6-25 Figure 6-13. Cleaning and Stripping Paint
6.51.3.3 C hemical conversion coating, detailed by Military Specification (MIL-SPEC) MIL-C-81706, Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys, is a chemical surface treatment used on aluminum alloys to provide a proper surface for paint finishing.
6.51.4 Special Processing of Intergranular Corrosion in Heat-Tre ated Aluminum Alloy Surfaces.
Intergranular corrosion is usually more severe in heat-treated aluminum alloys and exhibits a corrosive attack along grain boundaries of the alloyed aluminum, where grain boundaries differ from the metal within the grain. When in contact with an electrolyte, rapid corrosion occurs at the grain boundaries. In its most severe form, actual lifting of metal layers, or exfoliation, occurs. Mechanically remove all corrosion products and visible delaminated metal layers to determine the extent of damage and evaluate the remaining structural strength of the component. NDI inspection is necessary to determine the full extent of corrosion. For aluminum, eddy current NDI mapping is used. For stainless steel, penetrant or magnetic particle NDI is used.
6.51.4.1 Use m etal scrapers, rotary files, or abrasive wheels to remove all corrosion products and retain only structurally sound aluminum.
Note: A rotary file is recommended only for severe intergranular or exfoliation corrosion removal.
6.51.4.2 Rotary files should be sharp to cut metal without excessive smearing. A dull cutting tool will smear the metal over corrosion cracks or fissures, giving the appearance that it has removed the corrosion, but has not.
6.51.4.3 Use carbide-tip rotary files or metal scrapers, since they stay sharp longer.
Do not use abrasive blasting to remove intergranular corrosion.
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6.51.4.4 Inspection with a 5X to 10X magnifying glass or using a dye penetrant will help determine if all unsound metal and corrosion products are removed.
6.51.4.5 When corrosion is completely removed, blend or fair using a length-to-depth ratio of 20:1 in the area of corrosion removal. Blend where required, by using aluminum oxide impregnated, rubber-base wheels.
6.51.4.6 Chemical conversion coat exposed surfaces completely and restore paint coatings in the same manner as on any other aluminum surface. See Figures 6-14, Cleaning and Inhibiting Corroded Aluminum Surfaces;
6-15, Removing Excess Inhibitor Solution; and 6-16, Applying Wax to Cleaned Surfaces.
6.51.4.7 C orrosion damage beyond SRM limits should be repaired per a cognizant engineer or aircraft manufacturer’s FAA-approved instructions.
Figure 6-14. Cleaning and Inhibiting Corroded Aluminum Surfaces
6-27 Figure 6-15. Removing Excess Inhibitor Solution Fi gure 6-16. Applying Wax to Cleaned Surfaces
6.52 Repair of Aluminum Alloy Sheet Metal. After extensive corrosion removal, follow the below procedures:
1. If water can be trapped in blended areas, chemical conversion coat and fill the area with structural adhesive or sealant to the same level and contour as the original skin.
When areas are small enough that structural strength has not been significantly decreased, proceed to applying the protective finish.
2. When corrosion removal exceeds SRM limits, contact a cognizant engineer or the aircraft manufacturer for repair instructions.
3. Where exterior doublers exist, seal and insulate them adequately to prevent further corrosion.
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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 P repare 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 Id entify 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 P rotectively 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. See the safety procedures in Section 1, Safety Procedures.
6.54.5.2 D ilute a corrosion removing compound (SAE AMS1640, 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 A pply 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 M oderate 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 R emove 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 saltwater 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 HNO3 per gallon of water.
Prepare and store the solution in clean polyethylene or glass containers. Mix as follows:
1. Fill a container with a volume of water equal to 1/4 the desired total quantity of solution.
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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 HNO3 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 HNO3 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.
6.63.9 Clean reworked area using a solvent to provide a water breakfree surface. Do not use kerosene.
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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. Measu re 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 dihydrate (CaSO4.2H2O).
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 Fig ure 6-18. Removing Corrosion Products from Highly Stressed Steel Parts
6.72 C hemical 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.