Effect of pH on Thickness and Ion Content of the Oxide Film on Aluminum in NaCl Media

The effect of solution pH on some characteristic properties, i.e. , thickness and ion content, of the surface oxide film on high purity aluminum exposed to NaCl solutions have been analyzed by X-ray photoelectron spectroscopy (XPS). The aluminum oxide film thickness is strongly dependent upon the solution pH. In the active dissolution range, i.e. , pH <3 and >11, the oxide film is thinner than the native oxide film formed after polishing. Two local maxima were observed at pH ∼3.8 and ∼10.7 where the thickness of the film was 68 and 98 Å, respectively. We found only a slight thickness variation in the passive region of aluminum, which represented a film thickness of 40-45 Å. The comparison of the aluminum dissolution data with XPS results indicates that the oxide film is thicker at the limits of aluminum stability. The chloride content of the film maximized at pH ∼3.8 and exhibited a minimum at pH ∼10. The sodium concentration reached its maximum value at pH 11.6. Such a pH dependent ion concentration in the passive film is most likely the result of ion bonding to the OH-terminated surface and subsurface groups at the oxide film. © 2001 The Electrochemical Society. All rights reserved.