Page 184 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

162 CORROSION CAUSES

TABLE 3.6 Percentage of World’s Fleet by Class of Ship on
the Basis of Gross Tonnage
Class Percent
Refrigerated cargo 1
Chemical tankers 6
Bulk dry 30
Passenger/ferry 4
Oil tankers 28
Supply/tugs 2
Cargo/roll on/roll off 15
Fishing 2
Others/unknown 12

Table 3.6 shows the percentage of the world’s fleet by class of ship on the basis of
gross tonnage.

3.19 AIRCRAFT
According to the data presented in the 1999 edition of the Aviation and Aerospace
Almanac, the combined aircraft fleet operated by US airlines in 1998 totaled
7478 (23).
These include fixed-wing turboprop and turbojet and rotary wing aircraft. About
3973 are turbojet aircraft, which are divided into 29 different types for domestic and
international service. Table 3.7 shows a 1997 listing of the major US carrier fleets
with the number and average age of the fleets’ airplanes.

3.19.1 Corrosion Modes

Corrosion in an aircraft manifests itself in several different forms. Pitting and crevice
corrosion are the most common forms of corrosion in the 2000 and 7000 series alu-
minum alloys, which are the principal materials of construction. Pitting corrosion
produces deterioration of the airframe structure in localized areas and can have high
penetration rates. Pits often create stress concentrations, which may reduce the fatigue
life of a component. Crevice corrosion, by itself, is more destructive than pitting cor-
rosion. Crevice corrosion occurs when a corrosive fluid enters and is trapped between
two surfaces, such as a joint, a delaminated bond line, or under a coating. Both pitting
and crevice corrosion, when unchecked, can readily develop into exfoliation corro-
sion or intergranular SCC. Exfoliation corrosion is a form of intergranular corrosion
where corrosion attack occurs along the grain boundaries of elongated grains, causing
a leaf-like separation of the metal grain structure (Fig. 3.24). This form of corrosion
often initiates at unprotected end grains, such as at fastener holes and plate edges.
Figure 3.24 represents exfoliation corrosion around fastener holes in aluminum alloy
7075-T6 fuselage section.