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6.72.2 Other preparations, such as phosphoric acid or naval jelly, are effective rust converters, where tolerances are not critical and personnel can thoroughly rinse and neutralize residual acid.
6.72.3 These situations are generally not applicable to assembled aircraft. Chemical inhibitors on installed steel parts is not only undesirable, but very dangerous. The possibility of entrapping corrosive solutions, resulting in uncontrolled attack when such materials are used under field conditions, outweighs any advantage to be gained from their use.
6.73 Removal of Corrosive Products from High-Stressed Steel Parts. Do not use wire brushes on high-stressed steel parts. Any corrosion on the surface of a highly-stressed steel part is potentially dangerous; carefully remove corrosion products. Surface scratches or change in surface structure from overheating can cause sudden failure of these parts.
Instead, carefully use mild abrasive papers, such as fine grit aluminum oxide, or fine buffing compounds, such as rouge, on cloth buffing wheels to remove corrosion products.
• It is critical to not overheat steel surfaces.
• Abrasive blasting is also a satisfactory corrosion removal method for high-strength steel on aircraft exteriors.
• After careful removal of surface corrosion, immediately apply protective paint finishes.
6.74 Special Treatment of Stainless Steel Alloys. Do not use chemical cleaners on stainless steels. Stainless steels are of two general types: magnetic and nonmagnetic. Magnetic steels are of the ferritic or martensitic types and are identified by numbers in the 400 series. Corrosion often occurs on 400-series stainless steels and treatment is the same as specified in paragraph 6.73. Nonmagnetic steels are of the austenitic type and are identified by numbers in the 300 series. They are much more corrosion resistant than the 400-series steels, particularly in a marine environment.
6.74.1 Austenitic steels develop corrosion resistance by an oxide film which should not be removed even though the surface is discolored. The original oxide film is normally formed at time of fabrication by passivation. If this film is broken by accident or abrasion, it may not restore itself without repassivation.
6.74.2 If any deterioration or corrosion does occur on austenitic steels, and the structural integrity or serviceability of the part is affected, it will be necessary to remove the part.
6.75 Example of Process for Removal of Corrosion from Steel Parts. If possible, corroded steel parts should be removed from the aircraft. When impossible to remove the part, observe the aircraft preparations and safety precautions in paragraphs 6.1, 6.2, and 6.10.
Chemical removal or chemical conversion coatings are not allowed on steel parts.
6.75.1 Positively identify the metal as steel, as detailed in paragraph 6.17, and establish its heat-treated value.
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6.75.2 Clean area to be reworked.
Caution: Do not use acid-based strippers on high-strength steel parts.
6.75.3 Strip paint, if required.
6.75.4 Removing corrosion from steels treated to 220,000 psi and above should be accomplished only by hand sanding or dry abrasive blasting (see Section 6, Magnesium Alloys).
6.75.5 Mechanically remove all degrees of corrosion from steel parts heat treated below 220,000 psi as follows:
Caution: Use goggles or face shield to preclude injury from flying particles.
Protect adjacent areas to prevent additional damage from corrosion products removed by mechanical process.
6.75.5.1 Remove heavy deposits of corrosion products using a stainless steel hand brush. Abrasive blasting is an alternate method of corrosion removal. For abrasive blasting, use aluminum oxide (A-A-59316, Abrasive Materials; for Blasting) Type I, Grades A or B, grit sizes 25, 50, or 120; or size 13 glass beads (MIL-SPEC MIL-G-9954). Use an air pressure at the nozzle of 40 to 50 psi for direct-pressure machines when removing corrosion by abrasive blasting. Engineering approval from the aircraft manufacturer should be obtained prior to abrasive blasting metal thinner than 0.0625 inch.
6.75.5.2 Remove residual corrosion by hand sanding or with an approved hand-operated power tool.
6.75.5.3 The surface is highly reactive immediately following corrosion removal, so apply primer coats within 1 hour after sanding. After removing all corrosion visible through a magnifying glass, continue with paragraph 6.75.6.
6.75.6 Fair depressions resulting from rework using a blend ratio of 20:1. Clean rework area using 240 grit abrasive paper. Smooth with 300 grit and final polish with 400 grit abrasive paper.
6.75.7 Determine depth of faired depressions and confirm that the depth has not exceeded rework limits.
6.75.8 Clean reworked area with dry cleaning solvent. Do not use kerosene.
6.75.9 Apply protective finish or specific organic finish, as required.
6.75.10 Remove masking and protective covering.
6.76–6.79. RESERVED.
6-39 SECTION 8. PLATED PARTS
6.80 Chromium and Nickel Plated Parts. Nickel and chromium platings are used extensively as protective and wear-resistant coatings over high-strength steel parts, such as landing gear journals or shock strut pistons. Chromium and nickel plate provide protection by forming a somewhat impervious physical coat over the underlying base metal. When breaks occur in the surface, the protection is destroyed.
6.80.1 Reworking of chromium and nickel plated components is limited due to the critical requirements to which such components are subjected.
6.80.2 The rework should consist of light buffing to remove corrosion products and produce the required smoothness. This is permissible, provided the buffing does not reduce the plating’s depth below the minimum allowable thickness.
6.80.3 Whenever a chromium or nickel plated component requires buffing, coat the area with a corrosion preventive compound, if possible.
6.80.4 Remove and replace the component when buffing exceeds the minimum thickness, or when the base metal has sustained corrosive attack.
6.80.5 The removed component can be restored to serviceable condition by having trained personnel in a specialized shop completely stripping the old plating and replating it per acceptable methods and specifications.
6.81 Cadmium and Zinc Plated Parts. Cadmium plating is used extensively in aircraft construction as a protective finish over both steel and copper alloys. Protection is provided on a sacrificial basis in which the cadmium is the sacrificial material rather than the underlying base material. Properly functioning cadmium surface coatings may well show mottling, ranging from white-to-brown-to-black spots on their surfaces. These are indicative of the sacrificial protection being offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance’s sake. In fact, cadmium will continue to protect even when actual breaks in the coating develop and bare steel or exposed copper surfaces appear.
6.81.1 Where actual failures of the cadmium plate occur and corrosion products of the base metal initially appear, there may be some necessary mechanical cleaning of the area, but limit it to removal of the corrosion products from the underlying base material.
6.81.2 Under no condition should such a coating be cleaned with a wire brush. If protection is needed, apply a touch-up with primer or a temporary preservative coating. Restoration of the plate coating is impracticable in the field.
6.81.3 Zinc coatings offer protection in an identical manner to cadmium, and the corrective treatment for failure is generally the same as for cadmium plated parts.
6.82–6.89. RESERVED.
6-40 SECTION 9. OTHER METALS AND ALLOYS
6.90 Noble Metal Coatings—Cleanup and Restoration. Silver, platinum, and gold finishes are generally used in aircraft assemblies because of their resistance to ordinary surface attack and their improved electrical or heat conductivity. Silver plated electrodes can be cleaned of brown or black sulfide tarnish, as necessary, by placing them in contact with a piece of magnesium sheet stock while immersed in a warm water solution of common table salt mixed with baking soda or by using a fine grade abrasive mat or pencil eraser followed by solvent cleaning. If assemblies are involved, careful drying and complete displacement of water is necessary. In general, cleaning of gold or platinum coatings is not recommended in the field.
6.91 Copper and Copper Alloys. Copper and copper alloys are relatively corrosion resistant, and attacks on such components will usually be limited to staining and tarnish. Generally, such change in surface condition is not dangerous and should ordinarily have no effect on the function of the part. However, if it is necessary to remove such staining, a chromic acid solution of 8 to 24 ounces per gallon of water containing a small amount of battery electrolyte, not to exceed 50 drops per gallon, is an effective brightening bath. Staining may also be removed using a fine grade abrasive mat or pencil eraser followed by solvent cleaning.
6.91.1 Immerse the stained part in the cold solution. Surfaces can also be treated in place by applying the solution to the stained surface with a small brush.
6.91.2 Be careful to avoid any entrapment of the solution after treatment. The part should be cleaned thoroughly following treatment with all residual solution removed.
6.91.3 Serious copper corrosion is evident by the accumulation of green-to-blue copper salts on the corroded part. These products should be removed mechanically using a stiff bristle brush, brass wire brush, 400-grit abrasive paper or bead blast with glass beads, MIL-SPEC MIL-G-9954, size 13. When bead blasting, air pressure at the nozzle should be 20 to 30 psi for direct-pressure machines. Do not bead blast braided copper flexible lines. A surface coating should be reapplied over the reworked area. Again, chromic acid treatment will tend to remove the residual corrosion products.
6.91.4 Most brass and bronze structural parts will be protected by a cadmium surface plate. The mottling of the protective cadmium coat should not be removed, and mechanical surface cleaning should not be attempted unless actual copper corrosion products are beginning to appear. Under these conditions, any mechanical removal of the protective cadmium should be held to a minimum and limited to the immediate area of the copper attack.
6.92 Titanium Alloys. Titanium and its alloys are highly corrosion resistant because a protective oxide film forms on alloy surfaces upon contact with air. When titanium is heated, different oxides having different colors form on the surface. A blue oxide coating will form at 700 to 800 °F, a purple oxide at 800 to 950 °F, and a gray or black oxide at 1,000 °F or higher. Corrosive attack on titanium surfaces is difficult to detect. It may
6-41 show deterioration from the presence of salt deposits and metal impurities at elevated temperatures, so periodic removal of surface deposits is required.
6.92.1 Clean titanium surfaces until all traces of corrosion or surface deposits are removed using one of the following:
• Stainless steel wire brush;
• Hand sand with aluminum oxide abrasive paper or abrasive mat;
• Dry blast with glass beads, (MIL-SPEC MIL-G-9954) sizes 10, 11, 12, or 13, or aluminum oxide (A-A-59316, Type I) Grades A or B, using 40 to 50 psi air pressure at the nozzle for direct-pressure machines; or
• Hand polish with aluminum polish and soft cloth.
6.92.2 Chlorinated hydrocarbon cleaners should not be used on titanium alloys, which are subject to elevated temperatures in service. Such solvents can cause stress-corrosion in titanium.
6.93–6.99. RESERVED.
7-1 CHAPTER 7. SPECIAL PROBLEMS
7.1 Mercury Spills/Corrosion Damage.
7.1.1 The presence of mercury and mercury salts in air cargo is a definite possibility. Loading, unloading, and general shifting of such cargo can and does result occasionally in such situations as damaged containers, cartons, or electronic tubes, with subsequent possible leakage of mercury on aircraft structure.
7.1.2 Spillage of mercury or mercury compounds within an airplane requires immediate action for isolation and recovery to prevent possible corrosion damage and embrittlement of aluminum alloy structural components, 300-series and 400-series stainless steels, and unplated brass components, such as cable turnbuckle barrels.
7.1.3 Mercury, by the amalgamation process, can penetrate any break in the finish (anodize/alodine), paint, and seal coating of a metal structural component. Bright, polished, shining, or scratched surfaces will hasten the process, as well as moisture.
Mercury or mercury compounds attack the metal grain boundaries and seriously embrittle and reduce the strength of parts. Corrosive attack of freshly scratched aluminum alloy is very rapid, and complete penetration of sheet material is known to occur within 3 or 4 minutes.
7.1.4 Mercury is highly toxic and spreads very easily from one surface to another. It adheres to hands, shoes, clothes, and tools. The following precautions for mercury spills should be strictly followed:
7.1.4.1 Avoid contact with surfaces suspected of being contaminated. Use wood or fiber sheets to support body while working in the area.
WARNING: Do not attempt to pick up mercury by hand.
7.1.4.2 Wear wing socks (shoe protectors) and protective disposable clothing in contaminated area to prevent scratching metal surfaces. Properly dispose of socks after use.
7.1.4.3 Do not wear clothing used in contaminated areas on jobs in uncontaminated areas. Dispose of wing socks and protective clothing in an unused metal container outside of aircraft. Contact the waste management service or similar authority of the local state health department for instructions on proper disposal procedures for mercury.
7.1.4.4 Have personal clothing cleaned. Wash shoes with soap and water. Clean all tools used in the contaminated area with steam or hot water and soap. Discard any drill bits used on mercury-contaminated areas. Thoroughly clean vacuum cleaner, if used.
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7.1.4.5 Always wash hands thoroughly with soap and water after contacting mercury.
Keep hands away from mouth. Do not eat, smoke, or blow nose without first washing hands thoroughly.
7.1.4.6 Appreciable amounts of mercury will vaporize at normal temperatures.
Stagnant air will become dangerous to personal health.
WARNING: Always provide good ventilation while cleaning mercury-contaminated areas.
7.1.4.7 Do not use cleaning aids such as solvents, solids, or polishes on contaminated areas. Such materials may promote corrosion.
7.1.4.8 If hands become contaminated with mercury while working with cleaning equipment, do not touch any exposed metal in the surrounding area to prevent contamination.
7.1.5 Mercury spills, in some cases, have caused no adverse effects because of protective films of paint, dirt, grease, or oil. Do not rely on the fact that mercury spillage may not, in all cases, result in serious damage. Each instance of mercury or mercury compound spillage should be considered hazardous, and immediate action should be taken to safeguard the aircraft. Inspect for mercury and corrosion as follows:
7.1.5.1 Determine point at which mercury was spilled. Remove any mercury on the floor covering, and then remove covering.
7.1.5.2 When mercury is found on the floor, do not remove access/inspection plates, screws, rivets, bolts, etc., from the floor. Any hole which is left open in the contaminated area of the floor may allow mercury to spread to the structure underneath the floor.
7.1.5.3 Inspect the metal floor, seat tracks, cargo rails, and adjacent structure for mercury and corrosion. Inspect skin and internal structure below the point of spillage. Also inspect the lowest point in fuselage below cargo compartment floor if mercury spill has occurred in the cargo area. Mercury liquid will flow to the lowest level.
7.1.5.4 Inspect areas suspected of having mercury contamination using a l0-power magnifying glass. If corrosion of aluminum has started, there will be a light grayish “Christmas Tree” fuzz or gray powdery dust deposit, and severe structural damage may follow.
7.1.5.5 If corrosion is evident and cleanup cannot be completed immediately, coat contaminated area with corrosion preventative compound or engine oil. This helps to slow down the corrosion rate and also helps to prevent spreading of mercury contamination.
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7.1.5.6 When mercury spills occur in a lower cargo compartment, use a portable X-ray machine, if available, along the outside lower surface of the fuselage to check suspected hidden corrosion areas between the skin, stringers, and frames below the floor. Droplets of mercury will show on an X-ray image as small, white spots. Corrosion and embrittlement will appear as tree-like forms completely penetrating a structural component.
7.1.5.7 Inspect bronze/brass control cable turnbuckle barrels for mercury discoloration. Replace if the slightest discoloration is detected.
7.1.6 Removal procedure and precautions necessary when mercury is spilled are:
WARNING: Vacuum air passing over the mercury deposited in the glass container picks up mercury vapors and exhausts them from the vacuum cleaner.
Avoid breathing mercury vapors.
CAUTION: An air hose should not be used to dislodge mercury from aircraft under any circumstances.
7.1.6.1 Use a high-capacity vacuum cleaner with a trap-type glass container attached to the large vacuum hose. The size of the pickup hose from the container should be about 1/4 inch in diameter to increase the amount of suction applied to the mercury. Due to the weight of mercury, the container will catch the mercury before it can enter the vacuum cleaner hose.
7.1.6.2 An all rubber storage battery water syringe, or a medicine dropper, may be used to remove mercury if the trap-type glass container and vacuum cleaner are not available. Cellulose tape may be used to pick up very tiny particles of mercury.
7.1.6.3 General cleanup and inspection, using the equipment available, should be made immediately after spillage occurs or is detected.
7.2 Corrosion Protection for Agricultural Aircraft.
7.2.1 General. Practically all chemicals used in dusting and spraying operations are corrosive by nature and hasten deterioration of fabric, metal, and wood. It is essential to safe operation that precautions be taken to prevent corrosion and deterioration of wood, metal, and fabric. Cleanliness of the airplane is one of the most important precautions of all. If it were practicable and feasible to clean the airplane thoroughly, that is, give it a thorough dry washing inside and out after each day’s work, probably no special corrosive preventative measures would be necessary.
7.2.2 Precautions. Since thorough daily cleaning is not practicable, take the following precautions:
7.2.2.1 All fittings and metal structures should be covered with two coats of epoxy primer, a heavy preservative, conforming to MIL-SPEC MIL-C-16173,
7-4 Corrosion Preventative Compound, Solvent Cutback, Cold-Application, Grade 2 (paralketone), or equivalent material. This coating should be applied to items such as wing-root fittings, wing-strut fittings, control-surface hinges, horns, or mating edges of fittings and attach bolts.
7.2.2.2 Non-stainless steel control cables should be coated with paralketone or equivalent protective coating, or should be replaced with corrosion-resistant cables.
7.2.2.3 Periodic inspection of all critical portions of the aircraft structure should be made. Structural parts showing corrosion should be cleaned and refinished if the corrosion attack is superficial. If the part is severely corroded, it should be replaced.
7.2.2.4 Experience has shown that additional access openings, enabling ready inspection of lower and rearward portions of the fuselage, are particularly desirable.
7.2.2.5 Provide additional drainage and ventilation for all interiors to prevent collection of moisture.
7.2.2.6 At the time of recovering, both metal and wood airplane structural members should be coated with two coats of epoxy primer, followed by dope-proof paint or wrapping with cellophane tape.
7.2.2.7 Spray interiors of metal-covered wings and fuselages with an adherent corrosion inhibitor.
7.2.2.8 Wash exterior surfaces with clear fresh water at least once a week. Interior surfaces should also be washed, taking care to prevent damage to electrical circuits or other items subject to malfunctioning due to moisture.
7.2.2.9 Opening in the wings, fuselage, and control-surface members, such as tail-wheel wells or openings for control cables, should be sealed as completely as possible to prevent entry of dust or spray.
7.3–7.99. RESERVED.
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