Page 217 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION PROBLEMS IN THE US DEPT. OF DEFENSE 195

service life. In general the structural life of both commercial and military aircraft is
based on flight hours and number of fatigue cycles. In general, the life of aircraft
is fatigue-limited and corrosion is never considered to be a life-limiting factor. The
minimum KC-135 structural fatigue life-limited components are the fuselage and the
upper wing skin at 66,000–70,000 h, while the actual fleet hours are only 15,000. As
the KC-135 utilization averaged only 300–400 flight hours per aircraft in 1 year, it
appears that the fleet can easily remain in service until 2040.
However, severe corrosion has been experienced on the aluminum alloy compo-
nents of the KC-135 aircraft. This corrosion is the result of low utilization, where
the majority of the time is spent on the ground being exposed to the corrosive atmo-
spheric environments. In the 1950s, the KC-135 was never designed and constructed
with corrosion prevention as a primary concern. The original structural alloys were
aluminum alloys 2024-T3 and 2024-T4, 7075-T6, and 7178-T6, which are all sus-
ceptible to corrosion and SCC. The original construction was without any sealant
in the lap joints and fuselage skins that had spot-welded doublers attached to them.
The upper wing skins, which are made of the highly corrosion-susceptible aluminum
alloy 7178, were attached with high-strength steel fasteners, causing dissimilar metal
corrosion in certain areas.
A particularly severe problem is the corrosion of the fuselage lap joints, where the
voluminous corrosion products at the contact or faying surfaces of the lap joints cause
deformation of the skin (49, 50). Because of the resulting stress fatigue and stress
corrosion, cracks can nucleate near the fastener holes, jeopardizing the structural
integrity of the fuselage. Other corrosion problems on the KC-135 aircraft include dis-
similar metal corrosion and lap joint corrosion on the 7178 upper wing skin, lap joint
corrosion on the 7075-T6 fuselage crown section, and SCC of the 7075-T6 forged
frame section.

3.31.8 Navy
The Navy consists of several components, including ships, submarines, aircraft
weapons, and facilities such as buildings, piers, docks, and harbor structures. An
internal Navy study conducted in 1993 estimated the total cost of corrosion for all
naval systems at $2 billion per year (Sedriks, Office of Naval Research, Personal
Communication, July 2000).
The Navy fleet consists of various surface ship battle forces, including 11 aircraft
carriers, 106 surface combatants (i.e., cruisers, destroyers and frigates), 39 amphibi-
ous warfare ships, 34 combatant logistic ships, and 31 support/mine warfare ships
(total of 221 ships).
The surface ships are subject to extremely aggressive environments. An extensive
corrosion control program is required to maintain the fleet during dry-dock cycles.
The primary defense against corrosion is the diligent use of protective coatings. In
addition to coating, CP is used for the protection of underwater hull. The cost to main-
tain CP systems are low compared to the cost of maintaining the various protective
coating systems. Figure 3.29 shows a photograph of a destroyer, indicating the dif-
ferent shipboard coatings that are currently in use. The traditional coatings indicated