Diffusion processes determining the oxidation rate of multicomponent alloys

The duration of the selective oxidation period is a key factor determining the life time of the corrosion resistant alloys. Diffusion processes during the selective oxidation of an alloy containing a noble metal (Pt, Au) and a oxidizable metal (Ni, Cu or Zn) were theoretically analyzed by C. Wagner. He had shown the analytical method to calculate the oxidation rate (growth of the scale) as a function of initial concentration of the reacting metal. Wagner simplified the problem of alloy oxidation assuming the constant concentration at the alloy∣oxide interface. In this work we extend the Wagner model by introducing (i) the time dependence of the reacting metal concentration at the alloy∣oxide interface and the instantaneous rate constant, (ii) the composition dependent diffusivities of the alloy components and (iii) the finite geometry of the oxidized alloy. The model can be applied to predict the life time, i.e., the critical consumption of the reacting element and the evolution of the components concentration in alloy and at its boundaries. We present the comparison of Wagner’s results and our extended analysis.