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DO NOT ALLOW EXCESS TO DRY.
FIGURE 6-15. REMOVING EXCESS INHIBITOR SOLUTION
SOLVENT TYPE WAX- - SPONGE
APPLY IN SHADE FIGURE 6-16. APPLYING WAX TO CLEANED SURFACES the following procedures should be followed: REPAIR OF ALUMINUM ALLOY SHEET METAL. After extensive corrosion removal,
a. If water can be trapped in blended areas, chemical conversion coat and fill the blended area with structural adhesive or sealant to the same level and contour as the original skin. When areas are small enough that structural Chap 6 Par 651 165
strength has not been significantly decreased, no other work is required prior to applying the protective finish.
b. When corrosion removal exceeds the limits of the structural repair manual, contact a cognizant engineer or the aircraft manufacturer for repair instructions.
Where exterior doublers are allowed, it is necessary to seal and insulate them adequately to prevent further corrosion.
d. Doublers should be made from alclad, when available, and the sheet should be anodized (preferred) or chemical conversion coated 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.
f. 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.
g. 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.
h. 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 the protective paint finish to the reworked area.
CORROSION REMOVAL AROUND COUNTERSUNK FASTENERS IN ALUMINUM ALLOY.
Intergranular corrosion in aluminum alloys often originates at countersunk areas where steel fasteners are used. Removal of corrosion in a countersink is impossible to accomplish with the fastener in place.
a. When corrosion is found around a fixed fastener head, the fastener must be removed to ensure corrosion removal. It is imperative that all corrosion be removed to prevent further corrosion and 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.
Each time removable steel fasteners are removed from access panels, they should be inspected for material condition including the condition of the plating. If mechanical or plating damage is evident, replace the fastener.
Upon installation, one of the following fastener installation methods should be followed:
(1) 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, and set and torque the fastener within the working time of the sealant (this is the preferred method);
Chap 6 166 Par 652
fastener, set and torque the fastener; or (2) Apply the corrosion preventive compound to the substructure and primer set and torque the fastener. (3) Apply a coating of primer to the fastener, and while wet with 654- EXAMPLES OF REMOVING CORROSION FROM ALUMINUM AND ALUMINUM ALLOYS.
a. Prepare the aircraft for corrosion rework as provided in paragraph 612 and remove corrosion products as follows. Observe the work procedures of paragraph 601.
b. Positively identify the metal as aluminum.
Clean the area to be reworked. Strip paint if required.
d. Determine extent of corrosion damage as covered in paragraph 642. To remove light corrosion, proceed with paragraph 654.e. To remove moderate or severe corrosion, proceed with paragraph 654.f.
e. Remove light corrosion by hand rubbing the corroded surface with tools, abrasives or by chemical means as follows. Do not use the chemical removal process at temperatures above 100 °F or below 40 °F.
(1) Protectively mask adjacent areas to prevent brighteners from contacting magnesium, anodized aluminum, glass, plexiglass, fabric surfaces, and all steel. wear acid-resistant gloves, protective mask, and protective clothing when working with corrosion removing compounds. accidentally contact the skin or eyes, flush off immediately with plenty of If corrosion removing compounds clear water. Refer to safety procedures in paragraph 600.
Type I) with an equal volume of water. Mix the compound only in wood, plastic, or plastic lined containers. (2) Dilute corrosion removing compound (Specfication MIL-C-38334, The diluted solution of corrosion removing compound may be applied by flowing, mopping, sponging, brushing, or wiping.
(3) Apply 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 nylon mat. agitation with a nonmetallic, acid resistant brush or aluminum oxide abrasive
(4) Leave the solution on surface for approximately 12 minutes. Do not allow solution to dry on surface, as streaking will result. (On large exterior surfaces, remove solution by high pressure water rinse.)
(5) Wipe off solution with a clean, moist cloth; frequently rinse the cloth in clear water. Wipe the area several additional times with a fresh cloth dampened and rinsed frequently in clear water.
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(6) Dry the area with a clean, dry cloth and inspect for corrosion.
(7) Repeat the procedure outlined in paragraph 654.e. if any corrosion remains.
NOTE: IF CORROSION STILL REMAINS AFTER THE SECOND ATTEMPT,
MECHANICALLY REMOVE CORROSION AS DETAILED IN PARAGRAPH 654.f. OF THIS CHAPTER.
(8) After all corrosion has been removed, proceed with paragraph 654.f. (4).
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 TO PREVENT ADDITIONAL DAMAGE FROM CORROSION PRODUCTS REMOVED DURING MECHANICAL REMOVAL.
(1) Remove loose corrosion products by hand rubbing the corroded surface with tools or abrasives. Dry abrasive blasting using glass beads (Specification MIL-G-9954) sizes 10, 11, 12, 13, or grain abrasive (Specification MIL-G-5634) types I and III, may be used as an alternate method of removing corrosion from clad and nonclad 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 nonclad aluminum alloys. Engineering approval from the aircraft manufacturer should be obtained prior to abrasive blasting metal thinner than 0.0625 inch.
(2) Remove residual corrosion by hand sanding or with an approved hand-operated power tool. Corrosion removal using power tools is generally done vith flapbrush, rotary file, sanding pad, or abrasive wheel attachments. Rotary files should not be used on skin thinner than 0.0625. Select an appropriate abrasive from Table 6-1.
(3) Using a blend ratio of 20:1 (length to depth) blend and finish the corrosion rework area with progressively finer abrasive paper until 400-grit abrasive paper is used.
(4) Clean reworked area using dry cleaning solvent; do not use kerosene.
(5) Determine depth of faired depressions to ensure that rework limits have not been exceeded.
(6) Apply a chemical conversion coating to the corrosion rework area.
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(7) Apply paint finish to the corrosion rework area.
655.-659. RESERVED.
Chap 6 Par 654 169 through 180
7/25/91 AC 43-4A SECTION 6. MAGNESIUM ALLOYS 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 can be easily removed, this should be TREATMENT OF WROUGHT MAGNESIUM SHEETS AND FORGINGS. Corrosive attack on
a. Complete mechanical removal of corrosion products should be practiced of stiff bristle brushes and similar nonmetallic cleaning tools, particularly during treatment in place under field conditions. when practicable. Such mechanical cleaning shall normally be limited to the use trouble than the initial corrosive attack. tools, or contamination of treated surfaces by dirty abrasives, can cause more b. Any entrapment of steel particles from steel wire brushes, steel to magnesium panels, corrosion is likely to occur under the washers if corrective measures are not taken. c. When aluminum insulating washers are used and they no longer adhere magnesium panel. from all loose insulating washer locations in order to surface treat the When machine screw fasteners are used, they should be removed corrosion removal. the insulating washer and fastener should be removed to ensure complete (2) Where permanent fasteners other than machine screws are used, 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. (3) When located so that water can be trapped in the counterbored
661. REPAIR OF MAGNESIUM SHEET METAL AFTER EXTENSIVE CORROSION REMOVAL. The type sealant be used, since it serves a dual purpose of material separation and to form magnesium alloys in the contour, aluminum alloy may be utilized. When at all points where repairs are made. It is recommended that only noncorrosive still have to be sealed to prevent entrance of moisture between mating surfaces same general instructions apply when making repairs in magnesium as in aluminum this is done, it is necessary to insure effective dissimilar metal insulation. Vinyl tape will insure positive separation of dissimilar metals, but edges will doubler and skin being patched instead of only one coat. where it is difficult alloy skin, except that two coats of epoxy primer may be required on both the Chap 6 Par 660 181
7/25/91 AC 43-4A 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 areas. Engine cases are among the most common IN-PLACE TREATMENT OF MAGNESIUM CASTINGS. Magnesium castings, in examples of cast magnesium encountered in modern aircraft. Bellcranks, fittings, and numerous covers, plates, and handles may also be magnesium castings. When attack occurs on a casting, the earliest practicable treatment in salt water can develop "moth holes" and complete penetration overnight. is required if dangerous corrosive penetration is to be avoided. Engine cases separated in order to effectively treat the existing attack and prevent its If it is at all practicable, faying surfaces involved should be further progress. The same general treatment sequence detailed for magnesium skin should be followed. where engine cases are concerned, . baked enamel overcoats are usually involved rather than other topcoat 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 may be necessary in order manuals usually include dimensional tolerance limits for critical structural to evaluate the adequacy of structural strength remaining. Structural repair members. The FAA should be consulted if any questions of safety are involved.
magnesium parts should be removed from aircraft. When impossible to remove the part, make aircraft preparations detailed in paragraph 612. When using that procedure, observe the safety precautions and procedures of paragraph 600. EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM. If possible, corroded
a. Positively identify metal as magnesium. (Refer to paragraph 618)
b. Clean area to be reworked.
c. Strip paint if required.
To remove light corrosion, proceed with paragraph 663.e. To remove moderate or Determine extent of corrosion damage as detailed in paragraph 642.
severe corrosion, proceed with paragraph 663.f.
follows. assemblies, areas where the brush-on solution might become lodged, or local e° Do not use the following procedure for adhesive bonded parts or Remove light corrosion by light hand-sanding or chemically, as areas bared specifically for grounding or electrical bonding purpose.
(1) Remove loose corrosion with aluminum wool or abrasive mat, paper, or cloth.
(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.
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(3) Prepare corrosion treating solution in the following proportions: 1-1/2 pounds of sodium dichromate and 1-1/2 pints of concentrated nitric acid (HNO,) per gallon of water. Mix as follows, but prepare and store the solution in clean polyethylene or glass containers:
Fill a suitable container with a volume of water equal to 1/4 the desired total quantity of solution.
(ii) Add full quantity of sodium dichromate in proportions indicated and agitate solution until the chemical is dissolved.
(iii) Add water until quantity of solution is equal to approximately 2/3 the desired total quantity.
and mix thoroughly. (iv) Slowly add total volume of nitric acid (HNO,) to solution solution is reached and stir until entire solution concentration is equal. Add remaining water until total desired quantity of
(4) Remove remaining corrosion by swabbing the corroded surface 1 to 2 minutes with the nitric acid (HNO,) solution, then wipe dry.
brush, or abrasive mat and wipe dry. (5) Rinse thoroughly with clean water while scrubbing with a mop, has been removed. (6) Repeat the preceding sequence, as necessary, until all corrosion paragraph 663.g. (7) After all corrosion has been removed, proceed with
f. 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.
NOTE: DO NOT USE CARBON STEEL WIRE BRUSHES OR SILICONE CARBIDE ABRASIVES ON MAGNESIUM.
(1) Remove heavy corrosion products by hand brushing with a stainless steel or fiber brush followed by vacuum abrasive blasting with glass to 35 psi should be used for direct pressure machines. For suction type blast Specification MIL-G-5634 lypes 1 or Il1. (Specification MIL-G-9954) sizes 10, 11, 12, 13; or grain abrasive An air pressure at the nozzle of 10 equipment, use 50 percent higher pressure.
operated power tool. (2) Remove residual corrosion by hand sanding or with approved handapply corrosion treating solution. (3) After removing all corrosion visible through a magnifying glass, Chap 6 Par 663 183
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g. 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.
h. Determine depth of faired depressions to ensure that rework limits have not been exceeded.
i. Clean reworked area using a solvent to provide a water breakfree surface. Do not use kerosene.
Prepare and apply magnesium conversion coat conforming to MIL-M-3171, TYPE VI (DOW-19) as follows:
(1) Measure 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 dehydrate (CaS04.2H,0).
(4) Vigorously stir for at least 15 minutes to 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 (the color can vary from green-brown, brassy, 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 produce a brassy film, and 3 to 5 minutes a dark brown coating. Under adverse 1 to 5 minutes. Under these conditions, 1 to 2 minutes of treatment should conditions, and if the desired specified finish color is not produced in the specified time, the treatment may have to be prolonged up to 20 to 30 minutes in some instances until the proper finish is effected. For good paint adhesion, a dark brown color, free of powder, is considered best. The color may vary in using different vendors' materials. Too long exposure to the brush-on solution produces a coating which will powder and impair adhesion of applied paint finish/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.
Rinse with clean water, then allow to dry at ambient temperature for a minimum of 1 hour (more in high humidity areas).
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k. Apply primer and topcoat finish.
1. Remove masking and protective covering.
664.-669. RESERVED.
Chap 6 Par 663 185 through 190