EFFECTS OF ELECTROLYTE-COMPOSITION AND APPLIED POTENTIAL ON THE REPASSIVATION KINETICS OF PURE ALUMINUM

Abstract

Repassivation kinetics of aluminium wire electrode have been investigated as a function of applied potential in 0.5 M Na2SO4 solutions containing various concentration of chloride ion and hydrogen ion by means of potentiostatic current transients obtained from the abrading electrode. The logarithmic representation of current transients showed that the repassivation rate parameter n changed gradually from zero to -1 during the repassivation. The zero value of n means that the bare surface of pure Al is hardly repassivated in the early stage of repassivation. From the feature of the current transients obtained, the n, pit growth rate parameter b and induction time t(m) after which the growth of stable pits occurs were found to decrease with increasing applied potential in the applied potential range relatively lower than pitting potential. In contrast, the n, b and t(m) increased with increasing applied potential in the comparatively higher applied potential range above the pitting potential. The increases in the n, b and t(m) with increasing applied potential are attributed to the kinetic limitation of the transport of chloride ions to the bare surface of Al at the higher applied potentials. The pH dependency of the current transients showed that t(m) did not change even though the charge necessary for establishment of passivity increased with decrease of the pH, indicating the formation of more unstable reaction intermediates with decreasing pH of the electrolyte during the repassivation. From the experimental results, it is inferred that the repassivation of pure aluminium proceeds via the various sequential electrochemical reaction steps of the competitive adsorption of chloride ions and hydroxyl ions or water, precipitation of salt layers and formation of stable oxide films on the bare aluminium surface, and it depends crucially upon the applied potential and electrolyte composition, which reaction step is predominant.