Mg-rich primers ͑MRPs͒ provide cathodic and barrier-type protection to aluminum substrates similar to that provided by zinc-rich primers ͑ZRPs͒ 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 electrochemical impedance spectroscopy ͑EIS͒ data that account 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 an 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 an MRP was related to the interfacial resistance of the particles.Coatings containing chromate-based pigments have represented the prototype for the protection of aircraft structures made of aluminum alloys. 1,2 Pigments based on zinc chromate, zinc tetroxy chromate, and strontium chromate have been used to inhibit the corrosion of AA 2024-T3 structures by the U.S. Air Force. 3 The solubility of the chromate can result in its leaching from the polymeric binder, with environmental implications arising due to the toxic nature of chromates. 4,5 Federal regulations have been introduced by the U.S. government to curtail the use of chromate-based pigments, 6 and as such, there is ongoing research into alternatives to the chromatebased pigments. Alternative inhibitors that have been proposed include vanadates, phosphates, molybdates, borates, and silicates. 1,7-9 Commercial, nonchromate inhibitor pigments have been shown to be much less effective in corrosion protection as compared to the industrial standard of SrCrO 4 . 9 There has been recent progress in the application of an AlCo-Ce metallic coating that can provide cathodic protection as its open-circuit ͑OC͒ potential is as much as 750 mV lower than that of AA 2024-T3 depending on the Al-Co-Ce alloy. 10,11 The Co 2+ and Ce 3+ ions can be released from this metallic coating and provide additional inhibition of the AA 2024-T3 corrosion. There has also been recent progress in the development of a Mg-rich primer ͑MRP͒, and its applicability for the protection of aluminum structures has been demonstrated by Bierwagen and co-workers. 12-16 The application of MRPs does not require a chromate pretreatment, representing a Cr-free system.MRPs provide cathodic protection driven by the more active Mg particles compared to the Al substrate and also a barrier-type protection due to the insulation of the substrate from the environment by products of Mg hydroxide and hydroxy carbonate species that fill pores and voids. 13,17,18 The characterization of MRPs on Al substrates has been performed using OC potential evolution, potentiodynamic polarization, electrochemical impedance spect...