Pit propagation in pure aluminum investigated via the 1D artificial pit technique: Growth regimes, surface morphology and implications for stability criteria

The growth kinetics of various diameter aluminum artificial pits were investigated under potential control in 1 mol dm-3 HCl. Pits of diameter ≥ 50 μm display mass transport control at high potentials, Ohmic growth at intermediate values followed by a transition to a further region of potential-independent dissolution at ca. 100 mV prior to repassivation. X-ray computed tomographic examination of pit cross sections suggests the onset of Ohmic propagation coincides with a transition to non-uniform dissolution over the pit surface. By contrast 25 μm diameter pits show no evidence of dissolution below their limiting current density and appear to grow without surface roughening until 20 mV above the repassivation potential. Investigation into the nature of growth prior to this potential via step-potential procedures confirmed dissolution proceeds under mass transport control. Overall these observations support the hypothesis that sustained pit propagation requires a high degree of saturation, near 100%, in AlCl3. ©The Electrochemical Society.