Papers by Keyword: Oxidation Kinetics

Abstract: Several ferritic stainless steel grades are widely studied and used in solid oxide fuel cells (SOFCs) technology as interconnect materials. Their high-temperature oxidation behavior is interesting to evaluate their applicability at SOFCs operating conditions and to design degradation tests and models predicting the lifetime of a SOFC stack. In this work the AISI441 grade was oxidized in static air at 850°C to study its oxidation kinetic by weight gain measurements. It was found a parabolic growth with a rate constant of 9.42 x 10^-14 g²cm^-4s^-1. Data calculated using the diffusion coefficients of the species involved in the oxidation process resulted in higher weight gain. Discrepancies between the measurements and the model were corrected taking into account the chromium volatilization.

Abstract: The study of isothermal oxidation of Fe-Ni-Cr alloy was done at 900 °C for 500 hours. The effect of oxidation kinetics and oxide growth behavior on Fe-Ni-Cr alloy were investigated on heat-treated Fe-Ni-Cr alloy to understand the oxidation mechanism on different grain size of alloy. The grain size of Fe-Ni-Cr alloy was varying through heat treatment process at three different temperatures, namely 1000 °C, 1100 °C and 1200 °C for 3 hours soaking time followed by water quench. The heat-treated Fe-Ni-Cr alloy was experienced discontinuous isothermal oxidation test at 900 °C up to 500 hours exposure. The oxidation kinetics plot was calculated based on the weight change per surface area over time. The oxide surface morphology was characterized by using scanning electron microscope (SEM) equipped with energy dispersive x-ray (EDX) spectrometer. The heat treatment process recorded an increasing grain size alloy as the heat treatment temperature increase. 8H10 sample indicate the fine grain size, whereas 8H12 sample indicate the coarse grain size. The oxidation kinetics of all samples exhibit the weight gain pattern with fine grain 8H10 sample recorded the lowest weight gain compared to 8H11 and 8H12 samples. All samples were obeyed parabolic rate law indicating the oxide growth rate followed a diffusion-controlled mechanism. The oxide surface morphology of 8H10 sample displayed a continuous oxide scales with formation of grain boundary oxide along the grain boundary area. Similar oxide structure formed on 8H11 and 8H12 samples, except for the formation of crack on the grain boundary oxide on both samples. In addition, 8H12 sample also formed a porous oxide structure.

Abstract: The isothermal oxidation behavior of Zr₃[Al(Si)]₄C₆-ZrB₂-ZrC composite ceramics at 1000-1300 °C in air has been investigated. The oxidation kinetics of the composites and generally follows a parabolic law. At the same oxidation temperature and time, the weight gain per unit surface area, oxidation rate constant and oxide thickness of the composites are higher than those of monolithic Zr₃[Al(Si)]₄C₆ ceramic. With the incorporation of ZrB₂ and ZrC, the oxidation resistance of the composites becomes poor. The surfaces of the oxide layer have a loose and porous structure, consisting of mainly ZrO₂ and little mullite, and there are no dense oxide films preventing the inward diffusion of oxygen element effectively.

Abstract: The model explaining the occurrence of the electron concentration step front during oxidation of nitrogen-doped TiO_2-δ thin films is presented. This model is based on ambipolar chemical diffusion coefficient analysis, for which immobile and uniformly distributed nitrogen component is assumed. The diffusion species and oxygen activity (pressure) profiles are obtained by numerical and approximate analytical simulation of the chemical diffusion. The profiles indicate the presence of two separate singularities: the electron concentration step front, and the electron-hole recombination reaction front. The electron concentration step front relates to the singularity of the ambipolar diffusion of three types of charged species with essentially different diffusion coefficients.

Abstract: Stainless steels can form a protective oxide layer when exposed to a high temperature oxidising environment, this protective layer forms a diffusion barrier and slows the oxidation of the alloys in harsh environments. This characteristic has made stainless steels one of the most commonly used alloys for high temperature industrial applications. In this work, a systematic testing procedure has been used to investigate the high temperature oxidation of two commonly used grades of stainless steel, 316 and 310. Samples of each alloy have undergone isothermal testing in air at 1050°C, 1150°C and 1250°C for a range of time periods up to 8h. The oxidation kinetics were also investigated using thermo-gravimetric analysis in air at the same temperatures for 8h. The oxide layers formed on the samples were characterised using X-Ray diffraction, Scanning electron microscopy and energy dispersive spectroscopy. Information derived from oxide layer characterisation was used to explain any differences between the two alloys in terms of oxidation rate and overall alloy performance in the high temperature environment.

Abstract: Zirconium diboride is widely applied because of some excellent performances. The oxidation kinetics of ZrB₂-YAG-Al₂O₃ composite materials were researched, which helps to improve the performance of ultra-high-temperature composite materials. The results show the oxidation weight gain is decreased with increasing the content of YAG-Al₂O₃ and the molar ratio of Al₂O₃. The oxidation weight gain is increased with prolonging the oxidation time under 1300°Cæ, the oxidation weight gain ratio is decreased with prolonging the oxidation time. The effecting tendency of oxidation weight gain is not abvious with varying the contend of YAG-Al₂O₃ upon 1300°Cæ, however, the effecting tendency of oxidation weight gain is very abvious with varying the molar ratio of Al₂O₃.

Abstract: A systematic testing procedure has been employed to investigate the high temperature oxidation kinetics of AISI 316L. Thermo-gravimetric (TG) analysis was carried out at 950°C, 1050°C, 1150°C and 1250°C for 8h. Alongside this, isothermal furnace treatments were carried out on samples of the same material at the same temperatures for time periods of 0.5h, 1h, 2h, 4h and 8h. Changes in oxidation kinetics were observed on mass gain curves plotted from data derived from the TG analysis. When a change in oxidation kinetics was identified, the structure, thickness and composition of the oxides formed on the isothermal treatment samples at time periods before and after the change occurred could be studied. It was found that this systematic testing procedure provided a great deal of useful information allowing more meaningful conclusions to be made on the influence of oxide layer thickness, structure and composition on high temperature oxidation kinetics.

Abstract: Effect of ultrasonic treatment on the oxidation kinetics and microstructure of Fe₈₀Cr₂₀ alloy is investigated. The aim of this study is to improve the surface morphology and oxidation resistance of the Fe₈₀Cr₂₀ alloy powder as a raw material. Ultrasonic technique is very promise technology and not yet fully explored. The Fe₈₀Cr₂₀ material was treated by ultrasonic technique (UT) at various times of 3.5 h, 4.5 h, 5 h and followed by cold compaction with force of 13 Newton. The oxidation test was carried out using tube furnace under argon gas environment with 5 cycles, which has been considered. It was found that the UT has improved the surface morphology of the material effectively. Ultrasonic was revealed that the good interparticle bonding when experimented with ultrasonic treatment at 4.5 h. In the oxidation test, the lowest mass gain of 68 mg/cm² was found at the oxidation time of 100 h. Meanwhile, the lowest parabolic rate constant of 6.8198 x 10^-14 g²cm^-4s^-1 was revealed when ultrasonic for 4.5 h sample.

Abstract: Surface discharge plasma was used to activate air to produce reactive oxygen species (ROS) for promoting the oxidation of ammonium sulfite by air forced oxidation method. The effect of applied voltage and the air speed in discharge reactor on the generation of ozone as well as the oxidation kinetic of ammonium sulfite were analyzed and discussed. The ozone concentration increased with the discharge voltage, but decreased with the air speed, and a maximum mass of generated ozone was obtained at air speed of 15 m/s. Compared with air forced oxidation, the oxidation kinetic constant of ammonium sulfite by ROS injection had an increase by 2.67-fold, and the oxidation of ammonium sulfite followed a pseudo zero order kinetics. An increase in air speed accelerated the oxidation of ammonium sulfite, and the oxidation kinetic constant of ammonium sulfite was increased by 5-fold when the air speed increased by 2-fold.

Abstract: The oxidation behavior of SUS310S austenitic stainless steels was studied in isothermal conditions at different temperatures between 800^oC and 1100^oC for 96h in air. The oxidation kinetics was analyzed, the surface and cross-section of the oxide scale grown by oxidation were characterized by using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffusion (XRD) and X-ray photoelectron spectroscopy (XPS). The SUS310S steel has high oxidation resistance at 800^oC and with the increase of the temperature, the parabolic rate constants is constantly increasing. Examination of the morphology and composition of oxide layers reveals a double-layer structure, The inner layer is mainly chromium oxide (Cr₂O₃) and is covered by an uneven thinness outer layer of manganese-chromium or iron-chromium spinel oxide.