A Tracer Study on sCO2 Corrosion with Multiple Oxygen-Bearing Impurities

Abstract

Several modern power production systems utilize supercritical CO2 (sCO2), which can contain O2 and H2O as impurities. These impurities may degrade the compatibility of structural alloys through accelerated oxidation. However, it remains unclear which of these impurities plays a bigger role in high-temperature reactions taking place in sCO2. In this study, various model and commercial Fe‐ and Ni‐based alloys were exposed in 300 bar sCO2 at 750 °C to low levels (50 ppm) of O2 and H2O for 1,000 h. 18O-enriched water was used to enable the identification of the oxygen source in the post-exposure characterization of the samples. However, oxygen from the water did not accumulate in the scale, which consisted of Cr2O3 in the cases where a protective oxide formed. A 2wt.% Ti addition to a Ni-22%Cr model alloy resulted in the formation of thicker oxides in sCO2, while a 1wt.% Al addition reduced the scale thickness. A synergistic effect of both Al and Ti additions resulted in an even thicker oxide than what was formed solely by Ti, similar to observations for Ni-based alloy 282.

Review on the Corrosion Behaviour of Nickel-Based Alloys in Supercritical Carbon Dioxide under High Temperature and Pressure

“…When the content of O 2 is more than 2%, a thermodynamically stable and continuous chromia layer will be formed in S-CO 2 environments [81]. As for H 2 O, it tends to support the growth of oxides and induce evaporation of the surface oxide layer, which corresponds to the findings by Lehmusto [49].…”

“…Paradoxically, Cr 2 O 3 is supposed to form in the outer layer due to TiO 2 being thermodynamically more stable than Cr 2 O 3 . This is attributed to the lower metal activity of Ti and the higher diffusion rate of Ti ions outward through the chromium layer, thus this phenomenon is reasonable in both thermodynamics and kinetics [49]. In most situations, however, the TiO 2 layer is under the chromia and is able to resist corrosion.…”

“…Thus, studies have been made to explore the effect of impurities on corrosion behaviour of nickel-based alloys with two categories of S-CO 2 : research grade (RG) and industrial grade (IG). The purity of RG and IG conditions are 99.999% and 99.95%, respectively [49]. This section introduces the effect of various impurity contents on the corrosion behaviour of nickel-based alloys.…”

“…Due to the serious effect on the corrosion resistance of the nickel-based alloy that Reaction (7) will bring, some studies on the synergistic effect of H 2 O and O 2 have been conducted [35,49,70]. Oleksak et al [35] suggest that at high pressure, chromium volatilization will occur with the presence of H 2 O + O 2 .…”

“…The light microscopy images of cross section for some alloys are shown in Figure 7. It can be obviously seen that the amount of nodules on the surface and the internal oxidation in the matrix increased with the addition of O 2 and H 2 O. Lehmusto [49] used the method of isotope labeling. He replaced the O in H 2 O with O 18 .…”