Kinetics of the complexation reaction of iron with dissolved oxygen in liquid sodium

Abstract

This work aims to study the kinetics of pure iron corrosion by liquid sodium in the presence of dissolved oxygen. The corrosion rate of pure iron in liquid sodium was estimated from the mass loss in the samples as a function of immersion time. The parameters tested were dissolved oxygen content, temperature and velocity of the liquid sodium. The rate of pure iron dissolution in liquid sodium increases with dissolved oxygen content and follows a power law of order 2 as a function of the initial oxygen content (50−400ppm). The mechanism of iron corrosion by liquid sodium is activated in the temperature range between 525∘C and 600∘C. The experimental activation energy is estimated to be 168±10kJ/mol. The dissolution rate also increases with the liquid sodium agitation rate.Under the same experimental conditions, S. Meddeb et al. in [J. Nucl. Mater. 566 (2022) 153785] demonstrated that the iron dissolution is homogeneous and is explained by the formation of the NaFeO2 complex at the iron/liquid sodium interface. The corrosion kinetics measured is limited by the transfer of this complex through the mass transfer boundary layer. In the case of static sodium, the associated transfer coefficient is equal to (2.2 ± 0.8)·10−8 m/s. The proposed kinetic corrosion model estimates an excellent order of magnitude for our measurements of average iron thickness loss. Based on our experimental measurements, calculations lead to a diffusivity of the NaFeO2 complex in static liquid sodium of (1.6 ± 0.6)·10−9 m2/s. The Stokes-Einstein law can then be used to estimate an equivalent complex molecule radius of 2 · 10−9 m. These data, combined with the Stokes-Einstein equation, can be used in industrial applications to simulate the mechanism of iron corrosion in non-isothermal heat transfer loop systems.