CONTENTS Paragraph Page
CHAPTER 1. GENERAL1
101. Purpose of this Advisory Circular (AC)1
102. Background1
103. Military Specifications (MIL-SPEC)1
104. Scope and Arrangement2
105. Environmental Concerns3
106. Safety3
CHAPTER 2. CORROSION PRINCIPLES AN D DESCRIPTION5
201. Overview5
202. Corrosion Theory5
203. Factors Influencing Corrosion6
204. Environmental Conditions10
205. Corrosive Conditions10
206. Types of Corrosion15
CHAPTER 3. CORROSION CONTROL PROGRAM AND I NSPECTION20
301. General20
302. Preventive Maintenance Program20
303. Corrosion-Prone Areas21
304. Inspection Process28
305. Recognizing Avionics Corrosion31
306. Corrosion Effects on Nonmetals36
307. Corrosion Effects of Solder Flux37
308. Effects of Microbial Attack37
309. Effects of Insect and Animal Attack37
310. Effects of Dust/Lint Accumulation44
CHAPTER 4. CLEANING AND P RESERVATION45
401. General45
402. Avionics Corrosion Cleaning Facility45
403. Materials and Support Equipment Requirements45
404. Avionics Cleaning Equipment50
405. Avionics Cleaning Procedures54
406. Drying Equipment and Procedures61
407. Preservation64
408. Lubricants68
409. Packaging, Handling, and Storage68
CHAPTER 5. CORROSION REMOVAL, SURFACE TREATMENT, PAINTING, AND SEA
LING73
501. General73
502. Corrosion Removal Materials and Equipment73
503. Surface Treatment74
504. Protective Coatings77
505. Encapsulates83
CHAPTER 6. TREATMENT OF SPECIFIC AVIONICS EQUIPMENT99
601. General99
602. Repair of Avionics Equipment Housings, Mounting Racks, and
Storage Hardware99
603. Repair of Avionics Systems, Equipment, and Components111
CHAPTER 7. CORROSION CONTROL MEASURES FOR E LECTRICAL
BONDING/GROUNDING148
701. Electrical Bonding/Ground Connections148
702. Bonding/Grounding Surface Preparation149
CHAPTER 8. EFFECT AND T REATMENT OF CORROSION ON ELECTROMAGNETIC
INTERFERENCE SHIELDING DEVICES159
801. Definition/Description of Electromagnetic Interference (EMI)159
802. EMI Standards, Test Requirements, and Documents159
803. Aircraft System EMI Protection Requirements160
804. Types of EMI Protection162
805. EMI Protection Maintenance168
CHAPTER 9. EFFECT AND T REATMENT OF CORROSION ON ELECTROSTATIC
DISCHARGE SENSITIVE EQUIPMENT172
901. Definition/Description of Electrostatic Discharge (ESD)172
902. Sources of Static Charge172
903. Component Failure Modes174
904. Identification of ESD-Susceptible Equipment175
905. ESD Protection Requirements176
906. Corrosion Control Practices for ESF-Sensitive Devices180
CHAPTER 10. EMERGENCY ACTION FO R SERIOUS CORROSION OF AVIONICS
EQUIPMENT181
1001. General181
1002. Emergency Reclamation Team181
1003. Emergency Preparations181
1004. Emergency Cleaning Procedures182
1005. Emergency Drying and Preservation184
1006. Operator Level Emergency Cleaning Procedures186
1007. Avionics Repair Station Emergency Cleaning Procedures192
APPENDIX 1. CONSUMABLE SUPPLIES AND MATERIALS (10 pages)1
APPENDIX 2. SPECIFIC CONSUMABLE MATERIALS FOR CLEANING AND
CORROSION PREVENTION AND CONTROL (3 pages)1
APPENDIX 3. DEFINITION OF TERMS (8 pages)1
INDEX (11 pages)1
LIST OF FIGURES
Figure 2-1. Dust and Dirt Accumulation on Terminal Strip12
Figure 2-2. E vidence of Crevice Corrosion on Lower Fuselage Skin at an
Antenna Mounting16
Figure 3-1. Du st and Lint Accumulation in the Crown Area20
Figure 3-2. S urface Corrosion on an Avionics Mounting Rack22
Figure 3-3. C orrosion and Lint Accumulation on Cargo Door Micro-Switch23
Figure 3-4. C orroded Hardware on Terminal Board25
Figure 3-5. C orroded Shock Mount and Moisture Trap Area27
Figure 3-6. L int Accumulation on Avionics Rack Cooling Air Manifold28
Figure 3-7. C rown Area Inspection29
Figure 3-8. C orroded Hardware for Bonding Strap30
Figure 3-9. A nimal Droppings on Electrical Components38
Figure 3-10. R odent Droppings in Bilge Area39
Figure 3-11. B ird Nest in Engine Pylon40
Figure 3-12. L int Accumulation in Crown Area41
Figure 3-13. L int Accumulation on Electrical Connector42
Figure 3-14. L int Accumulation on Wiring Bundle43
Figure 3-15. L int Accumulation on Electrical Switch44
Figure 4-1. N FPA 704 Warning System Label50
Figure 4-2. E ngine Compartment Electrical Connectors59
Figure 4-3. E ngine Compartment Electrical Connectors60
Figure 4-4. A Water-Break-Free Surface Compared with One with Water Breaks61
Figure 4-5. T ypical Shipping Container for Avionics Equipment70
Figure 4-6. T ypical Shipping Container for Avionics Equipment71
Figure 5-1. C onformal Coating Removal94
Figure 6-1. E ngine Compartment Electrical Connectors101
Figure 6-2. B attery Compartment102
Figure 6-3. Mo isture Entrapment Area and Corroded Frame, Mounting Racks,
and Shock Mounts104
Figure 6-4. T erminal Board Installation106
Figure 6-5. Lint Accumulation on a Shelf110
Figure 6-6. L int and Debris Accumulation on a Shelf111
Figure 6-7. C orroded Bonding Strap and Attach Hardware113
Figure 6-8. C orroded Antenna Mounting Area and Electrical Plug114
Figure 6-9. N onmetallic Sealant Removal Tools115
Figure 6-10. C orroded Antenna Mounting Area116
Figure 6-11. B lade Antenna Installation118
Figure 6-12. B lade Antenna Installation118
Figure 6-13. F lush Mounted Antenna Radome120
Figure 6-14. M ultipin Connector with “Dog Bones” Installed140
Figure 6-15. E xternal Multipin Connector Corrosion141
Figure 6-16. E xternal Multipin Connector Corrosion142
Figure 6-17. C onnector Sealing Procedure143
Figure 7-1. S tud Bonding or Grounding to Flat Surface150
Figure 7-2. N ut Plate Bonding or Grounding to Flat Surface152
Figure 7-3. Bolt and Nut Bonding or Grounding154
Figure 7-4. C opper Jumper Connector to Tubular Structure155
Figure 7-5. B onding Conduit to Structure157
Figure 7-6. A luminum Jumper Connector to Tubular Structure157
Figure 8-1. T ypes of Electromagnetic Interference161
Figure 8-2. T ypical Metal Particle Filled Elastomer Gasket Installation164
Figure 8-3. E MI Spring Finger Installation on a Door165
Figure 8-4. B onding Using Beryllium Copper Pressure-Wiper Fingers166
Figure 8-5. C able Overbraid and Knitted Wire Mesh Tape167
Figure 9-1. T ypical ESD Workstation177
Figure 9-2. T ypical ESD Caution Labels179
LIST OF TABLES
Table 2-1. Effects of Airframe Fluid Intrusion12
Table 2-2. E ffects of Moisture and Fungi on Various Materials14
Table 2-3. G alvanic Series of Metals and Alloys in Seawater18
Table 3-1. E ffects of Corrosion on Avionics Equipment24
Table 3-2. Met als Most Commonly Used in Avionics Systems33
Table 3-3. N ature and Appearance of Corrosion Products on Metals34
Table 3-4. N onmetals Most Commonly Used in Avionics Systems36
Table 3-5. N ature and Appearance of Deterioration on Nonmetals37
Table 4-1. A vionics Cleaning Materials47
Table 4-2. R ecommended Cleaning Process Versus Type of Avionics Equipment51
Table 4-3. C leaning and Drying Restrictions56
Table 4-4. P reservative Compounds for Avionics Equipment67
Table 5-1. C ause and Prevention of Paint Defects81
Table 5-2. C orrosive Silicone Sealants, Adhesives, and Coatings92
Table 5-3. R emoval Method Preference on Specific Coatings Removal Methods95
Table 7-1. H ardware for Stud Bonding or Grounding to Flat Surface151
Table 7-2. H ardware for Nut Plate Bonding or Grounding to Flat Surface153
Table 7-3. H ardware for Bolt-and-Nut156
Table 9-1. T riboelectric Series (Partial)173
Table 9-2. T ypical Prime Charge Sources174
Table 9-3. T ypical Electrostatic Voltages174
Table 10-1. P riority Guide for Emergency Removal of Avionics Equipment182
Par 101 Page 1 CHAPTER 1. GENERAL
101. PURPOSE OF THIS ADVISORY CIRCULAR (AC). This AC contains methods, techniques, and practices acceptable to the Administrator for inspection, prevention, control, and repair of corrosion on avionics systems and equipment. The procedures in this AC are an acceptable means, but not the only means, of inspecting, preventing, controlling, and repairing avionics corrosion. This AC is intended to supplement the Original Equipment Manufacturer’s (OEM) published recommendations, or for use when there are no OEM repair or maintenance instructions. Operators having their own Federal Aviation Administration (FAA)-approved maintenance program may also include the guidance contained in this AC in the development of such programs. The contents of this document do not have the force and effect of law and are not meant to bind the public in any way, and the document is intended only to provide information to the public regarding existing requirements under the law or agency policies.
102. BACKGROUND.
a. Today’s avionics systems assume a major responsibility for the performance, safety, and success of commercial and general aviation. These avionics systems control the operation of flight-critical and flight-essential equipment, including navigation, communications, power distribution, flight and engine controls, displays, and wiring. The reliability of these complex and often interrelated systems in any environment is critical for safe operation.
NOTE: In this AC, use of the term “avionics systems” shall refer to any device that uses or conducts electrical power.
b. Corrosion is a major cause of avionics equipment failures, particularly when the equipment is installed in the aircraft. Studies have shown that 20 percent of avionics equipment failures are a direct result of corrosion. Even minute amounts of corrosion can cause intermittent malfunctions or complete equipment failures. Past experience has shown that avionics equipment designers have compromised the corrosion resistance in their designs by selecting incompatible or corrosion-prone materials in order to obtain certain electrical characteristics. These compromises can lead to corrosion problems that are aggravated by the exposure to various environmental conditions, including changes in temperature, pressure, humidity, dust, dirt, and industrial pollutants in the atmosphere.
c. The types of corrosion that occur on avionics equipment are similar to those found on airframe structures. These different types of corrosion are discussed in Chapter 2 and in AC 43-4, Corrosion Control for Aircraft. The primary difference between avionics and airframe corrosion is that a small amount of corrosion in avionics equipment can cause intermittent malfunction or complete failure, while the same amount on the airframe structure usually has little or no immediate effect.
103. MILITARY SPECIFICATIONS (MIL-SPEC).
a. Throughout this AC, materials recommended to prevent, control, and repair corrosion on avionics equipment will be listed. The materials listed will be identified using Military Specifications (MIL-SPEC). Appendices 1 and 2 list MIL-SPEC materials and equivalent commercially available products.
Par 103 Page 2
b. MIL-SPEC materials have undergone rigorous testing and qualification at government laboratories, plus years of in-service use on military aircraft and equipment in the harshest environments. These products are capable of providing an acceptable means of preventing and controlling avionics corrosion.
c. MIL-SPEC materials are not the only materials available on the commercial market.
New materials are being developed by manufacturers each day. These products may also provide an acceptable means of preventing, controlling, and repairing avionics corrosion. These new products may eventually be qualified to a MIL-SPEC.
104. SCOPE AND ARRANGEMENT. This AC provides basic avionics corrosion prevention and control maintenance information for use by all segments of aviation. The AC consists of ten chapters, three appendices, and an index. Summary of the contents within this AC are as follows:
a. Chapter 1, General.
b. Chapter 2, Corrosion Principles and Description. This chapter explains what avionics corrosion is, why it occurs, and the various forms it can take. Special emphasis is placed on the conditions causing corrosion and the peculiar aspects of environmental damage and fungal growth that apply to avionics equipment.
c. Chapter 3, Corrosion Control Program and Inspection. This chapter outlines a preventive maintenance program, explains how to recognize corrosion, and lists components most affected by corrosion.
d. Chapter 4, Cleaning and Preservation. This chapter describes the materials, equipment, and techniques recommended in the mechanical cleaning and preservation of avionics equipment.
e. Chapter 5, Corrosion Removal, Surface Treatment, Painting, and Sealing. This chapter describes materials and techniques used in the removal of avionics corrosion, and treatments and coatings that can be applied to various external and internal avionics equipment.
f. Chapter 6, Treatment of Specific Avionics Equipment. This chapter describes the materials and techniques recommended to remove corrosion from specific avionics equipment.
g. Chapter 7, Corrosion Control Measures for Electrical Bonding/Grounding. This chapter describes the materials and techniques for repairing or replacing existing bonding and grounding connections.
h. Chapter 8, Effect and Treatment of Corrosion on Electromagnetic Interference Shielding Devices. This chapter describes the electromagnetic environment in which avionics systems and equipment operate. The chapter reviews protection measures and techniques used to minimize electromagnetic interference (EMI).
i. Chapter 9, Effect and Treatment of Corrosion on Electrostatic Discharge Sensitive Equipment. This chapter describes the basic theory surrounding Electrostatic
Par 104 Page 3 Discharge (ESD) and outlines some of the methods currently available to keep ESD from occurring.
j. Chapter 10, Emergency Action for Serious Corrosion of Avionics Equipment. This chapter outlines the recommended emergency procedures to be followed after avionics equipment has been exposed to fire extinguishing agents, water immersion, or saltwater.
k. Appendix 1, Consumable Supplies and Materials. This appendix lists corrosion control materials and applications.
l. Appendix 2, Specific Consumable Materials for Cleaning and Corrosion Prevention and Control. This appendix serves as a supplement to Appendix 1 by providing a more detailed listing of selected products by product numbers (P/N) and manufacturer(s) of acceptable consumable materials for avionics cleaning and corrosion prevention and control.
m. Appendix 3, Definition of Terms. Contains a list of defined terms commonly used by avionics corrosion control personnel.
n. Comprehensive Index. The index locates specific subjects in this AC.
105. ENVIRONMENTAL CONCERNS. Federal and State laws concerning the environment and hazardous materials (HAZMAT) are constantly being revised and tightened. Additionally, local ordinances concerning the environment and HAZMAT vary from location to location.
Therefore, throughout this AC, when a material is required for a specific task and that material is an ozone depleting substance (ODS), the specific material will not be mentioned. For example, if a specific solvent is required for a cleaning operation, the term “approved solvent” will be used in place of a specific solvent call out.
106. SAFETY.
a. The following general safety precautions are not related to any specific task and may not appear elsewhere within this AC.
(1) Keep away from live circuits. Always remove power from, discharge, and ground a circuit before touching it.
(2) Do not service or adjust equipment alone. Maintenance personnel should not reach into, adjust, or service equipment, except in the presence of other personnel who are capable of rendering aid.
(3) Personnel working with or near high voltages should be familiar with modern methods of cardiopulmonary resuscitation (CPR).
(4) Personnel working in noise hazardous areas should wear proper hearing protection, and not exceed time limits for exposures to various sound intensities. Have periodic hearing ability checks.
Par 106 Page 4
(5) Use safety shields and glasses when working with power equipment. Adequate shielding for eyes and face should be used at all times.
b. Warnings and cautions contained within this AC are intended to notify personnel of potential equipment hazards and damage. Warnings are used to alert personnel of potential personal safety and health hazards. Cautions are used to alert personnel of conditions which could result in damage to equipment and property.
c. Responsibilities of supervisory personnel. The supervisor should receive training in and be knowledgeable about:
(1) Recognition and elimination of hazards,
(2) Occupational safety and health laws,
(3) Providing a safe workplace,
(4) Accident investigation and reporting procedures, and
(5) Proper inspection and maintenance methods for personal safety and protective equipment.
d. Supervisors should also review material safety data sheets (MSDS) for characteristics and hazards of the materials employees may be exposed to and ensure that personnel use required protective equipment.
e. Maintenance personnel should use appropriate protective equipment while exposed to hazardous conditions to prevent accidents, injuries, and occupational illness. Maintenance personnel should not use personal safety and protective equipment that is not in satisfactory and serviceable condition. All personnel should review MSDS for characteristics of materials that they may be exposed to. All personnel should review and comply with occupational safety and health requirements.
f. Many of the materials and procedures outlined in this AC are potentially hazardous to personnel and can cause damage to aircraft and equipment if used improperly. When using maintenance chemicals such as paint strippers, detergents, solvents, conversion coatings, and paint, follow the correct procedures and read all warnings, cautions, and notes.
107 through 200. RESERVED.
Par 201 Page 5 CHAPTER 2. CORROSION PRINCIPLES AND DESCRIPTION
201. OVERVIEW.
a. Maintenance of avionics equipment requires knowledge of aircraft electrical/electronic systems corrosion control. This knowledge requires the definitions and descriptions of the mechanisms that cause corrosion in avionics equipment. The definition of corrosion is “a chemical or electrochemical deterioration of a material, usually a metal, because of a reaction with its environment.” This deterioration can be complex because of the nature of the following:
(1) The different individual types of corrosion,
(2) Simultaneous attack by several types of corrosion, and
(3) The design characteristics and maintenance factors that make avionics systems susceptible to corrosion attack.
b. Corrosion can cause intermittent malfunctions, undesirable changes in electrical characteristics, or complete equipment failures. Avionics equipment does not have to be installed, operated, or located in a particularly harsh environment to be affected by corrosion.
Some forms of corrosion will be active in near ideal environments. Corrosion is the natural process of materials returning to their natural state. Avionics maintenance personnel should recognize that corrosion never sleeps and once started, corrosion continues its attack 24 hours a day, 365 days a year. Inadequate corrosion prevention and control will ultimately affect the equipment in down time and overall system reliability. Described in this advisory circular (AC) are methods to prevent and control corrosion of avionics equipment.
202. CORROSION THEORY.
a. Definitions of Terms:
(1) Element. A basic pure chemical substance. There are over 100 elements in nature, such as metals (e.g., titanium, gold, iron) and nonmetals (e.g., hydrogen, sulfur, nitrogen).
(2) Atom. The smallest unit of an element which is made up of a nucleus (protons and neutrons), and orbiting electrons.
(3) Electron. A negatively charged particle which orbits the nucleus of an atom.
Electrons flow through an electrolyte only in the presence of ions.
(4) Ions. An atom that is either positively or negatively charged. A charged atom is called an ion. When ions move through an electrolyte, an electric current is produced. Ions cannot move through metal conductors.
(5) Compounds. Substances made up of two or more elements that chemically combine.
(6) Anode. A conductive metal that has a tendency to corrode.
Par 202 Page 6
(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.
Par 203 Page 8
(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.