TO 1-1-700 - Robins Air Force Base

TO 1-1-700
3.11 PREVENTATIVE MAINTENANCE.
The two most important factors in preventing corrosion, and
the only ones which can be controlled by field personnel, are
the removal of the electrolyte and the application of protective
coatings. Since the extent of corrosion depends on the length
of time electrolytes are in contact with metals, corrosion can
minimized by frequent cleaning/washing. If non-corrosive
cleaners are used, the more frequently a surface is cleaned in a
corrosive environment the less the possibility of corrosive
attack. In addition, by maintaining chemical treatments, paint
finishes, lubricants, and corrosion preventive compounds
(CPC) in good condition, corrosion can be minimized. The
degradation of non-metallic materials can be minimized by
avoiding the use of unauthorized maintenance chemicals and
procedures. In addition, when repair or replacement of nonmetallic
materials is required use only approved materials.
Dedication to proper preventive maintenance practices maximizes
equipment reliability.
3.12 CORROSION EFFECTS ON GROUND ELECTRI-
CAL AND COMMUNICATIONS, ELECTRONIC AND
METEOROLOGICAL EQUIPMENT (C-E-M).
C-E-M and electrical equipment are easily damaged by contamination
with corrosion removal debris and by application
of improper corrosion control materials. In addition, corrosion
can severely damage and/or impair the proper operation of C-
E-M equipment by totally destroying and/or severely deteriorating
bonding/grounding connections, EMI shielding, conductive
paths for electrostatic discharge. Many of the
conventional corrosion treatment methods used on equipment
structural components are also used on areas adjacent to or
supporting C-E-M equipment, electrical equipment, wire bundles,
and other electrical parts. Personnel performing corrosion
control tasks on or around this equipment shall be familiar
with materials and procedures used for corrosion prevention
and control on electrical and electronic equipment to ensure
that no damage to electrical or C-E-M equipment will occur.
For more specific information, refer to TO 1-1-689-series, TO
1-1-691, TO 00-25-234, and TO 1-1A-14, and the system specific
equipment maintenance manuals.
3.12.1 Grounding and Bonding Connections. Electrical
bonding provides a low resistance electrical path between two
or more conductive units or components so that the low end of
the electrical circuits of each unit or component is at the same
electrical potential. Grounding is electrical bonding that uses
the primary structure to which an electronic system is mounted
as the return for its electrical circuit. Corrosion products are
non-conductive, so corrosion at a bonding/grounding connection
can create an open circuit at the connection and cause the
system to be inoperative.
3.12.2 Dissimilar Metal Junctions, Corrosion Effects.
Connecting two or more diverse electrical objects almost
always in a dissimilar metal couple which is susceptible to
galvanic corrosion that can rapidly destroy a bonding connection
if proper precautions are not observed. Aluminum alloy
jumpers/bonding straps are used for most bonding connections,
but copper jumpers/bonding straps are sometimes used
for bonding parts/components made of stainless/corrosion
resistant steel (CRES), cadmium plated low carbon steel, aluminum
alloy, brass, or some other metal. It is important to
choose bonding and associated hardware materials such that
the part in the connection that is most prone to corrode (the
anode) is the easiest to replace. After the grounding or bonding
connection has been made, overcoat the entire connection,
including all bare metal areas from which the finish system
was removed to provide good electrical contact, with a protective
sealant for all permanent bonding/grounding connections
and a film of water displacing corrosion preventive compound
(CPC) overcoated with heavy wax-like CPC for bonding/
grounding connections that must be frequently removed due to
frequent equipment maintenance or replacement. Consult
Chapter 7 of this manual for additional information on proper
bonding/grounding installation and sealing as well as the
proper materials for the hardware used. Consult TO 1-1A-8 for
assistance with hardware selection.
3.12.3 EMI Shielding. Radiated electromagnetic fields produced
by aircraft and ground radar antennas; ground C-E-M,
aircraft, and missile transmitters; certain poorly designed electronic
units; electric motors; lightning; or any other natural
effects can interfere with ground C-E-M systems causing electrical
and/or electronic malfunctions. This radiation is known
as EMI (electromagnetic interference). To prevent malfunctions
caused by EMI, electrically conductive shielding is either
built into the electronic device or must be added to access
panels, doors, or covers to: 1) prevent emission of EMI from
its own circuits and; 2) prevent susceptibility to outside EMI.
EMI seals and gaskets may also act as environmental seals in
certain locations especially around doors and access panels.
Since aluminum alloy surfaces oxidize very easily, thus
becoming much less conductive and/or non-conductive, other
materials have been used to make electrical contacts (i.e.,
beryllium-copper, titanium, silver plate, and tin-zinc coatings).
However, since these contacts must provide a conductive path
to an aluminum alloy or graphite/epoxy skin, galvanic corrosion
often occurs at the junction of these dissimilar metals.
When corrosion occurs, the conductive path is lost along with
the EMI protection, making the equipment susceptible to electrical
and electronic malfunctions caused by external radiation.
Examples of system malfunctions are microprocessor bit
errors, computer memory loss, false indicators (alarms, lights,
read-outs), CRT ripple, false signals and power loss. The
result of such malfunctions can be catastrophic (e.g., EMI
radiation was responsible for an aircraft jettisoning a bomb
while taking off from a carrier).
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