In design decisions involving the selection and use of materials, it is important to consider not only the electrical characteristics, but also factors such as basic resistance to corrosion, compatibility between metals, stability of organic materials and maintainability by fleet personnel. Material applications recommendations include:
Minimize dissimilar metals (galvanic) couples. The joining of dissimilar metals should be limited to those applications where similar metals cannot be used due to peculiar design requirements. If absolutely required, however, select the dissimilar mating metals to minimize the electrochemical potential of the joint. In all cases involving dissimilar metals it is essential that the junction area be sealed to minimize the presence of an electrolyte at the bimetallic interface. This sealing may be performed by the conformal coating on a PWB, or by application of sealant for couples located exterior to a WRA. Other couples within a WRA can be sealed using a non-acetic acid cure RTV (Room Temperature Vulcanizing), per MIL-A-46146. Where a dissimilar metals junction is designed for disassembly, such as a grounding or bonding connection, an easily replaceable anodic (consumable) part, such as an aluminum washer, should be included in the assembly.
Graphite is a material that is both a good conductor and a good lubricant. However, this material acts electrochemically like a noble metal and is cathodic (corrosive) to all structural metals. It is especially destructive when in contact with aluminum. Because of its extreme corrosiveness, graphite lubricants in any form should not be used on naval aeronautical equipment. When graphite is unavoidably present, as in graphite composite structural parts, ensure that insulation is applied to electrically isolate any avionic component from contact with the graphite composite.
ELECTRICAL CONNECTOR BOOTS ORGANIC COATINGS
Boots (rubber, plastic, elastomeric) have been used over ends of wire bundles, harnesses, connector backshells and similar applications to seal out moisture, provide strain relief and abrasion resistance. In applications where the harness breathes, boots can be used only for strain relief and abrasion resistance. Such boots must contain provisions for good drainage to prevent the boot from becoming a "bathtub" area. Boots intended to seal out moisture must be installed with the prescribed adhesives at both the cable and connector ends, and can be used only with harnesses that are already jacketed up to the junction with the boot.
Generally, the use of magnesium for any applications in a marine environment runs a very high risk of failure. If the use of magnesium is attempted, first priority in all design decisions must be toward maximizing protection from corrosive conditions. No metal more cathodic than aluminum should be mated to magnesium. If at all possible, do not use magnesium.
All working of aluminum, e.g., cutting, drilling, grinding, forming, etc., should be completed before surface treatment is applied. Of the common surface treatments used on aluminum, anodize provides the best protection, followed by Chemical Conversion Coating, MIL-C-5541, Class IA, and lastly by MIL-C-5541, Class 3. All of the foregoing coatings have insulative properties, decreasing in degree from anodize to Class 3. In most applications where electrical conductivity is required, the MIL-C-5541, Class 3, will provide adequate continuity and offer some environmental protection. Since even this coating provides some resistance to ground or attenuates weak signals, areas of this type should be kept to a minimum. For generally adequate electrical conductivity, per MIL-B-5087, all contamination and coatings except MIL-C-5541, Class 3, must be removed to bare aluminum at the point of contact. Treat cleaned area with MIL-C-5541, Class 3, if some other type of coating has been removed. Assemble electrical connection and seal entire junction area. The slight impedance due to the MIL-C-5541, Class 3, film is the compromise to achieve some corrosion protection and enhanced coating or sealant adhesion.
The preferred organic coating system for external surface protection of the structural metals used in the fabrication of avionic equipment is an Epoxy Topcoat, M I L-C-22750, over an Inhibited Epoxy Primer, MIL-P-23377. When aluminum is the base metal, the surface should be anodized or conversion coated prior to priming. While Polyurethane Topcoat, MIL-C-83286, also provides a tough, yet flexible coating, the health hazards involved in the application of this paint make it generally difficult for use in fleet avionics shops or paint facilities.
Because of the protection against moisture and
corrosion, primarily, and enhanced resistance of mounted components to shock
and vibration, secondarily, all PWBs for naval avionic equipment should have a
Ml L-146058 conformal coating. Figure 5-1 shows the results of corrosion on a PWB that was not protected by a conformal coating. As a production applied conformal coating, paraxylylene currently provides the best sealing. Other MIL-I-46058 materials have characteristics that may make the selection of a coating material other than paraxylylene valid under special circumstances. Of the conformal coatings that can be applied in a repair action in "fleet microminiature repair facilities" epoxy has been reported by fleet personnel as the preferred coating. Polyurethane is a close second. Acrylic, RTV and varnish types of coating have performance deficiencies that render them a poor design choice. Although the fleet microminiature repair technician cannot apply paraxylylene, he has a compatible patch coating available.
Utilize metal plating, where applicable, to provide sacrificial protection, barrier protection, as a third metal (such as tin) interposed between two otherwise incompatible metals, or as a substitute surface presenting a reduced galvanic voltage differential to a contacting metal. Use a nickel strike (thin plate) under gold. Because it promotes solderability and has outstanding corrosion resistance, gold is widely used as a thin plating over silver or copper. However, silver diffuses through gold, as does copper. Gold over either silver or copper accelerates corrosion of the less noble metal at the pores or pinholes in the gold.18 To avoid diffusion and corrosion, and still retain the strength of the base metal and the corrosion resistance of the gold, a plating of nickel should be placed on the copper, (instead of silver) under the gold. Use a nickel strike of at least 0.000400 inch under a gold plate of no less than 0.000030 inch. A continuous, non-porous gold plate is critical to preventing corrosion of the base metal. An experienced Materials Engineer (Metallurgist) should participate in the identification of metal applications to avoid problems such as red plague (a silver-copper galvanic corrosion product), purple plague (a gold-aluminum intermetallic compound), metallic whiskers, silver migration and other potential metallic problems.
Use the lowest acid content flux possible for soldering. The term "no acid or acid salts should be used" is applied to solder, and Military Specifications permit only non-corrosive and non-conductive fluxes.19 However, even "neutral" fluxes must have some acidity in order to remove metal oxides and provide a reducing atmosphere to prevent oxide formation, and "non-activated" fluxes can corrode metals.20 Thus, any flux residue must be completely removed by cleaning.
Use materials that are resistant to the fluids described
in paragraph 4.1 . Avoid those materials that outgas, support fungi, absorb moisture, and, in the case of seals, can be degraded in performance by the presence of fluids. All proposed organic materials should be reviewed by an experienced materials specialist.
In the case of metallic materials, the most corrosion resistant configuration (passivated) should be used with the minimum possible residual stressing. An experienced Materials Engineer (Metallurgist), who is aware of the characteristics of the fleet environment, should participate in the selection of metals that are resistant to the adverse features of that environment.
GASKETS, "O" RINGS AND SEALS
The initial costs of most gaskets, "O" rings or seals
are very small compared to the reliability and maintainability penalties
associated with failure and replacements of these items. Fluorocarbon and
fluorosilicone type materials are most resistant to the aircraft fluids
described in paragraph 4.1 .
A thin conductive gasket of silver, copper or graphite impregnated material is sometimes used in an attempt to enhance conductivity between a blade antenna aluminum base and the adjoining aircraft aluminum skin. This extreme galvanic couple results in nonconductive products of aluminum corrosion that greatly degrade system performance. A clean aluminum antenna base mounted against a clean fuselage skin (each treated per Chemical Conversion Coating, MIL-C-554 1, Class 3) and secured with metal screws provides the best long term mounting. Seal around coaxial exit from the fuselage (if access is possible) and around the periphery of the antenna base, with Sealant, Polysulfide, MIL-S-8802.
Like the conductive gasket, the design of an EMI gasket normally involves the use of highly cathodic materials (silver, monel, copper alloys or graphite) contacting anodic aluminum. This is fundamentally a metallurgically unsound condition! The survivability of such a design depends upon the effectiveness of a sealing system that precludes "breathing" of ambient air or any other method of moisture intrusion into the vicinity of the galvanic couple at the skin/ gasket interface. Design ingenuity must also take into account the problem of restoring this super sealed condition following fleet maintenance or equipment replacement.