Study of the spontaneous oxidation of sodium in air by EPMA and Monte Carlo simulations

Abstract

The growth of the corrosion layer occurring when a sodium surface is exposed to air under room conditions was studied considering the ratio between O − Kα and Na − Kα characteristic peak intensities from the X-ray emission spectrum collected in a scanning electron microscope for different exposure times of metallic sodium to air. Comparison between experimental results and Monte Carlo simulations leads to the conclusion that Na oxidation in air proceeds in two stages: at relatively short times (up to 6.5 s) a Na-rich layer of composition Na 2 O is formed. At longer times the oxide layer evolves through reaction with oxygen to a composition that can be described by the stoichiometry Na 2 O 2 . While the first of these processes seems to be limited by the reaction of the oxygen molecule with sodium, the second one appears to be limited by oxygen diffusion in the growing layer.