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
Par 309 Page 42 FIGURE 3-13. LINT ACCUMULATION ON ELECTRICAL CONNECTOR
Par 309 Page 43 FIGURE 3-14. LINT ACCUMULATION ON WIRING BUNDLE
Par 309 Page 44 FIGURE 3-15. LINT ACCUMULATION ON ELECTRICAL SWITCH
310.
EFFECTS OF DUST/LINT ACCUMULATION. Avionics equipment is subject to dust and lint accumulation. This condition is generally evident when the equipment has been installed for long periods of time and can become more severe with forced cooling air. In addition to the dust and lint accumulation from the movement of air, dust and lint can be attracted by magnetic fields from electric currents surrounding wiring and equipment. Dust can also accumulate on the surfaces of components and on the interior surfaces of components if that equipment has ventilating holes and louvers. The problem with accumulation of dust and lint is it will trap and hold moisture which can provide the electrolyte for corrosion and fungus growth (see Chapter 2, Paragraph 205, Corrosive Conditions). Additionally, dust and lint can degrade avionics equipment by being a conductor or an insulator. When dust and lint act as a conductor in the presence of moisture, they can provide a path for a current flow to either ground as a short or an unwanted circuit path between components. When dust and lint act as an insulator the avionics equipment can overheat causing premature failure. Severe accumulations of dust can appear as long stringy clumps, similar to Spanish moss hanging from trees, but on a smaller scale.
Figures 3-12 through 3-15 show examples of dust and lint accumulation. Frequent inspections and general cleaning of equipment will control the accumulation of dust and lint.
311 through 400. RESERVED.
Par 401 Page 45 CHAPTER 4. CLEANING AND PRESERVATION
401. GENERAL. The materials, equipment, and techniques described in this chapter are intended to assist the avionics technician at an avionics repair facility. This chapter discusses avionics cleaning and repair facility requirements, specialized support equipment to support cleaning, corrosion removal, drying of avionics equipment, and the different processes of cleaning, drying, preserving, packaging, handling, and shipping avionics equipment.
402. AVIONICS CORROSION CLEANING FACILITY. An avionics cleaning and repair facility should include as a minimum the following resources for the cleaning, drying, preserving, packaging, handling, and shipping of avionics equipment:
a. Adequate lighting and a temperature/humidity-controlled ventilation system.
b. Adequate space for safe operation of avionics cleaning and corrosion control equipment.
c. Operating instructions for each piece of equipment.
d. All hazardous materials (HAZMAT) and material safety data sheets (MSDS) for materials used.
e. Safety equipment and protective personal equipment as required by local, state, and Federal ordinances.
f. Personnel trained in the recognition of corrosion on avionics equipment as specified in this AC.
g. Personnel trained in the safe and proper operation of support equipment.
h. Quality assurance inspectors trained in the operational characteristics and restrictions of each piece of support equipment.
i. Avionics technicians who can recognize the various electrical and electronic equipment and components.
403. MATERIALS AND SUPPORT EQUIPMENT REQUIREMENTS.
a. General. Avionics technicians must understand the functions, capabilities, and restrictions associated with each piece of specialized support equipment, and the hazards of the cleaning and corrosion removal products and preservation materials. This knowledge will prevent injury to personnel and damage to avionics equipment and support equipment.
b. Materials. Consumable cleaning and corrosion removal products and preservation materials listed in Appendix 1 and Appendix 2 may be used as specified on avionics equipment if none were identified by the Original Equipment Manufacturer (OEM) or equipment operator.