Analysis of Oxides Formed on the Surface of the Alloy 690 in Hydrogenated Supercritical Water

The oxides formed on the surface of the alloy 690 in hydrogenated supercritical water at 400 °C for 1000 h were investigated using scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. The oxides on the surface of the alloy 690 exhibited multi-layer structure: an outer layer consisted of granular crystallites (NiO and NiFe₂O₄) and a compact inner layer (spinel and Cr₂O₃). Chemical analysis indicated that the outer layer was enriched in nickel but depleted in chromium, whereas the inner layer was enriched in chromium and iron but depleted in nickel. The inner layer was also characterized as layered structure by Fe-rich spinel on top of continuous Cr₂O₃ layer. Besides, Cr₂O₃ nodules were readily observed at the oxides/alloy interface.