The transition from the formation of mixed scales to the selective oxidation of the most-reactive component in the corrosion of single and two-phase binary alloys

The conditions for the transition from the formation of mixed scales to the exclusive oxidation of the component B, forming the most stable oxide, are examined for both single-phase and two-phase binary A-B alloys by taking into account the displacement of the alloy-scale interface due to the growth of the protective oxide. This procedure eliminates the inconsistencies arising from Wagner's classical treatment for single-phase alloys when the interdiffusion coefficient in the alloy is small with respect to the parabolic rate constant for outer-scale growth; but the same procedure leads to a significantly-improved treatment also for two-phase alloys. For the latter systems, the transition is shown to depend also on the solubility of B in the A-rich phase.Moreover, the exclusive growth of the most-stable oxide is more difficult than for single-phase alloys because it requires higher average concentrations of B in the alloy and may even become impossible if the parabolic rate constant of oxidation is large with respect to the interdiffusion coefficient in the alloy.