Effects of Cl-, NO3- and SO42- ions on anodic dissolution of pure aluminumin alkaline solution

The effects of the anions Cl-, NO3- and SO42- on the anodic dissolution of pure aluminum have been investigated in 0.01 M NaOH solution by using poten tiodynamic polarization experiments, a.c, impedance spectroscopy and potent iostatic current transients. It was found that the addition of Cl-, NO3- an d SO42- 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. Fitting was observed only in the pre sence of Cl- and NO3- ions, The reduced anodic dissolution rate by the addi tion of Cl- and NO3- ions below the pitting potential was traced back to th e 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 SO42- ions was accounted for by the reduced reacting surface area, by competitiv e adsorption of SO42- ions with OH- ions. The adsorption of SO42- ions does not cause pitting in all the applied anodic potential ranges. (C) 1999 Els evier Science Ltd. All rights reserved.