The effect of hydrophobic coatings on insulative skirts to attenuate galvanic corrosion between mechanically-fastened 6061‑T6 aluminum (Al) alloy and carbon-fiber reinforced polymer-matrix composites (CFR PMC) was studied in outdoor environments. The utilization of hydrophobic coatings on the insulative skirt can help to attenuate galvanic corrosion by disrupting the formation of a continuous electrolyte film. Our recent work* (Srinivasan et.al. 2016) showed a novel approach of utilizing hydrophobic coatings on insulative skirts (ranging from 0 inch to 8 inch in length) to mitigate galvanic corrosion between Al alloy and CFR PMC couples in laboratory humidity chamber corrosion tests (90% relative humidity (RH) and 30°C). Shorter skirt lengths (i.e., 0.25 inch) with a hydrophobic Siloxel ^TM coating was shown to be most effective in attenuating galvanic corrosion based on the capillary effect that wicked away large water droplets, generated from a mechanical sprayer.

During outdoor exposures, airborne salt particles are on the order of a few microns, and hence, may settled on the skirts as a relatively uniform distribution. To study the effectiveness (due to capillary wicking) of shorter skirt lengths during outdoor exposure, Siloxel™-coated G10 fiberglass insulating skirts of 0.25 inch and 0.5 inch were compared at rain forest (Manoa) and severe marine (MCBH) environments for 72 days. The natural salt particles deposited on the Siloxel™ coated skirts, and galvanic corrosion rates were higher for the shorter skirt length contrary to the laboratory humidity chamber results, indicating that the capillary wicking mechanism was not dominant in the field. In outdoor field environments, since natural salt particles (on the order of a few microns) are deposited on the substrate and not sprayed in solution as in the laboratory experiments, the capillary-wicking mechanism does not have a chance to operate in the absence of substantial rain.

To study the effectiveness of different coatings as a function of coating contact angle, CFR PMC/Al galvanic couples with 0.50 inch insulating skirts coated with either the Siloxel™ , an epoxy coating, or no coating were also exposed at the MCBH test site. The CFR PMC/Al galvanic specimens at the severe marine site were exposed in the sheltered and unsheltered conditions. The higher contact angle SiloXel^TM-coated G10 induced greater attenuating of galvanic corrosion in the sheltered condition; whereas, the epoxy-coated G10 induced greater attenuation in the unsheltered condition. In the unsheltered condition, the epoxy matrix of the G10 fiber glass substrate degraded due to exposure to sunlight causing the Siloxel™ coating (only several microns thick) to debond and crack resulting in a loss of hydrophobicity; whereas, the much thicker epoxy coating retained its integrity for the duration of the test and hence induced better attenuation.

*Raghu Srinivasan and Lloyd Hihara, “Utilization of Hydrophobic Coatings on Insulative Skirts to Attenuate Galvanic Corrosion between Mechanically-Fastened Aluminum Alloy and Carbon-Fiber Reinforced Polymer-Matrix Composites”, Electrochemistry Communications, Volume 72 (2016).