Papers by Keyword: Chromia

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Authors: Jing Ming Zhao, Kyu Hong Hwang, Jong Kook Lee, Min Cheal Kim
Abstract: Refractory castables containing calcium aluminate cement (CAC) are widely used in a range of furnace lining applications in the iron and steel, cement, glass, ceramic, and petrochemical industries. However, magnesia-phosphate cement (MPC) based material could be a new types of cement material, with many advantages such as rapid hydration, high early strength and circumstance suitability, which has very important value and wide application. In this study, MPC was used at Chromia/Alumina castable as binder addition instead of conventional calcium aluminate cement. Meanwhile, it also explains the relationship between the micro-mechanism and performance by micro methods such as SEM. The results shows that MPC based castables have good corrosion resistance, interface adhesiveness and abrasion resistance.
Authors: Céline Cabet, Gouenou Girardin, Fabien Rouillard, Jerome Chapovaloff, Krzysztof Wolski, Michèle Pijolat
Abstract: Nickel base alloys Haynes 230 and Inconel 617 are of interest for gas cooled reactors. At high temperature in impure helium, they generally form surface chromium-rich oxides. However above a critical temperature called TA, the scales are not stable anymore and the chromia destruction comes with a production of carbon monoxide. Reactivity tests on model alloys, with and without carbon, prove that chromia is reduced by the carbon from the alloy. TA vs P(CO) curves were also plotted for the two commercial alloys based on the experimental determination of TA in various atmospheres with increasing partial pressures of carbon monoxide. Unexpectedly, both materials exhibit an almost identical behavior although a basic equilibrium approach suggests that the chromia scale would be reduced in different conditions due to the thermodynamic particularity of the interfacial alloy/scale system.
Authors: Norinsan Kamil Othman, N. Othman, Azman Jalar
Abstract: The corrosion of four Fe-Cr alloys (Cr: 9, 12, 20, and 25%) with different Cr contents were subjected to cyclic oxidation furnace in Ar-10%O2 atmosphere at 950oC for 120 h. The samples after oxidation were investigated by using X-ray diffraction (XRD), optical metallography and SEM/EDS. The results indicated that increasing Cr content reduced the oxidation rate and the scale cracking. Alloys with less Cr content occurred breakaway oxidation due to formation of Fe-rich oxide scale. However, a thin protective chromia scale was observed on alloys with Cr > 20%. The corrosion behaviour on Fe-Cr alloy was then discussed based on experimental observation.
Authors: Stephen Osgerby, Hugh E. Evans, Stuart R.J. Saunders
Authors: Sébastien Chevalier
Abstract: High temperature reactivity of materials under oxidizing atmospheres is based on the formation of protective oxide scales. The protectiveness is obtained when the thermally grown oxide scales are dense, continuous and adherent to the metallic substrates (even during thermal shocks); as a matter of fact, the growth of the scale has to be governed by diffusion of species across the growing scale. The diffusing species are coming from the substrate (metallic ions) and/or from the oxidizing atmosphere (oxygen ions). The understanding of growth mechanisms can be reached by making two stage oxidation experiments, using oxygen isotopes. The experiment consists in oxidizing first the metallic substrate in 16O2, evacuating the oxygen after the desired time, and replacing it by 18O2. The distribution of oxygen isotopes given by secondary ion mass spectrometry (SIMS) across the oxide scale informs about the growth mechanisms: anionic transport, cations diffusion or mixed diffusion processes. The use of marker experiment is able to determine the oxygen diffusion coefficients within the growing scales. In this case, a thick scale is grown under 16O2, followed by a shorter diffusion experiment in 18O2. The distribution of 18O isotope across the scale by plotting ln (18O intensity) versus x (depth in oxide) allows determining the oxygen effective diffusion coefficient in the scale, according to the classical Fick’s law solution. A more accurate analysis of these profiles can provide information about bulk and grain boundary diffusion of oxygen. The results can be related to kinetics according to Wagner’s theory. The proposed work consists firstly in making a state of the art review about oxygen diffusion in thermally grown oxide scales, and secondly in connecting the so-obtained outcome (effective, bulk and grain boundary diffusion) to kinetics results. The proposed oxides are chromia, alumina and zirconia.
Authors: Zhao Jing Ming, Yang Zheng Xun, Hwang Kyu Hong, Park Sang Hwan
Abstract: Slagging coal gasifiers operate at temperatures as high as 1650°C in a reducing environment so that combustion chambers are lined with high chromia refractories. The quality of these refractories applied on IGCC should be a key factor that affects the entire cost of electric power production. In this study, low cost chromia-alumina castables were chosen as candidate refractories for IGCC. To enhance the workability of chromia-alumina castables, ultrafine alumina powder was added to improve the workability. It’s proved that the specific surface area and particles size distribution of ultrafine powders in matrix part greatly affect the flow values and microstructures.
Authors: E. Essuman, Gerald H. Meier, J. Zurek, Michael Hänsel, Lorenz Singheiser, W.Joe Quadakkers
Abstract: The oxidation behaviour of binary Fe-Cr alloys containing 10 and 20 mass % Cr, respectively, was studied in Ar-20%O2, Ar-7%H2O and in Ar-4%H2-7%H2O at temperatures between 800 and 1050°C. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the surface oxide scale, the present results reveal that the presence of water vapor also affects the transport processes in the alloy. The enhanced internal oxidation, which is likely the result of water vapor increasing the solubility and/or the diffusivity of oxygen in the alloy, explains the frequently observed effect that Fe(Ni)Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective oxidation in dry gases but breakaway type oxidation in steam. The temperature dependence of the change from protective to non-protective behaviour in Ar-H2O differs quantitatively, but not qualitatively from that in Ar-O2.
Authors: Mbaihoudou Kemdehoundja, Jean Luc Grosseau-Poussard, J.F. Dinhut, G. Bonnet
Abstract: Using Raman microprobe spectroscopy made it possible to study the buckling phenomenon in chromia films grown at 900°C in air from a Ni30at%Cr alloy. Blisters have been optically observed to be circular and, from the top view, the mean radius has been measured with an accuracy of about 1%m. An autofocus device allows the characterisation of the profile of each blister and the shift of the A1g Raman peak of chromia gave the local stress far from the blister and all along the buckled zone. From these observations, the induced spalling has been related to the blister morphology.
Authors: Marcia Silva, Sébastien Chevalier, G. Bonnet, J.C. Colson
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