6-1 CHAPTER 6. CORROSION REMOVAL TECHNIQUES SECTION 1. SAFETY PROCEDURES
6.1 General. The following general safety precautions contain guidelines for handling materials with hazardous physical properties and emergency procedures for immediate treatment of personnel who have inadvertently come into contact with a harmful material.
Materials with hazardous physical properties are referenced to pertinent safety precautions and emergency safety procedures. All personnel responsible for using or handling hazardous materials should be thoroughly familiar with the information in the following paragraphs.
6.1.1 Safety Precautions. When required to use or handle solvents, special cleaners, paint strippers, strong alkalis or acids, etchants, corrosion removers containing acids, or surface activation material, such as Bonderite 1200, observe the following safety precautions:
1. Avoid prolonged breathing of solvent or acid vapors. Solvents and acids should not be used in confined spaces without adequate ventilation or approved respiratory protection;
2. Never add water to acid. Always add acid to water;
3. Mix all chemicals per manufacturer instructions;
4. Clean water for emergency use should be available in the immediate work area before starting work;
5. Avoid prolonged or repeated contact of solvents, cleaners, etchants, acid, or conversion coating material, such as a Bonderite solution, with skin. Wear rubber or plastic gloves when using solvents, cleaners, paint strippers, etchants, or conversion coating materials. Wear goggles or plastic face shields and suitable protective clothing when cleaning, stripping, etching, or conversion coating overhead surfaces;
6. When mixing alkalis with water or other substances, use containers made to withstand chemical heat generated by this process;
7. Wash any paint stripper, etchant, or conversion coating material immediately from body, skin, or clothing;
8. Promptly flush out materials splashed in eyes with water, and get medical aid for injured persons;
9. Do not eat or keep food in areas where the food may absorb poisons. Always wash hands before eating or smoking;
10. Verify that areas are clear and remain clear of all potential ignition sources within 50 feet of any cleaning or treating operations where materials with a flash point of 140 °F or below are in use;
11. Suitable fire extinguishing equipment should be available to the cleaning/treating area;
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12. Where any flammable materials are in use, equipment should be effectively grounded;
13. If materials such as acid, alkali, paint remover, or conversion coatings are spilled on equipment or tools, treat immediately by rinsing with clean water if possible, or neutralizing acids with baking soda and alkalis with a 5 percent solution of acetic acid in water;
14. Do not use solvents with a flash point below 100 °F, such as methyl ethyl ketone (MEK) and acetone, in confined locations;
15. Clean all equipment after work is complete;
16. Implement all company safety precautions;
17. Check local environmental regulations for restrictions on the use of solvents, primers, and topcoats; and
18. Dispose of waste material per local environmental requirements.
6.1.2 E mergency Safety Procedures .
Note: Personnel should be thoroughly familiar with the following emergency safety procedures before using any materials referenced to an emergency safety procedure paragraph.
6.1.2.1 If exposed to physical contact with any of the following materials:
Methyl alcohol Xylene Methyl ethyl ketone Petroleum naphtha Methyl isobutyl ketone Chromates Toluene Dichromate Trichloroethylene Acetates Epoxy resin Cyclohexanone Methylene chloride Cellosolve Brush Bonderite Carbon tetrachloride
6.1.2.2 Treat as follows:
• If splashed into eyes, do not rub.
• Flush eyes immediately with water for at least 15 minutes. Lift upper and lower eyelids frequently to ensure complete washing.
• If splashed on clothing or large areas of the body, immediately remove contaminated clothing and wash body with plenty of soap and water. Also wash clothing before wearing again.
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• If splashed onto an easily accessible part of the body, immediately wash with soap and water.
• If suffering a headache or other obvious symptoms resulting from overexposure, move to fresh air immediately.
• If vapors are inhaled and breathing has slowed down or stopped, remove person from exposure and start cardiopulmonary resuscitation (CPR)/Automated External Defibrillator (AED) procedures at once. Call local emergency services and continue this treatment until emergency services arrives.
6.1.2.3 If exposed to physical contact with any of the following materials:
• Hydrofluoric acid.
• Nitric acid.
• Phosphoric acid.
• Phenol.
• Cresols.
• Tricresyl phosphate.
6.1.2.4 Treat as follows:
1. If splashed into eyes, quickly wipe eyelids with a soft cleaning tissue and immediately flush eyes with a gentle stream from a drinking fountain, cup, or other convenient water outlet while holding lids open. Call local emergency services and continue this treatment until emergency services arrives.
2. If splashed onto an easily accessible part of the body, immediately drench affected area with water until ambulance arrives.
3. If splashed onto clothing or large area of the body, immediately drench body and remove clothing while drenching until ambulance arrives.
4. If taken internally, begin the following treatment immediately:
• If the person is conscious, cause vomiting by placing finger in back of the person’s throat. Encourage the person to drink large quantities of water and repeatedly wash out the mouth.
• If the person is unconscious, do not give any liquid. Start CPR/AED procedures at once. Continue until an emergency service arrives. Use the procedure above if the person regains consciousness before emergency services arrives, use the procedure above for a conscious person.
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6.2 General Corrosion Control Work Procedures. The effectiveness of corrosion control depends on following basic work procedures. We recommend the following common work practices:
6.2.1 If rework procedures or materials are unknown, contact the aircraft manufacturer or authorized representative before proceeding.
6.2.2 The work areas, equipment, and components should be clean and free of chips, grit, dirt, and foreign materials.
6.2.3 Do not mark on any metal surface with a graphite pencil or any type of sharp, pointed instrument. Use temporary markings (markings soluble in water or methyl chloroform) for metal layout work or marking on the aircraft to indicate corroded areas.
6.2.4 Do not use graphite as a lubricant for any component. Graphite is cathodic to all structural metals and will generate galvanic corrosion in the presence of moisture, especially if applied in dry form.
6.2.5 Inspect footwear and clothing for metal chips, slivers, rivet cuttings, dirt, or sand, and remove all such material before walking or working on metal surfaces such as wings, stabilizers, or fuel tanks.
6.2.6 Do not abrade or scratch any surface unless following an authorized procedure. If surfaces are accidentally scratched, assess the scratch and take action to remove the scratch and treat the area, if necessary.
6.2.7 Do not polish coated metal surfaces for aesthetic purposes. Buffing removes a protective coating, and a brightly polished surface is normally not as corrosion resistant as an unpolished surface, unless it is protected by a coating such as wax or paint. A bare skin sheet polished to a mirror finish is more resistant than a bare, mill-finished sheet when both are given regular maintenance.
6.2.8 Protect surrounding areas when welding, grinding, or drilling, to prevent contaminating the areas with residue from these operations. In those areas where protective covering cannot be used, remove the residue by cleaning.
6.2.9 Replace severely corroded screws, bolts, and washers.
6.2.10 When a protective coating, such as cadmium plating, is damaged, immediately apply an appropriate protective finish to prevent additional corrosion damage.
6.3–6.9. RESERVED.
6-5 SECTION 2. CORROSION REMOVAL TECHNIQUES
6.10 General. When active corrosion is visually apparent, it is a good idea to start a positive inspection and rework program to prevent further deterioration of the structure. The following methods of assessing corrosion damage and procedures for rework of corroded areas are recommended during cleanup programs. In general, any rework would involve cleaning and stripping all finish from the corroded area, removing corrosion products, and restoring a surface protective film.
6.10.1 Repair of corrosion damage includes removal of all corrosion and corrosion products.
When the corrosion damage exceeds damage limits set by the aircraft manufacturer in the Structural Repair Manual (SRM), the affected part must either be replaced or an FAA-approved engineering authorization must be obtained for continued service for that part.
6.10.2 For corrosion damage on large structural parts in excess of that allowed in the SRM and where replacement is not practical, contact the aircraft manufacturer for rework limits and procedures.
6.11 Standard Methods. Several standard methods are available for corrosion removal, mechanical and chemical methods being normally used. Mechanical methods include:
1) hand sanding using an abrasive mat, abrasive paper, or aluminum oxide impregnated materials, or 2) powered mechanical sanding, grinding, and buffing using an abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats. However, the method used depends upon the metal and the degree of corrosion. Outlined in the following paragraphs is the removal method to use on each metal for each schedule degree of corrosion.
6.12 Preparations for Rework. All corrosion products should be removed completely when corroded structures are reworked. The corroding process will continue even though the affected surface is refinished. Before starting rework of corroded areas, carry out the following:
1. Position airplane in wash rack or provide washing apparatus for rapid rinsing of all surfaces.
2. Connect a static ground line to the airplane.
3. Prepare the aircraft for safe ground maintenance:
• Remove aircraft batteries, liquid oxygen (LOX) containers (if installed), and external hydraulic and electric power.
• Install all applicable safety pins, flags, and jury struts.
4. Protect pitot-static ports, louvers, air scoops, engine opening, wheels, tires, magnesium skin panels, and airplane interior from moisture and chemical brightening agents.
5. Protect surfaces adjacent to rework areas from chemical paint strippers, corrosion removal agents, and surface treatment materials.
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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