Par 6-179 Page 6-280 EXAMPLE OF REMOVING CORROSION FROM FERROUS METALS. If possible, corroded steel parts should be removed from the aircraft. When impractical to remove the part, follow the procedure below.
Prepare the area for rework.
Positively identify the metal as steel and establish its heat-treated value.
Clean the area and strip paint if required.
NOTE: Use of acid-based strippers, chemical rem overs, or chemical conversion coatings are not permitted on steel parts without engineering authorization.
Determine extent of corrosion damage.
Remove residual corrosion by hand sanding with mild abrasive ma ts, cloths, and papers, such as fine aluminum oxide grit.
Remove heavy deposits of corrosion products by approved mechanical methods for that particular form of steel and/or stainless steel.
Par 6-179 Page 6-281 Inspect the area for remaining corrosion. Rep eat procedure if any corrosion remains and the structural integrity of the part is not in danger, and the part meets the rework limits established by the manufacturer or FAA authorized DER.
Fair depressions using a blend ratio of 20:1. Clean area using 240-grit paper. Smooth area with 300-grit paper and give final polish with 400-grit paper.
Determine depth of faired depression to ensure that rework limits have not been exceeded.
Clean reworked area with dry cleaning solvent. Do not use kerosene.
Apply protective finish or specific organic finish as required.
NOTE: Steel surfaces are highly-rea ctive immediately following corrosion removal; consequently, primer coats should be applied within 1 hour after sanding.
Remove masking and protective coverings.
6-168.6-178. [RESERVED.]
Par 6-179 Page 6-282 SECTION 11. OTHER METALS AND ALLOYS
NOBLE METAL COATINGS - CLEANUP AND RESTORATION. Silver, platinum, and gold finishes are 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, 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.
COPPER AND COPPER ALLOYS are relatively corrosion resistant, and attack on such components will usually be limited to staining and tarnish. 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.
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.
Avoid any entrapment of the solution after treatment.
Clean the part thoroughly following treatment with all residual solution removed.
Serious copper corrosion is evident by the accumulation of green-to-blue copper salts on the corroded part. Remove these products mechanically using a stiff bristle brush, brass wire brush, 400-grit abrasive paper or bead blast with glass beads, (specification MIL-G-9954, size 13). Air pressure when blasting should be 20 to 30 psi for direct pressure machines. Do not bead blast braided copper flexible lines. Reapply a surface coating over the reworked area. Chromic acid treatment will tend to remove the residual corrosion products.
WARNING: Brushing, sanding, and abrasive blasting of copper and copper alloys can be dangerous due to the creation of toxic airborne particles. Take necessary precautions to ensure safety.
TITANIUM AND TITANIUM ALLOYS are highly corrosion resistant because an oxide film forms on their 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 1000 °F or higher. These coatings are protective discolorations and should not be removed.
Corrosive attack on titanium surfaces is difficult to detect. It may show deterioration from the presence of salt deposits and metal impurities at elevated temperatures so periodic removal of surface deposits is required. However, if corrosion develops on titanium, it usually occurs as pitting. Acceptable methods for corrosion removal are:
Par 6-193 Page 6-283 (and 6-40) Stainless steel wool or hand brush.
Abrasive mats, cloths, and papers with either aluminum oxide or silicon carbide grit.
Dry abrasive blasting using glass beads (spec. MIL-G-9954) sizes 10-13 or Aluminum oxide (spec.
MIL-G-21380, type I, grades A or B) at a blast pressure of 40 to 50 psi (if using suction equipment use 50 percent higher pressure).
WARNING: Dry abrasive blasting of titanium alloys creates sparking. Ensure that hazardous concentrations of flammable vapors are not present.
Par 6-193 Page 6-284 (and 6-40) Hand polish with aluminum polish and soft cloth.
Titanium surfaces are susceptible to hydrogen embr ittlement that can induce stress co rrosion and associated pitting.
Therefore, chemicals such as fire-resistant hydraulic fluids must be controlled. Chlorinated hydrocarbon solvents and chemical corrosion removers are prohibited from use on titanium and titanium alloys.
6-182.6-192. [RESERVED.]
Par 6-193 Page 6-285 (and 6-40) SECTION 12. PLATED PARTS
CHROMIUM AND NICKEL-PLATED PARTS.
Nickel and chromium platings are used extensively as protective and wear-resistant coatings over high-strength steel parts (landing gear journals, shock strut pistons, etc.).
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.
The amount of reworking that can be performed on chromium and nickel-plated components is limited.
The rework should consist of light buffing to remove corrosion products and produce the required smoothness. The buffing should not take the plating below the minimum allowable thickness.
Whenever a chromium or nickel-plated component requires buffing, coat the area with a corrosionpreventive compound, if possible.
When buffing exceeds the minimum thickness of the plating, or the base metal has sustained corrosive attack, the component should be removed and replaced.
The removed component can be restored to serviceable condition by having the old plating completely stripped and re plated in accordance with acceptable methods and specifications.
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 by a sacrificial process in which the cadmium is attacked rather than the underlying base material. Properly functioning cadmium surface coatings may show mottling, ranging from white to brown to black spots on their surfaces. These show the sacrificial protection being offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance sake. In fact, cadmium will continue to protect even when actual breaks in the coating develop and bare steel or exposed copper surfaces appear.
When the breakdown of the cadmium plating occurs and the initial appearance of corrosion products on the base metal develops, some mechanical cleaning of the area may be necessary but shall be limited to removal of the corrosion products from the underlying base material.
Under no condition should such a coating be cleaned with a wire brush. If protection is needed, a touch-up with primer or a temporary preservative coating should be applied. Restoration of the plate coating cannot be done in the field.
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.
However, the amount of zinc on aircraft structures is very limited and usually does not present a maintenance problem.
6-195.6-205. [RESERVED.]
Par 6-206 Page 6-286
SECTION 13. CORROSION PROOFING OF LAND PLANES
CONVERTED TO SEA PLANES
GENERAL. A special problem is encountered in the conversion of land planes to seaplanes.
In general, land planes do not receive corrosion proofing to the same extent as do seaplanes. Corrosion-proofing standards for land planes converted to seaplanes are divided into two classes, necessary minimum precautions and recommended precautions.
Regardless of such precautions, it is imperative that the exterior surfaces of seaplanes be washed with clear fresh water immediately following extended water operation, or at least once a day when operated in salty or brackish water. Wash interior surfaces of seaplanes exposed to spray, taking care to prevent damage to electrical circuits or other items subject to injury.
NECESSARY MINIMUM PRECAUTIONS. The following procedures are considered the minimum to safeguard the airworthiness of converted aircraft and are not in themselves intended to maintain airworthiness for an indefinite period.
Unless already protected , treat exposed fittings or fittings that can be reached through inspection openings with two coats of zinc chromate primer, paralketone, nonwater-soluble heavy grease, or comparable materials. This applies to items such as wing-root fittings, wing-strut fittings , control-surface hinges, horns, mating edges of fittings, and attached bolts.
Coat non-stainless control cables with grease or paralketone or other comparable protective coating, if not replaced with corrosion-resistant cables.
Inspect all accessible sections of aircraft structure.
Clean structural parts showing corrosion and refinish if corrosion attack is superficial.
If a part is severely corroded, replace with an adequately corrosion-proofed part.
RECOMMENDED PRECAUTIONS.
Recommended precautions are those which are suggested as a means of maintaining such aircraft in condition for safe operation over extended periods.
Provide additional inspection openings to assist in detecting corrosion. Experience has shown openings to allow inspection of the lower and rearward portion of the fuselage to be particularly desirable.
Incorporate additional provisions for free drainage and ventilation of all interiors to prevent collection of moisture (scoop-type marine drain grommets).
Protect the interior of structural steel tubing. This may be done by air and watertight sealing or by flushing with hot linseed oil and plugging the openings. Inspect tubing for missing sealing screws, presence of entrapped water, local corrosion around sealing screws, welded clusters, and bolted fittings, which may be indicative of entrapped moisture.
Slit the fabric of fabric-covered aircraft longitudinally on the bottom of the fuselage and tail structure for access to these sections. Coat the lower structural members with zinc chromate primer (two coats); follow by a coat of dope-proof paint or wrap with cellophane tape and rejoin the fabric. This precaution is advisable within a few months after start of operation as a seaplane.
Spray the interior of metal-covered wings and fuselages with an adherent corrosion inhibitor.
Par 6-220 Page 6-287 Place bags of potassium or sodium dichromate in the bottom of floats and boat hulls to inhibit corrosion.
Prevent the entry of water by sealing, as completely as po ssible, all openings in wi ngs, fuselage, control-surface members, openings for control cables, tail-wheel wells, etc.
6-209.6-219. [RESERVED.]
Par 6-220 Page 6-288 SECTION 14. HANDLING AND CARE OF AIRCRAFT RECOVERED FROM WATER IMMERSION.
GENERAL. Aircraft recovered from partial or total immersion in standing water or flash floods require an in-depth inspection and cleaning of both the exterior and interior areas.
Water-immersion increases the probability of corrosive attack, it removes lubricants, deteriorates aircraft materials, and destroys electrical and avionics components.
Sea water, because of salt content, is more corrosive than fresh water. However, fresh water may also contain varying amounts of salt and, as drying occurs, the salt concentration is incr eased and corrosive attack accelerated.
Prompt action is the most important factor following recovery of an aircraft from water-immersion.
Components of the aircraft which have been immersed, such as the powerplant, accessories, airframe sections, actuating mechanisms, screws, bearings, working surfaces, fuel and oil systems, wiring, radi os, and radar should be disassembled, as necessary, and the contaminants completely removed.
INITIAL FRESH WATER OR DETERGENT WASH.
As soon as possible after the aircraft is recovered from water-immersion, thoroughly wash all internal and external areas of the aircraft using a water/detergent solution as follows:
Mix liquid detergent (MIL-D-16791, type I) and isopropyl alcohol (TT-I-735) in ratio of eight parts detergent, to 20 parts of alcohol. Add the detergent/alcohol mixture to 72 parts of tap water and mix thoroughly. For use, a dd one part of the preceding concentrate to nine parts of tap water (warm water if available) and mix thoroughly.
If the above specified detergent/alcohol materials are not available, use water-emulsion cleaning compound (MIL-C-43616). Add one part compound to nine parts water. If the MIL cleaning compound is not available, use any available mild household detergent solution with fresh tap water.
RECIPROCATING ENGINES AND PROPELLERS.
Remove the propeller from the engine and the engine from the aircraft. The exterior of the engine and propeller should be washed with steam, or fresh water, preferably hot.
Major accessories, engine parts, etc., should be removed and all surfaces flushed with fresh water, preferably hot. If facilities are available, immerse the removed parts, time permitting, in hot water or hot oil, 180 °F, for a short time. Soft water is preferred.
Change the water frequently. All parts must be completely dried by air blast or other means. If no heatdrying facility is available, wipe the cleaned parts with suitable drying cloths.
The constant-speed propeller mechanism should be disassembled, as required, to permit complete decontamination. Clean parts with steam or fresh water, preferably hot. Dry the cleaned parts in an oven, but if a heat-drying facility is not available, wipe the cleaned parts with suitable drying cloths.
AIRFRAME. The salvable components of the fuselage, wings, empennage, seaplane and amphibian hulls and floats, and movable surfaces should be processed as follows:
Par 7-1 Page 7-289 The fabric from fabric-covered surfaces should be removed and replaced.
Clean the aircraft interior and exterior using steam under pressure with steam cleaning compound. Direct the steam into all seams and crevices where corrosive water may have penetrated. Avoid steam cl eaning electrical equipment, such as terminal boards and relays.
Areas that have been steam cleaned should be rinsed immediately with either hot or cold fresh water.
Touch up all scratches and scars on painted surfaces using zinc chromate primer or preservative.
Undrained hollow spaces or fluid entrapment areas should be provided temporary draini ng facilities by drilling out rivets at the lowest point. Install new rivets after drainage.
Remove and replace all leather, fabric upholstery, and insulation. Plastic or rubber foam that cannot be cleaned of all corrosive water must be replaced.
Par 7-1 Page 7-290 All drain plugs or drive screws in tubular structures should be removed and the structure blown out with compressed air. If water has reached the tubular inte riors, carefully flush with hot fresh water and blow out water with compressed air. Roll the structure as necessary to remove water from pockets. Fill the tubes with hot linseed oil, approximately 180 °F, drain oil and replace drain plugs or drive screws.
Clean sealed wood, metallic, and other non-metallic areas, excluding acrylic plastics, with warm water. Replace wood, metalite, and other porous materials expos ed to water-immersion unless surfaces are adequately sealed to prevent penetration by water. Virtually all solvents and phenolic ty pe cleaning agents are detrimental to acrylics and will either soften the plastic or cause crazing.
Remove instruments and radios and applicable cables and plumbing, and repair and inspect as necessary.
6-224.6-234. [RESERVED.]
Par 7-1 Page 7-291 CHAPTER 7. AIRCRAFT HARDWARE, CONTROL CABLES,
AND TURNBUCKLES
SECTION 1. RIVETS
GENERAL.
Standard solid-shank rivets and the universal head rivets (AN470) are used in aircraft construction in both interior and exterior locations. All protruding head rivets may be replaced by MS20470 (supersedes AN470) rivets. This has been adopted as the standard for protruding head rivets in the United States.
Roundhead rivets (AN430) are used in the interior of aircraft except where clearan ce is required for adjacent members.
Flathead rivets (AN442) are used in the interior of the aircraft where interferen ce of adjacent members does not permit the use of roundhead rivets.
Brazierhead rivets (AN455 and AN456) are used on the exterior surfaces of airc raft where flush riveting is not essential.
Countersunk head rivets MS20426 (supersedes AN426 100-degree) are used on the exterior surfaces of aircraft to provide a sm ooth aerodynamic surface, and in other applications where a smooth finish is desired.
The 100-degree countersunk head has been adopted as the standard in the United States. Refer to MIL-HD BK5 Metallic Materials and Elements for Fight Vehicle Structures, and U.S.A.F./Navy T./O. 1-1A-8, Structural Hardware.”
Typical rivet types are shown in table 7-10.
MATERIAL APPLICATIONS.
Rivets made with 2117-T4 are the most commonly used rivets in aluminum alloy structures. The main advantage of 2117 -T4 is that it may be used in the condition received without further treatment.
The 2017-T3, 2017-T31, and 2024-T4 rivets are used in aluminum alloy structures where strength higher than that of the 2117 -T4 rivet is needed. See Metallic Materials and Elements for Flight Vehicle Structures (MIL-HDBK-5) for differences between the types of rivets specified here.
The 1100 rivets of pure aluminum are used for riveting nonstructural parts fabricated from the softer aluminum alloys, such as 1100, 3003, and 5052.
When riveting magnesium alloy structures, 5056 rivets are used exclusively due to their corrosionresistant qualities in combination with the magnesium alloys.
Mild steel rivets are used primarily in riveting steel parts. Do not use galvanized rivets on steel parts subjected to high heat.
Corrosion-resistant steel rivets are used primarily in riveting corrosion-resistant steel parts such as firewalls, exhaust stack bracket attachments, and similar structures.