The importance of chemical transformations of adsorbed molecules for corrosion inhibition

This study investigates whether mercaptobenzimidazoles act as thiolates in inhibiting copper corrosion. To this end, we examined three mercaptobenzimidazole derivatives — 2-mercaptobenzimidazole (SH-BimH), 2-mercapto-1-methylbenzimidazole (SH-BimMe), and 2-(methylthio)benzimidazole (Me-S-BimH) — as corrosion inhibitors for copper in 3 wt% NaCl solution using a combined experimental and computational approach. Me-S-BimH has a thiol group (single bondSH) replaced by a methylthio group (single bondSCH ), which should prevent the formation of surface thiolates. In contrast, SH-BimMe has the same molecular formula as Me-S-BimH, but its methyl group does not cap the thiol group. Corrosion experiments reveal that after 1 h of immersion, Me-S-BimH is considerably less effective than SH-BimH and SH-BimMe at inhibiting copper corrosion. However, after 100 h of immersion, Me-S-BimH performs comparably to SH-BimH and SH-BimMe. This delayed effectiveness suggests that a molecular transformation activates Me-S-BimH over time. To explore this phenomenon, we performed a detailed DFT study of potential chemical transformations of adsorbed Me-S-BimH. Most transformations are exothermic, but only molecular deprotonation and Csingle bondS bond cleavage between the azole ring and the methylthio group exhibit sufficiently low activation barriers to occur at room temperature. Similar deprotonation and Csingle bondS bond cleavage reactions occur also for SH-BimH and SH-BimMe, leading to more strongly bound species than their intact molecular forms. Due to these transformations, Me-S-BimH and SH-BimH eventually result in the same strongly bound species, while SH-BimMe forms an analogous species. These findings may explain why, over time, all three compounds exhibit similar corrosion inhibition characteristics, and highlight the importance of chemical transformations of adsorbed molecules in corrosion inhibition.