Ac-impedance and Raman spectroscopy study of the electrochemical behaviour of pure aluminium in citric acid media

The electrochemical behaviour of pure aluminium in citric acid solution was investigated with a rotating disk electrode (RDE) at the open circuit potential (OCP). in the presence and in the absence of chloride, using electrochemical impedance spectroscopy and Raman spectroscopy. The impedance diagrams are composed of four frequency domains: at very high frequency (VHF), a Warburg-like: behaviour was attributed to the porosity effect of the external part of the oxide layer. A slightly depressed capacitive loop at high frequency (HF) was explained as the combination of two capacitive loops, one corresponding to the same phenomenon as in the VHF domain but insensitive to the porosity (barrier layer resistance in parallel to the oxide film capacity), the other one to the charge transfer resistance in parallel to the double layer capacity. An inductive loop at low frequencies (LF) appeared as due to the relaxation of oxygen species adsorbed at the film \ solution interface. while the very low frequencies (VLF) capacitive loop was ascribed to the coverage relaxation of adsorbed citrate species. possible modulation of the passive layer thickness was also envisaged as a phenomenon at the origin of this: latter loop. The diagrams shape is also insensitive to the addition of 104 ppm chloride inns to the solution. even though a diameter decrease of the HF capacitive loop is observed together with an increase of the frequency response. Raman Spectroscopy showed that citrate ions adsorb strongly to the electrode surface and that the time scale of this process is consistent with the frequency domain observed for the VLF capacitive loop. The interaction of citrate with the oxide la!:er is hindered by the addition of chloride ions to the electrolytic solution. (C) 2001 Elsevier Science Ltd. All rights reserved.