Abstract
Inhibition of the corrosion of a carbon steel (0.4% C) by 2-hexadecyl imidazoline and 2-hexadecyl imidazole has been evaluated by electrochemical techniques and correlated with surface tension measurements. Plots of inhibitor efficiency versus surfactant concentration produce S-shaped curves which are assumed to represent adsorption isotherms. A sharp increase in slope was observed at concentrations below the critical micellar concentration. This increase in slope is accounted for by changes in conformation of the adsorbed molecules: horizontal orientations (with respect to the surface) at lower concentrations reflecting cathodic behaviour and perpendicular orientations at the higher concentrations reflecting mixed cathodic and anodic behaviour. At increasing concentrations the inhibitory effect remained constant, suggesting complete saturation of the surface in a bilayered arrangement. Electrochemical impedance measurements, carried out for concentrations greater than the critical micellar concentration, corroborate this assumption: the inhibitors form a thick but adherent micellar film which acts as a diffusion barrier.
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