High-temperature oxidation of tungsten boride in oxygen and the effect of scale evaporation

The oxidation of tungsten boride in oxygen at pressures from 1 to 740 Torr over the temperature range of 500 to 1200°C was studied by means of thermogravimetric, petrographic, and x-ray analyses. Kinetic parameters of the oxidation were interpreted on the basis of the Frantzevitch oxidation equation, characterizing the volatility of the phases formed, the emf of a high-temperature galvanic cell, having a gas cathode and a metal anode, and the values of the electrical resistance of the film.     

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr₂O₃, except for a narrow band adjacent to the surface where NiCr₂O₄ particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr₂O₃ which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr₂O₃-rich surface bands.     

Kinetics of high-temperature oxidation of boron carbide in oxygen

Thermogravimetry and gas-adsorption chromatography were used to study the kinetics of formation of solid and gaseous products during the hightemperature oxidation of compact boron carbide in oxygen at 740 Torr. Oxidation resistance was observed at temperatures up to 1200°C. The main oxidation products were B₂O₃ and CO₂. Oxidation was paralinear; the carbon consumption exceeded the consumption of boron as compared to the ratio of these elements in the compound B₄C. This difference resulted in carbon depletion of the carbide layer in the substrate near the scale>.     

Influence of prior internal oxidation on the oxidation of dilute Co-Cr alloys in oxygen

The influence of an initial preinternal oxidation treatment in Co/CoO on the subsequent oxidation behavior of a series of dilute Co-Cr alloys (containing 0–1.5 wt. % Cr) in 10⁵ and 10³ Pa oxygen at 1473–1623 Khas been investigated. Particular emphasis has been placed on determining the solubility and mobility of Cr³⁺ ions in CoO. Use has been made of subsequent annealing in argon .     

Effect of oxygen partial pressure on the high-temperature oxidation of CVD SiC

The oxidation behavior of chemically vapor-deposited silicon carbide (CVD SiC) was studied at 1670-2010 K in O₂-Ar and CO₂-Ar. The oxidation kinetics in O₂-Ar was parabolic or linear parabolic, and was parabolic in CO₂-Ar. The activation energy for the parabolic rate constants (k_p) was 210-220 kJ/mol in O₂-Ar, and was 290-300 kJ/mol in CO₂-Ar. The oxygen partial pressure (P_O2) dependence of k_p was expressed as k_p∝ (P_O2)ⁿ, where n=0.08-0.13 and 0.37-0.53 in O₂-Ar and CO₂-Ar, respectively. Bubbles were formed at more than 1985 K and P_O2>5 kPa. The bubble formation temperature decreased with decreasing P_O2 at P_O2<5 kPa.     

The effect of purity on the high-temperature oxidation of hafnium

The oxidation kinetics of hafnium of different purities were studied in the temperature range of 800–1200°C. This paper concerns the structure studies of the oxidized samples. TGA, XRD, SEM methods, and microhardness measurements were used. Hafnium oxidation follows the parabolic rate law, changing with time to the linear one. Oxidation-reaction products are HfO_2(moncl) and -solid solution of oxygen in hafnium. Anomalous oxidation behavior of hafnium having a high impurity content was found in the range 800–950°C. Activation energies for the parabolic and linear oxidation stages were calculated.     

Oxide sublimation during the high-temperature oxidation of nickel alloys with protective coatings

It is shown that the oxidation of heat-resistant alloys is accompanied by a sublimation of the oxides MoO₃, WO₃, Al₂O₃, and CrO₃. The determinant in the sublimate is the molybdenum content, exceeding that in scale by a factor of 50–100. Protective coatings diminish the sublimation, rather than fully eliminate it. To diminish the molybdenum oxide sublimation decisively, it is expedient to alloy the coating with magnesium or magnesium oxide. Original Russian Text ? E.G. Ivanov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 4, pp. 415–418.     

The effect of purity on high-temperature oxidation of zirconium

The oxidation kinetics of zirconium of different purities were studied over the temperature range of 600–1300°C (- and -phases). The structure of the oxidized specimens was examined. TGA, XRD, EPMA, SEM, metallographic analysis, and microhardness measurements were carried out. Impurity elements were found to increase the oxidation rate of technical zirconium. The mechanism of the effect of impurity elements on zirconium oxidation was shown to differ for the - and -phases. Activation energies were calculated for the parabolic and linear stages of oxidation.     

The role of nickel in the high-temperature oxidation of Fe-Cr-Ni alloys in oxygen

The oxidation of several largely austenitic Fe-Cr-Ni alloys in 1 atm oxygen at 800–1200°C has been studied thermogravimetrically, metallographically, and in detail by electron probe micro analysis. Fe-Cr-Ni alloys of this type are protected by Cr₂O₃-healed scale, which thickens slower than on the corresponding binary Fe-Cr and Ni-Cr alloys, presumably because nickel and iron ions dope the Cr₂O₃ more effectively together than singly and/or because the alloy composition and ability to absorb cation vacancies are such as to produce a smaller vacancy activity gradient or level in the scale, or voids within it. The scale adhesion, as on Ni-Cr alloys, is generally good after long times, at least partly due to the convoluted alloy-oxide interface, in some cases to large intergranular Cr₂O₃-rich stringers, and possibly to the general specimen mechanical properties. Nonprotective stratified scale development is relatively unusual and often produces nickel-rich, alloy-particle-containing nodules, as on Fe-Ni alloys. Careful selection of ternary and more complex alloys with appropriate alloy interdiffusion coefficients and oxygen solubilities and diffusivities should permit development of materials with the best compromise between ease of Cr₂O₃ establishment, avoidance of breakaway, and readiness of scale healing.     

Influence of oxidation on the high-temperature mechanical properties of zirconia/nickel cermets

The influence of an oxidizing atmosphere on the high-temperature plasticity of zirconia/nickel cermets has been studied by conducting creep tests in air. The resulting microstructure has been characterized by scanning, conventional and high-resolution electron microscopy. Despite the large microstructural changes, the composites do not exhibit mechanical degradation.     

TP347H钢高温水蒸气氧化研究

研究了在600MW超临界机组屏式过热器TP347H管运行5000h后的高温蒸气氧化层。通过分析蒸气氧化层成分、相结构以及Cr的分布,并结合高温水蒸气存在的情况下氧化膜生长速度,提出了高温蒸气氧化层的形成机理。由Cr的扩散探讨了晶粒尺寸对TP347H管抗高温蒸气氧化性能的影响。结果表明:CrO2(OH)2的挥发导致了外层Fe的氧化物的形成,FeCr2O4内层的形成能够有效减缓蒸气氧化进程,进入稳态氧化阶段后氧化层厚度将保持不变;同时,随晶粒尺寸的减小TP347H的抗蒸气氧化性能提高。     

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Twenty-four ternary element additions were made to a binary TiAl alloy (Ti–34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al₂O₃ barrier being formed. This is probably due to the doping of those elements in TiO₂. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al₂O₃ layer as the second layer from the outer surface and the relatively massive precipitation of Al₂O₃ in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.     

高温空气氧化对高硅铝合金材料组织及物理性能的影响

采用高温空气氧化对Al-12Si与Al-30Si合金粉末进行预处理,结合包套热挤压,制备出Al2O3与SiO2增强的弥散强化型铝硅复合材料,通过金相及热物性测试,对材料显微组织、密度、气密性、热膨胀系数、导热系数进行了分析.结果表明合金粉末经高温空气氧化预处理后,Al-12Si晶粒长大不明显,而Al-30Si晶粒发生了明显长大;材料致密度随氧化时间延长略有下降,但均在97%以上,材料气密性均在1×10-9 Pa·m3/s左右;在相同工艺条件下,Al-30Si材料热膨胀系数明显低于Al-12Si材料,氧化时间对材料热膨胀系数影响不明显;导热系数随材料中硅含量增加而下降,随粉末氧化时间延长而增加,氧化24 h后,材料导热系数超过120 W/(m·K).     

Investigation of the effect of fraction composition and specific surface of anticorrosive pigments on the protective properties of epoxy coatings

The technological parameters of grinding of anticorrosive pigments are presented. The effect of nanosized particles of anticorrosive pigments on the protective properties of coatings based on epoxy varnish is studied. The structures of varnish-and-paint coatings are investigated by means of electron-scanning microscopy and atomic force microscopy.     

On the high-temperature oxidation of nickel

This paper summarizes on some of the extensive experimental data and corresponding models suggested to account for the oxidation mechanism of Ni in the temperature range 500-1400 °C. In addition it reports on in-house experimental data from investigations related to the oxidation of high-purity Ni from 500 to 1300 °C in the oxygen pressure range 1×10⁻⁴-1 atm based on TG, measurements of surface kinetics, two-stage oxidation, scanning electron microscopy, atomic force microscopy, secondary ion mass spectroscopy etc. The main part of this paper focuses on the more complex models suggested to account for experimental observations of the oxidation kinetics and the oxide morphology below 1000 °C.     

Duplex scale formation during high-temperature oxidation of Ni-0.1 wt.% Al alloy

The development of a duplex NiO scale microstructure on a Ni-0.1 wt.% Al alloy at 900°C has been examined, principally using secondary-ion mass spectrometry and analytical transmission electron microscopy. The¹⁸O-tracer distribution following sequential oxidation in¹⁸O₂/¹⁸O₂ showed that the inner NiO layer formed as a result of gaseous-oxygen penetration of the scale. The provision of pathways for oxygen transport as well as the role of Al, Si, and Ce segregation at oxide grain boundaries in influencing the growth rate and spallation behavior of the scale are discussed.     

Investigation of the high-temperature oxidation of Fe-Cr alloys by means of the isotope18O

The mechanism of oxidation at 1000°C of Fe-Cr alloys has been investigated. The samples were oxidized in two stages: first in an atmosphere of ¹⁶ O and subsequently in ¹⁶ O. Prior to each stage the samples had been marked with platinum wire markers. The concentration of ¹⁸ O in the scale was determined by means of two nuclear reactions: ¹⁸ O(,n) ²¹ Ne, and ¹⁸ O(p,n) ¹⁸ F independently. The results show that the dissociative mechanism of scale growth on Fe-Cr alloys is similar to that found earlier for Cu-Zn type alloys for which measurements were made using the radioactive sulfur isotope,³⁵ S.     

Anomalous high-temperature oxidation in the zirconium-copper system

The high-temperature oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu in oxygen is characterized by selective oxidation of zirconium while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₈Cu₅ phase. The oxidation of Zr₂Cu at elevated temperatures shows an anomalous decrease of the oxygen consumption rate in the temperature range 890-975 °C. The oxide layer consists of monoclinic ZrO₂ mainly, with preferentially oriented crystallites in depth region at 900 °C and tetragonal ZrO₂ on the surface below 600 °C, and small amounts of CuO and Cu₂O. The reaction kinetics obeys a parabolic rate law. The activation energy of 117.5 and 54.4 kJ/mol has been estimated for the oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu, respectively.