Cobalt(III) complexes of macrocyclic-bidentate type as a new group of corrosion inhibitors for iron in perchloric acid

A new class of corrosion inhibitors for iron demonstrated by group of cobalt complex compounds, of the general formula [Co^III(Rdtc)cyclam](ClO₄)₂ (1)-(4) where cyclam is 1,4,8,11-tetraazacyclotetradecane and Rdtc⁻ refer to piperidine-, 2-, 3- or 4-methylpiperidine-dithiocarbamates, respectively, has been studied. Investigation of inhibiting properties of the ligands themselves as well as their corresponding complexes with Co(III) ion, was performed in 0.1 M HClO₄ by potentiodynamic polarization and EIS measurements. Cobalt complexes inhibit the corrosion of iron better than the uncomplexed ligands. Polarization curves suggest that complexes and amino-ligands are mixed-type and cathodic-type inhibitors, respectively. The impedance of the inhibited substrate increased with increasing concentration of inhibitor. An equivalent circuit is suggested, based on the analysis of EIS spectra. Using molecular modeling calculations, the complex compounds were characterized in terms of their structural properties and inhibition activity.