01 March 2005
NAVAIR 01-1A-509-1
TM 1-1500-344-23-1
TO 1-1-689-1
from exposure. Hermetic sealing (liquid and vapor
Natural conditions in the environment which affect the
proof at normal temperatures and pressures) is
corrosion process are moisture, temperature, salt
recommended for moisture-critical items such as
atmospheres, ozone, sand, dust, and solar radiation.
capacitors and quartz crystals.
Corrosion may also be promoted by biological sources,
including animals, insects, and microorganisms. Man-
3-13.2. SALT ATMOSPHERES. Salt forms a strong
made conditions, which also affect the corrosion
electrolyte when dissolved in water, which causes rapid
process, include industrial pollution, manufacturing
corrosion of unprotected metal surfaces. The primary
operations, packaging, storage conditions, and
source of the world's salt is the ocean, which is 3.5% to
shipment. By understanding these conditions,
3.9% salt. Normal sea winds can carry from 10 to 100
maintenance personnel will be better able to prevent
pounds of seasalt per cubic mile of air. Since dissolved
aircraft damage.
salts are strong electrolytes, it is easy to understand
why corrosion is such a severe problem in shipboard
3-13. NATURAL ENVIRONMENTS.
and coastal environments.
3-13.1. MOISTURE. Moisture is present in air as a gas
3-13.3. OZONE. Ozone is a particularly active form of
(water vapor) or as finely divided droplets of liquid (mist
oxygen which is formed naturally during thunderstorms,
or fog). It often contains contaminants such as chlorides,
by arcing in electrical devices, and by photochemical
sulfates, and nitrates, which increase its corrosive effects.
reactions in smog. When ozone is absorbed by
Moisture enters all areas of an aircraft that air can enter.
electrolyte solutions in contact with metals, it increases
Enclosed areas which are not sealed allow air to enter
the rate of corrosion. It also oxidizes many nonmetallic
and leave as the pressure between the interior and
materials, being particularly harmful to natural and
exterior changes. These pressure differences occur
certain types of synthetic rubber. Rubber seals stored
when the aircraft changes altitude, when atmospheric
near welding equipment have experienced complete
pressure changes, and when the temperature of air
degradation.
inside an enclosed area changes.
3-13.4. SOLAR RADIATION. The two ranges of solar
3-13.1.1. Condensed Moisture. Moisture will condense
out of air when the air becomes too cool to hold all of the
radiation most damaging to materials are ultraviolet (the
moisture in it. The dew found on aircraft after a cool night
range that causes sunburn) and infrared (the range that
is the result of condensation. Condensed moisture
makes sunlight feel warm). On earth, maximum solar
usually evaporates as the surrounding air warms, but
radiation occurs in the tropics and equatorial regions,
leaves contaminants, including salts, behind. This can
but considerable damage occurs in the temperate zones
result in the build-up of soils and salt contamination.
as a result of solar heating, photochemical effects, and
Condensed moisture and its contaminants can also be
combinations of these two phenomena. Non-metals,
trapped in close fitting wettable joints, such as faying
especially organic and synthetic materials, are strongly
surfaces. Some gasket and packing materials will absorb
affected by sunlight. Both natural and synthetic rubber
several times their weight in water and, when heated,
deteriorate rapidly when exposed to sunlight. After
can transmit this retained moisture into the sealed area.
extended exposure, plastics darken, paints lose their
Moisture can accumulate in such areas through
protective characteristics, polymers undergo marked
successive cycles of warming and cooling. In addition,
decreases in strength and toughness, and colors fade,
moisture can be drawn along poor bond lines by capillary
removing essential color coding. Most electronic
action (wicking). Temperature and humidity conditions
equipment is housed in enclosed structures and is
can vary widely in separate sections of aircraft depending
protected from solar radiation. Extra care must be taken
on the success of environmental sealing, condensation,
in the selection and surface treatment of parts, such as
and location near heat-generating equipment.
cables and harnesses, that are exposed to exterior
environments.
3-13.1.2. Effect of Moisture. Electrolyte formation
3-13.5. TEMPERATURE. High temperature either
results from condensation of moisture. All non-metals
improves or impairs the performance of avionic
absorb some moisture, which may cause changes in
equipment, depending on conditions. Corrosion and
dimensional stability, dielectric strengths, ignition
other harmful processes (outgassing, decomposition)
voltages, and volume insulation resistances. In general,
increase as temperature rises. However, moderate
organic matrix composites are adversely affected by
increases in temperature may prevent condensation.
moisture and may suffer a loss of strength and stiffness
