Protective and Nanoporous Alumina Films Studied in situ by X-ray and Electrochemical Methods

Sammanfattning: In this thesis, the studies of native and anodic oxides on both aluminum model single crystal surfaces as well as on aluminum alloys found in applications are presented. The focus has been on the characterization in situ as the oxide is growing by an electrochemical process called anodization. For the investigations, a combination of various X-ray scattering and spectroscopy methods, as well as electrochemical methods were used. In addition, scanning electron microscopy and atomic force microscopy were used for ex situ characterization after the oxide growth.From thickness measurements of native oxide films, it was found that the film was thicker on the aluminum alloys than on the pure aluminum single crystal surfaces. It was also found that the thicknesses obtained from the electrochemical impedance spectroscopy were thinner than from X-ray reflectivity and X-ray photoelectron spectroscopy. The anodic oxide film was studied during anodization in sodium sulfate and citrate buffer solutions using a combination of X-ray reflectivity and electrochemical impedance spectroscopy. It was found with both techniques that the film thickness increased with anodization potential, but the thicknesses obtained from electrochemical impedance spectroscopy were thinner. The difference was attributed to a porous layer that is difficult to determine with impedance spectroscopy. It was also found that the electrochemical resistance of the oxide was less on the alloys than on the pure aluminum single crystals, which was partly attributed to the alloying elements. Finally, it was found that the anodic oxides formed in the buffer solution were less rough than the oxides formed in the sodium sulfate solutions. Generally, the approach provides the possibility of studying both the electrochemical and structural properties of anodic oxides, which in the future also can be used for corrosion studies of aluminum materials as well as other materials.The growth of self-ordered nanoporous anodic aluminum oxides was studied using grazing-incidence transmission small-angle X-ray scattering by which the interpore distance between the pores, domain size of ordered pores and thickness of the oxide could be followed during the anodization process. In 0.3 M sulfuric acid at 25 V and 0.3 M oxalic acid at 40 V the interpore distance and domain length increased with time, where the final interpore distance and domain length were larger in the case of oxalic acid at 40 V. In both electrolytes it was found that the single crystal orientation did not significantly influence the interpore distance or the domain length. However, it was found that the orientation had a strong influence on the growth rate, especially during anodization in oxalic acid. The electrodeposition of tin into the pores of the oxides was also studied with X-ray fluorescence and grazing-incidence transmission small-angle X-ray scattering, and it was possible to follow the increase of tin in the pores during the deposition. Generally, the method and the approach provides a tool for high statistical and temporal investigations during the growth as well as during possible applications of the nanoporous oxides, which is not obtained with standard microscopy methods that are classically used for studies of these systems.