disassembly would be required to gain access to hidden structure or components to accomplish visual inspections. They are also used to measure or estimate material loss due to corrosion and to verify the effectiveness of corrosion removal. Inspection of aircraft for corrosion is a continuing requirement and should be done on a daily basis.
Corrosion. Corrosion is the electrochemical deterioration of a metal because of its reaction with the surrounding environment. While new and better materials are continuously being developed, this progress is offset, in part, by a more aggressive operational environment. Corrosion is a complex phenomenon which can take many different forms and the resistance of aircraft materials to corrosion can drastically change with only a small environmental change.
b. Catastrophic Corrosion Events. Corrosion is most often thought of as a slow process of material deterioration, taking place over a significant period of time (examples being general corrosion, pitting, exfoliation, etc.). Other forms of corrosion degradation can occur very quickly, in days or even hours, wit!
catastrophic results. These torms such as stress corrosion cracking, environmental embrittlement, and corrosion fatigue) depend on both the chemical and mechanical aspects of the environment and can cause catastrophic structural failure without warn-
metals; however, the use of corrosion resistant metals and the application of protective coatings minimize aircraft corrosion.
Many other factors contribute to the amount and degree of aircraft corrosion such as the operational environment, materials used in the original design, and the amount of operational preservation provided during maintenance and repair.
d. Aircraft Environment. Aircraft exposed to salt air, heavy industrial pollution, and/or over water operations will experience more corrosion problems than aircrait operated in a ary environment. Corrosion is caused by the presence of salts in moist air or by some other abatement to corrosion present in the chemical content of the water or elements in the metal.
e. Manufacturers' Handbooks. Manufacturers' handbooks should be used as a general guide when an area is to be inspected for corrosion. Some manufacturers have developed videos explaining the process of corrosion and separate corrosion control manuals that are the basis for a complete aircraft corrosion inspection and program. On aircraft for which the manufacturer has not published a recommended corrosion inspection schedule and treatment program, the recommendations of Advisory Circular (AC) 43-4A, Corrosion Control for Aircraft, should be followed.
Chap 1 Par 12 Page 25
f. Uniform Etch Corrosion. Uniform etch corrosion results from a relatively uniform chemical attack on a metal surface (see Figure 1-19). On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if the attack is allowed to continue, the surface becomes rough and possibly frosted in appearance.
FIGURE 1-19. UNIFORM ETCH CORROSION (Courtesy of Thomas Flournoy) Chap 1 Page 26 Par 12
g. Pitting Corrosion. The most common form of corrosion on aluminum and magnesium alloys is pitting (see Figure 1-ZV) • it is first noticeable as a white or gray powdery deposit, which, when removed reveals uneven corrosive attack which is revealed as pits or holes in the surface.
FIGURE 1-20. PITTING CORROSION Galvanic Corrosion. Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte (see Figure 1-21). The electromotive force (EMF) ries lists metals in the order of their ability to corrode as follows.
1. Magnesium Alloys 12. Copper
2. Zinc (plate) 13. Monel 3 Beryllium 14. Stainless Steels Cadmium (plate) 15. Titanium Alloys
5. Aluminum Alloys* 16. Silver
6. Tin (plate) 17. Palladium 7 . Copper (plate) 18. Gold 8 • Nickel (plate) 19. Rhodium
9. Chromium (plate) 20. Platinum
10. Brass (yellow or naval) 21. Carbon/Graphite
11. Bronze 2024 (copper alloye) used mostly in fuselages and the 7000 lzinc alloy) series used mostly in flying and control surfaces have about the same corrosion potential. The exceptions are rolled, extruded, and forged aluminum (anisotropic grain structure) which have been heat treated of cold worked. These alloys are susceptible to exfoliation corrosion, Chap 1 Par 12 Page 27
FIGURE 1-21. GALVANIC CORROSION OF MAGNESIUM ADJACENT TO STEEL FASTENER (Courtesy of Thomas Flournoy) Chap 1 Page 28 Par 12
8/14/97 AC 43-204 The rate and degree of corrosion depends on the relative surface areas of the two metals and their location on the electromotive force series. Adjacent metals in the EMF series have low corrosion susceptibility whereas widely separated metals will corrode rapidly. For example, an aluminum fastener in contact with a relatively inert Monel (nickel, copper, steel) structure may corrode severely, while a Monel bracket secured to a large aluminum member would result in a relatively superficial attack on the aluminum sheet.
i. Concentration Cell or Crevice Corrosion. Concentration cell corrosion is corrosion of metals in a metal-to-metal joint;
corrosion at the edge of a joint, even through joined metals are identical; or corrosion of a spot on the metal surface covered by a foreign material (see Figure 1-22 for the three types of crevice corrosion).
LOW METAL ION CONCENTRATION HIGH OXYGEN CONCENTRATION
- HIGH METAL ION CONCENTRATION LOW OXYGEN CONCENTRATION
METAL ION CONCENTRATION CELL OXYGEN CONCENTRATION CELL
RIVETED LAP JOINT
• FOREIGN MATERIAL CREATES
PASSIVE FILM LOW OXYGEN REGION WHICH
PROTECTS EXPOSED PREVENTS THE RE-FORMATION
SURFACE OF PASSIVE FILM ACTIVE METAL
ACTIVE - PASSIVE CELL FIGURE 1-22. CONCENTRATION CELL CORROSION J. Intergranular Corrosion. Intergranular corrosion is an attack along the grain boundaries of a material. Rapid selective corrosion at the grain boundary can occur with subsequent delamination (see Figure 1-23). High-strength aluminum alloys such as 2014 and 7075 are more susceptible to intergranular corrosion if they have been improperly heat-treated and are then exposed to a corrosive environment.
Chap 1 Par 12 Page 29
FIGURE 1-23. INTERGRANULAR CRACKING AND CORROSION ON
A WINGSPAR CHORD Exfoliation Corrosion. Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the grain boundaries just below the sur- The lifting up or swelling is visible evidence of exfoliation corrosion (see Figure 1-24). Exfoliation is most prone to occur in rolled and wrought products such as extrusions, thick sheet, thin plate, and certain die-forged shapes. This is in contrast with cast products that tend to have a more homogeneous grain structure.
Chap 1 Page 30 Par 12
FIGURE 1-24. SEVERE EXFOLIATION CORROSION (Courtesy of Thomas Flournoy)
13.-199. RESERVED .
Chap 1 Par 12 Pages 31 through 42
CHAPTER 2. VISUAL INSPECTION PROCEDURES
200. GENERAL Generally, the inspection procedure to be used will be specified by the aircraft or component manufacturers or the FAA in documents such as maintenance or overhaul manuals, AD's, SSID's, or manufacturers' Service Bulletins. If, during maintenance, a condition is found on the aircraft being inspected which may recur on that aircraft or on another of its type, a certified inspector or other appropriate individual should create an inspection procedure covering the condition. Approval of such procedures should be in accordance with appropriate FARS.
201. MATERIAL CONTAINED IN A VISUAL INSPECTION PROCEDURE. Visual inspection procedures should all contain steps in a logical for- A procedure has four basic parts: (1) a basis for the inspection, (2) preparatory arrangements, (3) implementation of the inspection, and (4) evaluation of the results. Table 2-1 is a listing of all the elements to be considered when creating or reviewing an inspection procedure. Each of the items in the Table should be addressed in the procedure if relevant for the specific inspection.
Chap 2 Par 200 Page 43
AC 43 - 204 8/14/97 TABLE 2-1. TOPICS TO BE CONSIDERED IN PREPARING OR
REVIEWING A VISUAL INSPECTION PROCEDURE
BASIS FOR INSPECTION Reason for and purpose of the inspection including any relevant background information.
References to controlling documentation, for example: Advisory Circulars, Airworthiness Directives, Manufacturers and general NDT Manuals, Maintenance and other relevant manuals.
controlling dates: Effective date of the inspection, date before which the inspection should be accomplished, inspection interval, date at which the inspection no longer is necessary.
Effectiveness; aircraft, engine, or part number.
Time and labor required for the inspection.
Conditions under which the inspection is no longer necessary.
PREPARATION Preparation and cleaning of the item being inspected.
Any special requirements of the inspection such as unusual surface preparation (etching, paint removal) or the temperature range over which the inspection is effective.
Identification of any equipment necessary for the inspection together with provisions for periodic calibration thereof.
Identification of the specific materials approved for the inspection. These may be included in a referenced Qualified Products List.
IMPLEMENTATION specific description of the area to be inspected.
Instructions for carrying out the inspection.
Description of the defect to be detected, preferably with an illustration of a sample defect.
Postcleaning instructions if required.
Instructions for disposition of the article if it cannot be returned to service.
EVALUATION Procedure for establishing inspection reliability.
Accept/reject criteria.
Reporting requirements.
202. ACTIVITIES APPLYING TO ALL VISUAL INSPECTIONS. Some activities which represent good practice may not be specifically called out in the procedures, but should be completed if they are relevant. Examples of such good practice are:
Chap Page 44 Par 201
8/14/97 AU 43-204
a. Preliminary Inspection. A preliminary inspection of the overall general area should be performed for cleanliness, presence of foreign objects, derormed or missing fasteners, security of parts, corrosion, and damage. If the configuration or location of the part conceals the area to be inspected, it is appropriate to use visual aids such as mirror or borescope.
b. Precleaning. The areas or surface of parts to be inspected should be cleaned without damaging any surface treatment which may be present. Contaminants that might hinder the discovery of existing surface indications should be removed.
cleaning methods may remove indications of damage; care should be used if the cleaning tends to smear or hide possible indications of trouble. Surface coatings may have to be removed at a later time if other NDI techniques are required to verify any indications that are found. Some typical cleaning materials and methods used to prepare parts for visual inspection are detergent cleaners, alkaline cleaners, vapor degreasing, solvent cleaners, mechanical cleaning, paint removers, steam cleaning, and ultrasonic cleaning.
Corrosion Treatment. Any corrosion found in the preliminary inspection should be removed before starting a close visual inspection of any selected part or area. Manufacturers' handbooks, when available, are a good general guide for treatment of corrosion. Recommendations of AC 43-4A, if appropriate, should be used on aircraft for which the manufacturer has not published a recommended corrosion inspection schedule or treatment program.
The AC contains a summary of current available data regarding identification and treatment of corrosion on aircraft structure and engine materials. Examples of types of corrosion damage detectable by the visual method are also given.
d. Use of Visual Aids. When inspecting the area required, visual aids should be used as necessary. An inspector normally should have available suitable measuring devices: a flashlight and a mirror. Chapter 3 provides information on lighting techniques that can be used when inspecting for defects in different types of materials.
RECORD KEEPING. All defects found should be documented by Maintenance Record entry, written report, squawk sheet, photograph, or video recording for appropriate evaluation. Depending on the rules governing the facility and person performing the inspection, the report may be limited to simpiy reporting findings without any rejection or acceptance disposition. The type, location, and approximate size of any defects present should be documented. Based upon the particular inspection process specification (i.e., AD, Service Bulletin, Maintenance Manual requirement, or normal inspection discovery) the mechanic, repairman, or other authorized person should determine the particular acceptance, re- Chap 2 Par 202 Page 45
work, repair or rejection status of the part or structure being inspected.
a. Findings. One of the most important elements of the visual inspection process is the nature of the report of findings.
As a guide, the record of findings for any defects should be kept in a manner as to permit others who may be doing the same inspection to have the benefit of any previous information found and experience gained.
b. Maintenance of Records. The full value of visual inspection can be realized only if records are kept of the conditions found on parts inspected. The size and shape of the finding and its location should be recorded along with other pertinent information, such as rework performed or disposition. The inclusion of some permanent record of a defect on a report makes the report much more complete.
c. Illustrations as Records. It should be stressed that frequent and effective use of illustrations not only will enhance the effectiveness of the procedure but is indispensable in communicating to the inspector the nature of the defects to be found.
In addition to textual and anecdotal data, the following types of
(1) Sketches. The simplest record is a sketch of the part showing the location and extent of the defect. On large areas it may be sufficient to sketch only the critical area.
(2) Photography. Photographs (still or video recording) of defects can be taken for visual record purposes. Photographs produce a permanent and highly descriptive record since they show both size and location on the part. They are permanent, reproducible, and the required equipment is readily available. It is good practice to include a scale in the photo when practicable as well as some marking for identification. This is particularly necessary if the photograph is likely to become an exhibit involved in litigation.
chap 2 Page 46 Par 203