7/25/91 AC 43-4A with protective finishes. affected external aircraft surfaces are readily visible or available for EXTERNAL SKIN AREAS. External aircraft surfaces are ordinarily covered In addition, paint coatings may be applied. The inspection and maintenance. Much emphasis has been given to these areas in the of configurations or combinations of materials become troublesome and require past, and maintenance procedures are well established. Even here, certain types special attention if serious corrosion difficulties are to be avoided. Some of the common trouble areas, other than those attributed to engine exhaust deposits, are grouped as follows:
operational loads, moisture intrusion, and dissimilar metal skin corrosion (see Figure 4-4). areas around these fasteners are trouble spots. a. Steel, Titanium, CRES, and Nickel Alloy Fasteners. Fasteners and These areas are subject to high
ELECTROLYTE ENTERS
THROUGH CRACKS IN PAINT FILM
PAINT FILM (CATHODE) (ANODE)
INTERORANULAR
CORROSION STEEL CLADDING- FASTENER ALUMINUM - 7075- T6 FIGURE 4-4. INTERGRANULAR CORROSION OF 7075-T6 ALUMINUM
ADJACENT TO STEEL FASTENER Faying Surfaces and Crevices. Similar to corrosion around fasteners, usually detectable by bulging of the skin surface. corrosion in faying surfaces, seams, and joints is caused by the intrusion of moisture and other corrosive agents. The effect of this type of corrosion is Chap 4 Par 406 55
FIGURE 4-5. CRACK AND SKIN BULGING CAUSED BY CORROSION
c. Magnesium Skins. Properly surface treated, insulated, and painted magnesium skin surfaces give relatively little trouble from a corrosion standpoint 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.
skin with only protective finishes under the fastener heads, and fillet sealant or tape over the surface for insulation. Further, all paint coatings are inherently thin at abrupt changes in contour, such as at trimmed edges.
nagnesium's sensitivity to moisture, all of these conditions add up to a potential corrosion problem whenever magnesium is used.
surfaces, as well as other magnesium fittings or structural components, with Any inspection for corrosion should include all magnesium skin special attention to edges, areas around skin edges and fasteners, and cracked, chipped, or missing paint.
d. Spotwelded Skins. Corrosion of this type construction is chiefly the result of the entrance and entrapment of moisture or other corrosive agents between layers of the metal (see Figure 4-6).
(1) Spotwelded assemblies are particularly corrosion prone.
Corrosive attack causes skin buckling or spotweld bulging, and eventually spotweld fracture. Some of the corrosion may be caused originally by fabricating processes, but its progress to the point of skin bulging and spotweld fracture is the direct result of moisture or other corrosive agents Chap 4 56 Par 406
working its way through open gaps or seams. The use of weld-through sealing materials is expected to minimize this problem, but many in-service aircraft still have unsealed spotweld skin installed. This type of corrosion is evidenced by corrosion products appearing at the crevices through which the corrosive agents enter.
(2) Corrosion may appear at either external or internal faying surfaces, but it is usually more prevalent on external areas. More advanced corrosive attack causes skin buckling and eventual spotweld fracture. Skin buckling in this early stage may be detected by sighting or feeling along spotwelded seams or by using a straight edge.
(3) To prevent this condition, keep potential moisture entry points including gaps, seams, and holes created by broken welds filled with noncorrosive sealant.
CORROSIVE AGENTS ENTER-
AT UNSEALED SKIN EDGES
SPOT WELD
RIVET -
CORROSIVE AGENTS TRAVEL-
BETWEEN SKINS, AROUND
RIVETS AND WELDS
CAUSES BUCKLING OF OUTER SKIN CORRUSION BUILD UP HERE HERE FIGURE 4-6. SPOTWELDED SKIN CORROSION Piano-type Hinges. These are prime spots for corrosion due to dissimilar metal contact between the steel pin and aluminum hinge tangs. are also natural traps for dirt, salt, and moisture. Where this type of hinge They is used on access doors or plates, and actuated only when opened during an inspection, they tend to corrode and freeze in the closed position between inspections. When the hinge is inspected, it should be lubricated and actuated through several cycles to ensure complete penetration of the lubricant (see Chap 4 Par 406 57
•BARE STEEL HINGE PIN AL. ALLOY EXTRUSIONS
- HIDDEN CORROSION OCCURS HERE. JOINT FREEZES AND
LUGS BREAK OFF WHEN
HINGE IS ACTUATED FIGURE 4-7. HINGE CORROSION POINTS FIGURE 4-8. HINGE FAILURE CAUSED BY
CORROSION
f. 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 grey powder deposits or metal exfoliation. This is usually first evident as small raised areas or bumps under paint film.
(1) Treatment of this corrosive attack includes removal of all corrosion products, i.e., exfoliated metal is blended and polished not to exceed the limits set by the aircraft manufacturer. If corrosion products remain after the limits set by the aircraft manufacture have been reached, contact the aircraft manufacturer or the Federal Aviation Administration (FAA) for authorized limits. The treatment is not complete until the restoration of protective surface finishes is accomplished.
(2) Protect reworked areas with a chemical conversion coating, sealant primer, and top coat if applicable. Reworked areas should be carefully watched for any indications of renewed corrosive activity.
g. Organic Composites. Organic composites used in aircraft can cause different corrosion problems than those normally associated with all metal structure. Composites such as graphite/epoxy act as a very noble (cathodic) material, creating the potential for galvanic corrosion. The galvanic corrosion potential coupled with different methods of attachment (l.e., adhesive bonding, stepped structures, locking mechanical fasteners, etc.) lead to multicomponent galvanic couples with the problem being particularly aggravated by high humidity and salt water environments. Application of aircraft sealants over the Chap 4 58 Par 406
dissimilar metal/composite junctions will prevent galvanic corrosion if moisture is completely excluded. However, since complete exclusion of moisture is virtually impossible under extended periods of flight operation, the most effective method of eliminating the voltage potential is to provide a nonconductive layer such as fiberglass/epoxy and/or sealant between the composite and dissimilar metal surfaces.
WATER ENTRAPMENT AREAS. Corrosion will result from the entrapment of moisture. with the exception of sandwich structures, design specifications usually require that the aircraft have low point drains installed in all areas ineffective either due to location or because they are plugged by sealants, where moisture and other fluids can collect. In many cases, these drains are extraneous fasteners, dirt, grease, and debris. Potential entrapment areas are not a problem when properly located drains are functioning, and the aircraft is maintained in a normal ground attitude. However, the plugging of a single drain hole or the altering of the level of the aircraft can result in a corrosior problem if water becomes entrapped in one of these "bathtub" areas. Daily inspection of low point drains is a recommended practice.
airborne dirt and dust, bits of gravel from runways, and rain tends to remove the protective surfaces from these areas. Furthermore, cores of radiator ENGINE FRONTAL AREAS AND COOLING AIR VENTS. Constant abrasion by coolers, reciprocating engine cylinder fins, etc., due to the requirement for heat dissipation, may not be painted. Engine accessory mounting bases usually have small areas of unpainted magnesium or aluminum on the machined mounting surfaces. With moist and salt or industrial pollutant-laden air 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 and 4-10).
2 Chap 4 Par 406 59
ALL CADMIUM PLATED PARTS
•PUSH ROD HOUSINGS MAGNESIUM HOUSINGS 7
ACCESSORY MOUNTING BASES
ALL MATING SURFACES AIR INLET AREA
MAGNESIUM HOUSINGS FIGURE 4-9. RECIPROCATING ENGINE FIGURE 4-10. JET ENGINE FRONTAL
FRONTAL AREA CORROSION AREA CORROSION POINTS
409. ELECTRONIC PACKAGE COMPARTMENTS. Electronic and electrical package compartments cooled by ram air or compressor bleed air are subjected to the same conditions common to 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 and special design features incorporated to prevent water formation in enclosed spaces, this is still a trouble area that requires special attention.
d. Circuit breakers, contact points, and switches are extremely sensitive to moisture and corrosive attack and should be inspected for these conditions as thoroughly as design permits. If design features hinder Chap 4 60 Par 408
examination of these items while in the installed condition, inspection should be accomplished after component removal for other reasons.
b. Ireatment of corrosion in electrical and electronic components should be done only by, or under the direction of, qualified personnel familiar with the function of the unit involved as conventional corrosion treatment may be detrimental to some units.
c. Most corrosion that occurs on avionic equipment is similar to that which occurs on the basic airframe structure. The difference between avionic and airframe corrosion is that minute amounts of corrosion in avionic equipment on larger structures. can cause serious degradation or complete failure, while it would be unnoticed
d. Smog, smoke, soot, and other airborne contaminants are extremely corrosive to exposed avionic equipment. Many fumes and vapors emitted from factories or industrial complexes are highly acidic and greatly accelerate corrosion. An example is the corrosive effect of ozone, a product of many welding machines and large electrical motors. Complete degradation of rubber seals and damage to delicate components have occurred in equipment stored near ozone-producing equipment. Avionic shops and storage areas should have a filtered air-conditioning system.
Another man-made atmosphere is the aircraft environmental control These systems induce cooling air to the equipment. They may include a rilter system that extracts moisture, and in some cases contaminants, from the air that enters the equipment. Failure to replace and/or clean a filter, or eliminate a leaky environmental seal, may cause a moisture or contaminant buildup that could cause a corrosive atmosphere within the equipment.
f. The control of corrosion in avionic systems is similar to that in airframes, with appropriate modifications. The general differences in construction and procedures between airframe and avionics relative to corrosion control are:
(1) Less durable protection systems;
(2) Small amounts of corrosion can make equipment inoperative;
(3) Dissimilar metals are often in electrical contact;
(4) Stray currents can cause corrosion;
(5) Active metals and dissimilar metals in contact are often unprotected;
changes during flight; (6) Closed boxes can produce condensation during normal temperature
(7) Avionic systems have many areas to trap moisture;
Chap 4 Par 409 61
(8) Hidden corrosion is difficult to detect in many avionic systems;
(9) Many materials used in avionic systems are subject to attack by bacteria and fungi; and
(10) Organic materials are often used which, when overheated or improperly or incompletely cured, can produce vapors which are corrosive to electronic components and damaging to coatings and insulators.
410. MISCELLANEOUS TROUBLE AREAS. A variety of additional trouble spots exists, and some are covered by manufacturers' publications. Most aviation activities can add a favorite to the following list:
a. Examine all flexible hose assemblies for chafing, weather checking, hardening, discoloration, evidence of fungus, and torn weather protective coatings or sleeves. Replace those hoses that are found to be discrepant.
b. Irimmed edges of sandwich panels and drilled holes should have some type of corrosion protection. A brush treatment with an inhibitor solution or the application of a sealant along the edge, or both, is recommended. Any gaps or cavities where moisture, dirt, or other foreign material can be trapped should be filled with a sealant. The adjacent structure (not the sandwich) should have sufficient drainage to prevent moisture accumulation. Damage or punctures in panels should be sealed as soon as possible to prevent additional moisture entry--even if permanent repair has to be delayed.
c. Control cables may present a corrosion problem whether carbon steel or stainless steel is used. The presence of bare spots in the preservative coating is one of the main contributing factors in cable corrosion. Cable condition should be determined by cleaning the cable assembly, inspection for corrosion, and application of an approved preservative if no corrosion is found.
If external corrosion is found, relieve tension on the cable and check internal strands for corrosion. Cables with corrosion on internal strands should be replaced. Pay particular attention to sections passing through fairleads, around sheaves, and grooved bel crank arms. ixternal corrosion should be removed by a clean, dry, coarse rag or tiber brush. After complete corrosior removal, apply a preservative.
d. Topcoating materials (Buna - N, Polyurethane, and Epoxy) used in integral fuel cells are impervious to fuel but not completely impervious to moisture absorption. Since it is impossible to keep fuel completely free of water, moisture can penetrate through the topcoating materials and sometimes causes pitting or intergranular corrosion on aircraft structural parts. It has also been found that micro-organisms which live in the water entrained by fuel, particularly jet propellant types, feed on fuel hydrocarbon and hydrocarbon-type elastomeric coatings materials. These micro-organisms excrete organic acids, and dead micro-organisms act as a gelatinous acidified sponge which can deteriorate integral tank coatings and corrode the aircraft structure.
Microbial corrosion can be minimized by preventing as much water contamination of the fuel as possible with well-managed storage facilities, adequate filtration of fuel, and drainage of water contamination from integral fuel cells which keeps the water moving and reduces the chance for the colonies of microorganisms to develop. Micro-organic activity can be reduced by using a biocide Chap 4 62 Par 409
additive such as "Biobor JF" or equivalent. Solution strength and application frequency should be in accordance with the manufacturer's instructions.
Electrical connectors/components may be potted with a sealing compound to provide more reliability of equipment. The sealing compound prevents entrance of moisture into the area of connectors where the wires are attached to the pins.
(1) Rubber O-rings may also be used to seal moisture out of the mating area of pin connections and to prevent loss of pressurization in compartments containing bulkhead connectors.
(2) Moisture intrusion into electrical connectors can cause corrosion and an electrical failure. Suspected plugs should be disconnected, dissembled, solvent cleaned, and inspected for corrosion.
(3) When sealing provisions are not designed into the electrical component, these components can have moisture intrusion and internal corrosion.
f. Severe corrosion damage to the rear pressure bulkhead below the floor may occur as a result of contamination by fluids. Inspection for rear bulkhead corrosion may require extensive disassembly of components and fixtures to allow a thorough visual inspection. when inspection access holes are available, inspection by fiber optics is useful. Other nondestructive inspection (NDI) methods (x-ray, ultrasonic, and eddy current) are also available. However, these inspection techniques require specially trained personnel, NDI comparison standards, and suitable access. A regular inspection of the rear pressure bulkhead (both front and rear faces) below the floor level should be accomplished to prevent serious corrosion from occurring between the bulkhead and periphery doubler at the floor level. Such corrosion could weaken the bulkhead skin and cause sudden cabin pressure loss.
g. Some older aircraft have developed delaminations in cold bonded intrusion along the edge of the mating parts or around fasteners securing the mating parts together. Localized bulging of the skin or internal structural Corrosion between the delaminated surfaces is caused by moisture component, usually around the fasteners, is the first indication of a corrosion problem (see Figure 4-11). Skin cracks or dished or missing fastener heads may also indicate severe corrosion in bonded joints. Corrosion which occurs between skins, doublers, and stringers or frames will produce local bulging or pulled rivets (see Figure 4-12). Corrosion that occurs between the skins and doublers or tear straps away from backup structure such as stringer or frame will not produce local bulging. An external low frequency eddy current inspection may be used to determine the extent of corrosion in the skin. Lap joints should be opened with wedges to determine the full extent of corrosion damage (see Figures 4-13 and 4-14). Internal visual inspection should be used to detect delaminated doublers or tear straps. A penetrating water displacement corrosion inhibitor should be applied to faying surfaces after corrosion removal and repair.
Chap 4 Par 410 63
Flap and slat recesses and equipment installed in these areas, which are normally closed, may corrode unnoticed unless special inspections are performed.
FIGURE 4-11. SPAR CHORD LIFTING CAUSED BY CORROSION PRODUCTS Chap 4 64 Par 410
FIGURE 4-12. SKIN BULGING AROUND FASTENERS FIGURE 4-13. OPENING OF A CORRODED LAP JOINT FOR REPAIR Chap 4 Par 410 65
FIGURE 4-14. CLOSEUP VIEW OF A CORRODED LAP JOINT
411. FACTORS IN CORROSION CONTROL.
Corrosion Factors. The degree of severity, the cause, and the type of corrosion depend on many factors, including the size or thickness of the part, the material, heat treatment of the material, protective finishes, environmental conditions, preventative measures, and design.
(1) Thick structural sections are generally more susceptible to corrosive attack because of variations in their composition, particularly if the sections are heat-treated during fabrication. When large sections are machined or chem-milled after heat treatment, the corrosion characteristics of thinner sections may be different from those of thicker areas. Section size is based on Contre ind on structural requirements and cannot be changed for the purpose of controlling corrosion. From a maintenance standpoint, the correct approach is one of recognizing the need to ensure the integrity and strength of major structural parts and maintaining permanent protection over such areas at all times.
(2) In-service stresses and field repairs may affect the rates and types of corrosion. Aircraft structure under high cyclic stresses, such as helicopter main rotors, are particularly subject to stress-corrosion cracking.
Also areas adjacent to weld repaired items often have corrosion due to insufficient removal of the weld flux, or, for some steels, buildup of a magnetic field. Areas such as these should be closely inspected for signs of corrosion and, when found, proper treatment accomplished.
Chap 4 66 Par 410