Pit Propagation in Pure Aluminum Investigated via the 1D Artificial Pit Technique: Growth Regimes, Surface Morphology and Implications for Stability Criteria

Abstract

The growth kinetics of various diameter 1D 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 with a transition to a further region of potential-independent dissolution at ca. 100 mV prior to repassivation. Optical and x-ray computed tomography observations of pit cross-sections suggest Ohmic propagation is associated with non-uniform dissolution over the pit surface. By contrast narrow diameter pits (25 μm) show no evidence of dissolution below their limiting current density and appear to grow without surface roughening until 10 - 20 mV before repassivation. Investigation of the nature of pit growth prior to this point via potential perturbation techniques confirmed that dissolution occurs under mass transport control. These observations support the hypothesis that sustained pit propagation requires a high degree of saturation, close to 100%, in AlCl3.