Effect of water vapor on high-temperature oxidation of FeCr alloys

The suppression of protective chromia scale formation in water vapor containing service environments limits in many cases the upper application temperature of high-Cr martensitic and ferritic steels. The present paper discusses the mechanisms which are responsible for this technologically important effect, using results of oxidation tests with two types of FeCr model alloys in Ar-O₂, Ar-O₂-H₂O, and Ar(-H₂)-H₂O mixtures. The data shows that in atmospheres with a high ratio of water vapor to oxygen, Cr exhibits a higher tendency to become internally oxidized than in dry Ar-O₂, or e.g. air. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale and/or to enhance formation of volatile Cr species, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy, likely by incorporation of hydrogen.     

On the high-temperature corrosion of Fe-Cr alloys in sulfur vapor

The mechanism of sulfidation of Fe-Cr alloys ranging from 8 to 97 wt.% chromium was determined from studies of scale structures, surface morphologies of scale, and reaction kinetics. Although the kinetics of sulfidation were quite similar to those previously determined by Mrowec et al., the structures in the present work were different, being triplex in nature. The growth mechanism of each layer was determined, and the overall sulfidation behavior was compared to the oxidation behavior. Many similarities between the two corrosion processes were observed.     

Predicting the life of high-temperature structural components in power plants

The concept of time-dependent fracture mechanics has been used to develop the quantitative life-prediction methodology and inspection criteria for high-temperature structural components. As an example, the methodology was applied to steam pipes. Leak-before-break analyses were utilized to determine the flaw inspection criteria of steam pipes. Both static and cyclic loading conditions were included in the life-prediction analyses. Increasing the frequency of shutdowns was found to decrease the remaining life. The effects of operating pressures and temperatures and material properties on the life of steam pipes were quantified.     

The stability of interfaces in high-temperature metal-matrix composites

The interface between the matrix and the reinforcement is an area of great importance in the design of viable high-temperature metal-matrix composites (MMCs). Thorough understanding of the phenomena taking place at the interface is necessary to ensure reliable performance, but the thermodynamic data that would help to predict interfacial reactions are lacking for many systems. However, given certain knowledge, interfaces can be classified according to their stability, providing a tool for composites designers and an impetus for further fundamental work. This article describes the classification system and provides examples of interfacial behavior in a titanium-based material.     

Oxidation behaviour of Nimonic 263 in high-temperature supercritical water

The oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600–700°C under 25?MPa were carried out for up to 1000?h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni–Fe/Ni–Cr spinel oxide, Ni/Co hydroxide and TiO₂ and an inner layer of a mixture of NiCr₂O₄ and Cr₂O₃ while the innermost layer is made up of Cr₂O₃. The MoO₃ can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.     

The formation of aluminum oxide scales on high-temperature alloys

This paper is a brief review of the extensive literature relating to the formation of protective —Al₂O₃ scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.     

Behavior of oxide films on high-temperature alloys in carbonaceous gas atmospheres

Formation and deterioration of oxide films on several contemporary high-temperature alloys was studied in various carbonaceous gas atmospheres. Scanning electron microscopy and metallographic examination were applied to films on Fe-Cr-Ni and Ni-Cr-Al type alloy compositions exposed in CH ₄/H₂ and CO/CO₂ atmospheres at temperatures up to 900° C and pressures up to 900 psi (6.2 × 10⁶ N/m²). The effects of various preoxidation treatments were evaluated. Reduction of certain oxide phases is observed to promote catalytic gas decomposition. Al₂O₃ components in the films are observed to be stable under the reducing conditions experienced. Carbon uptake by various alloys is found to be quite sensitive to surface finish, with an observed increase in penetration with surface roughness.     

Nanometer bismuth oxide produced by resistance heating vapor oxidation

Bismuth oxide has wide applications in superconductive material, photoelectric material, electronic ceramic, electrolyte, and catalysts. To produce ultrafine bismuth oxide powders, some costly heating sources, such as plasma, high frequency induction, electron beam or laser, have to be used in the conventional vapor oxidation methods. The vapor oxidation method was improved by adding a reducing agent in the reaction system, where heating source was resistance tubular oven, instead of special heat source requirement. Nanometer bismuth oxide was prepared at 1 000-1 140 ~C, and the particle characteristics were investigated by XRD, SEM, DTA, laser sedimentograph. With low oxygen concentration （less than 20%） in the carrier gas, the bismuth oxide particle was near-sphere β-Bi2O3 with uniform and fine particle size （d0.5=65 nm, GSD= 1.42）; while with higher oxygen content （more than 50%）, the powders were mixture of Bi2O2CO3 and β-Bi2O3.     

Synthesis and high-temperature stability of titanium aluminide matrixin situ composites

A premixture of elemental powders of titanium and aluminum was supplied as a spray material for the direct fabrication of titanium aluminide matrixin situ composites by means of reactive low-pressure plasma spraying with a nitrogen and hydrogen mixed plasma gas. The aluminum content varied from 10 to 63 wt.% in the premixtures. The matrix of sprayed layers consisted of three kinds of titanium aluminides—Ti₃A1, TiAl, and TiAl₃—which begin to form on a low-carbon steel substrate immediately after deposition. The formation of nitrides, which act as a reinforcement, occurs both during the flight of liquid droplets and on the substrate. The nitrogen content is approximately 4 to 5 wt.% in the sprayed intermetallic matrix composites, regardless of the aluminum content of the premixtures. The kinds of titanium aluminides andin situ nitrides developed depend on the aluminum content of the premixtures. The homogeneity of the distribution of aluminum and titanium in sprayed intermetallic matrix composites has been improved by vacuum annealing. The predominant TiAl phase that formed in the sprayed intermetallic matrix composites with a Ti-36 wt.% AI premixture increases in quantity through annealing. Although some minor nitrides disappear through annealing, the principal reinforcement, Ti₂AlN, does not decompose, but increases in quantity. The hardness of sprayed intermetallic matrix composites varies with aluminum content of the premixtures, but is always greater than that of sprayed titanium aluminides containing no nitrides. Annealing does not reduce the hardness of sprayed intermetallic matrix composites. Sprayed and annealed intermetallic matrix composites with a Ti-36 wt.% Al premixture maintain their hardness of approximately 500 HV up to 800 K. Hence, reactive low-pressure plasma spraying offers a promising fabrication method for titanium aluminide matrixin situ composites, which are expected to excel in wear resistance applications at elevated temperatures.     

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. II. Scales grown in water vapor

The acoustic emission (AE) technique has been applied to identify scale cracking during thermal cycling of tubes of 2.25Cr-1Mo steel. The scale morphology and failure mode were investigated by light and electron optical methods. The scale formed at 600°C in water vapor consists of an outer magnetite and an inner, chromium-containing spinel layer. Cooling leads to tensile stresses in the scale that cause macro and microcrack formation in the scale. At constant-cycle parameters, a characteristic set of crack length and crack density is established. Changes in the cycle parameters also change the crack length and crack density. The experimental results can be described by a model developed by Hasselmann assuming a large number of noninteracting microcracks in a ceramic plate.     

Observations on the role of oxide scales in high-temperature erosion-corrosion of alloys

The results are presented of exposure to a controlled high-temperature erosive gas stream of a series of alloys, which were selected to represent the range of microstructures and mechanical properties available in commercial high-temperature alloys. Analysis of the kinetic and morphological data suggested that the high-temperature oxidation behavior of a given alloy plays a very important role in determining its erosion-corrosion behavior under the conditions studied. In terms of relative behavior, alloys which are weak but ductile at temperature, and which form tenacious oxide scales, exhibited the highest resistance to high-temperature erosion-corrosion. Simple models were developed to describe the expected interaction between high-temperature oxidation and erosion.     

碳对高温钛合金组织稳定性的影响

结合实验研究,分析了碳合金化对高温钛合金显微组织、高α+β相区、时效过程中α2和硅化物析出的影响机制及其碳化物的稳定性。α+β相区再结晶,碳化物主要在β转变组织中析出以及碳化物的百分数取决于初生α(αp)体积分数;β相区热处理,碳化物的析出阻碍β晶粒的长大。碳的加入延缓αp体积分数随温度的变化速度,扩大高温钛合金的高α+β相区;同时降低αp相Al的浓度,增加β转变组织中Al和Mo的浓度,导致αp相内α2颗粒直径逐渐减小,颗粒间距增大。β相与碳化物之间的包析反应是碳化物溶解的主要动力。     

Structure and high-temperature stability of compositionally graded CVD mullite coatings

Dense, uniform and crack-free mullite (3Al₂O₃·2SiO₂) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al₂O₃ nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and -alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.     

Ti-Al合金高温氧化膜的形态及形成

利用Pt丝标记法和定量成分分析等研究方法,研究Ti-Al(2%~75%Al,摩尔分数)合金在1 173 K的大气中经24 h氧化形成的氧化膜的组织形态和生长过程。结果表明:在氧化膜中观察到Al2O3存在3种不同分布形式,即当2%65%时,氧化膜由单一的Al2O3构成。Ti-Al合金高温氧化膜中Al2O3和TiO2的分布可分为4种组织形态。采用Pt标记可清晰观察到氧化膜内、外层,外层氧化膜的形成与Al离子向外扩散有关,内层氧化膜的形态受MO/M界面热力学平衡控制。     

The low-temperature oxidation of calcium by water vapor

The oxidation kinetics of calcium in water vapor have been studied over the temperature range 25–300°C. There is a change in the form of the oxidation kinetics with temperature, from essentially linear at temperatures below 150°C to logarithmic at 300°C. This is coupled with a change in the manner of growth of the oxide layer as well as the chemical composition of the reaction product. In addition, the oxidation rate decreases with temperature, reaching a minimum at about 150°C. At temperatures below 150°C oxidation appears to be a result of the formation of cracks or fissures in the oxide film. Above 150°C no single oxidation mechanism can be deduced.