Exfoliation
Exfoliation corrosion is a particular form of intergranular corrosion associated with high strength aluminum alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage.
Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.
Exfoliation of an aircraft component
Notice how the corrosion separates into distinct layers which have expanded to occupy a much larger area than the original, uncorroded part. Obviously, the structural integrity of this part disappeared long ago. (courtesy ) Mike Dahlager