Ion-exchange pigments in primer paints for anticorrosive protection of steel in atmospheric service: Cation-exchange pigments

Hexavalent chromium compounds (chromates) have been widely used as inhibitor pigments in the formulation of anticorrosive primers. However, their high toxicity and carcinogenic effects are forcing the development of effective chromate-free organic coatings. One such alternative is the use of ion-exchangeable pigments. Studies of these pigments have not been conclusive about their anticorrosive efficiency and controversy surrounds their functional mechanisms, exchange capacity and anticorrosive performance.This paper focuses on the anticorrosive protection afforded by cation-exchange pigments. Alkyd primer coatings formulated with non-toxic silica/calcium (Si/Ca) and bentonite/calcium (BT/Ca) pigments have been applied on carbon steel specimens, and the effect of these pigments on the protective properties of the coatings has been evaluated by means of natural and accelerated corrosion tests (humidity, salt spray and Kesternich, 0.2 L SO2). A traditional zinc chromate (ZC) pigment has also been used for comparative purposes. The cation-exchange capacity and inhibitor capacity of the Si/Ca and BT/Ca pigments has been evaluated, and the inhibiting power of both is seen to be less than that of the traditional ZC pigment. The formation of a calcium silicate film with good protective characteristics on the underlying steel is corroborated in the case of the Si/Ca pigment, whereas the anticorrosive properties of the BT/Ca pigment are deficient in the different tested environments.

► The suitability of cation-exchange pigments to replace toxic Cr(VI) is studied.
► The cation-exchange capacity and inhibitor power of Si/Ca and BT/Ca are evaluated.
► The anticorrosive protection of primer paints formulated with Si/Ca and BT/Ca is assessed.
► The exchange capacity of Si/Ca pigment, as well as the formation of a protective film, has been verified.
► BT/Ca presents very deficient anticorrosive properties.