Insights into corrosion inhibition mechanism of mild steel in 1 M HCl solution by quinoxaline derivatives: electrochemical, SEM/EDAX, UV-visible, FT-IR and theoretical approaches

Three quinoxaline-based heterocycles namely, 6-methyl-2,3-diphenylquinoxaline (Q-CH₃), 6-nitro-2,3-diphenylquinoxaline (Q-NO₂) and 2,3-diphenylquinoxaline (Q-H) were evaluated as inhibitor for mild steel (MS) in 1 M HCl. Inhibition effectiveness of the Q-H, Q-CH₃ and Q-NO₂ tested using different computational simulations and experimental methods. Results showed that inhibition effectiveness of Q-H, Q-CH₃ and Q-NO₂increases with their concentration. Polarization results showed that Q-H, Q-CH₃ and Q-NO₂displayed anodic-type behaviour. Inhibition efficiencies of Q-H, Q-CH₃ and Q-NO₂ followed the order: 87.6% (Q-NO₂) < 90.2% (Q-CH₃)< 92.4%(Q-H) for Q-CH₃. Presence of both electron withdrawing (-NO₂) and donating (-CH₃) substituents decrease the inhibition efficiency as compared to the parent compound however in decrease in protection power is more prominent in the presence of –NO₂ substituent. Q-H, Q-CH₃ and Q-NO₂inhibit corrosion by adsorbing on MS surface and their adsorption mode followed Langmuir adsorption isotherm. Adsorption of Q-H, Q-CH₃ and Q-NO₂ on metallic surface reinforced with SEM-EDS and UV-visible studies of MS surfaces. Interaction mechanism of Q-H, Q-CH₃ and Q-NO₂with MS surface and their mode of adsorption was studies using DFT and MD (MD) simulations, respectively. Negative sign of adsorption energies (E_ads) for Q-H, Q-CH₃ and Q-NO₂suggested that they adsorb spontaneously over MS surface.