Page 185 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

AIRCRAFT 163

TABLE 3.7 Major Carrier Jet Fleets in 1997
Carrier Number Age (year)
Alaska 75 7.4
America West 100 10.4
American 663 9.4
Continental 350 11.9
Delta 538 11.7
Federal Express 264 17.7
Northwest 412 18.9
TWA 200 19.9
United 592 11.5
United Parcel Service 144 16.5
US Air 408 12.3
Southwest 241

Figure 3.24 Exfoliation corrosion around fastener holes in Al alloy 7075-T6 (7).

Intergranular SCC occurs when stresses are applied perpendicular to the suscep-
tible grain boundaries. More so than pitting and crevice corrosion, susceptibility to
exfoliation corrosion, and intergranular SCC depends on alloy type, heat treatment,
and grain orientation. Other common forms of corrosion are fretting corrosion, which
occurs when two surfaces rub at high frequency and low amplitude in the presence
of a corrosive environment. Galvanic corrosion occurs when dissimilar metals such
as aluminum and steel are in direct contact. Isolation of different metals by proper
design and assembly can prevent both forms of corrosion.
There are many contributing causes of corrosion in commercial aircraft. The
first potential source of corrosion is in the basic design process. Materials selec-
tion, finishes, and structural configuration can have a significant impact on the
corrosion performance of an airplane. During the design phase, attention must be
paid to the basic principles of corrosion-conscious design, such as the selection of
corrosion-resistant materials, the avoidance of dissimilar metal-contact, crevices,
stresses, and poor drainage. In addition, the selection of sealants and finish sys-
tems is an important part of corrosion-conscious design. For example, the use
of corrosion-inhibiting primers and sealants on fasteners and faying surfaces has
become common practice for new airplanes, and the elimination of crevices is