New sugar-derived compounds as inhibitors of carbon steel against corrosion in acid solutions: experimental analyses and theoretical approaches†

The purpose of this study is to investigate the acid corrosion inhibition efficiency on carbon steel (CS) by utilizing two novel quinoxaline derivatives obtained from the reaction of recently synthesized D-mannose (MR₁ and MR₂) via nucleophilic substitution (SN₁). The synthesized compounds were recently characterized by ¹³C-NMR and ¹H-NMR spectroscopy. Electrochemistry testing was employed to evaluate their protective efficiency, whereas the surface was investigated using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results indicate that the two inhibitors MR₁ and MR₂ exhibit inhibition efficiencies of 95.3% and 94.8% at 10⁻³ M for MR₁ and MR₂, respectively. The impedance results indicated that the incorporation of MR₁ and MR₂ into the corrosive medium reduces charge capacitance, hence systematically enhancing the interface charge/discharge function and creating an adsorbed layer on the metal surface. Moreover, SEM, water contact angle, and XPS techniques corroborated the formation of a protective coating on the carbon steel substrate surface following the incorporation of MR₁ and MR₂. The chemical interaction mechanisms at the atomic scale were analysed using theoretical calculations, DFT calculations and MD simulations.