Benzothiazole-based ionic liquids as environment-friendly and high-efficiency corrosion inhibitors for mild steel in HCl: Experimental and theoretical studies

Two benzothiazole ionic liquids - (E)-3-ethyl-2-(4-hydroxystyryl) benzo[d]thiazol-3-ium iodide ([EHSBT]I) and (E)-3-ethyl-2-(4-hydroxystyryl)benzo[d]thiazol-3-ium bromide ([EHSBT]Br) generate the outstanding protection when employed as green corrosion inhibitors for the steel against aggressive acid solution at different temperatures. The corrosion efficiency of [EHSBT]I and [EHSBT]Br with the low dosage of 0.2 mM is 98.16 % and 89.68 %, respectively, at 303 K. The corrosion inhibition efficiency of [EHSBT]I decreases with the increased temperature and [EHSBT]Br is the opposite. Fortunately, both [EHSBT]I and [EHSBT]Br exhibit the inhibition efficiency of greater than 95 % even when it achieves 333 K, which is ascribed to the dominant chemical adsorption. Likewise, the electrochemical investigation presents the increasement of the charge transfer resistance and interfacial electric double layer thickness due to their adsorption on the substrate, restraining the metal dissolution and hydrogen evolution. The findings of the surface structure and appearance using UV-vis, FT-IR, XPS, SEM-EDS, AFM suggest that the inhibitors can form the complex film ([EHSTB]I-Fe, [EHSTB]Br-Fe) to shield the metal from corrosive particulates via chemical bonding. The confirmation of the active sites and adsorption configuration through theoretical calculation such as DFT and MD, along with RDF, promotes a further comprehension of the corrosion mitigation mechanism, which coincides with the experimental conclusion that the preparation of a stable protective film realizes the superior inhibition performance against high temperature acid corrosion.