The inhibition effect of 3-(4-Hydroxyphenylamino) propanenitrile (Para) and 3-(2-Hydroxyphenylamino) propanenitrile (ortho) on the corrosion of steel mild 1M HCl solution was studied by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy methods. The results revealed that corrosion rate depends on the molecular structure and the concentration of inhibitors. We found that the Para Hydroxyphenylamino propanenitrile is a good inhibitor and has been tested with efficiency reaches to 85% in 10- 3M. Polarization studies through electrochemical curves and spectroscopy of electrochemical impedance clearly show that 3-(4-Hydroxyphenylamino) propanenitrile is a mixed inhibitor. The adsorption of Ortho and Para inhibitors on the mild steel surface obeys Langmuir isotherm. Both thermodynamic parameters (adsorption heat ΔH°, adsorption free energy ΔG° and adsorption entropy ΔS°) and kinetic parameters (apparent activation energy Ea) were calculated and discussed. The quantum chemical parameters calculated using DFT at the B3LYP/6-31G* level of theory show a good correlation to the inhibition efficiency. The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), the separation energy (ΔE) and the dipole moment (μ) from the inhibitor to the metal surface explain well experimental data.
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