Measurement of Aluminium Oxide-Film Thickness: Barrier Oxide Film and Oxide Porous Layer
In the present investigation, holographic interferometry was utilized for the first time to measure in situ the thickness of the oxide film, alternating current (A.C.) impedance, and double layer capacitance of aluminium samples during anodization processes in aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out by the electrochemical impedance spectroscopy (EIS), in different concentrations of sulphuric acid (0.5-2.5 % H2SO4) at room temperature. In the mean time, the real-time holographic interferometric was used to measure the thickness of anodized (oxide) film of the aluminium samples in aqueous solutions. Also, mathematical models were applied to measure the alternating current (A.C.) impedance, and double layer capacitance of aluminium samples by holographic interferometry, during anodization processes in aqueous solution. Consequently, holographic interferometric is found very useful for surface finish industries especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film, the A.C. impedance, and the double layer capacitance of the aluminium samples in sulphuric acid (0.5-2.5 % H2SO4) can be determined in situ. Futhermore, a comparison was made between the electrochemical values obtained from the holographic interferometry measurements and from measurements of electrochemical impedance spectroscopy(EIS) on aluminium samples in sulphuric acid (0.5-2.5 % H2SO4). The comparison indicates that there is good agreement between the obtained electrochemical data from both techniques. However, there is a drastic difference between the measurement of the oxide film thickness by both techniques. The oxide film thickness of the aluminium samples in 0, 0.5, 1.0, 1.5, 2.0, 2.5% H2SO4 by the optical interferometry is in a micrometer scale. However, the oxide film thickness of the aluminium samples in 0, 0.5, 1.0, 1.5, 2.0, 2.5% H2SO4 by the E.I.Spectroscopy in a nanometer scale. This can be explained due to the fact that the E.I.Spectroscopy is useful technique to measure the electrochemical parameters and the thickness of the barrier (compact) oxide films. In contrast, the optical interferometry is found useful technique to characterize and measure the thickness of the porous oxide layer. Also, the optimum thickness of the oxide barrier film was detected to be equivalent to 0.612nm in sulphuric acid concentration of 2.5% H2SO4 by E.I. spectroscopy.
Andreas Öchsner and Graeme E. Murch
K. Habib et al., "Measurement of Aluminium Oxide-Film Thickness: Barrier Oxide Film and Oxide Porous Layer", Defect and Diffusion Forum, Vols. 273-276, pp. 283-293, 2008