Electrical properties of growing alumina scales

Abstract

Scales growing on platinum 22 wt.% aluminum specimens at 1100°C were investigated by the following electrical techniques: open-circuit potential and short-circuit current as a function of time and oxygen pressure; current-voltage relationship at several oxygen pressures; transients in the short-circuit current on rapidly changing the oxygen pressure; changes in the growth rate with applied potential. The growth characteristics of the scale were also investigated using kinetic and metallographic techniques. The apparent parabolic rate constant of oxidation decreases in the first hours of oxidation and stabilizes after 20–30 hr at 2 × 10⁻¹³ g²·cm⁻⁴·sec⁻¹. Transport in the scale is by lattice and grain-boundary diffusion and the changing kinetics are a consequence of changes in the oxide grain size. It was also determined that the electrical characteristics of the scale were affected by the hydrogen-water or carbon monoxide-carbon dioxide atmospheres used to obtain low oxygen pressures. Due to these complexities and to the difficulty in making a satisfactory oxide-gas electrode only qualitative observations could be made from the electrical measurements. These measurements are compatible with the scale having constant ionic conductivity in the pressure range 1 to 10⁻¹⁵ aim and predominantly electronic conductivity at lower oxygen pressures. The rate of oxidation can be accelerated or retarded by passing a current through the scale.