meta-Substituted aniline-N-salicylidenes as corrosion inhibitors of zinc in sulphuric acid

The corrosion of zinc in sulphuric acid containing different m-substituted aniline-N-salicylidenes has been studied with respect to inhibitor and acid concentration, period of exposure and temperature. 99% inhibitor efficiencies have been observed. It appears that the salicylidene part of the inhibitor including the iminic group in the molecule plays a dominant role in the inhibition. As far as the effect of exposure period and temperature is concerned, m-CNS has turned out to be the best one. For all the five inhibitors, the heats of adsorption and free energies of adsorption are negative, while the entropies of adsorption are positive. The free energies of adsorption are more negative in the case of very good inhibitors like m-TNS and m-CNS, indicating a strong interaction of the inhibitor molecules with the metal surface. The activation energies in the presence of these inhibitors are higher than that in plain acid. The adsorption of these inhibitors follows Langmuir adsorption isotherm. The conjoint effect of external cathodic current and the inhibitor is observed to be either synergistic or additive. Galvanostatic polarization studies indicate that these are mixed type inhibitors with predominant action on the local cathodes. Mechanism of the action of inhibitors has been provided.