A wall-jet electrode reactor and its application to the study of electrode reaction mechanisms Part III: Study of the mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid

Abstract

The mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid is determined from the analysis of the potentiostatic transient behaviour of the system aluminium–electrolyte under anodic and cathodic polarization and comparison of experimentally determined transients with calculated values derived from a candidate mechanistic scheme. It has been established, that the oxidation of aluminium in the development of a distinct surface morphology occurs according to the Al3+ ions being dissolved from the surface and removed to the bulk of the solution, hence forming pits. Al(Cl)3 is a solid intermediate. The morphology developed, is determined by the excess of Cl− ions created at the electrode surface, with respect to the bulk concentration. The accumulation of Cl− ions is governed by the ratio between the rate constant for the formation of Al(Cl)3, set by the flux of charges forced across the electrode–solution interface per unit surface area taking part in the active dissolution of aluminium and the mass transport rate of the Cl− ions. The reduction of H+ ions in the cathodic half period of the applied alternating current is mass transport controlled. The concomitant rise in interfacial pH causes Al3+ ions formed in the preceding anodic half period, which are not yet removed from the electrode–solution interface, to precipitate as aluminium.