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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 (i.e., 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, ultraviolet (UV) light, fungal growth, etc.
207 through 300. RESERVED.
Par 301 Page 20 CHAPTER 3. CORROSION CONTROL PROGRAM AND INSPECTION
301. GENERAL. Investigations over the past ten years have revealed that corrosion is a major factor in avionics equipment failures. As much as 20 percent of commercial/general aviation avionics failures are attributable to corrosion, and that figure rises to between 30 and 40 percent 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 3-1. 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;
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(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;
(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 where 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 303 Page 22 FIGURE 3-2. SURFACE CORROSION ON AN AVIONICS MOUNTING RACK
Par 303 Page 23 FIGURE 3-3. CORROSION AND LINT ACCUMULATION ON CARGO DOOR
MICRO-SWITCH
Par 303 Page 24 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.
Multipin 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.
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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.
FIGURE 3-4. CORRODED HARDWARE ON TERMINAL BOARD
e. E lectronics. 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 a 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.
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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.
(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.
Par 303 Page 27 FIGURE 3-5. CORRODED SHOCK MOUNT AND MOISTURE TRAP AREA
Par 303 Page 28 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. A 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:
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(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;
(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.
FIGURE 3-7. CROWN AREA INSPECTION
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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 10x 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 the applicable service manual for damage limits.
FIGURE 3-8. CORRODED HARDWARE FOR BONDING STRAP
d. W ater 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.