Prediction of the chromium oxide oxidation rate from the equilibrium constants and the mass transfer coefficients at high temperatures

Previously published experimental data on the oxidation/volatilization of chromium oxide covering a wide range of oxygen pressure (10^–6 to 1 atm) and temperature (900 to 1385°C) were used to predict its oxidation rate from the equilibrium constants and the mass transfer coefficients using the kinetic model originally developed by Bartlett. The experimental data available were found to be either surface-reaction-controlled or volatile-product-controlled which are the limiting cases of the model. The availability of the dimensionless fluid correlations and the force constants for the diffusing species enabled the prediction of the mass transfer rate for the various sample shapes and hydrodynamic conditions. The good agreement between the experimental and predicted oxidation rates covered four orders of magnitude range.Prepared for the U.S. Department of Energy under contract No. W-7405-Eng-82.     

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.     

Comparison of classical oxidation and laser oxidation of a chromium PVD coating on a pure-iron substrate

The oxidation of a chromium PVD coating on pure iron by a continuous 5-kW CO₂ laser beam in pure oxygen at 700°C for 20 min was compared with classical furnace oxidation. Laser oxidation induces faster oxidation kinetics, especially at the beginning of oxidation, without modifying the oxide nature (Cr₂O₃) and morphology. Oxygen-isotopic-exchange tests show that oxygen grain-boundary diffusion does not depend on the oxidation conditions, at least after 12 min oxidation. The effec of the laser treatment is discussed with respect to oxide nucleation, metastable-oxide formation, and oxide-formation-entropy evolution. The oxidation kinetics follow a parabolic law, and the oxide-growth mechanism was attributed to countercurrent oxygen and chromium diffusion along grain boundaries. Oxygen diffusion occurred by oxygen interstitials. The oxidation constant calculated from grain-boundary diffusion in the chromia scale is smaller than the experimental oxidation constant, suggesting the presence of particular short circuits (e.g., microcracks).     

The transition from internal oxidation to continuous-film formation during oxidation of dilute Ni-Si alloys

The oxidation of Ni-2.05Si and Ni-4.45Si was studied in oxygen over the range of 600°–1000°C for 18 hr. The oxidation kinetics did not follow a parabolic rate law, bur rather a power law of the form (w)ⁿ=kt was followed. The value of n ranged from 2.7 to 4.9 for Ni-2.05Si and from 3.0 to 6.4 for Ni-4.45Si. The low-silicon alloy exhibited extensive internal oxidation, whereas the higher-silicon alloy did not internally oxidize. In general, NiO containing little or no silicon formed as an exterior layer on both alloys. The internal oxidation zone in Ni-2.05Si was highly irregular in thickness, and in some areas there was no internal oxidation. The higher-silicon alloy formed a continuous layer of a silicon-rich oxide. X-ray diffraction did not detect silica (amorphous), and no evidence of Ni₂SiO₄ was observed, although EDAX analysis suggests that small amounts of the silicate might have formed. Theaverage thickness of the internal oxidation zone was found to agree well with calculated values based on oxygen solubility and diffusivity data. No enrichment of silicon occurred in the internal oxidation zone. Calculated values, 0.033 and 0.038 (depending on the model used), of the mole fraction of silicon required for the transition from internal oxidation to continuous silica film formation agreed well with experimental data obtained in both this study and with others reported in the literature.     

不同混合气氛下9Ni钢的高温氧化行为研究

对9Ni钢在不同氧分压及不同湿度条件下的高温氧化行为进行了研究。结果表明,9Ni钢的氧化行为会随着氧分压降低而受到阻碍,在低氧分压条件下没有Ni富集层产生,氧化进程受供氧量严重制约。而在不同湿度条件下,随着相对湿度的增大,氧化增重、氧化速率常数均增大;总氧化层厚度、Ni富集层厚度均增大,且增厚趋势趋于一致。潮湿环境下铁氧化层形成的大量裂纹、孔洞等缺陷,有利于气相物质的扩散,起到加速氧化的作用。     

Effect of grain size reduction on high temperature oxidation of binary two-phase alloys

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铁在双相Fe—Cu合金中的内—外氧化

三处Ｆｅ－Ｃｕ合金在氧分压低于氧化铜平衡分解压条件下的高温氧化结果表明,合金中同时发生了活泼组元铁的内氧化和外氧化,外氧化膜为单一铁的氧化物,内氧化区中氧化铁和金属铜继承了原始合金中两相的分布。内氧化区前沿的合金中未发现铁的贫化。这种氧化膜结构的单相合金中罕见而在双相合金中较为普遍,被认为是双相合金中两组元间有限地互溶度使得氧在合金中的过饱和程度比在固溶体合金中更强烈的结果。     

Kinetics of the oxidation of ilmenite

The oxidation of ilmenite (FeTiO₃) in air and dry oxygen was investigated over the temperature interval 600 to 970°C. Dense platelets of ilmenite crystals as well as powder samples of ilmenite were oxidized. The weight data were recorded employing a thermobalance. The oxidation kinetics of ilmenite platelets were parabolic except for the initial stages during which logarithmic kinetics were observed. For powder samples the logarithmic rate law was followed primarily. The logarithmic rate law was attributed to free penetration of oxygen through cracks and short-circuit paths. The activation energies associated with the logarithmic rate law were nearly one-half of those obtained from parabolic oxidation. The growth morphology of the products of oxidation of ilmenite was observed with a scanning electron microscope. The effect of growth morphology on the kinetics is discussed, and a probable reaction mechanism is suggested for the oxidation of ilmenite.     

氧化时间及铬含量对Fe-Cr钢高温氧化行为的影响

为了测定不同氧化时间以及铬含量对高温条件下钢材表面氧化铁皮组织和厚度的影响,将Fe-5Cr钢与Fe-10Cr钢在1000 ℃空气条件下氧化60~180 min,采用增重法绘制其氧化动力学曲线,并利用光学显微镜、能谱仪和X射线衍射仪对氧化铁皮的断面形貌和物相进行研究。结果表明,两试验钢氧化初期为“气-固”反应,中后期为“气相-氧化铁皮-固相”反应。两试验钢氧化铁皮结构均由外氧化层、内氧化层和内氧化区域组成。当氧化时间为180 min时,Fe-10Cr钢检测到了内氧化物Cr₂O₃。空气中氧元素向内扩散与基体中铬元素发生反应生成内氧化物Cr₂O₃,再与氧化铁皮层中的FeO发生固相反应生成尖晶石结构产物FeCr₂O₄。随着氧化时间的增加,由于内氧化物Cr₂O₃不断受到内氧化层的包裹而转为外氧化,内外氧化的转变使得基体不断被腐蚀,氧化铁皮厚度不断增加。     

An approximate calculation of the oxygen activity profile in solid-solution scales formed during the oxidation of binary alloys

A simplified method to calculate the oxygen activity profiles in solid-solution scales formed on binary alloys is proposed. This is based on Wagner's theoretical analysis but involves the solution of a single first-order differential equation by a process of successive approximations using the experimental data on the profiles of the scale composition. The agreement between the oxygen activity profiles calculated by this procedure and those obtained by the more general treatment is satisfactory for the systems examined. The method can therefore be used to obtain rapidly an approximate oxygen activity profile when the conditions for the application of the theory are satisfied.     

Electrolytic protection to reduce high-temperature oxidation

The imposition of a dc voltage across an oxide-ion-conducting solid electrolyte, upon which a gradient in the chemical potential of neutral oxygen (O, O₂) is maintained, causes coupled gradients in the electrochemical potentials of oxygen ions and electrons. The concept of balancing these gradients to provide oxidation protection at high temperatures is discussed. Experimental results show a significant reduction in the oxidation of a zircaloy substrate under an applied electric field near the open-circuit potential. The current densities required are small enough to allow the use of electrically-conducting-ceramic electrodes.Pacific Northwest Laboratory, (Operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830), P.O. Box 999, Richland, Washington 99352.     

High temperature oxidation of silicon carbide based materials

The process of high temperature oxidation of two silicon carbide based materials differing by methods of their production and properties has been studied up to 1500°C in air. The oxidation was performed under the isothermal conditions and at the programmed heat rate of 10° per minute. It was found that the oxidation resistance of the material was the function of the presence of extrinsic metals having close affinity for oxygen. It was also found that under heating up to 1500°C in air phase transitions occurred in the SiC surface layer.     

The oxidation and hot corrosion behavior of tungsten-fiber reinforced composites

The oxidation and hot corrosion behavior of a tungsten-fiber, reinforced Ni~ 20Cr alloy has been examined under the following exposure conditions: (a) pure oxygen at 1 atm pressure; (b) sulfidation in H₂–10 %H₂S; (c) presulfidation in H₂–10 %H₂S followed by oxidation in oxygen; and (d) oxidation in 1 atm oxygen after precoating with approximately 1 mg/cm² of Na₂SO₄. Rapid oxidation of the tungsten fibers causes considerable distortion of the matrix and catastrophic degradation of the matrix follows. Inter diffusion between the matrix and the fibers is also important. During sulfidation, only the matrix forms sulfides, the fibers remaining unaffected. Consequently, presulfidation, although having a dramatic effect on the oxidation of the matrix does not have a damaging effect on the fibres. Equally, the presence of sodium sulfate is not critical, although severe oxidation of the exposed tungsten fibers is still observed.     

Effect of mechanical stresses on heterogeneous oxidation of metals

The existence and influence of mechanical stresses, appearing at a metal-oxide interface, on the rate and regularities of metal oxidation is analyzed. The source of these stresses involves, in particular, the difference in thermal-expansion coefficients and densities of metal and oxide. From this point of view, the oxidation parameters of Al, Ti, Zr, and Mg are calculated. It is shown that the suggested approach well describes qualitatively, and in some cases quantitatively, the numerous experimental data concerning oxide-layer structures, complex multistep parabolic oxidation, transition from protective to nonprotective oxidation, etc. Results of the calculations indicate that for cylindrical vessels internal pressure may result in significant intensification of the oxidation process.     

High-temperature oxidation of Ti-4.32 wt.% Nb alloy

The oxidation kinetics of Ti-4.32 wt.% Nb (2.27 at.%) alloy in either air or oxygen under 1 bar pressure have been investigated in the temperature range of 1255–1471 K. For oxidation in oxygen the kinetics follow consecutively decreasing parabolic rate laws. The kinetics become nonparabolic after prolonged oxidation in air. X-ray data reveal the presence of TiO₂ (rutile) as the main oxide. The presence of Ti₂O, -TiN, and -TiN is found on the metal surface after oxidation in air. Microhardness and electron microprobe measurements are used to determine Ti, Nb, O, and N profiles across the oxidized alloy. The oxygen diffusion coefficient in the alloy can be expressed byD=65.6 exp(–234.3/RT) with the activation energy in kilojoules per mole. The oxidation behavior of the alloy is explained.Presently on academic leave at Max-Planck-Institut für Metallforschung, Institut für Werkstoffwissenschaften, Stuttgart, German Federal Republic.     

Effect of oxidation on droplet flattening and splat-substrate interaction in thermal spraying

The processes of oxidation that occur during particle inflight motion and during splat solidification in an oxygen-rich atmosphere were considered for the thermal spray process. The effect of oxidation on droplet flattening, splat-substrate mechanical and thermal interaction, splat morphology, and development of coating porosity and adhesion was studied. The influence of wetting and oxygen dissolution on flattening and splat-substrate adhesion was also investigated. The results from theoretical treatment agree with experimental observations.     

Influence of CeO2-coating on the high-temperature oxidation of chromium

The reactive-element effect on high-temperature oxidation of chromium metal was studied at oxygen pressures of 10⁵ Pa (1 atm) and 0.67 Pa and at temperatures of 800 and 1050°C under both isothermal and cyclic conditions. The reactive element, cerium, was applied as a cerium-oxide coating by sol-gel deposition. Growth and adherence of the chromia scale on the metallic substrate were investigated by kinetic weight-gain measurements, and the microstructure was characterized by scanning electron microscopy, transmission electron microscopy, EDX-analysis, X-ray diffraction, and Auger analysis. The study of the oxidation kinetics under isothermal conditions at 800°C clearly showed a reduction in the growth rate of the oxide film when cerium oxide was present. Improvement of the oxide-scale adhesion was also observed when ceria-coated chromium was subjected to thermal cycling. The applicability of a number of models which have been used to explain the reactive-element effect is discussed.     

The isothermal oxidation of Co-Cr alloys in 760 Torr oxygen at 1000°C

The isothermal oxidation of Co-Cr alloys containing 0–30% Cr in 760 Torr oxygen at 1000° C has been studied kinetically and by appropriate physical techniques. Chromium additions to cobalt increase the parabolic oxidation rate to an almost constant level from 2 to 15% Cr, while further additions to 20–30% Cr decrease the rate. All the alloys produce a virtually pure CoO layer outside a layer containing Co-Cr spinel particles in a Cr³⁺ -doped CoO matrix. The variation of oxidation rate with alloy chromium content is explained in terms of the complex interplay of doping, blocking of cation transport by voids and spinel particles and short circuiting by transport of dissociative oxygen across these voids and other processes, internal oxidation making a negligible direct contribution to weight gain. Complete spinel layers are never quite developed under the conditions studied, although formation of spinel does slow the oxidation rate. The improved protection eventually obtained at higher chromium levels is produced by the tendency to form a Cr₂O₃ healing layer.     

A mathematical model for internal oxidation

A mathematical model for internal oxidation kinetics was developed using numerical methods (finite difference) and computer techniques. The flexibility of the model permitted analysis of semi-infinite and finite situations with planar, cylindrical, and spherical geometries for systems with various amounts of local solute enrichment. Graphical results are presented for subscale thickness as a function of time and local enrichment as a function of position in the subscale. The model is also applied to internal oxidation with a discontinuous change in surface oxygen concentration; a graphical solution encompassing a wide range of possible experimental conditions is presented. The use of the model in analyzing nonisothermal internal oxidation problems is demonstrated.     

Compositional changes in the underlying alloy produced by the oxidation of Ni-Cr alloys

Compositional changes in the alloy beneath scales have been examined for the oxidation of Ni-27.4%Cr and Ni-40.2% Cr in 1 atm oxygen in the temperature range 1073–1473°K. Calculations of the rate of approach of the interfacial alloy composition to a constant value are compared with experimental data. Theoretical chromium depletion profiles obtained using both a finite difference analysis and an analytical expression are shown to be essentially equivalent and in good agreement with experimental measurements. The consequences of alloy depletion for the scaling behavior, when the protective scale is ruptured, are discussed.