Electronic structure and chemical bonding in smart anti-corrosion coatings

Abstract

Nitrilotris-(Methylenephosphonato)-Three-aqua-Iron(II) (FeNTP) is a linear organic-inorganic coordination polymer forming an anti-corrosion coating on the steel surface in aqueous medium with NTP-based corrosion inhibitors. The Cd-containing NTP-based inhibitor has higher protective properties than the inhibitor without cadmium. At the same time, the Fe–O bonds are more covalent, and the data from X-ray photoelectron and Mössbauer spectroscopy allow one to suppose that the Fe atoms within the polymer with Cd are in a different magnetic state. However, the causes of these phenomena and their relationship were not studied before. Here, for the first time, the electronic structure of FeNTP is calculated using density functional theory. A stable solution with a large magnetic moment at Fe atoms (4 μB) is found for FeNTP. The calculated properties correspond to the experimental ones obtained earlier: the Fe magnetic moment (estimated by XPS), the quadrupole splitting (Mössbauer spectroscopy), and the character of the chemical bond between Fe and O atoms (X-ray diffraction). Also, a second, nonmagnetic, solution is found, the calculated properties of which are identical to those of the Cd-containing FeNTP. The difference in electronic structure between these two states manifests itself in different Fe 3d occupancy, angular electron density distribution around the Fe atoms, and magnetic state. The interrelation between the magnetic state of the Fe atoms and their chemical bonds is described. This made it possible to reveal the mechanism for the effect of Cd doping of the protective layers on their anti-corrosive properties. Thus, an approach to developing the more effective and efficient corrosion inhibitors in the neutral aqueous media is proposed.