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

Abstract

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 (R _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 (ΔE = 4.53 eV for AEPAvs. 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.

Scheme 1. Synthetic pathway and the suggested mechanism of DOCA inhibitor.

Scheme 2. Synthetic pathway and the suggested mechanism of AEPA inhibitor.

Fig. 1. FT-IR spectra of the investigated inhibitors (DOCA and AEPA).

Fig. 2. ¹H NMR spectra of the investigated inhibitors (DOCA (a) and AEPA (b)).

Fig. 3. ¹³CNMR spectra of the investigated inhibitors (DOCA (a) and AEPA (b)).

Fig. 4. UV spectra of the investigated inhibitors (DOCA and AEPA).

Fig. 5. Potentiodynamic polarization curves for the corrosion of carbon steel in 1.0 M HCl in absence and presence of different concentrations of AEPA (a) and DOCA (b) compounds at 30 °C.

Fig. 6. Intermodulation spectra for carbon steel in 1.0 M HCl in absence and presence of different concentrations from AEPA compound at 30 °C.

Fig. 7. Nyquist plots, Bode and phase angle plots for steel in 1.0 M HCl solution without and with different concentrations of AEPA (a and c), and DOCA (b and d) compounds at 30 °C.

Fig. 8. The Langmuir adsorption model for AEPA and DOCA compounds on the steel surface in 1.0 M HCl using data obtained from PDP measurements at 30 °C.

Fig. 9. SEM and EDX images of the steel surface (a) polished and (b) after 24 hours immersion in 1.0 M HCl.

Fig. 10. SEM and EDX images of the steel surface after 24 hours immersion in 1.0 M HCl containing 10 mM of AEPA (a) and DOCA (b).

Fig. 11. Optimized structures of AEPA (a) and DOCA (b).

Fig. 12. Frontier molecular orbitals and energy gap of AEPA (a) and DOCA (b).

Fig. 13. Density of states of compounds AEPA (a) and DOCA (b).

Fig. 14. MEP maps of compounds AEPA (a) and DOCA (b).