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
JRZQTRINIDAD
CARACAS ¡ * BOGOTA 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
ORROSION SEVERITY ZON OMILD @ MODERATE * LOURENCO MANORES
• sEvEns DURBAN 4-24
Figure 4-18. South Pacific Corrosion Severity Map
SOUTH CHINA WAKE (SEVERE HAWAII SEA PHILIPPINES • SEVEREI SEVERITY ZONE 1 MODERATE CORROSION O MILO EW • 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 VLADI TOKYO NGHAI
OKINAWA (SEVERE)
O TAIWAN INDIA • HONG KONG
CALCUTTAMI AN HAINAN LE MAI
RANGOO SITO AY O
ENGA AIGON
INDIAN D
OCEAN SNAPORE MILD
CORROSION SEVERITY ZONE D MODERATE SEVERE INDONESIA (SEVERE pS 4-26
Figure 4-20. Europe and Asia Minor Corrosion Severity Map
CORROSION SEVERITY ZONE OMILO THENS
O CERATE O aveng 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 Compound, 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 high-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 Detergent 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 Cleaning 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, 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 Miscellaneous 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.
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4.13.2.9 Steam cleaning is not recommended for general use on aircraft. It erodes paint, crazes plastic, debonds adhesives, damages electrical insulation, and drives lubrication out of bearings.
4.13.3 Cleaning Procedures.
4.13.3.1 The following cleaning procedures are recommended:
• Remove/disconnect all electrical power;
• Ground aircraft;
• Aircraft wash personnel should wear protective gear, such as gloves, goggles, or aprons;
• Protect against water/cleaning compound intrusion, such as at closed doors, openings, cover vents, pitot static openings, or cover wheels.
Precautions should be taken to ensure these protections are removed after cleaning;
• Accomplish pre-wash lubrication. Lubricate per applicable maintenance manual;
• Mix cleaning solution per manufacturer recommendation;
• Use spray, not a stream, of water during aircraft wash;
• Do not use abrasive cleaning pads; and
• Rinse aircraft with fresh water to remove all cleaning compounds.
4.13.3.2 The following post-cleaning procedures are recommended:
• Remove all covers, plugs, and masking materials;
• Inspect and clear all drain holes;
• Inspect and open all known water trap areas for water accumulation and proper drainage;
• Lubricate aircraft per applicable maintenance manual; and
• Apply operational preservation.
4.13.4 Preservation. The day-to-day application of corrosion preventive compounds protects metal aircraft parts and components. They prevent corrosive materials from contacting and corroding bare metal surfaces. Many of these compounds also displace water and other contaminants from the surfaces. Some also lubricate as well as protect against corrosion. Corrosion preventive compounds vary in appearance and consistency from thick, black types to light oils. Some are water displacing and others are not. Thicker compounds provide the best corrosion protection, are longer lasting, and are more difficult to remove. Thinner materials provide some lubrication and do not crack, chip, or peel, but must be removed and replaced regularly to provide continuing protection.
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4.13.5 Surface Treatment.
4.13.5.1 An important step in the corrosion control process is surface treatment of a metal with a prescribed chemical to form a protective film. Chemical surface treatments, properly applied, provide corrosion resistance to the metal and improve the adhesion of subsequently applied paints. These surface treatments, also known as chemical conversion coatings, chromate conversion coatings, chemical films, or pretreatments, are aqueous acid solutions of active, inorganic compounds which convert aluminum or magnesium surfaces to a corrosion-resistant film.
4.13.5.1.1 Aluminum and aluminum alloy chemical conversion materials conform to MIL-SPEC MIL-C-81706, Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys.
4.13.5.1.2 Magnesium alloy chemical conversion materials conform to Aerospace Material Specification (AMS)-M-3171, Magnesium Alloy, Processes for Pretreatment and Prevention of Corrosion On.
4.13.5.1.3 Ferrous metals, stainless steel, and titanium treatment, prior to painting, are limited to corrosion removal and cleaning.
4.13.5.2 The surface should be prepared for application of the chemical conversion coatings.
4.13.5.2.1 Feather the edges of paint along the edge of areas that have been chemically stripped prior to pretreatment and repainting to ensure a smooth, overlapping transition between the old and new paint surfaces.
4.13.5.2.2 Clean the area with a fine or very fine clean abrasive mat saturated with water.
4.13.5.2.3 Rinse by flushing with fresh water. Particular attention should be given to fasteners and other areas where residues may become entrapped. At this stage in the cleaning, the surface should be without water beading or incomplete wetting, also called water breaks. A surface showing water breaks is usually contaminated with grease or oil, which will later interfere with conversion coating, sealing, and painting.
4.13.5.2.4 If the surface is not free of water breaks, re-clean the area with a solution of aircraft cleaning compound using an abrasive mat. Rinse thoroughly with water.
4.13.5.3 Chemical conversion coating is applied by brush, sponge stick moistener, or a non-atomizing sprayer to achieve a yellow-to-gold color for aluminum, usually 2 to 4 minutes, or greenish-brown, brassy, or brownish-yellow for magnesium, usually 30 seconds to 2 minutes.