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AC 43.13-1B 9/8/98 blasting with glass beads, (Spec. MIL-G-9954) sizes 10-13; or grain abrasive (Spec. MIL-G-5634), types I or III at an air pressure of 10 to 35 psi (if suction equipment is used, use a 50 percent higher pressure). Upon com pletion of blasting, inspect for the presence of corrosion in the blast area. Give particular at tention to areas where pitting has progressed into intergranular attack. This is necessary be cause abrasive blasting has a tendency to close up streaks of intergranular corrosion rather than remove them if the operator uses an im proper impingement angle. If the corrosion has not been removed in a total blasting time of 60 seconds on any one specific area, other mechanical methods of removal should be utilized. CAUTION: When blasting magne- sium alloys, do not allow the blast stream to dwell on the same spot longer than 15 seconds. Longer dwell times will cause excessive metal re- moval. k. Apply Chromic Acid Brush-on Pre treatment. (1) Chemical pretreatment such as the following chromic acid solution (Conversion coat conforming to Spec. MIL-M-3171, type VI) provides a passive surface layer with an inhibitive characteristic that resists corro sive attack and also provides a bond for subse quent coatings. Properly-applied magnesium pretreatment tend to neutralize corrosion me dia in contact with the surface. (2) The chromic acid brush-on pre treatment may be applied to all magnesium parts that require touch-up. This treatment is generally used in refinishing procedures or when parts and assemblies are too large to be immersed. This treatment is less critical to ap ply than the other brush-on treatments. It is relatively inexpensive and not as harmful when trapped in faying surfaces. eration: g. Inspect the reworked area to ensure that no corrosion products remain. If corrosion products are found, repeat method used and re-inspect. h. Fair depressions resulting from rework using a blend ratio of 20:1. Clean rework area using 240 grit abrasive paper. Smooth with 300 grit and finally polish with 400 grit abra sive paper. i. Determine depth of faired depressions to ensure that rework limits have not been ex ceeded. Refer to the manufacture’s specifica tions. j. Clean reworked area using a solvent to provide a water-break-free surface. Do not use kerosene or another petroleum base fuel as a cleaning solvent. Page 6-32 Par 6-152 (a) Solution Composition and Op 1 Distilled Water 1 gal. 2 Chromic Acid (CrO3) 1.3 oz. 3 (99.5 pure), Calcium Sulfate 1 oz. (CaSo4.2H2O) 4 Operating Temp. 70-90 °F. 5 Container: Stainless Steel, Alu minum, Vinyl, Polyethylene, or Rubber. NOTE: Good application requires proper preparation of the chromic acid coating solution and cleaning of the surface where the solution will be applied. A water-break test is recom- mended if the cleanliness of the sur- face is in doubt.PDF Image | AFS-640
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