Impedance study of imidazoline inhibitors against hydrogen sulphide corrosion of steel

Abstract Seven homologous imidazoline inhibitors, the derivatives of cyclopentylic and cyclohexylonaphthenic acids, have been synthesised. The effectiveness of these inhibitors for St3S carbon steel and 1H18N9T austenitic stainless steel was tested using the impedance method in 2 wt-% sodium chloride solution in the presence of a hydrocarbon phase. The solution was saturated with hydrogen sulphide. It was found that the inhibitors and their mix tures displayed high inhibition efficiencies, reaching 93% for carbon steel and 94% for the austenitic steel. Inhibitors with the shortest hydrocarbon side chains on the imidazoline ring were found to offer the lowest level of protection, the inhibition efficiency increasing with the length of the side chain. The capacitance measured on the surface decreased linearly as the coverage of the steel surface by inhibitor increased. The results of impedance measurements with commercial inhibitors have confirmed the conclusions from the tests on the pure chemical species.