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
Par 504 Page 78 applied, they will prolong the useful life of the base material by protecting it from corrosion and harmful agents. The application of a paint system involves three basic steps:
(a) Surface preparation;
(b) Application of primer or undercoat; and
(c) Application of one or more finish coats.
(2) Minor Paint Damage. Minor paint film damage generally occurs when maintenance personnel use hand and power tools on and around avionics equipment. Damage can also occur from equipment handling. The result of this damage is a protective finish that is chipped, scratched, or abraded. Short-term protection includes the application of a water-displacing corrosion preventive compound. Chapter 4 describes several water-displacing, corrosion preventive compounds that provide short and moderate term preservation. Long-term repair of a damaged protective coating is accomplished by touching up the paint system.
(3) Extensive Paint Damage. Extensive paint damage requires stripping of the old paint, evaluation for corrosion, cleaning, conversion coating for aluminum and magnesium, priming, and the application of one or two finish coats.
b. Cleaning and Surface Preparation. When the surface to be painted is contaminated, the paint system will not properly adhere. Almost all paint system adhesion failures such as peeling, flaking, etc., are the result of an improperly prepared (contaminated) surface.
Contaminants include oil, grease, dirt, moisture, or defective paint systems (loose or cracked paint). Proper cleaning and surface preparation includes removal of corrosion products and other contaminants. Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraph 502b, describe the equipment and procedures for cleaning and corrosion removal. Surface preparation (conversion coating) for aluminum and magnesium surfaces are described in paragraph 503.
Surfaces that are not to be painted should be masked with pressure-sensitive tape.
c. Painting Equipment and Materials. Painting equipment and accessory materials available for the application of protective paint coatings are listed in the following paragraphs.
(1) Painting Equipment.
(a) Spot Touch-up Spray Gun;
(b) Artist’s Air Brush;
(c) Paint Brush;
(d) Artist’s Paint Brush;
(e) Air Regulator and Metering Valve; and
(f) Paint Spray Booth.
Par 504 Page 79
(2) Painting Materials.
(a) Water-Reducible Epoxy Primer, Chemical and Solvent Resistant, conforming to MIL-P-85582. A low volatile organic compound (VOC) of not more than 340 g/1), environmentally compliant primer used to improve topcoat adhesion and provide a corrosion-inhibiting undercoating. This two-part primer kit should be mixed per the paint manufacturer’s instructions and applied over a properly prepared surface as described in Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraphs 502b and 503a.
NOTE: Local air pollution regulations may restrict the use of many primers, paint coatings, and thinners. Comply with all local air pollution regulations.
Most aircraft primers and topcoats are issued as two-part kits. Mix only the materials from the same two-part kit. The brand and batch numbers should be the same. Follow all mixing instructions provided by the paint manufacturer.
(b) Primer coating, Epoxy-polyamide, Chemical and Solvent Resistant, conforming to MIL-P-23377. A solvent-based primer used to improve topcoat adhesion and provide a corrosion-inhibiting undercoating. This two-part primer kit should be mixed per the paint manufacturer’s instructions and applied over a properly prepared surface described in Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraphs 502b and 503a.
(c) Epoxy-polyamide, conforming to MIL-C-22750, and aliphatic polyurethane, conforming to MIL-C-83286B. Several topcoats are available for the final or finish coat. The epoxy-polyamide and aliphatic polyurethane are solvent-based topcoats used to provide long-term protection to avionics equipment. An available environmentally compliant topcoat is Coating, Polyurethane, High Solids (VOC of 420 g/l), conforming to MIL-PRF-85285, Type 1. This topcoat is a water-reduced paint that provides long-term protection to avionics equipment. These topcoats are supplied as two-part kits and should be mixed per the paint manufacturer’s instructions. Selection of a topcoat color is normally based on the equipment’s location. Unless otherwise directed, refer to the OEM specification. The topcoat should be applied over a primed surface within 24 hours of the primer application. Applicable markings should be applied, using a stencil, with Lacquer, Acrylic Nitrocellulose, Lusterless, conforming to MIL-L-19538C in an appropriate color.
WARNING: Topcoats, Epoxy-Polyamide, MIL-C-22750; Polyurethane, Aliphatic, MIL-C-83286B; and Coating, Polyurethane High Solids, MIL-PRF-85285, Type 1, are toxic to the eyes, skin, and respiratory tract.
Skin and eye protection required. Avoid repeated or prolonged contact. Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
WARNING: Prolonged breathing of vapors from organic solvents or materials containing organic solvents is dangerous. Refer to the MSDS for warnings and required protective gear. Polyurethane, Aliphatic Thinner, MIL-T-81772 is flammable and toxic to the eyes, skin, and respiratory tract.
Skin and eye protection is required. Avoid repeated or prolonged contact.
Par 504 Page 80 Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
(d) Thinners are volatile solutions used to thin or reduce the paint topcoat to a desired consistency. The type of thinner that may be used, and the recommended quantity, are determined by the paint manufacturer and displayed on the paint container.
(3) Paint Problems. Certain discrepancies may appear on finish coatings due to faulty application methods or contamination of the surface to which the coating was applied. Table 5-1 lists the most common defects, probable causes, and prevention methods.
d. Application of Coatings. The methods used to paint, touch-up, or apply preservatives depends on the extent of the job, material, tools, facilities, and time available. Spraying is faster than other methods and results in a smoother surface finish. However, the time and labor required to set up spray equipment may not be justified by the amount of work to be done, or if extensive masking is required. Brushes are used where the use of spray guns is impractical, unsuitable, or not allowed.
(1) Paint Spray Booth. The paint spray booth should be used to conduct paint spray operations. The paint spray booth should, at a minimum, be equipped with an exhaust fan and filtration system capable of evacuating paint fumes and entrapping spray particles.
(2) The paint spray booth should comply with all applicable air pollution control requirements.
e. Application of Primer. Primers identified in subparagraph 504c are to be applied on avionics equipment as follows:
(1) The surface to be primed should be thoroughly cleaned and pre-treated per instructions in Chapter 4, paragraphs 403, 404, and 405, and Chapter 5, subparagraph 502b, which describe the equipment and procedures for cleaning and corrosion removal. Surface preparation (conversion coating) for aluminum and magnesium surfaces is described in paragraph 503.
(2) Mask all openings and areas not to be painted with pressure-sensitive tape and Kraft paper or other suitable barrier materials.
(3) Mix primer per manufacturer’s instructions. Thin (reduce) with applicable thinner as required.
Par 504 Page 81 TABLE 5-1. CAUSE AND PREVENTION OF PAINT DEFECTS Paint Application Defects Cause Prevention Webbing • Insufficient thinning
• Wrong thinning mixture
• Ensure sufficient mixing and thinning Dry Spots or Dulling - flattening out of the gloss in selective areas
• First coat applied too thin, especially in hot weather
• Poorly cleaned surface
• Wrong thinning mixture
• Ensure proper application
• Ensure proper preparation of the surface
• Ensure sufficient mixing and thinning Orange Peel - orange skin appearance
• Spraying at high viscosities
• Air pressure too high
• Poor spray technique, such as spraying too far from surface
• Ensure proper thinning
• Reduce air pressure
• Move closer to surface Blistering - looks like a bubble or swelled area in the paint film
• Trapped solvents
• Corrosion under the paint film
• Moisture in air supply line
• Prolonged exposure to high humidity
• Ensure sufficient drying time between coats
• Ensure proper surface preparation
• Ensure air supply line is free of water
• Avoid use of overly fast drying thinners when temperature is high Blushing - appears like milky gray cloud on paint film
• Occurs on humid days during or shortly after application
• Caused by condensation of moisture on the freshly painted surface when it is cooled to a temperature below the dew point.
The cooling can be caused by:
a. Evaporation of fast drying solvents
b. Air from the spray gun
c. Movement of item to area where the ambient temperature is below the dew point
• Apply paint only when paint area is warm enough to prevent cooling to dew point
• Do not subject item to ambient temperatures that are below the dew point until a minimum of 2 hours has elapsed after painting Fish-Eyed - appears like small crater-like openings just after spraying
• Oil or silicone materials in the air lines
• Dirty surfaces
• Ensure all equipment and surfaces to be painted are free of oil or silicones
• To remove fish-eyed paint, clean area with an approved solvent.
Blot dry using clean absorbent cloth. Repeat as necessary and repaint.