FIGURE 5-17. CORRODED AREA AFTER PAINT REMOVAL FROM WING SKIN FIGURE 5-18. ALODINE 1200 TREATMENT PRIOR TO PAINTING MAY HIGHLIGHT
PRESENCE OF REMAINING CORROSION OR CRACKS Chap 5 102 Par 504
Other Methods. In addition to visual inspection there are several NDI methods such as: liquid penetrant, magnetic particle, eddy current, x-ray, ultrasonic, and acoustical emission which may be of value in the detection of corrosion. These methods have limitations and should be performed only by qualified and certified NDI personnel. Eddy current, x-ray, and ultrasonic inspection methods require properly calibrated (each time used) equipment and a controlling reference standard to obtain reliable results.
(1) Liquid Dye Penetrant. Inspection for large stress-corrosion or corrosion fatigue cracks on nonporous ferrous or nonferrous metals may be accomplished by the use of liquid dye penetrant processes. The dye applied to a clean metallic surface will enter small openings, such as cracks or fissures by capillary action. After the dye has had an opportunity to be absorbed by any surface discontinuities, the excess dye is removed and a developer is applied to the surface. The developer acts like a blotter and draws the dye from the cracks or fissures back to the surface of the part, giving visible indication of the location of any fault that is present on the surface. The magnitude of the fault is indicated by the quantity and rate of dye brought back to the surface by the developer.
(2) Magnetic Particle Inspection. This method may be used for the detection of cracks or flaws on or near the surface of ferromagnetic metals (metals which are attracted by magnetism). A portion of the metal is magnetized, and finely divided magnetic particles (either in liquid suspension or dry) are applied to the object. Any surface faults will create discontinuities in the magnetic field and cause the particles to congregate on or above these imperfections, thus locating them.
(3) Eddy Current Inspection. Eddy current testing (primarily lowfrequency) can be used to detect thinning due to corrosion and cracks in multilayered structures. Low frequency eddy current testing can also be used to some degree for detecting or estimating corrosion on the hidden side of aircraft skins because, when used with a reference standard, the thickness of material which has not corroded can be measured. Low frequency eddy current testing can be used for estimating corrosion in underlying structure because the eddy currents will penetrate through into the second layer of material with sufficient sensitivity for approximate results. High-frequency eddy current testing is most appropriate for detection of cracks which penetrate the surface of the structure on which the eddy current probe can be applied (including flat surfaces and holes).
(4) X-Ray Inspection. X-ray inspection has somewhat limited use for the detection of corrosion because it is difficult to obtain the sensitivity requireo to detect minor or moderate corrosion. brietly, x-ray works by passing nigh energy rays generated by an x-ray machine through the material being inspected. This exposes the special film placed on the opposite side of the material. Areas of high density are indicated on the film as underexposed areas, while areas of low density are indicated on the film as overexposed Proper interpretation of the film will indicate whether defects are present. Moderate to servere corrosion or cracks can be detected using x-ray Chap 5 Par 504 103
inspection. However this method, like other NDI methods, requires a qualified and certified operator to obtain reliable results.
(5) Ultrasonic Inspection. Ultrasonic testing provides a sensitive detection capability for corrosion damage when access is available to a surface with a continuous bulk of material exposed to the corrosion. Ultrasonic inspection is commonly used to detect exfoliation, stress-corrosion cracks, and general material thinning. Ultrasonic digital thickness gages are not reliable for determining moderate or severe damage prior to removing the corrosion.
Highly trained personnel should conduct the examination if any useful
(6) Acoustic Emission Testing. This method using heat-generated emissions can be used to detect corrosion and moisture in adhesive-bonded metal honeycomb structures. Acoustic emission testing can detect corrosion initiation as well as advanced corrosion.
505.-599. RESERVED.
Chap 5 104 through 110 Par 504
CHAPTER 6. CORROSION REMOVAL TECHNIQUES SECTION 1. SAFETY PROCEDURES.
General safety precautions, outlined in the following paragraphs, contain guidelines for handling materials with hazardous physical properties and emergency procedures for immediate treatment of personnel who have inadvertently come into contact with one of the harmful materials.
Materials having hazardous physical properties are referenced to the pertinent safety precautions and emergency safety procedures. All personnel responsible for using or handling hazardous materials should be thoroughly familiar with the information in the following paragraphs.
a. Safety precautions. When required to use or handle any of the solvents, special cleaners, paint strippers (strong alkalies and acids), etchants (corrosion removers containing acids), or surface activation material (alodine 1200), observe the following safety precautions:
Avoid prolonged breathing of solvents' or acids' vapors.
Solvents and acids should not be used in confined spaces without adequate ventilation or approved respiratory protection;
(2) Never add water to acid. Always add acid to water;
(3) Mix all chemicals per the manufacturers' instructions;
(4) Clean water for emergency use should be available in the immediate work area before starting work;
(5) Avoid prolonged or repeated contact of solvents, cleaners, etchants (acid), or conversion coating material (alodine solution) with skin.
Rubber or plastic gloves should be worn when using solvents, cleaners, paint strippers, etchants, or conversion coating materials. Goggles or plastic face shields and suitable protective clothing should be worn when cleaning, stripping, etching, or conversion coating overhead surfaces;
(6) When mixing alkalies with water or other substance, use containers which are made to withstand heat generated by this process;
(7) Wash any paint stripper, etchant, or conversion coating material immediately from body, skin, or clothing;
(8) Materials splashed in the eyes should be promptly flushed out with water, and medical aid obtained for the injured person;
(9) Do not eat or keep food in areas where it may absorb poisons.
Always wash hands before eating or smoking;
Chap 6 Par 600 111
(10) Verify that the area within 50 ft. of any cleaning or treating operations where low flash point (140 °F or below) materials are being used is clear and remains clear of all potential ignition sources;
(11) Suitable fire extinguishing equipment should be available to the cleaning/treating area;
(12) Where any flammable materials are being used, equipment should be effectively grounded;
(13) If materials (acid, alkali, paint remover, or conversion coatings) are spilled on equipment and/or tools, treat immediately by rinsing with clean water, if possible, and/or neutralizing acids with baking soda and alkalies with a weak (5 percent) solution of acetic acid in water;
(14) Solvents with a low flash point (below 100 °F), such as methyl ethyl ketone (MEK) and acetone, should not be used in any confined locations;
(15) All equipment should be cleaned after work has been completed;
(16) Implement all company safety precautions;
(17) Check local environmental regulations for restrictions on the use of solvents, primers, and top coats; and
(18) Ensure disposal of waste material is per local environmental requirements.
b. Emergency safety procedures.
NOTE: PERSONNEL SHOULD BE THOROUGHLY FAMILIAR WITH THE FOLLOWING
EMERGENCY SAFETY PROCEDURES PRIOR TO USING ANY MATERIALS WHICH ARE REFERENCED TO AN EMERGENCY SAFETY PROCEDURE PARAGRAPH.
(1) If exposed to physical contact with any of the following materials:
Methy! alcohol Xylene Methyl ethyl ketone Petroleum naphthas Methyl isobutyl ketone Chromates Toluene Dichromates Trichloroethylene Acetates Epoxy resin Cyclohexanone Methylene chloride Cellosolve Brush alodine Carbon tetrachloride Treat as follows:
(i) If splashed into eyes, do not rub.
Chap 6 112 Par 600
(ii) Flush eyes immediately with water for at least 15 minutes. Lift upper and lower eyelids frequently to ensure complete washing.
(iii) If splashed on clothing or large areas of body, immediately remove contaminated clothing and wash body with plenty of soap and water. wash clothing before rewearing.
(ïv) If splashed onto an easily accessible part of the body, immediately wash with soap and water.
If suffering headache or other obvious symptoms resulting from overexposure, move to fresh air immediately.
If vapors are inhaled and breathing has slowed down or stopped, remove person from exposure and start artificial respiration at once.
Call ambulance and continue this treatment until ambulance arrives.
(2) If exposed to physical contact with any of the following materials:
Hydrofluoric acid Phenol Nitric acid Cresols Phosphoric acid Tricresyl phosphate Treat as follows:
If splashed into eyes, quickly wipe eyelids with a soft cleaning tissue and immediately flush eye with gentle stream from a drinking fountain, cup, or other convenient water outlet while holding lids open. Call an ambulance and continue the flushing procedure until an ambulance arrives.
(ii) If splashed onto an easily accessible part of body, immediately drench affected area with water until ambulance arrives.
(iii) If splashed onto clothing or large area of body, immediately drench body and remove clothing while drenching until ambulance arrives.
(iv) If taken internally, begin following treatment immediately:
(A) If the person is conscious, cause vomiting by placing finger in back of the person's throat. Encourage the person to drink large quantities of water and repeatedly wash out the mouth.
(B) If the person is unconscious, do not give any liquid. Start artificial respiration at once. Continue until ambulance arrives. If person regains consciousness before ambulance arrives, proceed as in subparagraph (A).
Chap 6 Par 600 113
601. GENERAL CORROSION CONTROL WORK PROCEDURES. The effectiveness of corrosion control depends on how well basic work procedures are followed. The following common work practices are recommended:
a. If rework procedures or materials are unknown, contact the aircraft manufacturer or authorized representative before proceeding;
o. Ihe work areas, equipment, and components should be clean and tree of chips, grit, dirt, and foreign materials.
c. Do not mark on any metal surface with a graphite pencil or any type of sharp, pointed instrument. Temporary markings (defined as markings soluble in water or methyl chloroform) should be used for metal layout work or marking on the aircraft to indicate corroded areas.
d. Graphite should not be used as a lubricant for any component.
Graphite is cathodic to all structural metals and will generate galvanic corrosion in the presence of moisture, especially if the graphite is applied in dry form.
Footwear and clothing should be inspected for metal chips, slivers, rivet cuttings, dirt, sand, etc., and all such material removed before walking or working on metal surfaces such as wings, stabilizers, fuel tanks, etc.
f. Do not abrade or scratch any surface unless it is an authorized procedure. If surfaces are accidentally scratched, the damage should be assessed and action taken to remove the scratch and treat the area.
g. Coated metal surfaces should not be polished for esthetic purposes.
Buffing would remove the protective coating and a brightly polished surface is normally not as corrosion resistant as a nonpolished surface unless it is protected by wax, paint, etc. A bare skin sheet polished to a mirror finish is more resistant than a bare mill finished sheet when both are given regular maintenance.
h. Protect surrounding areas when welding, grinding, drilling, etc., to prevent contaminating them with residue from these operations. In those areas where protective covering cannot be used, action should be taken to remove the residue by cleaning.
i. Severely corroded screws, bolts, and washers should be replaced.
When a protective coating, such as a cadmium plating on boits, screws, etc., is damaged, immediate action should be taken to apply an appropriate protective finish to prevent additional corrosion damage.
602.-609. RESERVED.
Chap 6 114 through 120 Par 601
1/25/91 AC 43-4A SECTION 2. CORROSION REMOVAL TECHNIQUES GENERAL. When active corrosion is visually apparent, a positive inspection and rework program is necessary to prevent any further deterioration of the structure. The following methods of assessing corrosion damage and procedures for rework of corroded areas could be used during cleanup programs.
in general, any rework would involve the cleaning and stripping of all finish from the corroded area, the removal of corrosion products, and restoration of surface protective film.
a. Repair of corrosion damage includes removal of all corrosion and corrosion products. When the corrosion damage exceeds the damage limits set by the aircraft manufacturer in the structural repair manual, the affected part must be replaced or an FAA-approved engineering authorization for continued service for that part must be obtained.
b. For corrosion damage on large structural parts which is in excess of that allowed in the structural repair manual and where replacement is not practical, contact the aircraft manufacturer for rework limits and procedures.
STANDARD METHODS. Several standard methods are available for corrosion removal. The methods normally used to remove corrosion are mechanical and chemical. Mechanical methods include hand sanding using abrasive mat, abrasive paper, or metal wool, and powered mechanical sanding, grinding, and buffing, using abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats.
is outlined in the following paragraphs. However, the method used depends upon the metal and the degree of corrosion. The removal method to use on each metal for each particular degree of corrosion
612. PREPARATIONS FOR REWORK. All corrosion products should be removed completely when corroded structures are reworked as the corroding process will continue even though the affected surface is refinished. Before starting rework of corroded areas, carry out the following:
rinsing of all surfaces. Position airplane in wash rack or provide washing apparatus for rapid
b. Connect a static ground line to the airplane.
c. Prepare the aircraft for safe ground maintenance.
and external hydraulic and electric power. (1) Remove the aircraft battery(s), LOX container (if installed),
(2) Install all applicable safety pins, flags, and jury struts.
d. Protect the pitot-static ports, louvers, airscoops, engine opening, chemical brightening agents. wheels, tires, magnesium skin panels, and airplane interior from moisture and
e. Protect the surfaces adjacent to rework areas from chemical paint strippers, corrosion removal agents, and surface treatment materials.
Спар 6 Par 610 121
7/25/91 AC 43-4A
613. PAINT REMOVAL.
using a medium grade abrasive mat. For larger areas, chemical paint removal is a. For small areas of metallic surfaces, paint may be removed by hand, the preferred method.
b. Phenolic and non-phenolic chemical paint removers containing methylene chloride are recommended for paint striping on metallic surfaces only.
Chemical paint strippers containing acid should not be used as hydrogen embrittlement on high strength steel and some stainless steels will occur. The following procedure is recommended when using chemical paint remover:
(1) Mask acrylic windows and canopies, other plastic parts, rubber noses, exposed wiring, composite surfaces, wheels and tires, and any other areas where the paint remover residue cannot be removed and refinishing cannot be accomplished;
(2) Remove sealants when required, by cutting away excess sealant with a sharp plastic scraper;
(3) Apply a thick, continuous coating of paint remover to cover the surface to be stripped;
(4) Allow paint remover to remain on the surface for a sufficient length of time to wrinkle and completely lift the paint. Reapply paint remover as necessary in the areas where paint remains tight or where the material has dried. Micarta scrapers, abrasive pads, or fiber brushes may be used to assist in removing persistent paint;
(5) Remove loosened paint and residual paint remover by washing and scrubbing the surface with fresh water and a stiff nylon bristle brush or an abrasive pad; and
(6) After thorough rinsing, remove masking materials and thoroughly clean the area with a solution of aircraft cleaning compound to remove paint remover residues.
c. For composite surfaces, paint removal should be done by mechanical removal techniques (scuff sanding) only. Composite surfaces include fiberglass, kevlar, carbon, graphite, and others. Due to the irregularities in composite surfaces (fiber weave), complete removal of the paint system can not be accomplished without surface fiber damage.
ine abre we used to prevent venoval of the base be required for removing corrosion from steel fasteners, side skins, or Chap 6 Par 613 122
7/25/91 AC 43-4A specified method for corrosion removal should be observed. irregularly shaped parts or surfaces. Whenever such special cases occur, the corrosion rework should be faired or blended with the surrounding surface. Fairing can be accomplished as follows: 615. FAIRING OR BLENDING REWORKED AREAS. All depressions resulting from
a. Remove rough edges and all corrosion from the damaged area. All TABLE 6-1.) dish-outs should be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. (SELECT THE PROPER ABRASIVE FOR FAIRING OPERATIONS FROM TABLE 6-1. ABRASIVES FOR CORROSION REMOVAL PROCESSED METALS OR MATERIALS TO BE RESTRICTIONS OPERATION ALUMINUM OXIDE ABRASIVE PAPER OR CLOTH CARBIDE SILICON GARNET ABRASIVE FABRIC OR PAD ALUMINUM STEEL STAINLESS PUMICE 350 MESH OR ABRASIVE WHEEL FINER ALLOYS FERROUS TO STEEL HEAT DOES NOT APPLY REMOVAL OR CORROSION 150 GRIT 180 GRIT FINE TO TREATÉD TO FAIRING FINER FINER ULTRAFINE
STRENGTHS TO 220,000 PSI AND FINISHING 400 ABOVE EXCEPT ALUMINUM ALLOYS SILICON CARBIDE ABRASIVE DO NOT USE FAIRING REMOVAL OR CORROSION FINER 150 GRIT OR 7/0 GRIT OR FINER ULTRAFINE VERY FINE AND CLAD ALUMINUM FINISHING 400 CLAD ALUMINUM SCRATCHES SANDING LIMITED TO THE REMOVAL OF MINOR FAIRING REMOVAL OR CORROSION 240 GRIT OR FINER 7/0 GRIT OR FINER ULTRAFINE VERY FINE AND × FINISHING 400 × ALLOYS MAGNESIUM CORROSION REMOVAL OR 240 GRIT VERY FINE AND
FAIRING FINER ULTRAFINE FINISHING 400 TITANIUM CLEANING 150 GRIT 180 GRIT
AND OR OR
FINISHING FINER FINER Chap 6 Par 614 123
7/25/91 AC 43-4A closely spaced multiple pits, intervening material should be removed to minimize catio of 20:1, length to depth (see Figures 6-1 and 6-2). b. Rework depressions by forming smoothly blended dish-outs, using a In areas having surface irregularity or waviness (see Figures 6-3 and 6-4). Steel nutplates and steel fasteners should be removed before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion (see Figure 6-5). All corrosion products must be removed during blending to prevent reoccurrence of corrosion (see Figures 6-6 and 6-7).
removal of corrosion products by any method should be blended out to prevent C. In critical and highly stressed areas, all pits remaining after stress risers which may cause stress-corrosion cracking. On noncritical removal.
PIT HAS BEEN CLEANED UP TO THE EXTENT THAT ALL LOOSE CORROSION PRODUCTS HAVE BEEN REMOVED.
CORROSION DAMAGE BEFORE REWORK
LONGITUDINAL ROUGH EDGES HAVE BEEN SMOOTHED AND ALL CORROSION HAS BEEN REMOVED. HOWEVER, DISH-OUT AFTER BLENDING
DEPRESSION HAS NOT BEEN SHAPED FIGURE 6-1. BLENDOUT OF CORROSION AS SINGLE DEPRESSION Chap 6 Par 615 124
LOCATION OF CORROSION PITS WIDTH OF CLEANED UP AREA (10 TIMES DEPTH MIN)
VIEW LOOKING DOWN
VIEW IN TRUE PERSPECTIVE
DEPTH OF CLEAN UP OF CORROSION
LENGTH OF CLEANED UP AREA (20 TIMES DEPTH MIN)
CROSS SECTIONAL VIEW
NOT ACCEPTABLE ACCEPTABLE FIGURE 6-2. TYPICAL EXAMPLE OF ACCEPTABLE CLEANUP OF CORROSION PITS
- TRANSVERSE - BEFORE REMOVAL CORROSION DAMAGE "MIN . 10D
- LONGITUDINAL •
BOTTOM OF DEPRESSION
AFTER CORROSION REMOVAL
DAMAGE REMOVED AND SURFACE SMOOTHED
WITH SHALLOW ELLIPTICAL DISH-OUT FIGURE 6-3. BLENDOUT OF MULTIPLE PITS IN CORRODED AREA Chap 6 Par 615 125