4-8 Figure 4-7. Hinge Corrosion Points
Fi gure 4-8. Hinge Failure Caused by Corrosion
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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.
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4.10.3 Most corrosion on avionic equipment is similar to basic airframe structure corrosion, but there is a difference: minute amounts of corrosion in avionic equipment can cause serious degradation or complete failure, while similar corrosion on larger structures would go unnoticed.
4.10.4 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 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 to help remove ozone from the enclosed shop or storage area.
4.10.5 Another man-made atmosphere is the aircraft environmental control system. These systems direct cooling air to the equipment, and may include a filter system that extracts moisture or contaminants from air entering the equipment. Always replace or clean filters, or eliminate leaky environmental seals, to prevent buildup of moisture or contaminants that could cause a corrosive atmosphere inside the equipment.
4.10.6 The control of corrosion in avionic systems is similar to that in airframes, with modifications. Factors affecting corrosion control in avionic equipment include:
• Less durable protection systems;
• Small amounts of corrosion can make equipment inoperative;
• Dissimilar metals are often in electrical contact;
• Stray currents can cause corrosion;
• Active metals and dissimilar metals in contact are often unprotected;
• Closed boxes can produce condensation during normal temperature changes during flight;
• Avionic systems have many areas to trap moisture;
• Hidden corrosion is difficult to detect in many avionic systems;
• Many materials in avionic systems are subject to attack by bacteria and fungi; and
• Organic materials, when overheated or improperly or incompletely cured, can produce vapors corrosive to electronic components and damaging to coatings and insulators.
4.11 Miscellaneous Trouble Areas. Additional trouble areas exist, some of which are covered by manufacturer publications. Below is a list of common trouble areas:
4.11.1 Examine and replace all flexible hose assemblies with chafing, weather checking, hardening, discoloration, evidence of fungus, and torn weather protective coatings or sleeves.
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4.11.2 Trimmed edges of sandwich panels and drilled holes should have some type of corrosion protection. We recommend a brush treatment with an inhibitor solution or applying a sealant along the edge, or both. With a sealant, fill gaps or cavities where moisture, dirt, or other foreign material can settle. Adjacent structures, not the sandwich, should have sufficient drainage to prevent moisture accumulation. Seal damage or punctures in panels as soon as possible to prevent additional moisture entry, even if permanent repair must be delayed.
4.11.3 Control cables may present a corrosion problem whether of carbon steel or stainless steel construction. Bare spots in the preservative coating is a main contributing factor in cable corrosion. Determine cable condition by cleaning the cable assembly, inspecting for corrosion, and applying 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 bell-crank arms.
External corrosion should be removed by a clean, dry, coarse rag or fiber brush. After complete corrosion removal, apply a preservative.
4.11.4 Topcoating materials in integral fuel cells, such as Buna-N, polyurethane, or epoxy, repel fuel but can absorb moisture through the topcoating materials and sometimes cause pitting or intergranular corrosion on aircraft structural parts. We have also found that microorganisms which live in water entrained by fuel, particularly jet propellant fuel types, feed on fuel hydrocarbon and hydrocarbon-type elastomeric coating materials.
Excreted organic acids and dead microorganisms act as a gelatinous acidified sponge which can deteriorate integral tank coatings and corrode the aircraft structure. Minimize microbial corrosion by preventing as much water contamination of the fuel as possible with well-managed storage facilities, adequate fuel filtration, and drainage of water contamination from integral fuel cells. This drainage keeps the water moving and reduces development of microorganism colonies. Reduce microorganic activity by using a biocide additive such as “Biobor JF” or equivalent. Use manufacturer instructions to determine solution strength and application frequency.
4.11.5 Electrical connectors or components may be potted with a sealing compound for improved reliability, and to prevent moisture entrance into connectors where wires are attached to pins.
4.11.5.1 Use rubber O-rings to keep moisture out of the mating area of pin connections and prevent loss of pressurization in compartments containing bulkhead connectors.
4.11.5.2 Moisture intrusion into electrical connectors can cause corrosion and an electrical failure. Disconnect and disassemble suspected plugs, clean with a solvent, and inspect for corrosion.
4.11.5.3 When electrical components cannot be sealed, these components can have moisture intrusion and internal corrosion.
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4.11.6 Severe corrosion damage to rear pressure bulkheads 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 enable a thorough visual inspection. When inspection access holes are available, inspection by fiber optics is useful. Other nondestructive inspection (NDI) methods, such as X-ray, ultrasonic, and eddy current, are also available. However, these inspection techniques require specially trained personnel, NDI comparison standards, and suitable access. Regular inspections of the rear pressure bulkhead, both front and rear faces, below the floor level should prevent serious corrosion buildup between the bulkhead and periphery doubler at the floor level. Such corrosion could weaken the bulkhead skin and lead to sudden cabin pressure loss.
4.11.7 Some older aircraft develop delaminations in cold-bonded joints. Corrosion between the delaminated surfaces stems from moisture intrusion along edges of mating parts or around fasteners securing mating parts together. Localized bulging of a skin or internal structural components, usually around the fasteners, is the first indication of a corrosion problem (see Figure 4-11, Spar Chord Lifting Caused by Corrosion Products). Skin cracks or dished or missing fastener heads may also indicate severe corrosion in bonded joints. Corrosion occurring between skins, doublers, and stringers or frames will produce local bulging or pulled rivets (see Figure 4-12, Skin Bulging Around Fasteners).
Corrosion that occurs between the skins and doublers or tear straps away from a backup structure, such as stringer or frame, will not produce local bulging. An external low frequency eddy current inspection should determine the extent of corrosion in the skin.
Open lap joints with wedges to determine the full extent of corrosion damage (see Figures 4-13, Opening of a Corroded Lap Joint for Repair, and 4-14, Close-Up View of a Corroded Lap Joint). Internal visual inspection should detect delaminated doublers or tear straps. Apply a penetrating water displacement corrosion inhibitor to faying surfaces after corrosion removal and repair.
4.11.8 Flap and slat recesses and equipment installed in these areas, normally closed, may corrode unnoticed without special inspections.
Figure 4-11. Spar Chord Lifting Caused by Corrosion Products Figure 4-12. Skin Bulging Around Fasteners 4-15
4-16 Figure 4-13. Opening of a Corroded Lap Joint for Repair
Fi gure 4-14. Close-Up View of a Corroded Lap Joint
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4.12 Factors in Corrosion Control.
4.12.1 Corrosion Factors. The severity, cause, and type of corrosion depend on many factors, including a part’s size or thickness, the material, heat treatment of the material, protective finishes, environmental conditions, preventative measures, and design.
4.12.1.1 Thick structural sections are generally more susceptible to corrosive attack because of variations in composition, particularly if sections are heat-treated during fabrication. When large sections are machined or chemically-milled after heat treatment, corrosion characteristics of thinner sections may differ from thicker areas. Section size adheres to structural requirements and cannot be changed to control corrosion. From a maintenance standpoint, the correct approach is to recognize the need to ensure the integrity and strength of major structural parts and maintaining permanent protection over such areas at all times.
4.12.1.2 In-service stresses and field repairs may affect rates and types of corrosion.
Aircraft structures under high cyclic stresses, such as helicopter main rotors, are particularly subject to stress-corrosion cracking. Areas next to weld-repaired items often have corrosion due to insufficient weld flux removal or buildup of a magnetic field. Closely inspect these areas for signs of corrosion and properly treat when found.
4.12.2 Corrosion Control in Design. Since corrosion is the deterioration of metal resulting from a reaction between a metal and its environment, some form of corrosion control or means to minimize corrosion should become part of an aircraft’s design phase before the aircraft enters operational service. The corrosion issues discussed in this AC provide information to reduce the rate of corrosive attack by corrosion control measures introduced early in design.
4.12.2.1 The nature of a material is a fundamental factor in corrosion. High-strength, heat-treatable aluminum and magnesium alloys are very susceptible to corrosion, while titanium and some stainless steel alloys are less susceptible in atmospheric environments. The aircraft manufacturer selects material for the aircraft based on material strength, weight, and cost, while corrosion resistance is often a secondary consideration. Corrosion control should be included in an aircraft’s preliminary design phase as early as possible.
4.12.2.2 More corrosion-resistant materials in any design normally involves additional weight to achieve the required strength. Weight consideration is a major factor in the construction of airframes, and the primary means of preventing corrosion is through protective coatings and proper maintenance procedures.
4.12.2.3 The use of corrosion-resistant alloys is not a cure-all for corrosion prevention.
A common mistake is to replace a corroded part with a corrosion-resistant alloy; sometimes the corrosion then shifts to another part and increases in severity.
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4.12.2.4 Protection against corrosion is minimized if the protected material is intrinsically resistant to corrosion. Aluminum copper alloys (2000 series) are known to have better stress-corrosion resistance and better fatigue strength properties than aluminum zinc alloys (7000 series), so aircraft designers use them more often as the primary structural materials.
4.12.2.5 Galvanic corrosion results from dissimilar metals in contact, and the galvanic series of metals and alloys (see Table 4-1, Galvanic Series of Metals and Alloys) is a factor to consider in aircraft repair. The further apart the potential voltages of the metals listed in Table 4-1 are, the greater the tendency for galvanic corrosion. The metals in Table 4-2, Grouping of Metals and Alloys, have small differences in electrical potential, so they are relatively safe to use in contact with one another. However, the coupling of metals from different groups will result in corrosion of the group having a lower potential voltage.
The electrode potentials as given in Table 4-1 are valid for pure materials at standard lab conditions. However, in-service conditions are different and potential voltages may shift to either side, positive or negative, as do alloying elements. Although this does not affect the general ranking, it can significantly affect the potential differences relevant for corrosion prevention and, in few cases, this may even invert ranking of two metals.
4-19 Table 4-1. Galvanic Series of Metals and Alloys Electrode Potential of Various Metals and Alloys (a) Metal or Alloy (b) PH stainless steels Potential, volts 0.025
0.1 N calomel scale (c) Magnesium -1.73 Zinc -1.10 7072, Alclad 3003, Alclad 6061, Alclad 7075 -0.96 5056, 7079-T6, 5456, 5083, 214, 218 -0.87 5052, 5652, 5086, 1099 -0.85 3004, B214, 1185, 1060, 1260, 5050 -0.84 1100, 3003, 615, 6053, 6061-T6, 6062-T6, 6063, 6363, Alclad 2014, Alclad 2024 -0.83 Cadmium -0.82 7075-T6, 356-T6, 360 -0.81 2024-T81, 6061-T4, 6062-T4 -0.80 355-T6 -0.79 2014-T6, 113, 750-T5 -0.78 2014-T4, 2017-T4, 2024-T3, and T4 -0.68 to-0.70 (d) Mild steel -0.58 Lead -0.55 Tin -0.49 Copper -0.20 Bismuth -0.18 Stainless steel (series 300, type 430) -0.09 Silver -0.08 Nickel -0.07 Chromium -0.49 to +0.018
(a) Data from Alcoa Research Laboratories.
(b) The potential of all tempers is the same unless temper is designated.
(c) Measured in an aqueous solution of 53g of sodium chloride + 3g hydrogen peroxide per liter at 25 degree c.
(d) The potential varies with quenching rate.