A MODEL FOR THE [100]CRYSTALLOGRAPHIC TUNNEL ETCHING OF ALUMINUM

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