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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.
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.