Page 186 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

164 CORROSION CAUSES

now required by “faying surface sealing” of all joints that are prone to corrosion.
Corrosion-inhibiting compounds are routinely applied in the final assembly of many
aircraft components, such as the inside fuselage crown and lower lobe, pressure bulk-
heads, pressure deck, under lavatories and galleys, wheel-wells, wing-empennage
cove areas, dry bays, empennage torque box interiors, and under fairings.
Another potential source of corrosion problems is in the manufacturing process
used to produce the aircraft. Specifically, the assembly and finishing processes can
determine whether a specific component will be subject to premature corrosion. Of
particular importance is the proper surface pretreatment and application of protec-
tive coatings and sealants, which must offer long-term durability to provide adequate
corrosion protection.
Once the airplanes are in the hands of operators, many factors, including operat-
ing conditions and maintenance practices, determine the corrosion performance of
the airplanes. Operational environments such as marine, tropical, high humidity, and
industrial can be extremely corrosive to the external structure of an aircraft. Further-
more, during operation, the protective surface finishes can deteriorate by chipping,
scratching, breaking around fasteners, abrasion, and aging. Environmental conditions
inside an airplane can be even more damaging. For example, lavatory spillage, galley
spillage, chemical spillage, animal waste, microbial growth, fire residue, and corro-
sive cargo such as fish (salt water) can create extremely corrosive conditions inside
an airplane. Condensation that forms on the inside of the fuselage is also a potential
source of internal corrosion. Boeing (Coating Industry Expert, Private Communi-
cation, June 2000) has conducted an inspection of airplanes with the most severe
moisture problems and found that as a result of moisture uptake in insulation blankets
in B-737-300 airplanes, the weight had increased by an average of 36 kg (79 lb).

3.20 RAILROAD CARS

Figure 3.25 shows a modern-day locomotive, a corroded locomotive, and recondi-
tioned railroad cars. Commodities transported by railroad are given in Table 3.8.
The largest costs to the industry are because of corrosion of the exterior and interior
of the railroad cars.
External corrosion of the cars is primarily because of atmospheric exposure. While
corrosion damage is still a concern, car appearance takes precedence; therefore, the
car manufacturers/lessees often choose to apply an exterior paint system to address
the aesthetic issues. The paint systems are typically “direct-to-metal” (DTM) epoxy
or epoxy with a urethane coat. This epoxy substrate adds protection against ultraviolet
radiation.
The most common method of internal corrosion prevention is the use of coating
systems and rubber linings for internal surfaces. The use of linings and coatings is
aimed not only at prolonging the service life of the car fleet, but also at precluding
the contamination of the transported commodity by corroding metal substrate. Con-
sidering that certain types of commodities such as chemicals are corrosive, corrosion
prevention measures are an absolute necessity.