Aluminum Anodization in the LiAlBr4-NaAlCl4-KAlCl4 Molten Salt

Abstract

The anodization of aluminum was investigated in the low-melting, mixed halide molten salt system LiAlBr4-NaAlCl4-KAlCl4 (30-50-20 m/o) as a function of temperature using rotating disk electrode voltammetry at a miniature Al electrode. At modest overpotentials, the anodization process proceeds under mixed kinetic/mass-transport control. However, at larger positive overpotentials and correspondingly higher rates, the anodization reaction transitions to a mass transport-limited process governed by the dissolution of a passive layer of AlCl3(s) on the electrode surface. The heterogeneous rate constant, referenced to the equilibrium potential, and the transfer coefficient of the Al anodization reaction were measured in the absence of passivation. The density and viscosity of this molten salt were determined as a function of temperature and were used with literature data to calculate the molar conductance of the melt. A Walden plot constructed from this data fell on the ideal line, indicating that the LiAlBr4-NaAlCl4-KAlCl4 molten salt exhibits the expected high ionicity.