Mixed-conduction model for stainless steel in a high-temperature electrolyte: Estimation of kinetic parameters of inner layer constituents

A quantitative procedure of determination of kinetic and transport parameters for individual alloy constituents during anodic film growth on stainless steels in light reactor water is developed. It is based on in-depth compositional data for oxides obtained from ex situ analyses using Auger electron spectroscopy and X-ray photoelectron spectroscopy. The growth of the inner compact layer is described as a sequence of interfacial reactions and transport driven by homogeneous diffusion-migration mechanism. Based on the mixed-conduction model for oxide films, a fitting procedure for the calculation of the in-depth distribution of the individual alloy constituents in the inner layer is put forward. The effects of temperature and applied potential on the kinetic and transport parameters in the inner layer are assessed. In addition, the growth of an outer layer consisting of crystallites with pores filled with electrolyte in-between is described formally as a diffusion process and the transport parameters characterizing this process are estimated. The estimates of the kinetic and transport parameters obtained are discussed in relation to the corrosion mechanism of the steel and the incorporation of electrolyte-originating species in the bilayer oxide film. Using the proposed quantitative procedure, the kinetic and transport parameters of long-term (up to 10,000 h) film growth and restructuring on AISI 304 stainless steel in simulated pressurized water ractor (PWR) with or without zinc addition are also estimated on the basis of a quantitative comparison of the model predictions and literature data on the in-depth concentration profiles of the constituent elements. The obtained values are discussed in terms of the effect of Zn on the growth rate of the inner and outer layers of the corrosion film.