Abstract
The protection afforded by organic coatings against corrosion for metallic structures is influenced by the ability of the coating to reduce the transport of water, ions, and oxygen from the environment to the substrate. The majority of protective organic coatings are two-layered with the topcoat as the barrier against the environment and the basecoat/primer providing adhesion between the topcoat and the substrate. Primers can also provide additional protection through the use of galvanic (zinc rich primers) or passivating (chromate primers) inhibitors. A standard testing method for coating systems is the QUV/Prohesion test. This test is a modification of the salt spray test and simulates weathering conditions. It involves alternating weekly between Prohesion and QUV exposures. The former includes a fog step of 1 hour salt spray at 25 °C and a dry step of 1 hour hot air at 35 °C. The latter involves 4 hours of UV light at 60 °C and 4 hours of condensation at 50 °C. Periodic monitoring of coating performance is done either by visual observation or by ex-situ electrochemical tests under immersion in a suitable electrolyte. The inclusion of embedded sensors within two-layered coating systems allowed in-situ monitoring during the QUV/Prohesion test. Results are presented where embedded sensors were used to monitor the changes associated with a standard Air Force urethane topcoat-epoxy primer system on a A2024-T3 substrate. In-situ results associated with the Prohesion dry and QUV condensation steps are presented from electrochemical impedance spectroscopy and electrochemical noise measurement tests. In-situ single-frequency impedance measurements were also used to monitor the changes in capacitance associated with water uptake under the varying conditions of the QUV/Prohesion test. Comparison of in-situ and ex-situ results will be presented.
