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Exfoliation corrosion is a more severe form of intergranular
corrosion that can occur along aluminum grain boundaries
in the fuselage empennage and wing skins of aircraft.
These grain boundaries in both aluminum sheet and plate
are oriented in layers parallel to the surface of the
material, due to the rolling process. The delamination
of these thin layers of the aluminum, with white corrosion
products between the layers, characterizes exfoliation
corrosion.
Exfoliation corrosion is often found next to fasteners
where an electrically insulating sealant or a sacrificial
cadmium plating has broken down, permitting a galvanic
action between the dissimilar metals. Where fasteners
are involved, exfoliation corrosion extends outward
from the fastener hole, either from the entire circumference
of the hole, or in one direction from a segment of the
hole. In severe cases, the surface bulges outward, but
in less severe cases, there may be no telltale blisters,
and you can only detect the exfoliation corrosion by
nondestructive inspection methods that are not always
very effective.
Controlled shot peening can be very effective in the
process of both identifying and repairing exfoliation
corrosion damage. Service manuals normally call for
the removal of the fasteners and then for the use of
rotary discs to sand away the corroded material, followed
by blending the area and polishing out the tool marks.
Aircraft structural engineers have used Metal Improvement
Company's controlled shot peening after removal of visible
exfoliation corrosion to compensate for the lower fatigue
strength of the newly reduced cross-section. The action
of peening, however, will cause the surface to blister
again, where deeper exfoliation corrosion is present.
The surface can then be redressed and repeened until
no further blistering occurs. Metal Improvement Company
calls this process Search Peeningsm. The
process provides both a reliable nondestructive testing
of the exfoliated material and a fatigue strength compensation
for any reduced cross section.
Metal Improvement Company can perform its Search Peening
process on-site at aircraft repair hangers to address
exfoliation corrosion. |
