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
Par 6-136 Page 6-271 manufacture’s instructions before use. Type I has a 1 year shelf life; therefore it shall not be used after 1 year from the date of manufacture.
Type II Powdered concentrate materials should be dissolved in the volume of water specified on the kit. These materials have an indefinite shelf life in the dry st ate. Once mixed, they should be used within 90 days.
Mix MIL-C-38334 in wood, plastic, or plastic-lined containers only. Wear acidresistant gloves, protective mask and protective clothing when working with this acid compound. If acid contacts the skin or eyes, flush immediately with water.
Apply MIL-C-38334 solution by flowing, mopping, sponging, brushing, or wiping.
When applying the solution to large areas, begin the application at the lowest area and work upward, applying the solution with a circular motion to disturb the surface film and ensure proper coverage. If pumping is required, pumps, valves, and fittings should be manufactured from 18-8 stainless steel or plastic.
CAUTION: When working with MIL-C-38334, keep the solution away from magnesium surfaces. The solution must be confined to the area being treated. All parts and assemblies including cadmium-plated items and hinges susceptible to damage from acid should be masked and/or protected. Also mask all openings leading to the primary structure that could trap the solution and doors or other openings that would allow the solution (uncontrolled) to get into the aircraft or equipment interior. It is a good practice to keep a wet rag on hand at all times, for removal of spills or splashes.
Allow the solution to remain on the surface for approximately 12 minutes and then rinse away with clean tap wa ter. For pitted or heavily-corroded areas the compound will be more effective if applied warm (140 °F) followed by vigorous agitation with a nonmetallic acid-resisting brush or aluminum oxide abrasive nylon mat. Allow sufficient dwell time, 12 to 15 minutes, before rinsing.
After each application examine the pits and/or corroded area to determine if another application is required with a 4 to 10 power magnifying glass. (Select the power depending on the distance available to make the inspection.) Corrosion still on the area will appear as a powdery crust slightly different in color than the uncorroded base metal. Darkening of area due to shadows and reaction from the acid remover should not be considered.
Once the corrosion has been removed and the area well-rinsed with clean water, a chromate conversion coating such as MIL-C-81706 or MIL-C-5541 alodine 1200, must be applied immediately thereafter.
Remove moderate to heavy corrosion with one of the following.
Powered Corrosion Removal.
Where the problem is severe enough to warrant the use of power tools, a pneumatic drill motor driving either an aluminumoxide-impregnated nylon abrasive wheel, flap brush or rubber grinding wheel may be used with an abrasive value to approximately 120 grit, as needed.
Corrosion-removal accessories, such as flap brushes or rotary files, should be used on one type of metal only. For example, a flap brush used to remove aluminum should not be used to remove magnesium, steel, etc.
Pneumatic sanders may be used with disk and paper acceptable for use on aluminum.
Par 6-148 Page 6-272 When mechanically removing corrosion from alumi num, especially aircraft skin thinner than
0.0625 inch, extreme care must be used. Vigorous , heavy, continuous abrasive grinding can generate enough heat to cause metallurgical change . If heat damage is suspected, hardness tests or conductivity tests must be accomplished to verify condition of the metal. The use of powered rotary files should be limited to heavy corrosion and should not be used on skin thinner than
0.0625 inch.
Blasting.
Abrasive blasting may be used on aluminum alloys using glass beads (Spec. MIL -G-9954) sizes 10 to 13, or grain abrasive (Spec. MIL -G-5634) types I and III may be used as an alternate method of removing corrosion from clad and non- clad aluminum alloys. Abrasive blasting should not be used 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 non-clad aluminum alloys. Engineering approval should be obtained prior to abrasive blasting metal thinner than 0.0625 inch.
When using abrasive blasting on aluminum alloys , do not allow the blast stream to dwell on the same spot longer than 15 seconds. Longer dwell times will cause excessive metal removal.
Intergranular exfoliation corrosion is not to be removed by abrasive blasting; however, blasting may be used with powered corrosion removal to determine whether all exfoliation corrosion has been removed.
Par 6-148 Page 6-273 Inspect the area for remaining corrosion. Repeat procedure if any corrosion remains.
NOTE: If corrosion remains after the second att empt, use a stronger method, e.g., chemical to mechanical.
Using a blend ratio of 20:1 (length to depth) blend and finish th e corrosion rework area with progressively finer abrasive paper until 400-grit paper is used.
Clean reworked area using dry cleaning solvent. Do not use ke rosene or any other petroleum base fuel as a cleaning solvent.
Determine depth of faired depressions to ensure that rework limits have not been exceeded.
Apply chemical conversion coating, MIL-C-81706, immediately after reworking. If 48 hours or more have elapsed since the conversion coating was first applied and the primer or final paint system has not yet been applied, then reapply the conversion coating before continuing.
NOTE: These solutions should not be allowed to co me in contact with magnesium or high-strength steels (180,000 psi). Do not permit solutions or materials to contact paint thinner, acetone or other combustible material: FIRE MAY RESULT.
Apply paint finish to area.
6-137.6-147. [RESERVED.]
Par 6-148 Page 6-274 SECTION 9. MAGNESIUM AND MAGNESIUM ALLOYS
GENERAL. Magnesium and magnesium alloys are the most chemically active of the metals used in aircraft construction and are the most difficult to protect. However, corrosion on magnesium surfaces is probably the easiest to detect in its early stages. Since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed, initial signs show a lifting of the paint films and white spots on the magnesium surface. These rapidly develop into snow-like mounds or even white whiskers. The prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided.
TREATMENT OF WROUGHT MAGNESIUM SHEETS AND FORGINGS. Corrosive attack on magnesium skins will usually occur around the 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 can be easily removed, this should be accomplished to ensure complete inhibition and treatment.
Complete mechanical removal of corrosion products should be practiced when practical. Mechanical cleaning should normally be limited to the use of stiff bristle brushes and similar nonmetallic cleaning tools.
Any entrapment of steel particles from steel wire brushes, steel tools, or contamination of treated surfaces, or dirty abrasives, can cause more trouble than the initial corrosive attack. The following procedural summary is recommended for treatment of corroded magnesium areas when accomplished under most field conditions.
When aluminum insulating washers are used and they no longer fasten tightly to magnesium panels, corrosion is likely to occur under the washers if corrective measures are not taken.
When machine screw fasteners are used, aluminum insulating washers must be removed from all locations to surface treat the magnesium panel.
Where permanent fasteners other than machine screws are used, the insulating washer and fastener must be removed.
When located so water can be trapped in the counter-bored area where the washer was located, use sealants to fill the counterbore. If necessary, fill several areas adjacent to each other. It may be advantageous to cover the entire row of fasteners with a strip of sealant.
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 that two coats of epoxy primer may be required on both the doubler and skin being patched instead of one. 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 non-corrosive type sealant be used, since it serves a dual purpose of material separation and sealing.
Par 6-148 Page 6-275
IN-PLACE TREATMENT OF MAGNESIUM CASTINGS. Magnesium castings, in general, are more porous and more prone to penetrating attack than wrought magnesium skin. However, treatment in the field is, for all practical purposes, the same for all magnesium. Bellcranks, fittings, and numerous covers, plates, and handles may also be magnesium castings. When attack occurs on a casting, the earliest practical treatment is required to prevent dangerous corrosive penetration. Engine cases in salt water can develop moth holes and complete penetration overnight.
If at all practical , faying surfaces involved shall be separated to treat the existing attack effectively and prevent its further progress. The same general treatment sequence as detailed for magnesium skin should be followed. Where engine cases are concerned, baked enamel overcoats are usually involved rather than other top coat finishes. A good air drying enamel can be used to restore protection.
If extensive removal of corrosion products from a structural casting is involved, a decision from the aircraft manufacturer or a DER may be necessary to evaluate the adequacy of stru ctural strength remaining.
Refer to the aircraft manufacturer if any questions of safety are involved.
EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM. If possible, corroded magnesium parts shall be removed from aircraft. When impossible to remove the part, the following procedure will be used.
Positively identify metal as magnesium.
Clean area to be reworked.
Strip paint if required.
Determine the extent of corrosion damage.
Remove light to moderate corrosion by one of the following means.
Non-Powered Corrosion Removal.
Non-powered removal can be accomplished using abrasive mats, cloth, and paper with aluminum oxide grit (do not use silicon carbide abrasive). Metallic wools and hand brushes compatible with magnesium such as stainless steel and aluminum, may be used.
When a brush is used the bristles should not exceed 0.010 inch in diameter. After using a brush, the surface should be polished with 400 grit aluminum oxide abrasive paper, then with 600 grit aluminum oxide abrasive paper.
Pumice powder may be used to remove stains or to remove corrosion on thin metal surfaces where minimum metal removal is allowed.
Chemical Corrosion Removal.
Chemical corrosion removal on magnesium alloys is usually done with a chromic acid pickle solution. Chemical corrosion removal methods are not considered adequate for areas that have:
Deep pitting,
Heavy corrosion and corrosion by products,
Previously had corrosion removed by mechanical means, or
Previously been sand blasted.