Effect of Impurity on the High-temperature Corrosion Performance of Alloys in Supercritical CO₂

In advanced power generation systems, the supercritical CO₂ (s-CO₂) Brayton cycle has been developed as a promising technology because of its high efficiency, compactness, and potential to complete carbon capture. Selecting appropriate alloys as the constructional materials is one of the crucial steps to the successful deployment and long-term safe operation of the s-CO₂ Brayton cycle. Current work investigates the corrosion behavior of candidate alloys in s-CO₂ environments with impurities. The corrosion products are characterized and the possible corrosion mechanism in the high-temperature s-CO₂ environment is discussed. Besides, the effect of impurity on the corrosion behavior of alloys is discussed.

Over the years, the supercritical carbon dioxide (s-CO₂) Brayton cycle has been developed as a promising working fluid to replace supercritical water (s-H₂O) Rankine cycle. It could be used in various energy systems, including Generation IV nuclear reactors, concentrated solar power plants, fossil fuel thermal power plants, waster heat recovery, etc. due to its merits of high thermal efficiency, simple physical footprint, compact equipment size, high flexibility on operation, simple layout, compact turbomachinery.¹ By far, several s-CO₂ based power plants are being built at the demonstration level,² and more research is being conducted worldwide to remove the technical barriers concerning characterization on the behavior of s-CO₂, design and operation, material selection, turbomachinery, and so on.³ Among these issues, materials knowledge gaps regarding the selection of appropriate constructional alloys greatly hinder the successful deployment of this attractive technology. The Brayton cycle is operated at high temperatures and pressures, yet the available information is limited to support the selection of appropriate alloys with acceptable long-term compatibility in s-CO₂.

The s-CO₂ Brayton cycles are designed with the operating pressure up to 30 MPa and the working temperature up to 800 °C.4 From corrosion aspects, s-CO₂ streams in such conditions arouse not only oxidation, but also carburization on alloys. Up to now, a lot of Fe- and Ni-based alloys as high-temperature structural components have been studied with wide temperature and pressure ranges, and several crucial aspects that may affect the corrosion process have been investigated.^5-7 The effect of s-CO₂ pressure has been studied by some researchers, and it is generally believed that pressure didn't play a decisive role in the corrosion process. For example, the variation on s-CO₂ pressure from 0.1 to 30 MPa only shows a minimal effect on the corrosion performance of several Fe- and Ni-based alloys after 500 hours of exposure at 750 °C.⁸ It was found that the alloy type and working temperature greatly affect the corrosion process. In one study, stainless steel (AL-6XN) and Ni-based alloys (PE-16, 230 and 625) were exposed to 20 MPa s-CO₂ at 650 °C for up to 3000 h. The results show that stainless steel had much higher corrosion rates than Ni-based alloys.⁷ In another study, ferritic steels (GR91, VM12), austenitic stainless steels (Corfer 22H, 304H, HR3C) and Ni-based alloys (617, 740H) were exposed to 20 MPa s-CO₂ environments, and the corrosion resistance followed the sequence of ferritic steels < austenitic stainless steels < Ni-based alloys.⁹ In addition, an increase of 50 °C on temperature increased the thickness of the formed oxide scale by a coefficient up to 4.⁹