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AC 43.13-1B 9/8/98 6-151. IN-PLACE TREATMENT OF MAGNESIUM CASTINGS. Magnesium castings, in general, are more porous and more prone to penetrating attack than wrought mag nesium skin. However, treatment in the field is, for all practical purposes, the same for all magnesium. Bellcranks, fittings, and numer ous covers, plates, and handles may also be magnesium castings. When attack occurs on a casting, the earliest practical treatment is re quired to prevent dangerous corrosive penetra tion. Engine cases in salt water can develop moth holes and complete penetration over night. a. If at all practical, faying surfaces in volved shall be separated to treat the existing attack effectively and prevent its further prog ress. The same general treatment sequence as detailed for magnesium skin should be fol lowed. Where engine cases are concerned, baked enamel overcoats are usually involved rather than other top coat finishes. A good air drying enamel can be used to restore protec tion. b. If extensive removal of corrosion products from a structural casting is involved, a decision from the aircraft manufacturer or a DER may be necessary to evaluate the ade quacy of structural strength remaining. Refer to the aircraft manufacturer if any questions of safety are involved. 6-152. EXAMPLE OF REMOVING CORROSION FROM MAGNESIUM. If possible, corroded magnesium parts shall be removed from aircraft. When impossible to remove the part, the following procedure will be used. a. Positively identify metal as magnesium. b. Clean area to be reworked. Page 6-30 c. Strip paint if required. d. Determine the extent of corrosion damage. e. Remove light to moderate corrosion by one of the following means. (1) Non-Powered Corrosion Removal. (a) Non-powered removal can be ac complished using abrasive mats, cloth, and pa per with aluminum oxide grit (do not use sili con carbide abrasive). Metallic wools and hand brushes compatible with magnesium such as stainless steel and aluminum, may be used. (b) When a brush is used the bristles should not exceed 0.010 inch in diameter. After using a brush, the surface should be pol ished with 400 grit aluminum oxide abrasive paper, then with 600 grit aluminum oxide abrasive paper. (c) Pumice powder may be used to remove stains or to remove corrosion on thin metal surfaces where minimum metal removal is allowed. (2) Chemical Corrosion Removal. (a) Chemical corrosion removal on magnesium alloys is usually done with a chromic acid pickle solution. Chemical corro sion removal methods are not considered ade quate for areas that have: 1 Deep pitting, 2 Heavy corrosion and corrosion by products, 3 Previously had corrosion re moved by mechanical means, or 4 Previously been sand blasted. Par 6-151PDF Image | AFS-640
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