Formation of Fine Clusters in High-Temperature Oxidation of Molten Aluminum

High-temperature oxidation of molten aluminum was investigated by high-resolution electron microscopes in order to determine the possibility of heterogeneous nucleation of aluminum grains on oxide for the grain refinement and structural uniformity of intensively melt-sheared aluminum alloys. High-resolution observations detect initial amorphous phase and gamma-alumina phase and show fine clusters with size of about 150 to 200 nm composed of extremely fine aluminum grains and gamma-alumina or amorphous aluminum oxide. Furthermore, high-resolution lattice images and diffraction patterns show no orientation relationship, although there is a specific orientation between gamma-alumina and aluminum along (111)[110] with high potency of heterogeneous nucleation. The volumetric shrinkage by the transformation of gamma- into alpha-alumina causes the surface oxide films to repeatedly rupture and leads to the creation of channels to the base melt surface for further oxidation of fresh metal. Based on the observations, the mechanism of high-temperature oxidation of molten aluminum and formation of the fine clusters as well as the possibility of the heterogeneous nucleation of aluminum grains are discussed.     

Morphology of Oxide Scale and Oxidation Kinetics of Low Carbon Steel

The oxidation kinetics and composition of oxide scales on low carbon steel(SPHC)were studied during isothermal oxidation.Thermogravimetric analyzer(TGA)was used to simulate isothermal oxidation process of SPHC for 240min under air condition,and the temperature range was from 500to 900 ℃.Scanning electron microscope(SEM)was used to observe cross-sectional scale morphology and analyze composition distribution of oxide scales.The morphology of oxide scale was classical three-layer structure.Fe2O3developed as whiskers at the outermost layer,and interlayer was perforated-plate Fe3O4 while innermost layer was pyramidal FeO.From the oxidation curves,the oxidation mass gain per unit area with time was of parabolic relation and oxidation rate slowed down.On the basis of experimental data,the isothermal oxidation kinetics model was derived and oxidation activation energy of SPHC steel was 127.416kJ/mol calculated from kinetics data.     

Wear Resistance of Plasma-Sprayed Coatings in Intensive Abrasive Wear Conditions

The abrasive wear resistance of plasma-sprayed oxide ceramic (Al2O3 and Cr2O3) and clad cermet ((Ti–Cr–C)–30% Ni and (TiC)–30% Ni) coatings was studied. The wear characteristics of the coatings such as wear rates, friction forces, friction coefficients, and wear groove depth determined with two methods were compared. The wear test methods included simulation of the friction process using loose abrasive particles and reciprocal ball-on-disk friction using a diamond indenter. The plasma-sprayed TiC-based coatings showed the lowest wear rate and can be applied to protect equipment parts subjected to intensive abrasive wear. The wear loss of the (Ti–Cr–C)–30% Ni and (TiC)–30% Ni coatings determined by friction against loose abrasive particles was 10–17 μm, while the wear loss of the oxide ceramic coatings was 20–42 μm, being 2–2.5 times higher on average.     

Formation of Coatings by Anodic Microarc Oxidation and Their Operation in Thermally-Stressed Assemblies

The effect of the range of electrophysical conditions on the course of anodic microarc oxidation is studied. Process parameters are optimized and a number of production procedures are developed for providing the preparation of efficient ceramic coatings that are effective heat protection for thermally stressed assemblies of internal combustion engines. Coating thickness may reach up to 200 (m that was previously considered impossible. The results form the basis of a production method for the ceramic heat-resistant coating by anodic microarc oxidation for the piston head of a high-power internal combustion engine.     

Diffusion in High-Temperature Ferrochrome Alloy Oxidation

Simulation has been applied to the diffusion characteristics such as the effective component diffusion coefficients in Fe 35 mass% Cr (0.5-3.0) mass% Al bearing surface oxides at the time of peeling. The simulation results are in agreement with experimental data on the layerwise phase compositions of the oxides. The model is described for the diffusion in a three-layer oxide region. The research elucidates the formation mechanism for multilayer oxides, and also gives estimates of their effects on heat resistance.     

The Effect of Oxide Grain Structure on the High-Temperature Oxidation of Fe,Ni and Cr

Abstract

Investigations have shown that the oxidation rate of metals and alloys is strongly influenced by the oxide grain size. This paper summarizes some of our data on the oxidation of Fe, Ni, Cr, and some alloys and relates the observed oxide grain structures to the oxidation kinetics and transport processes. Fine-grained polycrystalline oxide thickens most rapidly because the many oxide grain boundaries act as preferential paths for rapid diffusion. In contrast, monocrystalline oxide such as forms on some orientations of Ni and Cr thickens very slowly and shows the highest activation energy. Oxide grain size is found to depend on such factors as metal orientation, chemical pretreatment of the surface, cold work, and oxidation procedure.

Résumé

Plusieurs programmes de recherche ont montré que le taux d'oxydation des métaux et alliages dépend de la grosseur des grains des oxydes produits. Cet article présente certains des résultats obtenus en oxydant du Ni, du Cr et certains alliages; un rapport entre la structure des grains d'oxydes produits, d'une part, et d'autre part, la cinétique d'oxydation et les processus de transport présents, a été etabli. Les oxides polycristallins é grain serré facilitent l'oxydation, car leurs nombreux joints de grains offrent une route préférentielle à la diffusion rapide de l'oxygéne. Par contre, selon certaines orientations de Ni et de Cr, l'oxide monocristallin formé s'épaissit très lentement et présente les énergies d'activation les plus élevées. La grosseur des grains d'oxydes dépend de certains facteurs tels que l'orientation du metal oxyde, le prétraitement chimique de la surface, le travail à froid et la procédure d'oxydation.     

Characterization and High-Temperature Oxidation Behavior of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler Steels

Microstructure and mechanical properties of cold-spray coatings are usually required in order to explore the potential industrial application of the latter. This article demonstrates the successful formulation of Ni-20Cr and Ni-50Cr coatings on two boiler steels, namely, SAE 213-T22 and SA 516 steel by cold-spray process. The microstructure, coating thickness, phase formation, and microhardness properties of the coatings were evaluated. The coatings were subjected to cyclic heating and cooling cycles at an elevated temperature of 1173.15 K (900 °C) to ascertain their high-temperature oxidation behavior. Moreover, these cyclic exposures can give useful information regarding the adhesion of the coatings with the substrate steels. Of all the coatings, the Ni-50Cr coating on SA 516 steel had a maximum average hardness value of 469 Hv. As observed from the surface field emission–scanning electron microscopy (FE-SEM) analysis, the coatings were found to have nearly dense microstructure with the sprayed particles in interlocked positions. It was concluded that the cold-spray process is suitable for spraying the preceding powders onto the given boiler steels to produce nearly dense and low oxide coatings. The coatings, in general, were found to follow the parabolic rate of oxidation and were successful in maintaining their surface contact with their respective substrate steels.     

High-Temperature Oxidation of Bismuth- and Antimony-Based Sulfosalts

In this article, oxidation processes of Ag-Bi-Sb-based phases were investigated. Synthesized AgBiS₂ and AgSbS₂-Sb₂S₃-Sb samples were thermally analyzed in synthetic air by applying the simultaneous DSC-TGA analysis technique. The oxidation processes at P_O?=0.2 atm and T<1173 K were observed to take place in many-step sequence of various reactions with an overall reaction 2AgMeS₂ + 5.5O₂(g) ? 2Ag + Me₂O₃ + 4SO₂(g), where Me=(Bi,Sb). Oxidations of AgBiS₂, Sb₂S₃, and AgSbS₂ were observed to begin above 549±2K, 610±2K, and 733±2K, respectively. Furthermore, oxidation processes of AgBi₃S₅ and Ag₃SbS₃ were estimated, and thermodynamic functions for the overall oxidation reactions were calculated and discussed.     

等离子喷涂钛酸锶镧/金属复合涂层组织结构及性能的研究

钛酸锶镧(LST)作为一种新型固体氧化物燃料电池(SOFC)连接体材料,其导电性能远低于金属连接体,且受制备条件影响显著。本研究分别将不同体积分数的Ni、Fe及SS430不锈钢粉末与LST混合,通过大气等离子喷涂技术制备了相应的复合连接体涂层,并系统研究了材料组合对其组织结构、导电性能及稳定性的影响。研究结果表明,LST/Ni复合连接体涂层随着金属体积分数的增大,其导电率显著提高,达到甚至远超过LST烧结块体的水平;同时,由于热膨胀匹配的问题,过高的Ni比例使得复合涂层在经受热循环后发生明显的纵向开裂,显著降低其气密性。由于Fe颗粒在喷涂过程中的严重氧化,LST/Fe复合连接体涂层的导电性能没有得到明显改善。10%体积分数SS430混合喷涂粉末制备的LST/SS430复合涂层,具有高电导率和热稳定性,满足高性能SOFC连接体的要求。     

X70管线钢表面氧化铁皮缺陷形成原因分析

针对轧制X70管线钢板出现表面氧化铁皮缺陷的情况,南钢采用扫描电镜(SEM)和X射线能谱仪(EDS)分析测试手段,研究了板坯及钢板表面氧化铁皮形成机理和成因。结合现场工业试验,提出增加除鳞道次、适当提高开轧及终轧温度、强化一次氧化铁皮控制的措施,以达到减少X70管线钢板表面氧化铁皮缺陷和提高产品表面质量的目的。     

热轧带钢氧化铁皮的微观形貌和氧化动力学研究

 模拟了热轧带钢在不同温度、保温不同时间时,氧化铁皮的生成情况,并利用扫描电子显微镜(SEM)和X射线衍射对氧化铁皮的显微结构和形貌进行了分析研究。研究结果表明：热轧带钢在不同温度、保温不同时间,氧化铁皮都出现分层现象,临铁层为疏松的FeO,中间层为Fe3O4,外层为Fe2O3,中间层和外层致密;在同一温度下随着保温时间的增加,氧化铁皮厚度呈增加趋势;氧化铁皮形成的网状裂纹是氧化动力学曲线由抛物线规律向直线规律转变的主要原因。该模拟方法解决了精轧过程中无法直接测量氧化铁皮厚度的问题,为通过调整生产工艺参数控制氧化铁皮厚度提供了参考。     

低品位氧化锌烟气脱硫结垢试验研究

采用低品位氧化锌灰作为脱硫剂吸收模拟烟气中SO2。考察脱硫循环浆液的pH和氧化铅含量、矿浆浓度、不同吸收反应器对脱硫效果的影响,同时观察脱硫系统结垢情况。结果表明,在进口SO2浓度为3 000mg/m3、pH 6、矿浆浓度15%、浆液中氧化铅含量5%、液气比L/G=3L/m3、烟气量12m3/h、脱硫吸收反应器采用空塔喷淋的条件下,脱硫效率能够达到90%以上,且运行过程中不会出现结垢现象。     

航空发动机高温涂层腐蚀失效研究进展

航空发动机中高温涂层部件在不同的工况下会发生差异化的腐蚀现象。简要介绍了YSZ热障涂层、MCrAlY涂层的主要失效模式,重点梳理了这两类涂层的典型腐蚀失效形式,包括酸性腐蚀、熔盐腐蚀、CMAS腐蚀等。酸性腐蚀是涂层在潮湿环境且一般含有氯、硫条件下发生的弱酸性腐蚀,腐蚀温度为常温。熔盐腐蚀主要针对航空发动机在海洋环境下YSZ热障涂层及MCrAlY涂层面临熔融态盐(如NaCl、Na2SO4)发生的腐蚀,腐蚀温度一般为500~1 000 ℃。CMAS腐蚀主要是由沙漠、尘土环境下的复合氧化物引起陶瓷涂层失效,腐蚀温度一般高于1 200 ℃。最后对YSZ热障涂层及MCrAlY涂层的腐蚀失效机理及未来研究方向进行总结和展望。     

Kinetics and Reaction Mechanisms of High-Temperature Flash Oxidation of Molybdenite

The kinetics and reaction mechanism of the flash oxidation of +35/–53 μm molybdenite particles in air, as well as in 25, 50, and 100 pct oxygen higher than 800 K, has been investigated using a stagnant gas reactor and a laminar flow reactor coupled to a fast-response, two-wavelength pyrometer. The changes in the morphology and in the chemical composition of partially reacted particles were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and electron microprobe. High-speed photography was also used to characterize the particle combustion phenomena. The effects of oxygen concentration and gas temperature on ignition and peak combustion temperatures were studied. The experimental results indicate that MoS₂ goes through a process of ignition/combustion with the formation of gaseous MoO₃ and SO₂ with no evidence of formation of a molten phase, although the reacting molybdenite particles reach temperatures much higher than their melting temperature. This effect may be a result of the combustion of gaseous sulfur from partial decomposition of molybdenite to Mo₂S₃ under a high gas temperature and 100 pct oxygen. In some cases, the partial fragmentation and distortion of particles also takes place. The transformation can be approximated to the unreacted core model with chemical control and with activation energy of 104.0 ± 4 kJ/mol at the actual temperature of the reacting particles. The reaction was found to be first order with respect to the oxygen concentration. The rate constant calculated at the actual temperatures of the reacting particles shows a good agreement with kinetic data obtained at lower temperatures. The ignition temperature of molybdenite shows an inverse relationship with the gas temperature and oxygen content, with the lowest ignition temperature of 1120 K for 100 pct oxygen. Increasing the oxygen content from 21 to 100 pct increases the particle combustion temperature from 1600 K to more than 2600 K. A high oxygen content also resulted in a change of the reaction mechanism from relatively constant combustion temperatures in air to much faster transient combustion pulses in pure oxygen.