The fundamental source of corrosion problems in avionic
equipment is due to fluid intrusion throughout naval aircraft, especially in those aircraft
that are ship based - both fixed and rotary wing. Figure 1-1 shows an example of the effects of moisture and fluid intrusion in avionic equipment. It should be noted that this example demonstrated many design deficiencies that will be discussed throughout this Design Guide. For a variety of reasons, some of which are described in this Design Guide, there is no such thing as a continuously protected, humidity controlled environment inside of these aircraft. It follows, therefore, that the humid ambient air present in the aircraft will penetrate nonhermetically sealed or non-pressurized equipment housings. Thus, design must be based on the assumptions that:
a. A moisture film can be present on any uncoated metallic surface:
b. An electrolyte can be potentially present at any bimetallic couple;
c. Moisture will pool in any "bathtub"area; and
d. Fluids can reach equipment by running along (even wick upward in some cases) any conduit such as: a wire bundle, a waveguide, a control rod, or a hydraulic line.
Given the consideration that fluid intrusion is the fundamental source of
corrosion in avionic equipment, it should be recognized that avionic equipment
must survive for weeks or months in a down aircraft. In such cases the
equipment may not be operated during these periods, yet is exposed to a hot,
salt-laden, high relative humidity environment.
A secondary problem related to fluid intrusion is the build-up of cleanable films that result in disruption of electrical continuity in circuits throughout the avionic system. There is a growing belief in the military and industry among maintenance personnel that these cleanable films may be a major factor in the failure of avionic systems. Cleanable films are present on:
a. Edge connectors
b. Coaxial connectors
c. Multicontact connectors
d. Relay contacts
e. Switch contacts
Design weaknesses in corrosion resistance, including inadequate recognition of effects of cleanable films and moisture (fluid) intrusion, have caused problems in the past. It is the intent of this Design Guide to describe these design weaknesses, provide design or material application solutions and, hopefully, inhibit nature from taking its natural, but undesired, course.