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2-thiol, triphenylbenzyl,and tetrabenzyl phosphonium chloride on the corrosion of mild steel M. S. MORAD (0•26 wt-%C) in deaerated 3M HCl solution have been studied using the determination of polarisation curves as well as linear polarisation measurements. Adsorption isotherms and thermodynamic parameters for the adsorption process of the inhibitors were also determined and are discussed. Except for benzo(f)quinoline, which acts as an accelerator of corrosion, the other quinolines act as corrosion inhibitors. T he inhibition was found to be predominantly anodic with quinoline and 8-hydroxyquinoline, while quinoline-2-thiol is a mixed inhibitor. T he increase in inhibition eYciency with temperature, the resulting L angmuir adsorption isotherm and the high negative values of the standard f ree energy of adsorption DG°a denoted chemisorption. T riphenylbenzyland tetrabenzyl phosphoniumcompounds were found to be inhibitors of the mixed type. High negative values of the standard free energy of adsorption and the resulting T emkin isotherm indicated chemisorption of the phosphonium compounds. T he positive values of the standard enthalpy and entropy of adsorption indicated that adsorption of the inhibitors is associated with the desorption of H 2 O molecules f rom the electrode surface. High values of the standard energy of activation E* a of the inhibited corrosion process were interpreted in terms of deactivating coverage.

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