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Corrosion damage to aircraft fuselages is an example of atmospheric corrosion, a topic that is described in much detail in a separate module. Airports located in marine environments deserve special mention in this context. The risk and cost of corrosion damage are particularly high in aging aircraft. In the United States alone, aircraft corrosion is a multi-billion dollar problem. On some military aircraft types, corrosion maintenance hours are known to outstrip flight hours.
The current approach in dealing with corrosion is to remove it as soon as it is found and either repair the corroded structure or replace the component. This is costly in terms of increased maintenance time and decreased aircraft availability. Treating the corrosion with corrosion prevention compounds (CPCs) and leaving it in place until there is easier access to the affected areas during a scheduled service would increase aircraft availability. However this approach requires a detailed knowledge of the propagation rates of the specific corrosion type after treatment with CPC and currently this information is not available.
Corrosion manifests in many different forms. Concentration cell corrosion, or crevice corrosion, is the most common type found on airplanes, occurring whenever water is trapped between two surfaces, such as under loose paint, within a delaminated bond-line, or in an unsealed joint. It can quickly develop into pitting or exfoliation corrosion, depending on the alloy, form, and temper of the material being attacked.
Crevice corrosion damage in the lap joints of aircraft skins has become a major safety concern, particularly after the Aloha airlines incident. On April 28, 1988 a nineteen-year-old Boeing 737 aircraft, operated by Aloha airlines, lost a major portion of the upper fuselage near the front of the plane, in full flight at 24,000 feet. The Aloha incident marked a turning point in the history of aircraft corrosion.
In 1998, the combined commercial aircraft fleet operated by U.S. airlines was more than 7,000 airplanes. At the start of the jet age (1950s to 1960s), little or no attention was paid to corrosion and corrosion control. One of the concerns is the continued aging of the airplanes beyond the 20-year design life. Only the most recent designs (e.g., Boeing 777 and late-version 737) have incorporated significant improvements in corrosion prevention and control in design and manufacturing. The total annual direct cost of corrosion to the U.S. aircraft industry is estimated at $2.2 billion, which includes the cost of design and manufacturing ($0.2 billion), corrosion maintenance ($1.7 billion), and downtime ($0.3 billion). (reference)
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