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9/8/98 AC 43.13-1B SECTION9. MAGNESIUMANDMAGNESIUMALLOYS 6-148. GENERAL. Magnesium and mag nesium alloys are the most chemically active of the metals used in aircraft construction and are the most difficult to protect. However, cor rosion on magnesium surfaces is probably the easiest to detect in its early stages. Since mag nesium corrosion products occupy several times the volume of the original magnesium metal destroyed, initial signs show a lifting of the paint films and white spots on the magne sium surface. These rapidly develop into snow-like mounds or even white whiskers. The prompt and complete correction of the coating failure is imperative if serious struc tural damage is to be avoided. 6-149. TREATMENT OF WROUGHT MAGNESIUM SHEETS AND FORGINGS. Corrosive attack on magnesium skins will usu ally occur around the edges of skin panels, un derneath hold-down washers, or in areas physically damaged by shearing, drilling, abra sion, or impact. Entrapment of moisture under and behind skin crevices is frequently a con tributing factor. If the skin section can be eas ily removed, this should be accomplished to ensure complete inhibition and treatment. a. Complete mechanical removal of cor rosion products should be practiced when practical. Mechanical cleaning should nor mally be limited to the use of stiff bristle brushes and similar nonmetallic cleaning tools. b. Any entrapment of steel particles from steel wire brushes, steel tools, or con tamination of treated surfaces, or dirty abra sives, can cause more trouble than the initial corrosive attack. The following procedural summary is recommended for treatment of cor roded magnesium areas when accomplished under most field conditions. c. When aluminum insulating washers are used and they no longer fasten tightly to magnesium panels, corrosion is likely to occur under the washers if corrective measures are not taken. (1) When machine screw fasteners are used, aluminum insulating washers must be removed from all locations to surface treat the magnesium panel. (2) Where permanent fasteners other than machine screws are used, the insulating washer and fastener must be removed. (3) When located so water can be trapped in the counter-bored area where the washer was located, use sealants to fill the counterbore. If necessary, fill several areas adjacent to each other. It may be advantageous to cover the entire row of fasteners with a strip of sealant. 6-150. REPAIR OF MAGNESIUM SHEET METAL AFTER EXTENSIVE CORROSION REMOVAL. The same gen eral instructions apply when making repairs in magnesium as in aluminum alloy skin, except that two coats of epoxy primer may be re quired on both the doubler and skin being patched instead of one. Where it is difficult to form magnesium alloys in the contour, alumi num alloy may be utilized. When this is done, it is necessary to ensure effective dissimilar metal insulation. Vinyl tape will ensure posi tive separation of dissimilar metals, but edges will still have to be sealed to prevent entrance of moisture between mating surfaces at all points where repairs are made. It is recom mended that only non-corrosive type sealant be used, since it serves a dual purpose of material separation and sealing. Par 6-148 Page 6-29PDF Image | AFS-640
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