(v17preprint4)Aminic nitrogen- bearing polydentate Schiff base compounds as corrosion inhibitors for iron in acidic and alkaline media: A combined experimental and DFT studies

摘要

Quantum chemical calculations using the density functional theory (DFT) have been applied to the five kinds of polydentate Schiff base compounds (PSCs), act as inhibitors for iron in aerated 2.0 M HNO3 and 2.0 M NaOH media. The structural parameters, such as the frontier molecular orbital energy HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital), energy gap ΔE (ELUMO - EHOMO), the charge distribution, the absolute electronegativity (χ), the fraction of electrons transfer (ΔN) from inhibitors to iron, the dipole moment (μ), the global hardness (η) and the total energy (Etotal) were also calculated and correlated with their inhibition efficiencies (%IE). The inhibition effects of (PSCs) may be explained in terms of electronic properties. The results showed that the (%IE) of PSCs increased with the increase in EHOMO and decrease in ELUMO - EHOMO. The inhibitor molecules were first adsorbed on the iron surface and blocking the reaction sites available for corrosive attack; and the areas containing N and O atoms are most possible sites for bonding by donating electrons to the iron surface through interaction with π-electrons of the aromatic rings, and the azo methine group. Also, the adsorbed Schiff base molecules interact with iron ions in the corrosive media leading to neutral and cationic iron-Schiff base complexes. Adsorption process is spontaneous, exothermic and obeyed Temkin isotherm and regarded as physical than chemical mechanism. The polarization studied indicated that the inhibitors act as a mixed type inhibitor in HNO3; cause only inhibition of the cathode process in NaOH, and the magnitude of the displacement of the Tafel plot is proportional to its concentration. Models for the inhibition corrosion behaviour were developed based upon statistical analyses of the experimental data. Some thermodynamic and kinetic parameters (Kads, ΔGads) were estimated. Both experimental and quantum theoretical calculations are in excellent agreement.The inhibition efficiency increase in the order of: PSC_1 > PSC_2 > PSC_3 > PSC_4>PSC_5. Thus, DFT study gave further insight into the mechanism of inhibition action of PSCs. This research might provide a theoretical inhibition performance evaluation approach for homologous inhibitors.