01 March 2005
3-11.8.1. Solvents. Some encapsulants, conformal
revert. Usually, the process of potting involves the use
coatings, and acrylic plastics soften or dissolve when
of a mold to form the potting compound. These molds
they come in contact with cleaning solvents.
are made of plastic and may become the source of
moisture intrusion. All molds should be removed after
3-11.8.2. Heat. The heating of conformal coatings for
the potting compound has cured (refer to Volume III).
removal or repair may outgas corrosive vapors onto
3-11.4. LAMINATE CIRCUIT BOARDS. Laminate
metal components. Shrinkage elastomers (heat-
shrinkable tubing) can damage adjacent circuitry when
circuit boards typically use encapsulants or conformal
heat guns are applied to shrink the tubing. Certain oils
coatings as sealers. In some cases, the laminate is not
(such as silicones) and greases "creep" as temperature
sealed along the edge or at the contact tabs. This
allows the laminate board to absorb moisture and
surfaces, degradation of organic coatings, and attraction
delaminate. Overcleaning with abrasive materials may
damage the resin surface of the laminate board. High
temperatures caused by the burning of a circuit
3-11.8.3. Acid Creation. Some commercial conformal
component may char the laminate surface and increase
coating strippers contain acids that attack PCB laminates
and discolor or corrode copper. Certain room
3-11.5. RUBBERS AND ELASTOMERS. Rubbers and
temperature vulcanizing (RTV) silicone sealants contain
elastomers are used for insulation, seals, gaskets,
acetic acid that is highly corrosive to metal components
caps, tubing, films, and coatings. Natural rubber, silicone
(a list of authorized RTV silicone sealants is provided in
rubber, and polyurethane are normally susceptible to
Volume III). Degradation of polyvinylchloride (PVC)
attack by fungi, microbes, ozone and ultraviolet light.
gives off acid fumes that are corrosive to most materials
Polyurethane and silicone rubber are to some extent
used in avionics. Some solid film lubricants contain
permeable to moisture.
uninhibited molybdenum disulfide which, with moisture
and heat, may form sulfuric acid.
3-11.6. TAPES. Some pressure sensitive tapes are
effective moisture barriers. Problems arise with cloth or
3-11.8.4. Form Change. Some potting compounds
paper-based tapes. These materials absorb moisture
revert to liquid form under certain conditions. This
and support fungal attack. This is particularly true when
reversion causes a maintenance problem and reduces
wicking action takes place on the cloth or paper material.
the moisture protection in electrical connectors.
Some tapes deteriorate and outgas, emitting an acid
that is corrosive to metals.
3-11.8.5. Oil and Lubricants. Application of conformal
coatings, adhesives, and paint finishes is difficult when
3-11.7. OILS. The lubricants used in some electronic
silicone oil film is present on base material. Some solid
film lubricants containing graphite are corrosive. Graphite
equipment are capable of minimizing moisture intrusion
and moisture will promote galvanic corrosion in many
and corrosion attack. However, at high temperatures
metals. Some lubricants will attack neoprene, plastics,
and pressures, oils can chemically react with impurities
rubber, organic materials, and most paints.
to produce acids. Oil can hold a limited amount of water.
Excess water will separate from the oil if the holding
3-11.8.6. Plastics. Plastics can be damaged by solar
capacity is reached at a given temperature.
3-11.8. M A T E R I A L
darkening, discoloration, or color fading of the plastic.
Lower pressures and vacuum can cause outgassing
NONMETALLICS. The complexity of modern aircraft
and loss of plasticizers/ flexibilizers. This can cause a
and avionics makes it difficult to predict what problems
change in structural properties such as loss of strength.
may result from reactions between materials.
The damage is appears as embrittlement and crazing
of the surface, as well as loss of electrical properties.
the nonmetallic substances. This may result in the
release of chemicals or gases that react with other
components. In some cases, cleaning solutions, high
CORROSION. Corrosion of aviation equipment occurs
temperature, or lubricants will cause a reaction in
in both natural and man-made environments. In
nonmetallic substances. Common examples of
addition, the aircraft operational and maintenance
nonmetallic materials incompatibility are as follows:
environment contributes to unique corrosion conditions.