Corrosion resistance of AlN–SiC–TiB2 composite in air

The oxidation kinetics of AlN–SiC–TiB₂ composite in air has been investigated in terms of a theoretical analysis associated with the experiment data. The effects of temperature and temperature heating rate on the oxidation reaction have been discussed by using the “characteristic oxidation time”. The results show that the calculated results by our model can give a good agreement with the experimental data. From this study it is shown that, the “characteristic oxidation time” is a very useful parameter for comparing the property of oxidation resistance for different composites.     

Spectral and kinetic analysis of heterogeneous gas discharge plasma in the flow of an Al,H2O,and Ar mixture

Plasma excited by a longitudinal pulse-periodic discharge in a mixed flow of a buffer gas (Ar), oxidizer (H₂O), and aluminum dust is studied. The salient features of its kinetics are the atomic and molecular electronic channels of Al atom transformation, which were not taken into account earlier. They accelerate not only the oxidation process but also aluminum vaporization because the excitation and ionization energies are finally released on the surface of aluminum particles. This work is devoted to experimental and theoretical analysis of these channels.     

Oxidation mechanism of Si3N4-bonded SiC ceramics by CO, CO2 and steam

This paper presents a theoretical and experimental investigation into the oxidation reactions of Si3N4-bonded SiC ceramics. Such ceramics which contain a small amount of silicon offer increased oxidation and wear resistance and are widely used as lining refractories in blast furnaces. The thermodynamics of oxidation reactions were studied using the JANAF tables. The weight gain was measured using a thermogravimetric analysis technique to study the kinetics. The temperature range of oxidation measurements is from 1073 to 1573 K and the oxidation atmosphere is water vapour, pure CO and CO–CO2 gas mixtures with various CO-to-CO2 ratios. Thermodynamic simulations showed that the oxidation mechanism of Si3N4-bonded SiC ceramics is passive oxidation and all components contribute to the formation of a silica film. The activated energies of the reactions follow the sequence Si3N4>SiC>Si. The kinetic study revealed that the oxidation of Si3N4-bonded SiC ceramics occurred in a mixed regime controlled by both interface reaction and diffusion through the silica film. Under the atmosphere conditions prevailing in the blast furnace, this ceramic is predicted to be passively oxidized with the chemical reaction rate becoming more dominant as the CO concentration increases. © 1998 Chapman & Hall     

Application of Fenton oxidation to cosmetic wastewaters treatment

The removal of organic matter (TOC and COD) from a cosmetic wastewater by Fenton oxidation treatment has been evaluated. The operating conditions (temperature as well as ferrous ion and hydrogen peroxide dosage) have been optimized. Working at an initial pH equal to 3.0, a Fe(2+) concentration of 200 mg/L and a H(2)O(2) concentration to COD initial weight ratio corresponding to the theoretical stoichiometric value (2.12), a TOC conversion higher than 45% at 25 degrees C and 60% at 50 degrees C was achieved. Application of the Fenton oxidation process allows to reach the COD regional limit for industrial wastewaters discharges to the municipal sewer system. A simple kinetic analysis based on TOC was carried out. A second-order equation describes well the overall kinetics of the process within a wide TOC conversion range covering up to the 80-90% of the maximum achievable conversion.     

Verification and prediction of residual strength of C/SiC composites under non-stress oxidation

This article describes the strength behavior of C/SiC composites under non-stress oxidation. The oxidation kinetics model of a ceramic matrix composite contains both a reaction-controlled process and a diffusion-controlled process. The oxidation process has an effect on the fiber volume fraction, radius, and the composite interface shear stress. We obtained the residual strength of unidirectional C/SiC composites under 400–900 °C oxidation with non-stress environments by combining the mass loss rate model with the strength failure model at room temperature. This model considers the different components of the system to be subject to oxidation at different temperature ranges. Then, the influence of oxidation temperature, oxidation time, and fiber volume fraction on residual strength was studied. Experiments on C/SiC composites at 650 and 800 °C in an air environment with non-stress conditions were then performed, and the theoretical results of C/SiC composites were in good agreement with the experimental data.     

Oxidation of a Co-base Superalloy

The oxidation behavior of a cast polycrystalline Co-base superalloy was studied at temperatures from 900 to 1050℃ and analyzed by thermogravimetric analysis （TGA）, X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicate that a cast Co-base superalloy follows the subparabolic oxidation kinetics at 900 and 1000℃, which are controlled by the growth of the inner Cr-rich layer, and that after oxidation at 1050℃ for 200 h, it almost exhibits the linear oxidation kinetics possible due to the volatility of Cr-rich oxide. A mixed scale forms on the alloy after prolonged oxidation. The oxide scale formed at 900 and 1000℃ is composed of an outer layer of spinel and an inner continuous Cr-rich layer and at I050℃ is composed of a very discontinuous Cr-rich layer.     

Oxidation behaviour of single crystal nickel-based superalloys: intermediate temperature effects at 450–550°C

The oxidation behaviour of two single crystal Ni-based superalloys has been investigated at 450°C and 550°C. Isothermal oxidation was carried out for varying times and it was found that exposure resulted in a sub-micrometre thick oxide. The external and internal oxide kinetics were studied via high-resolution image analysis and both showed sub-parabolic growth rates. Thermogravimetric tests indicated that the overall oxidation growth obeys a near quartic power law while parabolic kinetics can describe the transient oxidation period. Characterisation of the resulting oxides was carried out using electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Results from thermodynamic modelling of the oxide formation are also presented to further assess the postulated mechanism of low-temperature oxidation in these Ni-based superalloys.     

原位红外光谱研究白油的热氧老化过程

采用原位红外研究了白油在氧气中的热氧老化过程．并通过热分析动力学研究了其热氧老化的反应速率及反应机理。研究表明．白油在热氧老化过程中主要发生断链和氧化反应。白油的断链为一级反应。在白油中加入多种不同类型添加剂,可以发现,白油的分解、断链和氧化均有不同程度的减弱。     

Diffusional effects for the oxidation of SiC powders in thermogravimetric analysis experiments

The oxidation behavior of SiC powders was studied using a thermogravimetric analysis (TGA). The effects of temperature (910–1010 °C), particle size (120 nm, 1.5 μm), and the initial reactant mass on the oxidation behavior of SiC were investigated. Kinetics analysis showed that intra-particle diffusion and chemical reaction controls the oxidation rate. Moreover, a new kinetics model was proposed to describe the oxidation rate of where all diffusion steps (bulk, inter- and intra-diffusion) and the chemical reaction may affect the overall reaction rate. The model was validated through a comparison with the experimental results obtained from the oxidation of SiC powders in TGA experiments. It was found that during the experiments, inter diffusion must also be taken account to describe adequately the oxidation rate. Numerical analysis indicated that inter-particle diffusion has a significant effect on the oxidation rate, especially for larger system.     

Temperature effect on the role of yttrium in oxidation behaviour of NiCrAl alloys

Temperature effect on the oxidation behaviour of NiCrAl and NiCrAl-Y alloys has been studied in the present work. Thermogravimetric analysis has been carried out at 900, 1000, 1100 °C for 210 h in order to investigate oxidation kinetics. Phase constitution of surface scales was determined by using X-ray diffraction, and microstructure was examined by using scanning electron microscope with energy dispersive spectra. Oxidation resistance of NiCrAl alloy has been found to be improved substantially by adding rare earth element yttrium at all the three experimental temperatures. But Y-rich inclusions result in higher initial oxidation rates as a result of oxidation of the inclusions.     

多晶石墨材料非等温氧化动力学及其氧化机理的研究

通过热重分析技术用模式函数和无模式函数法研究了多晶石墨材料的非等温氧化动力学, 并利用扫描电镜观察了该材料同一表面不同氧化程度的微观结构. 研究表明, 石墨氧化过程中, 活化能(E)随着氧化程度(α)增加而变化. 氧化从石墨表面的颗粒界面开始, 在氧化初期阶段, 表面原始空隙的尺寸随氧化程度的增加几乎不变. 石墨的非等温氧化过程主要分三个阶段: 氧化前期(α<20%), 氧化主要由碳氧化学反应控制; 氧化中期(20%<α<60%), 氧化为化学反应和气体扩散共同控制; 氧化后期(α>60%), 氧化主要由气体扩散控制. 运用无模式函数法研究石墨非等温氧化更具有可信度.     

Heat-affected zone oxidation properties in AISI 430 ferritic stainless steel welds

High temperature corrosion of ferritic stainless steel weldments has not previously been studied. This paper is an attempt to standardize a method for assessing the oxidation properties of the heat-affected zone (HAZ) of ferritic stainless steel welds. A weld thermal cycles simulator was used to model HAZ representative microstructure specimens, and the oxidation kinetics of both the base metal and selected HAZ specimens then studied by oxidizing the specimens at 950°C and 1 atmosphere in purified oxygen using a thermogravimetric rig. The oxidation mechanisms of both base metal and HAZ from representative specimens of the AISI 430 steel were studied using optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis. The oxidation kinetics and morphology of both HAZ and base metal samples are reported.     

Effect of doping Al on the liquid oxidation of Sn–Bi–Zn solder

The liquid oxidation behaviors of Sn–40Bi–2Zn and Sn–40Bi–2Zn–0.005Al solders were investigated from thermal dynamics and kinetics analysis. The characteristics of surface oxidation film at 170 °C were studied by thermo gravimetric analysis and X-ray photoelectron spectroscopy (XPS). Sn–40Bi–2Zn solder performed inferiorly in oxidation prevention performance, due to the formation of ZnO, which exhibits lower Gibbs free energy of formation and higher growth rate. Trace amount of Al addition, however, alleviated the oxidation behavior of Zn. XPS depth profile results indicated that the surface layer of Sn–40Bi–2Zn–0.005Al consisted of oxides of Al and Zn formed on the outer surface of the solder film and in the subsequent layer, mainly formed by the oxides of Sn, Bi. Al, basically formed as Al₂O₃, segregated towards the outer surface, seemed to deter the Zn oxidation on the solder surface.     

Contribution of solid electrochemical techniques to high temperature oxidation studiest†

Abstract

Solid electrochemical techniques have been applied to high temperature oxidation studies in experiments and concept. The principles of the parabolic oxidation law and solid electrolyte cells have been derived from the same origin developed by Carl Wagner. Oxygen sensors and pumps using solid electrolyte cells are very useful for the study of oxidation kinetics as a function of oxygen partial pressure. A tip-type solid electrolyte CO₂ sensor is very sensitive to detect the evolution of CO₂ gas, and is useful for the analysis of high temperature oxidation of metal carbides. A mechanistic model proposed for the reactive element effect of chromia-forming alloys has been confirmed by the measurements and data analysis of thermodynamic properties, electrical conductivity and diffusivity of YCrO₃ on the basis of solid electrochemical techniques and defect structures.     

Influence of TiC additions on the oxidation behaviour of Si3N4-based ceramics

The oxidation behaviour in air of Si3N4 ceramics containing Y2O3 and Al2O3 as sintering aids, with and without a dispersion of TiC particles, has been studied between 1200 and 1400 °C. The influence of TiC additions on the oxidation kinetics is discussed in comparison with the results obtained for the Si3N4–Y2O3–Al2O3 reference material. However, in all cases and within the experimental temperature range, the oxidation kinetics were observed to be of a parabolic type. The microstructure of the oxide scales formed, which has been characterized by scanning and transmission electron microscopy, was observed to depend on the oxidation temperature and the initial composition of the material. © 1998 Chapman & Hall     

Creep crack growth laws in heat-resistant steels

The conditions of various fundamental fields' domination at the creep crack tip have been considered. The creep crack growth properties and criteria are discussed. The relation between the creep crack growth kinetics and mechanisms have been studied for heat-resistant turbine steels over a wide test temperature and load range. In recent years much attention has been paid to the theoretical analysis of creep crack growth, though experimental investigations were generally performed for a comparatively narrow temperature and test time interval. The paper is concerned with the criteria and basic laws of creep crack growth. The results of our experimental investigation of the crack kinetics and crack growth mechanisms in heat-resistant steels in wide temperature and test time ranges are also presented.     

2D-C/SiC复合材料的氧化损伤及刚度模型

采用不同界面强度的2种2D-C/SiC试件,在空气环境中进行700℃无应力氧化试验。通过扫描电镜(SEM)分析发现：材料表面的氧化机制为反应控制,纤维均匀变细;内部的氧化机制为扩散控制,被氧化的纤维产生了缺口或者局部缩颈现象。纤维氧化使有效承载面积减小,导致材料的模量和强度下降。强界面材料模量高而强度低,断口整齐;弱界面材料模量低而强度高,纤维拔出较长。基于以上 SEM分析结果,建立了细观力学模型,对起始模量进行模拟计算,获得了与试验值比较吻合的结果。     

Oxidation mechanisms and kinetics of SiC-matrix composites and their constituents

The oxidation kinetics and mechanisms of SiC-matrix composites fabricated by chemical vapor infiltration, and of their constituents (C or SiC-fibers, C or BN interphases and SiC matrix) are studied on the basis of an experimental approach and modelling. The oxidation of carbon fibers is rate-controlled by a combined diffusion/chemical reaction mechanism at low temperatures and its rate reduced by a 1600°C heat treatment. The oxidation rate of the pyrocarbon is similar to that of the fibers when they have been heat-treated. The oxidation kinetics of both the SiC-based fibers and matrix are parabolic and assumed to be rate-limited by the diffusion of gaseous species in the silica scale. A full kinetics law is given. The occurrence of water in the atmosphere increases the oxidation rate of the fibers and decreases the activation energy, water becoming the main oxidizing agent. The oxidation of the BN-interphase is complex and strongly anisotropic, its kinetics depending on composition, structure and texture. Finally, the oxidation of SiC-matrix composites, depicted for 1D-SiC/C/SiC and 2D-C/C/SiC composites, involves both diffusion of gaseous species in the composite porosity and heterogeneous oxidation reactions. Oxidation occurs through the thickness of the composites at low temperatures which consumes the carbon-based constituents. Conversely, it tends to be limited to near the composite surface at high temperatures, due to the formation of silica-based phases healing the material porosity and preventing the in-depth oxidation of the carbon-based constituents.     

The oxidation kinetics of magnesium at low temperatures and low oxygen partial pressures

The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10^− 8 Pa and 1.3 × 10^− 5 Pa and at temperatures just above room temperature has been investigated in situ with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Quantitative analysis of the XPS spectra showed that the initially formed oxide has a higher Mg/O ratio (> 1.3) than bulk MgO. Ellipsometry measurements indicated that the band gap values of the oxide layers are considerably smaller than the value expected for bulk MgO ( 2.5 eV vs. 7.8 eV). From the XPS and ellipsometry data recorded as a function of oxidation time the oxidation kinetics have been determined. The kinetics has been described quantitatively with a coupled currents model, involving simultaneous transport of electrons and ions through the oxide layer.