Probing the effect of N-substituent unsaturation in benzimidazolone corrosion inhibitors on mild steel: Electrochemical performance and SCC-DFTB adsorption analysis on Fe(110)

Two N-substituted benzimidazolone derivatives, the N-propargyl- and N-allyl-substituted 1-(cyclohex-1-en-1-yl)benzimidazol-2-ones, CPB and ACPB, were evaluated as corrosion inhibitors for mild steel in 1.0 mol/L HCl. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) showed that both compounds act as mixed-type inhibitors, achieving inhibition efficiencies up to 96.9% (CPB) and 97.7% (ACPB) by EIS, and 98.9% (CPB) and 99.0% (ACPB) by PDP at 10 −4 M. ACPB outperformed CPB at all tested concentrations. Both inhibitors maintained high efficiency (>96%) over extended immersion and at temperatures up to 328 K, indicating robust film stability. Surface characterization by SEM confirmed the formation of a protective film on the inhibited surfaces. AFM showed that the inhibited surfaces retained near-polished roughness (R a = 11.28 nm for CPB; 21.65 nm for ACPB) compared to the severely corroded blank (R a = 644.9 nm). Self-consistent charge density functional tight-binding (SCC-DFTB) simulations on the Fe(110) surface revealed parallel adsorption geometries with C-Fe bond distances of 2.264 Angstrom (CPB) and 2.129 Angstrom (ACPB), and interaction energies of −1.045 eV and −1.108 eV, respectively. Projected density of states (PDOS) analysis confirmed orbital hybridization between the inhibitor molecules and the Fe surface. The slightly better performance of ACPB may be related to the weaker electron-withdrawing effect of the allyl group compared with propargyl, although the small differences in interaction energy and molecular descriptors prevent a definitive mechanistic assignment.