Abstract
Self-ordered porous anodic alumina (PAA) films are studied extensively due to a large number of possible applications in nanotechnology and low cost of production. Whereas empirical relationships between growth conditions and produced oxides have been established, fundamental aspects regarding pore formation and self-organization are still under debate. We present in situ structural studies of PAA films using grazing-incidence transmission small-angle X-ray scattering. We have considered the two most used recipes where the pores self-organize: 0.3 M H2SO4 at 25 V and 0.3 M C2H2O4 at 40 V. During anodization we have followed the evolution of the structural parameters: average interpore distance, length of ordered pores domains, and thickness of the porous oxide layer. Compared to the extensively used ex situ investigations, our approach gives an unprecedented temporal accuracy in determination of the parameters. By using of Al(100), Al(110) and Al(111) surfaces, the influence of surface orientation on the structural evolution was studied, and no significant differences in the interpore distance and domain length could be observed. However, the rate of oxide growth in 0.3 M C2H2O4 at 40 V was significantly influenced by the surface orientation, where the slowest growth occurs for Al(111). In 0.3 M H2SO4 at 25 V, the growth rates were higher, but the influence of surface orientation was not obvious. The structural evolution was also studied on pre-patterned aluminum surfaces. These studies show that although the initial structures of the oxides are governed by pre-patterning geometry, the final structures are dictated by the anodization conditions.
Highlights
GTSAXS has been used for studies of the two-step anodization process
2 h anodization were higher for samples anodized in oxalic acid
Aluminum and aluminum alloy materials are used in a vast number of applications
Summary
Aluminum and aluminum alloy materials are used in a vast number of applications due to their high strength to weight ratio, hardness and corrosion resistance.[1,2,3] The corrosion resistance of Al materials can in many cases be attributed to a thin (2–7 nm) native oxide lm on the material that is formed and is renewable in ambient and aqueous environments.[2,4,5,6,7] In harsher environments such as seawater, the protection of Al parts against corrosion needs to be improved, which can be achieved by electrochemically growing a thicker oxide by anodization.[2]During anodization, the applied anodic potential results in heterolytic dissociation of water at the oxide-electrolyte interface and an electric eld generated across the growing anodic aluminum oxide (AAO) lm. If the anodization is performed in an electrolyte in which anodic oxide is soluble, the produced AAO will be porous.[1,2] In this case, the migrating O2À/OHÀ ions forms oxide with Al3+ ions at the metal-oxide interface, and the migrating Al3+ ions are ejected into the solution without causing oxide formation at the oxide-electrolyte interface.[8] The porous-type oxide lm consists of a compact barrier layer in contact with the aluminum and a porous layer with nanometer-sized pores on top of the barrier layer. These pores are parallel, directed perpendicularly to the sample surface and extend through the entire porous layer
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