A model for the [100] crystallographic tunnel etching of aluminium

When high purity aluminium foil is subjected to DC electrochemical etching in hot aqueous chloride solution at temperatures above 60 degrees C, the [100] crystallographic tunnel growth of an etch pit is achieved. This crystallographic tunnel growth behaviour arises due to the following reason. The tunnel tip region where the active corrosion reaction takes place tends to be made of {111} planes which minimize the surface energy, whilst the subsequent tunnel wall is made of {100} planes, because the passive oxide film formed on the {100} planes, along which the biaxial elastic modulus is the smallest, would be least attacked by corrosion.