Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. Aluminum alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage. The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.
In this particular form of intergranular corrosion the expansive force of insoluble corrosion products tends to force the grains apart and leads to exfoliation corrosion, sometimes known as lamellar or layer corrosion. In extreme cases, the edges of the affected area are leaf like and resemble the separated pages of a wetted book that has become swollen and begun to open up. (reference)