AN EIS STUDY OF ALUMINUM BARRIER-TYPE OXIDE-FILMS FORMED IN DIFFERENT MEDIA

Impedance measurements were performed on aluminium in different aggressive (0.5 M NaCl) and non-aggressive (0.5 M Na2SO4, 0.16 M NH4-tartrate) electrolytes. Similar impedance responses of the formed passive film were found. The capacitive behaviour observed at high frequencies is related to the thickness and dielectric properties of the barrier oxide film. The impedance spectra obtained at low frequencies were interpreted in terms of a diffusion-controlled process within the oxide film. The Faradaic impedance given by the metal dissolution at the oxide film/electrolyte interphase is strongly affected by the anion type. This fact and the local pH developed at the interphase passive film/electrolyte determine the chemical stability of the film. The increased impedance observed in chloride solution under rotation conditions is explained by a local change of pH at the film/solution interphase. EIS can also be used to study pitting phenomena. In the stabilized active pitted area an ohmic control given by the presence of a salt film is responsible for the capacitive and low frequency inductive behaviour obtained.