Formation of Al2O3 Film and AlF3 Containing Al2O3 Film by an Anodic Polarization of Aluminum in Ionic Liquids

Anodic polarization of aluminum in three kinds of ionic liquids was studied by the constant-voltage rising rate method and constant-current method, with characterization by X-ray photoelectron spectroscopy. Both the capacity for passive-layer formation and the composition varied depending on the ionic liquid, applied voltage and holding time. A passive film consisting of Al2O3 and AlF3 in which the concentration of AlF3 became the highest at the Al2O3/Al interface was formed in 1-butyl-3-methylimidazolium trifluoromethylacetate (BMIm-TFA) at 40 V. In the case of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIm-TFSA), no passive film was formed at 40 V. However, a passive film in which the concentration of AlF3 reached its maximum at the electrolyte/Al2O3 interface was formed at 10 V, and the layer composition changed to pure Al2O3 when held at 10 V for 20 min. On the contrary, pure homogeneous Al2O3 film was formed in 1-ethyl-3-methylimidazolium lactate (EMIm-LAC) at 40 V. Although the oxygen source of the oxidation reaction was thought to be the small amount of water contained in the ionic liquids, the anion component of the ionic liquids appears to have had an important effect on the ability to form a passive layer and its composition. (C) The Electrochemical Society of Japan, All rights reserved.