Mg-rich primers (MRPs) have been shown to provide cathodic and barrier-type protection to aluminum substrates similar to that provided by zinc-rich primers (ZRPs) for steel. The former protection is driven by the more active Mg particles compared to the Al substrate and the latter by the insulation of the substrate from the environment by Mg containing products. A transmission line model was used to interpret ZRP EIS data that accounts for the contact impedance between zinc particles, the impedance associated with zinc dissolution, and the electrolyte resistance. Experiments were conducted for the evolution of the electrochemical behavior of a MRP on AA 2024-T3 under immersion in diluted Harrison's solution. The transmission line model was applicable to the MRP data. Comparison between the parameters of a ZRP and those for the MRP indicated that the loss of cathodic protection for a MRP was related to the interfacial resistance of the particles.
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.
Abstract Mg-rich primers provide cathodic protection of aluminum substrates similar to that provided by Zn-rich primers for steel substrates. This protection is due to the mixed potential that deve...
Protective multilayer coatings are designed, with each layer providing distinctive features to reduce the transport of water, ions, and oxygen from the environment to the substrate. The top coat provides the barrier, whereas the base coat/primer provides adhesion to the substrate as well as galvanic or passivation protection. The change in the coating capacitance during the ingress or egress of water is associated with the change in the dielectric property of the coating and can be monitored using the electrochemical impedance spectroscopy response of a high-frequency signal applied to the coating. The wetting and drying behaviors of epoxy and urethane single-layer coatings and epoxy primer/urethane top-coat two-layer coatings were experimentally measured. A hydrophilic room-temperature ionic liquid was used to simulate drying while maintaining a nonaqueous electrical contact to the coating. Embedded electrodes were used to access the top-coat–primer interface in the two-layer coating. Mathematical models were developed based on Fick's second law for water transport in single-layer and two-layer coatings. Comparison of measured and simulated water uptake demonstrated that the assumptions used in the single-layer model were appropriate while the assumptions used for transport across the interface between the layers in the two-layer coating were inadequate.
