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
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.
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
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
Do not use on fuel, hydraulic, or oxygen system
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
Q70. How should you remove moderate corrosion?
Q71. What is the most desirable method of mechanical
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
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.