Cathodic Protection of AA 2024-T3 by a Mg-rich Primer

Mg-rich primers are an alternate for chromate-based primers that are currently used by the Air Force for the protection of aluminum structures. The Mg pigments provide cathodic protection for the less active aluminum substrates and also provides a barrier type protection as the products from the Mg dissolution insulate the substrate from the environment. The cathodic protection is comparable to Zn-rich primers for steel substrates. Electrochemical impedance spectroscopic characterization of a Mg-rich primer and a standard Air Force urethane topcoat/Mg-rich primer system, both on AA 2024-T3 substrates, is presented. The systems were exposed to constant immersion in dilute Harrison’s solution. Embedded electrodes between the topcoat and primer were used to access the behavior of the Mg primer/substrate system as it functions beneath the topcoat. Experimental results demonstrated the effectiveness of the cathodic protection of the bare Mg-rich primer and the primer beneath a topcoat. The EIS data was analyzed using a Voigt element model to compare the behaviors of the bare primer and the primer beneath the topcoat. Interpretation of the data will focus on the evolution of the electrochemical properties of the primer and topcoat/primer systems as the cathodic protection is reduced during the consumption of Mg-rich pigments.