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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.
4-20 Table 4-2. Grouping of Metals and Alloys GROUP I Magnesium and Magnesium Alloys.
GROUP II Aluminum, Aluminum Alloys, Zinc, Cadmium, and Cadmium-Titanium Plate.
GROUP III Iron, Steels Except Stainless Steels; Lead, Tin, and their Alloys.
GROUP IV Copper, Brass, Bronze, Copper-Beryllium, Copper-Nickel Chromium, Nickel, Nickel Base Alloys, Cobalt Base Alloys, Carbon Graphite, Stainless Steels, Titanium, and Titanium Alloys.
NOTE: 1. Metals listed in the same group are considered similar to one another.
2. Metals listed in different groups are considered dissimilar to one another.
4.12.3 Protective Finishes. Base material with a protective finish is protected from corrosion and other forms of deterioration. These finishes have two categories: sacrificial and non-sacrificial. Sacrificial coatings include cadmium, zinc, and aluminum.
Non-sacrificial coatings include hard plating, such as chromium and nickel, chemical conversion coatings, sealant, primers, and topcoat.
4.12.4 Geographical Location and Environment. This factor concerns systems exposed to marine atmospheres, moisture, acid rain, tropical temperature conditions, industrial chemicals, and soils and dust in the atmosphere. Whenever possible, limit the requirement for operation of aircraft in adverse environments.
4.12.4.1 Moisture is present in the air as a gas, also called water vapor, or as finely divided droplets of liquid, as in mist or fog, and often contains contaminants such as chlorides, sulfates, and nitrates, which increase its corrosive effects.
Condensed moisture which evaporates will leave contaminants behind;
become trapped in close fitting, wettable joints, such as faying surfaces; and collect along poor bond lines by capillary action.
4.12.4.2 Salt particles, when dissolved in water, form strong electrolytes. Normal sea winds carry dissolved salt which makes coastal environments highly corrosive.
4.12.4.3 Industrial pollutants, such as carbon, nitrates, ozone, sulfur dioxide, and sulfates, contribute to the deterioration of nonmetallic materials and can cause severe corrosion of metals.
4.12.4.4 Warm, moist air normally found in tropical climates accelerates corrosion while cold, dry air normally found in arctic climates reduces corrosion.
4.12.5 Heat Treatment. Proper heat treatment of materials is a vital factor in maximizing resistance to corrosion.
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4.13 Preventative Maintenance.
4.13.1 Prevention. Corrosion prevention of aircraft structure depends on a comprehensive corrosion prevention and control plan, implemented from the start of operation of an aircraft, which includes:
4.13.1.1 Adequately trained personnel in:
• Recognition of corrosion-inducing conditions;
• Corrosion identification techniques;
• Corrosion detection, cleaning, and treating; and
• Lubrication and preservation of aircraft structure and components.
4.13.1.2 Scheduled inspection for corrosion.
4.13.1.3 Periodic, thorough cleaning, inspection, lubrication, and preservation. See Figures 4-15 through 4-20. Suggested intervals based on operating environment:
• Mild zones: every 90 calendar-days;
• Moderate zones: every 45 calendar-days; and
• Severe zones: every 15 calendar-days.
4.13.1.4 Prompt corrosion treatment after detection.
4.13.1.5 Accurate recordkeeping and reporting of material or design deficiencies to the manufacturer and the FAA.
4.13.1.6 Use of appropriate materials, equipment, and technical publications.
4.13.1.7 Maintenance of basic finish systems.
4.13.1.8 Keeping drain holes and passages open and functional. Sealants, leveling compounds, miscellaneous debris, or corrosion inhibitors should not block drain paths. See Figure 4-21, Blocked Drain Passages Resulted in Accumulation of Corrosion Contaminates and Moisture.
4.13.1.9 Replacing deteriorated or damaged gaskets and sealants, using noncorrosive type sealants, to avoid water intrusion and entrapment which leads to corrosion.
4.13.1.10 Minimizing aircraft exposure to adverse environments, such as hangar storage away from salt spray.
Figure 4-15. North America Corrosion Severity Map
FAIRBANKS * ANCHORAGE
CHICAGO NEW YORK
• DENVER
LOS ANGELE MIAMI
LP T HICC HAITI PUERTO
CORROSION SEVERITY ZONE MEXICO JAMAICA SOMINGO MILO
(1) MODERATE SEVERE 4-22
Figure 4-16. South America Corrosion Severity Map 70ZQTRINIDAD
CARACAS
• BOGOTA
A BELEM
GUAYAQU
A FORTALEZA RECIFE BRASILIA ¡ SALVADOR PAZ
SUNCION SAO PAULOT RIO DE JANIERO
SANTIAGO
• BUENOS MONTEVIDEO AIRES
CORROSION SEVERITY ZONE
• MILD _ MODERATE
• SEVERE 4-23
Figure 4-17. Africa Corrosion Severity Map ЛАВАТ.
ORROSION SEVERITY ZON OMILD OMAN SUNG @ MODERATE
• sevens DURBAN 4-24