Relationship between Al2O3 film dissolution rate and the pitting potential of aluminum in NaCl solution

The dissolution rate of the Al2O3 film, Rdis (mol/cm2 s), and the pitting potential, Ep, of polycrystalline Al electrodes were measured in 50 mM H3BO3/0.3 mM Na2B4O7 * 10 H2O solutions (pH 6.7) containing between 0 and 200 mM NaCl. Rdis was determined by analysis of the transient current, using the high-field growth model, following a large amplitude potential step to induce film growth in competition with film dissolution. The dissolution rate of the oxide film increased from 0.8(-0.2) X 10-12 mol/cm2 s in the absence of Cl- to a quasi-limiting value of 2.4(-0.2) X 10-12 mol/cm2 s at Cl- concentrations above -100 mM. Over the same concentration range, Ep shifted to more negative potentials with increasing Cl- concentration, also approaching a quasi-limiting value of ~-0.95 V vs. mercury/mercuric sulfate (saturated K2SO4) at Cl- concentrations greater than ~100 mM. The similar functional dependencies of Rdis and Ep on Cl- concentration suggest a strong correlation between oxide film dissolution and oxide breakdown.