chromium-rich precipitates. A marker experiment located the original water-metal interface as the outer-inner layer interface implying a mechanism where iron migrates outwards to form the outer layer and oxygen diffuses inwards to form the inner layer.