Microstructures of chromia scales grown in CO₂

Abstract

Scales grown on chromia forming alloys in CO 2 are less protective than those developed during exposure to oxygen or air. Reaction with CO 2 leads to faster chromia scale growth, the more rapid onset of breakaway and internal carburisation of the alloy. Conventional and laser Raman microscopy studies of Fe-Cr alloys show that local fluctuations in scale thickness are associated with varying degrees of chromium depletion. Local conversion of Cr 2 O 3 to spinel leads to rapid outward iron diffusion and nucleation of Fe-rich oxide nodules. A TEM investigation reveals that reaction in CO 2 produces finer grained Cr 2 O 3 , inward scale growth and more rapid scaling, as well as internal carburisation. Appropriate silicon additions to the alloys lead to the formation of a thin, glassy silica layer beneath the chromia, greatly slowed chromia thickening rates and the prevention of carburisation. Atom probe tomography is used to locate carbon within the chromia.

Effect of pressure on supercritical CO2 compatibility of structural alloys at 750 °C

“…Based on these results, an initial hypothesis that pressure plays a limited role can be tested by conducting long‐term experiments at 1 and 300 bar CO 2 . If behavior is similar in these conditions, the large compatibility database generated for the development of CO 2 ‐cooled nuclear reactors (1–20 kh exposures) can be utilized as well as the extensive work at 1 bar for a variety of applications including solid oxide fuel cells and oxy‐fired coal combustion . These 1 bar data sets including combinations of CO 2 , H 2 O, and O 2 are particularly useful for evaluating the compatibility for the direct‐fired systems where high levels of O 2 and H 2 O are expected and laboratory testing has not yet been developed to control impurities at >100 bar.…”

“…One of the key issues to be further explored is the possible ingress of C or carbonaceous species through the external scale and whether CO 2 pressure affects this transport. The explanation for the internal attack observed in CO 2 is that C penetrates the scale (likely by grain boundary transport ) and the C activity increases towards the metal‐scale interface due to the decreased O activity at this interface where Cr/Cr 2 O 3 or Al/Al 2 O 3 equilibrium exists . In an earlier study, C uptake was measured chemically to quantify internal carburization .…”

“…However, there is concern that Ni–Cr superalloys also could be internally carburized by the high carbon activity expected at the metal‐scale interface due to the low equilibrium oxygen partial pressure at that location . The goal of this initial assessment was to study the effect of CO 2 pressure on the short‐term oxidation behavior at 750 °C to determine if there was a discernable pressure effect that may allow the considerable understanding developed at lower CO 2 pressures to be used to understand and model long‐term sCO 2 behavior. These 500 h observations suggest little effect of pressure for the group of Fe‐ and Ni‐base alloys studied and that the reaction products formed in CO 2 at 125–300 bar are very similar to those formed in 1 bar CO 2 and dry air at 750 °C.…”

Corrosion Behaviors of Heat-Resisting Alloys in High Temperature Carbon Dioxide

“…Recently, as shown in Figure 17, David J. Young et al [101][102][103] analyzed the Cr 2 O 3 layer on the surface of Fe20Cr model alloy by atomic probe and found that carbon was enriched in the sub-monolayer level of grain boundary, which was supposed to be in the form of molecular oxide. In this form, moving is possible, and the penetration through oxide scale leads to carburization of the underlying alloy.…”

High-temperature corrosion resistance of nickel-base alloy 617 in supercritical carbon dioxide environment

“…Therefore, research on the corrosion performance of structural materials is meaningful and urgent. Research was conducted on high-temperature corrosion of 9%-12%Cr and austenitic heat-resistant steels and nickel base alloys in various environments [3][4][5][6][7][8][9][10][11][12]. Saunders reviewed the effect of water vapor on the corrosion behavior of steel and metal [13].…”