Improving metal dusting resistance of. transition‐metals and Ni‐Cu alloys

The present study focuses on a new technique for the prevention of metal dusting in carbonaceous gas environments at intermediate temperature. Preliminary laboratory metal dusting test was conducted for transition‐metals and Ni‐x%Cu binary alloys in a simulated 60%CO‐26%H₂‐11.5%CO₂‐2.5%H₂O (in vol.%) gas mixture at 650°C for 100 h. The metal dusting caused no coke deposition on transition‐metals of Cu, Ag, and Pt, while those of Fe, Co, and Ni have a large amount of coke and lost mass. Whether or not coking behavior of Ni‐Cu binary alloys formed any oxide scales in the simulated gas environment depended on the Cu content. Specimens containing low Cu were entirely covered with coke and showed rough metal surfaces due to the degradation of metal. Alloys of 20% and more Cu, on the contrary, had no coke deposition and smooth metal surfaces, suggesting alloys with an adequate Cu do not react with CO in the gas mixture without an oxide scale barrier. Based on these results, we conclude that Cu does not protect by formation of the oxide scale but has a “Surfactant‐Mediated Suppression” against metal dusting. This effect can be explained in terms of atomistic interaction of CO with transition‐metal surfaces by electronic structure analyses. The concept can be also useful for the practical material design of Ni‐Cr base alloy with excellent metal dusting resistance.     

The influence of surface condition on the metal dusting behavior of cast and wrought chromia forming alloys

The current work investigated the impact of surface condition on the metal dusting behavior of chromia forming alloys. Five commercial alloys were included in the study, wrought 800H, 353MA, and cast G4859, G4852 Micro, and ET45 Micro, these alloys have a chromium and nickel content in the range of 20–35 wt% and 32–45 wt%, respectively. The wrought alloys were tested in a pickled state and the cast alloys with a machined surface, all the alloys were tested using a laboratory ground surface condition for comparison. The exposures were performed using a gas with a composition of 44 vol% CO, 52 vol% H₂, 2 vol% CO₂, and 2 vol% H₂O at a temperature of 600 °C and a pressure of 5.5 bar. The samples were periodically characterized by measuring the mass loss, pit density, pit size, and pit depth. The results show that the pickled surfaces were sensitive toward metal dusting attack while the machined and the ground surfaces had better resistance. This shows that the surface pre‐treatment plays a crucial role.     

Occurrence of metal dusting – referring to failure cases

Twelve years of research on metal dusting at the Max‐Planck‐Institut for Iron Research have led to comprehensive knowledge on the mechanisms and kinetics of metal dusting and on ways and means for prevention of this corrosion phenomenon, – this knowledge is shortly summarized in this paper. Inspite of this state of understanding, the present opinion in industry is that metal dusting is mysterious and not predictable and cannot be controlled. This paper is intended to show that the occurrence of metal dusting and its reasons can be well understood, by describing and explaining five failure cases.     

Effect of experimental conditions on the metal dusting phenomenon in several commercial nickel‐base alloys

The corrosion phenomenon, metal dusting, has been observed in many high‐temperature industrial plants, such as ammonia, syngas and steam‐reforming industries. An experimental research programme has been carried out into the degradation resistance of wrought and cast commercial high‐temperature nickel‐base alloys in H₂/CO gas mixtures at 650 °C. The results in this paper are focused on the effect of the experimental set‐up on the response of the alloy during exposure to the gas mixture; in particular, the specimen holders had an important role on the onset of carbon transfer from the gas to the alloy surfaces. Specimens of most alloys exposed to the gas mixture while suspended from quartz hangers were shown to have good resistance to the environment for periods up to 5000 h. On the other hand, specimens of these alloys, exposed while located in porous refractory ceramic holders, gained large amounts of coke and underwent severe damage. A series of short tests were carried out in order to ascertain the main factors influencing the alloy reactivities in the experimental rig. The presence of specimens of a less‐resistant alloy and slight variations in gas composition (with the addition of CO₂) were observed to have a marked impact on the alloy reactivities. Factors such as catalysts, contaminants and gas composition were considered and are discussed.     

Effect of pre‐oxidation on coke formation and metal dusting of electroplated Ni3Al–CeO2‐based coatings in CO–H2–H2O

Pre‐oxidation was introduced to improve the resistance of electroplated pure, 5 µm CeO₂‐dispersed, and 9–15 nm CeO₂‐dispersed Ni₃Al coatings to coke formation and metal dusting in 24.4%CO–73.3%H₂–2.3%H₂O at 650 °C. Coke formation and metal dusting of pre‐oxidized Ni₃Al‐based coatings were retarded up to 200 h owing to a thin Al₂O₃ scale induced during pre‐oxidation. The long‐term effectiveness of pre‐oxidation nonetheless depended on the integrity of Al₂O₃ scale. The pure Ni₃Al coating suffered severe spallation after pre‐oxidation and thereby showed the worst long‐term resistance. Two pre‐treated 9–15 nm CeO₂‐dispersed Ni₃Al coatings exhibited the best long‐term resistance to carbon attack because nano‐CeO₂ particles maintained a full coverage of Al₂O₃ scale on the coatings. Two 5 µm CeO₂‐dispersed Ni₃Al coatings showed significant spallation after pre‐oxidation because of an overdoping effect and experienced coke formation and metal dusting during long‐term exposure.     

Effect of H2S on metal dusting of iron

A review is given on the effect of H₂S on metal dusting of iron which has been studied by gas carburisation in CO‐H₂‐H₂O‐H₂S and CH₄‐H₂‐H₂S mixtures at 500 and 700°C. The presence of H₂S in carburising gas atmospheres leads to sulphur adsorption on the iron surface, which retards carbon transfer. Segregation experiments and surface analyses have shown that sulphur segregates (and thus adsorbs) on cementite surfaces as well as on iron surfaces. The adsorbed sulphur also suppresses graphite nucleation and thus can stop the reaction sequence of metal dusting. Experiments by thermogravimetric analysis (TGA) have shown that the extent of retardation of metal dusting depends on temperature, carbon activity and H₂S content. The higher the carbon activity, the higher is the H₂S content required for suppression of metal dusting. At carbon activities a_C > a_C(Fe/Fe₃C) the metastable iron carbide, cementite (Fe₃C), occurs as an intermediate phase during metal dusting. Carburisation experiments in CO‐H₂‐H₂O‐H₂S mixtures at 500°C and X‐ray diffraction analysis (XRD) of carburised samples have revealed that at very high carbon activities a second iron carbide, Hägg carbide (Fe₅C₂), forms on the cementite surface. Microstructural investigations have shown that both metastable carbides decompose during metal dusting. Metal dusting experiments on iron at 700°C have been performed in CH₄‐H₂‐H₂S gas mixtures. By adding 15 ppm H₂S to the CH₄‐H₂ atmosphere the onset of metal dusting can be retarded for more than 350 hours. By means of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.     

Corrosion resistance of Ni‐50Cr HVOF coatings on 310S alloy substrates in a metal dusting atmosphere

Metal dusting attack has been examined after three 168 h cycles on two Ni‐50Cr coatings with different microstructures deposited on 310S alloy substrates by the high velocity oxy‐fuel (HVOF) thermal‐spray process. Metal dusting in uncoated 310S alloy specimens was found to be still in the initiation stage after 504 h of exposure in the 50H₂:50CO gas environment at 620 °C. Dense Ni‐50Cr coatings offered suitable resistance to metal dusting. Metal dusting was observed in the 310S substrates adjacent to pores at the interface between the substrate and a porous Ni‐50Cr coating. The porosity present in the as‐deposited coatings was shown to introduce a large variability into coating performance. Carbon formed by decomposition of the gaseous species accumulated in the surface pores and resulted in the dislodgement of surface splats due to stresses generated by the volume changes. When the corrosive gas atmosphere was able to penetrate through the interconnected pores and reach the coating–substrate interface, the 310S substrate was carburized, metal dusting attack occurred, and the resulting formation of coke in the pores led to local failure of the coating.     

金属氧化物半导体纳米晶液相法控制合成研究进展

对金属氧化物纳米晶的晶型、形貌及晶粒大小调控方法进行了综述,重点介绍了液相合成体系中表面活性剂、溶剂、杂质离子等因素对纳米晶形貌、晶型的调控作用以及各种方法的特点,提出发展绿色、高选择性合成方法制备单分散结构可控的金属氧化物半导体纳米晶仍是今后重要的研究方向之一。     

Recent advances in pseudocapacitor electrode materials: Transition metal oxides and nitrides

Faraday pseudocapacitors take both advantages of secondary battery with high energy density and supercapacitors with high power density, and electrode material is the key to determine the performance of Faraday pseudocapacitors. Transition metal oxides and nitrides, as the two main kinds of pseudocapacitor electrode materials, can enhance energy density while maintaining high power capability. Recent advances in designing nanostructured architectures and preparing composites with high specific surface areas based on transition metal oxides and nitrides, including ruthenium oxides, nickel oxides, manganese oxides, vanadium oxides, cobalt oxides, iridium oxides, titanium nitrides, vanadium nitrides, molybdenum nitrides and niobium nitrides, are addressed, which would provide important significances for deep researches on pseudocapacitor electrode materials.     

Coke formation during metal dusting of iron in CO‐H2‐H2O gas with high CO content

Iron carburisation and coke formation during metal dusting of iron have been investigated in the gas mixture of 75%CO‐24.8%H₂‐0.2%H₂O at 600°C and 700°C. In all cases, cementite is formed at the surface, together with a coke layer on the top. In the coke layer, two morphologies of graphite are identified: compact bulk graphite with a uniform thickness and a columnar structure, and filamentous carbon with iron‐containing phases at the tip or along its length. The examination of coke formation in different stages of reaction at 700°C reveals that the coke contains two layers. The inner layer is composed of filaments, while the outer layer consists of the compact columnar graphite. After 2 h reaction the top compact graphite layer has suffered a serious deformation and has formed fractures because of the growth of catalytic filamentous carbon underneath. These filaments grow outside from these fractures and finally cover the whole surface after 4 h reaction. At 600°C, however, the coke contains a thick bulk graphite layer and non‐uniformly distributed filaments on the top. The bulk graphite layer is composed of many graphite columns which are loosely piled and are vertical to the surface. Each graphite column consists of many fine graphite fibres in parallel with the columnar axis. Filaments grow outside preferably from the gaps among these graphite columns and along the grinding scratches. TEM analysis of the coke detects very convoluted filaments with iron‐containing particles at the tip or along their length. XRD and TEM analyses show that these particles are Fe₃C rather than metallic iron.     

AZ31B镁合金熔化极气体保护焊焊缝的组织和力学性能(英文)

使用熔化极气体保护焊对变形镁合金板材对接。在不同焊接工艺参数下观察焊缝中的气孔特征,分析气孔成因以及气孔对接头微观组织和力学性能的影响。显微组织检测结果表明:气孔主要分布在焊缝顶部或底部,而且气孔能相互连接甚至产生裂纹。然而,在合适的焊接工艺参数下,气孔的产生可以得到有效控制。拉伸测试结果表明:接头的平均抗拉强度接近甚至高于母材。由于第二相β-Mg17(Al,Zn)12的强化作用,焊缝区的硬度高于母材。     

α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.     

Recent trends in metal alloy electrolytic and electroless plating research: a review

Abstract

A short review of the recent electrolytic and electroless plating research trends for a variety of metal alloys deposited to improve corrosion resistance, wear characteristics and hardness, magnetic behaviour, catalytic properties and for other application areas, is presented. The review contains key references from the period 2008 to the present (mid 2011), although it is not exhaustive. One finding is that most of the studies highlighted appear to be focused on one or more application areas, rather than being mechanistic deposition process studies. In addition, applications other than corrosion and wear resistance seem to have inspired significantly more effort than hitherto.     

金刚石表面镀层在磨具中的作用机理

采用湿法镀（化学镀、电镀）或真空镀（蒸发镀、溅射镀、离子镀）方法,在金刚石表面镀覆一层铜、镍、钛、钼、钨等金属,或者它们的复合镀层。表面镀覆已经与单晶合成、分选、聚晶烧结、气相沉积一起,成为金刚石多品种专用化发展的五条重要途径之一。镀层赋予金刚石所需要的各种特有的物理化学性能,不仅可以提高金刚石颗粒强度以及与结合剂的结合强度,还有对金刚石表面的宏观隔离保护作用和对金刚石结构的微观侧面支撑作用．从而有利于防止金刚石在使用过程中被氧化和石墨化。特别是在树脂结合剂磨具中,镀层的热导率介于金刚石和树脂之间,可以有效地减缓磨削热脉冲对树脂造成的热损伤,从而显著延长其耐用度,使树脂磨具的使用寿命提高50％-100％。     

钢纤维增强有色金属基复合材料综述

有色金属基复合材料相对于传统有色金属材料而言,具有更好的抗氧化性、高耐热性、高比强度、高比模量、耐磨损和高使用寿命。在有色金属基复合材料的众多的增强体中,非金属纤维(C/C、SiC)与金属基质结合界面的相容性是制约金属基复合材料性能的关键问题,而金属纤维与金属基质之间良好的相容性能够有效改善金属材料的性能。金属纤维增强有色金属基复合材料的制备工艺主要有扩散粘结法、液态渗透法、压力铸造法、涂层热压法、双辊轧制法。 本文主要总结了钢纤维增强有色金属基(Al、Mg、Cu、Zn和Zr)复合材料的制备方法、微观组织、界面特征和机械性能,指出了钢纤维增强有色金属基复合材料进一步研究发展所需要解决的问题。     

金属氧化物表面与界面的第一性原理热力学:一个研究实例

通过第一性原理计算研究,能够揭示和阐明材料相关表面与界面热力学性质,这是通向纳米和界面材料设计自由的重要一步,即以理想性能为目标,科学设计制备工艺参数,针对性地调控材料微观结构.以SnO2纳米颗粒表面和原位内氧化制备获得的Ag-SnO2界面计算研究为例,系统介绍这一计算研究策略.基于缺陷热力学模型,构建不同环境温度和氧分压条件下所对应的热力学平衡状态的表面和界面结构模型,计算评估其相应性能.通过一系列的第一性原理能量学计算,进一步构建出平衡状态的表面与界面相图.这些相图能够充分描述制备和服役过程中,工况对材料表面和界面微观结构和相应性能的影响和作用关系.介绍和讨论如何应用这些相图理解和科学设计相应材料的制备工艺.     

金属板料冲压数值模拟中的宏观硬化模型研究现状

金属板料冲压数值模拟的精度取决于所应用的材料模型是否能够对材料的弹塑性行为提供准确描述。对于考察材料力学响应在变形过程中的变化,采用宏观硬化模型,并将微观结构特征整合在宏观内变量中,是一种合适的方法。文章结合微观结构演化,讨论了对金属材料宏观硬化的4种基本方式(各向同性硬化、运动硬化、旋转硬化、畸变硬化)及其限制;根据对不同应变路径变化下材料性能变化的描述,归纳了近年来较为典型的板料宏观硬化模型研究,并指出其不足之处及需要进一步研究的内容。同时,就金属板料冲压的宏观硬化模型研究需要注意的问题进行讨论。     

基于同步辐射成像技术的原位观察装置及金属凝固过程原位表征研究进展（英文）

金属材料的凝固过程对其最终显微组织和性能起着决定性的作用。因此,采用先进的原位表征技术探究金属的凝固过程具有重要意义。同步辐射X射线成像技术是直接观察金属凝固过程的有效手段。本文作者总结了与同步辐射光源相配套的凝固装置的发展,包括定向凝固装置、电阻炉、多场耦合装置、半固态成形装置、气动悬浮装置和激光增材制造装置。重点介绍了应用同步辐射X射线成像技术揭示不同原位环境下金属凝固行为的最新研究进展,并对该技术在凝固领域的前沿研究方向进行了展望。可以预见,同步辐射X射线成像技术的发展将在不同尺度上深入理解金属和其他材料的凝固过程发挥重要作用。     

金属离子在矿物浮选中的典型作用

在矿物浮选中,金属离子可以通过选择性活化或抑制矿物表面,从而发挥重要的作用.尽管此前大量工作报道了金属离子在浮选中的作用,但大多仅是针对某一特定类型金属离子或目的矿物进行研究分析,缺乏一个全面的综述.本文对金属离子在矿物浮选中的影响因素进行详细分类,总结金属离子在不同种类矿物浮选中的作用机理.此外,还对常见探索作用机理...     

Diamond machining of silicon: A review of advances in molecular dynamics simulation

Molecular dynamics (MD) simulation has enhanced our understanding about ductile-regime machining of brittle materials such as silicon and germanium. In particular, MD simulation has helped understand the occurrence of brittle–ductile transition due to the high-pressure phase transformation (HPPT), which induces Herzfeld–Mott transition. In this paper, relevant MD simulation studies in conjunction with experimental studies are reviewed with a focus on (i) the importance of machining variables: undeformed chip thickness, feed rate, depth of cut, geometry of the cutting tool in influencing the state of the deviatoric stresses to cause HPPT in silicon, (ii) the influence of material properties: role of fracture toughness and hardness, crystal structure and anisotropy of the material, and (iii) phenomenological understanding of the wear of diamond cutting tools, which are all non-trivial for cost-effective manufacturing of silicon. The ongoing developmental work on potential energy functions is reviewed to identify opportunities for overcoming the current limitations of MD simulations. Potential research areas relating to how MD simulation might help improve existing manufacturing technologies are identified which may be of particular interest to early stage researchers.