surface corrosion. characterized by discolorization and pitting to a depth of approximately, 1-mil (0.001 inch) maximum. This type of damage can normally be removed by light hand sanding. Moderate corrosion. This looks like light corrosion except that there may be some blisters or evidence of scaling and flaking of the coating or paint system. The pitting depths may be as deep as 10 mils (0.010 inch). This type of damage is normally removed by extensive hand sanding or light mechanical sanding. Severe corrosion. This type of corrosion has a general appearance that may be similar to moderate corrosion with severe intergranular corrosion, blistering exfoliation, scaling, or flaking. The pitting depths are deeper than 10 mils (0.010 inch). This damage must be removed by extensive mechanical sanding and grinding. Repairable damage. When corrosion damage exceeds the limits of the applicable MIM or SRM, it is classified as repairable damage. The use of the affected part may be continued after repair at a cognizant field activity (CFA). Nonrepairable damage. When corrosion damage exceeds the established repair limits and requires replacement of the affected parts or special depot-level repair, it is classified as nonrepairable damage. MECHANICAL CORROSION REMOVAL The most effective mechanical methods of removing corrosion with the least removal of the metal are vapor blasting, soft-grit blasting, and dry, vacuum blasting. For use on assembled aircraft, a portable unit, such as the VACU-Blast Dry Honing Machine, is the most desirable. VACU-Blast Dry Honer The VACU-Blast Dry Honing Machine is a portable, air-operated, self-contained, lightweight unit that uses the dry vacuum return system. Dry honing is the only approved blasting method of removing corrosion on assembled aircraft. With this machine, the work is visible, and metal removal can be held to closer limits. The machine is air-operated, and can be used in shore-based or shipboard operations. The dry honing machine (fig. 4-28) is composed of the following principal components mounted on a two-wheel carriage assembly: A hose rack and storage compartment is provided on the front of the dry honing machine for storage of hoses, brushes, and accessories. The dry honing machine can cause damage to aircraft components and systems if used improperly. Small quantities of abrasives will escape from the blast nozzle during normal use; therefore, the equipment must not be used where the abrasives may contaminate systems or components. The following are precautions you should use when working with this machine: Do not use on engines, gearboxes, or other oil lubricating systems. Do not use on fuel, hydraulic, or oxygen system components. Mask all vent susceptible systems when blasting near them to prevent possible contamination. Use only on exterior surfaces or parts that have been removed from the airframe to prevent possible contamination of interior areas. Do not use on airframe skins or structural parts that are exposed to more than 500°F in service. Do not blast Metallite or honeycomb panels. Q69. What must you do before starting corrosion removal? Q70.  How should you remove moderate corrosion? Q71.  What is the most desirable method of mechanical corrosion removal? Abrasive Wheel An abrasive wheel can be used to remove severe corrosion (intergranular or exfoliation) on thick metal. The abrasive wheel is composed of nonwoven nylon, resin reinforced. The wheel is mounted on a mandrel assembly and driven by a pneumatic drill motor. Eye protection must be worn when an abrasive wheel is operated. CAUTION After removal of exfoliation corrosion by abrasive wheel, VACU-Blast area with glass beads to ensure removal of all corrosion. Failure to do so will result in the formation of tiny bubbles or flakes. 4-40