Some Schiff bases as corrosion inhibitors for zinc in sulphuric acid

The inhibition of corrosion of zinc in sulphuric acid by Schiff bases aniline-N-benzylidene(ANB), aniline-N-salicylidene(ANS), aniline-N-p-methoxybenzylidene(ANPMB), p-methoxybenzylidene-4,4'-dimorpholine(p-MBDM) and salicylidene-4-4'-dimorpholine(SDM) has been studied with respect to inhibitor concentration, exposure period, and temperature. More than 99% inhibition is achieved with 0.5% concentration of inhibitor except in case of ANB whose efficiency is found to decrease with tinge and rise in temperature. The activation energies in inhibited acid are hi-her than that in plain acid. For all the inhibitors, the heats of adsorption and free energies of adsorption are negative, while the entropies of adsorption are positive. The free energies are more negative in the case of inhibitors like ANS, SDM, ANPMB and p-MBDM. It appears that ail efficient inhibitor is characterized by a relatively greater decrease in free energy of adsorption and less negative entropies of adsorption. Galvanostatic polarization data suggest that these are mixed type inhibitors with preferential action on local cathodes. Cathodic protection studies suggest that the protection is achieved at current densities much lower than that in plain acid. The conjoint effect of external cathodic current and the inhibitor is observed to be synergistic in most of the cases. The inhibitors appear to function through adsorption following Langumuir isotherm.