Quantum chemical calculations based on Hartree–Fock, AM1 and B3LYP functions were performed on the three Schiff-base compounds used as corrosion inhibitors for carbon steels in acid media. The quantum chemical calculations were conducted to determine the relationship between inhibition efficiency (IE) and the molecular structure of inhibitors. Several quantum chemical parameters, such as charge distribution, energy and distribution of the highest occupied molecular orbital (EHOMO) and lowest unoccupied molecular orbital (ELUMO), absolute electronegativity (χ) and the fraction of electrons (ΔN) transferred from inhibitors to the steel surface, were calculated and correlated with inhibition efficiencies. The results showed that the IE of Schiff-base compounds enhanced with increasing EHOMO or decreasing ELUMO; meanwhile, N atoms on Schiff-base molecules were the most probable sites for adsorption of inhibitor molecules on the metal surface. The inhibition mechanisms of three Schiff-base compounds were obtained from different calculation methods, and the results from each method are consistent.
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