Anodic oxidation mechanism of aluminum alloys in a sulfate-fluoride electrolyte

In recent years, fluoride containing electrolytes are used for the formation of porous structures on valve metals such as Ti, Nb, W, Ta and alloys thereof. The presence of fluoride ions allows the growth of nanoporous/nanotubular templates with well-defined structures. Very recently neutral sulfate-fluoride solutions have been successfully used to form nanoporous films on Al as well. As a continuation of this work, the present study reports on the initial stages of growth of porous anodic films on two Al alloys in fluoride-containing electrolytes by voltammetry and electrochemical impedance spectroscopy. A kinetic model based on the surface charge approach was employed to fit the impedance spectra and to estimate the main transport parameters of the oxide formation and dissolution process as depending on applied potential and alloy type.