TO 1-1-700 - Robins Air Force Base

TO 1-1-700
CHAPTER 10
CORROSION PRONE AREAS
10.1 COMMON AREAS.
There are certain corrosion prone areas common to shelters,
vans, radomes, and C-E-M equipment. Corrosion prone areas
should be cleaned, inspected, and treated more frequently than
less corrosion prone areas. The following paragraphs describe
the areas and contain illustrations to aid in inspections. However,
the list is not complete and should be expanded by referring
to the equipment system specific maintenance manuals
and maintenance inspection cards for each specific piece of C-
E-M equipment, shelter, or radome that show other possible
trouble spots.
10.2 DISSIMILAR METALS.
When two dissimilar metals make electrical contact in the
presence of an electrolyte, which metal will corrode and the
rate at which corrosion occurs depends on the difference in
their activities, that is, their positions in Figure 3-16 of Chapter
3 of this manual. The higher a metal is in this table and the
greater the difference in activity between the two metals, the
faster corrosion occurs. For example, magnesium would corrode
very quickly when coupled with gold in a humid atmosphere.
But aluminum would corrode very slowly, if at all, in
contact with cadmium. A flashlight battery (or dry cell) is an
example of galvanic corrosion put to practical use. A zinc
battery casing steadily corrodes supplying a steady flow of
electrons, but only when the switch is closed. When the switch
is open, there is no corrosion because electrons are not able to
leave the zinc anode. Pay particular attention to dissimilar
metal joints for signs of galvanic corrosion (see Figure 10-1).
C-E-M equipment, shelters, and radomes usually contain dissimilar
metals. The use of dissimilar metals in the selection of
materials and hardware i.e., screws, washers, and nuts, should
be eliminated wherever possible. Where contact between dissimilar
metals cannot be avoided, the choice of contacting
material and associated hardware material is important. Materials
should be chosen such that the part most prone to corrode
(anode) is the easiest and least expensive to replace. At bimetallic
junctions, where finishes are removed to provide good
electrical connection, a protective finish/sealant shall be reapplied
to the completed connection after it is made to prevent
corrosion. Attaching parts, such as nuts (standard, speed, and
self-locking), bushings, spacers, washers, screws (standard
and self-tapping), sleeves for shake-proof fastener studs,
clamps, bolts, etc., do not need to be painted in detail except
when dissimilar metal or wood contact is involved with the
materials being joined or exposure to a corrosive interior environment
or the exterior environment will occur in service.
However, all parts shall be installed wet with sealant.
10.3 FASTENERS.
There are hundreds of fasteners on/in C-E-M equipment and
shelter and radome surfaces, and areas around these fasteners
are trouble spots. These areas are subject to operational loads
and/or moisture intrusion which make the skin material highly
susceptible to corrosion at fastener locations (see Figure 10-2).
Equipment installation, shipping, or setup strains cause paint
to crack around the fasteners which provides a path for corrosive
materials to enter the joint between fastener heads and
skin panels. Any paint that is not highly flexible will crack to
some degree around fasteners. Fasteners should be installed
using a corrosive preventative compound, bonding agent, or
sealant to prevent corrosion between itself and the attaching
structure (see Figure 10-3). See Chapter 4 of TO 35E4-1-162
and Chapter 12 of this manual for sealant application.
Figure 10-1. Galvanic Corrosion Adjacent To Steel Fasteners
10-1