Exfoliation is a specific form of corrosion that travels along grain boundaries parallel to the surface of the part causing lifting and flaking at the surface. The corrosion products expand between the uncorroded layers of metal to produce a look that resembles pages of a book. This exfoliation corrosion (EC) is associated with sheet, plate and extruded products and usually initiates at unpainted or unsealed edges or holes of susceptible metals.

In details, exfoliation corrosion is a more severe form of intergranular corrosion, occuring 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.

EC is often found next to fasteners where an electrically insulating sealant or a sacrificial cadmium (Cd) 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.

For both identifying and repairing exfoliation corrosion damage, controlled shot peening can be very effective in the process. 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.