TO 35-1-3 - Robins Air Force Base

TO 35-1-3
c. Keeping all drains and drain holes open, and low parts
of the SE dry and free of salt deposits.
d. Initial use, inspection, and reapplication of the appropriate
CPCs following SE washing, repairs, and component
replacement.
e. Early detection and thorough removal of corrosion and
replacement of the damaged protective coatings and
sealants.
f. Sealing gaps, seams, and holes to prevent moisture
entry. This is especially important around spot-welded
areas.
g. Application/reapplication of sealants, where required, to
insulated dissimilar metals or prevent water intrusion
following SE repairs, and component replacements.
h. Technician awareness of the importance of corrosion
prevention during regular maintenance activities (while
performing daily repairs and maintenance tasks, always
look for signs of corrosion or corrosion damage).
i. Track causes of initial corrosion to the source and eliminate
the source.
1.7 TYPES OF CORROSION.
Corrosion is cataloged and classified in many ways. Occasionally,
different names are used for the same type of corrosion.
Common types of corrosion are described below.
1.7.1 Uniform Etch Corrosion. Uniform etch corrosion
results from a direct chemical attack on a metal surface and
involves only the metal surface. On a polished surface, this
type of corrosion is first seen as a general dulling or etching of
the surface, and, if the attack continues, the surface becomes
rough and possibly frosted in appearance. This type of corrosion
appears uniform because the anodes and cathodes are
very small and constantly shift from one area of the surface to
another. An example is the etching of metals by acids or by the
use of unapproved or improperly mixed cleaning compounds
(see Figure 1-5). The discoloration or general dulling of metal
created by exposure to elevated temperatures is not considered
to be uniform etch corrosion.
1.7.2 Galvanic Corrosion. Galvanic corrosion (see Figure
1-6) occurs when different metals are in contact with each
other and an electrolyte (such as salt water). It is usually recognizable
by a buildup of corrosion at the joint between the
metals. If two dissimilar metals that are widely separated in
Table 1-1 are in direct electrical contact, galvanic corrosion is
probably occurring. The less corrosion-resistant metal (the
higher metal in Table 1-1) becomes the anode and the more
corrosion-resistant metal (the lower metal in Table 1-1)
becomes the cathode. As stated earlier, the anode will corrode.
The farther apart the metals are in Table 1-1, the more severe
will be the corrosion of the anodic metal if the metals are
joined electrically and placed in an electrolyte. In most cases,
the galvanic series listed in Table 1-1 is a good predictor of
how metals will corrode in a galvanic couple. A major factor
in galvanic corrosion is the size of the anode and cathode. A
small anode in contact with a large cathode will corrode much
more severely than a large anode in contact with a small cathode.
Figure 1-5. Example of Uniform Etch
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