TO 35-1-3 - Robins Air Force Base

TO 35-1-3
sion. Improper dilution of cleaning chemicals, combining
cleaning compounds together, or the use of unauthorized
cleaning material can cause the cleaning solution to become an
electrolyte.
1.6.6.1 Moisture is present in air as a gas (water vapor) or as
finely divided droplets of liquid (mist or fog) and often contains
contaminants (such as chlorides, sulfates, and nitrates)
that increase its corrosive effects. Moisture will enter all areas
of SE that air can enter. All enclosed areas, that are not hermetically
sealed, will allow air to enter and leave as the difference
in pressure between the inside and outside the equipment
changes. These pressure differences occur when atmospheric
pressure changes and when the air temperature inside an
enclosed area changes. As the surrounding surfaces are heated,
moisture-laden air is drawn into the enclosed area or absorbed
in open cell foam. As the air around the equipment cool, moisture
will condense in the enclosed areas and settle in the lowest
area of the part. The enclosed areas are often hidden and/or
inaccessible. Moisture is difficult to remove and the subsequent
corrosion may be difficult to identify in the early stages.
1.6.6.2 Salt laden atmospheres pose a continual problem in
locations operating near coastal environments. When dissolved
in water, salt solutions form very effective electrolytes.
The ocean, which is 3.5% to 3.9% salt, is the world's primary
source of salt. Normal sea winds carry from 10 to 100 lbs of
sea salt per cubic mile of air. Bases located close to marine
environments are continually exposed to the salt laden atmosphere.
Because dissolved salts are strong electrolytes, it is
easy to understand why coastal environments are listed in the
moderate to severe corrosion prone categories and require frequent
rinsing and cleaning to remove the salt deposits.
1.6.6.3 Condensed moisture will usually evaporate as the
surrounding air warms but will leave behind its contaminants
(residues), including salts. This can result in the build-up of
soils and salt contamination. Salt-laden condensation may collect
at the lowest surface of the equipment. The lowest surfaces
are usually the last to dry. As the condensation continues
to collect at the lowest surfaces the contaminates/ electrolytes
become more concentrated. Condensed moisture and its contaminants
can also be trapped in close fitting, wettable joints,
such as faying surfaces. Some gasket and packing materials
will absorb several times their weight in water and, when
heated, can transmit this retained moisture into the sealed area.
Moisture can accumulate in such areas through successive
cycles of warming and cooling and cause corrosion on the
interior of the part resulting in premature structural failure as
shown in Figure 1-2.
1.6.6.4 Open cell foam, see Figure 1-3, is a sponge-like
material used mainly for sound suppression and insulation.
Humidity, condensation, rainfall, etc., is absorbed into the
foam and ultimately migrates to the mating metal surface to
which it is attached. The absorbed moisture is very slow in
drying out and provides the moisture/electrolyte against the
metal surfaces. When possible, open cell foam should not be
used. Closed cell types of foam may be suitable substitutes for
this material and will not absorb moisture. If open cell foam is
the only material suitable for your needs, prior to installation
in the SE, seal all open cell surfaces and sides of any perforations
in the foam with a polysulfide type sealant such as MIL-
PRF-81733. Sealing any tears in the foam or any unsealed
sides and edges of the foam will preclude any fluid absorption
by the foam.
Figure 1-2. Corrosion Accelerated by Condensed Moisture
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