AC 43-206

407. Preservation.....................................................................................................................55 408. Lubricants........................................................................................................................59 409. Packaging, Handling, and Storage ..................................................................................59 410. thru 500. RESERVED ....................................................................................................62 CHAPTER 5. CORROSION REMOVAL, SURFACE TREATMENT, PAINTING, AND SEALING ............................................................................... 63 501. General ............................................................................................................................63 502. Corrosion Removal Materials and Equipment................................................................63 503. Surface Treatment ...........................................................................................................64 504. Protective Coatings .........................................................................................................67 505. Encapsulates....................................................................................................................72 506. thru 600. RESERVED ....................................................................................................85 CHAPTER 6. TREATMENT OF SPECIFIC AVIONICS EQUIPMENT ...................................87 601. General ............................................................................................................................87 602. Repair of Avionics Equipment Housings, Mounting Racks, and Storage Hardware.....................................................................................................87 603. Repair of Avionics Systems, Equipment, and Components ...........................................98 604. thru 700. RESERVED ..................................................................................................128 CHAPTER 7. CORROSION CONTROL MEASURES FOR ELECTRICAL BONDING/GROUNDING ..................................................................................129 701. Electrical Bonding/Ground Connections ......................................................................129 702. Bonding/Grounding Surface Preparation......................................................................130 703. thru 800. RESERVED ..................................................................................................138 CHAPTER 8. EFFECT AND TREATMENT OF CORROSION ON ELECTROMAGNETIC INTERFERENCE SHIELDING DEVICES.............................................................................................................139 801. Definition/Description of EMI......................................................................................139 802. EMI Standards, Test Requirements, and Documents ...................................................139 803. Aircraft System EMI Protection Requirements ............................................................140 804. Types of EMI Protection...............................................................................................142 805. EMI Protection Maintenance ........................................................................................147 806. thru 900. RESERVED ..................................................................................................150 Page ii 05/30/01 AC 43-206 Section Page CHAPTER 9. EFFECT AND TREATMENT OF CORROSION ON ELECTROSTATIC DISCHARGE SENSITIVE EQUIPMENT.........................151 901. Definition/Description of ESD .....................................................................................151 902. Sources of Static Charge ...............................................................................................151 903. Component Failure Modes............................................................................................153 904. Identification of ESD-Susceptible Equipment..............................................................154 905. ESD Protection Requirements ......................................................................................154 906. Corrosion Control Practices for ESD-Sensitive Devices..............................................158 907. thru 1000. RESERVED ................................................................................................158 CHAPTER 10. EMERGENCY ACTION FOR SERIOUS CORROSION OF AVIONICS EQUIPMENT ...........................................................................159 1001. General ........................................................................................................................159 1002. Emergency Reclamation Team ...................................................................................159 1003. Emergency Preparations..............................................................................................159 1004. Emergency Cleaning Procedures ................................................................................160 1005. Emergency Drying and Preservation...........................................................................162 1006. Operator Level Emergency Cleaning Procedures .......................................................163 1007. Avionics Repair Station Emergency Cleaning Procedures .........................................169 1008. thru 1100. RESERVED. .............................................................................................171 APPENDIX 1. CONSUMABLE SUPPLIES AND MATERIALS (12 pages) ...............................................................................................................1 TABLE 1-1. AVIONICS CLEANING AND CORROSION REMOVAL CONSUMABLE MATERIALS ..................................................................3 APPENDIX 2. SPECIFIC CONSUMABLE MATERIALS FOR CLEANING AND CORROSION PREVENTION AND CONTROL (8 pages) .................................................................................................................1 TABLE 2-1. AVIONICS CLEANING AND CORROSION REMOVAL CONSUMABLE MATERIALS………… ..........…………………………1 TABLE 2-2. MANUFACTURERS AND ADDRESSES .................................................5 APPENDIX 3. DEFINITION OF TERMS (8 pages)...................................................................................................................1 COMPREHENSIVE INDEX (12 pages).................................................................................................................1 Page iii AC 43-206 05/30/01 LIST OF ILLUSTRATIONS Figure Page 2-1. DUST AND DIRT ACCUMULATION ON TERMINAL STRIP ...............................11 2-2. EVIDENCE OF CREVICE CORROSION ON LOWER FUSELAGE SKIN AT AN ANTENNA MOUNTING ................................................................................15 3-1. DUST AND LINT ACCUMULATION IN THE CROWN AREA ..............................19 3-2. SURFACE CORROSION ON AN AVIONICS MOUNTING RACK.........................21 3-3. CORROSION AND LINT ACCUMULATION ON CARGO DOOR MICRO-SWITCH .......................................................................21 3-4. CORRODED HARDWARE ON TERMINAL BOARD..............................................23 3-5. CORRODED SHOCK MOUNT AND MOISTURE TRAP AREA.............................24 3-6. LINT ACCUMULATION ON AVIONICS RACK COOLING AIR MANIFOLD......25 3-7. CROWN AREA INSPECTION ....................................................................................26 3-8. CORRODED HARDWARE FOR BONDING STRAP ...............................................27 3-9. ANIMAL DROPPINGS ON ELECTRICAL COMPONENTS ....................................33 3-10. RODENT DROPPINGS IN BILGE AREA ..................................................................34 3-11. BIRD NEST IN ENGINE PYLON................................................................................35 3-12. LINT ACCUMULATION IN CROWN AREA ............................................................36 3-13. LINT ACCUMULATION ON ELECTRICAL CONNECTOR....................................36 3-14. LINT ACCUMULATION ON WIRING BUNDLE .....................................................37 3-15. LINT ACCUMULATION ON ELECTRICAL SWITCH.............................................37 4-1. NFPA 704 WARNING SYSTEM LABEL ...................................................................43 4-2. ENGINE COMPARTMENT ELECTRICAL CONNECTORS....................................51 4-3. ENGINE COMPARTMENT ELECTRICAL CONNECTORS....................................52 4-4. A WATER-BREAK FREE SURFACE COMPARED WITH ONE WITH BREAKS............................................................................................................53 4-5. TYPICAL SHIPPING CONTAINER FOR AVIONICS EQUIPMENT.......................61 4-6. TYPICAL SHIPPING CONTAINER FOR AVIONICS EQUIPMENT.......................62 5-1. CONFORMAL COATING REMOVAL ......................................................................81 6-1. ENGINE COMPARTMENT ELECTRICAL CONNECTORS....................................89 6-2. BATTERY COMPARTMENT.....................................................................................90 6-3. MOISTURE ENTRAPMENT AREA AND CORRODED FRAME, MOUNTING RACKS, AND SHOCK MOUNTS ........................................................92 6-4. TERMINAL BOARD INSTALLATION......................................................................94 6-5. LINT ACCUMULATION ON A SHELF .....................................................................97 6-6. LINT AND DEBRIS ACCUMULATION ON A SHELF ............................................97 6-7. CORRODED BONDING STRAP AND ATTACH HARDWARE .............................99 6-8. CORRODED ANTENNA MOUNTING AREA AND ELECTRICAL PLUG ............99 6-9. NONMETALLIC SEALANT REMOVAL TOOLS...................................................100 6-10. CORRODED ANTENNA MOUNTING AREA ........................................................100 6-11. BLADE ANTENNA INSTALLATION ......................................................................102 6-12. BLADE ANTENNA INSTALLATION ......................................................................102 6-13. FLUSH MOUNTED ANTENNA RADOME .............................................................104 6-14. MULTI-PIN CONNECTOR WITH “DOG BONES” INSTALLED ..........................122 6-15. EXTERNAL MULTI-PIN CONNECTOR CORROSION..........................................123 6-16. EXTERNAL MULTI-PIN CONNECTOR CORROSION..........................................124 Page iv 05/30/01 AC 43-206 Figure Page 6-17. CONNECTOR SEALING PROCEDURE ..................................................................125 7-1. STUD BONDING OR GROUNDING TO FLAT SURFACE ...................................131 7-2. NUT PLATE BONDING OR GROUNDING TO FLAT SURFACE ........................133 7-3. BOLT AND NUT BONDING OR GROUNDING.....................................................135 7-4. COPPER JUMPER CONNECTOR TO TUBULAR STRUCTURE..........................135 7-5. BONDING CONDUIT TO STRUCTURE .................................................................137 7-6. ALUMINUM JUMPER CONNECTOR TO TUBULAR STRUCTURE ..................137 8-1. TYPES OF ELECTROMAGNETIC INTERFERENCE ............................................141 8-2. TYPICAL METAL PARTICLE FILLED ELASTOMER GASKET INSTALLATION........................................................................................143 8-3. EMI SPRING FINGER INSTALLATION ON A DOOR...........................................144 8-4. BONDING USING BERYLLIUM COPPER PRESSURE-WIPER FINGERS .........145 8-5. CABLE OVERBRAID AND KNITTED WIRE MESH TAPE..................................146 9-1. TYPICAL ESD WORKSTATION .............................................................................155 9-2. TYPICAL ESD CAUTION LABELS .........................................................................157 LIST OF TABLES Table Page 2-1. EFFECTS OF AIRFRAME FLUID INTRUSION........................................................11 2-2. EFFECTS OF MOISTURE AND FUNGI ON VARIOUS MATERIALS ...................13 2-3. GALVANIC SERIES OF METALS AND ALLOYS IN SEAWATER.......................16 3-1. EFFECTS OF CORROSION ON AVIONICS EQUIPMENT......................................22 3-2. METALS MOST COMMONLY USED IN AVIONICS SYSTEMS...........................29 3-3. NATURE AND APPREARANCE OF CORROSION PRODUCTS ON METALS...30 3-4. NONMETALS MOST COMMONLY USED IN AVIONICS SYSTEMS ..................32 3-5. NATURE AND APPEARANCE OF DETERIORATION ON NONMETALS...........32 4-1. AVIONICS CLEANING MATERIALS .......................................................................40 4-2. RECOMMENDED CLEANING PROCESS VERSUS TYPE OF AVIONICS EQUIPMENT ...........................................................................44 4-3. CLEANING AND DRYING RESTRICTIONS............................................................48 4-4. PRESERVATIVE COMPOUNDS FOR AVIONICS EQUIPMENT...........................58 5-1. CAUSE AND PREVENTION OF PAINT DEFECTS .................................................70 5-2. CORROSIVE SILICONE SEALANTS, ADHESIVES, AND COATINGS ................80 5-3. REMOVAL METHOD PREFERENCE ON SPECIFIC COATINGS REMOVAL METHODS..........................................................................82 7-1. HARDWARE FOR STUD BONDING OR GROUNDING TO FLAT SURFACE...132 7-2. HARDWARE FOR NUT PLATE BONDING OR GROUNDING TO FLAT SURFACE..................................................................................................134 7-3. HARDWARE FOR BOLT-AND-NUT BONDING OR GROUNDING TO FLAT SURFACE ..................................................................136 9-1. TRIBOELECTRIC SERIES (partial) ..........................................................................152 9-2. TYPICAL PRIME CHARGE SOURCES...................................................................152 9-3. TYPICAL ELECTROSTATIC VOLTAGES .............................................................153 10-1. PRIORITY GUIDE FOR EMERGENCY REMOVAL OF AVIONICS EQUIPMENT ....................................................................................160 Page v (and vi) 05/30/01 AC 43-206 CHAPTER 1. GENERAL 1. PURPOSE. This advisory circular (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 FAA-approved maintenance program may also include the guidance contained in this AC in the development of such programs. 2. 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% 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 43-4A, 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. 3. MILITARY SPECIFICATIONS. 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. b. Military Specification 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. Par 1 Page 1 AC 43-206 05/30/01 c. Military Specification 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. 4. 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. Introduction. 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 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. Par 3 Page 2 05/30/01 AC 43-206 l. Appendix 2. Specific Consumable Materials for Cleaning and Corrosion Prevention and Control. This 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. 5. ENVIRONMENTAL CONCERNS. Federal and State laws concerning the environment and hazardous materials are constantly being revised and tightened. Additionally, local ordinances concerning the environment and hazardous materials 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. 6. 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. (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 work place, Par 4 Page 3 AC 43-206 05/30/01 Par 6 Page 4 (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 and 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. 7. thru 200. RESERVED. 05/30/01 AC 43-206 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: the different individual types of corrosion, simultaneous attack by several types of corrosion, and 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 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. (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. Par 201 Page 5 AC 43-206 05/30/01 (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 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. Par 202 Page 6 05/30/01 AC 43-206 (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 a 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. (5) Silver is used as a protective plating material over copper in wave guides, miniature and micro-miniature 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. Par 203 Page 7 AC 43-206 05/30/01 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, paragraph 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, paragraph 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. (8) Certain oils, especially silicones and greases, creep as temperature increases, causing contamination of organic coatings and attraction of dust. d. Moisture Intrusion. Moisture intrusion can take several forms: accidental dousing, or immersion; or normally as a gas in the form of water vapor (i.e., humidity), or finely divided droplets of liquid (i.e., mist or fog). The normal type of moisture often contains pollutants such as particulates, smog, industrial contaminants, and chlorides from salt laden air. Except for hermetically sealed or pressurized avionics equipment, most avionics equipment breathes. This allows the free passage of this polluted moisture in and out of the equipment. The polluted moisture collects on the internal electrical and structural components through condensation. The following are methods of minimizing moisture intrusion. (1) Hermetic Sealing. Hermetic sealing provides the greatest resistance to moisture intrusion by providing adequate seals (solder or glass fusion joints) on the equipment and filling the interior compartment with a dry inert gas. Hermetically sealed avionics equipment provides for a seal integrity check by a built-in visual indicator. Cooling requirements, size, and container penetrations will often preclude this type of moisture intrusion protection. Par 203 Page 8 05/30/01 AC 43-206 (2) Pressurization of Equipment. The next best avionics equipment moisture intrusion protection is through pressurization of the equipment or the equipment compartment. The introduction of pressurized dry air into a semi-sealed area greatly reduces the intrusion of moisture. However, the additional weight required to achieve the necessary equipment rigidity largely precludes the use of this method of moisture intrusion reduction. (3) Sealing of Equipment. For avionics equipment that cannot be hermetically sealed or pressurized, protection from moisture intrusion can best be achieved by applying a sealant. Two types of sealant are available. Current technology uses polysulfide and RTV silicone sealants. Polysulfide sealants are two part component sealants, whereas RTV sealants are single component. For avionics equipment, RTV sealants that have a vinegar odor are not authorized. This type of sealant contains an acetic acid used for curing the sealant. The acid will cause corrosion. e. Manufacturing Process. (1) Surface Treatments. Avionics equipment cases are manufactured from aluminum, and occasionally magnesium, because of weight considerations, cooling efficiency, and cost. Surface treatments, commonly called pre-treatments, are an important first step in the overall protection of the equipment. The most common surface treatments for aluminum are anodizing and chemical conversion coating. Anodizing is normally applied by the original avionics equipment manufacturer on interior and exterior surfaces. Chemical conversion coating is normally a pre-treatment process applied by repair facilities. The MIL-SPEC for chemical conversion coating is MIL-C-5541, Class II and Class III. Class II is used for aircraft structural components, avionics cases, and at any location where electrical bonding is not a consideration. Class III is an avionics grade chemical conversion coating and provides adequate electrical continuity while providing some corrosion protection. The pre-treatment for the repair of magnesium is a chemical conversion coat conforming to MIL-M-3171, Type VI. (2) Organic Coatings. The exterior paint system on avionics equipment consists of a primer and a topcoat. The primer promotes adhesion and contains corrosion inhibitors. The topcoat provides durability to the paint system, including weather and chemical resistance. Environmental concerns, as well as local and state air pollution regulations have implemented strict controls on the amount of volatile organic compounds (VOC) (solvent) contained in primers and topcoats. The majority of all new avionics equipment is painted with VOC compliant water-based primers and topcoats. (3) Conformal Coatings. Conformal coatings offer the same protection to electrical components within the avionics equipment as the primer and topcoat offer to the exterior of the same equipment. Conformal coatings are generally a clear plastic coating applied over the electrical components, conforming to MIL-SPEC MIL-I-46058. Conformal coatings offer several advantages: protection from moisture and corrosion, and enhanced resistance to shock and vibration. Generally, there is no field level repair for conformal coated components. (4) Plating Systems. Metal plating is used in avionics equipment to provide sacrificial protection, barrier protection, and as a neutral nonreactive metal between two dissimilar metallic surfaces. Gold and tin are the two most widely used plating metals. Gold plating requires another metal (base) to be plated under the gold, usually nickel, silver, or copper. A nonporous gold plating is required to eliminate galvanic corrosion with the base-plated metal. Special corrosion conditions can be set up between gold and silver-copper plating if damaged. This special galvanic corrosion condition for gold is referred to as red plague. A special galvanic corrosion condition, known as purple plague, can form between gold and aluminum in the presence of silicone. Par 203 Page 9 AC 43-206 05/30/01 204. ENVIRONMENTAL CONDITIONS. a. Within the Aircraft. Maintenance personnel often assume that, once installed within the aircraft cockpit, cabin, and/or equipment bays, avionics equipment will be protected from water or fluid intrusion and, therefore, free of significant corrosion. In-service use has shown that this is not the case. Airframe flexing combined with in-service use and handling damage can lead to seal deterioration and violation of the water tightness of the airframe and avionics equipment. b. Operational Environments. The operational environment of today’s aircraft consists of two main conditions: periods of in-service use and periods of non-operation. When avionics equipment is operating, the heat generated by the equipment tends to drive off or at least minimize moisture intrusion or entrapment. The ability of corrosion to start or continue will be reduced. Conversely, when avionics equipment is not in operation moisture can collect on the electrical components, increasing the opportunity for corrosion to start. 205. CORROSIVE CONDITIONS. a. General. Corrosion is a major contributing cause to the reduction of avionics equipment reliability. Moisture is the single most important contributor to corrosion in avionics systems. The following paragraphs discuss environmental factors that have a major effect in the amount of corrosion on the equipment. b. Moisture. Moisture can be either a gas, water vapor (humidity), or finely divided droplets of liquid (mist or fog). This moisture generally contains industrial pollutants, particulates, smog, and chlorides from salt laden air. Moisture enters all areas of an aircraft and avionics equipment exposed to air. All enclosed areas that are not sealed allow moisture laden air to enter and leave while the difference in pressure between the inside and outside of the equipment changes. These pressure differences occur when the aircraft changes altitude, when atmospheric weather changes occur, and when the temperature changes within the equipment. Cooling air can also introduce moisture into the equipment. c. Condensed Moisture. Airborne moisture will condense when the air temperature drops below the dew point or when the air comes in contact with any surface colder than the dew point temperature. Water droplets on the outside of a cold drinking glass are an example of this condition. Condensed moisture usually evaporates as the surrounding air warms. When this occurs, the contaminants that were brought in by moisture-laden air and distributed on the equipment during condensation are left behind. d. Residual Contaminants. Residual contaminants usually consist of industrial pollutants, dusts, and salts. Industrial pollutants include: carbon from internal combustion engines, nitrates from agricultural fertilizers, ozone from electrical motors and welding processes, sulfur dioxide from turbine engines, industrial and ship exhausts stacks, and sulfates from automobile exhausts. Dusts include sand, dirt, and volcanic ash. Figure 2-1 shows dust and dirt accumulation. The contaminants found in industrial areas often contain a number of tar products, ashes, and soot. The primary sources of salt are the world’s oceans. The oceans contain between 3.5% and 3.9% salt. Normal sea winds can carry from 10 to 100 pounds of sea salt per cubic mile of air inland up to 100 miles. e. Other Fluids. Many fluids can be present in various areas of an airframe. Table 2-1 lists the type of fluid intrusion and possible effects. Fluids can be from external sources, internal leaks, or servicing spills. Some of these fluids are corrosive to metals, while others are destructive to seals. Destruction of seals can lead to fluid intrusion into areas that were considered, by the design, protected from corrosive fluids. Par 204 Page 10 05/30/01 AC 43-206 FIGURE 2-1. DUST AND DIRT ACCUMULATION ON TERMINAL STRIP TABLE 2-1. EFFECTS OF AIRFRAME FLUID INTRUSION Type of fluid intrusion Effects or deterioration Engine fuel Softening or swelling of some polymers. Hydraulic fluid Reduced paint adhesion, and introduction of insulated films on electrical connector contact surfaces. Lubricants Attack some seal and gasket material. Dielectric Coolant Attacks organic seals. Anti-icing fluids Increased condensation and attacks on electrical wiring. Aqueous contaminates (free water, urine, condensation, desiccants) Increased condensation causing pooling of fluids in bilge areas; corrosive attack of unprotected dissimilar metal couples; introduction of insulated films on electrical contact surfaces. Maintenance fluids (solvents, detergents, cleaners, strippers) Softening and/or reduced adhesion of some organic coatings and cracking of insulation on some electrical wiring. Par 205 Page 11 AC 43-206 05/30/01 f. Temperature. High temperature may improve or impair the performance of avionics equipment, depending on other conditions. Corrosion and other harmful processes (outgassing, decomposition, etc.) increase as the temperatures rise. In some instances, moderate rises in temperature prevent condensation of moisture laden air. Most fungal growth is inhibited by temperatures above 104° F (40
C). Low temperatures pose no direct threat of corrosion, except that as temperature drops, relative humidity rises. Extremely low temperatures can cause shrinkage and embrittlement of seals and gaskets, resulting in leakage and fluid intrusion. g. Pressure. Most avionics equipment is designed to operate at low pressure (high altitude). Low pressure causes outgassing of plastics and other organic materials, which can change the physical and chemical properties of those materials. Another problem created by pressure is pressure cycling (high altitude/low altitude). These varying pressure changes can cause breathing from leaky seals and gaskets. Breathing promotes condensation of moisture laden air and creates a corrosive environment. h. Micro-organism, Insect, and Animal Attacks. Condensed moisture can cause conditions that promote the growth of mold, bacteria, and fungi. Once mold, bacteria, and fungi are established, they absorb and hold moisture. As living creatures, they secrete wastes which are acidic. This acid is a strong electrolyte which corrosively attacks metal. Non-metals, such as sealants, provide nutrients which can accelerate their growth. Many factors determine the degree of the fungal attack. Table 2-2 lists materials and the deterioration from microbial attack. Damage to avionics systems and equipment can also be caused by small insects and animals. This condition is most prevalent in tropical environments. Avionics equipment in storage is also susceptible to this condition. Insects and animals may enter through vent holes, open cabinets, or torn packaging material. Once inside the equipment, they can build nests which will hold moisture. This moisture plus excretions can cause corrosion. Another type of damage possible on the interior of the avionics equipment happens when an insect or animal eats electrical insulation, varnishes, or conformal coatings. This removes the environmental protection coating, allowing direct corrosive attack on the underlying surface. i. Man-made Environments. Man-made environments include repair station work, equipment handling, packaging, storage, and shipment. Avionics equipment undergoing repair can be contaminated by the surrounding environment. Fumes and vapors from adjacent repair operations such as soldering, paint spraying, and solvent cleaning can become trapped in the equipment. Failure to remove soldering flux residues after a repair can cause corrosion. Removal of avionics equipment from an aircraft for maintenance or inspection can expose the equipment to various environments. Removal of components from the equipment without proper protection from the environment can subject that component to corrosion. Improper shipping containers such as in wooden or fiberboard containers can subject that material to corrosion from vapors released by the container. Changes in temperature and pressure while en route can allow the intrusion of moisture. Finally, the improper storage of avionics equipment awaiting installation can cause corrosion. Storage of avionics equipment on wooden shelving can subject the equipment to vapors released by the shelving material. Par 205 Page 12 05/30/01 AC 43-206 TABLE 2-2. EFFECTS OF MOISTURE AND FUNGI ON VARIOUS MATERIALS Part of Material Effects of moisture and fungi Fiber: washers, supports, etc. Moisture causes swelling that can lead to misalignment and binding of parts. Destroyed by fungi. Fiber: terminal strips and insulators Electrical leakage paths are formed causing flashovers and crosstalk. Insulating properties are lost. Destroyed by fungi. Laminated plastics: terminal strips and boards, switchboard panels, etc., tube sockets and coil forms Insulating properties are lost. Leakage paths cause flashovers and crosstalk. Delamination occurs and fungi grow on surface and around edges. Expansion and contraction under extreme temperature changes. Molded plastics: terminal boards, switchboard panels, connectors, tube sockets and coil forms, etc. Machined, sawed, or ground edges of surfaces support fungi, causing shorts and flashovers. Fungi growth can reduce resistance between parts mounted on plastic to such an extent that the parts are useless. Cotton linen, paper, and cellulose derivatives: insulation covering webbing, belting, laminations, dielectrics, etc. Insulating and dielectric properties are lost or impaired, causing arcing, flashovers, and crosstalk. Destroyed by fungi. Wood: cases, houses and plastic fillers, masts, etc. Dry rot, swelling, and delaminated housings caused by moisture and fungi. Leather: straps, cases, gaskets, etc. Moisture and fungi destroy tanning and protective materials, causing deterioration. Glass: lenses, windows, etc. Fungi grow on organic dust, insect track, insect feces, dead insects, etc. Dead mites and fungi growth on glass obscure visibility and corrode nearby metal parts. Wax: for impregnation Fungi-inhibiting waxes that are not clean support the growth of fungi, cause destruction of insulating and protective qualities, and permit entrance of moisture that destroys parts and unbalances electrical circuits. Metals High temperature and moisture vapor cause corrosion, etching of surfaces, and oxidation. This interferes with the operation of moving parts, screws, etc., and causes dust between terminals, capacitors, plates or air conductors, etc., which in turn leads to noise, loss of sensitivity, and arc-over. Metals, dissimilar Metals may have different corrosion potentials. When moisture is present, one of the metals (anode) corrodes. Soldered joints Residual soldering flux on terminal boards holds moisture which speeds up corrosion and growth of fungi. Par 205 Page 13 AC 43-206 05/30/01 206. TYPES OF CORROSION. a. General. Many different forms of corrosion occur on avionics equipment, depending on the type of material, configuration of the materials, and their environment. The rate and magnitude of the corrosion attack is also dependent upon those same materials and environment. A corrosion attack on equipment may involve several types of corrosion occurring simultaneously. This section describes the individual types of corrosion common to most avionics equipment. b. Uniform Surface Corrosion. Uniform surface corrosion is probably the most common type of corrosion and results from a direct chemical attack on a metal surface. This type of corrosion appears uniform because the chemical elements that make up an alloyed metal are different and thereby become anodes and cathodes. These anodes and cathodes are very small and constantly shift from one area of the surface to another. A dull or etched surface is usually the first indication of uniform surface corrosion. Continued attack is followed by roughness and a “frosty” or powdered surface. c. Crevice Corrosion. Crevice corrosion, also called concentration cell corrosion, occurs between the two mating surfaces in the presence of an electrolyte that has a different concentration/potential from one area to another. The electrolyte inside the crevice has a lower oxygen level and a higher metal ion concentration than the area just outside the crevice. As a result, the metal surfaces, even though they may be of the same material, have different potentials, and corrosion occurs. Figure 2-2 shows evidence of crevice corrosion on a lower fuselage skin at an antenna mounting. This type of corrosion may also occur when one of the mating surfaces is nonmetallic. There are three types of crevice/concentration cell corrosion: metal ion concentration cells, oxygen concentration cells, and active passive cells. (1) Metal Ion Concentration Cells. Stagnant electrolytes under mating/faying surfaces normally have a high concentration of metal ions compared to the metal just outside the mating/faying surface. The area of high metal ion concentration will be cathodic, while the area with a lower concentration will be anoxic and suffer corrosion. (2) Oxygen Concentration Cells. Electrolytes normally contain dissolved oxygen. Stagnant electrolytes under mating/faying surfaces contain less dissolved oxygen and are more anoxic than the adjacent area outside the mating surface. Corrosion occurs in the area of lower oxygen concentration. (3) Active Passive Cells. Metals which depend on tightly adhering oxide films for corrosion protection, such as an anodized surface on aluminum, are prone to a rapid corrosion attack by active passive cells. An active passive cell occurs when the oxide film is broken from a scratch. The difference in potential between the small area of exposed parent metal (anoxic area) and the larger oxide film (cathodic area) is high and the onset of corrosion is rapid. d. Pitting Corrosion. Pitting corrosion is a severe form of concentrated cell corrosion and is localized to a specific area. Pitting corrosion can be found on thin sheets of metal, such as plated PCB paths. The attack can be so severe that perforation of the plated PCB paths can occur. Pitting usually occurs along grain boundaries and at porous finished areas on the metal. Porous gold plating on copper contacts is a common location for pitting corrosion. The plating pores create small corrosion cells that continue to expand and deepen until a pit is created. Par 206 Page 14 05/30/01 AC 43-206 FIGURE 2-2. EVIDENCE OF CREVICE CORROSION ON LOWER FUSELAGE SKIN AT AN ANTENNA MOUNTING e. Fretting Corrosion. Fretting corrosion occurs when there is slight relative movement between two materials (usually metals) and an electrolyte is present. This corrosion is typical of close fitting, highly loaded interfaces. Fretting corrosion can occur on all metals, with aluminum, stainless steel, and titanium alloys being the most susceptible. These metals depend on an oxide surface film to inhibit further corrosion. With small movements between the two mating surfaces under pressure, the exposed oxide surface film is abraded away, exposing new parent metal. This new metal surface oxidizes again and the cycle repeats. The oxides that were abraded away locate themselves in the crevices of the parent metal. The oxides are harder than the parent metal, and, as the abrasion continues, they will act as an abrasive grit which further attacks the parent metal. f. Galvanic Corrosion. Galvanic corrosion occurs when different metals are in contact with each other in the presence of an electrolyte. Galvanic corrosion is characterized by a buildup of corrosion deposits on the mating active (anoxic) surface. The rate of corrosion of a galvanic couple is a function of the difference between the reactivity of the metals. The galvanic series for metals with salt water as the electrolyte is outlined in Table 2-3. The farther apart two metals are on the galvanic series chart, the faster the active (anode) metal will corrode in the presence of the electrolyte. In contrast, the closer the two metals are on the galvanic series chart, the slower the active (anode) metal will corrode in the presence of the electrolyte. Par 206 Page 15 AC 43-206 05/30/01 TABLE 2-3. GALVANIC SERIES OF METALS AND ALLOYS IN SEAWATER CORRODED END (ANOXIC, ACTIVE, OR LEAST NOBLE) Magnesium Alloys Zinc (plate) Beryllium Cadmium (plate) Uranium (depleted) Aluminum Alloys Indium Tin (plate) Stainless Steel 430 (active) Lead 1010 Steel Cast Iron Stainless Steel 410 (active) Copper (plate) Nickel (plate) AM 350 (active) Chromium (plate) Stainless Steel 350, 310, 304 (active) Stainless Steel 430, 410 (passive) Stainless Steel 13-8, 17-7 PH (active) Brass, Yellow, Naval Stainless Steel 316L (active) Bronze 220 Copper 110 Stainless Steel 347 (active) Copper-Nickel 715 Stainless Steel 202 (active) Monel 400 Stainless Steel 201 (active) Stainless Steel 321, 316 (active) Stainless Steel 309, 13-8, 17-7 PH (passive) Stainless Steel 304, 301, 321 (passive) Stainless Steel 201, 316L (passive) Stainless Steel 286 (active) AM355 (active) Stainless Steel 202 (passive) Carpenter 20 (passive) AM355 (passive) Titanium Alloys AM350 (passive) Silver Palladium Gold Rhodium Platinum Carbon/Graphite PROTECTED END (CATHODIC, PASSIVE, OR MORE NOBLE) Par 206 Page 16 05/30/01 AC 43-206 Par 206 Page 17 (and 18) g. Stress Corrosion. Stress corrosion or stress corrosion cracking occurs when stresses on a metal part and corrosion combine to produce damage greater than either one applied separately. The stresses on the part can be internal (residual) or externally applied. Stress corrosion cracking of metal parts occurs along (intergranular) or across (transgranular) boundaries. Stress corrosion failures can be catastrophic and occur without warning. For example, cracking can occur in stressed copper alloys exposed to ammonia and its compounds. h. Corrosion Fatigue. Corrosion fatigue is normal fatigue combined with corrosion. Normal fatigue (in a non-corrosive environment) is caused by repeated stress cycles at a level below the maximum stress the material can withstand. The fatigue (endurance) limit is the maximum cyclic stress at which a material will not sustain any fatigue damage (will not fracture). The combination of corrosion and fatigue reduces the fatigue limit of a material. Corrosion fatigue will eventually produce a failure regardless of how minimal the applied stress. i. Intergranular Corrosion. Intergranular corrosion is a chemical attack that occurs at the grain boundaries of a metal. A highly magnified view of a metal surface shows individual grains. Along the grain boundaries of the primary metal are individual grains of the metallic elements that make-up the alloy. These other metals have a different corrosion potential than the primary metal. Often, the grain boundaries are anoxic and tend to corrode more easily than the grains of the primary metal. When an electrolyte is present, rapid selective corrosion at the grain boundary occurs. j. Exfoliation Corrosion. Exfoliation corrosion is an advanced form of intergranular corrosion. Exfoliation corrosion causes the metal grains to separate at the grain boundaries due to the force of the expanding corrosion products. This type of corrosion is most often visible as a swelling or lifting of an exposed edge in extruded sections of metal. It is primarily found in aluminum sheets around steel fasteners. k. Nonmetallic Deterioration. Nonmetallic materials also deteriorate. This deterioration includes: swelling, distortion, disintegration, cracking, out gassing, and changes in electrical characteristics. The deterioration is caused by a change in the environment of the avionics equipment. These changes include: weather, moisture intrusion/entrapment, heat, UV light, fungal growth, etc. 207. thru 300. RESERVED. 05/30/01 AC 43-206 CHAPTER 3. CORROSION CONTROL PROGRAM AND INSPECTION 30l. GENERAL. Investigations over the past ten years have revealed that corrosion is a major factor in avionics equipment failures. As much as 20% of commercial/general aviation avionics failures are attributable to corrosion, and that figure rises to between 30% and 40% for the military. This excessive failure rate continues despite continued improvement in the overall reliability of avionics equipment and systems. The scope of this problem outlines the need for an effective preventive maintenance program. FIGURE 3-l. DUST AND LINT ACCUMULATION IN THE CROWN AREA 302. PREVENTIVE MAINTENANCE PROGRAM. a. Program Requirements. Successful avionics cleaning and corrosion prevention/control efforts depend on a coordinated, comprehensive preventive maintenance program. Everyone involved with the operation, repair, and maintenance of avionics equipment must take an “all hands” approach to cleaning, inspection, and corrosion prevention and control. Figure 3-1 shows dust and lint accumulation in the crown area. The basic philosophy of a preventive maintenance program should consist of the following: (1) Personnel adequately trained in the recognition of corrosion, including conditions, detection and identification, cleaning, treatment, and preservation; (2) Thorough knowledge of corrosion identification techniques; (3) Proper emphasis on the concept of “all hands” responsibility for corrosion control; (4) Inspection for corrosion, deteriorated seals, and proper routing on a scheduled basis; (5) Routine cleaning of all wiring and the exterior surfaces of components; Par 301 Page 19 AC 43-206 05/30/01 (6) Keeping drain holes open; (7) Early detection and repair of damaged protective coatings; (8) Prompt corrosion treatment after detection; (9) Accurate record keeping and reporting of material or design deficiencies; and (10) Use of appropriate materials, equipment, and technical publications. b. Corrosion Prevention Philosophy. Corrosion and environmental conditions are natural phenomena that adversely affect avionics equipment. Although they can never be totally eliminated, the problems these phenomena cause can be minimized so that they are more manageable. This can be achieved only by understanding the equipment failure mechanisms, implementing a preventive maintenance program, and using corrosion control techniques and materials. 303. CORROSION-PRONE AREAS. a. General. There are certain corrosion-prone areas common to all aircraft. For example, bilge areas are particularly susceptible to moisture intrusion/entrapment. The bilge area is where cables run, and wire bundles, coaxial cables, lights, and antennas are installed. It is almost impossible to prevent moisture intrusion/entrapment in these areas. Other corrosion-prone areas include areas in and around the engine exhaust, battery compartments, lavatories, buffets, galleys, entrances at cargo and passenger doors, and wheel wells and landing gear. b. Moisture and Other Fluid Intrusion Sources. Moisture, fluid intrusion, and fluid movement within the aircraft are caused by many factors, including: (1) The flexibility of aircraft structure, which prevents complete and effective airframe sealing at skin joints and around fasteners; (2) Numerous openings, such as equipment bay doors, access panels, ducts, and static pressure sensors, which can allow moisture intrusion; (3) Moisture from many sources, such as rain and aircraft wash, and fluids from leaking hydraulic, fuel, oil, and coolant lines; and (4) Moisture and fluid migration within the aircraft along air condition ducts, fuel and hydraulic lines, and cable and wire bundles. As a result of these conditions, moisture and other fluids can enter, migrate, and be trapped in areas of the aircraft where avionics equipment may be installed, which are normally considered protected. Par 302 Page 20 05/30/01 AC 43-206 FIGURE 3-2. SURFACE CORROSION ON AN AVIONICS MOUNTING RACK FIGURE 3-3. CORROSION AND LINT ACCUMULATION ON CARGO DOOR MICRO-SWITCH Par 303 Page 21 AC 43-206 05/30/01 TABLE 3-1. EFFECTS OF CORROSION ON AVIONICS EQUIPMENT Component Failure mode. Antenna Systems Short circuits or changes in circuit constants and structural deterioration. Chassis, housings, covers, and mount frames Contamination, pitting, loss of finish, and structural deterioration. Shock mounts and supports Deterioration and loss of shock damping effectiveness. Control box mechanical and electrical tuning linkage, and motor contacts Intermittent operation and faulty frequency selection. Water traps Structural deterioration. Relays and switching systems Mechanical failure, short circuits, intermittent operation, and signal loss. Plugs, connectors, jacks, and receptacles Short circuits, increasing resistance, intermittent operation, and reduced system reliability. Multi-pin cable connectors Short circuits, increasing resistance, intermittent operation, and water seal deterioration. Power cables Disintegration of insulation and wire/connector deterioration. Display lamps and wing lights Intermittent operation, mechanical and electrical failures. Wave guides Loss of integrity against moisture, pitting, reduction of efficiency and structural deterioration. Fluid cooling system lines Failure of gaskets, pitting, and power loss. Printed circuits and microminiature circuits Short circuits, increased resistance, and component and system failures. Batteries High resistance at terminals, failure of electrical contact points, and structural deterioration of mounting. Erroneous cockpit signals. Busbars Structural and electrical failures. Coaxial lines Impedance fluctuations, loss of signal, and structural deterioration of connectors. c. Structural. Structural parts include housing covers, supports, brackets, cabinets, and chassis, which require structural support and equipment protection. Corrosion on these parts should be cleaned and treated and the protective finish restored to eliminate long-term deterioration. Severe corrosive damage usually results from damage to the protective finish and subsequent attack to the exposed metal. The protective finish is usually damaged from an environmental attack, handling damage, or microbial growth. Figure 3-2 shows surface corrosion on an avionics mounting rack. Severe corrosion damage requiring major repairs should be accomplished by an authorized repair station. Minor corrosion damage and damaged protective coatings are normally repaired by an authorized certificated person. d. Electromechanical. Motion is an integral function of electromechanical switches, relays, potentiometers, motors, generators, and synchronous components. Storage or non-operation in certain environments tends to promote corrosion of these devices. The principal causes of malfunction are dust, condensation products, resultant corrosion products (oxides), and organic contaminate films. Failure of these devices does not normally occur during operation. The friction generated during operation usually keeps the critical surfaces clean enough to permit operation. On the other hand, during non-operations, insulating films form which prevent startup operation of the equipment. Page 22 Par 303 05/30/01 AC 43-206 FIGURE 3-4. CORRODED HARDWARE ON TERMINAL BOARD e. Electronics. Moisture and contaminates penetrate electronic equipment, causing many detrimental effects, such as corrosion. In most modern electronic systems, circuit areas have been minimized for faster signal processing and higher density. In addition, integrated circuits use low voltage for operation. This means that most circuit paths are thin, or small in cross-sectional area, and that individual circuit paths are close together. In these systems, trace amounts of moisture and contaminates may cause systems failures. Table 3-1 lists the typical effects of corrosion on avionics equipment. f. Special Considerations. The control of corrosion in avionics equipment is like that in the airframe. Procedures used on the airframe are applicable to avionics with appropriate modifications. Figures 3-4, 3-5, and 3-6 show examples of corrosion moisture traps. The general differences in construction and procedures for corrosion control on airframe and avionics are as follow: (1) Avionics rely on less durable protection systems. (2) Very small amounts of corrosion can make avionics equipment inoperative, as compared to airframes. (3) Dissimilar metals are often in electrical contact. (4) Stray electrical currents can cause corrosion. (5) Active metals and dissimilar metals in contact are often unprotected. Par 303 Page 23 AC 43-206 05/30/01 (6) Vented avionics boxes can be subject to condensation due to normal temperature changes during flight. (7) Avionics systems have many areas that trap moisture. (8) Corrosion on internal components is difficult to detect in many avionics systems. (9) Many materials used in avionics systems are subject to attack by bacteria and fungi. (10) Organic materials are often used which, when overheated or improperly or incompletely cured, can produce vapors. These vapors are corrosive to electronic components and damaging to coatings and insulators. FIGURE 3-5. CORRODED SHOCK MOUNT AND MOISTURE TRAP AREA Par 303 Page 24 05/30/01 AC 43-206 FIGURE 3-6. LINT ACCUMULATION ON AVIONICS RACK COOLING AIR MANIFOLD 304. INSPECTION PROCESS. a. General. Frequent corrosion inspections are essential to the overall success of a corrosion control program. Through detection, identification, and treatment, the costs resulting from corrosion are minimized. Without regular systematic inspections, corrosion will seriously damage avionics equipment. The following paragraphs describe some of the basic aspects of visual inspection for corrosion and the telltale signs associated with various types of corrosion damage. b. Inspection Factors. Calendar-based “check” maintenance inspections should be in accordance with the manufacturer’s or operator’s instructions. However, extreme environmental and operating conditions should be considered when determining the frequency of corrosion inspections. The following are factors to consider when developing or using a local inspection interval: (1) Operational environment; (2) Known corrosion-prone areas, such as battery components and compartments, and electrical bonds; (3) Length of storage time, with respect to the storage environment and the avionics equipment or component; Par 304 Page 25 AC 43-206 05/30/01 FIGURE 3-7. CROWN AREA INSPECTION (4) The amount of time the equipment is nonoperational, especially for low usage aircraft or equipment; (5) Non-pressurized avionics components and equipment bays; (6) Antennas and externally mounted avionics packages; and (7) Avionics equipment mounted in susceptible water entrapment and intrusion entry areas. c. General Inspection Procedures. The following general procedures are recommended for an avionics corrosion inspection: (1) Clean area or component to be inspected. (2) Visually examine the overall condition of the equipment or component using an appropriate light source. Suspected areas or corrosion-prone areas should be examined using a l0X magnifying glass and an appropriate light source to determine the extent of corrosion or if other damage exists. Miniature or microminiature circuit boards or avionics components should be examined using the appropriate microscope and light source, as necessary. (3) Refer to applicable service manual for damage limits. Par 304 Page 26 05/30/01 AC 43-206 FIGURE 3-8. CORRODED HARDWARE FOR BONDING STRAP d. Water Intrusion Inspection. Maintenance personnel who routinely inspect the aircraft and equipment should inspect the interior of equipment bays for evidence of water intrusion and entrapment. Any evidence of water stains or entrapped water will require a thorough inspection of mounted equipment and the surrounding structures for corrosion. Additionally, the source of water intrusion will need to be determined and eliminated. The following are suggested areas and procedures for a water intrusion inspection: (1) Verify installation of required fasteners in avionics equipment, airframe, and surrounding structure. (2) Inspect condition of gaskets, form-in-place seals, and pre-formed seals. (3) Verify drain holes are open and clear in avionics equipment and airframe structure. (4) Inspect condition of doors, electronic rack drip shields, and covers and panels for material condition, especially warping. (5) Prepare the compartment for a water intrusion test by installing “witness material” such as blotter paper, paper towels, etc. The witness material should be placed in all suspected areas of water intrusion in a manner that will indicate the leakage path. (6) Close or secure compartment doors, panels, or covers. CAUTION: Do not direct the stream of water at components with bearings and shaft seals. Par 304 Page 27 AC 43-206 05/30/01 (7) Apply fresh water with a hose in a stream to the exterior surface of the airframe for approximately five minutes. (8) Allow approximately three to five minutes for the water to penetrate and drain. (9) Open or remove compartment doors, panels, or covers. (10) Visually examine “witness” material for signs of water and source of water intrusion. (11) Accomplish repairs in accordance with applicable repair service manual. (12) Close or secure compartment doors, panels, or covers. (13) Repeat water intrusion test, subparagraph (5) and subsequent, along with the appropriate repairs until water intrusion leaks are no longer observed. 305. RECOGNIZING AVIONICS CORROSION. a. General. Recognizing the appearance of corrosion or corrosion products for specific metals is an important part of an avionics corrosion prevention and control program. Metals are susceptible to corrosion because all metals have a tendency to return to their natural forms. For example, iron tends to return to iron oxide (rust). Avionics systems make use of many metals not normally considered for airframe structures. Some of the rarer metals are found in avionics components, in transistors, miniature and microminiature circuits, and integrated circuits. Table 3-2 lists the metals most often used in electronics and avionics components. In addition to recognizing the appearance of corrosion or corrosion products for specific metals, maintenance personnel must be able to recognize a corrosive attack from solder flux, microbes, insects, and animal attack. Table 3-3 describes the appearance of corrosion for specific metals and different corrosive attacks. b. Corrosion Effects on Metals. Deterioration (corrosion) of a metal is caused by a chemical reaction with its environment. The corrosive effect can be accelerated by many of the factors that were discussed in chapter 2. No metal can have a perfect environmental integrity, and therefore will corrode. Corrosion of a specific metal will take on many different forms as the corrosive attack progresses. The following paragraphs provide a description of corrosion with respect to the most commonly used metals in avionics systems. (1) Iron and Steel. Iron and steel are used in avionics components as leads, magnetic shields, transformer cores, brackets, racks, and general hardware. Some of these components are plated with nickel, tin, or cadmium. Corrosion of steel is easily recognized because the corrosion product is red or black iron oxide (rust). When iron based alloys corrode, dark corrosion products usually form first. This material will promote further attack by absorbing moisture from any water source, including ambient air. The most practicable means of controlling corrosion on non-plated steel or iron is complete removal of the corrosion product (rust) by the least harsh method. Iron and steel surfaces are normally protected by applying a plating system, paint system, or application of a preservative compound. (2) Corrosion Resistant Steel. Stainless steel is used for mountings, racks, brackets, and hardware in avionics systems. Stainless steel is not readily susceptible to corrosion because of a tough chromium oxide film on the surface. However, exposure to a salt water environment will cause pitting. Stainless steel is corrosion resistant, but is susceptible to crevice corrosion. The corrosion product of Par 304 Page 28 05/30/01 AC 43-206 stainless steel is a roughened surface with a red, brown, or black stain. Corrosion treatment for stainless steel to remove the red, brown, or black stain should be limited to cleaning with stainless steel wool or a stainless steel brush. (3) Aluminum Alloys. Aluminum and aluminum alloys are widely used in avionics systems for electrical connectors and back shells, cabinets, housings, chassis, structures, and mounting fixtures. Corrosion of aluminum and aluminum alloys is indicated by a white or gray powder (aluminum oxide). In most environments, especially moist salt-laden air, aluminum alloys are subject to many types of corrosive attack and therefore require protection. Aluminum surfaces are protected by an entire paint coating system composed of a chemical conversion coat, primer, and topcoat. Painted aluminum surfaces, even with the recommended protection system, tend to mask any corrosion attack to the aluminum. Corrosion damage to the aluminum surface will show up in the paint coating as filiform corrosion showing signs of blistering, flaking, chipping, lumping, or other irregularities. TABLE 3-2. METALS MOST COMMONLY USED IN AVIONICS SYSTEMS Aluminum *Gold *Platinum Antimony Indium *Rhodium Arsenic *Iridium Selenium Beryllium Iron *Silver Bismuth Lead *Stainless Steel (CRES) *Brass Lead-Tin Alloy Steel *Bronze Magnesium *Tantalum Cadmium Mercury *Tin Cobalt *Monel Tungsten *Copper *Nickel Germanium *Palladium * Usually considered corrosion-resistant (4) Magnesium. Magnesium alloys are used throughout avionics systems because of their light weight. Magnesium alloys can be found in antennas, component structures, chassis, supports, and frames. Magnesium is highly susceptible to corrosion when exposed to any environment without a protective coating. Magnesium forms a strong (anoxic) galvanic cell with every other metal and is always the metal that will suffer the corrosive attack. Magnesium is subject to almost all types of corrosion mentioned in chapter 2, and once started, the corrosive attack will spread rapidly. Corrosion on magnesium alloys is recognized by white, powdery, snow-like mounds. When corrosion is found on magnesium, prompt corrective action is required. Magnesium surfaces are protected from corrosive attack by a paint coating system composed of a chemical conversion coat, primer, and topcoat. (5) Copper. Copper and copper based alloys are used extensively in avionics systems. Copper and copper based alloys are used for wiring, contacts, springs, connectors, and printed circuit boards. Copper and copper based alloys (brass and bronze) are quite resistant to corrosion. Copper is cathodic to most of the other metals used in avionics equipment. The corrosive attack to copper and copper based alloys is a uniform surface tarnish of a green-gray color that remains relatively smooth. This uniform surface tarnish is the result of the formation of a fine-grained, airtight copper oxide. The copper oxide offers good protection from further corrosion attack to the underlying metal in ordinary situations. However, exposure to moist salt-laden air or salt spray causes the formation of blue-green salts which indicate an active corrosive surface. Copper and copper based alloys are generally not protected by a paint coating system. Par 305 Page 29 AC 43-206 05/30/01 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. (6) Cadmium. Cadmium is used primarily in avionics equipment as a plating on hardware (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 Par 305 Page 30 05/30/01 AC 43-206 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 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. The “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. (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. Par 305 Page 31 AC 43-206 05/30/01 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 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 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 ultraviolet light. The light source should conform to MIL-L-9909. Par 306 Page 32 05/30/01 AC 43-206 308. 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 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 308 Page 33 AC 43-206 05/30/01 FIGURE 3-10. RODENT DROPPINGS IN BILGE AREA Page 34 Par 309 05/30/01 AC 43-206 FIGURE 3-11. BIRD NEST IN ENGINE PYLON Par 309 Page 35 AC 43-206 05/30/01 FIGURE 3-12. LINT ACCUMULATION IN CROWN AREA FIGURE 3-13. LINT ACCUMULATION ON ELECTRICAL CONNECTOR Par 309 Page 36 05/30/01 AC 43-206 FIGURE 3-14. LINT ACCUMULATION ON WIRING BUNDLE FIGURE 3-15. LINT ACCUMULATION ON ELECTRICAL SWITCH Par 309 Page 37 AC 43-206 05/30/01 310. 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. thru 400. RESERVED. Par 310 Page 38 05/30/01 AC 43-206 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 material and Safety Data sheets for materials used. e. Safety equipment and protective personal equipment as required by local, state, and federal ordi- nances. 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 s