Chett Boxley

Al2O3 film dissolution in aqueous chloride solutions

The dissolution rate of the Al 2 O 3 passive film in near neutral (∼pH 6.7) 50 mM H 3 BO 3 /0.3 mM Na 2 B 4 O 7 .10 H 2 O solutions containing between 0 and 150 mM NaCI is reported. Dissolution rates are determined by analysis of the voltammetric anodic growth curves, 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. Oxide dissolution rates increase from 3.5× 10 - 1 4 mol/cm 2 s in the absence of Cl - to 1.6 × 10 - 1 2 mol/cm 2 s in solutions containing 150 mM Cl - . No enhancement in dissolution rate is observed in NaBr solutions. The dependence of dissolution rate on Cl concentration is modeled assuming equilibrium adsorption of Cl - at the oxide/electrolyte interface, followed by desorption of an AlCl x complex. The results indicate that the oxide dissolution rate has a second-order dependence on the surface concentration of Cl - .

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

The dissolution rate of the Al 2 O 3 film, R dis (mol/cm 2 s), and the pitting potential, E p , of polycrystalline Al electrodes were measured in 50 mM H 3 BO 3 /0.3 mM Na 2 B 4 O 7 .10 H 2 O solutions (pH 6.7) containing between 0 and 200 mM NaCl. R dis 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) × 10 -12 mol/cm 2 s in the absence of Cl to a quasi-limiting value of 2.4(±0.2) × 10 -12 mol/cm 2 s at Cl concentrations above ∼100 mM. Over the same concentration range, E p shifted to more negative potentials with increasing Cl concentration, also approaching a quasi-limiting value of ∼-0.95 V vs. mercury/mercuric sulfate (saturated K 2 SO 4 ) at Cl - concentrations greater than ∼100 mM. The similar functional dependencies of R dis and E p on Cl - concentration suggest a strong correlation between oxide film dissolution and oxide breakdown.