Figure 5-4 shows dished and popped rivets; Figure 5-5 shows skin bulges or lifted surfaces; Figures 5-6 and 5-7 show cracks; and Figures 5-8 and 5-9 show corrosion products.
(1) The indications of corrosive attack can take several forms depending on the type of metal and the length of time the corrosion has had to develop. Corrosion deposits on aluminum and magnesium are generally a white powder, while ferrous metals vary from red to dark reddish brown stains.
(2) Sometimes the inspection areas are obscured by structural members, equipment installations, or for some other reason are awkward to check visually. Adequate access for inspection must be obtained by removing access panels and adjacent equipment, cleaning the area as necessary, and removing loose or cracked sealants and paints. Mirrors, borescopes, and fiber optics are useful in providing the means of observing obscure areas. Figures 5-10 through 5-18 depict some of these conditions.
FIGURE 5-2. CORROSION UNDER CHIPPED AND LOOSE PAINT ON WING SKIN Chap 5 94 Par 504
FIGURE 5-3. CORROSION INDICATED BY BLISTERING OF PAINT IN FUEL CELL FIGURE 5-4. POPPED RIVET HEADS RESULTING FROM CORROSION PRODUCTS Chap 5 Par 504 95
FIGURE 5-5. SKIN BULGING AROUND FASTENERS CAUSED BY PRESSURE FROM CORROSION
PRODUCTS FIGURE 5-6. CORROSION CRACKING BETWEEN FASTENERS ON A WING SPAR Chap 5 96 Par 504
FIGURE 5-7. FAYING SURFACE CORROSION ON WING SPAR CHORD WITH CORROSION
CRACKING ALSO VISIBLE ST 2 LH 1060 FIGURE 5-8. SEVERE CROWN STRINGER CORROSION Chap 5 Par 504 97
FIGURE 5-9. SPAR CHORD CORROSION COeR FIGURE 5-10. REMOVAL OF FILLET SEAL FROM INTERNAL EDGE OF LAP JOINT EXPOSED
FULL EXTENT OF CORROSION Chap 5 98 Par 504
FIGURE 5-11. REMOVAL OF INSULATION BLANKETS EXPOSED CORROSION FIGURE 5-12. CORROSION DEVELOPED UNDER LEVELING COMPOUND INSTALLED DURING
PREVIOUS REPAIR OF OVERWING EXIT Chap 5 Par 504 99
FIGURE 5-13. CORROSION BEHIND A STRUCTURAL COMPONENT ON A WING SPAR wIL 59.24 outba SEr. 12 - 18 FIGURE 5-14. CORROSION DAMAGE AROUND FASTENER HOLES AND IN FASTENER HOLES Chap 5 100 Par 504
FIGURE 5-15. SEVERELY CORRODED LAP JOINT FIGURE 5-16. LOCAL PAINT REMOVAL REQUIRED TO EXPOSE FULL EXTENT OF
CORROSION Chap 5 Par 504 101
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