A Mixed-Conduction Model for the Oxidation of Stainless Steel in a High-Temperature Electrolyte

The oxide films formed on AISI 316L(NG) in the temperature range have been characterized by impedance spectroscopy and ex situ analysis using Auger electron spectroscopy. Relatively thick films containing a high concentration of mobile defects form on stainless steel in a high-temperature borate electrolyte, but their impedance response is most probably controlled by the properties of a thin barrier sublayer. The ability of the mixed conduction model for passive films to reproduce the experimental impedance data in both alloy/oxide/electrolyte and alloy/oxide/inert metal configurations has been tested. A procedure for the calculation of the kinetic constants of the interfacial reactions of point defect generation/consumption, as well as those characterizing the transport rates of ionic/electronic defects in the oxide, has been developed. The effect of temperature on the kinetic and transport parameters has been assessed, and the relevance of these parameters for the corrosion behavior of stainless steel in a high-temperature electrolyte is discussed. The results show that the nature of the barrier layer does not change drastically with temperature, although the growth mechanism of the oxide film is different at than at room temperature.