A Close-up of the Effect of Iron Oxide Type on the Interfacial Interaction between Epoxy and Carbon Steel: Combined Molecular Dynamics Simulations and Quantum Mechanics

In the present study, the effect of different types of iron oxides, which naturally exist on steel substrate, on the interfacial interaction between an epoxy coating and a carbon steel substrate was studied at the molecular/atomic level by employing molecular dynamics (MD) simulations and quantum mechanics (QM) calculations. Three types of iron oxide, that is, ferrous oxide (FeO), ferric oxide (Fe2O3, hematite), and ferrous ferric oxide (Fe3O4, magnetite), were considered for modeling, and their binding energies were calculated and compared by altering the concentration of hydroxide groups on the surface. To probe the effect of curing agent on interfacial interactions, computations were performed for either uncured or aminoamide-cured epoxy resins. The effect of the acid–base properties of the iron oxide on the molecular bonding was theoretically investigated by imposing diverse iron hydroxide/oxide termination groups. Noticeably, MD and QM calculations confirmed rather well earlier experimental evaluations on iron oxide acid–base properties obtained through X-ray photoelectron spectroscopy measurements in the view of chemisorption of different epoxy compounds. However, the interaction behavior of cured epoxy with hydroxylated iron oxide surfaces was quantified mechanistically in the current work from a closer view. For instance, it was found that aminoamide-cured epoxy was adsorbed on all oxide substrates through a mechanism of electrostatic and donor–acceptor interactions, with binding energies of −113.6, −1035.9, and −304.4 kcal/mol respectively assigned to FeO, Fe2O3, and Fe3O4 iron oxides. In the case of a hydroxylated surface, aminoamide-conjugated epoxy adhesive was found in the hydrogen bond interface as well, which was evidence of strengthened binding at a surface populated by hydroxide groups. Moreover, theoretical explorations showed that the type of covalent linkage between the curing molecule and epoxy resin governs the extent of cured epoxy adhesion to the surface of iron oxide.