01 March 2005
NAVAIR 01-1A-509-3
TM 1-1500-344-23-3
TO 1-1-689-3
avionics experience corrosion between metal surfaces
tants. The copper ion concentration can be higher that
such as joints. In these environments, exclusion of
the other cathodic reactants (dissolved oxygen) allow-
electrolyte from the joint is essential to maintaining an
ing higher currents. Copper goes into solution through
EMI seal. Nonconductive environmental seals (gaskets,
either galvanic coupling to the conductive surface coat-
sealants, and coatings) must be used to exclude
ing or general corrosion of copper. Corrosion of the
moisture from the joint. Corrosion-induced changes in
aluminum flange also depends on the presence of
bulk resistivity of the EMI gasket may contribute to
aggressive ions in the solution. Initiation of corrosion
degradation of the EMI seal.
creates more acidic conditions further accelerating the
corrosion rate. In general, corrosion products are
7-5.2. CORROSION-INDUCED CHARACTERISTICS.
nonconductive materials that increase the electrical
Although no valid method of evaluating the effect of
resistance of the bond or joint between surfaces. This
corrosion on EMI shielding exists presently, certain
will result in an impedance high enough to destroy the
trends can be identified. The difficulty lies in the fact that
EMI shielding effectiveness of many avionic systems.
conductive gaskets may not be evaluated by simply
measuring EMI shielding performance before and after
7-6. PREVENTION, TREATMENT, AND CONTROL
environmental exposure. This is because the change in
OF CORROSION ON EMI SHIELDING.
EMI shielding is due to both corrosion and disassembly/
reassembly between EMI tests. EMI/EMP protection for
7-6.1. G E N E R A L . Changes in the shielding
enclosed avionic equipment can degrade in shielding
effectiveness can occur over time due to environmental
performance over time. This often occurs after only 3 or
exposure. These changes are usually the result of the
4 months of exposure to the field environment and
operational/ maintenance environment (salt solution,
normal use. Galvanic corrosion is usually the principal
vibration, pressure and temperature cycling) and
form of attack. In most military applications, the flange
complicate routine maintenance of avionic equipment.
is made of an aluminum alloy, such as 6061-T6. Steel,
In general, exclusion of moisture and electrolyte from
magnesium, and composite materials are occasionally
the joint is essential to maintaining an EMI seal. Since
used in flange construction. All of the common conduc-
EMI protection is a function of the electrical continuity
tive materials used in EMI gaskets are dissimilar to
of the joint, conductive environmental seals (gaskets,
varying degrees with aluminum. Gasket metals include
sealants, and coatings) must be used. Paints and
monel, beryllium copper, silver, carbon, nickel, and
sealants used in proximity of the joints to exclude
stainless steel. Composite fillers are substrates plated
moisture shall only be applied by qualified maintenance
with a more conductive material. These include tin plate
personnel and in accordance with this manual and the
on various substrates (beryllium copper, phosphor
applicable Maintenance Instruction Manual (MIM)
bronze, copperplated steel); silver on copper, alumi-
(Navy), Technical Orders (TO) (Air Force), and/or
num, nickel, and glass powders; and nickel on graphite
Technical Manual JM) (Army). In cases where
or carbon particles. Conductive composites offer sig-
conductive paths depend on the use of specific types
nificant advantages over aluminum, such as improved
of fasteners and other hardware, strict adherence
corrosion resistance and lighter weight. EMI attenua-
shall be paid to the specific fastener designated in the
tion properties are usually imparted to these compos-
applicable MIM (Navy), TO (Air Force), and/or TM
ites by filling the polymer with conductive fibers, flakes,
(Army). Proper and complete grounding/ bonding, with
or particles. Applying a conductive coating to the poly-
visual inspection after maintenance, is mandatory. In
mer surfaces is also employed. Galvanic series for
those instances where electrical bonding depends on
homogeneous materials, such as those contained in
bringing conductive surfaces into contact with threaded
Volume 1, Chapter 3, do not necessarily apply to
fasteners, standard torque values shall be used unless
composite fillers. Interaction between the plated sur-
otherwise specified.
faces and substrate can alter the potential and ability of
the surface to support cathodic reactions. In addition,
7-6.2. CORROSION PREVENTIVE MAINTENANCE
relative surface areas of the EMI gasket and aluminum
AND CONTROL. EMI gaskets should be periodically
will have some influence on the rate of dissolution of the
checked to ensure they are continuing to provide their
aluminum. The presence of copper ions accelerates
intended functions. Inspection intervals should balance
corrosion of aluminum by deposits that form local cath-
the need for proper operation and the environment,
odes and increase the concentration of cathodic reac-
with the destructiveness caused by surface wear of
