Investigation of the corrosion resistance characteristics of pigments in alkyd coatings on steel

Pigments are added to polymeric coatings for enhancing the protective characteristics of the barrier films that are deposited on metal surfaces to prevent corrosion. Electrochemical impedance spectroscopy (EIS) measurements were performed on carbon steel specimens coated with alkyd primers containing different pigments. Corrosion monitoring was carried out in immersion tests in 3.0 wt.% NaCl solution open to air at room temperature. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all the cases, though their evolution with time showed marked differences which derived directly from nature of the pigments contained in the alkyd primers. The impedance response of the coatings was found to correspond to a porous film presenting localized electrochemically reactive areas, though pigments may facilitate additional resistance to the protective film, the extent of which was found to depend on the nature of the pigment. In this way, the best results were found for the alkyd primer containing aluminium powder.