A mixed-conduction model for oxide films on Fe, Cr and Fe–Cr alloys in high-temperature aqueous electrolytes—II. Adaptation and justification of the model

Abstract

The aim of this two-part work is to propose a model for the corrosion mechanism of ferrous alloys in high-temperature aqueous environments. In this second part, the modifications to the mixed-conduction model (MCM) are discussed on the basis of experimental data presented in the first part for Fe, Cr and two Fe–Cr alloys (12 and 25 wt% Cr) in an aqueous solution at 200 °C. Application of the MCM to fit and predict experimental behaviour both at room temperature and at 200 °C is demonstrated. The major difference between the behaviour of films at room temperature and at 200 °C is that the mobility of ionic defects is much higher at the higher temperature. Estimates show that the ratio of the electronic and ionic diffusion coefficients ( D e/ D i) is of the order of 10 5 at room temperature and ≈30 at 200 °C for pure Fe. Such a large difference explains the higher growth rate and thickness of films formed on Fe at the higher temperature. It is also in agreement with the higher defect content and lower field strengths in high-temperature films. The application of the MCM to Fe–Cr alloys indicates that the diffusion coefficient of major ionic current carriers is smaller for the alloys than for pure Fe. Alloying with Cr thus lowers the ionic mobility in the passive film on a ferrous alloy also at 200 °C.