Supercritical water is the state of water when the temperature is above 374.15oC and the pressure is above 22.1MPa. Supercritical water oxidation (SCWO) can decompose and oxidize organic wastes into nontoxic products very quickly and thoroughly using supercritical water that becomes completely miscible with organics and O2. SCW coolant is a choice for fossil fuel power plants and nuclear reactors because of its large specific votume, good thermal conduction and high thermal efficiency. The SCW equipment serves in high temperatures and high pressures environment, so the severe reactor corrosion has become one of the key problems hindering the industrial application of SCWO. The corrosion experiments in SCW were conducted in self-developed SCW equipments. In this study, the corrosion behavior and the characterization of oxides grown on several alloys in SCW were investigated using scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction analyzer (XRD), X-ray photoelectron spectroscopy (XPS) combining with the calculation of SCW properties. The possible corrosion processes and mechanisms are also discussed.
The chosen alloys for study are (1) 316 stainless steel: it is cheap and has been widely used in industry and also in high temperature and pressure water environments as structural material; (2) nickel-based alloy 625: it is commonly used for constructing the SCWO systems and it is candidate material for SCW nuclear reactor and SCW coal gasification due to its excellent anti-oxidation against high temperature and high pressure environments; (3) ferritic/martensitic steel P92: it is used in main steam tubes of SCW fossil fuel power plants due to its anti-fatigue and anti-oxidation.
The main conclusions are as follows:
1. The SCW static equipments were designed and constructed which could operate at 700oC and 35MPa; the continuous SCWO equipment was upgraded which operated at 600oC and 30MPa.
2. The mass gain, morphologies and chemical compositions of oxides grown on 316 stainless steel exposed to 2% H2O2-containing sub-critical (350oC) and supercritical (400oC, 450oC, 500oC) water were investigated using the continuous SCWO equipment. The thickness of the oxides in SCW were between 0.6 μm and 2.5 μm and the structures were multi-layer consistent with the (Fe,Cr)2O3/Fe3O4+FeCr2O4/Cr2O3/Ni-enrichment/316 SS from the outer to inner layer. Ni enrichment was observed at the oxide/metal interface, especially at higher temperatures. TEM analyses showed that the outer oxide layer was (Fe,Cr)2O3 and the Cr content in (Fe,Cr)2O3 was enriched in the inner area of the oxide scales; the inner oxide layer was Fe3O4+FeCr2O4. The Fe-enriched outer layer was formed as a result of the Fe outward diffusion while the Cr-enriched inner layer was formed as a result of the O inward diffusion. α-Fe2O3 and Cr2O3 have the same space group and similar lattice constant, so α-Fe2O3 could contain some Cr and become (Fe,Cr)2O3 in the outer layer. The addition of Cr into Fe3O4 tends to transform the inverse spinel structure to the spinel structure. The FeCr2O4 spinel is much denser than the Fe3O4, which is believed to provide better oxidation resistance than the porous Fe3O4 layer.
3. The mass gain, morphologies and chemical compositions of oxides grown on nickel-based alloy 625 exposed to 400oC-500oC SCW were investigated using the continuous SCWO equipment. The oxides grown on nickel-based alloy 625 in 400oC-500oC SCW was thinner (0.72 μm-1.42 μm) and more protective compared with 316 stainless steel. The mass gain of alloy 625 was small once the oxides were formed. The analyses of the oxides grown on alloy 625 in SCW at different temperatures showed that the outer and inner oxide layer was Ni-enriched oxides and Cr-enriched oxides respectively. Multilayer structure was identified to consist of Ni(OH)2/NiO/NiCr2O4/Cr2O3/alloy 625 from outer to inner layer. The outer layer grew by dissolution and precipitation mechanism, while the Cr-enriched inner layer was formed by oxygen diffusing inward.
4. The corrosion behavior of P92 in 400oC-600oC and 25 MPa deaerated SCW after 500 h oxidation was studied using SCW static equipments. The mass gain diagram of P92 indicated a parabolic oxidation law. The thickness of the oxides grown on P92 were 3.8 μm (400oC), 11.2 μm (500oC), 18.3 μm (550oC) and 45.3 μm (600oC). TEM analyses showed that the outer layer was Fe3O4 and inner layer was Fe3O4+FeCr2O4 at 400oC SCW after 500 h oxidation. The element W in P92 became W3O8 and embedded in Fe3O4, which may produce detrimental effect to the oxidation resistance of P92. The oxide was thick (45.3 μm) at 600oC and resulted in cracks at the surface. The mass gain at 600oC was larger than those at 400oC-550oC because the cracks became effective channels for oxygen inward diffusion. The outer large Fe3O4 oxides may grow from the small Fe3O4 oxides at the outer/inner layer interface through Fe outward diffusion while the inner layer Fe3O4+FeCr2O4 was formed by oxygen inward diffusion.
5. The SCW properties and the effect of SCW pressures on the metal corrosion rate were studied through the calculated data. The effect of the SCW temperature and pressure on the dissociation of HCl was also studied. Increasing the pressure will increase the metal corrosion rate in sub-critical and supercritical water. At the area near the supercritical point, the effect of pressure on the corrosion rate was greater. The dissociation constant of HCl decreased when the SCW temperature increased or the SCW pressure decreased, which would decrease the corrosion.
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