Effects of Cl −, NO −3 and SO 2−4 ions on anodic dissolution of pure aluminum in alkaline solution

Abstract

The effects of the anions Cl −, NO − 3 and SO 2− 4 on the anodic dissolution of pure aluminum have been investigated in 0.01 M NaOH solution by using potentiodynamic polarization experiments, a.c. impedance spectroscopy and potentiostatic current transients. It was found that the addition of Cl −, NO − 3 and SO 2− 4 ions in 0.01 M NaOH solution reduces the anodic dissolution rate of pure aluminium in the passive potential region on the polarization curves and potentiostatic current transients. Pitting was observed only in the presence of Cl − and NO − 3 ions. The reduced anodic dissolution rate by the addition of Cl − and NO − 3 ions below the pitting potential was traced back to the formation of resistive insoluble compounds. Above the pitting potential, resistive transitory compounds would be formed up to the induction time and then dissolved into the solution as complex anions, thus causing pitting. By contrast, the decreased anodic dissolution rate by the addition of SO 2− 4 ions was accounted for by the reduced reacting surface area, by competitive adsorption of SO 2− 4 ions with OH − ions. The adsorption of SO 2− 4 ions does not cause pitting in all the applied anodic potential ranges.