Par 6-89 Page 6-259 Replace any defective, damaged, twisted, or bulging hoses.
SANDWICH PANELS. Inspect edges of sandwich panels for damage to the corrosion protection finish or sealant and for corrosion.
CONTROL CABLES.
Inspect control cables for bare spots in the preservative coating and corrosion.
If external corrosion is found, relieve tension on the cable and check internal strands for corrosion. Cables with corrosion on internal strands s hould be replaced. External co rrosion should be rem oved by a clean, dry, coarse rag or fiber brush. A preservative should be applied after removal of external corrosion.
INTEGRAL FUEL CELLS.
Inspect top coat finish for breaks, peeling, lifting of surface, or other damage.
Inspect aircraft structure for top coat finish damage from pitting or intergranular corrosion.
ELECTRICAL CONNECTORS.
Inspect electrical connectors for breaks in potting compound and corrosion of pins and wires.
If the electrical connector is suspected of having moisture intrusion, disassemble the connector, clean the connector, and inspect it for corrosion.
6-78.6-88. [RESERVED.]
Par 6-89 Page 6-260 SECTION 6. CORROSION REMOVAL PROCEDURES
GENERAL. The general safety precautions, outlined in the following paragraphs, contain guidelineGeneral safety precautions for handling materials with hazardous physical properties. The following paragraphs are outlined in the following paragraphs. They also address emergency procedures for immediate treatment of personnel who have inadvertently come into contact with one of the harmful materials. Materials having hazardous physical properties are referenced to the 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.
SAFETY PRECAUTIONS.
Chemical. When required to use or handle any of the solvents, special cleaners, paint strippers (strong alkalies and acids), etchants (corrosion removers containing acids), or su rface activation material (Alodine 1200), observe the following safety precautions:
Avoid prolonged breathing of solvent s or acid's or acid vapors.
Never add water to acid. Always add acid to water.
Mix all chemicals per the manufacturer’s instructions.
Clean water for emergency use should be available in the immediate work area before starting work.
Avoid prolonged or repeated contact with the skin of solvents, cleaners, etchants (acid), or conversion coating material (Alodine solution) with skin. Rubber or plastic gloves should be worn. Goggles or plastic face shields and suitable protective clothing should be worn when cleaning, stripping, etching, or conversion coating overhead surfaces.
When mixing alkalies with water or other substance, use containers that are made to withstand heat generated by this process.
Wash any paint stripper, etchant, or conversion coating material immediately from body, skin, or clothing.
Materials splashed in to the eyes should be promptly flushed out with water, and medical aid obtained for the injured pe rson immediately.
Do not eat or keep food in areas where it may absorb poisons poisons may be absorbed .
Always wash hands before eating or smoking.
Verify that the area within 50 feet of any cleaning or treating operations where low flash point (140 °F or below) materials are being used, is clear and remains clear of all potential ignition sources.
Suitable fire fire-extinguishing equipment should be available to the cleaning/treating area.
Equipment should be effectively grounded where any flammable materials are being used.
If materials (acid, alkali, paint remover, or conversion coatings) are spilled on equipment and/or tools, treat immediately by rinsing with clean water, if possible, and /or neutralizing acids with baking soda and alkalies with a weak (5 percent) solution of acetic acid in water.
Par 6-91 Page 6-261 In confined location, do not use solvents with a low flash point, (below 100 ºF) such as Methyl Ethyl Ketone (MEK) and acetone.
All equipment should be cleaned after work has been completed.
Implement all company safety precautions.
Check and follow all applicable restrictions and requirements on the use of solvents, primers, and top coats.
Check and fEnsure disposal of waste material is per local environmental requirements ollow all applicable restrictions and requirements for use and disposal of waste material.
Blasting. The following precautions should be taken when using any type of blasting equipment:
Operators should be adequately protected with complete face and head covering equipment, and provided with pure breathing air.
Static-ground the dry abrasive blaster and the material to be blasted.
Magnesium cuttings and small shavings can ignite easily and are an extreme hazard. Fires of this metal must be extinguished with absolutely dry talc, calcium carbonate, sand, or graphite by applying the powder to a depth of 1/2 inch over the metal.
Titanium alloys and high tensile -tensilestrength steel create sparks during dry abrasive blasting. Care should be taken to ensure that hazardous concentrations of flammable vapors do not exist.
CORROSION CONTROL WORK PROCEDURES.
The effectiveness of corrosion control depends on how well basic work procedures are followed. The following common work practices are recommended:
Par 6-91 Page 6-262 If rework procedures or materials are unknown, contact the aircraft manufact urer or FAA authorized Designated Engineering Representative (DER) before proceeding.
The work areas, equipment, and components should be clean and free of chips, grit, dirt, and foreign materials.
Do not mark on any metal surface with a graphite pencil or any type of sharp, pointed instrument.
Temporary markings (defined as markings soluble in water or methyl chloroform) should be used for metal layout work or marking on the aircraft to indicate corroded areas.
Graphite should not be used 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 the graphite is applied in dry form.
Footwear and clothing should be inspected for metal chips, slivers, rivet cuttings, dirt, sand, etc., and all such material removed before walking or working on metal surfaces such as wings, stabilizers, fuel tanks, etc.
Do not abrade or scratch any surface unless it is an authorized procedure. If surfaces are accidentally scratched, the damage s hould be assessed and action taken to remove the scratch and treat the area.
Coated metal surfaces should not be polished for aesthetic purposes. Buffing would remove the protective coating and a brightly polished surface is normally not as corrosion resistant as a non-polished surface unless it is protected by wax, paint, etc or paint.
Par 6-91 Page 6-263 (and 6-20) Protect surrounding areas when welding, grinding, drilling, etc., to prevent contaminating them or drilling, to prevent contamination with residue from these operations. In those areas where protective covering cannot be used, action should be taken to remove the residue by cleaning.
Severely corroded screws, bo lts, and washers should be replaced. When a protective coating, such as a cadmium plating on bolts, or screws, etc., is damaged, immediate action should be taken to ly apply an appropriate protective finish to prevent additional corrosion damage.
6-92.6-112. [RESERVED.]
Par 6-113 Page 6-264
SECTION 7. BASIC CORROSION REMOVAL TECHNIQUES
GENERAL. When active corrosion is found, a positive inspection and rework program is necessary to prevent any further deterioration.
The following methods of assessing corrosion damage and procedures for rework of corroded areas could be used during cleanup programs.
In general, any rework would involve the cleaning and stripping of all finish from the corroded area, removal of corrosion products, and restoration of surface protective film.
Repair of corrosion damage includes removal of all corrosion and corrosion products. When the corrosion damage is severe and exceeds the damage limits set by the aircraft or parts manufacturer, the part must be replaced.
If manufacturer information and limits are not available, then a DER must be consulted before the aircraft or part is returned to service.
PREPARATIONS FOR REWORK. All corrosion products should be removed completely when corroded structures are reworked. Before starting rework of corroded areas, carry out the following:
Document corrosion damage.
Position the aircraft in a wash rack or provide washing apparatus for rapid rinsing of all surfaces.
Connect a static ground line from the aircraft to a grounding point.
Prepare the aircraft for safe ground maintenance.
Remove battery(s), liquid oxygen generator container (if installed), and external hydraulic and electric power.
Install all applicable safety pins, flags, and jury struts.
Protect the pitot-static ports, louvers, airscoops, engine opening, wheels, tires, magnesium skin panels, and airplane interior from moisture and chemical brightening agents.
Protect the surfaces adjacent to rework areas from chemical paint strippers, co rrosion removal agents, and surface treatment materials.
FAIRING OR BLENDING REWORKED AREAS.
All depressions resulting from corrosion rework should be faired or blended with the surrounding surface. Fairing can be accomplished as follows:
Remove rough edges and all corrosion from the damaged area. 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 st abilizers. (Select the proper abrasive for fairing operations from table 6-1.)
In critical and highly stressed areas, all pits remaining after the removal of corrosion products should be blended out to prevent stress risers that may cause stress corrosion cracking. (See figure 6-14.) On a non-critical structure, it is not necessary to blend out pits remaining after removal of corrosion products by abrasive blasting, since this results in unnecessary metal removal.
Par 6-113 Page 6-265 TABLE 6-1. Abrasives for corrosion removal.
METALS OR RESTRICTIONS OPERAT ION ABRASIVE PAPER OR CLOTH ABRASIVE ALUMINUM STAINLESS PUMICE 350 ABRASIVE
MATERIALS TO BE
PROCESSED
ALUMINUM OXIDE
SILICON
CARBIDE GARNET
FABRIC OR PAD
STEEL MESH OR FINER WHEEL
FERROUS ALLOYS
CORROSION
REMOVAL
OR FAIRING 150 GRIT
OR FINER 180 GRIT
OR FINER
FINE TO
ULTRA FINE
X X X X FINISHING 400 X X X
ALUMINUM ALLOYS
EXCEPT CLAD
DO NOT USE
SILICON
CARBIDE
CORROSION
REMOVAL
OR FAIRING 150 GRIT
OR FINER 7/0 GRIT
OR FINER
VERY FINE AND
ULTRA FINE X X X ALUMINUM ABRASI VE FINISHING 400 X X CLAD
ALUMINUM
SANDING
LIMITED TO THE
REMOVAL OF MINOR
CORROSION
REMOVAL
OR FAIRING 240 GRIT
OR FINER
7/0 GRIT
OR FINER
VERY FINE AND
ULTRA FINE
X X SCRATCHES FINISHING 400 X
MAGNESIUM ALLOYS
CORROSION
REMOVAL
OR FAIRING 240 GRIT
OR FINER
VERY FINE AND
ULTRA FINE X X X FINISHING 400 X X
TITANIUM
CLEANING AND
FINISHING 150 GRIT
OR FINER 180 GRIT
OR FINER
X X X
Rework depressions by forming smoothly blended dish-outs, using a ratio of 20:1, length to depth. (See figure 6-15.) In areas having closely spaced multiple pits, intervening material should be removed to minimize surface irregularity or waviness. (See figure 6-16.) Steel nut-plates and steel fasteners should be removed before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion. All corrosion products must be removed during blending to prevent reoccurrence of corrosion.
CORROSION REMOVAL BY BLASTING.
Abrasive blasting is a process for cleaning or finishing ferrous metals by directing a stream of abrasive particles against the surface of the parts. Abrasive blasting is used for the removal of rust and corrosion and for cleaning prior to painting or plating. The following standard blast-cleaning practices should be adopted.
The part to be blast-cleaned should be removed from the aircraft, if possible. Otherwise, areas adjacent to the part should be masked or protected from abrasive impingement and system (hydraulic, oil, fuel, etc.)
contamination.
Parts should be dry and clean of oil, grease, or dirt, prior to blast cleaning.
Close-tolerance surfaces, such as bushings and bearing shafts, should be masked.
Blast-clean only enough to remove corrosion coating.
Proceed immediately with surface treatments as required.
CLEANERS, POLISHES, AND BRIGHTENERS. It is important that aircraft be kept thoroughly clean of contaminating deposits such as oil, grease, dirt, and other foreign materials.
Par 6-117 Page 6-266
F IGURE 6-14. Typical example of acceptable cleanup of corrosion pits.
Materials. Do not use harmful cleaning, polishing, brightening, or paint-removing materials. Use only those compounds that conform to existing government or established industry specifications or that have been specifically recommended by th e aircraft manufacturer.
Observe the product manufacturer’s recommendations concerning use.
Chemical Cleaners. Chemicals must be used with great care in cleaning assembled aircraft. The danger of entrapping corrosive materi als in faying surfaces and crevices counteracts any advantages in their speed and effectiveness. Use material s that are relatively neutral and easy to remove.
Removal of spilled battery acid. The battery, battery cover, battery box and adjacent areas will be corroded if battery acid spills onto them. To clean spilled battery acid, brush off any salt residue and sponge the area with fresh water. For lead-acid batteries, sponge the area with a solution of 6 ounces of sodium bicarbonate (baking soda) per gallon of fresh water.
Apply generously until bubbli ng stops and let solution stay on the area for 5 to 6 minutes, but do not allow it to dry. For nickel-cadmium batteries, sponge the area with a solution of 6 ounces of monobasic sodium phosphate per gallon of fresh water. Sponge area again with clean fresh water and dry surface with compressed air or clean wiping cloths.
STANDARD METHODS. Several standard mechanical and chemical methods are available for corrosion removal. Mechanical methods include hand sanding using abrasive mat, abrasive paper, or metal wool; and powered mechanical sanding, grinding, and buffing, using abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats. The method used depends upon the metal and degree of corrosion. The removal method to use on each metal for each particular degree of corrosion is outlined in the following section.
PIT HAS BEEN CLEANED UP TO THE EXTENT THAT
ALL LOOSE CORROSION PRODUCTS HAVE BEEN REMOVED.
CORROSION DAMAGE BEFORE REWORK
LONGITUDINAL
ROUGH EDGES HAVE BEEN SMOOTHED AND ALL CORROSION HAS BEEN REMOVED. HOWEVER, DEPRESSION HAS NOT BEEN SHAPED. DISH-OUT AFTER BLENDING FIGURE 6-15. Blendout of corrosion as a single depression.
- TRANSVERSE -
CORROSION DAMAGE
BEFORE REMOVAL Ç5D + LONGITUDINAL MIN
BOTTOM OF DEPRESSION
AFTER CORROSION REMOVAL
DAMAGE REMOVED AND SURFACE SMOOTHED
WITH SHALLOW ELLIPTICAL DISH-OUT FIGURE 6-16. Blendout of multiple pits in a corroded area.
Par 6-117 Page 6-267
Par 6-132 Page 6-268
6-119.6-131. [RESERVED.]
Par 6-132 Page 6-269 SECTION 8. ALUMINUM AND ALUMINUM ALLOYS
GENERAL. Aluminum and aluminum alloys are the most widely used material for aircraft construction. Aluminum appears high in the electro-chemical series of elements and corrodes very easily. However, the formation of a tightly-adhering oxide film offers increased resistance under most corrosive conditions.
Most metals in contact with aluminum form couples that undergo galvanic corrosion attack. The alloys of aluminum are subject to pitting, intergranular corrosion and intergranular stress corrosion cracking. In some cases the corrosion products of metal in contact with aluminum are corrosive to aluminum. Therefore, aluminum and its alloys must be cleaned and protected.
SPECIAL TREATMENT OF ANODIZED SURFACES. Anodizing is the most common surface treatment of aluminum alloy surfaces.
The aluminum sheet or casting is made the positive pole in an electrolyte bath in which chromic acid or other oxidizing agents produce a supplemental protective oxide film on the aluminum surface. The anodized surface coating offers the alloy a great deal of protection as long as it is not damaged. Once the film is damaged, it can only be partially restored by chemical surface treatment.
Therefore exercise care to avoid breaking of the protective film, particularly at the edges of the sheet.
REPAIR OF ALUMINUM ALLOY SHEET METAL.
After extensive corrosion removal the following procedures should be followed:
If water can be trapped in blended areas, chemical conversion coat in accordance with MIL-C-81706 and fill the blended area with structural adhesive or sealant to the same level and contour as the original sk in. When areas are small enough that structural strength has not been significantly decreased, no other work is required prior to applying the protective finish.
When corrosion removal exceeds the limits of the structural repair manual, c ontact a DER or the aircraft manufacturer for repair instructions.
Where exterior doublers are installed, it is necessary to seal and insulate them adequately to prevent further corrosion.
Doublers should be made from alclad, when available, and the sheet should be anodized (preferred) or a chemical conversion coat applied after all cutting, drilling, and countersinking has been accomplished.
All rivet holes should be drilled, countersunk, surface treated, and primed prior to installation of the doubler.
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.
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 moisture.
Remove all excess sealant after fa steners are installed.
Apply a fillet sealant bead around the edge of the repair.
After the sealant has cured apply the protective paint finish to the reworked area.
Par 6-132 Page 6-270
CORROSION REMOVAL AROUND
COUNTERSUNK FASTENERS IN ALUMINUM ALLOY . Intergranular corrosion in aluminum alloys often originates at countersunk areas where steel fasteners are used.
When corrosion is found around a fixed fastener head, the fastener must be removed to ensure corrosion removal. All corrosion must be removed to prevent further corrosion and loss of structural strength. To reduce the recurrence of corrosion, the panel should receive a chemical conversi on coating, be primed, and have the fasteners installed wet with sealant.
Each time removable steel fasteners are removed from access panels, they should be inspected for condition of the plating. If mechanical or plating damage is evident, replace the fastener. One of the following fastener installation methods should be used:
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, set and torque the fastener within the working time of the sealant (this is the preferred method).
Apply the corrosion preventive compound to the substructure and fastener, set and torque the fastener.
Apply a coating of primer to the fastener, and while wet with primer, set and torque the fastener.
EXAMPLES OF REMOVING CORROSION FROM ALUMINUM AND ALUMINUM ALLOYS.
Positively identify the metal as aluminum.
Clean the area to be reworked. Strip paint if required.
Determine extent of corrosion damage.
Remove light to moderate corrosion with one of the following.
Non-Powered Corrosion Removal.
The removal of corrosion products by hand can be accomplished by use of aluminum grit and silicon carbide abrasive, such as non-woven, non-metallic, abrasive mat (Spec. MIL-A-9962), abrasive cloth, and paper. Aluminum wool, fiber bristle brushes, and pumice powder are also acceptable methods.
Stainless steel brush (Spec. H-B-178, type III, class 2) may be used as long as the bristles do not exceed 0.010 inch in diameter. After use of this brush the surface should be polished with 60 grit aluminum oxide abrasive paper, then with 400 grit aluminum oxide paper. Care should be exercised in any cleaning process to avoid breaking the protective film.
Steel wool, emery cloth, steel wire brushes (except stainless steel brush) copper alloy brushes, rotary wire brushes, or severe abrasive materials should not be used on any aluminum surface.
Chemical Corrosion Removal.
The corrosion removal compound aluminum pretreatment MIL-C-38334, an acid material, may be used to remove corrosion products from aluminum alloy materials or items (e.g., skins, stringer, ribs in wings, tubing, or ducts). MIL-C-38334 is available in two types:
Type I Liquid concentrate materials should be diluted in accordance with the