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(7) Cathode. A dissimilar conductive material (usually a metal) which has less tendency to corrode.
(8) Electrolyte. A conductive liquid (usually water) that contains ions in solution. For example, saltwater is an electrolyte made up of water containing sodium and chlorine ions. The electrolyte solution is capable of carrying an electric current between the anode and cathode.
(9) Electron Conductor. Electrical contact between the anode and the cathode; for example, the different elements that make up an alloyed metal, or a fastener holding two pieces of metal (anode and cathode) together.
(10) Galvanic Couple. A cell consisting of two dissimilar metals (an anode and a cathode) in contact with each other through an electrolyte solution.
b. When a metal corrodes, electrons are lost from the atoms in the metal part, and these atoms become metal ions in the electrolyte. Once in solution, positively charged metal ions can combine with the negatively charged ions to form corrosion products.
c. A metal will corrode only when all four of the following exist: there is an anode, there is a cathode, there is an electrolyte containing ions, and the anode and cathode are connected by an electron conductor. Elimination of any one of these four items will slow the corrosion process.
203. FACTORS INFLUENCING CORROSION.
a. Basic Design. There are many design decisions and compromises to be made in the course of developing avionics equipment. The design specifications leave room for a wide range of engineering practices to meet not only the performance, cost, and schedule, but also the reliability and maintainability requirements. Each piece of avionics equipment is designed to withstand its intended operational environment. However, some design compromises have to be made to provide the unique electrical, mechanical, and thermal characteristics of the equipment.
These compromises can cause the equipment to be vulnerable to corrosion, especially during inoperative periods. Good design practices include:
(1) Shoe Box Type Lid Construction. When access to the equipment is from the top, use a shoe box type lid construction. Fasteners securing the shoe box lid should be from the sides and not through the top.
(2) Limited Openings in the Equipment Housing. To minimize moisture intrusion, keep the number of penetrations into the equipment to a minimum. When penetration is required, use “O” rings and gaskets for sealing. For wiring entry, use “L” type electrical connectors and mount them horizontally (through the vertical sides), and well above the bottom of the housing.
(3) Proper Electrical Connector Mounting. Electrical and coaxial connectors should be mounted horizontally (through the vertical sides). When electrical and coaxial connectors are mounted on the top of the equipment, there should be a raised area on the upper side of the equipment where the connector is mounted, and an “L” type connector should be used. Electrical
Par 203 Page 7 wiring should incorporate a drip loop into the wiring harness so the wiring is leading up to the connector.
(4) Proper Printed Circuit Board Mounting. Printed circuit board should be mounted vertically with the electrical connection also in a vertical position.
(5) Low Point Drains. Low point drains should be incorporated so that any moisture will drain from the equipment when the aircraft is in the flight position and when it is parked on the ground.
(6) Eliminating Moisture Traps. Avoid moisture traps or bathtub areas on the interior areas of the equipment. Design in drain paths to the low point drains. Avoid moisture traps in electrical wire bundles where anti-chaffing material or boots are incorporated.
(7) Cooling Air Systems. Cooling air systems should incorporate a system to remove moisture and particulate matter from the conditioned air. This is especially important when the conditioned air is forced directly towards active electronic elements.
(8) Proper Bonding and Grounding. Electrical bonding and grounding should be accomplished using straps rather than sliding housing-to-rack or tapered pin on housing-to-rack electrical contacts. Straps should be located for ease of maintenance and properly sealed because of the dissimilar metal (galvanic) couple.
(9) Proper Equipment Mounting. Avionics equipment should be mounted in such a manner that will allow sufficient airflow around the equipment and keep the equipment at least 1/2 inch above the compartment floor.
b. Material Selections and Uses. Proper material selection is critical for protecting avionics equipment against the environment. Many types of corrosion that occur in avionics equipment also occur in the airframe structure. However, the range of material call outs in avionics equipment is greater than in the airframe. Several new types of corrosion problems are therefore unique to avionics equipment. The following indicate the uses of different materials in the construction of various electrical and electronic components.
(1) Copper and copper-based alloys are generally used in avionics systems as contacts, springs, leads, connectors, printed circuit boards (PCB), conductors, and wire.
(2) Iron and steel are used as component leads, magnetic shields, transformer cores, brackets, racks, and general hardware.
(3) Magnesium alloys are used extensively throughout avionics systems as antenna structures, chassis, supports, and frames (radar).
(4) Nickel and tin plating are used for protective coatings and for material compatibility purposes. Tin is also one of the components of solder. Tin plating is also used on radio frequency (RF) shields, filters, and automatic switching devices.
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(5) Silver is used as a protective plating material over copper in wave guides, miniature and microminiature circuit boards, tank circuits, and RF shielding.
(6) Aluminum and aluminum alloys are widely used, because of their light weight, in equipment housings, chassis, mounting racks, supports, and electrical connector shells.
(7) Cadmium is used as a sacrificial coating on ferrous hardware, such as bolts, nuts, washers, and screws.
(8) Ion vapor deposition (IVD) of aluminum is also used as a sacrificial coating on hardware and is a nonhazardous replacement for cadmium.
(9) Gold is commonly used on electrical connectors, contacts, and edge connectors where the lowest electrical resistance is required.
c. Material Compatibility. Due to the complexity of the material process used in modern electronic assemblies, it is sometimes difficult to predict if potential problems will be created by the reaction between two or more nonmetallic materials in a circuit assembly. Incompatibility of materials can result in the release of chemicals or gases that will react with other circuit components. In some cases, the incompatibility of cleaning solutions will cause reactions in substances that are corrosive to associated circuitry. The following list contains several of the potential problems:
(1) The heating of conformal coating for the purpose of removal or repair may cause an outgassing that can be corrosive to metal components.
(2) Some commercial coating strippers contain acids that attack PCB laminates and discolor or corrode copper.
(3) Certain room temperature vulcanizing (RTV) silicone sealants contain acetic acid that is highly corrosive to metal components in avionics circuits. See Chapter 5, subparagraphs 505d(4) through (6), and Table 5-2.
(4) Some potting compounds revert to a liquid form under certain conditions. This reversion process reduces moisture protection in electrical connectors. See Chapter 5, subparagraph 505b.
(5) Degradation of polyvinyl chloride gives off acetic fumes, which are corrosive to most metals used in avionics equipment.
(6) Shrinkable elastomers (heat shrink) tubing, although not a problem directly, can cause damage to adjacent circuitry when heat guns are applied to shrink the tubing.
(7) Some dry film lubricants contain graphite, which is an excellent lubricant, but graphite is also corrosive. Graphite is cathodic to metals and, in the presence of moisture, promotes galvanic corrosion. Other dry film lubricants contain molybdenum disulfide which in the presence of moisture and heat can form a corrosive sulfuric acid.
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(8) Certain oils, especially silicones and greases, creep as temperature increases, causing contamination of organic coatings and attraction of dust.
d. Moisture Intrusion. Moisture intrusion can take several forms: accidental dousing, or immersion; or normally as a gas in the form of water vapor (i.e., humidity), or finely divided droplets of liquid (i.e., mist or fog). The normal type of moisture often contains pollutants such as particulates, smog, industrial contaminants, and chlorides from salt laden air. Except for hermetically sealed or pressurized avionics equipment, most avionics equipment breathes. This allows the free passage of this polluted moisture in and out of the equipment. The polluted moisture collects on the internal electrical and structural components through condensation. The following are methods of minimizing moisture intrusion.
(1) Hermetic Sealing. Hermetic sealing provides the greatest resistance to moisture intrusion by providing adequate seals (solder or glass fusion joints) on the equipment and filling the interior compartment with a dry inert gas. Hermetically sealed avionics equipment provides for a seal integrity check by a built-in visual indicator. Cooling requirements, size, and container penetrations will often preclude this type of moisture intrusion protection.
(2) Pressurization of Equipment. The next best avionics equipment moisture intrusion protection is through pressurization of the equipment or the equipment compartment.
The introduction of pressurized dry air into a semi-sealed area greatly reduces the intrusion of moisture. However, the additional weight required to achieve the necessary equipment rigidity largely precludes the use of this method of moisture intrusion reduction.
(3) Sealing of Equipment. For avionics equipment that cannot be hermetically sealed or pressurized, protection from moisture intrusion can best be achieved by applying a sealant.
Two types of sealant are available. Current technology uses polysulfide and RTV silicone sealants. Polysulfide sealants are two part component sealants, whereas RTV sealants are single component. For avionics equipment, RTV sealants that have a vinegar odor are not authorized.
This type of sealant contains an acetic acid used for curing the sealant. The acid will cause corrosion.
e. Manufacturing Process.
(1) Surface Treatments. Avionics equipment cases are manufactured from aluminum, and occasionally magnesium, because of weight considerations, cooling efficiency, and cost.
Surface treatments, commonly called pre-treatments, are an important first step in the overall protection of the equipment. The most common surface treatments for aluminum are anodizing and chemical conversion coating. Anodizing is normally applied by the original avionics equipment manufacturer on interior and exterior surfaces. Chemical conversion coating is normally a pre-treatment process applied by repair facilities. The MIL-SPEC for chemical conversion coating is MIL-C-5541, Class II and Class III. Class II is used for aircraft structural components, avionics cases, and at any location where electrical bonding is not a consideration.
Class III is an avionics grade chemical conversion coating and provides adequate electrical continuity while providing some corrosion protection. The pre-treatment for the repair of magnesium is a chemical conversion coat conforming to SAE-AMS-M-3171, Type VI.
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(2) Organic Coatings. The exterior paint system on avionics equipment consists of a primer and a topcoat. The primer promotes adhesion and contains corrosion inhibitors. The topcoat provides durability to the paint system, including weather and chemical resistance.
Environmental concerns, as well as local and state air pollution regulations have implemented strict controls on the amount of volatile organic compounds (VOC) (solvent) contained in primers and topcoats. The majority of all new avionics equipment is painted with VOC compliant water-based primers and topcoats.
(3) Conformal Coatings. Conformal coatings offer the same protection to electrical components within the avionics equipment as the primer and topcoat offer to the exterior of the same equipment. Conformal coatings are generally a clear plastic coating applied over the electrical components, conforming to MIL-SPEC MIL-I-46058, as revised. Conformal coatings offer several advantages: protection from moisture and corrosion, and enhanced resistance to shock and vibration. Generally, there is no field level repair for conformal coated components.
(4) Plating Systems. Metal plating is used in avionics equipment to provide sacrificial protection, barrier protection, and as a neutral nonreactive metal between two dissimilar metallic surfaces. Gold and tin are the two most widely used plating metals. Gold plating requires another metal (base) to be plated under the gold, usually nickel, silver, or copper. A nonporous gold plating is required to eliminate galvanic corrosion with the base-plated metal. Special corrosion conditions can be set up between gold and silver-copper plating if damaged. This special galvanic corrosion condition for gold is referred to as red plague. A special galvanic corrosion condition, known as purple plague, can form between gold and aluminum in the presence of silicone.
204. ENVIRONMENTAL CONDITIONS.
a. Within the Aircraft. Maintenance personnel often assume that, once installed within the aircraft cockpit, cabin, and/or equipment bays, avionics equipment will be protected from water or fluid intrusion and, therefore, free of significant corrosion. In-service use has shown that this is not the case. Airframe flexing combined with in-service use and handling damage can lead to seal deterioration and violation of the water tightness of the airframe and avionics equipment.
b. Operational Environments. The operational environment of today’s aircraft consists of two main conditions: periods of in-service use and periods of non-operation. When avionics equipment is operating, the heat generated by the equipment tends to drive off or at least minimize moisture intrusion or entrapment. The ability of corrosion to start or continue will be reduced. Conversely, when avionics equipment is not in operation moisture can collect on the electrical components, increasing the opportunity for corrosion to start.
205. CORROSIVE CONDITIONS.
a. General. Corrosion is a major contributing cause to the reduction of avionics equipment reliability. Moisture is the single most important contributor to corrosion in avionics systems. The following paragraphs discuss environmental factors that have a major effect in the amount of corrosion on the equipment.
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b. Moisture. Moisture can be either a gas, water vapor (humidity), or finely divided droplets of liquid (mist or fog). This moisture generally contains industrial pollutants, particulates, smog, and chlorides from salt laden air. Moisture enters all areas of an aircraft and avionics equipment exposed to air. All enclosed areas that are not sealed allow moisture laden air to enter and leave while the difference in pressure between the inside and outside of the equipment changes. These pressure differences occur when the aircraft changes altitude, when atmospheric weather changes occur, and when the temperature changes within the equipment.
Cooling air can also introduce moisture into the equipment.
c. Condensed Moisture. Airborne moisture will condense when the air temperature drops below the dew point or when the air comes in contact with any surface colder than the dew point temperature. Water droplets on the outside of a cold drinking glass are an example of this condition. Condensed moisture usually evaporates as the surrounding air warms. When this occurs, the contaminants that were brought in by moisture-laden air and distributed on the equipment during condensation are left behind.
d. Residual Contaminants. Residual contaminants usually consist of industrial pollutants, dusts, and salts. Industrial pollutants include: carbon from internal combustion engines, nitrates from agricultural fertilizers, ozone from electrical motors and welding processes, sulfur dioxide from turbine engines, industrial and ship exhausts stacks, and sulfates from automobile exhausts. Dusts include sand, dirt, and volcanic ash. Figure 2-1 shows dust and dirt accumulation. The contaminants found in industrial areas often contain a number of tar products, ashes, and soot. The primary sources of salt are the world’s oceans. The oceans contain between 3.5% and 3.9% salt. Normal sea winds can carry from 10 to 100 pounds of sea salt per cubic mile of air inland up to 100 miles.
e. Other Fluids. Many fluids can be present in various areas of an airframe. Table 2-1 lists the type of fluid intrusion and possible effects. Fluids can be from external sources, internal leaks, or servicing spills. Some of these fluids are corrosive to metals, while others are destructive to seals. Destruction of seals can lead to fluid intrusion into areas that were considered, by the design, protected from corrosive fluids.
Par 205 Page 12 FIGURE 2-1. DUST AND DIRT ACCUMULATION ON TERMINAL STRIP TAB LE 2-1. EFFECTS OF AIRFRAME FLUID INTRUSION Type of Fluid Intrusion Effects or Deterioration Engine fuel Softening or swelling of some polymers.
Hydraulic fluid Reduced paint adhesion, and introduction of insulated films on electrical connector contact surfaces.
Lubricants Attack some seal and gasket material.
Dielectric Coolant Attacks organic seals.
Anti-icing fluids Increased condensation and attacks on electrical wiring.
Aqueous contaminates (free water, urine, condensation, desiccants) Increased condensation causing pooling of fluids in bilge areas;
corrosive attack of unprotected dissimilar metal couples; introduction of insulated films on electrical contact surfaces.
Maintenance fluids (solvents, detergents, cleaners, strippers) Softening and/or reduced adhesion of some organic coatings and cracking of insulation on some electrical wiring.
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f. Temperature. High temperature may improve or impair the performance of avionics equipment, depending on other conditions. Corrosion and other harmful processes
(e.g., outgassing, decomposition, etc.) increase as the temperatures rise. In some instances, moderate rises in temperature prevent condensation of moisture laden air. Most fungal growth is inhibited by temperatures above 104 °F (40 °C). Low temperatures pose no direct threat of corrosion, except that as temperature drops, relative humidity rises. Extremely low temperatures can cause shrinkage and embrittlement of seals and gaskets, resulting in leakage and fluid intrusion.
g. Pressure. Most avionics equipment is designed to operate at low pressure (high altitude). Low pressure causes outgassing of plastics and other organic materials, which can change the physical and chemical properties of those materials. Another problem created by pressure is pressure cycling (high altitude/low altitude). These varying pressure changes can cause breathing from leaky seals and gaskets. Breathing promotes condensation of moisture laden air and creates a corrosive environment.
h. Microorganism, Insect, and Animal Attacks. Condensed moisture can cause conditions that promote the growth of mold, bacteria, and fungi. Once mold, bacteria, and fungi are established, they absorb and hold moisture. As living creatures, they secrete wastes which are acidic. This acid is a strong electrolyte which corrosively attacks metal. Non-metals, such as sealants, provide nutrients which can accelerate their growth. Many factors determine the degree of the fungal attack. Table 2-2 lists materials and the deterioration from microbial attack.
Damage to avionics systems and equipment can also be caused by small insects and animals.
This condition is most prevalent in tropical environments. Avionics equipment in storage is also susceptible to this condition. Insects and animals may enter through vent holes, open cabinets, or torn packaging material. Once inside the equipment, they can build nests which will hold moisture. This moisture plus excretions can cause corrosion. Another type of damage possible on the interior of the avionics equipment happens when an insect or animal eats electrical insulation, varnishes, or conformal coatings. This removes the environmental protection coating, allowing direct corrosive attack on the underlying surface.
i. Man-Made Environments. Man-made environments include repair station work, equipment handling, packaging, storage, and shipment. Avionics equipment undergoing repair can be contaminated by the surrounding environment. Fumes and vapors from adjacent repair operations such as soldering, paint spraying, and solvent cleaning can become trapped in the equipment. Failure to remove soldering flux residues after a repair can cause corrosion. Removal of avionics equipment from an aircraft for maintenance or inspection can expose the equipment to various environments. Removal of components from the equipment without proper protection from the environment can subject that component to corrosion. Improper shipping containers such as in wooden or fiberboard containers can subject that material to corrosion from vapors released by the container. Changes in temperature and pressure while en route can allow the intrusion of moisture. Finally, the improper storage of avionics equipment awaiting installation can cause corrosion. Storage of avionics equipment on wooden shelving can subject the equipment to vapors released by the shelving material.
Par 205 Page 14 TABLE 2-2. EFFECTS OF MOISTURE AND FUNGI ON VARIOUS MATERIALS Part of Material Effects of Moisture and Fungi Fiber: washers, supports, etc. Moisture causes swelling that can lead to misalignment and binding of parts. Destroyed by fungi.
Fiber: terminal strips and insulators Electrical leakage paths are formed causing flashovers and crosstalk.
Insulating properties are lost. Destroyed by fungi.
Laminated plastics: terminal strips and boards, switchboard panels, etc., tube sockets and coil forms Insulating properties are lost. Leakage paths cause flashovers and crosstalk. Delamination occurs and fungi grow on surface and around edges. Expansion contraction under extreme temperature changes.
Molded plastics: terminal boards, switchboard panels, connectors, tube sockets and coil forms, etc.
Machined, sawed, or ground edges of surfaces support fungi, causing shorts and flashovers. Fungi growth can reduce resistance between parts mounted on plastic to such an extent that the parts are useless.
Cotton linen, paper, and cellulose derivatives: insulation covering webbing, belting, laminations, dielectrics, etc.
Insulating and dielectric properties are lost or impaired, causing arcing, flashovers, and crosstalk. Destroyed by fungi.
Wood: cases, houses, plastic fillers, masts, etc.
Dry rot, swelling, and delaminated housings caused by moisture and fungi.
Leather: straps, cases, gaskets, etc.
Moisture and fungi destroy tanning and protective materials, causing deterioration.
Glass: lenses, windows, etc. Fungi grow on organic dust, insect track, insect feces, dead insects, etc. Dead mites and fungi growth on glass obscure visibility and corrode nearby metal parts.
Wax: for impregnation Fungi-inhibiting waxes that are not clean support the growth of fungi, cause destruction of insulating and protective qualities, and permit entrance of moisture that destroys parts and unbalances electrical circuits.
Metals High temperature and moisture vapor cause corrosion, etching of surfaces, and oxidation. This interferes with the operation of moving parts, screws, etc., and causes dust between terminals, capacitors, plates, or air conductors, etc., which in turn leads to noise, loss of sensitivity, and arc-over.
Metals, dissimilar Metals may have different corrosion potentials. When moisture is present, one of the metals (anode) corrodes.
Soldered joints Residual soldering flux on terminal boards holds moisture which speeds up corrosion and growth of fungi.
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206. TYPES OF CORROSION.
a. General. Many different forms of corrosion occur on avionics equipment, depending on the type of material, configuration of the materials, and their environment. The rate and magnitude of the corrosion attack is also dependent upon those same materials and environment.
A corrosion attack on equipment may involve several types of corrosion occurring simultaneously. This section describes the individual types of corrosion common to most avionics equipment.
b. Uniform Surface Corrosion. Uniform surface corrosion is probably the most common type of corrosion and results from a direct chemical attack on a metal surface. This type of corrosion appears uniform because the chemical elements that make up an alloyed metal are different and thereby become anodes and cathodes. These anodes and cathodes are very small and constantly shift from one area of the surface to another. A dull or etched surface is usually the first indication of uniform surface corrosion. Continued attack is followed by roughness and a frosty or powdered surface.
c. Crevice Corrosion. Crevice corrosion, also called concentration cell corrosion, occurs between the two mating surfaces in the presence of an electrolyte that has a different concentration/potential from one area to another. The electrolyte inside the crevice has a lower oxygen level and a higher metal ion concentration than the area just outside the crevice. As a result, the metal surfaces, even though they may be of the same material, have different potentials, and corrosion occurs. Figure 2-2 shows evidence of crevice corrosion on a lower fuselage skin at an antenna mounting. This type of corrosion may also occur when one of the mating surfaces is nonmetallic. There are three types of crevice/concentration cell corrosion:
metal ion concentration cells, oxygen concentration cells, and active passive cells.
(1) Metal Ion Concentration Cells. Stagnant electrolytes under mating/faying surfaces normally have a high concentration of metal ions compared to the metal just outside the mating/faying surface. The area of high metal ion concentration will be cathodic, while the area with a lower concentration will be anoxic and suffer corrosion.
(2) Oxygen Concentration Cells. Electrolytes normally contain dissolved oxygen.
Stagnant electrolytes under mating/faying surfaces contain less dissolved oxygen and are more anoxic than the adjacent area outside the mating surface. Corrosion occurs in the area of lower oxygen concentration.
(3) Active Passive Cells. Metals which depend on tightly adhering oxide films for corrosion protection, such as an anodized surface on aluminum, are prone to a rapid corrosion attack by active passive cells. An active passive cell occurs when the oxide film is broken from a scratch. The difference in potential between the small area of exposed parent metal (anoxic area) and the larger oxide film (cathodic area) is high and the onset of corrosion is rapid.
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d. Pitting Corrosion. Pitting corrosion is a severe form of concentrated cell corrosion and is localized to a specific area. Pitting corrosion can be found on thin sheets of metal, such as plated PCB paths. The attack can be so severe that perforation of the plated PCB paths can occur.
Pitting usually occurs along grain boundaries and at porous finished areas on the metal. Porous gold plating on copper contacts is a common location for pitting corrosion. The plating pores create small corrosion cells that continue to expand and deepen until a pit is created.
FIGURE 2-2. EVIDENCE OF CREVICE CORROSION ON LOWER FUSELAGE SKIN
AT AN ANTENNA MOUNTING
e. F retting Corrosion. Fretting corrosion occurs when there is slight relative movement between two materials (usually metals) and an electrolyte is present. This corrosion is typical of close fitting, highly loaded interfaces. Fretting corrosion can occur on all metals, with aluminum, stainless steel, and titanium alloys being the most susceptible. These metals depend on an oxide surface film to inhibit further corrosion. With small movements between the two mating surfaces under pressure, the exposed oxide surface film is abraded away, exposing new parent metal. This new metal surface oxidizes again and the cycle repeats. The oxides that were abraded away locate themselves in the crevices of the parent metal. The oxides are harder than the parent metal, and, as the abrasion continues, they will act as an abrasive grit which further attacks the parent metal.
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f. Galvanic Corrosion. Galvanic corrosion occurs when different metals are in contact with each other in the presence of an electrolyte. Galvanic corrosion is characterized by a buildup of corrosion deposits on the mating active (anoxic) surface. The rate of corrosion of a galvanic couple is a function of the difference between the reactivity of the metals. The galvanic series for metals with saltwater as the electrolyte is outlined in Table 2-3. The farther apart two metals are on the galvanic series chart, the faster the active (anode) metal will corrode in the presence of the electrolyte. In contrast, the closer the two metals are on the galvanic series chart, the slower the active (anode) metal will corrode in the presence of the electrolyte.