Exfoliation Corrosion

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.

Cases of 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.

Solution for Exfoliation Corrosion

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 Curtiss-Wright’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. Curtiss-Wright calls this process Search Peening. The process provides both a reliable nondestructive testing of the exfoliated material and a fatigue strength compensation for any reduced cross section.

Curtiss-Wright can perform its Search Peening process on-site at aircraft repair hangers to address exfoliation corrosion.

Controlled Shot Peening
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