New Approach for Chromate Free Coatings in Aircraft Applications: Modified Zinc Pigments

Alternatives for the environmentally harmful Cr(VI)-based protective coatings for aluminum alloys are highly demanded, especially for aircraft application a. So far, suitable alternative pigments providing inhibiting effects which are comparable to the performance of chromates have not been found. Well known pigments used in corrosion protection coatings are zinc pigments for steel substrates - offering cathodic protection to the substrate and a densification of the coating due to the volume expansion of its corrosion products. Furthermore, it is generally assumed that zinc corrosion products also offer an inhibiting effect. Due to their lower electrochemical potential zinc does not provide cathodic protection to aluminum substrates. To transfer the positive protection effects of zinc to the application of protective coatings on aluminum substrates, zinc pigments have been successfully modified by alloying with magnesium. Mg-Zn alloys were prepared in different compositions with special attention to the intermetallic phases like MgZn, and Mg₂Zn₃. Pigments were prepared and included in a coating system. Pickled samples of AA 2024 unclad were coated and the corrosion behavior investigated. A durability of more than 10,000 hours in salt spray test could already be achieved. Additional measurements showed a significant influence of the pigment micro-structure on the electrochemical behavior of the intermetallic pigments.