Dissolution of the native oxide film on polycrystalline and single-crystal aluminum in NaCl solutions

The rate of dissolution, R-dis, of the native Al2O3 film on electrochemically polished, single-crystal [(100), (110), and (111) orientations] and polycrystalline Al electrodes is reported. Measurements were made in 50 mM H3BO3/0.3 mM Na2B4O7.O10 H2O solutions (pH similar to 6.7) containing between 0 and 75 mM NaCl. R-dis is determined by analysis of voltammetric curves recorded during film growth, using a model in which oxide growth is governed by high-field transport of ions within the film, while oxide dissolution at the oxide/electrolyte interface occurs by a potential-independent mechanism. R-dis is observed to be dependent upon the surface orientation in the order: polycrystalline Al. Al(111); Al(110). Al(100). Small enhancements in both the rate of ion transport within the oxide film and R-dis at polycrystalline Al are attributed to lowering of activation barriers at grain boundaries. R-dis is also shown to increase with increasing Cl- concentration, reaching a limiting value at; 40 mM, regardless of electrode crystallinity or orientation. The dependence of R-dis on Cl- concentration is consistent with adsorption of Cl- at the oxide/electrolyte interface, followed by desorption of an AlClx complex. (C) 2004 The Electrochemical Society.