Thermodynamic Model for Estimating the Composition of Passive Films and Flade Potential on Fe-Cr Alloys in Aqueous Solutions

A thermodynamic model of a passive film is developed, in which the formation of the film on the surface of an Fe-Cr alloy in an aqueous solution is considered to be a result of the stable solid-phase chemical and adsorption equilibrium at the alloy-inner passive film layer interface. In the calculations, the Cr(2)O(3) content in the passive film is determined by both the Gibbs energy change (Delta G < 0) in the chemical oxidation of the alloy components by the water oxygen and the change in the surface Gibbs energy (Delta G(S) > 0) of the alloy. The Delta G(S) change results in the negative adsorption of chromium atoms, which shifts the 3Fe + Cr(2)O(3) <-> 3FeO + 2Cr equilibrium toward the FeO formation in the passive film. Calculations showed that the enrichment of the passive film in chromium oxide should sharply increase in a chromium content range of 10-20% in the alloy, which agrees with the known data of X-ray photoelectron spectroscopy of the passive films. A formula is derived for estimating the Flade potential of Fe-Cr alloys, which relates the Flade potentials of individual Fe and Cr components to the FeO and Cr(2)O(3) contents in the passive film.