01 March 2005
NAVAIR 01-1A-509-1
TM 1-1500-344-23-1
TO 1-1-689-1
3-7.2. Paint coatings can mask the initial stages of
faster corrosion occurs. For example, magnesium
corrosion. Since corrosion products occupy more volume
would corrode very quickly when coupled with gold in
than the original metal, the paint surfaces may become
a humid atmosphere. But aluminum would corrode
blistered, flaked, chipped, or appear lumpy.
very slowly, if at all, in contact with cadmium. A
flashlight battery is an example of galvanic corrosion
which influence metal corrosion and the rate of corrosion
casing steadily corrodes, supplying a steady flow of
are outlined below.
electrons, but only when the switch is closed. When
the switch is open, there is no corrosion because
3-8.1. TYPE OF MATERIAL. The best time to prevent
electrons are not able to leave the zinc anode.
corrosion is at the design stage. Proper material selection
3-8.4. ANODE AND CATHODE SURFACE AREA.
is critical for the protection of equipment against harmful
The rate of corrosion also depends on the size of the
environmental effects. Most pure metals are not suitable
parts in contact. If the surface area of the corroding
for aircraft construction and are used only in combination
metal (the anode) is smaller than the surface area of
with other metals, and sometimes non-metals, to form
the less active metal (the cathode), corrosion will be
alloys. The metals most commonly used in aircraft
rapid and severe. But, when the corroding metal is
construction are aluminum, steel, titanium, and
larger than the less active metal, corrosion will be slow
magnesium. Cadmium, nickel, chromium, and silver
and superficial. For example, an aluminum fastener in
are sometimes used as protective platings. Metals have
contact with a relatively inert monel structure may
a wide range of corrosion resistance. The most active
corrode severely, while a monel bracket secured to a
metals (those which tend to lose electrons easily), such
large aluminum member would result in a relatively
as magnesium and aluminum, corrode easily and are
superficial attack on the aluminum sheet (see
(those which do not lose electrons easily), such as gold
and silver, do not corrode easily and are listed at the
3-8.5. PRESENCE OF ELECTROLYTES. Electrically
conducting solutions are easily formed on metallic
3-8.2. HEAT TREATMENT AND GRAIN DIRECTION.
Most alloys are made up entirely of small crystalline
water accumulate. Dirt, salt, acidic stack gases, and
regions called grains. When heat treated during
engine exhaust gases can dissolve on wet surfaces,
manufacturing or repair, heavy sections of metals do
not cool uniformly and, as a result, tend to vary in
thereby increasing the rate of corrosion.
3-8.6. ELECTROLYTE CONCENTRATION. In the
another. This can cause corrosion if one area is more
active than another. Alloys which are fabricated by
same way that metals can corrode when exposed to
rolling, extruding, forging, or pressing have properties
different concentrations of oxygen in an electrolyte,
which depend highly on direction (parallel to grain
corrosion will also occur if the concentration of the
elongation vs. cross grain). Corrosion can occur on
electrolyte on the surface varies from one location to
surfaces of those regions which are less resistant and
another. This corrosive situation is known as a
also at grain boundaries, resulting in the formation of
concentration cell.
pits and intergranular corrosion. For example, exposed
end grains corrode much more easily than flattened
3-8.7. AVAILABILITY OF OXYGEN. When some of the
elongated surfaces in sheet stock. This explains why
electrolyte on a metal surface is partially confined (such
exfoliation occurs at the edge of aircraft skin sections or
as between faying surfaces or in a deep crevice), metal
next to countersunk fasteners.
in this confined area corrodes more rapidly than other
metal surfaces of the same part outside this area. This
3-8.3. DISSIMILAR METALS. When two dissimilar
type of corrosion is called an oxygen concentration cell
or differential aeration cell. Corrosion occurs more
metals make electrical contact in the presence of an
rapidly than would be expected because the reduced
electrolyte, the rate at which corrosion occurs depends
oxygen content of the confined electrolyte causes the
on the difference in their activities, that is, their positions
adjacent metal to become anodic to the metal surfaces
in Figure 3-4. The greater the difference in activity, the
exposed to the air.