High temperature oxidation of tungsten wires in O2-Ar mixtures

The rate of volatilization of tungsten wires in natural convection by high temperature oxidation was measured in O₂-Ar mixtures containing between 20 and 150 ppm O₂ at 0.789 atm total pressure. At constant oxygen partial pressure the rate generally decreases with increasing temperature between 2450 and 3200 K. The magnitude of the rate ranges between 1.2×10^?6 and 1.7×10^?5 g/cm²?s, depending on temperature and oxygen partial pressure. The oxidation reaction is gas-transport controlled as demonstrated by supplemental measurement in helium oxygen mixtures, by experiments in 0.395 atm of argon, and by comparison with previous measurements on surface-controlled oxidation of tungsten. The experimental results are in reasonable agreement with a volatile oxide counter diffusion model.     

Reduction of hematite compacts by H2-CO gas mixtures

The reduction behaviour of hematite compacts by H₂-CO gas mixtures was investigated at 1073-1223 K. The total porosity, pore size distribution and surface area of the compact was measured using mercury pressure porosimeter. The reduction tests were carried out using Cahn balance. The reduction behaviour could not be described in terms of a single rate-determining step; the reduction process was initially controlled by the chemical reaction at the oxide/iron interface, controlled by the intraparticle diffusion through the reduced layer towards the end of reduction, and the mixed control, in between. Over the whole range, the reduction rate decreased with CO content in the gas mixture. The chemical reaction rate constants were two to three times higher for H2 reduction than those of CO reduction, and the effective diffusivities of H2 reduction were three to four times higher than those of CO reduction. Values of activation energy for chemical reaction were found to be 19.8-42.1 kJ/mol depending on the gas compositions; 100% CO showing the lowest.     

Reduction of solid wustite in H2/H2O/CO/CO2 gas mixtures

The reduction of dense wustite in H₂/H₂O/CO/CO₂ gas mixtures has been carried out at temperatures between 1073 and 1373 K. The critical conditions for the formation of porous iron product morphologies have been identified and the results discussed in relation to the breakdown of dense iron layers on wustite surfaces. Formerly with the University of Queensland.     

Microstructural features produced by the reduction of wustite in H2/H2O gas mixtures

The systematic study of the reduction of pure wustites (FeO) between 600 and 1100°C in H₂/H₂O gas mixtures has revealed a number of important morphological changes. It has been shown that dense wustite can decompose to form a highly porous wustite before iron nucleation takes place. The product morphologies of iron formed on the wustite on reduction have been classified into three types, (a) porous iron, (b) porous wustite covered by dense iron, and (c) dense wustite covered by dense iron.     

The gaseous reduction of solid calciowustites in Co/Co2 and H2/H2O gas mixtures

The reduction of calciowustites has been carried out in CO/CO₂ and H₂/H₂O gas mixtures at temperatures between 1073 and 1373 K. The effect of lime additions to the wustite is to extend considerably the range of gas conditions over which porous iron morphologies are observed compared to those found on reduction of pure wustite. The products obtained at low oxygen potentials consisted of porous iron containing a dispersion of CaO particles, at intermediate oxygen potentials a two phase structure consisting of porous iron and dicalcium ferrite was formed, and at high oxygen potentials a dense iron layer over the oxide surface is observed throughout reduction. F. NAKIBOGLU, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, Brisbane, Australia D.H. St. JOHN, formerly Postdoctoral Fellow, University of Queensland     

Corrosion of 310 stainless steel in H2- H2O- H2S gas mixtures: Studies at constant temperature and fixed oxygen potential

Corrosion of SAE 310 stainless steel in H₂-H₂O-H₂S gas mixtures was studied at a constant temperature of 1150 K. Reactive gas mixtures were chosen to yield a constant oxygen potential of approximately 6 × 10^-13 Nm^-2 and sulfur potentials ranging from 0.19 × 10^-2 Nm^-2 to 33 × 10^-2 Nm^-2. The kinetics of corrosion were determined using a thermobalance, and the scales were analyzed using metallography, scanning electron microscopy, and energy dispersive X-ray analysis. Two corrosion regimes, which were dependent on sulfur potential, were identified. At high sulfur potentials (P _{S 2} ± 2.7 × 10^-2 Nm^-2) the corrosion rates were high, the kinetics obeyed a linear rate equation, and the scales consisted mainly of sulfide phases similar to those observed from pure sulfidation. At low sulfur potentials (P _{S 2} ± 0.19 × 10^-2 Nm^-2) the corrosion rates were low, the kinetics obeyed a parabolic rate equation, and scales consisted mainly of oxide phases. Thermochemical diagrams for the Fe-Cr-S-O, Fe-Ni-S-O, Cr-Ni-S-O, and Si-Cr-S-O systems were constructed, and the experimental results are discussed in relation to these diagrams. Based on this comparison, reasonable corrosion mechanisms were developed. At high sulfur potentials, oxide and sulfide phases initially nucleate as separate islands. Overgrowth of the oxide by the sulfide occurs and an exchange reaction governs the corrosion process. Preoxidation at low oxygen potentials and 1150 K is beneficial in suppressing sulfidation at high sulfur potentials. Formerly a Senior Scientist with Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720.     

The effects of impurity elements on the reduction of wustite and magnetite to iron in CO/CO2 and H2/H2O gas mixtures

The reduction of dense wustite and magnetite samples in CO/CO₂ and H₂/H₂O gas mixtures has shown that impurity elements in solid solution in the oxides can significantly affect the reaction mechanisms operative during reduction and the conditions for porous iron growth. In general, the presence of P, Mg, Ti, Si, Ca, K, and Na in wustite favors, in order of increasing strength, the formation of the porous iron product morphology. Aluminum, on the other hand, significantly reduces the range of gas conditions over which the porous iron microstructure may be obtained. S. GEVA, formerly Research Assistant, Department of Mining and Metallurgical Engineering, University of Queensland. D.H. St. JOHN, formerly Senior Lecturer, Department of Mining and Metallurgical Engineering, University of Queensland.