1/25/91 AC 43-4A C. If the results of paragraph 618.b. are negative (zinc not present), hydroxide (NaOH) on a fresh spot. Check for the following: confirm the magnesium and aluminum tests by placing a drop of 10 percent sodium
(1) No reaction which will identify the metal as magnesium.
(2) A reaction that produces a colorless spot will identify the metal as a bare-aluminum alloy.
cadmium chloride on a fresh spot. further test to distinguish the different alloys by placing a drop of 10 percent d. If an aluminum alloy is identified as outlined in paragraph 618.c., the metal as 7075 or 7178 bare-aluminum alloy. A dark gray deposit forming within a few seconds will identify metal as 7075 or 7178 clad-aluminum alloy. (2) A dark gray deposit forming within 2 minutes will identify the clad will identify the metal as 2024 aluminum alloy (a faint deposit will form after 15 or 20 minutes). (3) No deposit formation in the time specified for 7075 or 7178 chloride (Cuci, 2H,0) in 100 cubic centimeters of HCL and placing a drop of the e. Confirm the austenitic steel test by dissolving 10 grams of cupric solution on a fresh spot. After 2 minutes, add three or four drops of distilled water to the drop of HCL solution and dry the surface. The appearance of a brown spot identifies the metal as an austenitic steel.
for a plating material as detailed in paragraph 617. f. If no reaction was noted as outlined in paragraph 618.a or e., test the plating should be removed by mechanical abrasion and the base metal g. If step f. reveals the presence of plating on the nonmagnetic metal, identified by the visual and/or chemical methods outlined in paragraph 617.
h. Clean and refinish as required in paragraph 620.
for metals used in aircraft construction are: chemical conversion coatingther surface treatments include lacquer and chromate films. phosphate treatments for steels, and chromate treatments for aluminum. these surface treatments may be accomplished by the following procedures: 619. SURFACE TREATMENT TESTING. The most common types of surface treatment The identification of
a. Phosphate treatment.
zinc, cadmium, or aluminum can be confirmed by placing a drop of 20 percent yellow precipitate will form. molybdate solution. If the metal surface has had a phosphate treatment, a nitric acid (HNO,) on the surface and following this with two drops of ammonium Chap 6 Par 618 131
7/25/91 AC 43-4A aluminum, or magnesium are highly colored and are indicative of the application b. Chromate treatment. Surface chromate treatments on zinc, cadmium, of these treatments. However, a bleached chromate treatment may have been applied and then coated with lacquer to mask any residual iridescence for the sake of appearance. If so, visual detection of the chromate is impossible. test for lacquer, proceed as directed in paragraph 619.c. It should be noted that the bleaching process used in a bleached chromate treatment lowers the corrosion resistance provided by the chromate film.
Lacquer finish. To test for lacquer, place a drop of concentrated sulfuric acid on the surface. If lacquer is present, the spot will rapidly turn brown. brown with no effervescence. If the metal is zinc, there will be no rapid effervescence. If the If lacquer is not present, the spot will not turn metal is cadmium, there will be no reaction.
a drop of 5 percent aqueous solution of lead acetate on the surface. d. Chromate film. To detect a chromate film on zinc and cadmium, place metal has been treated, the surface will show no discoloration for 10 seconds.
If there is no surface
620. POSTIDENTIFICATION CLEANING AND REFINISHING.
NOTE: THE SOLUTIONS USED IN THE CHEMICAL SPOT TESTS ARE EXTREMELY CORROSIVE.
After identification of the metal is completed, clean the area as follows:
a. Blot any remaining chemicals with a dry cloth.
b. Swab the area several times with a water moistened cloth.
Test the surface by placing a piece of litmus paper on the moistened surface. If the litmus paper changes color, repeat steps a. and b. until no color change occurs.
d. Dry surface thoroughly.
Remove corrosion, if present, and refinish the surface, as applicabie.
621.-625. RESERVED.
Chap 6 Par 619 132 through 140
7/25/91 AC 43-4A SECTION 3. MECHANICAL CORROSION REMOVAL BY BLASTING
626. GENERAL. Abrasive blasting is a process for cleaning or finishing with the following requirements being made: as painting or plating. removal of rust and corrosion and for cleaning prior to further processing, such metals, plastics, and other materials by directing a stream of abrasive particles against the surface of the parts. Abrasive blasting is used for the Standard blast cleaning practices should be adopted blasting is preferred. a. Any form of blast cleaning equipment may be used, but in-cabinet recovery are provided for the abrasives. b. External gun blasting may be used if adequate confinements and
627. SAFETY PRECAUTIONS. Operators should be adequately protected with Magnesium creates a fire hazard when abrasive blasted. Dry abrasive blasting of complete face and head covering equipment, and provided with pure breathing air. titanium alloys and high tensile strength steel creates sparking. Care should 55307 be taken to assure that no hazardous concentration of inflammable vapors exists.
Static ground the dry abrasive blaster and the material to be blasted.
from abrasive impingement and system (hydraulic, oil, fuel, etc.) contamination. possible. a. The part to be blast-cleaned should be removed from the aircraft, if Otherwise, areas adjacent to the part should be masked or protected blast cleaning. b. Parts should be clean of oil, grease, dirt, etc., and dry prior to should be masked. c. Close-tolerance surfaces, such as bushings, bearing shaft, etc., immediately with finishing requirement using surface treatments as required. d. Blast-clean only enough to remove corrosion coating. Proceed
628.-639. RESERVED.
Chap 6 Par 626 141 through 150
SECTION 4. CORROSION DAMAGE AND REWORK LIMITS
640. DISCUSSION. Corrosion evaluation will be required after general inspection and cleaning to determine the nature and extent of repair or rework.
Local blending of corroded areas may be required to determine the total extent of the corrosion problem (see Figures 6-1, 6-2, 6-3 and 6-4). Corrosion damage classifications are defined as follows:
Light corrosion. Characterized by discoloration or pitting to a depth of approximately 0.001 inch maximum. This type of damage is normally removed by light, hand sanding or a minimum of chemical treatment.
D. Moderate corrosion. Appears similar to light corrosion except there may be some blisters or evidence of scaling and flaking. Pitting depths may be as deep as 0.010 inch. This type of damage is normally removed by extensive mechanical sanding.
c. Severe corrosion. General appearance may be similar to moderate corrosion with severe blistering exfoliation and scaling or flaking. Pitting depths will be deeper than 0.010 inch. This type of damage is normally removed by extensive mechanical sanding or grinding. Severe corrosion damage beyond the limits of the aircraft structural repair manual will require FAA-approved engineering authorization and may include the following typical corrosion trimming out of cracked and corroded areas (see Figure 6-8) or spot facing of fastener locations (see Figure 6-9).
FIGURE 6-8. SEVERE CORROSION REQUIRING TRIMMING OUT OF DAMAGED AREA Chap 6 Par 640 151
FIGURE 6-9. CORROSION AT FASTENER HOLES REMOVED BY SPOT FACING REMOVAL OF CORROSION. There are two basic methods of corrosion removal, mechanical and chemical. The method used depends upon the type of structure, its location, the type and severity of corrosion, and the availability of maintenance equipment. Mechanical methods of corrosion removal are the most Coper on ianed neta wit contribute to ture Corsion if brunes are lo If brushes are used they should be stainless steel or nonmetallic. Mechanical methods of corrosion removal are used on all three levels of corrosion damage. Chemical methods of corrosion removal are limited to light corrosion and only in areas where the chemicals cannot migrate to other areas.
642. DETERMINING DEGREE OF CORROSION DAMAGE. Determine degree of corrosion damage (light, moderate, or severe) with a depth dial gauge, straight edge, or a molding compound. The depth of corrosion cannot be measured until all the corrosion is removed. Before measurements are made, visually determine if corrosion is in an area which has previously been reworked. If corrosion is in the recess ot a faired or blended area, measure damage to include the material which has previously been removed. The following method outlines the process for taking measurements with the depth gauge.
a. Remove loose corrosion products, if present.
b. Position depth gauge as illustrated in Figure 6-10 and determine the measurement reading.
Chap 6 152 Par 641
7/25/91 AC 43-4A NOTE: The base of the depth gauge should be flat against the undamaged surface on each side of the corrosion. When taking measurements on concave or convex surfaces, place the base perpendicular to the radius of the surface as shown in Figure 6-10.
c. Take several additional depth readings.
d. Select deepest reading as being the depth of the corrosion damage.
643. the allowable damage limit chart in the manufacturer's repair manual. removed from any damaged surface may be determined from criteria contained in DETERMINING REWORK LIMITS. The maximum allowable amount of material criteria is given, contact the aircraft manufacturer for cleanup limits.
644. DETERMINING MATERIAL THICKNESS REDUCTION AFTER CORROSION CLEANUP. The amount of material which may be removed from a part or panel during corrosion cleanup is usually available in the manufacturer's allowable damage limit charts. To ensure that the allowable limits are not exceeded, an accurate measurement should be made of the material removed or material thickness remaining in the reworked area.
Measurement of panel thickness after rework can be made using an ultrasonic tester. This method requires a qualified NDI operator and suitable test standards for calibration.
made using a depth dial gauge (see Figure 6-10). If the depth dial gauge will give accurate results may be used to determine material removed. In the eveni not work, clay impressions, liquid rubber, or other similar means which will b. Measurement of the depth of blended pits (material removed) can be repair manual, contact the manufacturer or the FAA. repaired or replaced. If replacement or repair criteria is not contained in the that material removal limits have been exceeded, the area or part should be Chap 6 Par 642 153
7/25/91 AC 43-4A
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
7/25/91 AC 43-4A
(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