Kinetics and mechanism of the low-pressure,high-temperature oxidation of tungsten silicides-III

Oxidation studies were performed on silicon-coated tungsten which had been treated to form various intermetallic compounds on the surface. Two classifications of behavior were noted (1) a passivated state in which a protective layer of SiO₂ formed at low temperatures and (2) a combustion state in which volatile oxides such as SiO, WO₂, and WO₃ formed. A simple kinetic model has been proposed to account for the abrupt transition from one state to the other. The temperature-pressure conditions under which the transition occurs have been delineated by a thermodynamic study. The combustion state is characterized by the linear growth of W₅Si₃ on WSi₂ surfaces and by the growth of a solid solution of silicon in tungsten on W₅Si₃ surfaces. The oxygen consumption on W₅Si₃ surfaces followed first-order kinetics with an activation energy of 19±1 kcal/mole. The activation energy for oxygen consumption on the solid solution was the same as for pure tungsten, 28±1 kcal/mole, but the kinetic order of the solid solution was 0.9 compared to 0.7 for pure tungsten.     

Kinetics and mechanism of high-temperature oxidation of dilute cobalt-chromium alloys

Kinetics of oxidation of Co-Cr alloys containing 0.4%–15% Cr was studied as a function of temperature (1273–1573 K) and oxygen pressure (4 × 10²–10⁵Pa). The oxidation process was found to be approximately parabolic and faster than that for pure cobalt. The scales are double-layered and consist of a compact outer CoO layer and a porous inner layer containing CoO slightly doped by chromium and spinel CoCr₂O₄. The oxidation mechanism was investigated by means of platinum markers and the¹⁸O isotope. The scale on the alloys containing less than 1% Cr grows exclusively by outward diffusion of cobalt, while that on the alloys containing more chromium—with a significant contribution of inward oxygen transport from atmosphere. This transport is not a lattice diffusion, but proceeds presumably through microfissures resulting from the secondary process of perforating dissociation of the outer scale layer.     

Kinetics and mechanism of high-temperature oxidation of copper covered by bismuth thin films

The oxidation kinetics of copper covered by thin films of bismuth were studied by TGA, X-ray diffraction, X-ray micro-elemental, coulombmetric methods, and by electron and optical microscopy. At 1003 K catastrophic oxidation of copper coated by bismuth thin films was observed. The parabolic rate constant of copper oxidation (Kp) depends markedly on the thickness of the bismuth film and is more than 1000 times greater than that of bare copper. The mechanism of catastrophic copper oxidation in contact with bismuth is discussed.     

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>.     

Kinetics and mechanism of high-temperature sulfur corrosion of Fe-Cr-Al alloys

The influence of aluminium on the kinetics and mechanism of high-temperature sulfidation of Fe-Cr alloys containing 20 at.% chromium has been investigated. It has been found that the addition of aluminum greatly improves the scaling resistance of Fe-Cr alloys against attack by sulfur vapors at high temperatures.     

Kinetics and mechanism of the accelerated oxidation of low carbon scrap steels

Especially with accelerated oxidation, the kinetic curves of steel strips in CO₂-CO gas mixtures at 900° C do not adhere to either the parabolic or the linear growth law. Linear kinetics is essentially obeyed in the oxidation of untreated steel, whereas mixed parabolic or linparabolic laws determine the oxidation of steels treated with oxidation promoters. The observed kinetics are best explained with the Zener diffusion model, which assumes that the activation energy for diffusion is spent in deforming the crystal lattice elastically in the vicinity of the saddle point. The model results in a strong dependence of diffusivity on the melting point and elastic moduli of the diffusion medium (oxide scale). A diffusion coefficient that decreases exponentially with melting point and the Tammann differential relation for oxide growth account adequately for the observed kinetics.     

Some aspects of the mechanism of high-temperature oxidation of nickel in SO2

A number of investigations on the mechanism of reaction of nickel with SO₂ has been summarized. The calculation results of the equilibrium gas composition in homogeneous SO₂+O₂ mixtures are described over wide ranges of temperatures (500–1100°C) and initial gas compositions. The Ni–O–S phase diagram at 540°C has been compared with data on the stability of interaction products under conditions close to equilibrium. The catalytic activity of NiO has been verified to accelerate the attainment of thermodynamic equilibrium in the SO₂–O₂–SO₃ system. The most effective catalytic activity of NiO occurred at 650–800°C. A monolayer (6 Å) of NiSO₄ was detected on the scale surface by ESCA. This surface phase is assumed to be formed either as an activated complex on the NiO catalyst or as the locally stable NiSO₄ phase. Both assumptions lead to a possible recognition of the sulfate intermediate mechanism.     

Mechanical aspects of high-temperature oxidation

Under service conditions it is most often not high temperature corrosion alone which causes failure of components but the interaction with simultaneous mechanical stresses. A key role is played in this case by the protective surface oxide scales which may crack and heal. This role is discussed in the form of model considerations for the beginning of scale failure under tensile, compressive and more complex stress situations. Discussion leads to the formulation of rules for a protective scale with optimum behaviour under the effect of stresses.     

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.     

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.     

The oxidation behavior of high-temperature aluminides

The kinetics and mechanisms of oxidelayer formation on Ti-Al-X alloys, specifically Ti-25Al-18Ta, Ti-30Al-2.7Nb, and Ti-43Al-11Nb have been studied. The activation energies for the oxide formation of each of the alloys used in this work were determined, and the values obtained were compared with those obtained by other researchers for identical alloys. Author’s Note: All compositions are given in atomic percent unless otherwise indicated. For more information, contact R.G. Reddy, University of Alabama, Department of Metallurgical and Materials Engineering, A129 Bevill Buiding, P.O. Box 870202, Tuscaloosa, Alabama 35487-0202; (205) 348-1740; fax (205) 348-2164.     

Kinetics of high-temperature oxidation and embrittlement factors of zirconium alloys in tests simulating LOCA-type accidents in nuclear power plants

Tests of tube samples of fuel claddings of zirconium alloys of different chemical composition were performed with simulation of emergency situations at nuclear power plants with coolant loss including heating, high-temperature oxidation in steam, and cooling. Kinetics of high-temperature oxidation and structural phase changes under heating and cooling were studied, structure and analysis of fractures were studied quantitatively, and mechanic properties (residual ductility) of the samples after cooling were determined. The principal causes of alloy embrittlement were determined. Irrespective of the observed changes in the oxidation kinetics and microstructure character, the residual ductility of all the studied alloys was zero.     

The high-temperature oxidation,reduction, and volatilization reactions of silicon and silicon carbide

A thermochemical analysis was made of the oxidation, reduction, and volatilization reactions which occur in the Si-O-C system. One characteristic feature is the high SiO(g) and SiO(g) + CO(g) pressures at the Si(s)-SiO₂ and SiC(s)-SiO₂(s) interfaces. Active oxidation with weight losses and passive oxidation with weight gains were found on oxidizing Si(s) and SiC(s) in low oxygen pressures above 1000°C. Rapid oxidation was related to the SiO(g) and SiO(g) + C0(g) pressures at the Si(s) or SiC(s)-SiO₂(s) interfaces.     

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.     

The high-temperature oxidation resistance of iron-silicon-aluminum alloys

Silicon or chromium can be used as an oxygen getter in iron-aluminum alloys to prevent the internal oxidation of aluminum. This suppresses the formation of the iron oxide nodules that tend to destroy binary iron-aluminum alloys during high-temperature oxidation. Alloys of iron containing aluminum and silicon in varying proportions were heated in flowing air for 50 hr at 1093°C. Of the alloys tested, one containing 6% aluminum and 1 % silicon was the most resistant to oxidation.     

Static and dynamic cyclic oxidation of 12 nickel-, cobalt-, and iron-base high-temperature alloys

Twelve typical high-temperature nickel-, cobalt-, and iron-base alloys were tested by 1 hr cyclic exposures at 1038, 1093, and 1149°C and 0.05 hr exposures at 1093°C. The alloys were tested in both a dynamic burner rig at Mach 0.3 gas flow and in static air furnace for times up to 100 hr. The alloys were evaluated in terms of specific weight loss as a function of time, and X-ray diffraction analysis and metallographic examination of the posttest specimens. A method previously developed was used to estimate specific metal weight loss from the specific weight change of the sample. The alloys were then ranked on this basis. In general the burner-rig test was more severe than a comparable furnace test and resulted in an increased tendency for oxide spalling due to volatility of Cr in the protective scale and the more dratic cooling due to the air-blast quench of the samples. Increased cycle frequency also increased the tendency to spall for a given test exposure. The behavior of the alloys in both types of tests was related to their composition, particularly their Cr and Al contents and their tendency to form four types of scales: NiO or CoO, Cr₂O₃-chromite spinel, -Al₂O₃-aluminate spinel, or ThO₂-blocked Cr₂O₃. The alloys with the best overall behavior formed -Al₂O₃-aluminate spinels.     

On modeling the oxidation of high-temperature alloys

Oxidation of high-temperature alloys represents complex, strongly coupled, non-linear phenomena which include: (i) diffusion of oxygen in the alloy; (ii) an oxidation reaction in which the reaction product causes substantial permanent, anisotropic volumetric swelling; (iii) high-temperature elastic–viscoplastic deformation of the base alloy and the oxide; and (iv) transient heat conduction. We have formulated a continuum-level chemo-thermomechanically coupled theory which integrates these various nonlinear phenomena. We have numerically implemented our coupled theory in a finite-element program, and have also calibrated the material parameters in our theory for an Fe–22Cr–4.8Al–0.3Y heat-resistant alloy experimentally studied by Tolpygo et al. Using our theory, we simulate the high-temperature oxidation of thin sheets of FeCrAlY and show that our theory is capable of reproducing the oxide thickness evolution with time at different temperatures, the permanent extensional changes in dimensions of the base material being oxidized and the development of large compressive residual stresses in the protective surface oxide which forms. As an application of our numerical simulation capability, we also consider the oxidation of an FeCrAlY sheet with an initial groove-like surface undulation, a geometry which has been experimentally studied by Davis and Evans. Our numerical simulations reproduce (with reasonable accuracy) the shape-distortion of the groove upon oxidation measured by these authors. This example has obvious ramifications for delamination failure of a ceramic topcoat on a thermally grown oxide layer in thermal barrier coatings.     

The high-temperature oxidation resistance of Co-Al alloys

Most Ni and Co-base alloys used for high-temperature service rely on the production of a compact, stable Cr₂O₃ scale for their oxidation resistance. However, as operating temperatures have risen above 900–950° C, the loss of Cr₂O₃ as the volatile CrO₃ has led to an inadequate life span of these alloys, particularly in rapidly flowing, turbulent gas streams. As a result of this, it has been necessary to examine the possibility of using Al₂O₃ as the protective scale. Al₂O₃ has a lower growth rate than Cr₂O₃, it is nonvolatile, and, unlike Cr-containing systems, it is less likely to form compound oxides such as spinels. In this study, the amount of Al which must be present in the Co-Al system to form a continuous layer of Al₂O₃ in the temperature range 800–1000° C has been determined. The quantity was found to rise from about 7–10 wt. % at 800° C to 10–13 wt. % at 900° C and 13 wt. % at 1000° C. Notice has also been taken of the abilities of the alumina-forming alloys to re-form a protective oxide in the event of spalling, blistering, or any other disruptions of the scale, and some cyclic-oxidation checks have been conducted on the Co13Al alloy at 900 and 1000° C.This work has been partly supported by the Science Research Council and one of us (G.N.I.) wishes to thank them for the award of a Science Research Council Research Studentship     

Decarburization of Fe-Cr-C steels during high-temperature oxidation

Isothermal oxidation treatments were carried out on Fe-Cr-C steels. The steels containing 0.08, 0.15, 0.17, 0.88, 1.51, and 12.77wt.% Cr and 0.10, 0.49, 1.19, 0.18, 1.05, and 1.63 wt.% C were oxidized in ambient air at temperatures of 900, 1000, and 1200°C. Steels containing 13.22, 12.90, 12.52, and 12.77wt.% Cr and 0.15, 0.30, 0.50, and 1.63 wt.% C were heated (1100°C/3hr) in a flowing atmosphere of O₂-N₂-He in a SETARAM thermobalance. Evidence of decarburization of the steels is given by metallographic observations, by direct measurements of carbon diffusivities from the decarburization profiles in the oxidized samples, and by the results of kinetics measurements.³ Carbon diffusion coefficients were measured by the standard sectioning method in the samples oxidized in air. A. generalized equation for carbon diffusivity in Fe-Cr-C alloys is developed in terms of N_Cr[wt.%], N_C[wt.%], and T[K].