The present work considers the effects of $Cl^-$, $SO_4^{2-}$ and $NO_3^-$ ions on the formation and dissolution of the surface film on pure aluminium in alkaline solutions, by using potentiodynamic polarization experiment, abrading electrode technique, potentiostatic current transient method and a.c. impedance method.In order to characterize the difference in passivating film growth kinetics between the native oxide-free aluminium surface and the native oxide-covered aluminium surface, two kinds of potentiostatic anodic current transients were measured just after interrupting the abrading action on the specimen and just after applying an anodic potential to the specimen in alkaline solutions containing $Cl^-$, $SO_4^{2-}$ and $NO_3^-$ ions. With increasing $Cl^-$ ion concentration, passive current density on potentiodynamic polarization curves decreased and repassivation parameter increased below pitting potential. These results mean that chloride complexes formed by incorporation of $Cl^-$ ions into the surface film help passivation of pure aluminium in alkaline solutions before the surface film is broken-down. The additions of $SO_4^{2-}$ and $NO_3^-$ ions in alkaline solution demonstrated the same effects on the passive current density and on the repassivation parameter as $Cl^-$ ion below pitting potential, indicating that $SO_4^{2-}$ and $NO_3^-$ ions impede the dissolution of the surface film as a consequence of reducing the reacting surface area by their adsorption on the specimen. The induction time on the bare surface, after which pits grow, was observed to be shorter than that on the native oxide-covered surface. This implies that critical $Cl^-$ concentration within the film is attained more rapidly on the bare aluminium surface than on the native oxide-covered aluminium surface, resulting from direct formation of chloride complexes by electrochemical reaction between $Cl^-$ ions and aluminium metal on the bare surface. From the a.c. impedance measurement, the surface film formed by the application of anodic potential on pure aluminium was found to be composed of more resistive thin films in the presence of $SO_4^{2-}$ ions in alkaline solution as compared with those in the absence of $SO_4^{2-}$ ions in the alkaline solution.
