Galvanic corrosion on aircraft around mechanical fasteners represents a significant portion of total platform maintenance costs and contributes towards reduced operational readiness. Historical approaches to controlling galvanic corrosion involve protecting the anode (e.g. usually the aluminum airframe), but minimal efforts have been made at limiting the galvanic contribution at the cathode. There is a need to improve the galvanic corrosion control tool set across these platforms, and technologies that aid in controlling the available cathodic current density are a new approach. Luna Labs is developing a sol-gel based coating to mitigate galvanic corrosion around fasteners and other dissimilar materials by providing excellent physical and electrical barrier protection to corrosive environments. This sol-gel coating results in a highly cross-linked inorganic/polymer hybrid film with excellent impact resistance, flexibility, and toughness. It is also inherently chrome-free and non-hazardous. The coating is designed for drop-in fastener application and easy integration with common aircraft components. Corrosion tests have indicated a reduction in observed galvanic corrosion by 2-3 times less compared to that of bare fasteners. This reduction in corrosion will directly translate to cost savings through decreased maintenance of aircraft components and improved operational readiness. A technology development status update will be provided.
Aircraft reliability and maintainability are critical to their availability and operation. One of the biggest threats to aircraft availability is corrosion of aircraft systems. The annual cost of corrosion for Air Force aviation and missiles is estimated to be $5.67B dollars, or 23.6% of total maintenance costs for this equipment, and 2.1 million non-available hours [1]. It is estimated that for defense acquisition programs 60 – 80% of the total life cycle cost is associated with the operation and support (O&S) phase [2]. Corrosion increases demand on labor, degrades operational readiness, and is a safety issue. Modern aircraft necessitate advanced corrosion solutions in order to protect and maintain their readiness. In particular, cathodic fasteners located adjacent to anodic aluminum alloys can lead to significant galvanic corrosion of the aluminum. Cathodic fastener materials may include corrosion resistant steel (CRES), titanium, copper-beryllium and nickel-based alloys, mated to aluminum structures composed of alloys 7075-T6, 2024-T3, 7050-T7451, and sometimes carbon fiber reinforced polymer (CFRP) composites. To protect against corrosion on aircraft, aluminum alloys are routinely chromate conversion coated, primed with chromated epoxy-polyamide (MMS 423, MIL-PRF-85582 TY I Class C2), and top-coated with polyurethane (MMS 420, MIL-PRF-85285 TY I). Similarly, fasteners may be coated with sacrificial metals (Cd, Zn-Ni, IVD Al), pretreated with conversion coatings, and wet installed with sealants or primers. However, metallic coatings can become worn during repeated handling and fastener installation, and defects in the primer/paint system are inevitable during operations. Some fastener locations can be especially susceptible to corrosion as sea water and other chloride-containing electrolytes collect in these areas, resulting in a severe occluded environment and aggressive attack.
