TO 35-1-3 - Robins Air Force Base

TO 35-1-3
Figure 1-1. Diagram of a Simplified Corrosion Cell and an Example of Raised Corrosion Product
1.5.2 Corrosion Under Painted Surfaces. Paint coatings
can mask the initial stages of corrosion. Because corrosion
products occupy more volume than the original metal, frequently
inspect paint surfaces for irregularities such as blisters,
flakes, chips, and lumps.
1.6 FACTORS INFLUENCING CORROSION.
Both natural and man-made environments cause corrosion of
SE. Natural conditions that affect the corrosion process are
moisture, temperature, salt atmospheres, ozone, sand, dust,
solar radiation, insects and birds, and microorganisms. Manmade
conditions that affect the corrosion process are industrial
pollution, manufacturing operations, storage conditions, and
shipment. By understanding these conditions, maintenance
personnel will be better able to prevent SE damage. Factors
that influence metal corrosion and the rate at which it occurs
are outlined in Table 1-1.
1.6.1 Type of Metal. The metals most commonly used in
SE construction are aluminum, steel, and to some extent magnesium.
Cadmium, nickel, chromium, and silver are sometimes
used as protective plating. Metals have a wide range of
corrosion resistance. The most active metals (i.e., those that
tend to lose electrons easily), such as magnesium and aluminum,
corrode easily and are listed at the top of Table 1-1. The
most noble metals (i.e., those that do not lose electrons easily),
such as gold and silver, do not corrode easily and are listed at
the bottom of Table 1-1.
1.6.2 Dissimilar Metal Coupling (Galvanic Corrosion).
When two dissimilar metals make electrical contact in the
presence of an electrolyte, the rate at which corrosion occurs
depends on the difference in their location on the galvanic
scale (see Table 1-1). The greater the distance between the
metals, the faster corrosion occurs. For example, magnesium
corrodes very quickly when coupled with gold in a humid
atmosphere. However, pure aluminum corrodes very slowly, if
at all, when in contact with cadmium. It is essential in the
repair of SE that compatible metals and metal fasteners, washers,
etc., be selected for repair and refurbishment processes. If
dissimilar metal coupling must occur due to the equipment
design or configuration, contact the appropriate system equipment
specialist for the appropriate processes and materials to
insulate the incompatible metals.
1.6.2.1 Graphite or carbon fiber composites are materials
which consist of reinforcing fibers in a matrix made of organic
resin, usually epoxy. They are an important class of SE construction
materials because of their high strength-to-weight
ratios and high stiffness. Although graphite or carbon/carbon
composites appear as a plastic, the graphite material is considered
a metal. Care should be exercised when attaching fasteners
through the fibers. Use the appropriate sealant to insulate
the composite from the metal parts.
1.6.2.2 Since carbon is the least active metal in the galvanic
series, it will accelerate the corrosion of any metal to which it
is coupled. Insulation, with sealant or other nonconductive
coatings, between graphite or carbon epoxy composites and
other metals surfaces or fasteners is necessary to prevent dissimilar
metal attack on the attached part.
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