Exploring the corrosion inhibition of aluminium by coatings formulated with calcium exchange bentonite

Enviromentally friendly calcium-exchange derived from naturally occurring sodium-bentonite clays (Wyoming) are shown to significantly enhance resistance to corrosion protection in organic coatings applied on aluminium under aggressive environment typical from industrial areas. Two pigments classified as ion-exchange were also studied for comparison (Shieldex® and Al-Zn-vanadate hydrotalcite) together with zinc chromate as reference corrosion protection pigment. Electrochemical impedance spectroscopy (EIS) is used to study the corrosion protection in the metal/coating interface of pigmented alkyd coatings and a blank coating (without corrosion inhibitor pigment) in combination with visual inspection. The protection performance of these specimens was studied using outdoor exposure (two atmospheres with different aggressiveness) and accelerated tests (condensing humidity, salt spray and Kesternich tests, respectively). Results have shown strong dependence of the coating performance with the aggressive environment (e.g. Cl−, H+, SO2) for all coatings formulated with ion-exchange pigments. The corrosion protection of the underlying aluminium substrate provided by calcium-exchange bentonite coating was shown under the presence of cationic aggressive agents in accelerated corrosion tests (specifically in Kesternich test). However, poor performance was observed for this coating using chlorides as an aggressive agent. Therefore, the presence of bentonite pigment improves the corrosion protection due to the cation-exchange mechanism.