The sacrificial and barrier mechanisms of protection afforded to the aluminum alloy (AA)2024-T351 (UNS A92024) substrate by a Mg-rich primer (MgRP) were investigated. Test methods were developed to estimate the total residual stored Mg anode capacity and electrically "well-connected" Mg in the primer, as sensed electrochemically, after various environmental exposures. The residual barrier properties after depletion of the Mg primer were also assessed. To determine the quantity of Mg that was electrically and ionically well connected to the AA2024-T351 substrate, as sensed electrochemically, and to monitor coating barrier characteristics after partial MgRP utilization, a full-immersion testing protocol was designed. The testing regimen included an open-circuit hold to assess galvanic coupled potentials between the MgRP and 2024-T351, electrochemical impedance spectroscopy to assess coating barrier properties, potentiostatic holds to asses Mg anodic dissolution charge, and x-ray diffraction to assess the total elemental Mg remaining in the MgRP. Changes in these parameters are reported after full immersion in 50 mM sodium chloride (NaCl) solution as a function of MgRP pigment volume fraction (PVC) with or without a topcoat. X-ray diffraction was used to estimate the total amount of Mg in the MgRP before and after full immersion. Preliminary findings suggest two possible modes of protection: long range protection of remote defects and local or short range Mg pigment-based protection of local and buried defects. Both modes of protection are mediated by the high ionic and electrical resistance of the coating system as a function of MgRP PVC, primer polymer, and topcoat properties. A method for estimating the remaining capacity of the coating for each mode of protection is developed. Future testing will extend these methods to both field and accelerated test environments.
