Boeing airplanes are designed to provide corrosion protection throughout their design service life of 20 years. To meet this objective, Boeing considers the following criteria:

1. Causes of corrosion.
2. Types of corrosion.
3. Methods of corrosion control.

1. CAUSES OF CORROSION
Corrosion is the destruction of metal by electrochemical reaction with its environment. Figure 1 illustrates some typical sources of the corrosion that affects airplanes. As shown in figure 2, three conditions must exist simultaneously for corrosion to take place:

1. The presence of an anode and a cathode. This occurs when two dissimilar metals or two regions of differential electrolyte concentration create a difference in electrical potential.
2. A metallic connector between the anode and cathode.
3. An electrolyte such as water.

Eliminating these three conditions in airplanes is restricted by practicality, functionality, and feasibility. Dissimilar metal contact cannot always be avoided because of weight, cost, and functional issues, but the potential for corrosion can be minimized by using surface treatments, plating, painting, and sealing. Water cannot be avoided, but it can be controlled with drain paths, drain holes, sealants, and corrosion-inhibiting compounds. Controlling the presence of water is usually the most effective means of preventing corrosion (see "Controlling Nuisance Moisture in Commercial Airplanes," Aero no. 5, January 1999).

2. TYPES OF CORROSION
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.

All forms of concentration cell corrosion can be very aggressive, and all result from environmental differences at the surface of a metal. The most common form is oxygen differential cell corrosion. This occurs because moisture has a lower oxygen content when it lies in a crevice than when it lies on a surface. The lower oxygen content in the crevice forms an anode at the metal surface. The metal surface in contact with the portion of the moisture film exposed to air forms a cathode. As shown in figure 3, chloride ions (Cl) migrate to the anode and create an acidic and corrosive condition. The most effective way to eliminate this kind of corrosion is to keep water out of the joint.

Two of the most destructive forms of corrosion are stress corrosion cracking (SCC), also known as environmental assisted stress corrosion, and exfoliation corrosion. SCC occurs rapidly and follows the grain boundaries in aluminum alloys (fig. 4). Exfoliation corrosion also follows grain boundaries (fig. 5). It occurs in multiple planes, causing a leaf-like separation of the metal grain structure. Both forms of corrosion cause a loss of load-carrying capability. The most effective way to control this kind of corrosion is to use materials that are not susceptible to SCC at design stress levels or have a grain structure that is not susceptible to exfoliation.

Pitting corrosion (fig. 6) results in a local loss of material. Although very little metal is removed, the pits can act as stress risers that lead to fatigue failure if located in a critical load path.

General corrosion (fig. 7) affects structure the least. It consumes metal uniformly and at a relatively slow rate. However, if left unattended over a long period, general corrosion can remove enough metal to cause structural concerns.

Galvanic corrosion occurs when two metals with different electrical potentials are electrically connected in the presence of an electrolyte. This can occur at a macro scale, such as an aluminum-nickel-bronze bushing in an aluminum fitting, or on a micro scale, such as an aluminum-copper intermetallic at the surface of an aluminum alloy.

3. METHODS OF CORROSION CONTROL
Proper design for corrosion control must include consideration of proper material selection, finish selection, drainage, sealants, galvanic coupling of materials, application of corrosion-inhibiting compounds, access for maintenance, the use of effective corrosion control programs in service, and consideration of environmental issues.

Material selection.
Selecting the proper material is essential for long-term corrosion control. Aluminum is the most widely used airplane material (fig. 8). Aluminum and low-alloy steels are the two groups of airplane materials most susceptible to corrosion.

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. 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 are 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.

Finish selection.
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 cadmium plated 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 are Skydrol-resistant epoxies formulated for general use, for resistance to fuel, or for use on exterior aerodynamic surfaces. In some areas, Skydrol-resistant epoxy or polyurethane topcoats are applied over the primer for functional reasons.

Exterior surfaces of the fuselage and vertical stabilizer are painted with a Skydrol-resistant, decorative polyurethane topcoat over a urethane-compatible epoxy primer that resists filiform corrosion.

Drainage.
Effective drainage of all structure is vital to prevent fluids from becoming trapped in crevices. The entire lower pressurized fuselage is drained by a system of valved drain holes. Fluids are directed to these drain holes by a system of longitudinal and cross-drain paths through the stringers and frame shear clips. Examples of Boeing design concepts that accomplish this are shown in figures 9, 10, and 11.

Sealants.
Boeing effectively eliminates the potential for joint crevice corrosion by sealing the fay surfaces with a polysulfide. The polysulfide sealant 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. Examples of this sealing are shown in figures 12 and 13. Nonaluminum fasteners on the exterior of the airplane and those that penetrate the pressurized portion of the fuselage are installed with sealant. Bushings in aluminum and low-alloy steel fittings are also 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.

Galvanic coupling of materials.
Boeing groups materials into four categories (table 1) of differing galvanic properties. 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. Figure 14 shows the 777 carbon fiber-reinforced plastic (CFRP) floor beam design and corrosion-protection methods. An aluminum splice channel is used to avoid attaching the floor beam directly to the primary structural frame.

Application of corrosion-inhibiting compounds.
Although finishing, sealing, and drainage provide most of the corrosion protection for airplane design, corrosion-inhibiting compounds (CIC) offer additional protection, especially when periodically reapplied in service. Boeing initially applied CICs in chronic corrosion areas such as the lower lobe of the fuselage and since then has expanded their use to other areas. In current-production airplanes, CICs are applied to most aluminum structure as shown in figure 15.

CICs are petroleum-based compounds dispersed in a solvent and are either water displacing or heavy duty. Water-displacing CICs are sprayed on structure to penetrate faying surfaces and to keep water from entering crevices. These CICs must be reapplied every few years, depending on the environment in which the airplane has been operated. Heavy-duty CICs 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.

Access for maintenance.
Boeing designs all structure to allow access for frequent maintenance and corrosion inspections. Easy access is critical for regular inspections, which are an operator’s first step in combating corrosion.

Effective corrosion control programs.
A comprehensive, in-service corrosion control program is necessary to maximize the corrosion protection designed into Boeing airplanes. Corrosion prevention and control programs (CPCP) for each Boeing airplane were developed under the direction of the International Airworthiness Assurance Working Group. This group developed a mandatory CPCP to establish minimum in-service maintenance procedures for aging airplanes. Following these procedures is necessary to control corrosion and so ensure structural integrity and airworthiness for continued flight safety, regardless of airplane age. These corrosion control programs will also minimize the need for corrosion-related maintenance. Regulatory authorities, operators, and airframe manufacturers are developing CPCPs for next-generation airplanes that integrate existing inspection programs.

Environmental issues.
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. Boeing continues to test new materials and processes that limit the emission of hazardous and toxic materials into the atmosphere and water as well as the exposure of personnel to these materials.

DAVID BANIS
ENGINEER
MATERIALS TECHNOLOGY
BOEING COMMERCIAL AIRPLANES GROUP

J. ARTHUR MARCEAU
ENGINEER (RETIRED)
MATERIALS TECHNOLOGY
BOEING COMMERCIAL AIRPLANES GROUP

MICHAEL MOHAGHEGH
ENGINEER
STRUCTURES ENGINEERING
BOEING COMMERCIAL AIRPLANES GROUP