The model of the heterogeneous reaction between the alloy and oxidant is shown. The alloy reacts with oxidant and forms oxide scale. The reaction rate is controlled by the interdiffusion in alloy and the chemical diffusion through the compact scale. In this work we extend the Wagner model by introducing i) the variable instantaneous rate constant, ii) the composition dependent diffusivities of the alloy components and iii) the finite geometry of the oxidized alloy. The model allows us to predict the life time of the alloy and the evolution of the components concentration. The comparison of Wagner’s results and our extended analysis is shown.