Molecular structure, dynamics and adsorption behavior of water molecules and ions on [0 1 0] surface of γ-FeOOH: A molecular dynamics approach

The interaction between chloride ions and passivate film of steel bars determines the corrosion resistance of iron materials. In this study, the molecular structure, dynamics properties and adsorption behavior of water molecules and ions on passivate film surface is systematically investigated by molecular dynamics simulation method. This interfacial model is constructed for NaCl solution with 1.5 mol/L near the γ-FeOOH [0 1 0] surface. The FeO6 octahedron, exhibiting strong hydrophilic nature within the surface, provides a large amounts of oxygen sites to form H-bonds with the neighboring water molecules. The surface water molecules, distributed in the first adsorbed layer, not only show a comparably high magnitude of dipole moment but also tend to develop an H-down orientation with respect to the γ-FeOOH substrate. Solid oxygen and hydrogen atoms in the γ-FeOOH substrate, which replace the surrounding water molecules, change the hydration structure of ions and the adsorbed ions mobility. Furthermore, the sodium and chloride ions can be adsorbed by the iron hydroxyl surface in different ways. On the one hand, by forming NaO bond with solid oxygen in the iron hydroxyl, the sodium ions can penetrate into the vacancy areas between the neighboring FeO6 octahedrons. On the other hand, chloride ions form H-bonds with the neighboring FeOH groups and associate with sodium ions on the surface to form the cation-anion ionic pairs. Both H-bond connection and the cation-anion ionic pairs restrict the chloride ions in the γ-FeOOH substrate. Additionally, due to the attraction of the solid substrate, the diffusion coefficient of solution species within the surface dramatically decrease by 70% and the resident time of water and ions in the vicinity of γ-FeOOH substrate is elongated more than twice as compared with that in bulk solution. Hopefully, this paper can provide valuable insights and interpret the corrosion mechanism of iron materials at the molecular level.