Incorporation of Zn into oxide films on stainless steel in simulated PWR conditions

The ultimate goal of this work is to develop a deterministic model for activity incorporation and to increase the theoretical understanding of the oxide film build-up and breakdown, controlling the localised corrosion phenomena of structural materials in Light Water Reactors. In the present communication, the kinetic parameters of the incorporation of Zn aquoions during the initial stages of film growth on stainless steel in simulated Pressurised Water Reactor (PWR) conditions have been estimated. Also, a description for the effect of this process on film growth and restructuring is presented. To achieve that, the oxidation of AISI 316L(NG) in simulated PWR water (2.2 ppm Li, 1200 ppm B) with or without addition of 1 ppm Zn at 280oC for up to 72 h has been characterised in-situ by electrochemical impedance spectroscopy, both at the corrosion potential and under anodic polarisation up to 0.5 V vs. the reversible hydrogen electrode (RHE). Additional tests were performed in simulated PWR coolant with the addition of 0.01 M Na2B4O7 to exclude the effect of pH excursions probably due to hydrolysis reactions. The thickness and in-depth composition of the oxide films formed at open circuit and at 0.5 V vs. RHE in the investigated electrolytes have been assessed by X-ray photoelectron spectroscopy (XPS). The kinetic and transport parameters characterising the oxide layer growth have been estimated using a calculational procedure based on the Mixed Conduction Model for oxide films.