FIGURE 5-1. CORROSION FOUND AFTER REMOVING CARGO DOOR THRESHOLD COVERS PRIMARY APPROACH. The primary approach to corrosion detection is corrosion inspections on a regularly scheduled basis. Early detection and treatment reduce costs, out of service time, and the possibility of flight or flight related incidents. All corrosion inspections should start with a thorough cleaning of the area to be inspected. A general visual inspection of the area follows using a flashlight, inspection mirror, and a 5-10x magnifying glass. The general inspection should look for obvious defects and suspected areas. A detailed inspection of damage or suspected areas found during the general inspection follows. The detailed inspection can be one or more of the following.
504. NONDESTRUCTIVE INSPECTION (NDI).
a. Visual Inspection. Visual inspection is the most widely used technique and is an effective method for the detection and evaluation of corrosion. Visual inspection employs the eyes to look directly at an aircraft surface, or at a low angle of incidence to detect corrosion. Using the sense of touch of the hand is also an effective inspection method for the detection of hidden well developed corrosion. Other tools used during the visual inspection are mirrors, borescopes, optical micrometers, and depth gauges. The following shows the type of corrosion damage detectable using the visual inspection method: Figures 5-2 and 5-3 show chipped, missing, and lifted paint;
Chap 5 Par 502 93
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