CORROSION CONTROL/INSPECTION continued
a. Microbial Induced Corrosion
Operations in high temperature and high humidity areas are susceptible to microbial attack. Bacteria living adjacent
to metals, can cause corrosion by depleting the oxygen supply or by releasing corrosive products. Fungi may
damage organic materials by retaining moisture on a metal surface and clogging drainage holes, and secreting
corrosive fluids. Micro-organisms, when transferred from a hot/wet area to a dry/cold area, may become dormant
and appear to die. Then, when returned to a hot/wet climate, the micro-organisms will again become active.
1.49.10. Corrosive Environments
Moisture is present in the air and often contains contaminants such as chlorides, sulfates, and nitrates. Condensed
moisture after it evaporates, will leave its contaminants on surfaces of the aircraft. Accumulations of moisture may
result from cycles of warming and cooling, and drawn along poor bond lines by capillary action.
Generally, corrosion will increase as temperatures rise, but a moderate increase may reduce corrosion by prevent-
ing condensation. Temperatures above 104 F (40 C) will inhibit mold and bacteria growth. Temperatures held at the
lower end of the range will generally reduce the rate of corrosion. Warm, moist air, normally found in tropical climates
tends to accelerate corrosion while cold, dry air normally found in artic climates tends to reduce corrosion rates.
Corrosion does not occur in very dry conditions. Certain areas within an aircraft, such as the cockpit and air
conditioned equipment bays, may be subjected to climatic dry air that has been cooled by air conditioners. This
reduces its ability to hold moisture, and when ducted into interior area of the aircraft, can release sufficient moisture
to accelerate corrosion.
Salt dissolved in water forms a strong electrolyte. Normal winds carry 10 to 100 pounds of sea salt per cubic mile of
air. Since dissolved salts are strong electrolytes, salt water environments are highly corrosive to aircraft.
Ozone is an active form or oxygen which is formed naturally during thunderstorms, by arcing in electrical devices,
and by photochemical reactions in smog. When ozone is absorbed by electrolyte solutions in contact with metals, it
increases the rate of corrosion.
Sand and dust are present in many areas, but particularly in industrial areas where they often contain tar products,
ashes, and soot. Dust is also found in the tropic zones during times of little or not rainfall. Sand and dust are extreme
problems in the deserts, since dry, powdery sand and dust are carried by the wind. During sandstorms, they can
penetrate sealed equipment as well as many internal areas of airframes. Sand and dust are hygroscopic and, when
present on internal or external surfaces of aircraft or electronic parts, can absorb and hold moisture. Dust from
volcanic areas contain chlorides and sulfates, which are extremely corrosive in the presence of moisture. Although
small amounts of sand or dust may be unnoticed by operation personnel they may be sufficient to promote corrosion.
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