Weight change and oxygen consumption measurements were used to study the oxidation of molybdenum from 550 ~ to 1704~ for pressures of 5 to 76 Torr. For temperatures of 550~176 two processes occurred simultaneously, oxide scale formation and molybdenum trioxide volatility. Above 800~ at pressures up to 76 Tort molybdenum trioxide volatilized as fast as it formed. At 900~ and 76 Torr using 1.2 cm 2 samples the primary chemical reaction gave a rate of about 10 is at. molybdenum/cm2/sec. Above this temperature for 1.2 cm 2 specimens the reaction was limited by gaseous diffusion of oxygen. Little change was found in the rate of oxidation to 1615~ Pressure had only a small effect on the rate of reaction for these reaction conditions. However, in the chemically controlled region pressure had an important effect on the rate of oxidation. To extend the temperature region where the primary chemical reaction was rate controlling, samples of small area were used. A sample having a total area of 0.12 cm 2 gave a reaction rate of 8 x 10 is at./cm2/sec at 1410~ For these very fast reactions, appreciable temperature rises occurred, and the actual sample temperature had to be estimated. A log K vs. 1/T plot of the primary chemical

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...

A kinetic study was made of the oxidation of high-purity silicon carbide using a dynamic-type reaction system. Two types of oxidation behavior were found. Passive oxidation occurred for conditions where silica films were formed on the surface. Active oxidation occurred for conditions where volatile silicon monoxide was formed. The transition conditions between the two types of oxidation were studied as a function of oxygen pressure and temperature. The transition temperatures and pressures were related to thermochemical conditions for the reaction of silicon carbide with silica to form silicon monoxide and carbon monoxide.

The oxidation of high purity chromium was studied over the temperature range 700°–1100°C using the vacuum microbalance method. Below 900°C conventional oxidation curves were obtained which can be fitted to the parabolic rate law. Above 900°C and for a film thickness of approximately 4800Aå, the rate of oxidation increased in an unusual manner. This increase in the rate of oxidation disappeared on further oxidation. At temperatures of 1050°C and higher a large increase occurred in the rate of oxidation, suggesting that the oxide film was no longer protective for film thicknesses greater than 42,000Aå.A logarithmic plot of the parabolic rate law constant vs. 1/T shows two straight lines separated by a transformation region. This gives 37,500 cal/mole and −15.3 entropy units for heat of activation and entropy of activation between 700° and 900°C and 59,400 cal/mole and +6.2 entropy units for 1000°–1100°C.The rate of evaporation and the rate of oxidation of Cr are equal at about 950°C. This corresponds to the transformation region between the two mechanisms of oxidation. It is concluded that the failure of Cr in oxidation is closely related to the high vapor pressure of Cr above 900°C.

The high vacuum behavior of chromium at temperatures between 600 ~ and 1015~ was studied. Using decarburized specimens, chromium acts as a getter to oxygen, while above 825~ chromium vaporizes at an appreciable rate. The vapor pressure of chromium was determined between 885 ~ and 1015~ and a value of 93.9 -4-0.2 kcal per mole was found for /xH~. Oxide and nitride films have no appreciable effect on the vapor pressure curve in this temperature and thickness range. The reaction of carbon with the surface oxide occurs at temperatures of 800~ and higher with the carbon being removed as carbon monoxide.The oxidation reaction was studied over the temperature range 700 ~ to 900~ Except for the initial period of the reaction the data may be fitted by the parabolic rate 1 law. A plot' of log K/T vs. ~ gives a heat of activation of 66,300 cal/mole. Application of the transition state reaction rate theory to the rate data gives positive values of 10.7 to 13.5 entropy units for the entropy of activation.A comparison of the rate data with that of other metals shows that chromium has a good oxidation resistance although inferior to beryllium and the Ni-Cr series of alloys.