Effect of low oxygen concentration on the oxidation behavior of Ni-based alloys 625 and 825 in supercritical water

•Oxidation of alloy 625 and 825 in supercritical water containing 5000 ppm oxygen.•Pitting corrosion was found on surfaces of both alloy 625 and 825.•The oxide scales on alloys composed of Fe2O3, NiFe2O4 and NiO.•Transpassive dissolution occurred and Cr was oxidized to its hexavalent compound.•Penetration rate of alloy 625 and 825 was 28 μm/year and 31.5 μm/year, respectively.

In this study, the oxidation behavior of Ni-based alloys 625 and 825 in supercritical water containing 5000 ppm of dissolved oxygen at 500 °C was investigated by scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray diffraction, and laser Raman spectroscopy. Alloy 625 and 825 showed a pitting-like phenomenon after 500 h of exposure. The structure and composition of the oxides formed on alloy 625 and 825 were similar, namely, an outer layer of Fe oxides, spinel structures, and Ni(OH)2; and an inner NiO layer. The mechanism for the formation of the oxide films was proposed, and the factors affecting corrosion of Ni-based alloys were discussed in detail. In addition, the corrosion resistance performance of alloy 625 and 825 was compared.

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