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6.13 Paint Removal. Refer to the Original Equipment Manufacturer (OEM) maintenance manuals for specific guidelines.
6.13.1 For small areas of metallic surfaces, remove paint by hand using a medium grade abrasive mat. For larger areas, chemical paint removal is the preferred method.
6.13.2 Phenolic and non-phenolic chemical paint removers containing methylene chloride are recommended for paint stripping on metallic surfaces only. Do not use chemical paint strippers containing acid because the strippers will cause hydrogen embrittlement on high-strength steel and some stainless steels. The following procedure is recommended when using chemical paint remover:
1. Mask acrylic windows and canopies, plastic parts, rubber hoses, exposed wiring, composite surfaces, wheels and tires, and other areas where paint remover residue cannot be removed and refinishing cannot be accomplished;
2. Remove any sealants by cutting away excess sealant with a sharp plastic scraper;
3. Apply a thick, continuous coating of paint remover to cover the painted surface;
4. Leave the paint remover on the surface for enough time to wrinkle and completely lift the paint. Reapply paint remover as necessary in areas where paint remains tight or where the material has dried. Micarta scrapers, abrasive pads, or fiber brushes can be used to assist in removing persistent paint;
5. Remove loosened paint and residual paint remover by washing and scrubbing the surface with fresh water and a stiff nylon bristle brush or an abrasive pad;
and
6. After thorough rinsing, remove masking materials and thoroughly clean the area with a solution of aircraft cleaning compound to remove paint remover residues.
6.13.3 For composite surfaces, paint removal should be done by mechanical removal techniques (scuff sanding) only. Composite surfaces include fiberglass, Kevlar, carbon, and graphite.
Due to irregularities in composite surfaces, such as a fiber weave, a paint system cannot be completely removed without damaging surface fibers.
6.13.4 Paint can be mechanically removed by hand or with fine or very fine abrasive mats or flap brushes on power tools. If using power tools, be careful to prevent removal of the base material.
6.14 Special Techniques. In special instances, a schedule or specific method to remove corrosion may be needed. Depending upon rework criteria, corrosion in a hole may be removed by enlarging the hole. Abrasive blasting may remove corrosion from steel fasteners, side skins, or irregularly shaped parts or surfaces. Whenever such special cases occur, observe the specified method for corrosion removal.
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6.15 Fairing or Blending Reworked Areas. Fair or blend with a surrounding surface all depressions resulting from corrosion rework. Fairing can be accomplished as follows:
6.15.1 Remove rough edges and all corrosion from damaged areas. All dish-outs should be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. Select the proper abrasive for fairing operations from Table 6-1, Abrasives for Corrosion Removal.
6.15.2 Rework depressions by forming smoothly-blended dish-outs, using a length-to-depth ratio of 20:1 (see Figures 6-1, Blendout of Corrosion as Single Depression Width, and 6-2, Typical Example of Acceptable Cleanup of Corrosion Pits). In areas having closely-spaced multiple pits, remove intervening material to minimize surface irregularity or waviness (see Figures 6-3, Blendout of Multiple Pits in Corroded Area, and 6-4, Profile of Reworked Corroded Areas in Regions of Limited Access). Remove steel nutplates and steel fasteners before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion (see Figure 6-5, Steel Particles from Fasteners or Nutplates May Cause Future Corrosion Problems in Aluminum Structure). Remove all corrosion products during blending to prevent reoccurrence of corrosion (see Figures 6-6, Incomplete Removal of Corrosion Products Resulted in Reoccurrence of Corrosion, and 6-7, Incomplete Removal of Corrosion Products Resulted in Reoccurrence of Corrosion).
6.15.3 In critical and highly stressed areas, pits may remain after removal of corrosion products by any method. These pits should be blended out to prevent stress risers, which may cause stress-corrosion cracking. But if the pits are on noncritical structures, use of abrasive blasting is not required to blend out the pits, since this results in unnecessary metal removal.
6-8 Table 6-1. Abrasives for Corrosion Removal Metals or Materials to be Processed Restrictions Operation Abrasive Paper or Cloth Abrasive Fabric or Pad Aluminum Stainless Steel Pumice 350 Mesh or Finer Abrasive Wheel Aluminum Oxide Silicon Carbide Garnet Ferrous Alloys Does Not Apply to Steel Heat Treated to Strengths to 220,000 psi and Above Corrosion Removal or Fairing 150 Grit or Finer 180 Grit or Finer Fine to Ultrafine
X X X X Finishing 400 Grit X X X Aluminum Alloys Except Clad Aluminum Do Not Use Silicon Carbide Abrasive Corrosion Removal or Fairing 150 Grit or Finer 7/0 Grit or Finer Very Fine and Ultrafine
X X X Finishing 400 Grit X X Clad Aluminum Sanding Limited to the Removal of Minor Scratches Corrosion Removal or Fairing 240 Grit or Finer 7/0 Grit or Finer Very Fine and Ultrafine X X Finishing 400 Grit X Magnesium Alloys Corrosion Removal or Fairing 240 Grit or Finer Very Fine and Ultrafine
X X X Finishing 400 Grit X X Titanium Cleaning and Finishing 150 Grit or Finer 180 Grit or Finer X X X
Figure 6-1. Blendout of Corrosion as Single Depression Width
AT WOE CO ARE HO DUCTS AVE BE HAT
CORROSION DAMAGE BEFORE REWORK
LONGITUDINAL
ROUGH EDGES HAVE CORROSION HAS BEEN REMOVED DEPRESSION HAS NOT BEEN SHAPED, N REMOVED, HOWEVER, L DISH-OUT AFTER BLENDING 6-9
Figure 6-2. Typical Example of Acceptable Cleanup of Corrosion Pits
LOCATION OF WIDTH OF CLEANED CORROSION PITS UP AREA (10 TIMES DEPTH MIN)
VIEW LOOKING DOWN
VIEW IN TRUE PERSPECTIVE
DEPTH OF CLEAN UP OF CORROSION
LENGTH OF CLEANED UP AREA (20 TIMES DEPTH MIN!
CROSS SECTIONAL VIEW
NOT ACCEPTABLE ACCEPTABLE Figure 6-3. Blendout of Multiple Pits in Corroded Area
- TRANSVERSE CORROSION DAMAGE 10D
BEFORE REMOVAL MIN
- LONGITUDINAL -
BOTTOM OF DEPRESSION
AFTER CORROSION REMOVAL
DAMAGE REMOVED AND SURFACE SMOOTHED
WITH SHALLOW ELLIPTICAL DISH-OUT 6-10
6-11 Figure 6-4. Profile of Reworked Corroded Areas in Regions of Limited Access Fi gure 6-5. Steel Particles from Fasteners or Nutplates May Cause Future Corrosion Problems in Aluminum Structure
6-12 Figure 6-6. Incomplete Removal of Corrosion Products Resulted in Reoccurrence of Corrosion Fi gure 6-7. Incomplete Removal of Corrosion Products Resulted in Reoccurrence of Corrosion
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6.16 Chemical Testing.
6.16.1 A serious problem in corrosion control is identification of the metal on which corrosion occurs. This identification is important because all metals possess certain chemical characteristics common only to themselves, and these characteristics vary greatly from metal to metal and from alloy to alloy of the same metal. Since these characteristics are common to all metals and their alloys, chemical cleaning solutions and chemical protective films will react differently with different metals. In some cases, this reaction produces adverse reactions which can severely weaken or destroy the structural capabilities of the metal.
6.16.1.1 The primary method of identifying metal is in the aircraft SRM. When the SRM is limited or when more information is required, such as with heat-treat and protective finishes, the best source of identification is the aircraft manufacturer, where an aircraft drawing can be reviewed.
6.16.1.2 Use chemical testing when all other methods have been exhausted and when the following precautions are followed:
• Personnel should become thoroughly familiar with the safety precautions and emergency safety procedures, prior to performing any chemical testing.
• Chemical spot testing should be accomplished by qualified personnel only.
• Fasteners should not be identified by chemical spot tests.
• High-strength steel should not be identified by chemical spot tests.
6.16.2 Use test samples for chemical testing for metals before testing the actual part. Determine the preliminary surface preparation and primary classification of the metal by the following procedure:
1. On the surface to be tested, choose an area where there is no corrosion and remove paint, if present, from a 1-inch square. Paint may be removed using hand sanding or an approved paint remover.
CAUTION: Take adequate precautions to prevent paint remover from contacting composite parts.
2. Clean the surface of the test area.
3. Tentatively identify the exposed metal surface by visually comparing it with samples of previously identified materials, if available.
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4. Identify the metal as ferrous or nonferrous by placing a magnet on the exposed surface. Magnetic attraction classifies the base metal as a ferrous magnetic material, which can be either iron or steel. The absence of magnetic attraction classifies the base metal as either an austenitic stainless steel or a nonferrous metal, such as aluminum or magnesium.
5. Have a qualified person test magnetic metals for hardness before chemical spot testing. If the metal is nonmagnetic, proceed with paragraph 6.18.
6.17 Chemical Spot Analysis of Magnetic Metals. Aircraft manufacturers usually use magnetic ferrous alloys, such as high-strength steels and some stainless steels, in aircraft construction. These magnetic alloys can be plated with chromium, nickel, zinc, cadmium, silver, or a combination of these platings.
6.17.1 If a magnetic alloy has a cadmium, zinc, or chromium plating, that plating will exert magnetic attraction. Nickel plating will show slight magnetic attraction even if the substrate or base metal is not magnetic.
6.17.2 If positive identification of the metal plating is necessary, the identification should be made after accomplishing a hardness test.
CAUTION: Do not perform a chemical spot test on steels heat-treated to 220,000 psi and above.
6.17.3 Place a drop of 10 percent hydrochloric acid (HCl) on the prepared metal surface. Ensure that the surface is dry before applying the acid.
• A rapid reaction producing a dark deposit indicates that the metal is zinc.
• A slow or no reaction indicates that the metal may be cadmium, chromium, nickel, or steel.
6.17.4 After 1 minute, add a drop of sodium sulfide (Na2S) to the drop of HCl.
CAUTION: Adding Na2S to any acid produces a poisonous gas. Provide adequate ventilation when performing these identification tests. Do not mix large quantities of Na2S and acid.
• A white precipitate identifies the metal as zinc.
• A yellow ring formed around a white precipitate identifies the metal as cadmium.
• A black ring formed around a white precipitate identifies the metal as iron or steel.
• A black precipitate indicates that the metal is chromium or nickel.
6.17.5 Confirm the cadmium, zinc, iron, or steel test by placing a drop of 20 percent nitric acid (HNO3) on a fresh spot. After 1 minute, add a drop of Na2S to the drop of HNO3.
• A white precipitate identifies the metal as zinc.
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• A yellow precipitate identifies the metal as cadmium.
• A black spot identifies the metal as iron or steel.
6.17.6 Confirm the chromium test by placing a drop of 10 percent HCl on a fresh spot. Add a drop of concentrated sulfuric acid (H2SO4) to the drop of HCl. A color change to green after 1 or 2 minutes identifies the metal as chromium.
6.17.7 Confirm the nickel test by placing a drop of dimethylglyoxime (C4H8N2O2) solution on a fresh spot. Add a drop of ammonium hydroxide (NH4OH) to the drop of C4H8N2O2 solution. A pink-to-red precipitate identifies the metal as nickel.
6.17.8 Clean and refinish, as detailed in paragraph 6.20.
6.18 Chemical Spot Analysis of Nonmagnetic Metals. The most common nonmagnetic metals in aircraft construction are aluminum, magnesium, and austenitic steels, generally used as 18-8 stainless steel, where the “18-8” indicates a composition of 18 percent chromium and 8 percent nickel. Positively identify these nonmagnetic metals by the following procedure:
6.18.1 Place a drop of 10 percent HCl on the prepared metal surface and allow to stand for 1 minute. Ensure the surface is dry before applying the acid.
Note: Zinc deposits on nonmagnetic metals will react with 10 percent HCl but will not produce a black spot.
• A rapid or violent reaction that produces a black spot indicates the metal is magnesium.
• A slow reaction indicates the metal is aluminum.
• No reaction indicates the metal is an austenitic steel or a nonmagnetic plating material.
6.18.2 If a reaction did not produce a black spot as noted in the first bullet under paragraph 6.18.1, determine if zinc is present, as detailed in paragraph 6.17.5.
6.18.3 If the results of paragraph 6.18.2 are negative (zinc is not present) confirm the magnesium and aluminum tests by placing a drop of 10 percent sodium hydroxide (NaOH) on a fresh spot. Check for the following:
• No reaction will identify the metal as magnesium, or
• A reaction that produces a colorless spot will identify the metal as a bare-aluminum alloy.
6.18.4 If an aluminum alloy is identified as outlined in paragraph 6.18.3, further test to distinguish the different alloys by placing a drop of 10 percent cadmium chloride (CdCl2) on a fresh spot.
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• A dark gray deposit forming within a few seconds will identify the metal as 7075 or 7178 bare-aluminum alloy.
• A dark gray deposit forming within 2 minutes will identify the metal as 7075 or 7178 clad-aluminum alloy.
• No deposit formation in 2 minutes will identify the metal as 2024 aluminum alloy. A faint deposit will form after 15 or 20 minutes.
6.18.5 Confirm the austenitic steel test by dissolving 10 grams of cupric chloride (CuC12 2H2O) in 100 cubic centimeters of HCl and placing a drop of the solution on a fresh spot. After 2 minutes, add three or four drops of distilled water to the drop of HCl solution and dry the surface. The appearance of a brown spot identifies the metal as an austenitic steel.
6.18.6 If no reaction was noted as outlined in paragraph 6.18.1 or 6.18.5, test for a plating material, as detailed in paragraph 6.17.
6.18.7 If paragraph 6.18.6 reveals plating on a nonmagnetic metal, the plating should be removed by mechanical abrasion and the base metal identified by the visual and/or chemical methods outlined in paragraph 6.17.
6.18.8 Clean and refinish as required in paragraph 6.20.
6.19 Surface Treatment Testing. Chemical conversion coatings, phosphate treatments for steels, and chromate treatments for aluminum are the most common surface treatment for metals in aircraft construction. Other surface treatments include lacquer and chromate films. Identify these surface treatments by the following procedures:
6.19.1 Phosphate Treatment. Confirm the presence of a phosphate treatment on steel, zinc, cadmium, or aluminum by placing a drop of 20 percent HNO3 on the surface, then following with two drops of ammonium orthomolybdate ((NH4)2MoO4) solution. A yellow precipitate will form.
6.19.2 C hromate Treatment . Surface chromate treatments on zinc, cadmium, aluminum, or magnesium are highly colored and indicate the application of these treatments. However, a bleached chromate treatment may have been applied and then coated with lacquer to mask any residual iridescence for the sake of appearance. If so, visual detection of the chromate is impossible. To test for lacquer, proceed as directed in paragraph 6.19.3. Note that the bleaching process in a bleached chromate treatment lowers the corrosion resistance provided by the chromate film.
6.19.3 L acquer Finish. To test for lacquer, place a drop of concentrated H 2SO4 on the surface. If lacquer is present, the spot will rapidly turn brown with no effervescence or bubbling. If lacquer is not present, the spot will not turn brown. If the metal is zinc, there will be no rapid effervescence. If the metal is cadmium, there will be no reaction.
6.19.4 C hromate Film. To detect a chromate film on zinc and cadmium, place a drop of 5 percent aqueous solution of lead acetate (Pb(C2H3O2)2) on the surface. If the metal has
6-17 been treated, the surface will show no discoloration for 10 seconds. If there is no surface treatment, an immediate dark spot will appear.
6.20 Post-Identification Cleaning and Refinishing.
Note: The solutions used in chemical spot tests are extremely corrosive.
6.20.1 After identification of the metal is completed, clean the area as follows:
1. Blot any remaining chemicals with a dry cloth.
2. Swab the area several times with a water-moistened cloth.
3. Test the surface by placing a piece of litmus paper on the moistened surface.
If the litmus paper changes color, repeat steps 1 and 2 until no color change occurs.
4. Dry surface thoroughly.
5. Remove corrosion, if present, and refinish the surface, as applicable.
6.21–6.25. RESERVED.