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To give a basic understanding of the experimentally observed difference between Cr2O3 and Al2O3 scales on carbon permeation, we employed the first-principles calculation methods to predict atomistic structures, formation energies, and carbon binding energies of Σ11 10 1 ¯ 1 / 10 1 ¯ 1 ¯ tilt grain boundaries (GB) in both α-Al2O3 and α-Cr2O3 with a corundum structure. Owing to different surface terminations, we predicted two distinct kinds of stable atomistic structures for the GB: one with measurable voids and high formation energy, and the other with a compact interface and low formation energy. The predicted GB structures agree with experimental images. No significant structural difference was found for the same GB in α-Al2O3 and α-Cr2O3. Moreover, we predicted that atomic carbon would bind to the Σ11 10 1 ¯ 1 / 10 1 ¯ 1 ¯ GB in α-Cr2O3 appreciably more strongly than in α-Al2O3. Therefore, our computational results suggest that chemical affinity rather than geometric structure of the GBs is related to different carbon permeation behaviors in Al2O3 and Cr2O3 scales. [ABSTRACT FROM AUTHOR]

The microstructure of the oxides formed on F91 steel upon breakaway oxidation in air and CO2 at 700 °C was characterized and compared. It was observed that the Cr-rich corundum layer became humped and disrupted upon breakaway oxidation in air in the first stage. In contrast, spinel was found to form in CO2 without any humping of the Cr-rich layer upon breakaway oxidation. Whether or not the humping and disrupting of the Cr-rich layer were present was related to the form of oxide. Specifically, corundum oxide was facile in air, whereas spinel oxide was facile in CO2. [ABSTRACT FROM AUTHOR]

Upon exposure to CO2 at 550 °C, F91 tends to form rapidly growing scales consisting of an outer Fe oxide and an inner Fe-Cr spinel oxide. A comparative study has been carried out between the pre-oxidized and non-pre-oxidized F91, to determine the influence of pre-oxidation upon the oxidation behavior of F91 in CO2. Formation of a rapidly growing scale and carburization could be inhibited by a pre-oxidation treatment in air prior to oxidation in CO2. Although during exposure to CO2, a fast growing scale still would form locally, pre-oxidation changed its structure. Effects of pre-oxidation time on the oxidation resistance in CO2 are investigated. [ABSTRACT FROM AUTHOR]

The oxidation behavior of three commercial superalloys, CrMoV, F91 and Mar-M247, was studied at 550 °C in laboratory air for 1000 h. Mar-M247 superalloy showed the best oxidation resistance, which is attributed to the formation of a scale rich in Cr2O3 and Al2O3, followed by F91 and CrMoV. A thick duplex oxide formed on CrMoV alloy and spallation was observed. The results for CrMoV alloy showed that calculated Fe diffusion in magnetite was 200 times faster than literature values for Fe diffusion in Fe3O4, which is attributed to grain-boundary diffusion and the effect of impurity on diffusion. F91 initially formed a protective chromium-rich oxide layer followed by formation nodules, leading breakaway oxidation. The oxide nodules consisted of a duplex structure with different morphologies and oxide phases from duplex oxide scale in CrMoV. [ABSTRACT FROM AUTHOR]