The formation of oxide film on the surface is expected to affect the mechanical behavior of the materials 41, 42. 40 performed sCO 2 corrosion tests on high-Ni alloys pipes that were internally pressurized, and found that the internal oxidation rate was higher due to the internal pressure. 39 assessed the SCC of 316 and alloy 230 in sCO 2 using U-bend samples, and reported that the stress acting on the U-bend did not promote SCC nor significantly change the oxidation products. It is reported that sCO 2 accelerated the cracking of materials at constant load 38. In practice, chemical corrosion and mechanical stress are acting on the materials simultaneously, which may result in the stress corrosion cracking (SCC) of the materials and plays one of the most significant roles in component failures of light water reactors (LWRs) 37. Few mechanical tests on AFA steels have been carried out in sCO 2. However, the early research mainly focused on the general corrosion behavior of AFA steels, the mechanical property of AFA steels was mostly tested in air 19, 20, 21, 22, 23, 24, 36. The general corrosion resistance of steels was also enhanced in the lead-bismuth eutectic with the increasing of Al addition, while the continuous Al-rich oxide film formed only when the oxygen concentration was low 34, 35. Moreover, with the addition of Al, the formation of Ni-Al phases 20, 21, 22, 23, 24, 32, 33 in the materials also increases the creep strength of materials, which future improves the application potential for AFA steels in high-temperature sCO 2 environment. The oxide film of AFA-OC6 in sCO 2 was mainly composed of thin and continuous Al 2O 3 and (Cr, Mn) 3O 4 at low temperatures or after short exposure time, while the oxide film showed a complex multilayer structure as the temperature and exposure time increased 31.
30 compared the CO 2 pressure compatibility of several commercial Fe- and Ni-based structural alloys, and found that the mass gain of Al-containing materials was the lowest. Alumina (Al 2O 3) owns a lattice of corundum type, which is the same as Cr 2O 3, while the thermodynamic stability of Al 2O 3 is higher 22 and is expected to offer better protection to the materials 27, 28, 29 exposed to high temperature and corrosive environment. Previous research showed that the mass gain of AFA steels in 800 ☌ air 25 and supercritical water 26 is quite low for the reason that a continuous Al 2O 3 layer was formed. Thus, the alumina-forming austenitic (AFA) stainless steels that initially developed to improve the creep resistance 19, 20, 21, 22, 23, 24 have attracted more and more attention. To solve this problem, a material that not only owns high oxidation resistance in sCO 2, but also keeps the advantages of easy processing and low cost, is needed. Large area spallation of oxide film and many porosities were observed on the surface of 310 and 316 stainless steels exposed to sCO 2 for only 500 h 10, which cannot meet the requirements for the applications in sCO 2-cooled nuclear reactor, especially the cladding materials. But the stability of these Cr oxide films in high-temperature sCO 2 is still insufficient 10, 16, 17, 18. For the austenitic stainless steels and F/M steels exposed to low-temperature environment (such as subcritical water), Cr 2O 3 and Cr-containing oxide layers are formed on the surface, which plays the most important protective role 15. The thickness of oxide film on T22 steel was beyond 32 μm after 200 h exposure in 550 ☌ sCO 2 9. High-temperature corrosion resistance of F/M steels is poor 14. Among them, Ni-based alloys have high radioactive residue, but their economic cost is too high to be applied on a large-scale 13.
However, the failure of materials under the operating environment has gradually become one of the key issues that limit the development of the sCO 2 system 8.Ĭurrently, the conventional structural and cladding materials that may be used in sCO 2 cooled nuclear reactor mainly include ferritic/martensitic (F/M) steel 9, austenitic stainless steel 10, 11, and nickel-based alloy 12. The sCO 2 cooled nuclear reactor has become one of the most promising Generation IV nuclear reactors 3, 4, 5, 6, 7. With the advantages of high compact ability, good compressibility, and high heat transfer efficiency 1, 2, supercritical carbon dioxide (sCO 2) has been considered as a potential fluid for different energy systems, such as nuclear reactors.
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