Par 6-20 Page 6-247
FIGURE 6-11. Galvanic corrosion of magnesium adjacent to steel fastener.
FIGURE 6-12. Stress corrosion cracking.
Salt solutions and sea water cause stress corrosion cracking of high strength, heat - strength, heat- treated steel and aluminum alloys.
Methyl alcohol-hydrochloric acid solutions will cause stress corrosion cracking of some titanium alloys.
Magnesium alloys may stress-corrode in moist air.
Stress Corrosion may be reduced by
• applying protective coatings,
• stress relief heat treatments,
• using corrosion inhibitors, or
• controlling the environment.
Par 6-20 Page 6-248 FATIGUE CORROSION. Fatigue corrosion involves cyclic stress and a corrosive environment. Metals may withstand cyclic stress for an infinite number of cycles so long as the stress is below the endurance limit of the metal. Once the limit has been exceeded, the metal will eventually crack and fail from metal fatigue. However, when the part or structure undergoing cyclic stress is also exposed to a corrosive environment, the stress level for failure may be reduced many times.
Thus, failure occurs at stress levels that can be dangerously low depending on the number of cycles assigned to the life -limited part.
Fatigue corrosion failure occurs in two stages. During the first stage the combined action of corrosion and cyclic stress damages the metal by pitting and crack formations to such a degree that fracture by cyclic stressing will eventually occur, even if the corrosive environment is completely removed.
The second stage is essentially a fatigue stage in which failure proceeds by propagation of the crack (often from a corrosion pit or pits) and. It is controlled primarily by stress concentration effects and the physical properties of the metal. Fracture of a metal part , due to fatigue corrosion, generally occurs
Par 6-29 Page 6-249
at a stress level far below the fatigue limit atof an uncorroded part, even though the amount of corrosion is relatively small.
FRETTING CORROSION. Fretting corrosion,(also known as wear corrosion or friction oxidation) can occur at the interface of two highly -loaded surfaces which are not supposed to move against each other. However, vibration may cause the surfaces to rub together resulting in an abrasive wear known as fretting. (See figure 6-13.) The protective film on the metallic surfaces is removed by this rubbing action. With continued rubbing, metal particles sheared from the surface of the metal readily combine with oxygen to form metal oxide. As these oxides accumulate, they cause damage by abrasive action and increased local stress. The most common example of fretting corrosion is the smoking rivet found on engine cowling and wing skins. This is one corrosion reaction that is not driven by an electrolyte, and in fact, moisture may inhibit the reaction.
Par 6-29 Page 6-250
Application of a lubricant or installation of a fretting -resistant material between the two surfaces can reduce fretting corrosion.
FIGURE 6-13. Fretting corrosion.
6-23.6-28. [RESERVED.]
Par 6-29 Page 6-251
SECTION 3. CORROSION PROTECTION MEASURES FOR BASIC
MATERIALS
GENERAL. In the repair of aircraft, apply corrosion proofing of the same type or equivalent to that originally applied unless the repair would result in increased susceptibility to corrosion, in which case use additional corrosion protection measures. The following is a list of the most commonly used corrosion - used corrosion-proofing techniques.
ANODIZING AND RELATED PROCESSES. In anodizing, aluminum alloys are placed in an electrolytic bath causing a thin film of aluminum oxide to form on the surface of the aluminum. This is resistant to corrosion and affords a good paint base. However, other processes, which do not provide as good a corcorrosive protection as anodizing, are good paint bases. The processes are:
Alkaline cleaning followed by chromic acid dip;
Alcoholic phosphoric acid cleaner; and
Alkaline dichromate treatment.
PLATING. Steels are commonly plated with other metals to prevent corrosion. Plating is accomplished by placing the article in an electrolytic bath. Metals used in plating vary in the corrosion protection they afford steel. For instance, in platings that corrode before steel, such as zinc or cadmium, slight breaks or cracks throughout the plating will not result in rusting of the exposed steel. With the surface metal corroded, the steel is protected.
However, when the steel corrodes faster than the plate metal, such as chromium, the amount of protection depends on the tightness of the plating. The process of pPost-plate bake treatment to relieve hydrogen embrittlement is a necessary part of replating procedures for high-strength steel parts. High-strength nuts and bolts are highly susceptible to failure from hydrogen embrittlement, at the many stress risers and notches in their design, through normal usage of sustained and dynamic stressesembrittlement. Because of the potential failures of embrittled parts, careful control over the heat treatment, grinding, preplate cleaning, plating, and post-plate baking of high-strength parts is necessary.
PHOSPHATE RUST-PROOFING. This process is commercially known as Parkerizing, Bonderizing, Granodizing, etc. The coating placed on the part is used to protect steel parts after machining and before painting.
CHROME-PICKLE TREATMENT. Magnesium parts which have been immersed or brushed with a solution of nitric acid and sodium dichromate will be protected for temporary storage. The coating will also serve as a bond for subsequent organic finishes. Sealed chrome-pickle treatment is used on magnesium parts for long term protection.
Diluted chromic acid is a touch-up treatment. It is less critical to apply and can be applied over previously -applied thin chromate films.
DICHROMATE TREATMENT. This treatment consists of boiling magnesium parts in a solution of sodium dichromate. It provides good paint base and protective qualities on all standard wrought magnesium alloys except the magnesium-thorium alloys. Acid pickling of the magnesium surface prior to application of the dichromate treatment is required if maximum corrosion resistance of the finish is expected.
STANNATE IMMERSION TREATMENT.
Stannate immersion treatment deposits a layer of tin. It is a protective paint base for magnesium alloy parts which contain inserts and fasteners of a dissimilar metal such as
Par 6-50 Page 6-252 brass, copper, or steel. This treatment cannot be used with part s containing aluminum insert s or fasteners because the high alkalinity of the bath attacks the aluminum.
GALVANIC ANODIZING TREAT-MENT. An electrolytic process that provides a paint base and corrosion -preventive film on magnesium alloys containing manganese.
CLADDING. Aluminum alloys which are susceptible to corrosion are frequently clad with pure aluminum. Slight pits, scratches, or other defects through the cladding material must be avoided, since the aluminum alloy core will corrode rapidly.
METAL SPRAYING. Metal is melted and sprayed on the surface to be protected. The surface must be properly prepared and thoroughly cleaned to prevent peeling of the sprayed coat.
SHOT-PEENING. Shot-peening and other treatments, by which the surface can be placed in compression, are effective in preventing stress corrosion.
Par 6-50 Page 6-253 ORGANIC COATINGS. Zinc chromate primer, enamels, chlorinated rubber compounds, etc., are organic coatings commonly used to protect metals.
DOPE PROOFING. When doped fabrics are applied over an organic finished metal structure, the dope will have a tendency to loosen the finish on the metal. For this reason, organic coatings on the metal are usually covered with a dope -proof paint, with metal foil, or with cellulose tape to prevent the dope from soaking through.
TUBE INTERIORS. Protect the interiors of structural steel and aluminum tubing against corrosion. A small amount of water entrapped in a tube can corrode entirely through the tube thickness in a short period. Coat the tube interior by flushing with hot linseed oil, paralketone, or other approved corrosion inhibitor. The flushing liquid is usually introduced through small holes drilled in the tubing. Allow the flushing liquid to drain and plug the holes with a screw or by other means to prevent entry of moisture. Air and watertight sealing of the tubing will also give adequate protection against corrosion if the tubing is internally dry before being sealed.
6-43.6-49. [RESERVED.]
Par 6-50 Page 6-254 SECTION 4. CORROSION PREVENTIVE MAINTENANCE
GUIDELINES: ALL AIRCRAFT. Corrosion prevention depends on a comprehensive prevention and control plan, implemented from the start of operation of an aircraft, which includes:
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.
Inspection for corrosion on a scheduled basis.
Thorough cleaning , inspection, lubrication, and preservation at prescribed intervals.
Prompt corrosion treatment after detection.
Accurate record-keeping and reporting of material or design deficiencies to the manufacturer and the Federal Aviation Administration (FAA).
Use of appropriate materials , equipment, and technical publications.
Maintenance of the basic finish systems.
Keeping drain holes and passages open and functional.
Sealants, leveling compounds, miscellaneous debris, or corrosion inhibitors should not block drain paths.
Replacing deteriorated or damaged gaskets and sealants (using non-corrosive type sealants) to avoid water intrusion and entrapment that leads to corrosion.
Minimizing the exposure of aircraft to adverse environments by keeping the aircraft in a hangar.
Periodic and frequent inspection of areas where there are foamed plastics or other absorbent material.
Daily draining of fuel cavities to remove accumulated water and other foreign matter.
Daily wipe-down of exposed critical surfaces of hydraulic cylinders.
GUIDELINES: AIRCRAFT OPERATING OVER SALT WATER. In addition to the inspection and treatment prescribed above, the following treatment shall be applied:
Remove all traces of salt water and salt water residue by thoroughly washing the aircraft with fresh water.
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.
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
Par 6-63 Page 6-255 paint. Apply with a cloth or a soft brush soaked in the mixture.
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.
Where maximum corrosion protection is desired on stationary parts, use exte rior surface corrosion preventive compound, Specification MIL-C-16173, grade I.
Wipe the exposed portion of the landing gear shock strut piston with a cloth soaked in the applicable hydraulic fluid.
Par 6-63 Page 6-256 Most parts of landing gear wheels are made from magnesium or aluminum alloys which corrode rapidly unless carefully protected. When the aircraft operates near salt water and off coral beach es, the corrosion can be very rapid.
Inspect wheels to determine the paint condition.
Refinish portions of a wheel where paint has deteriorated, peeled, or chipped.
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 6-257 SECTION 5. VISUAL CORROSION INSPECTION GUIDE FOR AIRCRAFT
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.
EXHAUST TRAIL AREAS.
Visually inspect paint in areas of the exhaust trails for damage.
Visually inspect under fairings, around rivet heads, and in skin crevices, for corrosion in areas of engine exhaust trail.
BATTERY COMPARTMENTS AND BATTERY VENT OPENINGS.
Inspect battery compartment for electrolyte spillage, corrosion, and condition of protective paint.
Inspect area around battery vent for corrosion.
LAVATORIES AND GALLEYS.
Inspection 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.
BILGE AREAS.
Inspect bilge areas for waste hydraulic fluids, water, dirt, loose fasteners, drill chips, and other debris.
Remove any foreign material from bilge and inspect for corrosion.
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.
EXTERNAL SKIN AREAS.
Inspect external skin surfaces for damage to protective finishes and corrosion.
Inspect around fasteners for damage to protective finishes and corrosion.
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 i ndicate severe corrosion in bonded joints.
Inspect area around spot welds for bulges, cracks, or corrosion.
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.
Par 6-89 Page 6-258 Inspect thick alloy skin surfaces for pitting, intergranular corrosion, and ex foliation of the metal. Look for white or gray deposits around countersunk fastener heads and raised areas or bumps under the paint film.
Inspect composite skins for corrosion of attachment fasteners.
WATER ENTRAPMENT AREAS. Inspect area around edge of drain holes for corrosion and ensure that drain holes are not blocked by debris.
ENGINE FRONTAL AREAS.
Inspect reciprocating engine cylinder fins, engine cases, and cooling air ducts for damage to exterior finish and corrosion.
Inspect radiator cooler cores for corrosion.
ELECTRONIC PACKAGE COMPARTMENTS.
Inspect circuit-breakers, contact points, and switches for evidence of moisture and corrosive attack.
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.
FLEXIBLE HOSE ASSEMBLIES.
Inspect hose assemblies for chafing, weather-checking, hardening, discoloration, evidence of fungus, torn weather protective coatings or sleeves, and corrosion of fittings.