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b. MIL-SPEC materials have undergone rigorous testing and qualification at government laboratories, plus years of in-service use on military aircraft and equipment in the harshest environments. These products are capable of providing an acceptable means of preventing and controlling avionics corrosion.
c. MIL-SPEC materials are not the only materials available on the commercial market.
New materials are being developed by manufacturers each day. These products may also provide an acceptable means of preventing, controlling, and repairing avionics corrosion. These new products may eventually be qualified to a MIL-SPEC.
104. SCOPE AND ARRANGEMENT. This AC provides basic avionics corrosion prevention and control maintenance information for use by all segments of aviation. The AC consists of ten chapters, three appendices, and an index. Summary of the contents within this AC are as follows:
a. Chapter 1, General.
b. Chapter 2, Corrosion Principles and Description. This chapter explains what avionics corrosion is, why it occurs, and the various forms it can take. Special emphasis is placed on the conditions causing corrosion and the peculiar aspects of environmental damage and fungal growth that apply to avionics equipment.
c. Chapter 3, Corrosion Control Program and Inspection. This chapter outlines a preventive maintenance program, explains how to recognize corrosion, and lists components most affected by corrosion.
d. Chapter 4, Cleaning and Preservation. This chapter describes the materials, equipment, and techniques recommended in the mechanical cleaning and preservation of avionics equipment.
e. Chapter 5, Corrosion Removal, Surface Treatment, Painting, and Sealing. This chapter describes materials and techniques used in the removal of avionics corrosion, and treatments and coatings that can be applied to various external and internal avionics equipment.
f. Chapter 6, Treatment of Specific Avionics Equipment. This chapter describes the materials and techniques recommended to remove corrosion from specific avionics equipment.
g. Chapter 7, Corrosion Control Measures for Electrical Bonding/Grounding. This chapter describes the materials and techniques for repairing or replacing existing bonding and grounding connections.
h. Chapter 8, Effect and Treatment of Corrosion on Electromagnetic Interference Shielding Devices. This chapter describes the electromagnetic environment in which avionics systems and equipment operate. The chapter reviews protection measures and techniques used to minimize electromagnetic interference (EMI).
i. Chapter 9, Effect and Treatment of Corrosion on Electrostatic Discharge Sensitive Equipment. This chapter describes the basic theory surrounding Electrostatic
Par 104 Page 3 Discharge (ESD) and outlines some of the methods currently available to keep ESD from occurring.
j. Chapter 10, Emergency Action for Serious Corrosion of Avionics Equipment. This chapter outlines the recommended emergency procedures to be followed after avionics equipment has been exposed to fire extinguishing agents, water immersion, or saltwater.
k. Appendix 1, Consumable Supplies and Materials. This appendix lists corrosion control materials and applications.
l. Appendix 2, Specific Consumable Materials for Cleaning and Corrosion Prevention and Control. This appendix serves as a supplement to Appendix 1 by providing a more detailed listing of selected products by product numbers (P/N) and manufacturer(s) of acceptable consumable materials for avionics cleaning and corrosion prevention and control.
m. Appendix 3, Definition of Terms. Contains a list of defined terms commonly used by avionics corrosion control personnel.
n. Comprehensive Index. The index locates specific subjects in this AC.
105. ENVIRONMENTAL CONCERNS. Federal and State laws concerning the environment and hazardous materials (HAZMAT) are constantly being revised and tightened. Additionally, local ordinances concerning the environment and HAZMAT vary from location to location.
Therefore, throughout this AC, when a material is required for a specific task and that material is an ozone depleting substance (ODS), the specific material will not be mentioned. For example, if a specific solvent is required for a cleaning operation, the term “approved solvent” will be used in place of a specific solvent call out.
106. SAFETY.
a. The following general safety precautions are not related to any specific task and may not appear elsewhere within this AC.
(1) Keep away from live circuits. Always remove power from, discharge, and ground a circuit before touching it.
(2) Do not service or adjust equipment alone. Maintenance personnel should not reach into, adjust, or service equipment, except in the presence of other personnel who are capable of rendering aid.
(3) Personnel working with or near high voltages should be familiar with modern methods of cardiopulmonary resuscitation (CPR).
(4) Personnel working in noise hazardous areas should wear proper hearing protection, and not exceed time limits for exposures to various sound intensities. Have periodic hearing ability checks.
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(5) Use safety shields and glasses when working with power equipment. Adequate shielding for eyes and face should be used at all times.
b. Warnings and cautions contained within this AC are intended to notify personnel of potential equipment hazards and damage. Warnings are used to alert personnel of potential personal safety and health hazards. Cautions are used to alert personnel of conditions which could result in damage to equipment and property.
c. Responsibilities of supervisory personnel. The supervisor should receive training in and be knowledgeable about:
(1) Recognition and elimination of hazards,
(2) Occupational safety and health laws,
(3) Providing a safe workplace,
(4) Accident investigation and reporting procedures, and
(5) Proper inspection and maintenance methods for personal safety and protective equipment.
d. Supervisors should also review material safety data sheets (MSDS) for characteristics and hazards of the materials employees may be exposed to and ensure that personnel use required protective equipment.
e. Maintenance personnel should use appropriate protective equipment while exposed to hazardous conditions to prevent accidents, injuries, and occupational illness. Maintenance personnel should not use personal safety and protective equipment that is not in satisfactory and serviceable condition. All personnel should review MSDS for characteristics of materials that they may be exposed to. All personnel should review and comply with occupational safety and health requirements.
f. Many of the materials and procedures outlined in this AC are potentially hazardous to personnel and can cause damage to aircraft and equipment if used improperly. When using maintenance chemicals such as paint strippers, detergents, solvents, conversion coatings, and paint, follow the correct procedures and read all warnings, cautions, and notes.
107 through 200. RESERVED.
Par 201 Page 5 CHAPTER 2. CORROSION PRINCIPLES AND DESCRIPTION
201. OVERVIEW.
a. Maintenance of avionics equipment requires knowledge of aircraft electrical/electronic systems corrosion control. This knowledge requires the definitions and descriptions of the mechanisms that cause corrosion in avionics equipment. The definition of corrosion is “a chemical or electrochemical deterioration of a material, usually a metal, because of a reaction with its environment.” This deterioration can be complex because of the nature of the following:
(1) The different individual types of corrosion,
(2) Simultaneous attack by several types of corrosion, and
(3) The design characteristics and maintenance factors that make avionics systems susceptible to corrosion attack.
b. Corrosion can cause intermittent malfunctions, undesirable changes in electrical characteristics, or complete equipment failures. Avionics equipment does not have to be installed, operated, or located in a particularly harsh environment to be affected by corrosion.
Some forms of corrosion will be active in near ideal environments. Corrosion is the natural process of materials returning to their natural state. Avionics maintenance personnel should recognize that corrosion never sleeps and once started, corrosion continues its attack 24 hours a day, 365 days a year. Inadequate corrosion prevention and control will ultimately affect the equipment in down time and overall system reliability. Described in this advisory circular (AC) are methods to prevent and control corrosion of avionics equipment.
202. CORROSION THEORY.
a. Definitions of Terms:
(1) Element. A basic pure chemical substance. There are over 100 elements in nature, such as metals (e.g., titanium, gold, iron) and nonmetals (e.g., hydrogen, sulfur, nitrogen).
(2) Atom. The smallest unit of an element which is made up of a nucleus (protons and neutrons), and orbiting electrons.
(3) Electron. A negatively charged particle which orbits the nucleus of an atom.
Electrons flow through an electrolyte only in the presence of ions.
(4) Ions. An atom that is either positively or negatively charged. A charged atom is called an ion. When ions move through an electrolyte, an electric current is produced. Ions cannot move through metal conductors.
(5) Compounds. Substances made up of two or more elements that chemically combine.
(6) Anode. A conductive metal that has a tendency to corrode.
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(7) Cathode. A dissimilar conductive material (usually a metal) which has less tendency to corrode.
(8) Electrolyte. A conductive liquid (usually water) that contains ions in solution. For example, saltwater is an electrolyte made up of water containing sodium and chlorine ions. The electrolyte solution is capable of carrying an electric current between the anode and cathode.
(9) Electron Conductor. Electrical contact between the anode and the cathode; for example, the different elements that make up an alloyed metal, or a fastener holding two pieces of metal (anode and cathode) together.
(10) Galvanic Couple. A cell consisting of two dissimilar metals (an anode and a cathode) in contact with each other through an electrolyte solution.
b. When a metal corrodes, electrons are lost from the atoms in the metal part, and these atoms become metal ions in the electrolyte. Once in solution, positively charged metal ions can combine with the negatively charged ions to form corrosion products.
c. A metal will corrode only when all four of the following exist: there is an anode, there is a cathode, there is an electrolyte containing ions, and the anode and cathode are connected by an electron conductor. Elimination of any one of these four items will slow the corrosion process.
203. FACTORS INFLUENCING CORROSION.
a. Basic Design. There are many design decisions and compromises to be made in the course of developing avionics equipment. The design specifications leave room for a wide range of engineering practices to meet not only the performance, cost, and schedule, but also the reliability and maintainability requirements. Each piece of avionics equipment is designed to withstand its intended operational environment. However, some design compromises have to be made to provide the unique electrical, mechanical, and thermal characteristics of the equipment.
These compromises can cause the equipment to be vulnerable to corrosion, especially during inoperative periods. Good design practices include:
(1) Shoe Box Type Lid Construction. When access to the equipment is from the top, use a shoe box type lid construction. Fasteners securing the shoe box lid should be from the sides and not through the top.
(2) Limited Openings in the Equipment Housing. To minimize moisture intrusion, keep the number of penetrations into the equipment to a minimum. When penetration is required, use “O” rings and gaskets for sealing. For wiring entry, use “L” type electrical connectors and mount them horizontally (through the vertical sides), and well above the bottom of the housing.
(3) Proper Electrical Connector Mounting. Electrical and coaxial connectors should be mounted horizontally (through the vertical sides). When electrical and coaxial connectors are mounted on the top of the equipment, there should be a raised area on the upper side of the equipment where the connector is mounted, and an “L” type connector should be used. Electrical
Par 203 Page 7 wiring should incorporate a drip loop into the wiring harness so the wiring is leading up to the connector.
(4) Proper Printed Circuit Board Mounting. Printed circuit board should be mounted vertically with the electrical connection also in a vertical position.
(5) Low Point Drains. Low point drains should be incorporated so that any moisture will drain from the equipment when the aircraft is in the flight position and when it is parked on the ground.
(6) Eliminating Moisture Traps. Avoid moisture traps or bathtub areas on the interior areas of the equipment. Design in drain paths to the low point drains. Avoid moisture traps in electrical wire bundles where anti-chaffing material or boots are incorporated.
(7) Cooling Air Systems. Cooling air systems should incorporate a system to remove moisture and particulate matter from the conditioned air. This is especially important when the conditioned air is forced directly towards active electronic elements.
(8) Proper Bonding and Grounding. Electrical bonding and grounding should be accomplished using straps rather than sliding housing-to-rack or tapered pin on housing-to-rack electrical contacts. Straps should be located for ease of maintenance and properly sealed because of the dissimilar metal (galvanic) couple.
(9) Proper Equipment Mounting. Avionics equipment should be mounted in such a manner that will allow sufficient airflow around the equipment and keep the equipment at least 1/2 inch above the compartment floor.
b. Material Selections and Uses. Proper material selection is critical for protecting avionics equipment against the environment. Many types of corrosion that occur in avionics equipment also occur in the airframe structure. However, the range of material call outs in avionics equipment is greater than in the airframe. Several new types of corrosion problems are therefore unique to avionics equipment. The following indicate the uses of different materials in the construction of various electrical and electronic components.
(1) Copper and copper-based alloys are generally used in avionics systems as contacts, springs, leads, connectors, printed circuit boards (PCB), conductors, and wire.
(2) Iron and steel are used as component leads, magnetic shields, transformer cores, brackets, racks, and general hardware.
(3) Magnesium alloys are used extensively throughout avionics systems as antenna structures, chassis, supports, and frames (radar).
(4) Nickel and tin plating are used for protective coatings and for material compatibility purposes. Tin is also one of the components of solder. Tin plating is also used on radio frequency (RF) shields, filters, and automatic switching devices.
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(5) Silver is used as a protective plating material over copper in wave guides, miniature and microminiature circuit boards, tank circuits, and RF shielding.
(6) Aluminum and aluminum alloys are widely used, because of their light weight, in equipment housings, chassis, mounting racks, supports, and electrical connector shells.
(7) Cadmium is used as a sacrificial coating on ferrous hardware, such as bolts, nuts, washers, and screws.
(8) Ion vapor deposition (IVD) of aluminum is also used as a sacrificial coating on hardware and is a nonhazardous replacement for cadmium.
(9) Gold is commonly used on electrical connectors, contacts, and edge connectors where the lowest electrical resistance is required.
c. Material Compatibility. Due to the complexity of the material process used in modern electronic assemblies, it is sometimes difficult to predict if potential problems will be created by the reaction between two or more nonmetallic materials in a circuit assembly. Incompatibility of materials can result in the release of chemicals or gases that will react with other circuit components. In some cases, the incompatibility of cleaning solutions will cause reactions in substances that are corrosive to associated circuitry. The following list contains several of the potential problems:
(1) The heating of conformal coating for the purpose of removal or repair may cause an outgassing that can be corrosive to metal components.
(2) Some commercial coating strippers contain acids that attack PCB laminates and discolor or corrode copper.
(3) Certain room temperature vulcanizing (RTV) silicone sealants contain acetic acid that is highly corrosive to metal components in avionics circuits. See Chapter 5, subparagraphs 505d(4) through (6), and Table 5-2.
(4) Some potting compounds revert to a liquid form under certain conditions. This reversion process reduces moisture protection in electrical connectors. See Chapter 5, subparagraph 505b.
(5) Degradation of polyvinyl chloride gives off acetic fumes, which are corrosive to most metals used in avionics equipment.
(6) Shrinkable elastomers (heat shrink) tubing, although not a problem directly, can cause damage to adjacent circuitry when heat guns are applied to shrink the tubing.
(7) Some dry film lubricants contain graphite, which is an excellent lubricant, but graphite is also corrosive. Graphite is cathodic to metals and, in the presence of moisture, promotes galvanic corrosion. Other dry film lubricants contain molybdenum disulfide which in the presence of moisture and heat can form a corrosive sulfuric acid.
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(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.
(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 SAE-AMS-M-3171, Type VI.
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(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, as revised. 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.
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.
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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 205 Page 12 FIGURE 2-1. DUST AND DIRT ACCUMULATION ON TERMINAL STRIP TAB LE 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.
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f. Temperature. High temperature may improve or impair the performance of avionics equipment, depending on other conditions. Corrosion and other harmful processes
(e.g., 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. Microorganism, 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.