Quantcast Clad Aluminum Surfaces

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205.200 Figure 4-26.—Hinge corrosion points. structures in avionic equipment, and the skin of the aircraft. Because of its wide use, you must be able to recognize and take the proper corrective action whenever corrosion is detected or suspected. Aluminum and its alloys are subject to a wide range of corrosive attack, varying from general etching of the surfaces to penetrating attacks along the internal grain boundaries of the metal. The corrosion products (fig. 4-27) appear as white-to-gray powdery deposits that have greater volume than the original metal. In its early stages, aluminum corrosion is evident as a general etching, pitting, or roughness of the surface. The surface attack progresses quite slowly at first; however, the attack will accelerate if the corroding material is not given immediate attention. Paint coatings mask evidence of corrosion, but because the corrosion products have a greater volume, corrosion will show up as blisters, flakes, chips, lumps, or other irregularities in the paint coating. Often, white or gray streaks of corrosion products become readily apparent at breaks in the paint film. Maintenance personnel should investigate such signs further to determine the extent that corrosion has progressed. There are three types of aluminum surfaces insofar as corrosion removal is concerned. They are clad, anodized, and exfoliated aluminum surfaces. Clad Aluminum Surfaces. Pure aluminum has considerable corrosion resistance compared to aluminum alloys. but it has little or no structural strength. An extremely thin sheet of pure aluminum laminated onto each side of an aluminum alloy sheet improves the corrosion resistance with little impairment of strength. The trade name of this aluminum laminate, as originated by the Aluminum Company of America, is Alcad. From this trade name the adjective clad and the verb cladding have been derived. An example of clad aluminum is the surface of unpainted aircraft. Not all aircraft sheet aluminum is clad, especially those alloy sheets from which small brackets, gussets, and fittings are made. The pure aluminum is very soft, and fabrication processes would severely damage or destroy the clad surfaces. To remove corrosion from clad surfaces, the corroded areas should be hand polished with MIL-P-6888 metal polish. It effectively removes stains and produces a high-gloss, lasting polish on unpainted clad surfaces. During the polishing operation, you should take care to avoid mechanical 4-36



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