Electrochemical characterisation of the ability of dicarboxylic acid salts to the corrosion inhibition of mild steel in aqueous solutions

Electrochemical measurements (electrochemical impedance spectroscopy (EIS), polarisation and open circuit potential (OCP) measurements) were used to study the corrosion protection of mild steel in aerated neutral solution of selected dicarboxylates. It was shown that all investigated carboxylates are effective in the presence of air. Most of the carboxylates act predominantly at local defects in the primary oxide layer by forming of weakly soluble Fe(III) compounds, in agreement with the pore plugging concept. As known, carboxylates, which can also be adsorbed on the oxide covered mild steel surface, should own an enhanced efficiency. It was found that the adsorption ability of dicarboxylates their selves are weak, but the corrosion protective effect could be improved considerably, if mixtures of dicarboxylates with monocarboxylates are used. The additional inhibition effect was ascribed to the ability of monocarboxylates to adsorb stronger on the oxide covered mild steel surface than dicarboxylates. The sebacate/caprinate mixture provides the best protection comparable with the synergistic effect observed with carboxylate in the presence of benzotriazole, too.

► Corrosion protection of mild steel in neutral aerated solutions.
► Salts of dicarboxylic acids-formation of weakly soluble Fe(III) compounds and adsorption on the primary oxide layer.
► Mixture of salts of dicarboxylic and monocarboxylic acids – synergistic effect leads to a passivation of the mild steel surface.
► Interpretation of impedance spectra using diffusion elements.