Ortho-, meta-, and para-aminophenol-N-salicylidenes as corrosion inhibitors of zinc in sulfuric acid

Purpose - The evaluation of Schiff bases derived from o-, m- and p-aminophenols and salicylalclehyde as corrosion inhibitors of zinc in sulfuric acid and to study their action mechanism. Design/methodology/approach - The effect of various parameters on the behaviour of these inhibitors has been studied using the weight loss and polarization measurements. Findings - In general, the ortho isomer was highly effective as a corrosion inhibitor because it formed a chelate with a six-membered ring and moreover the ortho isomer possessed pronounced electromeric effect. These inhibitors obey the Langmuir adsorption isotherm. The almost constant performance with temperature in the case of ortho and para isomers in 0.5 M sulfuric acid suggested strong adsorption bonds. The thermodynamic parameters suggested that this strong interaction of the inhibitor molecules with the metal surface resulted in spontaneous adsorption. It may be concluded that a good inhibitor is characterised by a relatively greater decrease in free energy of adsorption, lower entropy of adsorption and higher heat of adsorption. Polarization data indicated that all these isomers were predominantly cathodic inhibitors. The conjoint effect of external cathodic current and these inhibitors was either synergistic or additive. Research limitations/implications - Even more powerful Schiff bases need to be synthesised and evaluated as corrosion inhibitors with a number of metals and alloys in diverse media, which may be effective at low concentrations. Originality/value - Very few inhibitors exhibit such excellent inhibitive effect on zinc in aggressive corrosive media. Rarely do we find such detailed studies.