The Formation of Porous Oxides on Metals

The accepted mechanism of paralinear oxidation that an inner, compact layer of oxide forms at a parabolic rate and transforms to an outer, porous layer at a linear rate has been tested in the case of tungsten. Samples were oxidized at 750° and 800°C for increasing times, and the amount of porous oxide present on each sample was estimated by the method of krypton sorption. The rate of porous oxide formation is not constant, but instead decreases with time to reach an almost constant value so that the results are not in accord with the paralinear model. A similar conclusion is reached by metallographic examination of the partially oxidized samples. Results for the oxidation at 600°C are used to illustrate the difficulty of testing the paralinear rate equation by an analysis of kinetic data alone.