Abstract
Organic coatings represent the most widely used means of mitigating the corrosion of metallic structures. A coating's primary role is as a physical barrier, reducing the transport of water, ions, and oxygen from the environment to the substrate. Coatings can also provide cathodic protection when pigmented with active metals or can passivate surfaces when pigmented with inhibitors. A standard testing method for characterizing the performance of a coating system 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 h salt spray at 25°C and a dry step of 1 h hot air at 35°C. The latter involves 4 h of ultraviolet (UV) light at 60°C and 4 h 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 in situ the changes associated with a standard United States Air Force urethane topcoat-epoxy primer system on an aluminum alloy (AA)2024-T3 (UNS A92024) 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. Ex situ measurements were also performed and these were compared with the in situ results to demonstrate the coating properties under in situ and ex situ conditions.
