Electrochemical Diagnostic Cycle Testing of Magnesium and Magnesium Oxide-Pigmented Primers on AA2024-T351

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Abstract
Intact Mg-rich and MgO-rich primer (MgRP and MgORP) coatings on aluminum alloy 2024-T351 substrate were evaluated for their ability to protect remote scratches (coating defects) as well as for coating barrier properties using a laboratory diagnostic cycle test. These coatings consist of a pretreated substrate and a Mg-containing pigmented primer with and without a UV-inert pigmented topcoat. For MgRP, Mg2+ and electrons are available upon Mg oxidation, while for MgORP only Mg2+ is available upon MgO chemical dissolution. The thickness and the pigment volume concentration of the MgORP was less than the MgRP as characterized with scanning electron microscopy/energy dispersive spectroscopy imaging, resulting in a larger reservoir and Mg capacity in MgRP than the Mg2+ capacity in MgORP. Diagnostic electrochemical cycle testing assessed the anode capacity of Mg pigment available for sacrificial anode-based cathodic protection of the substrate during potentiostatic holds during full-immersion testing in 5 wt% NaCl. The MgO had no capacity for sacrificial anode-based cathodic protection, but it did exhibit beneficial effects associated with the electrochemical behavior of coated and bare AA2024-T351; this was attributed to presence of Mg2+ in solution and the redeposited corrosion product. During exposure, electrochemical impedance spectroscopy monitored the barrier properties of the coatings, which were affected by primer type and the presence of topcoat. MgRP is shown to protect AA2024-T351 via both sacrificial anode-based cathodic protection and Mg2+ redeposition. MgORP is shown to function as a corrosion preventative coating for AA2024-T351 through chemical release of Mg2+, which beneficially alters the electrochemical corrosion behavior of the AA2024-T351 alloy. This investigation serves as a foundation for future investigations into MgORP and spent MgRP in the case where Mg has been oxidized as a result of long-term field exposure.

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