Kinetics of the Oxidation of Pure Tungsten from 500° to 1300°C

Abstract

Kinetic studies were made on the oxidation of tungsten from 500 ~ to 1300~ for time periods up to 6 hr, and for oxygen pressures from 0.1 atm to 0.00132 arm. The rate data were fitted to the parabolic rate law. A number of deviations and transitions were observed. For all of the experiments the initial slopes of the parabolic rate law plots were smaller than the final values found for thick films. A transition in the rate of oxidation was observed for weight gains of 2500-4000 ~g/cm ~ at temperatures of 650~176 Photographs of the oxidized surface above 650~ show that oxidation occurs in a preferential manner at the edges. Pressure had a strong effect on the rate of oxidation for the experiments above 950~ At 1200~ weight loss curves were observed for pressures as high as 0.1 atm. Above 1200~ the oxidation reaction is similar to the combustion of graphite. The rate of oxidation is limited by the volatility of WO~, the diffusion of oxygen to the surface, and the diffusion of W~O~ away from the surface.The behavior of tungsten and its surface oxides in oxidizing and neutral atmospheres and in vacuum at high temperature is an interesting scientific problem. This paper presents results of an extensive study of the following problems: (a) the effect of time and temperature on the rate of oxidation of tungsten from 500 ~ to 1300~ (b) the effect of pressure on the time course of oxidation at four temperatures, (c) physical structure and crystal structure of the oxide scale, and (d) mechanism of reaction.The oxidation of tungsten below 500~ has been studied by Gulbransen and Wysong (1) and by Gorbounova and Arslamb6kov (2). Gulbransen and Wysong (1) found the oxidation to follow the parabolic rate law between 400 ~ and 500~ Deviations were found to occur at 550 ~ and below 400~ A heat of activation of 45,650 cal/mole was calculated from the parabolic rate law constant. Tungsten oxides were found to volatilize at temperatures as low as 800~ for thick oxide films. Thin oxide films were found to require a higher temperature for volatilization of WO,. Gorbounova and Arslamb~kov (2) found that the heat of activation for the temperature range of 390~176 was dependent on the surface preparation. Oxidation experiments on electrolytically polished samples gave a heat of activation of 46,500 cal/mole from the parabolic rate law constant while studies on mechanically polished samples gave a heat of activation of 41,000 cal/mole. Dunn (3) studied the oxidation of tungsten from 700 ~ to 1000~ and found the parabolic rate law was followed. The temperature variation of the parabolic rate law constant was found to follow an exponential law of the Arrhenius type. An inflection was noticed at 850~176and was attributed to a phase change in the oxides.Scheil (4) studied the oxidation rate at 500 ~ and 700~ over long periods of time and found a linear rate law. This was interpreted as evidence for the presence of a nonprotective film.An intermediate type of rate law was suggested by Nachtigall (5) and Kieffer and Kolbl (6).Webb, Norton, and W...

On the oxidation mechanism of pure tungsten in the temperature range 600–800°C

“…This agrees with the numerous discrepancies reported in the literature concerning kinetics, composition and structure of the oxide scale [14 23]. Although the change from parabolic to linear kinetics has been described during oxidation in the temperature range 500 700 °C [14,18], a parabolic behavior was not found in our study over the entire temperature range. Furthermore, our results differ from the data reported in previous works in which kinetics obeyed parabolic laws during short term exposure in the interval 400 900 °C [14,20,25].…”

“…Although the change from parabolic to linear kinetics has been described during oxidation in the temperature range 500 700 °C [14,18], a parabolic behavior was not found in our study over the entire temperature range. Furthermore, our results differ from the data reported in previous works in which kinetics obeyed parabolic laws during short term exposure in the interval 400 900 °C [14,20,25]. This means that the oxide scale was unable to provide effective protection from the early stages of oxidation, as deduced by the lateness for achieving the steady state.…”

“…Moreover, the influence of impuri ties cannot be ruled out because the reduction kinetics of non sag (NS) doped tungsten blue oxide is affected strongly by small amounts of impurities such K, Al or Si [27]. Usually, the typical yel low color of WO 3 was not observed during short term exposures, up to 6 h, between 500 and 700 °C [14], which would confirm that WO 3 formed once a continuous dark layer was established. Kellet external yellow scale in a sample oxidized 4 h at 700 °C [20].…”

Solar production of WO3: a green approach

“…During this set of experiments, it was found that some of the formed tungsten oxide sublimes. This phenomena was previously observed in the same temperature range by Gulbransen and Andrew [23] and Baur et al [24]. During experimentation some of the volatilized tungsten oxide was deposited on the reactor walls forming a yellowish thin film.…”

“…These cracks promote the oxygen transport inside the tungsten, helping the reaction take place inside tungsten bars leading to an increment in the thickness of the film. The cracking mechanism was previously described by Gulbransen and Andrew [23]. According to these authors, the formation of tungsten oxides is a cyclic mechanism.…”