Par 407 Page 66
(5) Internal surfaces of tuned tanks;
(6) Relay and circuit breaker contacts; and
(7) Fuses.
f. Preservative Materials. Preservatives are materials that can take the place of more permanent coating materials such as paint, but require removal and repeated application on a scheduled basis. Some preservatives may also act as water-displacing compounds. Table 4-4 contains a list of preservatives and water-displacing compounds for use on avionics components.
Preservatives must be applied over water-displacing, corrosion preventive compound, MIL-PRF-81309, Type II, to accomplish a complete water-displacing and preservative system on all areas exposed to elements and moisture.
g. How to Preserve. The application of preservatives for specific avionics components is covered in Chapter 5. The following are general application procedures that apply in most cases:
(1) Ensure surface to be preserved is free of dirt, grime, other contaminants, standing water, and corrosion products. Remove corrosion products as outlined in Chapter 5. Clean and dry the surface as appropriate with a cleaner and dryer as outlined earlier in this chapter;
(2) When necessary, apply pressure-sensitive tape to protect areas and components not to be preserved. Subparagraph 407e refers to areas and components not to be preserved;
(3) Water-displacing preservative compounds that conform to MIL-PRF-81309, Type III, (Avionics), MIL-PRF-81309, Type II, and MIL-DTL-85054 (AMLGUARD) should be applied with an even, thin film to the surface. Ensure complete coverage of dissimilar metal contact areas, crevices, and water entrapments areas. Avoid excessive application. When MIL-DTL-85054 is used, apply a second coat after 30 minutes;
(4) Non-water-displacing preservative compound that conforms to MIL-PRF-16173, Grade 4 should receive a film of water-displacing preservative compound as described in subparagraph 407g(3) to the part or component prior to the application of MIL-PRF-16173, Grade 4, to remove any moisture. MIL-PRF-16173, Grade 4, should be applied with a brush or sprayed in a thin, even film to nonmoving external areas. Fasteners may be dipped in MIL-PRF-16173, Grade 4, prior to installation; and NOTE: MIL-PRF-16173, Grade 4, may be thinned for spraying as required with an approved solvent or a general purpose oil.
(5) Remove pressure-sensitive tape as required.
Par 407 Page 67 TABLE 4-4. PRESERVATIVE COMPOUNDS FOR AVIONICS EQUIPMENT Description Characteristics Application Restrictions Corrosion Preventive Compound, Water-Displacing, Ultra-Thin Film, MIL-PRF-81309, Type III, avionics Grade 2.
General preservative for avionics equipment, internal areas of electrical connectors, and solder receptacles and joints. Contains water-displacing properties.
Apply by spraying an even, thin film to the surface. Can be removed with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
Not intended for use on exterior of avionics equipment. Deposits a thin film which must be removed for proper function of contacts points and other electromechanical devices where no slipping or wiping action is involved. Do not use around oxygen, oxygen fittings, or oxygen regulators, since fire or explosion may result.
Corrosion Preventive Compound, Water-Displacing, Ultra-Thin Film, MIL-PRF-81309, Type II.
General preservative for internal and external areas of chassis, equipment covers, hardware, mounting brackets, latches, hinges, terminal boards, busbars, ground straps, and internal/external areas of junction boxes.
Apply by spraying an even, thin film to the surface. Can be removed with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
Do not use on interior surfaces of electrical connectors and receptacles.
Do not use on interior surfaces of coaxial connectors. Do not use around oxygen, oxygen fittings, or oxygen regulators, since fire or explosion may result.
Corrosion Preventive Compound, Water-Displacing, Clear, MIL-DTL-85054.
General preservative for external surfaces exposed to elements and moisture, including: chassis, equipment covers, mounting racks, equipment racks, shelving, brackets, radar plumbing, antenna hardware, latches, terminal boards, busbars, ground straps, junction boxes, fasteners, external connectors, coaxial connectors, and receptacles.
Apply by spraying an even thin film or brush onto the surface. Material presents a thin, non-tacky, clear film.
Can be removed with an approved solvent or dry cleaning solvent, MIL-PRF-680, Type II, Isopropyl Alcohol, TT-I- 735, or by applying Corrosion Preventive Water-Displacing Compound, MIL-DTL-85054, and wiping.
Do not use on interior surfaces of electrical equipment and hardware.
Do not use on moving/sliding surfaces. Do not use on interior surfaces of electrical connectors, coaxial connectors, or receptacles. Do not use around oxygen, oxygen fittings, or oxygen regulators, since fire or explosion may result.
Corrosion Preventive Compound, Solvent Cutback, Cold Application, MIL-PRF-16173, Grade 4.
General preservative for external surfaces exposed to elements and moisture, including: mounting racks, shelving, brackets, radar plumbing, shock mounts, rigid mounts, antenna hardware, general hardware, hinges, fasteners, ground straps, and exterior surfaces of electrical connectors, coaxial connectors, and receptacles.
Apply by brush or spraying an even, thin film to the surface.
Material presents a semi-transparent film.
Can be removed with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
Do not use on interior surfaces of avionics equipment. Do not use on interior of electrical connectors, coaxial connectors, or receptacles.
Do not use around oxygen, or oxygen regulators, since fire or explosion may result.
Par 408 Page 68
408. LUBRICANTS.
a. General. Lubrication of equipment performs several important functions: it prevents wear between moving parts, provides a barrier to corrosion, and is a short-term preservative.
Particular attention should be given to lubrication points around hinges, latches, etc., for prevention of lubrication breakdown. Lubrication breakdown includes caking of the lubricant, an indicator of contamination.
b. Requirements. Maintenance personnel should refer to the applicable maintenance manual for specific lubrication requirements, materials, and frequency.
409. PACKAGING, HANDLING, AND STORAGE.
a. General. An avionics corrosion control program must include procedures for packaging, handling, and storage of avionics equipment and components. These components will be rendered useless if the packaging, handling, and storage procedures are not followed.
Materials used for the packaging, handling, and storage must be compatible with the avionics equipment and the anticipated environment. Figures 4-5 and 4-6 show typical storage and shipping containers. These containers are not intended for storage in the elements.
b. Packaging and Storage Materials Guidelines.
(1) Certain packaging materials containing wood, cotton, foam, and paper are susceptible to mold and fungal attack. (See Chapter 3, paragraph 308.) These materials and other items such as shredded newspaper, excelsior, and cardboard may give off sulfurous or acidic vapors that can promote a corrosive attack on electronic components.
(2) Use only metal or preserved wooden shelves for storing avionics equipment and components. Inadequately preserved wood can produce corrosive vapors.
(3) Provide closed-cell polyethylene foam 1/2 inch thick as a cushioning for equipment on shelves, pallets, etc. Do not use an open-cell foam or sponge rubber, horse hair, or similar material that will hold moisture.
(4) Use conductive plastic or metal caps for electrical connector protection.
(5) Use cellular plastic film cushioning material (bubble wrap) for short-term protection during transportation of equipment and to protect against handling and shock damage.
(6) Never place bubble wrap in direct contact with ESD-sensitive devices. The electronic device or component should first be placed in a conductive bag. For equipment requiring ESD protection, see Chapter 9.
(7) Use plastic bags for short-term protection of noninstalled small components and microminiature PCBs against moisture and contamination.
(8) Use unicellular polypropylene packing foam and water vapor proof packaging material for long-term protection of miniature and microminiature circuit components, laminated
Par 409 Page 69 circuit boards, and critical avionics components against moisture and contamination. Figures 4-5 and 4-6 show typical shipping containers for avionics equipment.
c. Handling. Damage can occur to avionics equipment because of incorrect packaging methods and rough handling between the manufacturer, aircraft, and avionics repair facilities.
The best method of avoiding handling damage when transporting equipment is to use the specially designed cushioned shipping container from the OEM. A method that can be used if the original shipping container is not available is to use cellular plastic film cushioning material (bubble wrap). Bubble wrap is primarily used to absorb shock and is not intended as a water vapor proof packaging material. Bubble wrap should be placed around the component to be protected in two layers, with the second layer rotated 90 degrees from the original layer. The corners should be left open to allow the component and packing material to breath and avoid condensation and water entrapment. Secure the alternating layers of bubble wrap using masking tape or other pressure-sensitive tape.
d. Package and Storage. Packaging of avionics equipment is normally a function of a supply or receiving and shipping department. In many cases, the packaging of avionics equipment is accomplished by aircraft or avionics maintenance personnel. Packaging methods employed by these personnel are an important consideration for the equipment due to the length of time in storage awaiting repair or transit between aircraft and repair stations.
(1) Proper packaging should include provisions for the length of time the equipment will be in storage. Equipment should be packaged for long-term storage if the length of storage time is not known.
(2) Proper packaging materials, when specifically designed shipping containers are not available, should include: 6 mm thick (minimum) anti-static zip lock plastic-type bags. These bags are adequate for short-term storage and protection from moisture and contamination. They should be used during maintenance or repair operations on laminated circuit boards and small electrical/electronic components. They should also be used as part of a longer storage packaging and sealing system. For long-term storage, water vapor proof, heat-sealed barrier material provides excellent protection against moisture and contamination. Cushioning material such as unicellular polypropylene packaging foam should be used in conjunction with the plastic bags and water vapor proof, heat-sealed barrier material when protection from shock and handling is required.
(3) For equipment requiring ESD protection, see Chapter 9.
e. Electrical Connector and Wave Guide Caps. Metal caps and blank off plates, preferably moisture and vapor proof, are the preferred methods of protecting electrical connectors and wave guides from contamination and damage. Conductive plastic caps are another acceptable method of protection. Use only plastic caps which cover by surrounding the connectors or wave guide. Do not use push-in type caps or covers. These type caps or covers can easily be overlooked during assembly or become foreign object debris (FOD). When metal or conductive plastic caps and blank off plates are not available, electrical connectors and wave guides may be capped off with pressure-sensitive tape conforming to MIL-T-22085, Type II.
Other type tapes should not be used to seal or cap-off electrical connectors or wave guides.
Par 409 Page 70
f. Desiccants. Desiccants are normally packaged with equipment packaged for storage or shipment. The purpose of the desiccant is to absorb moisture and lower the relative humidity.
The following describes some of the problems and considerations for using desiccants.
(1) Desiccants may be ineffective for the following reasons:
(a) Moisture may condense as water when the desiccant becomes saturated.
(b) Desiccant is not placed in the proper location.
FIGURE 4-5. TYPICAL SHIPPING CONTAINER FOR AVIONICS EQUIPMENT (
c) Rapid changes in ambient temperature may produce precipitation before the desiccants can react.
(d) Not enough desiccant was used for the area to be protected.
(2) The following considerations apply to desiccants:
Par 409 Page 71 CAUTION: Do not use loose desiccants in packing of avionics equipment.
Loose desiccant may contaminate and cause damage to the packaged equipment.
(a) Desiccant material should be contained in rupture-resistant sturdy bags.
(b) Desiccant bags should be secured to prevent movement.
(c) Desiccant bags should not be placed on or permitted to come in contact with unprotected surfaces.
(d) Desiccants should be reactivated prior to reuse.
(e) Desiccant bags should not be removed from their sealed container until ready for use.
FIGURE 4-6. TYPICAL SHIPPING CONTAINER FOR AVIONICS EQUIPMENT
Par 409 Page 72
(3) If a desiccant bag should break open during transit, clean the avionics equipment immediately upon discovery. Do not turn moving parts any more than absolutely necessary until all desiccant particles have been removed. Work out the desiccant particles with a clean acid brush and not more than 10 psi dry air pressure. An alternate method is to use a clean acid brush and a vacuum cleaner. For equipment with ESD protection, see Chapter 9.
g. Humidity Indicators. Humidity indicators should be placed in containers when desiccants are used. A humidity indicator is used to determine if a desiccant is sufficiently active to maintain an acceptable relative humidity within the container.
CAUTION: Do not place humidity indicator in direct contact with metal.
Chemicals used in the indicator may cause corrosion.
410 through 500. RESERVED.
Par 501 Page 73 CHAPTER 5. CORROSION REMOVAL, SURFACE TREATMENT, PAINTING, AND
SEALING
501. GENERAL. This chapter outlines the materials, equipment, and techniques involved in corrosion control. Maintenance personnel should analyze each corrosion problem and select the correct corrosion removal and preservation materials. Where possible, follow-up actions should be conducted to ensure that all corrosion has been removed and proper protection has been applied. An avionics corrosion control program is an important function in maintaining any aircraft and aircraft component. The program requires knowledge of the science and technology of avionics corrosion control. Preventive maintenance must occur as part of all maintenance functions performed on avionics systems. Whenever equipment is removed from the aircraft for bench check or repair, covers and housings should be inspected and treated for corrosion.
Avionics technicians must ensure that corrosion, repair, treatment, and preventive maintenance becomes a normal part of their maintenance and repair procedure.
502. CORROSION REMOVAL MATERIALS AND EQUIPMENT.
a. General. Avionics technicians and repair stations should review Appendix 1 of this AC to convert military specification corrosion repair materials and equipment mentioned in these chapters to the commercial equivalent.
b. Corrosion Removal. When corrosion is detected, corrective action is required. When the corrosion is within repairable limits specified in the applicable Original Equipment Manufacturer’s (OEM) manual or other directive, corrective action should be initiated. This should consist of paint removal (as required), cleaning, corrosion removal, treatment, and the application of protective coatings and preservation. The mildest method of corrosion removal should always be used. The following paragraphs list approved methods for use on avionics equipment.
WARNING: Prolonged breathing of vapors from organic solvents or materials containing organic solvents is dangerous. Refer to the appropriate material safety data sheet (MSDS) for warnings and required protective gear. Chemical paint removers are toxic to the skin, eyes, and respiratory tract. Avoid breathing the vapors. Use only with adequate ventilation. Avoid skin and eye contact. Wear gloves and goggles while handling. If eye contact is made, wash immediately with large amounts of water. If skin contact is made, wash immediately with soap and water.
CAUTION: Epoxy paint removers are harmful to rubber and plastic products, including wiring insulation. Exercise care to avoid contact with such surfaces. Mask those adjacent areas which are not to be stripped with pressure-sensitive tape.
(1) Chemical Paint Removers. Epoxy paint removers conforming to QPL-81294 may be used to chemically remove paint from metal surfaces. This paint remover should be applied by brush. Care must be observed to ensure that the paint remover does not contact any part of the body. Observe all cautions and warnings. Whenever a chemical paint remover has been used,
Par 502 Page 74 ensure that the surface is washed with a detergent and water mixture and thoroughly rinsed prior to the application of other coatings. Properly dispose of hazardous wastes generated during the stripping process.
(2) Cleaning. Chapter 4, paragraph 405 outlines the materials and procedures for cleaning avionics equipment.
(3) Corrosion Removal Equipment and Methods.
(a) Hand Rubbing/Abrasion. The nature of some surfaces, such as chrome-, nickel-, gold- and silver-plated contacts, restricts the use of highly abrasive methods. Tarnish and light corrosion can be removed from such surfaces by hand rubbing with a pencil eraser, brushes, and nonabrasive pads. Surfaces such as covers, connectors, receptacles, antenna mounts, equipment racks, chassis, etc., may have light to moderate corrosion removed by an abrasive mat or cloth.
(b) Portable Mini-Abrasive Unit. The portable mini-abrasive unit is a handheld miniature abrasive tool used to remove light corrosion products from small avionics components, such as printed circuit boards (PCB), edge connector pins, small avionics structural components, etc. Care should be taken not to remove the thin plating from these surfaces. Monobasic sodium phosphate conforming to AWWA B504-18 is generally the specified abrasive material used in portable mini-abrasive units. Refer to the manufacturer’s manual for other possible material choices. The portable mini-abrasive unit should only be operated in a blast cleaning booth or other similar structure. Contamination of other equipment or components by monobasic sodium phosphate can occur if it is allowed to blow freely into the surrounding shop area.
(c) Handheld Abrasive Tools. The handheld abrasive tool is used to remove corrosion products from larger components such as avionics equipment structures and housings.
Glass beads conforming to MIL-G-9954 are generally the specified abrasive material used in handheld abrasive tools. Refer to the manufacturer’s manual for other possible approved abrasive material choices. This unit should also be operated only in a blast cleaning booth or other similar structure. Contamination of other equipment or components by glass beads can occur if they are allowed to blow freely into the surrounding shop area.
503. SURFACE TREATMENT.
a. Chemical Conversion Material Procedures. Chemical conversion is an extremely important part of the corrosion control process. Properly applied, chemical treatments impart considerable corrosion resistance to the basic metal and greatly improve the adhesion of subsequently applied paints.
WARNING: Chemical film materials are strong oxidizers and a fire hazard when in contact with organic materials such as paint thinners. Do not store or mix surface treatment materials in containers previously containing flammable products. Rags contaminated with chemical film materials should be treated as hazardous materials and disposed of accordingly.
Par 503 Page 75
(1) Treatment Application Particulars. The material for the treatment of aluminum alloys conforms to MIL-DTL-81706. Class 1A chemical conversion material provides superior corrosion protection. Class 3 chemical conversion material provides less corrosion protection and is used where low electrical resistance is required, such as mounting areas for antennas. The material for the treatment of magnesium alloys conforms to SAE-AMS-M-3171, Type VI. These chemical conversion materials may be in the form of dry crystals or pre-mixed. When dry crystals are provided, they must be mixed with an appropriate amount of distilled water. When the chemical conversion material is pre-mixed, no additional distilled water need be added.
(2) Distinguishing Between Magnesium and Aluminum. The methods to distinguish between magnesium and aluminum are as follows:
(a) Magnesium may be distinguished from aluminum by spot testing with a silver nitrate solution. When silver nitrate crystals are provided, dissolve a few crystals (approximately 1/4 teaspoon) of silver nitrate conforming to A-A-59282 in approximately one ounce of distilled water. When a pre-mix is provided, that solution is ready for use.
WARNING: Silver nitrate, A-A-59282, is corrosive and toxic to the eyes, skin, and respiratory tract. Skin and eye protection is required. Use only in a well-ventilated area.
(b) Select or make a clean, bare spot on the metal. Place one drop of solution on the spot. If that area turns black, the material is magnesium. Aluminum and its alloys will not show any reaction to silver nitrate solution.
b. Aluminum Surface Treatment. When an aluminum alloy needs to be surface treated, clean the surface to obtain a water break-free surface as described in Chapter 4, subparagraph 405m. Apply chemical conversion coating on the aluminum surface as follows:
NOTE: Metal portions of brushes should be wrapped with masking tape prior to use when applying conversion coating material in order to protect against contamination from the metal wrapping of the brush.
NOTE: Chemical conversion material conforming to MIL-DTL-81706 is the only chemical conversion treatment for aluminum used on avionics equipment.
WARNING: Chemical conversion material is flammable and toxic to the eyes, skin, and respiratory tract. Skin and eye protection is required. Avoid repeated or prolonged contact. Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
CAUTION: Chemical conversion solutions can become contaminated if brought in contact with glass containers, ferrous metals (other than 300-series stainless steels), or copper alloys. Stainless steel, polyethylene, and polypropylene containers should be used. Discard all contaminated solutions.
Par 503 Page 76
(1) Wet the surface with water.
(2) While the surface is still wet, apply the chemical conversion material using an artist’s brush. Continue to apply the chemical conversion material until an iridescent golden color is obtained. This usually takes 2 to 4 minutes.
(3) Once the iridescent golden color is obtained, immediately rinse the chemical from the surface with fresh water. Rinsing thoroughly is important to stop the chemical action and minimize solution entrapment. Failure to adequately rinse may accelerate corrosion and reduce paint adhesion.
(4) Remove all excessive chemical conversion material that may be trapped in pools within the aircraft or component.
(5) After thorough rinsing, allow the coated surface to air dry for a minimum of 30 minutes undisturbed. Do not wipe the area with a cloth or brush until dry, since premature wiping would remove the soft coating. The coating is soft until dry.
NOTE: As a chemical conversion solution approaches its shelf life, or at temperatures below 50 °F, more time may be required to form good films.
This is indicated by the proper golden color.
(6) Any difficulties in properly obtaining the iridescent golden color can usually be attributed to insufficiently cleaned metal surfaces, or to depleted or contaminated solution.
Chemical conversion materials generally have a shelf life of one year. Contaminated or overaged chemical conversion material may not form good films. Suspected overage chemical conversion material may still be used if the iridescent golden color can be obtained on the aluminum in under 4 minutes.
c. Magnesium Surface Treatment. When a magnesium alloy needs to be surface treated, clean the surface to obtain a water break-free surface as described in Chapter 4, subparagraph 405m. Apply chemical conversion coating on the magnesium alloy surface as follows:
WARNING: Chemical conversion material is flammable and toxic to the eyes, skin, and respiratory tract. Skin and eye protection is required. Avoid repeated or prolonged contact. Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
CAUTION: Chemical conversion solutions can become contaminated if in contact with glass containers, ferrous metals (other than 300-series stainless steels), or copper alloys. Stainless steel, polyethylene, and polypropylene containers should be used. Discard all contaminated solutions.
NOTE: Chemical conversion material conforming to SAE-AMS-M-3171 is the only chemical conversion treatment for magnesium used in avionics.
Par 503 Page 77
(1) Wet the surface with water.
(2) While the surface is still wet, apply the chemical conversion material using an artist’s brush. Continue to apply the chemical conversion material until a greenish-brown or brass-yellow color is obtained. This usually takes 1 to 5 minutes.
(3) Once the greenish-brown or brass-yellow color is obtained, immediately rinse the chemical from the surface with fresh water. Rinsing thoroughly is important to stop the chemical action and minimize solution entrapment. Failure to adequately rinse may accelerate corrosion and reduce paint adhesion.
(4) Remove all excessive chemical conversion material that may be trapped in pools within the aircraft or component.
(5) After thorough rinsing, allow the coated surface to air dry for a minimum of 30 minutes undisturbed. Do not wipe the area with a cloth or brush until dry, since premature wiping would remove the soft coating. The coating is soft until dry.
NOTE: As a chemical conversion solution approaches its shelf life, or at temperatures below 50 °F, more time may be required to form good films.
This is indicated by the proper greenish-brown or brass yellow.
(6) Any difficulties in properly applying chemical conversion materials can usually be attributed to insufficiently cleaned metal surfaces, or to depleted or contaminated solution.
Contaminated or overaged chemical conversion material may not form good films. Suspected overage chemical conversion material may still be used if the greenish-brown or brass-yellow color can be obtained on the magnesium in under 30 minutes.
d. Treatment of Other Metals. The treatment of other metals is limited to corrosion removal and cleaning.
e. Post-Treatment of Conversion Coated Surfaces. Before painting or applying sealant, allow chemical conversion coating to dry (usually 1 hour). The coating is soft until dry. Do not wipe the area with a cloth or brush until dry, since wiping will remove the soft coating.
504. PROTECTIVE COATINGS.
a. General. Protective coatings (generally paint) are susceptible to damage by handling, accidental scratching, and corrosion. The function of boxes, chassis, housings, and frames are to enclose, protect, and secure the vital internal components of the avionics unit. Therefore, with proper maintenance of the protective coating, the structural integrity and protection of the avionics unit can be maintained.
(1) Painted Surfaces. Painted surfaces on avionics equipment will withstand a normal amount of abrasion from handling and hand tools. When the painted surface becomes chipped, scratched, or scuffed, the loss of the protective coating allows the base metal to become more susceptible to corrosion. Maintenance personnel should pay particular attention to the use of tools around, and handling of, avionics equipment. When protective coatings are properly