High-performance corrosion inhibitors for carbon steel in hydrochloric acid: electrochemical and DFT studies

This research examines the corrosion inhibition efficiency of two novel compounds, AEPA and DOCA, on carbon steel in 1.0 M hydrochloric acid. Both AEPA and DOCA demonstrated excellent electrochemical performance as corrosion inhibitors, with inhibition efficiencies exceeding 93% at a concentration of 10 mM, as confirmed through potentiodynamic polarization (PDP), electrochemical frequency modulation (EFM), and electrochemical impedance spectroscopy (EIS) techniques. EIS analysis revealed a marked increase in charge transfer resistance (<i>R</i> _ct), reaching 387.55 Ω cm² for AEPA and 345.80 Ω cm² for DOCA, indicating the formation of a robust protective layer on the carbon steel surface. Adsorption studies confirmed that both inhibitors follow the Langmuir isotherm model, suggesting monolayer chemisorption. The calculated adsorption equilibrium constants with corresponding Gibbs free energy values of -29.53 kJ per mol (AEPA) and -29.30 kJ per mol (DOCA), respectively, indicating spontaneous and strong adsorption interactions. Theoretical insights from density functional theory (DFT) calculations revealed that AEPA possesses a higher HOMO energy (-5.65 eV) and a lower LUMO energy (-1.12 eV) compared to DOCA (HOMO: -6.70 eV, LUMO: -0.85 eV), resulting in a smaller energy gap (Δ<i>E</i> = 4.53 eV for AEPA<i>vs.</i> 5.85 eV for DOCA). This suggests that AEPA has a greater electron-donating ability and stronger interaction with the metal surface. The integration of experimental and theoretical approaches provides a comprehensive understanding of their inhibition mechanisms and highlights their potential for practical applications in corrosion protection.