Anticorrosive behaviour of alkyd paints formulated with ion-exchange pigments

Abstract

Hexavalent chromium compounds (chromates) have been widely used as inhibitive pigments in the formulation of anticorrosive paints. However, their high toxicity and carcinogenic effects are forcing the development of effective chromate-free organic coatings. One such alternative, which is very attractive from a scientific point of view, is the use of ion-exchangeable pigments (IEPs). The few studies conducted with this type of pigment are not conclusive about their anticorrosive efficiency and controversy surrounds their functioning mechanisms, interchange capacity and anticorrosive performance. In the present research, which focuses on the anticorrosive protection of this type of pigment, alkyd paint coatings formulated with vanadate-hydrotalcite (HT/V) (anionic) and calcium/silica (Ca/Si) (cationic) IEPs have been applied on low carbon steel specimens. A traditional zinc chromate pigment has also been used for comparative purposes. The effect of these non-toxic pigments on the protective properties of coatings has been tested by means of natural and accelerated corrosion tests (humidity, salt spray and Kesternich, 0.2 l SO 2) and electrochemical impedance spectroscopy (EIS). None of the IEPs equalled the anticorrosive behaviour of the zinc chromate in the different tests. The anionic pigment (HT/V) seems to present good behaviour in chloride environments (salt spray, NaCl solutions, etc.) while the cationic pigment (Ca/Si) performs well in the humidity condensation and SO 2 tests.