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c. General Inspection Procedures. The following general procedures are recommended for an avionics corrosion inspection:
(1) Clean area or component to be inspected.
(2) Visually examine the overall condition of the equipment or component using an appropriate light source. Suspected areas or corrosion-prone areas should be examined using a 10x magnifying glass and an appropriate light source to determine the extent of corrosion or if other damage exists. Miniature or microminiature circuit boards or avionics components should be examined using the appropriate microscope and light source, as necessary.
(3) Refer to the applicable service manual for damage limits.
FIGURE 3-8. CORRODED HARDWARE FOR BONDING STRAP
d. W ater Intrusion Inspection. Maintenance personnel who routinely inspect the aircraft and equipment should inspect the interior of equipment bays for evidence of water intrusion and entrapment. Any evidence of water stains or entrapped water will require a thorough inspection of mounted equipment and the surrounding structures for corrosion. Additionally, the source of water intrusion will need to be determined and eliminated. The following are suggested areas and procedures for a water intrusion inspection:
(1) Verify installation of required fasteners in avionics equipment, airframe, and surrounding structure.
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(2) Inspect condition of gaskets, form-in-place seals, and pre-formed seals.
(3) Verify drain holes are open and clear in avionics equipment and airframe structure.
(4) Inspect condition of doors, electronic rack drip shields, and covers and panels for material condition, especially warping.
(5) Prepare the compartment for a water intrusion test by installing witness material, such as blotter paper, paper towels, etc. The witness material should be placed in all suspected areas of water intrusion in a manner that will indicate the leakage path.
(6) Close or secure compartment doors, panels, or covers.
CAUTION: Do not direct the stream of water at components with bearings and shaft seals.
(7) Apply fresh water with a hose in a stream to the exterior surface of the airframe for approximately five minutes.
(8) Allow approximately three to five minutes for the water to penetrate and drain.
(9) Open or remove compartment doors, panels, or covers.
(10) Visually examine witness material for signs of water and source of water intrusion.
(11) Accomplish repairs in accordance with applicable repair service manual.
(12) Close or secure compartment doors, panels, or covers.
(13) Repeat water intrusion test (subparagraph d(5) and subsequent), along with the appropriate repairs until water intrusion leaks are no longer observed.
305. RECOGNIZING AVIONICS CORROSION.
a. General. Recognizing the appearance of corrosion or corrosion products for specific metals is an important part of an avionics corrosion prevention and control program. Metals are susceptible to corrosion because all metals have a tendency to return to their natural forms. For example, iron tends to return to iron oxide (rust). Avionics systems make use of many metals not normally considered for airframe structures. Some of the rarer metals are found in avionics components, in transistors, miniature and microminiature circuits, and integrated circuits.
Table 3-2 lists the metals most often used in electronics and avionics components. In addition to recognizing the appearance of corrosion or corrosion products for specific metals, maintenance personnel must be able to recognize a corrosive attack from solder flux, microbes, insects, and animal attack. Table 3-3 describes the appearance of corrosion for specific metals and different corrosive attacks.
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b. Corrosion Effects on Metals. Deterioration (corrosion) of a metal is caused by a chemical reaction with its environment. The corrosive effect can be accelerated by many of the factors that were discussed in Chapter 2. No metal can have a perfect environmental integrity, and therefore will corrode. Corrosion of a specific metal will take on many different forms as the corrosive attack progresses. The following paragraphs provide a description of corrosion with respect to the most commonly used metals in avionics systems.
(1) Iron and Steel. Iron and steel are used in avionics components as leads, magnetic shields, transformer cores, brackets, racks, and general hardware. Some of these components are plated with nickel, tin, or cadmium. Corrosion of steel is easily recognized because the corrosion product is red or black iron oxide (rust). When iron-based alloys corrode, dark corrosion products usually form first. This material will promote further attack by absorbing moisture from any water source, including ambient air. The most practicable means of controlling corrosion on non-plated steel or iron is complete removal of the corrosion product (rust) by the least harsh method. Iron and steel surfaces are normally protected by applying a plating system, paint system, or application of a preservative compound.
(2) Corrosion Resistant Steel. Stainless steel is used for mountings, racks, brackets, and hardware in avionics systems. Stainless steel is not readily susceptible to corrosion because of a tough chromium oxide film on the surface. However, exposure to a saltwater environment will cause pitting. Stainless steel is corrosion resistant, but is susceptible to crevice corrosion.
The corrosion product of stainless steel is a roughened surface with a red, brown, or black stain.
Corrosion treatment for stainless steel to remove the red, brown, or black stain should be limited to cleaning with stainless steel wool or a stainless steel brush.
(3) Aluminum Alloys. Aluminum and aluminum alloys are widely used in avionics systems for electrical connectors and back shells, cabinets, housings, chassis, structures, and mounting fixtures.
Corrosion of aluminum and aluminum alloys is indicated by a white or gray powder (aluminum oxide).
In most environments, especially moist salt-laden air, aluminum alloys are subject to many types of corrosive attack and therefore require protection. Aluminum surfaces are protected by an entire paint coating system composed of a chemical conversion coat, primer, and topcoat. Painted aluminum surfaces, even with the recommended protection system, tend to mask any corrosion attack to the aluminum. Corrosion damage to the aluminum surface will show up in the paint coating as filiform corrosion showing signs of blistering, flaking, chipping, lumping, or other irregularities.
Par 305 Page 33 TABLE 3-2. METALS MOST COMMONLY USED IN AVIONICS SYSTEMS Aluminum *Gold *Platinum Antimony Indium *Rhodium Arsenic *Iridium Selenium Beryllium Iron *Silver Bismuth Lead *Stainless Steel (CRES) *Brass Lead-Tin Alloy Steel *Bronze Magnesium *Tantalum Cadmium Mercury *Tin Cobalt *Monel Tungsten *Copper *Nickel Germanium *Palladium *Usually considered corrosion-resistant
(4) Magnesium. Magnesium alloys are used throughout avionics systems because of their light weight. Magnesium alloys can be found in antennas, component structures, chassis, supports, and frames. Magnesium is highly susceptible to corrosion when exposed to any environment without a protective coating. Magnesium forms a strong (anoxic) galvanic cell with every other metal and is always the metal that will suffer the corrosive attack. Magnesium is subject to almost all types of corrosion mentioned in Chapter 2, and once started, the corrosive attack will spread rapidly. Corrosion on magnesium alloys is recognized by white, powdery, snow-like mounds. When corrosion is found on magnesium, prompt corrective action is required. Magnesium surfaces are protected from corrosive attack by a paint coating system composed of a chemical conversion coat, primer, and topcoat.
(5) Copper. Copper and copper-based alloys are used extensively in avionics systems. Copper and copper-based alloys are used for wiring, contacts, springs, connectors, and printed circuit boards.
Copper and copper-based alloys (brass and bronze) are quite resistant to corrosion. Copper is cathodic to most of the other metals used in avionics equipment. The corrosive attack to copper and copper-based alloys is a uniform surface tarnish of a green-gray color that remains relatively smooth. This uniform surface tarnish is the result of the formation of a fine-grained, airtight copper oxide. The copper oxide offers good protection from further corrosion attack to the underlying metal in ordinary situations.
However, exposure to moist salt-laden air or salt spray causes the formation of blue-green salts which indicate an active corrosive surface. Copper and copper-based alloys are generally not protected by a paint coating system.
Par 305 Page 34 TABLE 3-3. NATURE AND APPEARANCE OF CORROSION PRODUCTS ON METALS Alloy Type of attack to which alloy is susceptible Appearance of corrosion product Aluminum alloy Surface, pitting, and intergranular corrosion White or gray powder Titanium Highly corrosion resistant; extended or repeated contact with chlorinated solvents may result in embrittlement;
cadmium-plated tools can cause embrittlement of titanium No visible corrosion products Magnesium alloy Highly susceptible to pitting corrosion White powder, snow-like mounds, and white spots on surface Carbon and low alloy steel (1000-8000 series) Surface oxidation and pitting Reddish-brown oxide (rust) Stainless steel (300-400 series) Intergranular corrosion; some tendency towards pitting in a marine environment (300 series more corrosion-resistant than 400 series) Rough surface; may show red, brown, or black stain Nickel base alloy (Inconel) Generally has good corrosion-resistance qualities;
sometimes susceptible to pitting Green powdery deposit Copper base alloy (Monel) Surface and intergranular corrosion Blue or blue-green powder deposit Cadmium (used as a protective plating for steels) Uniform surface corrosion White to brown to black mottling of the surface Chromium (used as a wear-resistant plating for steels) Subject to pitting in chloride environments Chromium, being cathodic to steel, promotes rusting of steel where pits occur in the coating Silver Will tarnish in presence of sulfur Brown to black film Gold Highly corrosion-resistant Deposits cause darkening of reflective surfaces Tin Subject to whisker growth Whisker-like deposits Electroless nickel (used as a plating on aluminum connectors) Pitting and flaking of surface plating Nickel, being cathodic to Aluminum, does not corrode itself, but promotes corrosion of the aluminum base metal where pits occur in the plating.
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(6) Cadmium. Cadmium is used primarily in avionics equipment as a plating on hardware (e.g., nuts, bolts, etc.) and electrical connectors. It is also used to provide a compatible surface for parts in contact with other material. Cadmium, when plated over steel, is anoxic to the steel and protects the steel as a sacrificial coating. Corrosion on cadmium is evidenced by white to brown to black mottling of the surface. Cadmium plating on steel hardware is still protecting the steel until signs of rust on the steel appear. Care should be taken not to remove any of the cadmium plating adjacent to a rusted area on the base material.
(7) Silver. Silver is normally used as plating material over copper in wave guides, miniature and microminiature circuits, wiring, and contacts. It is also used on radio frequency (RF) shielding. Silver does not corrode in a normal sense, but it will tarnish in the presence of sulfur. The tarnish (silver sulfide) appears as a brown to black film. Corrosion treatment should be limited to cleaning.
(8) Red Plague. When silver plating over copper is damaged, there can be an accelerated corrosive attack of the underlying copper. Red plague is readily identifiable by the presence of a brown-red powder deposit on the exposed copper. In the case of wiring, the problem is compounded by the wire insulating material. The insulating material can prohibit detection of the damaged silver plating until the damage to the copper is extensive.
(9) Gold. Gold is the best plating material for electrical connections because of its corrosion resistance and the ease with which it can be soldered. Gold is used on printed circuits, semiconductor leads and contacts, and is usually plated over nickel, silver, or copper. Gold is a noble metal and does not normally corrode; however, a slight tarnish will appear as a darkening of its normally reflective surface. When gold is plated over silver or copper, accelerated corrosion can occur at pin holes or pores in the gold plating. This tarnishing of the gold over silver is readily identified by a brown to black film. The tarnishing of the gold over copper is identified as a blue-green film. The methods employed in tarnish removal are critical on gold-plated components because the plating is very thin (typically 0.00015 inch thickness).
(10) Purple Plague. Purple plague is a brittle gold-aluminum corrosion product formed when a gold-plated component and an aluminum component are mechanically attached or bonded together. Microelectronic circuit failures can occur at the interconnecting mechanical bond as this gold-aluminum corrosion product grows.
(11) Tin. The use of tin in solder is a well-known application. Tin is also used as a plating on RF shields, filters, crystal covers, and automatic switching devices. Tin is the best solder and corrosion-resistant coating of the available metallic coatings. The problem with tin is its tendency to grow whiskers on tin-plated wire and other tin-plated devices. Tin whiskers can grow to an extent that they will cause shorting across microelectronic circuits.
(12) Black Plague. Black plague is a black substance that forms in the liquid cooling systems of high power radars. This substance adheres to the walls of tubing and components in the cooling system and affects the heat transfer characteristics of the system. Corrosion removal for these components is the same as for the base metal.
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(13) Nickel. Nickel is primarily used as an electroless nickel coating and is subject to pitting corrosion. Flaking of the nickel coating can also occur when the underlying metal corrodes.
306. CORROSION EFFECTS ON NONMETALS. Deterioration of nonmetallic subassemblies and other hardware costs commercial and private operators millions of dollars per year in replacement material costs and loss of equipment availability. In most cases, the deterioration of the nonmetallic material permits the intrusion of moisture into the equipment.
This deterioration creates physical swelling, distortion, mechanical failure through cracking, altering of electrical characteristics, etc. The most common nonmetals used in avionics systems and the nature and appearance of their deterioration are listed in Tables 3-4 and 3-5 respectively.
TABLE 3-4. NONMETALS MOST COMMONLY USED IN AVIONICS SYSTEMS Acrylics Encapsulates Paper Adhesives Felt Plastics Asbestos Glass Polymers Ceramics Graphite Potting Compounds Cloth Laminates Primers Conformal Coatings Leather RTV Cork Lubricants Sealants Elastomers Paint Tapes
Par 306 Page 37 TABLE 3-5. NATURE AND APPEARANCE OF DETERIORATION ON NONMETALS Material Type of attack to which material is susceptible Appearance of deterioration Acrylics UV light, moisture, solvents Discoloration, cracking Adhesives Dirt, UV light, solvent, moisture Cracking, peeling Ceramic Extreme heat Discoloration, cracking Cloth Dry rot, mildew Discoloration, tears, dust Conformal coating Moisture, scratches Peeling, flaking, bubbling Cork Moisture, mildew, dry rot Discoloration, dust, peeling Elastomers Heat, UV light, excessive cycling Cracks, crazing, discoloration Encapsulant UV light, moisture Cracking, peeling, disbonding Felt Moisture, mildew Discoloration, looseness Glass Heat Cracks, discoloration Laminates UV light, moisture, solvents Discoloration, disbonding, delamination Paint Moisture, heat, humidity Bubbling, peeling, cracking Plastic UV light, heat, humidity Discoloration, cracks, deformation Polymers Extreme heat, solvents Discoloration, deformation Potting Compound UV light, moisture, heat Discoloration, cracks, deformation RTV (noncorrosive) Moisture, UV light, heat Peeling, disbonding, discoloration Sealants Moisture, UV light, heat Peeling, disbonding, discoloration
307. CORROSION EFFECTS OF SOLDER FLUX. Solder flux residues may be conductive and corrosive. They are often tacky, collecting dust which can absorb moisture and create current leakage paths. Solder flux resin appears as an amber-colored globule, drip, or tail at or near the solder joint. Under ultraviolet (UV) light, traces of flux appear as a fluorescent yellow to light brown residue. When soldering, use the lowest acid content flux possible (even neutral fluxes have some acid in order to remove metal oxides) and provide a reducing atmosphere to prevent oxide formation during soldering. After soldering, all flux residue must be completely removed by cleaning. Complete removal can be verified by the use of the UV light.
308. EFFECTS OF MICROBIAL ATTACK. Bacteria and fungi not only feed on organic material, but also release acids which are corrosive. Bacteria and fungi may be found on encapsulates, conformal coated circuit boards, rubber gaskets, thermoplastics, optical lenses, etc.
The presence of bacteria and fungi can be readily identified by damp, slimy, and bad-smelling growths. These growths vary in color from black, blue-green, and green to yellow.
309. EFFECTS OF INSECT AND ANIMAL ATTACK. Small insects and animals may enter packaged equipment and feed on various organic materials such as polyethylene and wire
Par 309 Page 38 insulation. This attack can result in system or equipment failure. The presence of nests, holes in packaging, and excrement indicate animal or insect attack. This problem is generally more severe in equipment that is in storage or has been out of service for a long period of time.
Figures 3-9, 3-10, and 3-11 show the effects of animals. Frequent inspection of the equipment or shipping containers is the best method of controlling this problem.
FIGURE 3-9. ANIMAL DROPPINGS ON ELECTRICAL COMPONENTS
Par 309 Page 39 FIGURE 3-10. RODENT DROPPINGS IN BILGE AREA
Par 309 Page 40 FIGURE 3-11. BIRD NEST IN ENGINE PYLON
Par 309 Page 41 FIGURE 3-12. LINT ACCUMULATION IN CROWN AREA