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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 G eographical 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 Preventive 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
NEW YORK
CHICAGO
• DENVER
LOS ANGELE MIAMI HAIT
CORROSION SEVERITY ZONE MILO MEXICO JAMAICA SOMINGO
(1) MODERATE ) SEVERE 4-22
Figure 4-16. South America Corrosion Severity Map 7022 TRINIDAD
CARACAS
• BOGOTA BELEN
SUAYAQU
A FORTALEZA RECIFE
BRASILIA P SALVADOR PAZ
SAO PAULONE RIO DE JANIERO
SUNCION
SANTIAGO
• BUENOS MONTEVIDEO AIRES
CORROSION SEVERITY ZONE MILD _ MODERATE
• SEVERE 4-23
Figure 4-17. Africa Corrosion Severity Map ПАВАТ /
CORROSION SEVERITY ZON OHILD @ MODERATE *I LORENCO MANCHES DURBAN 4-24
Figure 4-18. South Pacific Corrosion Severity Map SOUTH • WAKE.
CHINA (SEVERE HAWAII SEA PHILIPPINES * (SEVERE) GUAM SEVERITY ZONE • MODERATE CORROSION O MILO
• FEVERE
SOLOMONS INDIAN
OCEAN CORAL SEA ¡SEVERE) *
AUSTRALIA
SOUTH PACIFIC SIDNEY
PERTH TASMAN
MELBOU WELLINGTON
CHRIST CHURC
TASMANIA 4-25
Figure 4-19. Asia Corrosion Severity Map VLADIV TOKYO
THE INGHAI
OKINAWA (SEVERE)
CHUNKING
O TAIWAN
INDIA CHONG KONG
CALCUTZAHITI RANCOO AY O
ENGA AIGON
OCEAN INDIAN D CUALA IMPUR SHERAPORE MILD
CORROSION
SEVERITY ZONE ) MODERATE
SEVERE INDONESIA (SEVERE DE 4-26
Figure 4-20. Europe and Asia Minor Corrosion Severity Map
CORROSION SEVERITY ZONE OMILO
(1) MOCERATE O Evens 4-27
4-28 Figure 4-21. Blocked Drain Passages Resulted in Accumulation of Corrosion Contaminates and Moisture
4.13.2 Cleaning Compounds for Metals.
Note: More is not always better, and mixing more cleaning compound increases the pH of a solution, which can do more harm than good. Always mix a cleaning compound using manufacturer recommendations.
4.13.2.1 Cleaning compounds work by dissolving soluble soils, emulsifying oily soils, and suspending solid soils. There are several cleaning compounds, and each cleans a surface using one or more of the below mechanisms.
4.13.2.2 Highly alkaline cleaning compounds (with a pH greater than 10) are not recommended. However, moderately alkaline cleaners (with a pH between 7.5 and 10) conforming to Military Specification (MIL-SPEC) MIL-C-85570, Cleaning Compounds, Aircraft, Exterior, Types I and II, are recommended.
Both types contain detergents, foaming agents, and solvents, and work the same as a detergent solution.
4.13.2.3 A hi gh-gloss spot cleaner conforming to MIL-SPEC MIL-C-85570, Type III, is recommended for cleaning exhaust track areas of high-gloss paint systems.
This material contains solvents, detergents, and suspended abrasive matter to remove soil by wearing away the surface that holds it.
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4.13.2.4 Thixotropic, or viscous, cleaner conforming to MIL-SPEC MIL-C-85570, Type V is recommended for cleaning wheel wells and replacement of some solvent cleaning where water rinsing can be used. This cleaner contains solvents, detergents, and some thickening agents. When applied undiluted to an oily or greasy surface, the cleaner clings long enough to emulsify the soil, usually about 5 to 15 minutes, then can be rinsed away with fresh water.
Note: Solvents for cleaning operations is becoming more and more limited due to environmental regulations. Determine local requirements for waste disposal.
4.13.2.5 Solvent emulsion cleaners conforming to MIL-SPECs MIL-C-43616, Cleaning Compounds, Aircraft Surface, and MIL- C-85704, Cleaning Compound, Turbine Engine Gas Path, become emulsions when diluted. The solvent in the cleaner softens oily soils so that they can be emulsified by the detergent and rinsed away.
4.13.2.6 D etergent solution cleaners conforming to MIL-SPEC MIL-D-16791, Detergents, General Purpose (Liquid, Nonionic), dissolve in water and clean by dissolving soluble salts, emulsifying low-viscosity oils, and suspending easily removed dirt and dust. They are not very effective on grease, but are excellent cleaners for interior lightly soiled areas, plastics, and instrument glass covers.
4.13.2.7 C leaning solvents dissolve oily and greasy soils so that they can be easily wiped away or absorbed on a cloth. Solvents differ significantly in cleaning ability, toxicity, evaporation rate, effect on paint, and flammability. Cleaning solvents are best for localized spot application only. A dry cleaning solvent conforming to MIL-SPEC MIL-PRF-680, Performance Specification:
Degreasing Solvent, Type II, is the most common cleaning solvent used on aircraft, due to its low toxicity, minimal effect on paint, and relative safety.
Other solvents, such as alcohols, ketones, chlorinated solvents, and naphtha, are specialized materials and have restricted use. Refer to manufacturer maintenance and cleaning procedures for specific applications.
4.13.2.8 Mi scellaneous cleaning agents include:
• Plastic polish, which contains mild abrasive matter to polish out scratches in canopy materials;
• Alkaline chemicals used to neutralize specific acidic soils;
• Sodium bicarbonate for electrolyte spills from sulfuric acid batteries; and
• Monobasic sodium phosphate and boric acid for electrolyte spills from nickel-cadmium batteries.