- STIFFENE
IDGE FAC
ADJACENT STRUCTURE
STIFFENER
EDGE FACE FIGURE 6-4. PROFILE OF REWORKED CORRODED AREAS IN REGIONS OF LIMITED ACCESS FIGURE 6-5. STEEL PARTICLES FROM FASTENERS OR NUTPLATES MAY CAUSE FUTURE
CORROSION PROBLEMS IN ALUMINUM STRUCTURE Chap 6 126 Par 615
FIGURE 6-6. INCOMPLETE REMOVAL OF CORROSION PRODUCTS RESULTED IN
REOCCURRENCE OF CORROSION FIGURE 6-7. INCOMPLETE REMOVAL OF CORROSION PRODUCTS RESULTED IN
REOCCURRENCE OF CORROSION Chap 6 Par 615 127
616. CHEMICAL TESTING.
A serious problem encountered in corrosion control is the identification of the metal on which corrosion occurs. The importance of this identification arises from the fact that all metals possess certain chemical characteristics that are common only to themselves and which vary greatly from metal to metal and from alloy to alloy of the same metal. Since these characteristics are common to all metals and their alloys, chemical cleaning solutions and chemical protective films will react differently with various In some cases, this produces adverse reactions which can severely weaken or destroy the structural capabilities of the metal.
(1) The primary method of determining material identification is in the aircraft structural repair manual. When the structural repair manual is limited or when more information is required, 1.e., neat-treat and protective finishes, the best source of material identification is the aircraft manufacturer, where an aircraft drawing can be reviewed.
(2) Chemical testing can be used when all other methods have been exhausted and when the following precautions are followed:
Personnel should become thoroughly familiar with the safety precautions and emergency safety procedures, prior to performing any chemical testing.
(її) Chemical spot testing should be accomplished by qualified personnel only.
(iii) Fasteners should not be identified by chemical spot tests.
(iv) High strength steel should not be identified by chemical spot tests.
Chemical testing for each type of metal should be accomplished on test samples before any tests are accomplished on the actual part. The preliminary surface preparation and primary classification of the metal may be determined by the following procedure.
(1) On the surface to be tested, choose an area where there is no corrosion and remove paint (if present) from a 1-inch square. Paint may be removed using hand sanding or an approved paint remover.
CAUTION: ADEQUATE PRECAUTIONS SHOULD BE TAKEN TO PREVENT PAINT REMOVER FROM CONTACTING COMPOSITE PARTS.
(2) Clean area of surface to be tested.
(3) Tentatively identify the exposed metal surface by visually comparing it with samples of previously identified materials, if available.
Chap 6 128 Par 616
7/25/91 AC 43-4A on the exposed surface. (4) Identify the metal as ferrous or nonferrous by placing a magnet Magnetic attraction classifies the base metal as a ferrous magnetic material (iron or steel).
magnesium, etc.) metal as either an austenitic stainless steel or a nonferrous metal (aluminum, The absence of magnetic attraction classifies the base
(5) Hardness test magnetic metals by a qualified person prior to chemical spot testing. If the metal is nonmagnetic, proceed with paragraph 618.
CHEMICAL SPOT ANALYSIS OF MAGNETIC METALS. The magnetic metals usually employed in aircraft construction are ferrous alloys (high strength steels and some stainless steels). These magnetic alloys, when plated, are generally plated with either chromium, nickel, zinc, cadmium, silver, or with a
a. If a magnetic alloy has been plated with cadmium, zinc, or chromium, attraction even if the substrate or base metal is not magnetic. it will exert magnetic attraction. Nickel plating will show slight magnetic identification should be made after accomplishing a hardness test. If positive identification of the metal plating is necessary, the CAUTION: DO NOT PERFORM A CHEMICAL SPOT TEST ON STEELS HEAT-TREATED TO 220,000 PSI AND ABOVE.
c. Place a drop of 10 percent hydrochloric acid (HCL) on the prepared metal surface. Ensure that the surface is dry before applying acid.
metal is zinc. (1) A rapid reaction producing a dark deposit indicates that the chromium, nickel, or steel. (2) A slow or no reaction indicates that the metal may be cadmium, CAUTION: THE ADDITION OF SODIUM SULFIDE (Na,S) TO ACID PRODUCES A POISONOUS GAS. ADEQUATE VENTILATION SHOULD BE PROVIDED WHEN THESE TESTS ARE BEING PERFORMED. DO NOT ALLOW LARGE QUANTITIES OF SODIUM SULFIDE (Na,S) AND ACID TO BE MIXED.
d. After 1 minute, add a drop of Na,S to the drop of HCL.
(1) A white precipitate identifies the metal as zinc.
metal as cadmium. (2) A yellow ring formed around a white precipitate identifies the Chap 6 Par 616 129
(3) A black ring formed around a white precipitate identifies the metal as iron or steel.
(4) A black precipitate indicates that the metal is chromium or nickel.
Confirm the cadmium, zinc, iron, or steel test by placing a drop of 20 percent nitric acid (HNO,) on a fresh spot. After 1 minute, add a drop of NaS to the drop of HNO.
(1) A white precipitate identifies the metal as zinc.
(2) A yellow precipitate identifies the metal as cadmium.
(3) A black spot identifies the metal as iron or steel.
f. Confirm the chromium test by placing a drop of 10 percent HCL on a fresh spot. Add a drop of concentrated sulfuric acid (H,S04) to the drop of HCL.
A color change to green after 1 or 2 minutes identifies the metal as chromium.
g. Confirm the nickel test by placing a drop of dimethylglyoxime solution on a fresh spot. Add a drop of ammonium hydroxide (NOH) to the drop of dimethyiglyoxime solution. A pink to red precipitate identities the metal as nickel.
h. Clean and refinish as detailed in paragraph 620.
618. nonmagnetic metals used in aircraft construction are aluminum, magnesium, and CHEMICAL SPOT ANALYSIS OF NONMAGNETIC METALS. The most common austenitic steels (generally used as 18-8 stainless steel). The positive identification of these nonmagnetic metals is accomplished by the following procedure:
a. Place a drop of 10 percent HCL on the prepared metal surface and allow to stand for 1 minute. Ensure that the surface is dry before applying acid. (Zinc deposits on nonmagnetic metals will react with 10 percent HCL but will not produce a black spot.)
(1) A rapid or violent reaction that produces a black spot indicates that the metal is magnesium.
(2) A slow reaction indicates that the metal is aluminum.
No reaction indicates that the metal is an austenitic steel or a nonmagnetic plating material.
b. If a reaction did not produce a black spot as noted in paragraph
618.a. (1), determine if zinc is present as detailed in paragraph 617.e.
Chap 6 130 Par 617
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