Electrochemical study of the adsorption and inhibiting properties of halogen derivatives of aniline on iron in sulfuric acid

The adsorption of halogen derivatives of aniline on an iron electrode in sulfuric acid medium has been studied in relation to their inhibiting properties using electrochemical a.c. impedance and d.c. polarization techniques, as well as quantum chemical calculations of chemical structure and distribution of electron charges in the organic molecules. It is established that the adsorption process is described by Frumkin's isotherm. The adsorption parameters characterizing the interaction forces between the molecules in the adsorbed layer, the free energy of adsorption, the maximum surface excess of adsorbed species, the area occupied by each molecule etc. have been calculated on the basis of this isotherm. It is shown that the inhibition efficiency, as well as the free energy of adsorption, increases in the sequence: aniline < p-Cl-aniline < p-Br-aniline < p-I-aniline. It was found that the adsorption behaviour of the substances studied is characterized by two adsorption states: vertical and planar orientation of the molecules. Combining data from electrochemical measurements with data from quantum chemical calculations the correlation between the chemical structure of the aniline derivatives and their adsorption capability and inhibition efficiency has been ascertained.