Par 6-51 Page 6-17
a. Remove all traces of saltwater and saltwater residue by thoroughly washing the aircraft with freshwater.
(1) After drying, coat the propeller, hubs, blades, and other unpainted or unprotected parts of the engine and its installation parts by spraying or rubbing lightly with corrosion-preventive compound, Specification MIL-C-16173, Grade 4.
(2) Apply this mixture on parts that move or require some lubrication and on all fittings subject to corrosion such as landing gear retracting plungers, control surface hinges, control cables, exposed rivets and bolts, and other similar parts not protected by paint. Apply with a cloth or a soft brush soaked in the mixture.
(3) Wipe off excess mixture. When applying the mixture, take care that as little as possible is deposited on exhaust pipes or collector rings to avoid a fire hazard when the engine is started. Keep the ignition wires, propeller anti-icer feed hose, tires, and other rubber parts free of the mixture.
b. Where maximum corrosion protection is desired on stationary parts, use exterior surface corrosion-preventive compound, Specification MIL-C-16173, Grade 1.
c. Wipe the exposed portion of the landing gear shock strut piston with a cloth soaked in the applicable hydraulic fluid.
d. Most parts of landing gear wheels are made from magnesium or aluminum alloys which corrode rapidly unless carefully protected. When the aircraft operates near saltwater and off coral beaches, the corrosion can be very rapid. Inspect wheels to determine the paint condition.
e. Refinish portions of a wheel where paint has deteriorated, peeled, or chipped.
f. Except for friction and bearing surfaces, apply a protective coating to all parts of wheels and brake assemblies.
6-52.–6-62. [RESERVED.]
Par 6-63 Page 6-18 SECTION 5. VISUAL CORROSION INSPECTION GUIDE FOR AIRCRAFT
6-63. GENERAL. This guide provides a general inspection checklist for those parts or surfaces that can be visually inspected without disassembly of the aircraft. It is intended for use in establishing corrosion inspection areas for which the manufacturer has not provided a recommended corrosion inspection program. The manufacturer’s recommended corrosion inspection program will take precedence over this guideline. These inspections should be accomplished in conjunction with other preventive maintenance.
6-64. EXHAUST TRAIL AREAS.
a. Visually inspect paint in areas of the exhaust trails for damage.
b. Visually inspect under fairings, around rivet heads, and in skin crevices for corrosion in areas of engine exhaust trail.
6-65. BATTERY COMPARTMENTS AND BATTERY VENT OPENINGS.
a. Inspect battery compartment for electrolyte spillage, corrosion, and condition of protective paint.
b. Inspect area around battery vent for corrosion.
6-66. LAVATORIES AND GALLEYS. Inspect areas around lavatories, sinks, and ranges for spillage and corrosion. Pay particular attention to floor area and the area behind and under lavatories, sinks, and ranges.
6-67. BILGE AREAS.
a. Inspect bilge areas for waste hydraulic fluids, water, dirt, loose fasteners, drill chips, and other debris.
b. Remove any foreign material from bilge and inspect for corrosion.
6-68. WHEEL WELLS AND LANDING GEAR. Inspect wheel well area and landing gear components for damage to exterior finish coating and corrosion. Particular attention should be given to exposed surfaces of struts, oleos, arms, links, and attaching hardware; axle interiors, exposed position indicator switches and other electrical equipment; crevices between stiffeners, ribs, and lower skin surfaces; magnesium wheels, particularly around bolt heads, lugs, and wheel web areas; and exposed rigid tubing at “B” nuts and ferrules under clamps, and tubing identification tapes.
6-69. EXTERNAL SKIN AREAS.
a. Inspect external skin surfaces for damage to protective finishes and corrosion.
b. Inspect around fasteners for damage to protective finishes and corrosion.
Par 6-69 Page 6-19
c. Inspect lap joints for bulging of skin surface, which may indicate the presence of corrosion between the faying surfaces. Skin cracks and/or dished or missing fastener heads may also indicate severe corrosion in bonded joints.
d. Inspect area around spot welds for bulges, cracks, or corrosion.
e. Inspect piano-type hinges for corrosion. When piano hinges are inspected, they should be lubricated and actuated through several cycles to ensure complete penetration of the lubricant.
f. Inspect thick alloy skin surfaces for pitting, intergranular corrosion, and exfoliation of the metal. Look for white or gray deposits around countersunk fastener heads and raised areas or bumps under the paint film.
g. Inspect composite skins for corrosion of attachment fasteners.
6-70. WATER ENTRAPMENT AREAS. Inspect area around edge of drain holes for corrosion and ensure that drain holes are not blocked by debris.
6-71. ENGINE FRONTAL AREAS.
a. Inspect reciprocating engine cylinder fins, engine cases, and cooling air ducts for damage to exterior finish and corrosion.
b. Inspect radiator cooler cores for corrosion.
6-72. ELECTRONIC PACKAGE COMPARTMENTS.
a. Inspect circuit breakers, contact points, and switches for evidence of moisture and corrosive attack.
b. Treatment of corrosion in electrical and electronic components should be done by or supervised by qualified personnel familiar with the function of the unit involved.
6-73. FLEXIBLE HOSE ASSEMBLIES.
a. Inspect hose assemblies for chafing, weather-checking, hardening, discoloration, evidence of fungus, torn weather protective coatings or sleeves, and corrosion of fittings.
b. Replace any defective, damaged, twisted, or bulging hoses.
6-74. SANDWICH PANELS. Inspect edges of sandwich panels for damage to the corrosion protection finish or sealant and for corrosion.
6-75. CONTROL CABLES.
a. Inspect control cables for bare spots in the preservative coating and corrosion.
b. 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. External corrosion
Par 6-75 Page 6-20 should be removed by a clean, dry, coarse rag or fiber brush. A preservative should be applied after removal of external corrosion.
6-76. INTEGRAL FUEL CELLS.
a. Inspect top coat finish for breaks, peeling, lifting of surface, or other damage.
b. Inspect aircraft structure for top coat finish damage from pitting or intergranular corrosion.
6-77. ELECTRICAL CONNECTORS.
a. Inspect electrical connectors for breaks in potting compound and corrosion of pins and wires.
b. If the electrical connector is suspected of having moisture intrusion, disassemble the connector, clean the connector, and inspect it for corrosion.
6-78.–6-88. [RESERVED.]
Par 6-89 Page 6-21 SECTION 6. CORROSION REMOVAL PROCEDURES
6-89. GENERAL. General safety precautions for handling materials with hazardous physical properties are outlined in the following paragraphs. They also address emergency procedures for immediate treatment of personnel who have inadvertently come into contact with harmful materials. All personnel responsible for using or handling hazardous materials should be thoroughly familiar with the information in the following paragraphs.
6-90. SAFETY PRECAUTIONS.
a. Chemical. When required to use or handle solvents, special cleaners, paint strippers (strong alkalis and acids), etchants (corrosion removers containing acids), or surface activation material (Alodine 1200), observe the following safety precautions:
(1) Avoid prolonged breathing of solvent or acid vapors.
(2) Never add water to acid. Always add acid to water.
(3) Mix all chemicals per the manufacturer’s instructions.
(4) Clean water for emergency use should be available in the immediate work area before starting work.
(5) Avoid prolonged or repeated contact with the skin of solvents, cleaners, etchants (acid), or conversion coating material (Alodine solution). Rubber or plastic gloves should be worn. Goggles or plastic face shields and suitable protective clothing should be worn when cleaning, stripping, etching, or conversion coating overhead surfaces.
(6) When mixing alkalis with water or other substance, use containers that are made to withstand heat generated by this process.
(7) Wash any paint stripper, etchant, or conversion coating material immediately from body, skin, or clothing.
(8) Materials splashed into the eyes should be promptly flushed out with water, and medical aid obtained immediately.
(9) Do not eat or keep food in areas where poisons may be absorbed. Always wash hands before eating or smoking.
(10) Verify that the area within 50 feet of any cleaning or treating operations where low flashpoint (140 °F or below) materials are being used is clear and remains clear of all potential ignition sources.
(11) Suitable fire-extinguishing equipment should be available to the cleaning/treating area.
Par 6-90 Page 6-22
(12) Equipment should be effectively grounded where any flammable materials are being used.
(13) If materials (acid, alkali, paint remover, or conversion coatings) are spilled on equipment and/or tools, treat immediately by rinsing with clean water, if possible, and/or neutralizing acids with baking soda and alkalis with a weak (5 percent) solution of acetic acid in water.
(14) In confined location, do not use solvents with a low flashpoint (below 100 °F), such as methyl ethyl ketone (MEK) and acetone.
(15) All equipment should be cleaned after work has been completed.
(16) Check and follow all applicable restrictions and requirements on the use of solvents, primers, and top coats.
(17) Check and follow all applicable restrictions and requirements for use and disposal of waste material.
b. Blasting. The following precautions should be taken when using any type of blasting equipment:
(1) Operators should be adequately protected with complete face and head covering equipment, and provided with pure breathing air.
(2) Static-ground the dry abrasive blaster and the material to be blasted.
(3) Magnesium cuttings and small shavings can ignite easily and are an extreme hazard.
Fires of this metal must be extinguished with absolutely dry talc, calcium carbonate, sand, or graphite by applying the powder to a depth of 1/2 inch over the metal.
(4) Titanium alloys and high-tensile-strength steel create sparks during dry abrasive blasting. Care should be taken to ensure that hazardous concentrations of flammable vapors do not exist.
6-91. CORROSION CONTROL WORK PROCEDURES. The effectiveness of corrosion control depends on how well basic work procedures are followed. The following common work practices are recommended:
a. If rework procedures or materials are unknown, contact the aircraft manufacturer or FAA-authorized Designated Engineering Representative (DER) before proceeding.
b. The work areas, equipment, and components should be clean and free of chips, grit, dirt, and foreign materials.
c. Do not mark on any metal surface with a graphite pencil or any type of sharp, pointed instrument. Temporary markings (defined as markings soluble in water or methyl chloroform) should be used for metal layout work or marking on the aircraft to indicate corroded areas.
Par 6-91 Page 6-23
d. Graphite should not be used as a lubricant for any component. Graphite is cathodic to all structural metals and will generate galvanic corrosion in the presence of moisture, especially if the graphite is applied in dry form.
e. Footwear and clothing should be inspected for metal chips, slivers, rivet cuttings, dirt, sand, etc., and all such material removed before walking or working on metal surfaces such as wings, stabilizers, fuel tanks, etc.
f. Do not abrade or scratch any surface unless it is an authorized procedure. If surfaces are accidentally scratched, the damage should be assessed and action taken to remove the scratch and treat the area.
g. Coated metal surfaces should not be polished for aesthetic purposes. Buffing would remove the protective coating, and a brightly polished surface is normally not as corrosion resistant as a nonpolished surface unless it is protected by wax or paint.
h. Protect surrounding areas when welding, grinding, or drilling to prevent contamination with residue from these operations. In those areas where protective covering cannot be used, remove the residue by cleaning.
i. Severely corroded screws, bolts, and washers should be replaced. When a protective coating, such as a cadmium plating on bolts, or screws, is damaged, immediately apply an appropriate protective finish to prevent additional corrosion damage.
6-92.–6-112. [RESERVED.]
Par 6-113 Page 6-24 SECTION 7. BASIC CORROSION REMOVAL TECHNIQUES
6-113. GENERAL. When active corrosion is found, a positive inspection and rework program is necessary to prevent any further deterioration. The following methods of assessing corrosion damage and procedures for rework of corroded areas could be used during cleanup programs. In general, any rework would involve the cleaning and stripping of all finish from the corroded area, removal of corrosion products, and restoration of surface protective film.
a. Repair of corrosion damage includes removal of all corrosion and corrosion products.
When the corrosion damage is severe and exceeds the damage limits set by the aircraft or parts manufacturer, the part must be replaced.
b. If manufacturer information and limits are not available, then a DER must be consulted before the aircraft or part is returned to service.
6-114. PREPARATIONS FOR REWORK. All corrosion products should be removed completely when corroded structures are reworked. Before starting rework of corroded areas, carry out the following:
a. Document corrosion damage.
b. Position the aircraft in a wash rack or provide washing apparatus for rapid rinsing of all surfaces.
c. Connect a static ground line from the aircraft to a grounding point.
d. Prepare the aircraft for safe ground maintenance.
(1) Remove battery(s), liquid oxygen (LOX) generator container (if installed), and external hydraulic and electric power.
(2) Install all applicable safety pins, flags, and jury struts.
e. Protect the pitot-static ports, louvers, airscoops, engine opening, wheels, tires, magnesium skin panels, and airplane interior from moisture and chemical brightening agents.
f. Protect the surfaces adjacent to rework areas from chemical paint strippers, corrosion removal agents, and surface treatment materials.
6-115. FAIRING OR BLENDING REWORKED AREAS. All depressions resulting from corrosion rework should be faired or blended with the surrounding surface. Fairing can be accomplished as follows:
a. Remove rough edges and all corrosion from the damaged area. All dish-outs should be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. (Select the proper abrasive for fairing operations from Table 6-1.)
Par 6-115 Page 6-25 TABLE 6-1. ABRASIVES FOR CORROSION REMOVAL
METALS OR
MATERIALS TO BE
PROCESSED
RESTRICTIONS OPERATION ABRASIVE PAPER OR CLOTH ABRASIVE FABRIC OR PAD
ALUMINUM STAINLESS STEEL PUMICE 350 MESH OR FINER
ABRASIVE WHEEL
ALUMINUM OXIDE
SILICON
CARBIDE GARNET
FERROUS ALLOYS
CORROSION
REMOVAL
OR FAIRING 150 GRIT OR FINER 180 GRIT
OR FINER
FINE TO ULTRA FINE
X X X X FINISHING 400 X X X
ALUMINUM ALLOYS EXCEPT CLAD
ALUMINUM
DO NOT USE
SILICON
CARBIDE
ABRASIVE
CORROSION
REMOVAL
OR FAIRING 150 GRIT OR FINER 7/0 GRIT
OR FINER
VERY FINE AND ULTRA FINE
X X X FINISHING 400 X X CLAD
ALUMINUM
SANDING
LIMITED TO THE
REMOVAL OF MINOR
SCRATCHES
CORROSION
REMOVAL
OR FAIRING 240 GRIT OR FINER 7/0 GRIT
OR FINER
VERY FINE AND ULTRA FINE X X FINISHING 400 X
MAGNESIUM ALLOYS
CORROSION
REMOVAL
OR FAIRING 240 GRIT OR FINER
VERY FINE AND ULTRA FINE
X X X FINISHING 400 X X
TITANIUM CLEANING AND
FINISHING 150 GRIT OR FINER 180 GRIT
OR FINER X X X
Par 6-115 Page 6-26
b. In critical and highly stressed areas, all pits remaining after the removal of corrosion products should be blended out to prevent stress risers that may cause stress corrosion cracking.
(See Figure 6-14.) On a noncritical structure, it is not necessary to blend out pits remaining after removal of corrosion products by abrasive blasting, since this results in unnecessary metal removal.
FIGURE 6-14. TYPICAL EXAMPLE OF ACCEPTABLE CLEANUP OF
CORROSION PITS
c. Re work depressions by forming smoothly blended dish-outs, using a ratio of 20:1, length to depth. (See Figure 6-15.) In areas having closely spaced multiple pits, intervening material should be removed to minimize surface irregularity or waviness. (See Figure 6-16.)
Steel nut-plates and steel fasteners should be removed before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion. All corrosion products must be removed during blending to prevent reoccurrence of corrosion.
FIGURE 6-15. BLENDOUT OF CORROSION AS A SINGLE DEPRESSION REMOVED.
CORROSION DAMAGE BEFORE REWORK
LONGITUDINAL
ROUGH EDGES HAVE BEEN SMOOTHED AND ALL CORROSION HAS BEEN REMOVED. HOWEVER, DEPRESSION HAS NOT BEEN SHAPED. DISH-OUT AFTER BLENDING Par 6-115 Page 6-27
Par 6-115 Page 6-28 FIGURE 6-16. BLENDOUT OF MULTIPLE PITS IN A CORRODED AREA 6-
116. CORROSION REMOVAL BY BLASTING. Abrasive blasting is a process for cleaning or finishing ferrous metals by directing a stream of abrasive particles against the surface of the parts. Abrasive blasting is used for the removal of rust and corrosion and for cleaning prior to painting or plating. The following standard blast-cleaning practices should be adopted.
a. The part to be blast-cleaned should be removed from the aircraft, if possible.
Otherwise, areas adjacent to the part should be masked or protected from abrasive impingement and system (hydraulic, oil, fuel, etc.) contamination.
b. Parts should be dry and clean of oil, grease, or dirt prior to blast cleaning.
c. Close-tolerance surfaces, such as bushings and bearing shafts, should be masked.
d. Blast-clean only enough to remove corrosion coating. Proceed immediately with surface treatments as required.
6-117. CLEANERS, POLISHES, AND BRIGHTENERS. It is important that aircraft be kept thoroughly clean of contaminating deposits such as oil, grease, dirt, and other foreign materials.
a. Materials. Do not use harmful cleaning, polishing, brightening, or paint-removing materials. Use only those compounds that conform to existing government or established industry specifications or that have been specifically recommended by the aircraft manufacturer.
Observe the product manufacturer’s recommendations concerning use.