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Corrosion |
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Stress Corrosion Corrosion Fatigue Fretting Corrosion Heat Treatment |
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Corrosion of magnesium alloys.
Magnesium is the most chemically active of the metals used in the aircraft construction and is, therefore, the most difficult to protect. When a failure in the protective coating does occur, the prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided. Magnesium attack is probably the easiest type of corrosion to detect in its early stages since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed. The beginning of attack shows as a lifting of paint films and white spots on the magnesium surface. These rapidly develop into snow like mounds or even "white whiskers". Re-protection involves the removal of corrosion products, the partial restoration of surface coatings by chemical treatment, and re-application of protective coatings. Treatment of Wrought Magnesium Sheet and Forgings. Magnesium skin attack will usually occur around edges of skin panels, underneath hold down washers, or in areas physically damaged by shearing, drilling, abrasion or impact. If the skin section can be removed easily, this should be done to assure complete inhibition and treatment. If insulating washers are involved, screws should be loosened, at least sufficiently to permit brush treatment of magnesium under the insulating washer. Complete mechanical removal of corrosion products should be practiced insofar as procticable. Such mechanical cleaning should be limited to the use of stiff, hog-bristle brushes and similar nonmetallic cleaning tools, particularly if treatment is to be performed under field conditions. Any entrapment of steel particles from steelwire brushes or steel tools, or contamination of treated surfaces by dirty abrasives, can cause more trouble than the initial corrosive attack. Corroded magnesium may generally be treated as follow : (1) Clean and strip the paint from the area to be treated. ( Paint stripping procedures). (2) Using a stiff, hog-bristle brush, break loose and remove as much of the corrosion products as practicable. Steel wire brushes, caborundum abrasives, or steel cutting tools should not used. (3) Treat the corroded area liberally with a chromic acid solution, to which has been added sulphuric acid, and work into pits and crevices by brushing the area while still wet chromic acid, again using a nonmetallic brush. (4) Allow the chromic acid to remain in place for 5 to 20 minutes before wiping up the excess with a clean, damp cloth. Do not allow the excess solution to dry and remain on the surface, as paint lifting will be caused by such deposits. (5) As soon as the surfaces are dry, restore the original protective paint. Treatment of Installed Magnesium Castings. Magnesium castings, in general, are more pronous and more prone to penetrating attack than wrought magnesium skins. However, treatment is for all practical purposes, the same for all magnesium areas. Engine cases, bellcranks, fittings, numerous covers, plates, and handles are the most common magnesium coatings. When attack occurs on coating, the earliest practicable treatment is required if dangerous corrosive penetration is to be avoid. In fact, engine cases submerged in salt water overnight can be completely penetrated. If it is at all practicable, parting surfaces should be separated to effectively treat the existing attack and prevent it further progress. The same general treatment sequence in the preceding paragraph for magnesium skin should be followed. If extensive removal of corrosion products from a structural casting is involved, a decision from the manufacturer may be necessary to evaluate the adequacy of structural strength remaining specific structural repair manuals usually include dimentional tolerance limits for critical structural members and should be referred to if any question of safety is involved.
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| Corrosion of fereous metals. | ||
| One of the most familiar types of corrosion is ferrous oxide,.. | ||
| Corrosion of aluminum and aluminum alloys. | ||
| Corrosion attack on aluminum surfaces is usually quite obvious, | ||
| Corrosion of magnesium alloys. | ||
| Magnesium is the most chemically active of the metals used,. | ||
| Treatment of titanium and titanium alloys. | ||
| Attack on titanium surfaces is generally difficult to detect | ||
| Protection of dissimilar metal contacts. | ||
| Certain metals are subject to corrosion when placed in contact with other metals | ||
| Processes and materials used in corrosion control | ||
| Aircraft parts are almost always given some type surface finish | ||
| Chemical treatment. | ||
| Parco Lubrizing in a chemical treatment for iron and steel parts | ||
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Copyright @ 2007. |
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