The protection of occluded regions such as aircraft lap joints by corrosion prevention compounds (CPC) requires the CPC to wick into such regions, wet the surface, and displace water that may be present. This work considers both thermodynamic (wettability) and kinetic (wicking rate) aspects of CPC behavior when applied to simulated lap joints assembled by pin-and-collar type fasteners and rivets. Wicking rate was determined in situ using fiber optic sensors assembled into lap joints. The wettability of CPC was assessed via measurements of the dynamic contact angle. All of the commercially available CPC evaluated demonstrated ability to wick into occluded regions although the wicking rate was highly dependent on the conditions within the joint. CPC showed good thermodynamic and kinetic wetting ability on dry, pristine high strength aluminum alloy surfaces. When water is present and/or the surface is corroded, however, wetting by the CPC became more difficult relative to the pristine condition as indicated by increased contact angles and reduced wicking kinetics. In addition, it was found that the gap of the joint has a strong effect on the wicking kinetics of water and CPC. Wicking generally was slower in tight lap structures and increased with gap to a maximum and then decreased with further increases in gap opening height.
Corrosion prevention compounds (CPC) are relatively inexpensive temporary corrosion
control products commonly used on commercial and military aircraft 1, 2. They are designed to suppress existing corrosion activity and prevent new corrosion sites from forming. Generally, the protection of boldly exposed surfaces by CPC is attributed to the formation of a barrier film 3. Therefore, on boldly exposed surfaces, CPC functions similarly to conventional organic coatings although by design they can be conveniently applied on top of the (failed) existing coating system. However, some CPC are being used with the intent of protecting occluded regions within aircraft structures. Under these circumstances, the CPC must possess the ability to penetrate or wick into the occluded region and displace water that may be present.
Recent work has shown that currently available aerospace qualified CPC provided limited corrosion protection inside occluded regions such as aircraft fuselage lap joints 4. Furthermore, it has been demonstrated that water penetration into occluded sites is rapid (ca. mm/min), whereas egress is very slow (ca. mm/day) with the result that these locations remain wet long after the boldly exposed surfaces become dry 4. As a result, occluded sites, which are inherently difficult to inspect, can develop severe corrosion resulting in increased maintenance costs, decreased fleet readiness, and adversely impacting structural integrity. To be effective at preventing corrosion, the CPC must be able to wick into and displace water from occluded regions.
