Anodizing

Abstract: Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.

Abstract: The conditions for the self-organized formation of ordered hexagonal structures in anodic alumina were investigated for both oxalic and sulfuric acid as an electrolyte. Highly ordered pore arrays were obtained for oxidation in both acids. The size of the ordered domains depends strongly on the anodizing voltage. This effect is correlated with a voltage dependence of the volume expansion of the aluminum during oxidation and the current efficiency for oxide formation. The resulting mechanical stress at the metal/oxide interface is proposed to cause repulsive forces between the neighboring pores which promote the formation of ordered hexagonal pore arrays.

Abstract: Self-ordering of the cell arrangement of the porous structure of anodic alumina has been studied in a sulfuric acid solution. Ordering of the cell arrangement was dependent on the applied potential, and a highly ordered structure was obtained under anodization at a constant potential of 25 to 27 V. Self-ordering of the porous structure proceeded with the growth of the oxide layer under anodization at an appropriate potential, and a porous film with an almost ideal hexagonal honeycomb structure was formed over an area of several micrometers after a long period of anodization.

Abstract: Anodization of titanium and its alloys is an important surface treatment, especially for adhesion applications, but is not as well studied as for aluminium alloys. This paper deals with the morphological, structural and physicochemical characterization of anodic oxide films grown on titanium and Ti–6Al–4V (TA6V) in chromic acid solution without (CA) or with (CA/HF) hydrofluoric acid addition. Several investigations methods are used: high-resolution scanning electron microscopy (HR-SEM), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), nuclear reaction analysis (NRA) and wetting angle measurements. The occurrence and morphology of the nanoporous structure for CA/HF anodization are described. The compact films grown in CA solution are amorphous and the porous films grown in the CA/HF solution are partially crystalline. The thickness and morphology of the films are described and discussed as a function of the anodizing conditions and of the composition of the underlying substrate. The composition of the film appears to be TiO2+Al2O3 (with Ti/Al atomic ratio ∽5), with incorporation of fluorine from the solution in the porous films and of small quantities of vanadium in the films that are grown. The specific role played by the Cr(VI) and F species on the film growth-and-dissolution formation process is discussed and a growth mechanism is proposed. Copyright © 1999 John Wiley & Sons, Ltd.