Page 309 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION CONTROL IN AIRCRAFT 287

Corrosion control can be accomplished in the design and manufacturing phase as
well as in the operation and maintenance phase of the aircraft. Proper design for corro-
sion control must include the selection of materials, coatings, sealants, and corrosion
inhibitors. It is also necessary to avoid dissimilar metal contacts, access for mainte-
nance, and proper drainage.

4.23.1 Material Selection
High-strength aluminum alloys are most widely used in airplanes because of their
high strength-to-weight ratio. However, these alloys and the low-alloy, high-strength
carbon steels are most susceptible to corrosion. Clad aluminum alloy sheets and
plates are used where weight and function permit, while corrosion-resistant alloys
and tempers are used to increase the resistance of the alloys to exfoliation corrosion
and SCC. For example, aluminum alloy 7055-T7751 plate, which is not susceptible
to exfoliation corrosion, has replaced the alloy 7150-T651 plate on upper wing
skins. Major structural forgings of aluminum alloys and steel may be shot-peened
to improve their fatigue and stress corrosion life. Titanium alloys such as Ti6Al–4V
are used in environments such as floor structures under entryways, galleys, and
lavatories. Where possible, stainless steels are used. However, a number of highly
loaded structural components such as landing gears and flap tracks have to be made
of low-alloy, high-strength steel. FRP are corrosion resistant and are widely used.
Carbon fiber-reinforced plastics (CFRP) can cause galvanic corrosion in attached
aluminum structures.
Application of CFRP is in the Boeing 777 CFRP floor beam design where an
aluminum splice channel is used to avoid attaching the floor beam directly to the
primary structural frame.

4.23.2 Coating Selection
The most practical way of combating corrosion is the use of appropriate coating. The
coating for aluminum alloys consists of an appropriate surface such as an anodized
surface with a corrosion-inhibiting primer. Anodizing is done using phosphoric acid.
The other corrosion inhibiting primers are Skydrol-resistant epoxies formulated for
general use, resistance to fuel and hydraulic fluids, or for use on exterior aerodynamic
surface. 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. Titanium and stainless steel are cadmium plated
and primed if they are attached to aluminum or steel parts. This is done to prevent
galvanic corrosion of aluminum or the steel.

4.23.3 Drainage
Effective drainage of the entire plane structure is important in preventing fluids from
becoming trapped in crevices. The entire lower pressurized fuselage is drained by a
system of valved drain holes. The fluids are directed to the drain holes by a system
of longitudinal and cross-drained paths through the stringers and frame shear clips.