Papers by Keyword: Oxidation Mechanism

Abstract: High temperature oxidation dynamic behaviors and mechanisms for 30Cr25Ni20Si heat-resistant steel were investigated at 800, 900 and 1000°C. The oxide layers were characterized by scanning electron microscopy (SEM-EDS), X-ray diffractometer (XRD). The results showed that the oxidation rate of test alloys is increased with increasing the oxidation time. The oxidation dynamic curves at 800 and 900°C follow from liner to parabolic oxidation law. The transition point is 10 h. At 1000°C, the steel exhibits a catastrophic oxidation, and the oxidation mass gain value at 50 h is 0.77 mg/cm². This suggests that the steel at 900°C has formed a dense protective surface oxidation film, effectively preventing the diffusion of the oxygen atoms and other corrosive gas into the alloy. Therefore, at the first stage of oxidation, chemical adsorption and reaction determine the oxide film composition and formation process. At the oxide film growth stage, oxidation is controlled by migration of ions or electrons across the oxide film. When the spinel scale forms, it acts as a compact barrier for O element and improving the oxidation resistance.

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: Oxidation behavior of FGH91 superalloy in air and the temperature range of 540 to 980 °C for period up to 100h has been studied. The results indicate that oxidation kinetics obeyed parabolic law, and the activation energy of oxidation was calculated to be 98.2 KJ/mol. The scales on the surface were determined by SEM, EDS and XRD, the results show that the oxidation scales were composed of three layers: the outer layer was composed of Cr₂O₃ layer with a small amount of TiO and NiCr₂O₄ spinel; the intermediate layer was made up of Al₂O₃ and matrix alloy; and a small amount of titanium-nitride disperse in the inner layer. Besides, γ’-free layer was formed under the oxidation scales due to the impoverishment of Al in this layer, which was induced by the formations of Al₂O₃.

Abstract: A layered structure of different copper oxides was produced by thermal oxidation of copper. The structure and microstructure of the different layers were investigated by scanning electron microscopy and transmission electron microscopy. From the substrate to the free surface, three different layers are formed, namely, Cu₂O layer, CuO layer and CuO nanowires. Based on our observations, a possible formation mechanism of layered copper oxides and CuO nanowires was proposed. The results shed light on the oxidation process of metals and provide insight into the synthesis of copper oxides and CuO nanowires.

Abstract: To verify the Si emission phenomenon during oxidation of SiC, the behavior of Si atoms was investigated using HfO₂/SiC structures. At low oxygen pressure, i.e. the oxidation condition predominant to active oxidation, Si emission into oxide layer and the growth of SiO₂ on the oxide surface were clearly observed by TOF-SIMS. On the other hand, the growth of SiO₂ on the surface was suppressed under an ordinary pressure. These results evidence the Si emission during oxidation that is proposed in the Si and C emission model.

Abstract: First-principles calculations have been carried out to investigate the incipient oxidation mechanism of FeCrAl alloy. It is found that Al,Y,Cr atoms energetically prefer surface sites, and the driving force of Y segregation to surface is strongest. The surface segregation of Y, Al and Cr will suppress the outward diffusion of Fe, form the tight coherent films of Al₂O₃,Cr₂O₃ and RE oxides which can restrain oxygen inward diffusion, as a result, leading to the decrease of the growth of oxide films. The O adsorption process at Fe surface are found to be spontaneous, and our calculations predict Al, Cr, Y segregation at Fe surface is beneficial for decreasing the oxidation rate of FeCrAl alloy. The interaction between O and Fe, Cr, Al, Y atoms exists both ionic and covalent binding characteristics. Also Al, Y alloying increases ionic and covalent binding between Al, Y and O, which speed selective oxidation of Al and Y, and hence improves the oxidation performance of FeCrAl containing Y alloy.

Abstract: A layered structure of different iron oxides was produced by thermal oxidation of iron. The structure and microstructure of different layers were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Selected area electron diffraction (SAED) was used to identify the structures of the different oxide layers. Two different structures of Fe₂O₃ were found to co-exist. Based on our observation, a possible oxidation mechanism for iron was proposed. The results shed light on the oxidation process of metals and provide insight into the synthesis of iron oxides.

Abstract: Aluminum nanoparticles were prepared by laser and inducting heating methods, then the phase, structure and thermal properties of them were characterized by X-ray diffraction, electron microscope and simultaneous thermal analyzer, respectively. The results showed that there was great difference between micro- and nanoparticles in oxidation behavior. Aluminum micron particles had a very strong endothermic peak at the temperature of about 660°C and the absorption heat enthalpy is 328J/g.The release heat enthalpies of aluminum nanoparticles oxidized by oxygen are 4.219 kJ/g and 3.584 kJ/g. Because of the size decreases to nano-size, the aluminum nanoparticles can be oxidized directly by oxygen and release a large amount of heat, which is beneficial to the propellants when it been burn. However, the micro-particles should absorb heat which other components released, and release heat slowly after melted, which are not as good as the nano-ones.

Abstract: The isothermal oxidation behaviour under static atmosphere of a new directionally solidified Ni-base superalloy was investigated. The results showed that the oxidation kinetics curves of the alloy follow parabolic law in the temperature range of 750-950°C. The diffusion activation energy Q is to be about 244.86 kJ•mol-1 and the oxidation within this temperature range is mainly controlled by Cr3+ diffusion among Cr2O3 oxidation film. The oxidation resistance grade of the alloy is perfect anti-oxidation within 750-850°C, and anti-oxidation within 900-950°C. The oxidation film can be divided into TiO2, Cr2O3+TiTaO4+NiCr2O4, Al2O3 and TiN layers, from the surface to inside. Priority oxidation and the inside oxidation of Al are considered with the oxidation of Ti and the depletion of Cr, and it is the key point for the improving of oxidation resistance.

Abstract: An overview on parameters influencing the oxidation behaviour of carbon based materials in oxidizing gases is presented in order to support the development of advanced carbon containing materials with high oxidation resistance. Facilities for testing the oxidation behaviour, as operated in FZJ, are explained. Results of exemplary oxidation tests in air at 700°C on diverse new developed materials are presented: Ti and Zr dopings and coatings were found less efficient, whereas Si coatings/dopings significantly reduce oxidation rates. Low oxidation rates of 3D-CFCs without doping/coating, which were manufactured under temperature treatment of > 2200°C and used high purity starting materials, point out the relevance of the latter parameters. Future work on oxidation resistance of carbon based materials is shortly discussed.