Analyses of oxide films grown on Alloy 625 in oxidizing supercritical water

Morphologies, microstructures and chemical composition of oxide films grown on Alloy 625 at 400 °C, 450 °C and 500 °C in oxidizing supercritical water containing 2.0% H2O2 were investigated using weight measurement, grazing incidence X-ray diffractometry, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. It was found that the mass gain of the alloy in oxidizing supercritical water increased with exposure time. The smallest mass gain was obtained at 450 °C, agreeing well with the smallest oxide film thickness at this temperature. The average thickness of the oxide films was 1.28 μm, 0.72 μm and 1.42 μm at 400 °C, 450 °C and 500 °C, respectively after 250 h exposure. The size of oxide crystals on the alloy surface gradually grew with increasing exposure temperature. A discontinuous and thin Cr2O3 layer was formed at 400 °C in oxidizing supercritical water as a result of leaching of Cr3+ as Cr6+, while a continuous Cr2O3 layer was formed at 450 °C and 500 °C. Duplex oxide layer structure was observed at all three temperatures, which was identified to consist of Ni(OH)2/NiO/NiCr2O4/Cr2O3/Alloy 625 from outer to inner layer. The growth mechanism of oxide films on Alloy 625 in oxidizing supercritical water seems to be similar to that in high temperature water, namely the Ni(OH)2/NiO outer layer growth by dissolution and precipitation mechanism and the Cr2O3 inner layer formation by oxygen diffusing inward and reacting with the retained Cr.