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AC 43.13-1B 9/8/98 (b) When mechanically removing corrosion from aluminum, especially aircraft skin thinner than 0.0625 inch, extreme care must be used. Vigorous, heavy, continuous abrasive grinding can generate enough heat to cause metallurgical change. If heat damage is suspected, hardness tests or conductivity tests must be accomplished to verify condition of the metal. The use of powered rotary files should be limited to heavy corrosion and should not be used on skin thinner than 0.0625 inch. (2) Blasting. (a) Abrasive blasting may be used on aluminum alloys using glass beads (Spec. MIL-G-9954) sizes 10 to 13, or grain abrasive (Spec. MIL-G-5634) types I and III may be used as an alternate method of removing cor rosion from clad and non-clad aluminum al loys. Abrasive blasting should not be used to remove heavy corrosion products. Direct pres sure machines should have the nozzle pressure set at 30 to 40 psi for clad aluminum alloys and 40 to 45 psi for non-clad aluminum alloys. Engineering approval should be obtained prior to abrasive blasting metal thinner than 0.0625 inch. (b) When using abrasive blasting on aluminum alloys, do not allow the blast stream to dwell on the same spot longer than 15 seconds. Longer dwell times will cause ex cessive metal removal. Intergranular exfolia tion corrosion is not to be removed by abrasive blasting; however, blasting may be used with powered corrosion removal to determine whether all exfoliation corrosion has been re moved. f. Inspect the area for remaining corro sion. Repeat procedure if any corrosion re mains. NOTE: If corrosion remains after the second attempt, use a stronger method, e.g., chemical to mechanical. g. Using a blend ratio of 20:1 (length to depth) blend and finish the corrosion rework area with progressively finer abrasive paper until 400-grit paper is used. h. Clean reworked area using dry cleaning solvent. Do not use kerosene or any other pe troleum base fuel as a cleaning solvent. i. Determine depth of faired depressions to ensure that rework limits have not been ex ceeded. j. Apply chemical conversion coating, MIL-C-81706, immediately after reworking. If 48 hours or more have elapsed since the con version coating was first applied and the primer or final paint system has not yet been applied, then reapply the conversion coating beforecontinuing. NOTE: These solutions should not be allowed to come in contact with mag- nesium or high-strength steels (180,000 psi). Do not permit solutions or materials to contact paint thinner, acetone or other combustible material: FIRE MAY RESULT. k. Apply paint finish to area. 6-137.6-147. [RESERVED.] Page 6-28 Par 6-136PDF Image | AFS-640
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