Relationship between chemical structure of imidazoline inhibitors and their effectiveness against hydrogen sulphide corrosion of steels

Seven homologous imidazoline type inhibitors were synthesised, all being derivatives of cyclopentyl- and cyclohexylnaphthenic acids. The protective effectiveness of these compounds was investigated by applying the potential sweep method to the carbon steel St3S and austenitic steel 1H18N9T in a 2% solution of sodium chloride in contact with a hydrocarbon phase and saturated with hydrogen sulphide gas. It was found that the inhibitors investigated and their mixtures present high protective effectiveness at a concentration of 25 ppm, reaching values of 99% for carbon steel and 80% for austenitic steel at 40 degreesC. This protective effectiveness was improved by increasing the length of the substituent chain. Consequently, the least effective compounds were those with the shortest chain substituted at the imidazoline ring. Inhibitors under study were chemisorbed on the electrode surface of the steel and this chemisorption process obeyed the Frumkin adsorption isotherm. Values of the Gibbs free energy of adsorption of inhibitors were determined, and also the equilibrium constants of adsorption and constants of attraction. (C) 2000 IoM Communications Ltd.