In Situ Embedded Sensor Monitoring of a United States Air Force Primer Beneath a Topcoat Exposed to Atmospheric Humidity and Thermal Conditions

Two-layered United States Air Force organic coatings are designed with the topcoat functioning as a barrier against the environment and the primer providing adhesion between the topcoat and the substrate, as well as promoting chromate passivation. In standard testing methods such as salt spray and QUV/Prohesion, periodic testing of coatings is performed using ex situ electrochemical experiments. The primary purpose of these accelerated testing procedures is to rank coating performance under simulated weathering conditions. The conventional approach for these electrochemical impedance spectroscopy measurements requires that a conductive medium be in direct contact with the exposed coating. As such, the influence of the atmospheric humidity on the performance of the coating is lost. The inclusion of embedded sensors within two-layered coating systems allows in situ monitoring of the primer/substrate system beneath the topcoat electrochemical properties. The influences of atmospheric humidity and temperature on the electrochemical properties of the primer/substrate system of an Air Force urethane topcoat/chromate epoxy primer is reported here. The experimental procedure involved exposure of coated aluminum alloy (AA)2024-T3 (UNS A92024) substrates to different humidity levels at several temperatures. Electrochemical impedance spectroscopy was conducted to assess the electrochemical properties of the primer/ substrate beneath the topcoat. The analysis presented demonstrated the changes in the barrier and dielectric properties of the primer in response to changing atmospheric humidity and thermal conditions.