HIGH-TEMPERATURE OXIDATION OF METALS UNDER TIME-DEPENDENT GAS-PRESSURE

A theor. study on the high-temp. oxidn. of thick oxide films grown under a time-dependent gas partial pressure is reported.  The diffusion across the film is assumed to be the rate-limiting step of the overall reaction, and Wagner's (1933) hypotheses were used as the starting point for the reasoning.  A general formulation for the oxidn. under no time const. gas pressure, in terms of a time-dependent answer function is given.  The effect of an external elec. field on the reaction rate is reported when a const. c.d., J, is applied to the oxide scale.  If the oxide is a good electronic conductor at J = 0 and t » τ, the reaction rate assumes formally the well-known Wagner's expression of the rate const. but with a gas partial pressure at oxide/gas interface which is a time function.  If the oxide is a good ionic or electronic conductor, the effect due to an external elec. field, J ≠ 0, on the reaction rate is the same as predicted by the Wagner's theory.