6-13
6.16 Chemical Testing.
6.16.1 A serious problem in corrosion control is identification of the metal on which corrosion occurs. This identification is important because all metals possess certain chemical characteristics common only to themselves, and these characteristics 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 different metals. In some cases, this reaction produces adverse reactions which can severely weaken or destroy the structural capabilities of the metal.
6.16.1.1 The primary method of identifying metal is in the aircraft SRM. When the SRM is limited or when more information is required, such as with heat-treat and protective finishes, the best source of identification is the aircraft manufacturer, where an aircraft drawing can be reviewed.
6.16.1.2 Use chemical testing 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.
• Chemical spot testing should be accomplished by qualified personnel only.
• Fasteners should not be identified by chemical spot tests.
• High-strength steel should not be identified by chemical spot tests.
6.16.2 Use test samples for chemical testing for metals before testing the actual part. Determine the preliminary surface preparation and primary classification of the metal 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: Take adequate precautions to prevent paint remover from contacting composite parts.
2. Clean the surface of the test area.
3. Tentatively identify the exposed metal surface by visually comparing it with samples of previously identified materials, if available.
4. Identify the metal as ferrous or nonferrous by placing a magnet on the exposed surface. Magnetic attraction classifies the base metal as a ferrous magnetic material, which can be either iron or steel. The absence of magnetic
6-14 attraction classifies the base metal as either an austenitic stainless steel or a nonferrous metal, such as aluminum or magnesium.
5. Have a qualified person test magnetic metals for hardness before chemical spot testing. If the metal is nonmagnetic, proceed with paragraph 6.18.
6.17 Chemical Spot Analysis of Magnetic Metals. Aircraft manufacturers usually use magnetic ferrous alloys, such as high-strength steels and some stainless steels, in aircraft construction. These magnetic alloys can be plated with chromium, nickel, zinc, cadmium, silver, or a combination of these platings.
6.17.1 If a magnetic alloy has a cadmium, zinc, or chromium plating, that plating will exert magnetic attraction. Nickel plating will show slight magnetic attraction even if the substrate or base metal is not magnetic.
6.17.2 If positive identification of the metal plating is necessary, the identification should be made after accomplishing a hardness test.
CAUTION: Do not perform a chemical spot test on steels heat-treated to 220,000 psi and above.
6.17.3 Place a drop of 10 percent hydrochloric acid (HCl) on the prepared metal surface. Ensure that the surface is dry before applying the acid.
• A rapid reaction producing a dark deposit indicates that the metal is zinc.
• A slow or no reaction indicates that the metal may be cadmium, chromium, nickel, or steel.
6.17.4 After 1 minute, add a drop of sodium sulfide (Na2S) to the drop of HCl.
CAUTION: Adding Na2S to any acid produces a poisonous gas. Provide adequate ventilation when performing these identification tests. Do not mix large quantities of Na2S and acid.
• A white precipitate identifies the metal as zinc.
• A yellow ring formed around a white precipitate identifies the metal as cadmium.
• A black ring formed around a white precipitate identifies the metal as iron or steel.
• A black precipitate indicates that the metal is chromium or nickel.
6.17.5 Confirm the cadmium, zinc, iron, or steel test by placing a drop of 20 percent nitric acid (HN03) on a fresh spot. After 1 minute, add a drop of Na2S to the drop of HN03.
• A white precipitate identifies the metal as zinc.
• A yellow precipitate identifies the metal as cadmium.
• A black spot identifies the metal as iron or steel.
6-15
6.17.6 Confirm the chromium test by placing a drop of 10 percent HCl on a fresh spot. Add a drop of concentrated sulfuric acid (H2S04) to the drop of HCl. A color change to green after 1 or 2 minutes identifies the metal as chromium.
6.17.7 Confirm the nickel test by placing a drop of dimethylglyoxime (C4H8N2O2) solution on a fresh spot. Add a drop of ammonium hydroxide (NH40H) to the drop of C4H8N2O2 solution. A pink-to-red precipitate identifies the metal as nickel.
6.17.8 Clean and refinish, as detailed in paragraph 6.20.
6.18 Chemical Spot Analysis of Nonmagnetic Metals. The most common nonmagnetic metals in aircraft construction are aluminum, magnesium, and austenitic steels, generally used as 18-8 stainless steel, where the “18-8” indicates a composition of 18 percent chromium and 8 percent nickel. Positively identify these nonmagnetic metals by the following procedure:
6.18.1 Place a drop of 10 percent HCl on the prepared metal surface and allow to stand for 1 minute. Ensure the surface is dry before applying the acid.
Note: Zinc deposits on nonmagnetic metals will react with 10 percent HCl but will not produce a black spot.
• A rapid or violent reaction that produces a black spot indicates the metal is magnesium.
• A slow reaction indicates the metal is aluminum.
• No reaction indicates the metal is an austenitic steel or a nonmagnetic plating material.
6.18.2 If a reaction did not produce a black spot as noted in the first bullet under paragraph 6.18.1, determine if zinc is present, as detailed in paragraph 6.17.5.
6.18.3 If the results of paragraph 6.18.2 are negative (zinc is not present) confirm the magnesium and aluminum tests by placing a drop of 10 percent sodium hydroxide (NaOH) on a fresh spot. Check for the following:
• No reaction will identify the metal as magnesium, or
• A reaction that produces a colorless spot will identify the metal as a bare-aluminum alloy.
6.18.4 If an aluminum alloy is identified as outlined in paragraph 6.18.3, further test to distinguish the different alloys by placing a drop of 10 percent cadmium chloride (CdCl2) on a fresh spot.
6-16
• A dark gray deposit forming within a few seconds will identify the metal as 7075 or 7178 bare-aluminum alloy.
• A dark gray deposit forming within 2 minutes will identify the metal as 7075 or 7178 clad-aluminum alloy.
• No deposit formation in 2 minutes will identify the metal as 2024 aluminum alloy. A faint deposit will form after 15 or 20 minutes.
6.18.5 Confirm the austenitic steel test by dissolving 10 grams of cupric chloride (CuC12 2H20) in 100 cubic centimeters of HCl and placing a drop of the 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.
6.18.6 If no reaction was noted as outlined in paragraph 6.18.1 or 6.18.5, test for a plating material, as detailed in paragraph 6.17.
6.18.7 If paragraph 6.18.6 reveals plating on a nonmagnetic metal, the plating should be removed by mechanical abrasion and the base metal identified by the visual and/or chemical methods outlined in paragraph 6.17.
6.18.8 Clean and refinish as required in paragraph 6.20.
6.19 Surface Treatment Testing. Chemical conversion coatings, phosphate treatments for steels, and chromate treatments for aluminum are the most common surface treatment for metals in aircraft construction. Other surface treatments include lacquer and chromate films. Identify these surface treatments by the following procedures:
6.19.1 Phosphate Treatment. Confirm the presence of a phosphate treatment on steel, zinc, cadmium, or aluminum by placing a drop of 20 percent HN03 on the surface, then following with two drops of ammonium orthomolybdate ((NH4)2MoO4) solution. A yellow precipitate will form.
6.19.2 Chromate Treatment. Surface chromate treatments on zinc, cadmium, aluminum, or magnesium are highly colored and indicate the application 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. To test for lacquer, proceed as directed in paragraph 6.19.3. Note that the bleaching process in a bleached chromate treatment lowers the corrosion resistance provided by the chromate film.
6.19.3 Lacquer Finish. To test for lacquer, place a drop of concentrated H2SO4 on the surface. If lacquer is present, the spot will rapidly turn brown with no effervescence or bubbling. If lacquer is not present, the spot will not turn brown. If the metal is zinc, there will be no rapid effervescence. If the metal is cadmium, there will be no reaction.
6.19.4 Chromate Film. To detect a chromate film on zinc and cadmium, place a drop of 5 percent aqueous solution of lead acetate (Pb(C2H3O2)2) on the surface. If the metal has
6-17 been treated, the surface will show no discoloration for 10 seconds. If there is no surface treatment, an immediate dark spot will appear.
6.20 Post Identification Cleaning and Refinishing.
Note: The solutions used in chemical spot tests are extremely corrosive.
6.20.1 After identification of the metal is completed, clean the area as follows:
1. Blot any remaining chemicals with a dry cloth.
2. Swab the area several times with a water-moistened cloth.
3. Test the surface by placing a piece of litmus paper on the moistened surface.
If the litmus paper changes color, repeat steps 1 and 2 until no color change occurs.
4. Dry surface thoroughly.
5. Remove corrosion, if present, and refinish the surface, as applicable.
6.21–6.25. RESERVED.
6-18 SECTION 3. MECHANICAL CORROSION REMOVAL BY BLASTING
6.26 General. Abrasive blasting is a cleaning or finishing process which directs a stream of abrasive particles against a surface, and can be used with metals, plastics, and other materials. Abrasive blasting can remove rust and corrosion and can clean a surface before further processing, such as painting or plating. Use standard blast cleaning practices, to include the following:
• Use any form of blast cleaning equipment, but in-cabinet blasting is preferred.
• Use external gun blasting if adequate abrasive confinement and recovery exists.
6.27 Safety Precautions. Operators should adequately protect themselves with complete face and head covering equipment, and use pure breathing air. Magnesium creates a fire hazard when abrasive blasted. Dry abrasive blasting of titanium alloys and high tensile strength steel creates sparking. Take care that no hazardous concentration of flammable vapors exists. Static ground the dry abrasive blaster and the material to be blasted.
Note: Refer to the OEM maintenance manual for abrasive blasting guidance.
6.27.1 Remove the part from the aircraft for blast cleaning, if possible. Otherwise, mask or protect areas adjacent to the part against abrasive impingement and system contamination. Examples are hydraulic, oil, or fuel systems.
6.27.2 Parts should be clean and dry prior to blast cleaning.
6.27.3 Mask close-tolerance surfaces, such as bushings or bearing shafts.
6.27.4 Blast clean only enough to remove corrosion coating. Proceed immediately with finishing requirement using surface treatments, as required.
6.28–6.39. RESERVED.
6-19 SECTION 4. CORROSION DAMAGE AND REWORK LIMITS
6.40 Discussion. Evaluate corrosion 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:
6.40.1 Corrosion Level 1:
1. Damage occurring between successive inspections that is within allowable damage limits;
2. Damage occurring between successive inspections that does not require structural reinforcements, replacement, or new damage tolerance-based inspections;
3. Corrosion occurring between successive inspections that exceeds allowable limits but can be attributed to an event not typical of operator usage of other aircraft in the same fleet; or
4. Light corrosion occurring repeatedly between inspections that eventually requires structural reinforcements, replacement, or new damage tolerance-based inspections.
6.40.2 Corrosion Level 2:
1. Corrosion occurring between any of the two successive corrosion inspection tasks that requires a single rework or blend-out which exceeds the allowable limit; or
2. Corrosion occurring between successive inspections that is widespread and requires a single blend-out approaching the allowable rework limit, i.e., it is not light corrosion as provided for in Corrosion Level 1, definition 3.
6.40.3 Corrosion Level 3: Corrosion occurring during the first or subsequent accomplishments of a corrosion inspection task that the operator determines to be an urgent airworthiness concern.
6-20 Figure 6-8. Severe Corrosion Requiring Trimming Out of Damaged Area
Fi gure 6-9. Corrosion at Fastener Holes Removed by Spot Facing
6.41 R emoval of Corrosion. Two basic methods of corrosion removal include mechanical and chemical removal. The method used depends upon the type and location of a structure, the type and severity of corrosion, and the availability of maintenance equipment. Mechanical methods of corrosion removal are the most commonly used, and include sanding, buffing, grinding, and section removal. Avoid the use of soft metal wire
6-21 brushes (with copper alloys in the wires) because residual traces of copper on cleaned metal will contribute to future corrosion. If using brushes, they should be stainless steel or nonmetallic. Mechanical methods of corrosion removal are used on all three levels (light, moderate, and severe) of corrosion damage. However, use chemical methods of corrosion removal only with light corrosion and only in areas where the chemicals cannot migrate to other areas.
6.42 Determining Degree of Corrosion Damage. Use the following equipment to decide to measure corrosion damage: depth dial gauge, straight edge, or a molding compound.
Corrosion must be removed before the corroded area can be measured. Before making measurements, see if corrosion is in a reworked area which has previously been repaired.
If corrosion is in a recess of a faired or blended area, measure damage to include the material which has previously been reworked. The following method outlines the procedure for taking measurements with the depth gauge:
1. Remove loose corrosion products, if present.
2. Position depth gauge, as illustrated in Figure 6-10, Corrosion Damage and Rework Measurement Using Depth Dial Gauge, and determine the measurement reading.
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.
3. Take several additional depth readings.
4. Select the deepest reading as the depth of the corrosion damage.
6.43 Determining Rework Limits. The allowable damage limit chart of the manufacturer’s repair manual documents the maximum amount of material that can be removed from any damaged surface. If no criteria are given, contact the aircraft manufacturer for cleanup limits.
6.44 Determining Material Thickness Reduction After Corrosion Cleanup. Allowable material removal from a part or panel during corrosion cleanup is usually in the manufacturer’s allowable damage limit charts. An accurate measurement should be made of the material removed or material thickness remaining in the reworked area to prevent exceeding allowable limits.
6.44.1 Ultrasonic testers can measure panel thickness after rework. This method requires a qualified nondestructive inspection (NDI) operator and suitable test standards for calibration.
6.44.2 Measurement of the material removed, which is the depth of blended pits, can be made using a depth dial gauge (see Figure 6-10). If the depth dial gauge will not work, clay impressions, liquid rubber, or other similar means of getting accurate results may be used to determine material removed. In the event that material removal limits have been
exceeded, the area or part should be repaired or replaced. If replacement or repair criteria is not contained in the repair manual, contact the manufacturer.
Figure 6-10. Corrosion Damage and Rework Measurement Using Depth Dial Gauge
ADJUSTABLE ON IN CREMENTS: OF 0.000 DIAL FACE LOCK BASE POINT "A" „POINT "B"
CORROSION PIT POINT "C" CONICAL POINT NOTE
• DEPTH DIAL GAUGE 0 TO 0,500 INCH RANGE. DETAIL I
• TAKE MEASUREMENT READINGS PER-
PENDICULAR TO THE RADIUS OF CONCAVE OR CONVEX SURFACES. SEE DETAIL I. TAKE READINGS AT VARIOUS POINTS
• 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 DEEFEST READING TO BE POINT "C." DETERMINE DEPTH AS FOLLOWS:
DEPTH = C - (A + B)
6.45-6.49. RESERVED.
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6-23 SECTION 5. ALUMINUM AND ALUMINUM ALLOYS
6.50 Treatment. In general, corrosion of aluminum can be treated more effectively in place than by removing structural parts from the aircraft. 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 depending on whether the aluminum surfaces are bare in use or protected by paint coatings.
6.51 Processing of Aluminum Surfaces.
6.51.1 Bare Aluminum Surfaces. While few unpainted aircraft operate in maritime conditions, we include general information on the nature of Alclad surfaces and their treatment. Pure aluminum has greater corrosion resistance than the stronger aluminum alloys.
Manufacturers usually take advantage of this by laminating a thin sheet of relatively pure aluminum, 1 to 5 mils thick, over the base higher-strength aluminum alloy surface.
Corrosion protection is good, and operators can maintain the Alclad surface in a polished condition (see Figure 6-12, Polishing and Brightening Alclad Surfaces). In cleaning such surfaces, however, take care to prevent staining and marring of the exposed aluminum.
More importantly, from a protection standpoint, avoid unnecessary mechanical removal of the protective Alclad layer and exposing the more susceptible base material.
6.51.2 Additional Processing of Aluminum Surfaces Prior to Paint Finishes. Before applying paint to aluminum surfaces, more severe cleaning procedures and more thorough corrective treatments can be used before painting. Application of a paint finish requires proper prepaint treatment for good paint adhesion.
6.51.3 Special Treatment of Anodized Surfaces. Anodizing is the most common surface treatment of aluminum alloy surfaces. Manufactured or reworked parts or components undergo tank processing during and frequently before fabrication from sheet stock. The aluminum sheet or casting becomes a positive pole in an electrolytic bath, in which chromic acid or similar oxidizing agent produces a supplemental protective oxide film on the aluminum surface. 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, so avoid unnecessary destruction of the oxide film while processing anodized surfaces.
6.51.3.1 Do not use steel wool, steel wire brushes, copper alloy brushes, or severe abrasive materials on any aluminum surface. Aluminum wool, fiber bristle brushes, and mild abrasives are acceptable tools for cleaning anodized surfaces, but exercise care in any cleaning process to avoid unnecessary breaking of the protective film, particularly at the edges of the aluminum sheet. See Figures 6-11, Removing Oil and Surface Dirt, and 6-13, Cleaning and Stripping Paint.
6.51.3.2 Tampico fiber brushes are preferred and are adequate to remove most corrosion. Producing a buffed or wire brush finish by any means should be prohibited. Take every precaution to maintain as much of the protective
6-24 coating as practicable. Otherwise, treat anodized surfaces in the same manner as other aluminum finishes. Vacuum blasting is an acceptable corrosion removal method to remove surface corrosion. Vacuum blasting should not be used to remove intergranular corrosion.
Figure 6-11. Removing Oil and Surface Dirt
Fig ure 6-12. Polishing and Brightening Alclad Surfaces