Application of the Rosenburg kinetic method for determination of the parabolic rate constants for metal oxidation

The Rosenburg continuous kinetic method is proposed for the determination of the parabolic rate constants of metal oxidation as a function of temperature and oxidant pressure. Using this method, it is possible to make numerous measurements in a continuous manner on a single metal sample left in situ in the furnace, thus eliminating systematic errors due to differences between samples. Moreover, using this method it is possible to determine directly from a given kinetic curve such important parameters as the coefficients of chemical diffusion and self-diffusion of the more mobile species in the studied compound and the total equilibrium defect concentration. The latter parameter has been inaccessible up to now by the experimental method. The limits of applicability of this method are given in the paper. As an example of this method, the kinetics of cobalt oxidation are investigated in the range of temperature 1000–1250°C and oxygen pressure 10^–3–1 atm; the results compare favorably with those obtained by other authors. The method is, however, applicable to certain other systems, namely, metal oxides and sulfides.     

On the influence of sol-gel derived CeO2 coatings on high-temperature oxidation of Co, Ni and Cu

The effects of CeO₂ coatings on high-temperature oxidation of Co, Ni and Cu have been investigated as a function of temperature at oxygen pressures from 1×10⁻⁴ to 1 atm. The oxidation mechanisms for Co and Cu are essentially unaffected by CeO₂ coatings, whereas the oxidation rate of Ni decreases by approximately one order of magnitude. The oxygen pressure dependence does not change markedly with CeO₂ coatings for any of the metals studied. For oxidation of Ni plus CeO₂ coatings, the temperature dependence is less marked at lower temperatures, whereas essentially the same behavior is observed for Co and Cu with and without the coating. Differences in the effects of CeO₂ coatings for the three metal systems have been attributed to the relative influence of grain boundary transport on the overall rates of oxidation.     

High-Temperature Oxidation Behavior of Ti2AlC in Air

The isothermal oxidation behavior of bulk Ti₂AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti₂AlC had excellent oxidation resistance. The oxidation of Ti₂AlC obeyed a cubic law with cubic rate constants, k_c, increasing from 2.38×10^-12 to 2.13×10^-10 kg³/m⁶/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner -Al₂O₃ layer and a discontinuous outer TiO₂ (rutile) layer formed on the Ti₂AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti₂AlC substrate during thermal cycling.     

Kinetics and mechanism of the gold corrosion dissolution in hypochlorite solutions

Kinetics and mechanism of the gold metal dissolution by means of corrosion in chloride-hypochlorite solutions are studied. The dependences of the dissolution rate on the solution pH, sodium hypochlorite concentration, and temperature are determined. Conditions of the gold passivation surface during its corrosion in the studied solutions are discussed. The first-order rate constants of the gold dissolution (k_i = 0.079–0.4030 s^–1) at temperatures from 277 to 304 K and others are calculated. The equilibrium constants of the dissolution reactions in acid and alkaline chloride-hypochlorite solutions are different: 1.2 × 10⁶ and 2.39 × 10², respectively. The activation energy calculated from the temperature dependence of the rate constants (53.43 kJ/mol) evidences a kinetic control of the gold dissolution. Quantitative data on the composition of surface adsorption films, formed by oxidation gold dissolution products, are obtained using the Auger spectroscopyTranslated from Zashchita Metallov, Vol. 41, No. 1, 2005, pp. 26–33.Original Russian Text Copyright © 2005 by Kozin, Prokopenko, Bogdanova.     

The growth and structure of oxide films on Fe. II. Oxidation of polycrystalline Fe at 240–320°C

The influence of surface pretreatment and metal orientation on the oxidation of coarse-grained polycrystalline Fe has been studied at 240 to 320°C in 5×10^–3 Torr O₂ using electron diffraction, electron microscopy, and Mössbauer spectroscopy to complement kinetic data. Consistent with previous studies on Fe single crystals, differences in oxidation kinetics for surfaces covered with an electropolish film from those with a similar thickness prior oxide formed by dry oxidation at room temperature are interpreted in terms of differing densities of leakage paths in the oxide layers. The more complex kinetics for electropolished polycrystalline Fe are a result of the leakage path density, the degree of oxide separation, and the extent of -Fe₂O₃ formation varying with substrate orientation. Where adherent Fe₃O₄ layers are formed on polycrystalline and single-crystal Fe surfaces, the parabolic rate constants give an activation energy which is consistent with a previous value of 32 kcal · mole^–1, suggesting that at these low temperatures the transport mechanism for magnetite growth is cation diffusion via easy diffusion paths in the oxide.     

High-temperature oxidation of titanium carbide in oxygen

The kinetics of titanium carbide oxidation in oxygen over the temperature range of 600–1200°C and oxygen pressure from 0.1 to 740 Torr have been studied with a vacuum microbalance. Layer-by-layer x-ray analysis, petrography, metallography, and gas chromatography have been used to analyze the oxidation products. A paralinear nature of the oxidation of material was established, and the rate constants of the process were calculated for the corresponding parabolic and linear portions of the kinetic curves. It was shown that a gaseous product, CO₂, formed, as well as a solid product, TiO₂ (rutile), both stoichiometric and nonstoichiometric. The lower oxides, Ti₃O₅, Ti₂O₃, TiO, were noted in the scale at temperatures from 700 to 800° and low oxygen pressures, their relative quantity rising with decreasing pressure. Based on x-ray analysis and microhardness measurements, it was concluded that titanium oxicarbides formed in the TiC, directly adjacent to the scale. A possible oxidation mechanism of titanium carbide is proposed.     

The influence of an intermediate annealing treatment on the oxidation of copper and nickel

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics of copper and nickel in 1 atm oxygen at 800 and 1100°C, respectively, using a semiautomatic microbalance. Changes in scale morphology and composition have been investigated using various physical techniques. The outer CuO layer formed on copper during oxidation dissociates very rapidly on annealing to give CU₂O and oxygen since the partial pressure of oxygen in the gas is below the dissociation pressure of CuO but above that of Cu₂O at 800°C. The CuO layer is quickly re-formed on reoxidation in oxygen. There are relatively few other changes in the oxide morphologies of either metal during annealing, although the small grains present in the scale adjacent to the metal after oxidation are able to grow. During reoxidation both metals show a reduction in oxidation rate constant because of the decrease in total cation vacancy concentration in the scale and the reduced cation vacancy gradient across the scale brought about by the reduction in oxygen partial pressure at the oxide-gas interface during annealing. The reoxidation rate constants following annealing approach those recorded prior to annealing as the equilibrium cation vacancy levels in the scales are reestablished in the oxidizing environment. Rosenberg's method for analysis of the kinetics of reoxidation has enabled the equilibrium concentrations and diffusion coefficients of cation vacancies in CU₂O and NiO during oxidation in 1 atm oxygen at the appropriate temperatures to be estimated approximately. These show reasonable agreement with literature values.     

The electrical properties of niobium pentoxide scales during their formation on niobium

A study of the electrical properties of compact scales growing on niobium has revealed several new properties of the oxidation reaction. The growth of the oxide follows two parabolic rates, with the latter approximately four times the former. Both the rate constants and the time of transition between the rates are sensitive to oxygen pressure, but it is the latter property that largely explains the extreme sensitivity of the oxidation reaction to oxygen pressure reported previously. Concomitant with the change to the faster oxidation rate is the development of a voltage across the scale (metal negative). The voltage has a value of 0.3 V at 1 atm and decreases approximately linearly with the logarithm of pressure to 0 V at 10^–3 atm. The oxidation rate is essentially insensitive to induced changes in the voltage. These properties of the oxidation reaction cannot be explained by the usual formalization of point defect theory.This investigation was completed under the auspices of the National Research Council of Canada.     

成分设计优化TiC/Hastelloy复合材料的抗氧化性能

TiC/Hastelloy复合材料是极具应用前景的中温固体氧化物燃料电池连接体材料,而抗氧化性能是影响其应用的关键性能之一。通过无压反应渗透工艺分别制备出含有50vol%和58vol%金属基体的TiC/Hastelloy复合材料。高金属含量使复合材料中的Cr含量增加,促进连续Cr₂O₃氧化层的形成,抑制Ni和Ti原子的外扩散,进而优化复合材料的抗氧化性能。氧化膜中Ti和Ni的氧化物含量降低,复合材料的氧化增重由2.03 mg·cm^-2降低到0.55 mg·cm^-2。同时,为了抑制Cr挥发,在含有58vol%金属基体的TiC/Hastelloy复合材料中引入Co。在氧化过程中,Co和金属基体中的Fe在Cr₂O₃氧化层中具有较快的扩散速率,可以在Cr₂O₃氧化层外侧原位形成CoFe₂O₄层。     

The duplex oxidation of vacuum annealed 316 stainless steel in CO2/CO gas mixtures between 500 and 700°C

The duplex oxidation of vacuum annealed 316 stainless steel has been studied at 650°C (923 K) and 700°C (973 K) in CO₂/CO gas mixtures of varying oxygen potential covering the range of Fe₃O₄ stability at these temperatures. The weight gain curves obtained were interpreted in terms of the usual 2-stage process of duplex scaling on 18/8 type stainless steels. The primary rate at 650° C showed an approximate dependence on the oxygen partial pressure of the oxidising gas of K_p π P_o2^11.15. At 700°C however the primary rate appeared nominally independent of oxygen partial pressure. Primary duplex oxidation rate constants were also measured at constant P_o2 over the temperature range 500–700°C (773–973 K). An Arrhenius relationship was found between the primary rate and temperature with an associated activation energy of 40 ± 6 K cal mol^?1 (167 ± 25 kJ mol^?1). In general there was no obvious correlation between the composition of the inner spinel and primary rate with the exception that low oxidation rate constants in both the primary and the secondary or healing stage were associated with spinels containing less than 10 wt. % Ni and more than 35 wt. % Cr, the nickel content appearing the more significant.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

Oxidation properties of Co-Mn alloys at temperatures from 1273 to 1473 K

The oxidation behavior of Co alloys containing up to 23.0 % Mn was investigated at temperatures of 1273–1473 K in an oxygen pressure range of 10–10⁵ Pa using thermogravimetry, electron-probe microanalysis, and optical microscopy.Oxidation followed a parabolic rate law, except in the early reaction stage. For the growth of the (Co, Mn)O scale, parabolic rate constants increased with increasing manganese content, whereas with further additions of manganese the rate constants decreased due to an intermediate layer of (Co, Mn)₃O₄. This critical manganese content increased with increasing temperature. The scale structures were classified into three groups: group I, a single-layer scale; group II, an inner-outer double layer; and group III, an inner-intermediate-outer triple layer. The inner layer consisted of (Co, Mn)O; the outer layer was a mixture of (Co, Mn)O and (Co, Mn)₃O₄; the intermediate layer was composed of (Co, Mn)₃O₄. Cation concentration profiles were measured across the external scales with single- or multilayer structures. Diffusional analysis indicated that manganese moved 1.25 times faster than did cobalt in the (Co, Mn)O scale. Activation energies for oxidation are 118–126 kJ/moles for groups I and II, and for the alloys in group III they are 290–367 kJ/moles. Parabolic rate constants and scale structures are discussed on the basis of the Co-Mn-O phase diagram.     

The influence of ceria surface additions on manganese oxidation at high temperatures

The kinetics, scale composition, and growth mechanism of ceria-coated and blank specimens of manganese oxidation in air were examined. The scale growth obeys the parabolic rate law at 700°C for all specimens. Lower parabolic rate constants for coated specimens are attributed to the presence of a CeO₂ external scale. It constitutes a limiting factor of the oxygen activity at the gas-oxide interface. This lower-oxygen activity leads to a less-metal-deficient state of the scale. Due to this, the inner-MnO scale becomes more adherent to the substrate. Preheating at 700°C, in hydrogen (P_{O 2}=10^–24 atm), was performed in order to be placed in the MnO stability domain and try to introduce cerium in the manganese-oxide scale. This pretreatment promotes macroscopic bonding in the layer formed during subsequent oxidation in air. It ensures a better scale adherence. A new diffusional-transport mechanism in manganosite is proposed in accordance with all experimental observations of the literature and with the cerium-manganese-oxygen system studied in the present work. This model considers the high Mn³⁺ stability in octahedral sites of the MnO oxygen ion body. Low-oxygen partial pressure conditions permit the formation of an adherent inner-MnO scale on coated specimens. A CeO₂ scale formed above the MnO scale; MnO is present as a minor component in this scale and it is located mainly at the internal interface. The difficulties in forming the cerium-orthomanganite are attributed to the very high stability of MnO related to this wide range of nonstoichiometry and to the low manganese diffusivity through the cerium-containing scale.     

The Oxidation of Two-Phase Cu-Cr Alloys Under 10-19 atm O2 at 700-900°C

The oxidation of three Cu-Cr alloys containing25, 50, and 75 wt.% Cr and having a two-phasemicrostructure has been studied at 700-900°C inH₂-CO₂ mixtures under10^-19 atm O₂, i.e., below thestability of the copper oxides. At variance with theresults obtained by oxidizing the same materials in air,the alloys corroded quite slowly and formed onlyexternal chromium-oxide scales rather than showinginternal oxidation of chromium. In view of the quitesmall solubility of chromium in copper, lower than thecritical value calculated for the transition from theinternal to the external oxidation of chromium, this result is attributed mainly to a supply ofchromium from the small Cr-rich particles present in thetwo-phase eutectic mixture. The corrosion kineticsfollowed the parabolic rate law to a reasonableapproximation, but the rate constants changed ratherirregularly as functions of temperature and alloycomposition. The scaling rates were generally largerthan those measured for pure chromium under the sameoxygen pressure, but much smaller than those of the same alloysin air. These results are examined with specialreference to the two-phase nature of thealloys.     

The high temperature oxidation of nickel

The oxidation of pure Ni has been studied over a temperature range of 1173–1573 K under 1.01 bar oxygen. The oxidation kinetics, the oxide growth and the metal recess obey parabolic time relationships. The dependence of the parabolic rate constant for oxidation can be expressed as: K_p = 7.95 × 10⁻² exp (− 220.9/RT) with the activation energy in kJ mole⁻¹. An attempt is made to calculate K_p in terms of diffusion parameters. These calculated values compare well with those measured experimentally.     

The location of hydrogen in the kinetics of oxidation of ferrous alloys by superheated steam

The rate of emission of hydrogen from the metal surface and from the oxide surface has been determined during the oxidation of ferrous alloys at 501°C. The kinetics were in accordance with the assumption of a parabolic rate law, and the rate constants were in agreement with those calculated from the thickness of the oxide layer at the end of the reaction. The proportion of hydrogen emitted from the metal to that from the oxide surface showed a dependence on the partial pressure of steam for the 9%Cr-1%Mo alloy at 501°C. The ratio was dependent on the alloy composition, and also on temperature, as shown by results for the 9%Cr-1%Mo alloy at 450 and 552°C. For this alloy, the activation energy and rate constants are consistent with a rate-determining step dependent on cation diffusion. The oxide film is almost impermeable to hydrogen gas. A possible oxidation mechanism is suggested.     

Oxidation of cobalt at high temperature

Precise values of parabolic rate constants of cobalt oxidation have been determined over a wide range of temperature (950–1300°C) and oxygen pressure (6.58× 10^?4?0.658 atm). The dependence of the calculated values of parabolic rate constants k″_p on oxygen pressure and temperature can be described by the following empirical equation: $$k''_p = const. \cdot {\text{p}}_{O_2 }^{{\text{1/n}}} \cdot exp ( - {\text{E}}_{\text{k}} /RT)$$ The exponent 1/n decreases with an increase in temperature from 1/3.40 at 950°C to 1/3.96 at 1300°C, whereas the activation energy E_k decreases with an increase in the oxygen pressure from 41.7 to 38.1 kcal/mole.     

Internal oxidation of Ag-in alloys: Stress relief and the influence of imposed strain

The kinetics of internal oxidation of silver-indium alloys containing 3.5, 5.9, and 9.8 at.% In in air at temperatures 773 to 973 K were established by TGA with no load applied to the specimens. Silver nodules free of oxide particles were observed to form at the surface during internal oxidation. The volume of these silver nodules was comparable to the total volume increase caused by internal oxidation. The alloys were also creep tested during oxidation in air at creep rates varying from 10^–7 to 5×10^–5 s^–1 at 773, 873, and 973 K. The parabolic rate constants k_p for the internal oxidation of the solute were determined from the measured widths of the internal oxidation zones. A small increase in k_p was observed with increased strain rate. The large volume change associated with internal oxide formation resulted in a stress gradient between the stress-free surface and the internal oxidation front which is under a high compressive stress. Stress relief occurred by transport of silver to the surface. A Nabarro-Herring creep type mechanism based on lattice diffusion of Ag cannot account for the high rate of silver transport to the surface. Pipe-diffusion controlled creep is proposed as the mechanism of stress accommodation by silver diffusion.     

The oxidation behavior of an Fe61B15Zr8Mo7Co5Y2Cr2 bulk metallic glass at 650 °C in various oxygen-containing environments

The oxidation behavior of an Fe₆₁B₁₅Zr₈Mo₇Co₅Y₂Cr₂ bulk metallic glass (Fe7-BMG) was investigated in three oxygen-containing atmospheres over the oxygen partial pressure range of 10³–10⁵ Pa at 650 °C. The oxidation kinetics of the Fe7-BMG followed the parabolic rate law in all cases, indicating that solid-state diffusion is the rate-controlling step during oxidation. The oxidation rate constants (k_p values) increased with increasing oxygen partial pressure, implying a typical scaling behavior with a p-type semiconductivity. Duplex scales formed on the Fe7-BMG consisted of an outer layer of mostly iron oxides (Fe₂O₃/Fe₃O₄) and minor amounts of B₂O₃, and a heterophasic inner layer of mostly tetragonal-ZrO₂ (t-ZrO₂) intermixed with non-oxidized intermetallic phases.     

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.