Papers by Keyword: Chromia

Abstract: This chapter primarily reviews the nature of water vapour when it presents in bulk gas. The change in a ratio between water vapour and corresponding dissociated hydrogen, which determine the thermodynamic stability of the oxide formation, is analysed when the oxidation kinetics are linear and parabolic. When water vapour reaches the solid/gas interface, chromium species volatilisation and oxidation controlled by surface reaction can occur. The adsorbed water vapour can be further incorporated into the oxide possibly in the form of hydrogen defects. The role of these defects on altering the defect structure of the oxide is discussed. Finally, characteristics of the oxide scale on stainless steels formed in the atmosphere containing water vapour are reviewed.

Abstract: This chapter is dedicated to the description of high temperature oxidation of both chromia and alumina forming alloys. The defect structures of iron and chromium are firstly reviewed. The effects of elements on stainless steel oxidation behaviour are further addressed. For the chromia-forming stainless steel, the oxidation rate is reduced with the increased silicon content but not in a monotonic manner. Titanium and niobium can reduce breakaway oxidation of Fe–18Cr–10Ni austenitic stainless steel. Titanium can enhance the adhesion of scale to the Fe–18Cr by mechanical keying effect of TiO₂ formed at the steel/scale interface. For the alumina-forming stainless steel, the formation of alumina and its transformation during oxidation are reviewed. Chromium can be added to reduce the critical aluminium content in the steels in order to form alumina at high temperatures. The addition of reactive elements with appropriate level can improve scale adhesion and reduce the steel oxidation rate. Refractory element like molybdenum can increase strength of material but also accelerate the oxidation rate of the steels containing reactive elements. The development of new alumina-forming austenitic alloy grades is finally described.

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.

Abstract: The effect of chromia additive on defects in yttria stabilized zirconia (YSZ) nanopowders was investigated in this work. It was found that positrons are trapped at vacancy-like misfit defect at grain boundaries and at larger defects situated at triple points. Moreover, a long-lived ortho-positronium contribution was found in YSZ nanopowder without chromia. Addition of chromia prolongs the lifetime of positrons trapped at vacancy-like misfit defects which indicates segregation of Cr ions at grain interfaces and interaction of Cr with vacancy-like misfit defets. Moreover addition of chromia completely suppresses formation of positronium.

Abstract: In the present study the oxidation behaviour of a number of candidate alloys for heat exchanging components was investigated in model gas mixtures containing high amounts of CO₂ and/or water vapour in the temperature range 550-700°C up to exposure times of 1000 h. During exposure in Ar/CO₂ and Ar/CO₂/H₂O base gas mixtures at 550-650°C the oxidation rates and scale compositions of martensitic 9-12%Cr steels were similar to those previously observed in steam environments. Thin and protective Cr-rich oxide scales which are commonly found during air oxidation was observed locally on the specimens surfaces after oxidation in Ar-(1-3%)O₂-CO₂. The tendency for protective chromia base scale formation increased when 3% oxygen was added, especially for the 12%Cr steel. When iron base oxide scales were formed on the metal surface, the martensitic steels tended to exhibit carburisation whereby the extent was reduced by increasing the water vapour and oxygen contents. All three studied austenitic alloys exhibited very slow scale growth rates at 550°C, however, at and above 600°C the steels with lower Cr content started to form two-layered iron rich surface oxide scales whereby the outer oxide was prone to spallation upon thermal cycling. The high-Cr austenitic steel 310N and the nickel base alloy 617 formed very thin, Cr-rich oxide scales at all used test temperatures and atmospheres. For those two materials the oxidation behaviour in gases containing water vapour in combination with intentionally added oxygen was affected by formation of volatile chromium oxyhydroxide.

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.

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.

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.

Abstract: Chromia scales grown on several chromia-forming metallic substrates in various conditions were characterised by photoelectrochemistry (PEC), highlighting the presence of two semiconductor phases signed by their respective bandgaps (3.0 and 3.5 eV) with variations of semiconduction type (n, p and insulator, more or less doped). The protective character of the scale was clearly demonstrated when the highest bandgap phase (3.5 eV), identified as the external subscale was close to an insulator.

Abstract: For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.