Inhibitory behaviour and adsorption stability of benzothiazole derivatives as corrosion inhibitors towards galvanised steel†

An in-depth understanding of corrosion inhibitor behaviour(s) at the metal–solution interface governed by unique molecular features is the key premise to realising molecular tailoring for pronounced metal protection. This study investigated the distinct adsorption behaviours induced by merely replacing the chemical functionality upon benzothiazole, i.e., 2-mercaptobenzothiazole (2-MBT) and 2-aminobenzothiazole (2-ABT), towards electro-galvanised steel (ZE) corrosion using both experimental and theoretical approaches. Electrochemical results confirm that both inhibitor candidates act as corrosion inhibitors for ZE in NaCl solution. The underlying interactions of the inhibitor molecule with the targeting metal, dissolved metal ions and corrosion products were explored by means of X-ray photoelectron spectroscopy, focused ion beam scanning electron microscopy and Raman spectroscopy. It is suggested that 2-MBT facilitates the precipitation upon the ZE by complexing with the released Zn²⁺ in solution, while 2-ABT promotes preferentially thin inhibitor film formation initiated by chemisorption. Density functional theory (DFT) reveals that at high concentrations the molecules tend to adsorb vertically (slightly tilted) at the surface, where the presented heteroatoms enhance surface–molecule interaction. In addition, DFT suggests that the strong binding strength of 2-MBT could facilitate the formation of complexes with displaced Zn. Based on the proposed mechanisms, the adsorption stability upon polarised ZE surfaces was determined, which reveals that 2-MBT forms a thick inhibitor layer at a relatively high polarisation state, whereas 2-ABT dissociates from the surface with the increasing value of surface overpotential. The findings of this study provide structural understanding that underpins inhibitor tailoring and molecular design to achieve the desired inhibition properties.