||NAVMAT P-4855-2: Design Guidelines for Prevention and Control of Avionic Corrosion
4.5 Corrosion and Deterioration of Subassemblies/Circuit Component
There are a number of problems and forms of deterioration that can take place with water and contamination present and around avionic subassemblies. These problems are identified below.
This plating has two primary advantages-its corrosion resistance and it solderability. As a noble metal, gold does not chemically combine to form an oxide, for example, and it is cathodic to all other metals. The problem occurs when gold is so thinly plated over a base metal that porosity occurs. For obvious economic reasons only the minimum required gold is used. The resulting galvanic cell at a pinhole in the gold is particularly aggressive for two reasons. First, gold is at the extreme cathodic end of the electromotive series, which creates an exceptionally high driving voltage-especially when in contact with a very anodic metal such as aluminum. Second, the very large cathodic (gold) area surrounding the small pinhole anodic area is the worst possible galvanic configuration. To prevent pinholing the plating process must be carefully controlled to ensure plating purity, and with regular inspections to verify a continuous, non-porous coating. It is recommended that the minimum plating thickness be no less than 0.000030 inch for solderability and no less than 0.000150 inch for engineering applications (wear and environmental protection).
Although gold is resistant to chemically combining with other elements, it does form intermetallic compounds with certain elements which can cause problems. These include:
a. Gold dissolves in tin/lead solder to form a brittle, intermetallic compound with the tin, which looks (and acts) like a cold solder joint. Careful control of soldering time and temperature, as well as gold thickness and purity are required to prevent this problem.
b. If gold is plated over copper, the copper can diffuse into the gold. This diffusion acts to accelerate the corrosion of the copper at any pinhole in the gold.
c. Like copper, silver also can diffuse into gold. Prevention of diffusion in both cases can be achieved by use of a nickel strike (thin plate) prior to gold plating. This nickel plate should be at least 0.000400 inch thick.
d. In the presence of silicon, gold can alloy with aluminum, forming a brittle intermetallic compound known as "purple plague".
Wire wrap becomes subject to corrosion and
contamination on motherboards mounted horizontally, allowing moisture to
condense on the board as well as collecting dust and dirt. A potentially
bad situation exists when a motherboard is mounted very near the bottom of an
equipment housing. Instances have occurred when an equipment is mounted
on the bottom of an equipment bay and the water collection in the bottom of
the bay was deep enough to allow water to rise into the equipment housing
through the drain holes to a level that was above the motherboard. The
result each time was a corroded motherboard. Figure 4-8 shows an example of a motherboard that was exposed to standing water. This is a particular problem because it is essentially impossible to clean wire wrap. Prevention of the condition is achieved through drainage of the equipment bay and mounting of the motherboard well clear of the bottom of the housing. This prevention is the only viable solution, currently, to wire wrap corrosion.
PRINTED WIRING BOARD DISSIMILAR METAL JUNCTIONS
There are a number of dissimilar metal junctions on a PWB under the conformal coating, just as there are dissimilar metals mated at various grounding and bonding junctions, antenna base/ aircraft skin interfaces, and at many other junctures throughout the avionic system. While a few dissimilar metal junctions might be avoided in the total avionic system, basically the necessity for the use of a variety of metals to achieve the highest possible level of system performance ensures that the large majority of the dissimilar metal junctions are essential to the design. When fluid (an electrolyte) is present at a dissimilar metal couple, an accelerated form of corrosion (galvanic) can occur. The most practical means of preventing such corrosion (as well as other forms of corrosion) is to exclude the presence of an electrolyte. The use of conformal coatings and sealants, especially combined with desiccants, and drainage are techniques that can inhibit the introduction of an electrolyte at the dissimilar metal junction.
WORN EDGE CONNECTORS
A common metallic configuration for edge connectors involves gold plating over a copper base. Gold is a soft metal with poor abrasive wear characteristics. Particulate contamination causes rapid wear of gold, and soon the copper will diffuse into the gold. As previously noted, deterioration of electrical continuity at edge connectors normally is the major single cause of WRA units becoming non-serviceable. Preventive design involves:
a. Nickel plating 0.000400 inch thick under the gold plate of 0.000150 inch for wear resistance.
b. Locating the edge connectors so they are subjected to minimal contamination and providing a continuing seating pressure.
c. Application of Water-Displacing Corrosion Preventive Compound, MIL-C-81309, Type III.
External corrosion of the connector shell frequently
is the cause for replacement, although no functional failure may be
evident. Also, interior corrosion due to condensation from the
"breathing" process results in many functional failures and attendant
replacement. The angle of the connector mounting to the equipment
housing and the angle of the wire bundle exiting the back of the connector are
crucial to vulnerability, as previously discussed. Externally mounted
connectors (in wheel wells, on sponsons, etc.) frequently require additional
external protection from erosion damage to the protective coating or
plating. Figure 4-9 is an example of multicontact connectors mounted in a wheel well. Note the extensive surface corrosion, vertical vice horizontal mounting, and the lack of drip loops-all these problems on a non-environmentally sealed electrical connector! Besides the proper mounting of connectors and limiting the entry angle of the wire bundles into the backshell to prevent side loads and moisture intrusion, fluid effects on the connector can be minimized by a protective system on the exterior of the connector shell.
Cadmium plated shells have survived better than nickel plated shells in the fleet environment, apparently due to the compatibility of cadmium and aluminum. The nickel plating is subject to micro cracking and, it being cathodic to the underlying aluminum shell, will cause severe galvanic corrosion of the aluminum at cracks or other areas of mechanical damage to the plating. The cadmium plating continues to protect the aluminum shells much longer when the cadmium is, itself, protected. In production a chromate treatment over the cadmium plate can extend the life of the plating by over seven times. Application of Water Displacing Corrosion Preventive Compound, MIL-C-85054, over chromate will further enhance the life of the connector.
The newer cadmium/nickel diffused coating and cadmium
over nickel-phosphorous electroless plating are performing excellently in
laboratory testing and in the limited fleet applications. The use of IVD
(Ion Vapor Deposition) aluminum coated connector shells also show significant
improvement over cadmium plated shells. Similarly, developmental work is
underway on corrosion proof composite connector shells. While these
approaches have not yet been proven in the fleet environment, it is clear that
the nickel plated connector has been proven to be inadequate and should not be
utilized in this environment. Figure 4-10 shows an example of nickel plated multicontact feed through connectors and the effect of corrosion. It should be noted that the plating system has flaked or chipped off, exposing the base metal.
The fundamental need for dissimilar metals junctions, as previously described, requires special awareness of the need to control the particularly aggressive form of corrosion that is inherent with bimetallic couples. Electronic design is unique in the wide variety of metals that are utilized because of particular physical properties. Some of the more common metals and their uses in avionic equipment include:
a. Gold is common on electrical connector contacts, edge connectors, leaf- type relays, miniature coaxial connectors, PWB runs, semiconductor leads, and microminiature and hybrid circuits.
b. Silver is commonly used as a protective coating on relay contacts, waveguide interiors, wire, high frequency cavities, tank circuits, r.f. shields and r.f. gaskets.
c. Magnesium alloys find use for radar antenna dishes and lightweight structures such as hardware, chassis, supports and frames. It is particularly susceptible to corrosion, however.
d. Iron and steel (ferrous alloys) are used as component leads, magnetic shields, transformers, brackets, racks and general hardware.
e. Aluminum and aluminum alloys are widely used in equipment housings, chassis, mounting racks, supports, frames and electrical connector shells.
f. Copper and copper-based alloys are generally used as the base metal in PWB edge connectors, contacts, springs, leads, electrical connectors, PWB runs and for wire.
g. Cadmium is used to coat ferrous hardware, such as bolts, washers, screws and electrical connectors as a sacrificial protective coating.
h. Nickel and tin-plating are used for barrier type protective coatings and for compatibility purposes between two otherwise incompatible metals. Tin is widely used in solder and tin-plating is common in r.f. shields, filters, small enclosures and automatic switching devices. Nickel plating as a layer between copper and gold overplate is used to inhibit the diffusion of copper into the gold.
The foregoing metals combine to form literally thousands of galvanic couples within avionic equipment. In the presence of a highly conductive electrolyte, such as sea water, it is not possible to assemble other metals to aluminum without protection.17
The potential for galvanic corrosion is inherent in avionic design, and for this reason care must be taken to seal and preserve any bimetallic couple in the initial design phase.
COMMON CORROSION PROBLEMS
The previous sections primarily have described the manner by which
electrolytes (moisture) become present throughout the avionic systems and
equipment in conjunction with the extensive presence of galvanic couples. While
galvanic corrosion is a serious cause of equipment malfunction and even
destruction, other types of corrosion also result because of material selection
and contamination entrapping features of hardware design. These include:
a. Surface Contamination
Stack gases as well as industrial fumes and vapors contained in smog,
soot and various airborne contaminants combine with moisture to form acidic
surface contamination. The hygroscopic dust, dirt and lint hold moisture on
the contaminated surfaces, thereby keeping the reactions active. A form of
corrosion can result characterized by a very general surface attack or
localized pitting. The more anodic metals, when uncoated, are most
susceptible to such deterioration. Painting of exposed metal surfaces is the
most practical form of control of this problem.
b. Under Cushioning Material
Foam-type cushioning material is used for operational shock mounts on
certain equipment. In many cases, foam is used in the lids or tops of
avionic equipment as a cushioning pad to hold vertically mounted PWBs in
place. As the cushioning foam deteriorates in service, the adhesion to the
base plates also deteriorates. Any moisture absorbed in the foam is then
held against the bared metal plates. Similarly, felt pads and other organic
cushions can act to hold moisture on a metallic surface and support
corrosion. This problem is controlled by selection of materials that will
not deteriorate (revert) or hold moisture.
c. Cracks in Plating
One of the reasons for plating is to provide a more corrosion-resistant
metallic surface on a part fabricated of a metal that had been selected for
properties other than corrosion resistance. Some platings act sacrificially
to perform the protection, while others act as a barrier to prevent moisture
from reaching the corrodible base metal. When a barrier plating
cracks-whether due to flexure, differential expansion, mechanical damage,
etc.-not only is the base metal exposed to corrosion, but if the plating is
incompatible (cathodic) to the base metal, the corrosion is accelerated. A
sealant or Water Displacing Corrosion Prevention Compound, such as
MIL-C-85054, provides flexible protection over cracked plating. Sacrificial
plating should be protected at the time of production with a chromate
coating. This greatly extends the life of the plating.
d. Crevice Corrosion
Areas between joined metallic surfaces, under washers, along engaged
fastener threads or other areas deprived of oxygen are subject to formation
of a localized anodic area in the presence of an electrolyte. The resulting
corrosion damage can be very insidious because it occurs only in hidden
areas. This form of corrosion can occur in metals normally not considered
subject to corrosion, such as stainless steel. It is recommended a
metallurgist participate in the selection of materials and metallic
e. Stress Corrosion
When a metal is under tensile stress and also subjected to a corrosive
environment, an accelerated corrosion attack can occur which results in
early failure by cracking. Press fit bushings, tapered bolts and severe
metal forming are examples of creating the high residual tensile stresses
that lead to stress corrosion in the marine environment. Selection of proper
alloys and heat treatments prevent stress corrosion. It is recommended a
metallurgist review the use of metals which will be under significant
f. Intergranular Corrosion
Most metals are characterized by a microscopic grain structure. The grain
structure is such that one area may be anodic to another. Also, the grain
boundaries are anodic to the grains in aluminum alloys. Thus, in a damp
environment, corrosion sometimes can proceed along subsurface grain
boundaries. Proper selection of alloys and heat treatments is required to
prevent this form of corrosion. The metallurgist can assist in preventing