It has been suggested by Galvele [26] that stable pits are formed when, as a result
of the hydrolysis of dissolved metal ions, the internal pH in a pit falls below a threshold
value. In light of the Pourbaix diagram for aluminium [12] this behaviour may be
explained by the creation of a pit environment that results in the production of soluble
corrosion products. Galvele also demonstrated, using diffusion equations, that
for an open pit the internal pH will be dependent upon the rate of production of
metal ions and the distance that any hydrogen ions generated by hydrolysis have
to travel to leave the pit, expressed as the pit current density at the base of the pit
and the pit depth respectively.