3-5
ALLOY TYPE OF ATTACK TO WHICH
ALLOY IS SUSCEPTIBLE
APPEARANCE OF
CORROSION PRODUCT Chromium (Plate) Pitting; promotes rusting of steel where pits occur in plating No visible corrosion products;
blistering of plating due to rusting and lifting Silver Will tarnish in the presence of sulfur Brown-to-black film Gold Highly corrosion resistant Deposits cause darkening of reflective surfaces
3.3–3.99. RESERVED.
4-1 CHAPTER 4. CORROSION-PRONE AREAS AND PREVENTATIVE MAINTENANCE
4.1 General. This chapter lists corrosion problem areas common to most aircraft. Be aware to clean, inspect, and treat corrosion-prone areas more frequently. The list is not intended for a specific aircraft, but it can be used to set up a maintenance inspection program. This chapter also includes factors in corrosion control and preventative maintenance.
4.2 Exhaust Trail Areas. Both jet and reciprocating engine exhaust gas deposits are very corrosive. Inspection and maintenance of exhaust trail areas should include attention to areas indicated in Figure 4-1, Exhaust Trail Area Corrosion Control Points, and inspection should include removal of fairings and access plates in the exhaust gas path.
4.2.1 Gaps, seams, hinges, and fairings are areas where exhaust trail deposits may be trapped and normal cleaning methods cannot reach these areas.
4.2.2 Exhaust deposit buildup on the upper and lower wing, aft fuselage, and in the horizontal tail surfaces will be considerably slower and sometimes completely absent on certain aircraft models.
Figure 4-1. Exhaust Trail Area Corrosion Control Points
4.3 Battery Compartments and Battery Vent Openings. In spite of protective paint systems and extensive sealing and venting, battery compartments continue to be corrosion problem areas. Fumes from overheated battery electrolyte are difficult to contain and can spread to internal structures where unprotected surfaces can come under corrosive attack. For lead-acid batteries, frequent cleaning and neutralization of acid deposits with sodium bicarbonate solution will minimize corrosion. If the battery
4-2 installation includes external vent openings on the aircraft skin, these areas should be included in inspection and maintenance procedures. If aircraft have batteries with electrolytes of either sulfuric acid or potassium hydroxide, electrolyte leakage will cause corrosion. Consult applicable maintenance manuals to determine the battery type installed and recommended maintenance practices. Clean nickel cadmium compartments with ammonia or boric acid solution, dry thoroughly, then paint with an alkali-resistant varnish.
4.4 Lavatories, Buffets, Galleys, Doorways, and Cockpit Floorboards. These areas, particularly deck areas behind lavatories, sinks, and ranges, are potential trouble areas where spilled food and waste products may collect. Even though some contaminants are not corrosive, they will attract and retain moisture, and promote corrosive attack. Pay particular attention to bilge areas and aircraft structure located under galleys and lavatories. Clean these areas frequently and maintain protective sealant and paint finishes.
Figure 4-2. Typical Damage to Floor Around Lavatories, Buffets, and Galleys
4.5 B ilge Areas. On all aircraft, the bilge area is a common trouble area. The bilge is a natural sump or collection point for waste hydraulic fluids, water, dirt, loose fasteners, drill chips, and other debris. Residual oil quite often masks small quantities of water which settle to the bottom and set up a hidden potential corrosion cell. Except for water displacing corrosion-preventing compounds, keeping bilge areas free of extraneous material, including water and oil, will insure the best protection against corrosion. A good vacuum cleaner and clean wiping cloths are handy tools to clean such areas.
4.6 Wheel Wells and Landing Gear. The wheel well area probably receives more punishment than any other area of the aircraft. It is exposed to mud, water, salt, gravel,
4-3 and other flying debris from runways during flight operations. See Figure 4-3, Landing Gear and Wheel Corrosion Points.
4.6.1 Aircraft wheels and wheel well areas should receive frequent cleaning, lubrication, and paint touch-up. Because of many complicated shapes, assemblies, and fittings in the area, complete coverage with a protective paint film is difficult, and preservative coatings tend to mask trouble rather than prevent it. Do not use preservative coatings on landing gear wheels because of the heat generated from braking.
4.6.2 During inspection of this area, pay particular attention to these trouble areas:
• Aluminum and high-strength steel.
• Exposed surfaces of struts, oleos, arms, links, and attaching hardware, such as bolts or pins.
• Axle interiors.
• Exposed position indicator switches and other electrical equipment.
• Crevices between stiffeners, ribs, and lower skin surfaces, typical water and debris traps.
• Magnesium wheels, particularly around bolt heads, lugs, and wheel web areas.
• Exposed rigid tubing, especially at “B” nuts and ferrules under clamps and tubing identification tapes.
Figure 4-3. Landing Gear and Wheel Corrosion Points
- TYPICAL DEBRIS TRAPS
SWITCHES -STRUTS
TIE DOWN POINTS
• UNPLATED HIGH
SPRINGS- INSIDE AXLES STRENGTH STEEL
- WATER TRAPS 1 DISSIMILAR METAL
CONTACTS
UNDER FERRULES AND
IDENTIFICATION TAPES
- WHEEL BEARING RACES 4-4
4-5
4.7 External Skin Areas. External aircraft surfaces are ordinarily covered with protective finishes, and operators can also apply paint coatings. Affected external aircraft surfaces are readily visible or available for inspection and maintenance. Much emphasis has been given to these areas in the past, and maintenance procedures are well established. Even here, certain configurations or combinations of materials become troublesome and require special attention to avoid serious corrosion difficulties. Some common trouble areas are listed below, other than those attributed to engine exhaust deposits:
4.7.1 Steel, Titanium, Corrosion Resistant Steel (CRES), and Nickel Alloy Fasteners.
Fasteners, and areas around these fasteners, are trouble areas. These areas are subject to high operational loads, moisture intrusion, and dissimilar metal skin corrosion. See Figure 4-4, Intergranular Corrosion of 7075-T6 Aluminum Adjacent to Steel Fastener.
Figure 4-4. Intergranular Corrosion of 7075-T6 Aluminum Adjacent to Steel Fastener
4.7.2 Faying Surfaces and Crevices. Similar to corrosion around fasteners, corrosion in faying surfaces, seams, and joints results from intrusion of moisture and other corrosive agents.
The effect of this type of corrosion is usually detectable by bulging of the skin surface.
4-6 Figure 4-5. Crack and Skin Bulging Caused by Corrosion
4.7.3 Magnesium Skins. Properly surface-treated, insulated, and painted magnesium skin surfaces give relatively little corrosion-related trouble if the original surface is maintained. However, trimming, drilling, and riveting destroy some of the original surface treatment, which may not be completely restored by touch-up procedures.
4.7.3.1 Some aircraft have steel fasteners installed through a magnesium skin, with only protective finishes under the fastener heads and fillet sealant or tape over the surface for insulation. Paint coatings are inherently thin at abrupt changes in contour, such as at trimmed edges. With magnesium’s sensitivity to moisture, these conditions add up to a potential corrosion problem whenever magnesium is used.
4.7.3.2 Corrosion inspections should include all magnesium skin surfaces and magnesium fittings or structural components, with special attention to edges, areas around skin edges and fasteners, and cracked, chipped, or missing paint.
4.7.4 Spot-Welded Skins. Corrosion of this type is chiefly the result of the entrance and entrapment of moisture or other corrosive agents between layers of metal. See Figure 4-6, Spot-Welded Skin Corrosion.
4.7.4.1 Spot-welded assemblies are particularly corrosion prone, and corrosive attack causes skin buckling or spot-weld bulging and fracture. This type of corrosion is evidenced by corrosion products appearing at crevices through which the corrosive agents enter. Some corrosion may originate from fabricating processes, but its progress to the point of skin bulging or spot-weld fracture is the direct result of moisture or other corrosive agents moving through open
4-7 gaps or seams. Weld-through sealing materials should minimize this problem, but many in-service aircraft have unsealed spot-weld skin.
4.7.4.2 Corrosion may appear at either external or internal faying surfaces, but is usually more prevalent on external areas. More advanced corrosive attack causes skin buckling and eventual spot-weld fracture. Detect skin buckling in this early stage by sighting or feeling along spot-welded seams or by using a straight edge to show variations along the skin.
4.7.4.3 To prevent this condition, keep potential moisture entry points (such as gaps, seams, and holes created by broken welds) filled with a noncorrosive sealant.
Figure 4-6. Spot-Welded Skin Corrosion
4.7.5 Piano-Type Hinges. These are prime spots for corrosion due to dissimilar metal contact between the steel pin and aluminum hinge tangs. These hinges are also natural traps for dirt, salt, and moisture. Hinges tend to corrode and freeze in the closed position between inspections where they are part of access doors or plate assembly and actuated only when opened during an inspection. When the hinge is inspected, lubricate it and work it through several cycles to enhance lubricant penetration. See Figures 4-7, Hinge Corrosion Points, and 4-8, Hinge Failure Caused by Corrosion.
4-8 Figure 4-7. Hinge Corrosion Points
Fi gure 4-8. Hinge Failure Caused by Corrosion
4-9
4.7.6 Heavy or Tapered Aluminum Alloy Skin Surfaces. Heavy or thick sections of most heat-treated aluminum alloys are susceptible to pitting or intergranular corrosion and exfoliation of the metal. When inspecting external skin surfaces, especially around countersunk fastener heads, look for white or gray powder deposits or metal exfoliation.
This usually first appears as small, raised areas or bumps under paint film.
4.7.6.1 Treatment of this corrosive attack includes removal of all corrosion products.
An example of this product removal is exfoliated metal, which is blended and polished within the limits set by the aircraft manufacturer. If corrosion products remain after reaching limits set by the aircraft manufacturer, contact the aircraft manufacturer for authorized limits. The treatment is not complete until protective surface finishes are fully restored.
4.7.6.2 Protect reworked areas with a chemical conversion coating, sealant primer, and topcoat, if applicable. Watch reworked areas for any indications of renewed corrosive activity.
4.7.7 Organic Composites. Organic composites used in aircraft can cause different corrosion problems than all-metal structures. Composites such as graphite/epoxy act as a very noble, or cathodic, material, creating the electrical potential for galvanic corrosion. The galvanic corrosion potential, coupled with different attachment methods, such as adhesive bonding, stepped structures, or locking mechanical fasteners, leads to multicomponent galvanic couples aggravated by high humidity and salt water environments. Applying aircraft sealants over dissimilar metal/composite junctions will prevent galvanic corrosion if moisture is completely displaced from the junction. Since moisture displacement is virtually impossible during extended flight operations, the most effective method of eliminating the voltage potential is to provide a nonconductive layer between the composite and dissimilar metal surfaces, such as a fiberglass/epoxy, sealant, or a combination or the two. Application of sealants over dissimilar metal/composite junctions may mask corrosion if the seal is compromised.
4.8 Water Entrapment Areas. Corrosion will result from entrapment of moisture. Except for sandwich structures, design specifications usually require that the aircraft have low-point drains in all areas where moisture and other fluids can collect. In many cases, these drains are ineffective either because of location or because they are plugged by sealants, extraneous fasteners, dirt, grease, and debris. Potential entrapment areas are not a problem when properly located drains are functioning and the aircraft has a normal ground attitude and is resting on its landing gear. Plugging a single drain hole or altering the level of the aircraft can result in a corrosion problem if water becomes entrapped in one of these “bathtub” areas, and we recommend a daily inspection of low-point drains.
4.9 Engine Frontal Areas and Cooling Air Vents. Constant abrasion by airborne dirt and dust, bits of gravel from runways, and rain all tend to remove protective surfaces from engine frontal areas and cooling air vents. Radiator cooler cores or reciprocating engine cylinder fins, designed for heat dissipation, may not be painted. Engine accessory mounting bases usually have small areas of unpainted magnesium or aluminum on machined mounting surfaces. With moist and salt or industrial pollutant-laden air
4-10 constantly flowing over these surfaces, they are prime sources of corrosive attack.
Inspection of such areas should include all sections in the cooling air path, with special attention to obstructions and crevices where salt deposits may build up during marine operations. See Figures 4-9, Reciprocating Engine Frontal Area Corrosion, and 4-10, Jet Engine Frontal Area Corrosion Points.
Note: When applicable, refer to the engine manufacturer’s specific instructions for further guidance.
Figure 4-9. Reciprocating Engine Frontal Area Corrosion
4-11 Figure 4-10. Jet Engine Frontal Area Corrosion Points
4.10 E lectronic Package Compartments. These areas are cooled by ram air or compressor bleed air and are subject to the same conditions as engine and accessory cooling vents and engine frontal areas. While the degree of exposure is less because of a lower volume of air passing through the compartment and special design features preventing water formation in enclosed spaces, this is still a trouble area needing special attention.
4.10.1 As circuit breakers, contact points, and switches are extremely sensitive to moisture and corrosive attack, inspect for these conditions as thoroughly as the design permits. If design features hinder examination of these items while installed, inspect the items if components are removed for other reasons.
4.10.2 Only qualified personnel familiar with the function of the unit involved should perform or direct the treatment of corrosion in electrical and electronic components, as conventional corrosion treatment may damage some units.