As the average age of commercial airplanes currently in service increases, concerns continue to be raised about existing maintenance programs and how effectively they can help ensure the continued airworthiness of older airplanes. The Federal Aviation Administration (FAA) and other agencies around the World are working with industry representatives on programs to address these concerns. The purpose of these programs is to determine what actions should be taken to mitigate the effects of age on systems in older airplanes. (reference)
Aluminum alloys are the most widely used airplane material. Clad aluminum sheet and plate are used where weight and function permit, such as for fuselage skins. Corrosion-resistant aluminum alloys and tempers are used to increase resistance to exfoliation corrosion and SCC. An example of such a change is the replacement of 7150-T651 aluminum plate on upper wing skins with 7055-T7751 plate, which is not as susceptible to corrosion, a long departure from the 7075-T6 alloys still in used in less new aircraft. Major structural forgings are shot peened to improve the fatigue life of aluminum and steel parts and to reduce susceptibility to SCC.
Corrosion-resistant titanium alloys should also be considered for use in severe corrosion environments, such as floor structure under entryways, galleys, and lavatories. Corrosion resistant steels are used wherever possible, but a number of highly loaded structural parts, such as landing gear and flap tracks, are made from high-strength, low-alloy steel. Magnesium alloys are no longer used for primary structure. Fiber-reinforced plastics are corrosion resistant, but plastics reinforced with carbon fibers can induce galvanic corrosion in attached aluminum structure.
The most practical and effective means of protecting against corrosion involves finishing surfaces with an appropriate protective coating. For aluminum alloys, the coating system usually consists of a surface to which a corrosion-inhibiting primer is applied. In recent years it has become common practice not to seal the anodized layer. Although this reduces the corrosion resistance of the anodized layer, the primer adheres better to the unsealed surface. As a result, it is less likely to chip off during manufacture and service, producing improved system performance. For low-alloy steel parts, the coating system consists of cadmium plating to which a corrosion-inhibiting primer is applied.
Stainless steel parts are usually cadmium plated, until an adequate replacement to cadmium is found, and primed if they are attached to aluminum or alloy steel parts. This is to prevent the stainless steel from galvanically corroding the aluminum or alloy steel. For the same reason, titanium parts are primed if they are attached to aluminum or alloy steel parts. The corrosion-inhibiting primers used should be hydraulic fluid resistant when formulated for general use, for resistance to fuel, or for use on exterior aerodynamic surfaces. In some areas, hydraulic fluid resistant epoxy or polyurethane topcoats should be applied over the primer for functional reasons.
Exterior surfaces of the fuselage and vertical stabilizer should be painted hydraulic fluid resistant, decorative polyurethane topcoat over a urethane-compatible epoxy primer that resists filiform corrosion.
Effective drainage of all structure is vital to prevent fluids from becoming trapped in crevices. The lower pressurized fuselage should be drained by a system of drain holes with a system of longitudinal and cross-drain paths through the stringers and frame shear clips.
The potential for joint crevice corrosion can be eliminated by sealing the fay surfaces with a polysulfide sealant that is typically applied to such areas as the skin-to-stringer and skin-to-shear tie joints in the lower lobe of the fuselage, longitudinal and circumferential skin splices, skin doublers, the spar web-to-chord and chord-to-skin joints of the wing and empennage, wheel well structure, and pressure bulkheads. Non-aluminum fasteners on the exterior of the airplane and those that penetrate the pressurized portion of the fuselage are installed with sealant. Fillet seals can also be used for corrosion protection. They are used in severe corrosion environments if electrical bonds or the peripheries of antennas and other removable assemblies are present.
The objective is to avoid coupling materials from different groups unless required by economic and weight considerations. If dissimilar metal coupling is required, proper finishing and sealing techniques and guidelines are used to prevent corrosion. For example, graphite fibers, which are used to reinforce some plastic structure, present a particularly challenging galvanic corrosion combination. The fibers are good electrical conductors and they produce a large galvanic potential with the aluminum alloys used in airplane structure. The only practical, effective method of preventing corrosion is to keep moisture from simultaneously contacting aluminum structure and carbon fibers by finishing, sealing, using durable isolating materials such as fiberglass, and providing drainage.
Although finishing, sealing, and drainage provide most of the corrosion protection for airplane design, corrosion prevention compounds (CPCs) offer additional protection, especially when periodically reapplied in service. CPCs are petroleum-based compounds dispersed in a solvent and are either water displacing or heavy duty. Water-displacing CPCs are sprayed on structure to penetrate faying surfaces and to keep water from entering crevices. These CPCs must be reapplied every few years, depending on the environment in which the airplane has been operated. Heavy-duty CPCs are sprayed on as well, but they form a much thicker film and have much less penetrating ability. They are used on parts of the airplane most prone to corrosion.
Easy access is critical for regular inspections, which are an operatorís first step in combating corrosion.
A comprehensive, in-service corrosion control program is necessary to maximize the corrosion protection. Corrosion prevention and control programs should be developed under the direction of a recognized authority. These corrosion control programs will also minimize the need for corrosion-related maintenance.
A challenge in future efforts to prevent corrosion is the fact that many materials and processes currently in use will have to be modified to meet national and local environmental regulations.