Quantum Chemical Studies on Adsorption of Imidazole Derivatives as Corrosion Inhibitors for Mild Steel in 3.5% NaCl Solution

Adsorption of benzimidazole, 2-methylbenzimidazole and 2aminobenzimidazole on mild steel in 3.5% NaCl solution was studied using density function theory (DFT) calculations. In this regard, charge transfer resistance (Rct) and double layer capacitance (Cdl) obtained by electrochemical impedance spectroscopy (EIS) were used to calculate surface coverage and to build prediction models. When prediction models were only based on quantum chemical parameters of imidazoles, the most effective parameters were frontier orbital energies meaning that dominant mechanism of adsorption was chemisorption. However, models based on imidazoles and iron quantum chemical parameters revealed that the most effective parameter is dipole moment meaning that dominant mechanism of adsorption is physisorption. Prediction models based on inhibitor-Fe(OH)2(H2O)4 and inhibitor-Fe(OH)3(H2O)3 systems presented dipole moment and electron donating power as the most effective parameters for θR and θC based calculations, respectively. Such a behavior could be attributed to chemical interaction of inhibitors with Fe(OH)2(H2O)4 and Fe(OH)3(H2O)3 on the mild steel surface resulting in a porous thick layer which could not effectively block the surface but could decrease double layer capacitance.