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.
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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.
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4.13.5.4 Immediately rinse the part thoroughly with fresh water. Do not wipe with a rag or cloth.
4.13.5.5 Allow the chemical conversion coated surface to dry a minimum of 30 minutes and a maximum of 4 hours before painting.
4.13.6 Sealants. Refer to MIL-SPEC MIL-PRF-81733, Sealing and Coating Compound, Corrosion Inhibitive.
4.13.6.1 Sealants are one of the most important tools for corrosion prevention and control. They prevent the intrusion of moisture, salt, dust, and aircraft fluids, which can lead to extensive corrosion. For sealants to be effective, it is critical to choose the correct sealant for a specific area or situation, and that it is applied correctly.
4.13.6.2 Sealants are useful in the following areas:
• Fuel tank;
• Pressure areas;
• Weather sealing;
• Firewalls;
• Electrical;
• Acid-resistant areas;
• Windows;
• High temperature applications; and
• Aerodynamic sealing.
4.13.6.3 There are numerous sealing compounds available with different properties and intended use. Refer to the aircraft manufacturer manual for specific information about selection of the sealing compound and proper application.
Observe the warning and cautions of the manufacturer when using sealing compounds. Sealing compounds generally are divided into two major types:
those requiring a curing agent and those which cure in air.
4.13.6.3.1 Polysulfide, polythioether, and polyurethane sealing compounds consist of the base, or prepolymer, and the accelerator, or curing agent. When thoroughly mixed, the catalyst cures the base to a rubbery solid. Rates of cure depend on the type of base, catalyst, temperature, and humidity. A full cure may take as long as 7 days.
4.13.6.3.2 Silicone sealing compounds generally consist of one component which cures by reaction with moisture in the air. If silicones are applied too thickly or in such a way as to prevent moisture from entering the material, they may not
4-33 c ure at all. Many silicone sealing compounds also produce acetic acid, which has a vinegar smell, while curing, and can lead to severe corrosion problems.
Limit silicone sealing compounds on aircraft to those noncorrosive products conforming to MIL-SPEC MIL-A-46146, Adhesives-Sealan ts, Silicone, RTV, Noncorrosive (For Use with Sensitive Metals and Equipment).
4.13.6.3.3 Some s ealing compounds need a special primer or adhesion promoter before applying the sealant to develop a good adhesive bond with the surface. Use only those primers or adhesion promoters recommended by the product manufacturer.
4.13.6.4 Sealant application procedures:
4.13.6.4.1 Remove corrosion, apply a chemical surface treatment, and prime all surfaces, except internal fuel tank surfaces. If the surfaces have been contaminated following surface treatment, clean the area with cleaning solvent and a clean cloth. Dry the surfaces immediately with a clean cloth. Do not allow solvents to evaporate from the surface.
4.13.6.4.2 Mask off the area being sealed to prevent sealant from contacting adjacent areas during application and post-application smooth-out. Examples of where masking may be beneficial are fillet sealing of exterior surface lap and butt seams.
4.13.6.4.3 When required by the manufacturer, apply a thin coating of an adhesion-promoting solution. Allow to dry by evaporation without touching the area for 30 minutes to 1 hour before applying sealant.
4.13.6.4.4 Spatula type sealants may be applied with a nonmetallic spatula or scraper.
Avoid entrapping air. Work sealant into recesses by sliding the edge of the scraper firmly back over the recesses between taped areas next to the sealant application area. Smoothing will be easier if the nonmetallic scraper is first dipped in water.
4.13.6.4.5 If using a brush to apply a sealant, brush it on and smooth until the desired thickness is reached.
4.13.6.4.6 Applying sealant with a caulking gun will not usually require masking and is especially adaptable to filling seams or applying form-in-place gaskets.
4.13.6.4.7 Use a solid, continuous pattern to apply sealant with a spray gun.
4.13.6.4.8 Allow sealant to dry or cure per manufacturer recommendations.
4.13.6.4.9 When required, prime sealant as soon as it no longer feels tacky, then topcoat as necessary.
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4.13.6.4.10 Apply faying surface sealant between the contacting surface of two or more parts. Use this sealant for all assembly and, where possible, reassembly.
Where at all possible, use it in conjunction with fillet sealing. There are two types of faying surface seal installations, removable and permanent. The removable type is for access doors, removable panels, inspection plates, windows, etc. The sealant is normally applied to the substructure and a parting agent applied on the removable panel during sealant cure. The permanent type is for sealing between parts of a structure that is permanently fastened together with a high-adhesion sealant.
4.13.6.5 Aircraft maintainers commonly use fillet, or seam, seals. Fillet seals cover structural joints or seams along stiffeners, skin butts, walls, spars, and longerons, and seal around fittings and fasteners. Fillet sealant should be used in conjunction with faying surface sealing and in place of it if the assembly sequence restricts the use of faying surface sealing.
4.13.6.6 Injection sealing primarily fills voids created by structural joggles, gaps, and openings. Use only sealants recommended by the aircraft or equipment manufacturer, and inject sealant into the area using a sealant gun. This method produces a continuous seal, normally impossible while fillet sealing. Clean the voids of all dirt, chips, burrs, grease, and oil before injection sealing.
4.13.6.7 The fastener sealing method varies depending on the type of fastener, which is sealed either during or after assembly. To seal a permanent fastener during assembly, apply the sealant to the hole or dip the fastener into the sealant, and install the fastener while the sealant is wet. For removable fasteners, start the fastener in the hole and apply sealant to the lower side of the fastener head or countersink. To seal after assembly, apply the sealant to the fastener head after installation.
4.13.6.8 Seal fuel cells per the aircraft manufacturer’s maintenance manual procedures.
4.13.7 Paint Finishes and Touchup Procedures.
4.13.7.1 Paint systems protect exposed surfaces against corrosion and other forms of deterioration. Operational uses for schedule paint schemes include:
• High visibility requirements;
• Identification markings;
• Abrasion protection; and
• Specialty coatings, such as walkway coatings.
4.13.7.2 Aircraft paint systems have a primer coat and topcoat. The primer promotes adhesion and contains corrosion inhibitors, while the topcoat provides durability to the paint system, including weather and chemical resistance, and coloring for operational requirements.
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4.13.7.3 Some aircraft surfaces (such as Teflon-filled, rain erosion, or walkways) need specialized coatings because they have service exposure or operational requirements. For these surfaces, refer to the aircraft-specific manufacturer maintenance manuals.
4.13.7.4 The Environmental Protection Agency (EPA), joined by local air pollution control districts, has implemented rules which limit the volatile organic content (VOC), or solvent content, of paints applied to aircraft and ground support equipment. Understand and obey these rules to avoid large fines.
4.13.7.5 Much of a paint finish’s effectiveness and adherence to a surface depends on careful preparation of the surface before touchup and repair.
4.13.7.5.1 Aged paint surfaces must be scuff-sanded to ensure adhesion of freshly overcoated, applied paint. In sanding the surface, completely roughen it by hand sanding or with power tools.
4.13.7.5.2 For final preparation, ensure that surfaces are corrosion free, prepared, the surrounding paint feathered, cleaned, and conversion coated. Replace any seam sealants when necessary. Mask areas, as required, to prevent overspray.
4.13.7.6 Spray application for touchup, overcoat and total repaint. Below are preparation tips:
4.13.7.6.1 Thin primers, as required, with a thinner recommended by the paint manufacturer, stir, and apply in even coats. Thickness varies among primers, but generally, the total dry film thickness is 0.6 to 0.9 mils, or 0.0006 to
0.0009 inches. The remaining film thickness should be see-through. Allow the primer to air dry before applying the topcoat; be sure to read the paint manufacturer’s recommendations. You should normally apply a topcoat less than 24 hours after primer application.
4.13.7.6.2 Also, treat topcoats with a thinner per the paint manufacturer’s recommendations, stir the topcoat, and apply it in even coats. Topcoat thickness varies for each topcoat, but generally, the total dry film thickness is
1.5 to 2.0 mils, or 0.0015 to 0.002 inches. Air dry the topcoat per paint manufacturer’s instructions.
• Apply Teflon-filled, or anti-chafe, coatings over a primer per the manufacturer’s instructions.
• Apply walkway compounds over a primer, per the manufacturer’s instructions.
4.14–4.99. RESERVED.
5-1 CHAPTER 5. INSPECTION REQUIREMENTS
5.1 General. Corrosion inspections should be a part of daily or preflight maintenance inspections, except for special requirements in trouble areas. Overemphasizing a particular corrosion problem when discovered, and forgetting about corrosion until the next crisis, is an unsafe, costly, and troublesome practice. Inspection for corrosion is a continuing requirement, and requires conducting daily inspections. If a special crew or group handles corrosion control, they should schedule maintenance checks so that these crews can accomplish inspections and rework with access plates removed and components disconnected or out of the way.
5.1.1 Most manufacturer inspection handbooks are complete enough to cover all parts of the aircraft or engine, but no part or area of the aircraft should go unchecked. Use these handbooks as a general guide when inspecting an area for corrosion.
5.1.2 Trouble areas, however, are a different matter. Experience shows that certain combinations of conditions result in corrosion in spite of routine inspection requirements.
These trouble areas may be peculiar to particular aircraft models, in conditions similar to most aircraft.
5.1.3 Flight routes and bases of operation will expose some airplanes to more corrosive conditions than others. An aircraft’s operational environment may affect the environment’s corrosion severity, placing the operational environment in a mild, moderate, or severe category. Figures 4-15 through 4-20 indicate the corrosion severity of operational environments around the world. A particular area may be corrosive because of many factors, including airborne industrial pollutants, chemicals used on runways and taxiways to prevent ice formation, humidity, temperatures, or prevailing winds blowing from a corrosive environment.
5.2 Frequency of Inspections. In addition to routine maintenance inspections, observe these special requirements:
• A ircraft operating in a severe environment should be inspected every 15 calendar- days.
• Aircraft operating in a moderate environment should be inspected every 45 calendar- days.
• Aircraft operating in a mild environment should be inspected every 90 calendar-days.
• The aircraft should be washed prior to any inspection for corrosion.
• Crews familiar with corrosion problems and treatment should perform corrosion checks.
• Operators of infrequently flying aircraft should develop a calendar-based corrosion inspection and repair program, rather than basing the program on flight hours.
Because of uncertain operating environments, make adjustments to the calendar-time inspection interval after analysis of corrosion inspection findings.
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5.3 Recommended Depth of Inspection. Generally speaking, inspection requirements ensure adequate inspection of all compartments and interior aircraft cavities. Adequate maintenance is assured for most operating conditions when such general requirements are observed, along with a periodic check of a list of common trouble areas. To assist in complete coverage, the following summary is included:
5.3.1 D aily and Preflight Inspection . Check engine compartment gaps, seams, and faying surfaces in the exterior skin. Check all areas not requiring removal of fasteners or panels, such as bilge areas, wheel and wheel well areas, battery compartments, fuel cell and cavity drains, engine frontal areas, including intake vents, and engine exhaust areas.
5.3.2 Indepth Inspections. Remove screw-attached panels, access plates, and removable skin sections, as necessary, to thoroughly inspect internal cavities. Remove questionable heavy, internal preservative coatings for at least a spot check of the area underneath the coating. Inspect the aircraft interior in corrosion-prone areas, such as around lavatories, galleys, under floors, or baggage compartments. See Figure 5-1, Corrosion Found After Removing Cargo Door Threshold Covers.
5.3.3 Annual Inspections. Conduct corrosion inspections during annual inspections or other scheduled indepth inspections during which areas of the aircraft not normally accessible will be available for corrosion inspection.
5.3.4 Corrosion Preventive Compounds. Use the Original Equipment Manufacturer (OEM)-recommended corrosion preventive or compounds, such as LPS3, Dinol AV5, equivalent products, or advanced developments of these compounds, to effectively reduce the occurrence of corrosion. Review the results of corrosion inspections to help establish the effectiveness of corrosion preventive compounds and determine the reapplication interval of them.
Figure 5- 1. Corrosion Found After Removing Cargo Door Threshold Covers
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5.4 Primary Approach. Corrosion detection is primarily detected through corrosion inspections done on a regular schedule. Early detection and treatment reduce costs, out-of-service time, and the possibility of flight or flight-related incidents. All corrosion inspections should start with a thorough cleaning of the area to be inspected. Then, visually inspect the area with a flashlight, inspection mirror, and a 5X to 10X magnifying glass, looking for obvious defects and suspected areas. A detailed inspection follows of damage or suspected areas found during the general inspection. The detailed inspection can be one or more of the following items:
5.4.1 Nondestructive Inspection (NDI).
5.4.1.1 Visual Inspection. Visual inspection is the most widely used technique and is an effective method for detection and evaluation of corrosion. Visual inspection employs the eyes to look directly at an aircraft surface, or at a low angle of incidence, to detect corrosion. Using the sense of touch of the hand is also an effective inspection method for the detection of hidden, well-developed corrosion. Other tools such as mirrors, borescopes, optical micrometers, and depth gauges can be used for visual inspections. The following shows the type of corrosion damage detectable during visual inspection: Figures 5-2, Corrosion Under Chipped and Loose Paint on Wing Skin, and 5-3, Corrosion Indicated by Blistering of Paint in Fuel Cell, show chipped, missing, and lifted paint; Figure 5-4, Popped Rivet Heads Resulting from Corrosion Products, shows dished and popped rivets; Figure 5-5, Skin Bulging Around Fasteners Caused by Pressure from Corrosion Products, shows skin bulges or lifted surfaces; Figures 5-6, Corrosion Cracking Between Fasteners on a Wing Spar, and 5-7, Faying Surface Corrosion on Wing Spar Chord with Corrosion Cracking Also Visible, show cracks; and Figures 5-8, Severe Crown Stringer Corrosion, and 5-9, Spar Chord Corrosion, show corrosion products.
5.4.1.1.1 Indications of corrosive attack can take several forms depending on the type of metal and the length of time the corrosion has developed. Corrosion deposits on aluminum and magnesium are generally a white powder, while ferrous metals vary from red to dark reddish-brown stains.
5.4.1.1.2 Sometimes structural members or equipment installations block inspection areas, or for some reason, the areas are awkward to check visually. Ensure adequate access for inspection by removing access panels and adjacent equipment, by cleaning the area as necessary, and by removing loose or cracked sealants and paints. Mirrors, borescopes, and fiber optics are useful in providing the means of observing obscure areas. Figures 5-10 through 5-18 depict some of these conditions.