Local Effects of Organic Inhibitor Molecules on Passivation of Grain Boundaries Studied In Situ on Copper

The effects of two organic corrosion inhibitors, 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzimidazole (MBI), on grain boundary passivation was investigated on copper at the nanometer scale in NaOH solution using electrochemical scanning tunneling microscopy (ECSTM). Global electrochemical analysis by cyclic voltammetry showed that the organic surface layers, pre-formed after reductive dissociation of the native oxide in the presence of the inhibitors, block the formation of a Cu(I) surface oxide and thus passivation, but do not entirely suppress residual reactivity. Local ECSTM analysis in initial metallic, subsequently oxidized, and final reduced states confirmed residual intergranular reactivity except for coherent twins. On coincidence site lattice (CSL) and random boundaries, residual dissolution with accumulation of corrosion products or residual passivation was observed, depending on the barrier effect of the pre-formed inhibitor layer on oxide formation. For low Ʃ CSLs, no difference of barrier effect was observed between MBT and MBI. For more reactive high Ʃ CSLs and random boundaries, pre-adsorbed MBT formed a stronger barrier against passivation by oxide growth than pre-adsorbed MBI. The results provide deeper understanding of how passivation is altered by a pre-formed surface layer of organic corrosion inhibitor, including locally at different grain boundary types.