Thermal Degradation of a Mg-Rich Primer on AA 2024-T3

Abstract

The mitigation of corrosion on aluminum aerospace alloys has been predominantly by organic coatings that include strontium chromate pigments. These pigments are excellent in passivating the aluminum surface but are toxic and are in the process of being regulated out of the aerospace industry. Passivating replacements such as vanadates, molybdates and phosphates are not as protective as chromates. Cathodic protection of aluminum alloys have been demonstrated using a coating pigmented with Mg particles. The protection provided by Zn-rich primers for steel is analogous to that provided by Mg-rich primers for aluminum. A transmission-line model was used to interpret Zn-rich primer EIS data that accounted for the contact impedance between the Zn particles, the impedance associated with the Zn dissolution, and the percolation resistance of the coating. This model was shown to be applicable to the EIS data associated with a Mg-rich primer on AA 2024-T3 immersed in dilute Harrison's solution (DHS). The degradation of the protection of the Mg-rich primer was influenced by the interfacial impedance while that of the Zn-rich primer was influenced by both the contact and interfacial impedance. The difference between the degradations of Mg-rich and Zn-rich primer systems was based on results obtained using the transmission line model. The applicability of the model to Mg-rich primers is further accessed in this effort where the model was used to interpret the EIS results associated with the thermal degradation of Mg-rich primer on AA 2024-T3 immersed in DHS.