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Tungsten rod and sheet were found to oxidize linearly in oxygen from 600 ~ to 850~ at oxygen pressures ranging from 20 to 500 psia. Oxidation rate increased with increased oxygen pressure at temperatures 750~176Theoretical considerations indicate that an equilibrium adsorption process occurs prior to the rate-determining step. The assumption of a linear change of adsorption energy with increase of surface coverage Mlowed observed oxidation rates to be corrected for pressure effect. The activation energy was 48 kcal. Tungsten sheet volatilization losses were appreciable above 800~ Only tungsten rod data were free from volatilization losses at 850~ Photographs are included showing the effect of shearing samples at room temperature prior to oxidation on the final physical appearance of the oxide. Cold shearing produces exfoliated tungsten oxide.

The oxidation of Mond nickel was studied between 1000 ~ and 1200~ at oxygen pressures from 6.5 x 10 -3 to 20.4 arm. The oxidation rate was controlled by diffusion through the oxide and had a gross activation energy of 50 _ 4 kcal/mole. Least squares straight line fitting over the entire pressure range indicated the rate was proportional to Po21/5. However, the rates above 1 arm appeared insensitive to pressure. On this basis, the measured rates below 1 atm more closely follow the PO21/4 dependence expected from electrical conductivity studies in NiO. The oxide oriented in a preferred manner with 100 planes parallel to the polycrystalline nickel base.In the early work of Wagner and Griinewald (1) on the oxidation of nickel, the observed parabolic rate was derived on the basis of diffusion of nickel ions by a vacancy exchange mechanism through a uniform layer of NiO. Below 1000~ the parabolic relation has been observed by other investigators (2-4). The role of cation vacancies has been confirmed by electrical and thermoelectric studies on NiO (5-7), which is a p-type semiconductor, and by the close correspondence of the activation energies for oxidation and for diffusion of radioactive nickel in NiO (8,9). Nevertheless, there are some anomalies. Gulbransen and Andrew (10) have observed that at 900~ and higher, parabolic rate law plots of the data show increasing values of the rate constant with time. The dissociation of two electron holes from each cation vacancy and a resulting oxidation rate and electrical conductivity dependence on Poe 1/6 which was assumed by Wagner and Griinewald are not clearly supported by electrical conductivity measurements or oxidation rate measurements.

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Co was oxidized from 800 ~ to 1200~ in 0.013-27.2 atm O2. It oxidized in accordance with the parabolic rate law above 950~ and formed the single oxide, CoO, above 900~ Pressure increase accelerated the rate of oxidation. However, the oxidation rate eventually ceased to increase with increase of oxygen pressure at temperatures below 1150~ Theoretical considerations employing a vacancy saturation mechanism correlated the data. Photomicrographs of the oxide layer are included. Activation energy for the diffusion process is 58,000 cal,