Effect of Mercapto and Methyl Groups on the Efficiency of Imidazole and Benzimidazole-based Inhibitors of Iron Corrosion.

We report on the combined experimental and computational study of imidazole- and benzimidazole-based corrosion inhibitors containing methyl and/or mercapto groups. Electrochemical measurements and long-term immersion tests were performed on iron in NaCl solution, whilst computational study explicitly addresses the molecular level details of the bonding on iron surface by means of density functional theory calculations (DFT). Experimental data were the basis for the determination of inhibition efficiency and mechanism. Methyl group combined with mercapto group has a beneficial effect on corrosion inhibition at all inhibitor concentrations. The beneficial effect of mercapto group combined with benzene group is not so pronounced as when combined with methyl group. The latter is in stark contrast with the behaviour found previously on copper, where the effect of methyl group was detrimental and that of mercapto and benzene beneficial. Explicit DFT calculations reveal that methyl-group has a small effect on the inhibitor-surface interaction. In contrast, the presence of mercapto group involves the strong S-surface bonding and consequently the adsorption of inhibitors with mercapto group is found to be more exothermic.