Corrosion resistance of new epoxy–siloxane hybrid coatings. A laboratory study

Abstract

Traditional multilayer epoxy/polyurethane type anticorrosive paint systems are widely employed in the protection of steel structures due to their high efficiency against atmospheric corrosion. However, the use of isocyanate in the curing process, and the high volatile organic compound (VOC) content of such systems, makes it necessary to search for new isocyanate-free paints. Hybrid organic–inorganic coatings, such as epoxy–siloxane coatings, represent a step forward in the field of paint coatings for atmospheric corrosion protection. These new isocyanate-free hybrids present low VOC levels – due to the high solid content associated with their low viscosity – along with good heat and UV radiation stability and excellent chemical resistance. With the new polysiloxane inorganic resins it is hoped to improve the anticorrosive behaviour of traditional organic binders. This work assesses the anticorrosive performance of one-layer epoxy–siloxane coatings compared to traditional two-layer epoxy/polyurethane coatings, in paint systems with and without an epoxy or silicate-type zinc-rich primer. The anticorrosive properties of these coatings applied on steel substrates have been evaluated using a wide range of experimental techniques, namely measurement of water vapour and oxygen permeability in free films, 3-year outdoor testing, accelerated corrosion testing (condensing humidity, Kesternich, salt fog and Prohesion), and electrochemical impedance spectroscopy (EIS). Hybrid epoxy–siloxane films show the lowest oxygen and water vapour permeability, capacitance values, and the highest ionic resistance values, along with excellent behaviour in humidity and Kesternich tests, outperforming traditional epoxy/polyurethane paints, while the behaviour of both types of coatings is similar in the salt fog, Prohesion and short-term outdoor tests. This type of paint requires the presence of zinc-rich primers to optimise its anticorrosive behaviour.