Papers by Keyword: Oxidation Kinetics

Abstract: The oxidation kinetics of SiC fiber-reinforced SiC matrix composites with a BN interphase (SiC/BN/SiC) and the constituent fibers was characterized by thermogravimetric analysis and microstructural characterization at temperatures (816-1538°C) and oxygen partial pressures (0.1% to 5% O₂) relevant to the hypersonic flight and re-entry environments. TGA of the SiC fibers showed that oxidation of the thin BN surface layer led to initially rapid oxidation kinetics and formation of a relatively thick silica scale at very short times under most test conditions. At longer times the fiber oxidation kinetics were representative of silica formation on pure SiC. Oxidation of the composites was conducted on coupons with the SiC seal coat removed on one edge to simulate damage to the composite, allowing ingress of oxygen to the fiber tows. Microscopy was conducted to determine the distance of oxygen ingress into the coupon. At the lower temperatures and oxygen partial pressures the exposed edge did not seal off by silica formation, yet the BN interphase areas were only minimally oxidized. At the intermediate temperatures silica formed at the exposed surface limiting further oxidation of the exposed fibers and BN interphase areas. Finally at the highest temperature and lowest oxygen partial pressure, active oxidation of SiC occurred for both the fibers and coupons resulting in irregular material attack. Implications for use of SiC/BN/SiC materials for hypersonic vehicle thermal protection systems are summarized.

Abstract: In this research, TP304 was treated by shot blasting under the pressure of 0.65MPa. The stainless steel shots used in the shot blasting experiment were in different sizes, separately with diameter 0.6mm and 1.2mm. To investigate the effect of shot blasting on the oxidation resistance behavior, steam oxidation experiments have been conducted to both original and shot blasting treating TP304s in 650°C and 25MPa. Then oxidation kinetics curve of each sample was measured by discontinuous weight method, morphology of oxide film was observed through SEM(scanning electron microscope), the component of the oxidation was measured by XRD(X-ray Diffraction), and the variation of Cr content in the oxide film was analyzed by EDS(energy-dispersive X-ray spectroscopy). According to the results, Oxidation kinetics curve in high temperature and pressure, expressed in original and shot blasting treating TP304s, both follow the principle of quasi-parabola and shot blasting can largely enhance TP304’s steam oxidation resistance in high temperature and pressure.

Abstract: Discrete mass gain method is adopted to determine the oxidation kinetic behaviors of 2Cr12NiMoWV steel in water vapor at 540°C and 17.5 MPa. Results indicate that the kinetic curve of the oxidation experiences a dramatic turning where the oxidation has lasted 20h, namely, the oxidation is faster than before the oxidation point of 20h, then oxidation rate is greatly reduced;, and after the oxidation has lasted for 300h, its reaction is inhibited to enter into a relatively stable period. The test analysis of oxide film under scanning electronic microscopy and electronic probe microanalyser shows that the formation process of the oxide film is as follows: needle-like oxide nuclei → short-strip oxide → fine equiaxed crystal grains → large equiaxed crystal grains or columnar crystal particles. The alloying elements involved in oxidation represent the law as follows: Fe, Mo and Si react with vapor at the initial stage of oxidation reaction to form Mo and Si-contained Fe-rich oxide. As the oxidation reaction proceeds, Cr gathered at the interface between the surface oxide film and the matrix, is internally oxidized. The Mo, Si and Cr in the surface oxide will be continuously volatilized with the occurrence of oxidation reaction.

Abstract: In this thesis,a series of oxidation experiments on galena are conducted under different condition such as pH is 2,temperature is 50°C and 25°C.After the experiment is completed,The rate constant,reaction order，reaction reaction rate and energy of activation are also analyzed and computed.The effects of temperature and oxidized medium on lead ore oxidation speed were studied. Under the condition of 50°C, The rate of oxidation reaction is increase when concentration is 100mg/l.The higher the concentration the more sulfuric acid ion,the easier to produce sediment of lead sulfate , so the reaction will be impeded .the reaction will be extremely slow or even can’t carry on at 25°C.

Abstract: The article uses ferric sulfate as oxidant, and makes Galena oxidation test in the oxidation of different concentrations of medium, when pH is 2, and temperature is 50oC and 25oC. It analyzes the effects of the rate of Galena oxidation from temperature and oxidation of medium concentrations. In the conditions of 50oC, Fe2(SO4)3 as an oxidant, the solution concentration of 100mg / l, the reaction is quick. When the higher the concentration of solution, the higher the solution concentration of sulfate ions, the easier it is generated PbSO4 precipitation, impeding the reaction. In 25oC, the reaction is very slow or even not to.

Abstract: In the present work, intermetallics Fe3Si and Fe3 (Si1-xAlx) (x=0.33, 0.67, 0.80) produced by arc melting, were evaluated regarding the oxidation at 900°C in air. The test preformed was the isothermal and cyclic oxidation. The microstructures of oxide films were analyzed by XRD, SEM, EDS, EPMA. The result showed that the four kinds of intermetallics had good oxidation resistance at 900°C.The addition of Al element can increase the oxidation resistance of Fe3Si based intermetallics. The oxidation kinetics of the four intermetallics follows the rule of the parabola. The main oxidation product of Fe3Si is SiO2.The kinetics of oxidation reaction is mainly controlled by Si diffusion. The oxidation product of three Al alloying Fe3Si based intermetallics is Al oxide. Al moves outward from the substrate to form a protective oxide film which restrained the diffusion of Si and Fe outward.

Abstract: Stainless steel/TiC nanocomposite powder were prepared by high-energy ball-milling method using 316 stainless steel powder, carbon and titanium powder as raw materials. Microstructure of the nanocomposite powder was investigated with XRD and TEM techniques. The results showed that the stainless steel/TiC nanocomposite powder obtained when the ball-milling time was more than 40 hours. DSC analysis method was used to study the characteristics of oxidation resistance and the oxidation reaction kinetics of the nanocomposites powder. Results show that the oxidant resistance of nanocomposite powder was improved, the activation energy of oxidation reaction increased.

Abstract: Ni-Cr coating and Ni-Cr/ZrO2 gradated coating were deposited on C45 carbon steel by high velocity arc spraying and micro-plasma spraying to solve the high temperature oxidation problem of medium carbon steel components. The oxidations of Ni-Cr coating, Ni-Cr/ZrO2 gradated coating and C45 carbon steel substrate were carried out for up to 108 hours in air atmosphere at 1100°C. The oxidation behaviors were investigated after detailed examinations by thermal gravimetric analysis (TGA), x-ray diffraction (XRD) and scanning electron microscopy (SEM). Ni-Cr coating and Ni-Cr/ZrO2 gradated coating show similar and low dynamics curve near to the logarithms function. Surface observations with SEM and XRD reveal that the oxidizing surface of the C45 carbon steel substrate is mainly a loose spherical structure consisted of mainly Fe2O3 and Fe3O4. The surface structures of Ni-Cr coating and Ni-Cr/ZrO2 gradated coating after 108 hours oxidization are rather denser than that of C45 carbon steel, which can effectively improve the properties of the oxidation resistance of C45 carbon steel substrate.

Abstract: The performance of Ni-based alloys at high temperature strongly depends on the formation of a protective chromia scale which prevents from fast growth NiO scale. The evolution of the oxide scales will be shown to be strongly dependent on the chromium content especially for the shortest oxidation times. Indeed, transient stages will be reported to condition the subsequent oxidation behavior at longer times until the oxidation rate constant becomes invariant. To this end, a comparative study on the outward and inward diffusion of species upon oxidation at 800°C for short times (0.5 / 1 / 2 / 4 / 8 h) has been evaluated on commercial available Ni20Cr and Ni30Cr alloys through in-situ Raman spectroscopy and supported by post-mortem X-ray diffraction (XRD) as well as scanning electron microscopy coupled to energy dispersive spectrometry (FEG-ESEM / EDS) investigations.

Abstract: 316 stainless steel nano-grain powders were prepared by high-energy ball-milling method. The evolutions of grain size and microstructure of the stainless steel powders with the change of ball-milling conditions were investigated by XRD, SEM. DSC method was used to analyze the oxidation resistance of the powders after ball milling. The law of the change in oxidation weight gain and DSC oxidation peak of samples at the different heating rates were analyzed. Oxidation kinetics studies showed that the oxidant resistance of 316 stainless steel nano-crystalline powder was improved, the activation energy of oxidation reaction increased.