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4. Doublers should be made from Alclad, when available, and the sheet should be anodized (this is preferred) or chemical conversion coated after finishing all cutting, drilling, and countersinking.
5. Drill, countersink, surface treat, and prime all rivet holes prior to installation of the doubler.
6. Apply a suitable sealing compound in the area to be covered by the doubler.
Apply sufficient thickness of sealing compound to fill all voids in the area being repaired.
7. Install rivets wet with sealant. Sufficient sealant should be squeezed out into holes so that all fasteners, as well as all edges of the repair plate, will be sealed against entrance of moisture.
8. Remove all excess sealant after fasteners are installed. Apply a fillet sealant bead around the edge of the repair. After the sealant has cured, apply a protective paint finish to the reworked area.
6.53 Corrosion Removal Around Countersunk Fasteners in Aluminum Alloy. In aluminum alloys, intergranular corrosion often originates at countersunk areas where steel fasteners are used. Corrosion removal in a countersink is impossible with the fastener in place.
6.53.1 When corrosion is found around a fixed fastener head, remove the fastener to ensure corrosion removal, prevent further corrosion, and prevent loss of structural strength. To reduce the reoccurrence of corrosion, the panel should receive a chemical conversion coating, be primed, and have the fasteners installed wet with sealant.
6.53.2 Each time removable steel fasteners are removed from access panels, inspect them for material condition, including the condition of the plating. If mechanical or plating damage is evident, replace the fastener. Upon installation, follow one of the below fastener installation methods:
6.53.2.1 A preferred method is to brush a corrosion preventive compound on the substructure around and in the fastener hole, start the fastener, apply a bead of sealant to the fastener countersink, then set and torque the fastener within the working time of the sealant;
6.53.2.2 Apply the corrosion preventive compound to the substructure and fastener, set and torque the fastener; or
6.53.2.3 Apply a coating of primer to the fastener, and, while wet with primer, set and torque the fastener.
6.54 Examples of Removing Corrosion from Aluminum and Aluminum Alloys.
6.54.1 Prepare the Aircraft. Prepare the aircraft for corrosion rework, as provided in paragraph 6.12, and remove corrosion products as follows. Observe the work procedures of paragraph 6.2.
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6.54.2 Identify the Metal. Positively identify the metal as aluminum.
6.54.3 Clean the Area. Clean the area to be reworked. Strip paint, if required.
6.54.4 Determine the Extent of the Corrosion. Determine extent of corrosion damage, as covered in paragraph 6.42. To remove light corrosion, proceed with paragraph 6.54.5. To remove moderate or severe corrosion, proceed with paragraph 6.54.6.
6.54.5 Light Corrosion Removal. Do not use the chemical removal process at temperatures above 100 °F or below 40 °F. Remove light corrosion by hand rubbing the corroded surface with tools, abrasives, or by chemical means, as follows:
6.54.5.1 Protectively mask adjacent areas to prevent brighteners from contacting magnesium, anodized aluminum, glass, Plexiglas, fabric surfaces, and all steel. Wear acid-resistant gloves, protective mask, and protective clothing when working with corrosion removing compounds. If corrosion removing compounds accidentally contact the skin or eyes, flush off immediately with plenty of clear water. Refer to safety procedures in Section 1, Safety Procedures.
6.54.5.2 Dilute a corrosion removing compound (SAE AMS1604, Corrosion Removing Compound, for Aircraft Surfaces, Type I) with an equal volume of water. Mix the compound only in wood, plastic, or plastic-lined containers.
Apply the diluted solution of corrosion removing compound by flowing, mopping, sponging, brushing, or wiping.
6.54.5.3 Apply the diluted solution to large areas with a circular motion to disturb the surface film and ensure proper coverage. The diluted solution should be applied starting at the lowest area and working upwards. The solution will be more effective if applied warm, 140 °F maximum, followed by vigorous agitation with a nonmetallic, acid resistant brush or aluminum oxide abrasive nylon mat.
6.54.5.4 Leave the solution on the surface for about 12 minutes. Do not let the solution dry on the surface; streaking will result. On large exterior surfaces, remove solution by high-pressure water rinse.
6.54.5.5 Wipe off solution with a clean, moist cloth. Frequently rinse the cloth in clear water. Wipe the area several times with a fresh cloth, dampened and rinsed frequently in clear water.
6.54.5.6 Dry the area with a clean, dry cloth and inspect for corrosion.
6.54.5.7 Repeat the procedure outlined in paragraph 6.54.5 if any corrosion remains.
Note: If corrosion still remains after the second attempt, mechanically remove corrosion, as detailed in paragraph 6.54.6.
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6.54.5.8 After all corrosion has been removed, proceed with paragraph 6.54.6.4.
6.54.6 Moderate and Severe Corrosion Removal. Mechanically remove moderate or severe corrosion by the appropriate methods, as follows:
Note: Wear goggles or face shield to protect against corrosion particles that break loose and fly off. Protect adjacent areas and prevent additional damage from corrosion products removed during mechanical removal.
6.54.6.1 Remove loose corrosion products by hand rubbing the corroded surface with tools or abrasives. Dry abrasive blasting using MIL-SPEC MIL-G-9954, Glass Beads: For Cleaning and Peening, use glass bead sizes 10, 11, 12, 13, or A-A-1722, Grain, Abrasive (Soft Blasting), Types I and III may be used as an alternate method to remove corrosion from clad and nonclad aluminum alloys.
Do not abrasively blast to remove heavy corrosion products. Direct-pressure machines should have the nozzle pressure set at 30 to 40 psi for clad aluminum alloys, and 40 to 45 psi for nonclad aluminum alloys. Get engineering approval from the aircraft manufacturer before abrasive blasting metal thinner than 0.0625 inches.
6.54.6.2 Remove residual corrosion by hand sanding or with an approved hand-operated power tool. Power tools can generally be used to remove corrosion with flap-brush, rotary file, sanding pad, or abrasive wheel attachments. Do not use rotary files on skin thinner than 0.0625 inches. Select an appropriate abrasive from Table 6-1.
6.54.6.3 Using a blend length-to-depth ratio of 20:1, finish the corrosion rework area with progressively finer abrasive paper until getting to 400 grit abrasive paper.
6.54.6.4 Clean the reworked area using dry cleaning solvent; do not use kerosene.
6.54.6.5 Determine depth of faired depressions to confirm rework limits have not been exceeded.
6.54.6.6 Apply a chemical conversion coating to the rework area.
6.54.6.7 Apply paint finish to the rework area.
6.55–6.59. RESERVED.
6-31 SECTION 6. MAGNESIUM ALLOYS
6.60 Treatment of Wrought Magnesium Sheets and Forgings. Corrosive attack on magnesium skins will usually occur around edges of skin panels, underneath hold-down washers, or in areas physically damaged by shearing, drilling, abrasion, or impact.
Entrapment of moisture under and behind skin crevices is frequently a contributing factor. If the skin section is easy to remove, do this to ensure complete inhibition and treatment.
Note: Refer to the OEM maintenance manual for treatment of magnesium sheets and forgings.
6.60.1 Mechanically remove corrosion products when practicable. Use stiff bristle brushes and similar nonmetallic cleaning tools for such mechanical cleaning, particularly during treatment in field conditions.
6.60.2 Any entrapment of steel particles from steel wire brushes, steel tools, or contamination of treated surfaces by dirty abrasives can cause more trouble than the initial corrosive attack.
6.60.3 When aluminum insulating washers no longer adhere to magnesium panels, corrosion is likely to occur under the washers if corrective measures are not taken.
6.60.3.1 Remove machine screw fasteners from all loose insulating washer locations to surface treat the magnesium panel.
6.60.3.2 Where permanent fasteners are in place, remove the insulating washer and fastener for complete corrosion removal.
6.60.3.3 When water can collect in a counterbored area where the washer was located, use sealants to fill the counterbore. If necessary to fill several areas adjacent to each other, it may be advantageous to cover with a strip of sealant.
6.61 Repair of Magnesium Sheet Metal After Extensive Corrosion Removal. The same general instructions apply when making repairs in magnesium as in aluminum alloy skin, except two coats of epoxy primer may be needed on both the doubler and skin being patched instead of only one coat. Where it is difficult to form magnesium alloys in the contour, aluminum alloy may be utilized. When this is done, it is necessary to ensure effective dissimilar metal insulation. Vinyl tape will ensure positive separation of dissimilar metals, but edges will still have to be sealed to prevent entrance of moisture between mating surfaces at all points where repairs are made. It is recommended that only noncorrosive type sealant be used, since it serves a dual purpose of material separation and sealing.
6.62 In-Place Treatment of Magnesium Castings. Magnesium castings, in general, are more porous and more prone to penetrating attack than wrought magnesium skin. Treatment in the field is usually the same for all magnesium areas. Engine cases are among the most common examples of cast magnesium in modern aircraft. Bell cranks, fittings, and numerous covers, plates, and handles may also be magnesium castings. When attack
6-32 occurs on a casting, apply the earliest practicable treatment to avoid dangerous corrosive penetration. Engine cases in salt water can develop “moth holes” and complete penetration overnight.
6.62.1 If it is at all practicable, separate faying surfaces involved to effectively treat the existing attack and prevent its further progress. Follow the same general treatment sequence detailed for magnesium skin. Where engine cases are concerned, baked enamel overcoats are usually involved rather than other topcoat finishes. A good air drying enamel can restore protection.
6.62.2 If extensive removal of corrosion products from a structural casting is involved, request the aircraft manufacturer to evaluate the adequacy of remaining structural strength. SRMs usually include dimensional tolerance limits for critical structural members. The FAA should be consulted if any questions of safety are involved.
6.63 Example of Removing Corrosion from Magnesium. If possible, corroded magnesium parts should be removed from the aircraft. When impossible to remove the part, make aircraft preparations detailed in paragraph 6.12. When using that procedure, observe the safety precautions and procedures of Section 1.
6.63.1 Positively identify metal as magnesium (see paragraph 6.18).
6.63.2 Clean area to be reworked.
6.63.3 Strip paint, if required.
6.63.4 Determine extent of corrosion damage, as detailed in paragraph 6.42. To remove light corrosion, proceed to paragraph 6.63.5. To remove moderate or severe corrosion, go to paragraph 6.63.6.
6.63.5 Do not use the following procedure for adhesive bonded parts or assemblies, areas where the brush-on solution might become lodged, or local areas bared specifically for grounding or electrical bonding purpose. Remove light corrosion by light hand sanding or chemically, as follows:
6.63.5.1 Remove loose corrosion with aluminum wool or abrasive mat, paper, or cloth.
6.63.5.2 Mask off other materials and parts, especially rubber parts, bearings, and cast or pressed inserts to prevent contact with the treating solution or its fumes.
6.63.5.3 Prepare corrosion treating solution in the following proportions: 1.5 pounds of sodium dichromate and 1.5 pints of concentrated HN03 per gallon of water.
Prepare and store the solution in clean polyethylene or glass containers. Mix as follows:
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1. Fill a container with a volume of water equal to 1/4 the desired total quantity of solution.
2. Add full quantity of sodium dichromate in proportions indicated and agitate solution until the chemical is dissolved.
3. Add water until quantity of solution is equal to approximately 2/3 the desired total quantity.
4. Slowly add total volume of HN03 to the solution and mix thoroughly.
5. Add remaining water until total desired quantity of solution is reached and stir until entire solution concentration is equal.
6.63.5.4 Remove remaining corrosion by swabbing the corroded surface 1 to 2 minutes with the HN03 solution, then wipe dry.
6.63.5.5 Rinse thoroughly with clean water while scrubbing with a mop, brush, or abrasive mat and wipe dry.
6.63.5.6 Repeat the preceding sequence, as necessary, until all corrosion has been removed.
6.63.5.7 After all corrosion has been removed, proceed with paragraph 6.63.7.
6.63.6 Mechanically remove moderate or severe corrosion. Wear goggles or a face shield to preclude injury from corrosion particles breaking loose and flying off. Protect adjacent areas to prevent additional damage from corrosion products removed when using this procedure.
6.63.6.1 Remove heavy corrosion products by hand brushing with a stainless steel or fiber brush followed by vacuum abrasive blasting with MIL-SPEC MIL-G-9954 glass bead sizes 10, 11, 12, 13 or A-A-1722 grain abrasive Types I or III. An air pressure at the nozzle of 10 to 35 psi should be used for direct-pressure machines. For suction type blast equipment, use 50 percent higher pressure.
6.63.6.2 Remove residual corrosion by hand sanding or with an approved hand-operated power tool.
6.63.6.3 After removing all corrosion visible through a magnifying glass, apply a corrosion treating solution.
6.63.7 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.
6.63.8 Determine depth of faired depressions and confirm the depth does not exceed rework limits.
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6.63.9 Clean reworked area using a solvent to provide a water breakfree surface. Do not use kerosene.
6.63.10 Prepare and apply magnesium conversion coat conforming to AMS-M-3171, Magnesium Alloy, Processes for Pretreatment and Prevention of Corrosion On, Type VI (DOW-19) as follows:
1. Measure 1 gallon of distilled water into a clean polyethylene or glass container.
2. Add 1.3 ounces (dry) of chromium trioxide or 1.3 ounces of technical grade chromic acid.
3. Add 1 ounce of calcium sulfate dehydrate (CaS04.2H20).
4. Vigorously stir for at least 15 minutes; ensure that the solution is saturated with calcium sulfate. Let chromate solution stand for 15 minutes prior to decanting.
5. Prior to use, decant solution (avoid transfer of undissolved calcium sulfate) into suitable usage containers (polyethylene or glass).
6. Apply solution by swabbing until the metal surface becomes a dull color, which can vary from green-brown, brassy, or yellow-brown to dark brown.
Under optimum conditions of temperature at 70 °F or above and fresh materials, the time required to properly apply magnesium pretreatment is usually 1 to 5 minutes. Under these conditions, 1 to 2 minutes of treatment should produce a brassy film, and 3 to 5 minutes of treatment a dark brown coating. Under adverse conditions, and if the desired specified finish color is not seen in the specified time, the treatment may have to be prolonged to 20 to 30 minutes until the proper finish is seen. For good paint adhesion, a dark brown color, free of powder, is considered best. The color may vary in using different vendor materials. Too long exposure to the brush-on solution produces a coating which will powder and impair adhesion of applied paint finishes/films. Use caution in swabbing on the solution. Severe rubbing of the wet surface will damage the coating.
Note: High pressure spraying or rubbing abrasion will damage the fresh coating.
7. Rinse with clean water, then allow to dry at ambient temperature for a minimum of 1 hour. Allow more time in high humidity areas.
6.63.11 Apply primer and topcoat finish.
6.63.12 Remove masking and protective covering.
6.64–6.69. RESERVED.
6-35 SECTION 7. FERROUS METALS
6.70 General. One of the most familiar kinds of corrosion is red iron rust, generally resulting from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but red rust is not a protective coating. Its presence actually promotes additional attack by attracting moisture from the air and acting as a catalyst in causing additional corrosion to take place.
6.70.1 Red rust first shows on bolt heads, hold-down nuts, and other unprotected aircraft hardware. Red rust will often occur under nameplates secured to steel parts. Its presence in these areas is generally not dangerous and has no immediate effect on the structural strength of any major components. However, it is indicative of a general lack of maintenance and possible attack in more critical areas.
6.70.2 When paint failures occur, or mechanical damage exposes highly-stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous in these areas and should be removed and controlled.
6.71 Mechanical Removal of Iron Rust. The most practicable means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means (see Figures 6-17, Removing Corrosion Products from Ordinary Steel Surfaces, and 6-18, Removing Corrosion Products from Highly-Stressed Steel Parts). On high-strength steel, corrosion removal by hand sanding is recommended. The use of powered tools is not recommended on high strength steel because of the danger of local overheating and the formation of notches, which could lead to failure. However, in any such use of abrasives, residual iron rust usually remains in the bottom of small pits and other crevices.
6.71.1 The best method to use on exterior surfaces is abrasive blasting, which can remove nearly all rust.
6.71.2 Paint the cleaned metal surface as soon as possible after corrosion removal. In any event, do not allow the surface to become wet before painting.
6-36 Figure 6-17. Removing Corrosion Products from Ordinary Steel Surfaces
F igure 6-18. Removing Corrosion Products from Highly-Stressed Steel Parts
6.72 Chemical Surface Treatment of Steel Surfaces.
Note: Secure engineering authorization to use chemical surface treatment on high-strength steel parts.
6.72.1 There are acceptable methods for converting iron rust to phosphates and other protective coatings. Parco Lubrizing and phosphoric zinc preparations are examples of such treatment. However, these processes require shop-installed hot tanks and are impracticable for use in the field.
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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.