2-10. Pitting Corrosion.
strengthen some metal and is an agent in some
lubricants. At the same time, carbon is highly cathodic
to all metal alloys and is extremely corrosive when in
a. Pitting occurs most commonly on aluminum and
contact with high potential end alloys as shown in
stainless steel products. It is a form of localized
corrosion in which small areas of the metal corrode
preferentially to the adjacent areas. This type of
corrosion causes cavities or pits in the metal surface.
c. Galvanic corrosion is sometimes desirable as in
the case of galvanized steel. This is steel with a zinc
b. The cause of pitting corrosion is salt, either
coating. The zinc coating protects the steel by serving as
introduced through sea water or on the highway in
a barrier between the shell and the corrosive world. The
winter months. To avoid pitting, clean the vehicle after
zinc coating also serves as a sacrificial metal due to the
exposure to salt as soon as is practical.
galvanic process whereby the natural corrosion process
concentrates on the weaker zinc to the benefit of the
steel. Maintenance technicians must keep galvanic
corrosion in mind whenever replacing nuts, bolts, metal
2-11. The Differential Aeration Effect.
screws, clamps, etc. These bin items should always
approximate the metals they will come in contact with
a. Air carries moisture but also dries moisture from
on the vehicle.
the surface of metals. Whenever the design of the
vehicles is such that moisture can become trapped,
while areas immediately surrounding the trapped
moisture are ventilated and, therefore, quickly dried,
Ozone and Ultraviolet Radiation.
the difference between the two metal areas (one
dry/one moist) will set up an electrochemical cell. The
a. Ozone is a blue, gaseous form of oxygen formed
naturally in our atmosphere by the effects of lighting
process and an intense, localized corrosion. This
dynamic is called the "differential aeration effect"
and/or by normal oxygen's exposure to ultraviolet
which can be translated into the phrase "result of
radiation from the sun. From the high atmosphere
where it is concentrated, the ozone falls to earth. Ozone
is the primary enemy of tires and other rubber
b. Crevice corrosion in rocker panels is the most
products. It destroys the two most important properties
commonly cited example of the differential aeration
of rubber, its strength and elasticity. Ozone exposure
effect. This dynamic is also responsible for poultice
dries out rubber so that it becomes brittle and easily
worn through. Ozone oxygen atoms exist in greater
quantities on hot, desert-like surfaces of the earth
where their effects on tires, rubber seals, and other
automotive products are most pronounced. Most Army
2-12. Poultice Corrosion.
tires and other rubber products are manufactured with
Poultice corrosion is rust which occurs when deposits
time and exposure. Commercial preservatives such as
of mud or other debris hold stagnant moisture in
Armor All or Agemaster can be applied as required.
contact with a metal surface. Because the mud or debris
is moisture-laden, this condition accelerates the rusting
b. Ultraviolet radiation is a corrosive agent.
process. As with pitting corrosion, the solution is to
Ultraviolet radiation is everywhere in the rays of the
keep the vehicle as clean as possible at all times.
sun. Aside from contributing to the formation of ozone,
ultraviolet radiation breaks down paints and resins. It
also breaks down the polymer bonds or chemical
structures of rubbers and plastics. With prolonged
2-13. Crevice Corrosion.
exposure to direct sunlight, these materials will lose
their strength, dry out, and crack. If prolonged
Crevice corrosion is caused by a design problem that
exposure to ultraviolet radiation cannot be avoided,
manufacturers are being asked to keep in mind during
provide a preventive maintenance watch on all such
the planning of new vehicles. Crevice corrosion is
products and periodic replacement, as necessary.
intense accelerated rusting in joints and recesses. It is
Rubber products may be protected using commercial
caused by the differential aeration effect outlined
above. The solution for crevice corrosion is rustproof-
ing of the joint or recess.
possible to forestall the effects of ultraviolet radiation.