A WALL-JET ELECTRODE REACTOR AND ITS APPLICATION TO THE STUDY OF ELECTRODE-REACTION MECHANISMS PART III - STUDY OF THE MECHANISM OF THE AC ELECTROLYTIC GRAINING OF ALUMINUM IN HYDROCHLORIC-ACID

The mechanism of the a.c. electrolytic graining of aluminium in hydroc hloric acid is determined from the analysis of the potentiostatic tran sient behaviour of the system aluminium-electrolyte under anodic and c athodic polarization and comparison of experimentally determined trans ients with calculated values derived from a candidate mechanistic sche me. It has been established, that the oxidation of aluminium in the de velopment of a distinct surface morphology occurs according to Al + 3 Cl- (k(1)) under right arrow Al(Cl)(3) + 3e(-) Al(Cl)(3) (k(2)) under right arrow Al3+ + 3 Cl- the Al3+ ions being dissolved from the surfac e 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 b y the ratio between the rate constant for the formation of Al(Cl)(3), set by the flux of charges forced across the electrode-solution interf ace per unit surface area taking part in the active dissolution of alu minium 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 c auses Al3+ ions formed in the preceding anodic half period, which are not yet removed from the electrode-solution interface, to precipitate as aluminium.