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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