This treatment of a pit is very useful as it describes
the competitive nature of production and loss of hydrogen ions inside the pit. If a
pit is able to attain and sustain an internal pH that is lower than a threshold, then
the pit will be able to propagate indefinitely. If on the other hand the pit is unable
to attain and/or sustain a pH below this threshold pH, it will be passivated by the
production of aluminium oxides/hydroxides at the corroding surface. However, this
treatment was simplistic in many aspects, it assumed a cylindrical shape with only
the bottom of the cylinder corroding, it took no account of pit caps and their effect
on diffusion into and out of a pit, and also the effect of transport control and/or
ohmic control on the internal pitting current density was ignored [26].