FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 1. General | 6-4. Micro-organisms
9/27/01
AC 43.13-1B CHG 1
c. Corrosion is quickened by high-
temperature environments that accelerate
chemical reactions and increase the concentra
tion of water vapor in the air.
d. Electrolytes (electrically-conducting
solutions) form on surfaces when condensa
tion, salt spray, rain, or rinse water accumulate.
Dirt, salt, acidic gases, and engine exhaust
gases can dissolve on wet surfaces, increasing
the electrical conductivity of the electrolyte,
thereby increasing the rate of corrosion.
e. When some of the electrolyte on a
metal surface is partially confined, (such as
between faying surfaces or in a deep crevice)
the metal around this area corrodes more rap
idly. This type of corrosion is called an oxygen
concentration cell. Corrosion occurs more
rapidly because the reduced oxygen content of
the confined electrolyte causes the adjacent
metal to become anodic to other metal surfaces
on the same part that are immersed in electro
lyte or exposed to air.
structive are sulfuric acid (battery acid), halo
gen acids (hydrochloric, hydrofluoric, and hy
drobromic), nitrous oxide compounds, and or
ganic acids found in the wastes of humans and
animals.
b. Alkalies, as a group, are not as corro
sive as acids. Aluminum and magnesium al
loys are exceedingly prone to corrosive attack
by many alkaline solutions unless the solutions
contain a corrosion inhibitor. Substances par
ticularly corrosive to aluminum are washing
soda, potash (wood ashes), and lime (cement
dust).
c. The major atmospheric corrosive
agents are oxygen and airborne moisture.
Corrosion often results from the direct action
of atmospheric oxygen and moisture on metal
nd the presence of additional moisture often
accelerates corrosive attack, particularly on
ferrous alloys. The atmosphere may also con
tain other corrosive gases and contaminants,
particularly industrial and marine salt spray.
f. Slime, molds, fungi, and other living
organisms (some microscopic) can grow on
damp surfaces. Once they are established, the
area usually remains damp, increasing the pos
sibility of corrosion.
g. Manufacturing processes such as ma
chining, forming, welding, or heat treatment
can leave residual stress in aircraft parts and
can cause cracking in a corrosive environment.
d. The corrosiveness of water depends
on the type and quantity of dissolved mineral
and organic impurities and dissolved gasses
(particularly oxygen) in the water. One char
acteristic of water that makes it corrosive is its
conductivity. Physical factors, such as water
temperature and velocity also have a direct
bearing on its corrosiveness.
6-4. MICRO-ORGANISMS.
6-3. COMMON CORROSIVE AGENTS.
Substances that cause corrosion are called cor
rosive agents. The most common corrosive
agents are acids, alkalies, and salts. The at
mosphere and water, the two most common
media for these agents, may also act as corro
sive agents.
a. Bacteria may be either aerobic or an
aerobic. Aerobic bacteria require oxygen to
live. They accelerate corrosion by oxidizing
sulfur to produce sulfuric acid. Bacteria living
adjacent to metals may promote corrosion by
depleting the oxygen supply or by releasing
metabolic products. Anaerobic bacteria, on the
other hand, can survive only when free oxygen
a. Any acid will severely corrode most
of the alloys used in airframes. The most de-
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