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ADJUSTABLE
DIAL FACE LOCK
DIAL INDICATOR MARKED IN INCREMENTS OF 0.0DOE BASE POINT "A" POINT "B"
CORROSION PIT
CONICAL POINT "C" POINT NOTE
• DEPTH DIAL GAUGE 0 TO 0.500 INCH DETAIL I RANGE.
• TAKE MEASUREMENT READINGS PER-
PENDICULAR TO THE RADIUS OF CONCAVE OR CONVEX SURFACES. SEE TAKE READINGS A DETAIL I. ARIOUS POINT
• TAKE MEASUREMENT READINGS ON
EDGES ADJACENT TO CORROSION
DAMAGE OR BLENDED DEPRESSION (POINTS "A" AND "B"'). TAKE
SEVERAL READINGS AROUND CENTER OF CORROSION DAMAGE OR BLENDED DETAIL II
DEPRESSION (SEE DETAIL II) AND
CONSIDER THE DEEPEST READING TO BE POINT "C." DETERMINE DEPTH AS FOLLOWS :
DEPTH = C - (A + B) 2 FIGURE 6-10. CORROSION DAMAGE AND REWORK MEASUREMENT USING DEPTH DIAL GAUGE
645.-649. RESERVED.
Chap 6 Par 644 154 through 160
7/25/91 AC 43-4A SECTION 5. ALUMINUM AND ALUMINUM ALLOYS TREATMENT. In general, corrosion of aluminum can be more effectively treated in place rather than removing structural parts from the aircraft for corrosion treatment. Treatment includes the mechanical removal of the corrosion products, the inhibition of residual materials by chemical means, and the restoration of permanent surface coating. Details of treatment vary dependinc on whether the aluminum surfaces are to be left bare in use or are to be protected by paint coatings.
651. PROCESSING OF ALUMINUM SURFACES.
Bare aluminum surfaces. While few unpainted aircraft are used under marine conditions, some general information is included on the nature of alclad surfaces and their treatment. Relatively speaking, pure aluminum has greater corrosion resistance than the stronger aluminum alloys. Advantage is taken of this by laminating a thin sheet of relatively pure aluminum, one to five mils thick, over the base higher strength aluminum alloy surface. The protection obtained is good, and the alclad surface can be maintained in a polished condition (see Figure 6-1z). in cleaning such surfaces, however, care should be taken to prevent staining and marring of the exposed aluminum, and more of the protective alclad layer and the exposure of more susceptible, but stronger, aluminum alloy base material. important from a protection standpoint, to avoid unnecessary mechanical removal
b. Additional processing of aluminum surfaces prior to paint finishes.
Aluminum surfaces that are to be subsequently painted can be exposed to more severe cleaning procedures and can also be given more thorough corrective treatment prior to painting. Application of a paint finish requires proper prepaint treatment for good paint adhesion.
surface treatment of aluminum alloy surfaces. Tank processing is accomplished c. Special treatment of anodized surfaces. Anodizing is the most common positive pole in an electrolytic bath in which chromic acid or other oxidizing agents produce a supplemental protective oxide film on the aluminum surface. fabrication from sheet stock. The aluminum sheet or casting is made as a during manufacture or rework of a part or component and frequently prior to its Aluminum oxide is naturally protective and anodizing merely increases the thickness and density of the natural oxide film. When this coating is damaged in service, it can only be partially restored by chemical surface treatment.
destruction of the oxide film. Therefore, any processing of anodized surfaces should avoid unnecessary abrasive materials should not be used on any aluminum surface. Aluminum wool, Steel wool, steel wire brushes, copper alloy brushes, or severe fiber bristle brushes, and mild abrasives are acceptable tools for cleaning anodized surfaces, but care should be exercised in any cleaning process to avoid aluminum sheet (see Figures 6-11 and 6-13). unnecessary breaking of the protective film, particularly at the edges of the Chap 6 Par 650 161
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(2) Tampico fiber brushes are preferred and are adequate to remove most corrosion. prohibited. Take every precaution to maintain as much of the protective coating Producing a buffed or wire-brush finish by any means should be aluminum finishes. Vacuum blasting is an acceptable corrosion removal method to as practicable. Otherwise, treat anodized surfaces in the same manner as other remove surface corrosion. Vacuum blasting should not be used to remove intergranular corrosion.
- EMULSION CLEANER
CLEANING NOT EFFECTIVE
WITHOUT SCRUBBING FIGURE 6-11. REMOVING OIL AND SURFACE DIRT USE MIL - C- 5410 FIRST IF NECESSARY _C
MILD ABRASIVE C
OR POLISH ele FIGURE 6-12. POLISHING AND BRIGHTENING ALCAD SURFACES Chap 6 Par 651 162
MASKING PAPER MASKING TAPE- L BRISTLE BRUSH:
FIGURE 6-13. CLEANING AND STRIPPING PAINT
(3) Chemical conversion coating (Specification MIL-C-81706) is a chemical surface treatment used on aluminum alloys to inhibit corrosion and to provide a proper surface for paint finishing.
d. Special processing of intergranular corrosion in heat-treated aluminum alloy surfaces. _ Intergranular corrosion is usually more severe in heat-treated aluminum alloys and is a corrosive attack along grain boundaries of the alloyed aluminum, where the grain boundaries differ from the metal within the grain. When in contact with an electrolyte, rapid corrosion occurs at the grain boundaries. In its most severe form, actual lifting of metal layers (exfoliation) occurs. The mechanical removal of all corrosion products and visible delaminated metal layers should be accomplished in order to determine the component. the extent of the damage and to evaluate the remaining structural strength of
(1) Use metal scrapers, rotary files, or abrasive wheels to assure that all corrosion products are removed and that only structurally sound aluminum remains.
NOTE: THE USE OF A ROTARY FILE IS RECOMMENDED ONLY FOR SEVERE INTERGRANULAR OR EXFOLIATION CORROSION REMOVAL.
(2) Rotary files should be sharp to insure that they cut the metal without excessive smearing. A dull cutting tool will smear the metal over corrosion cracks or fissure and give the appearance that corrosion has been removed, when, in fact, it may not have been.
Chap 6 Par 651 163
(3) Carbide tip rotary files or metal scrapers should be utilized since they stay sharp longer. Abrasive blasting should not be used to remove intergranular corrosion.
(4) Inspection with a 5- to 10-power magnifying glass or the use of dye penetrant will assist in determining if all unsound metal and corrosion products have been removed.
(5) When complete removal of corrosion has been accomplished, blend or fair using a ratio of 20:1 (length to depth) in the area of corrosion removal. Blending, where required, can best be accomplished by using aluminum oxide impregnated, rubber-base wheels.
(6) Chemical conversion coat the exposed surfaces completely and restore paint coatings in the same manner as on any other aluminum surface (see Figures 6-14, 6-15, and 6-16).
(7) Corrosion damage beyond the limits set in the structural repair manual should be repaired per a cognizant engineer or aircraft manufacturer's instructions that are FAA approved.
STIFF BRISTLE BRUSH
CHROMIC ACID ALODINE, ETC.
FIGURE 6-14. CLEANING AND INHIBITING CORRODED ALUMINUM SURFACES.
Chap 6 164 Par 652
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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