Determination of the field strength and realization of the high-field anodization of aluminum

Abstract

Previous studies suggested that high-field anodization of aluminum can be realized given that aluminum is anodized at a high voltage just below the breakdown value. However, increasing the applied voltage cannot guarantee the enhancement in electric field strength across the barrier layer of porous anodic alumina (PAA) due to a concurrent increase of the barrier-layer thickness. Here, we report comparative studies of aluminum anodization in a highly concentrated (0.75 M) and a dilute (0.1 M) oxalic acid solution. Special attention is given to the field strength during anodization. To calculate the field strengths, the barrier-layer thickness of PAA was determined by a re-anodizing technique. Electrochemical impedance spectra (EIS) were also used to measure the barrier-layer thickness to improve the reliability of the field strength measurements. Both routes can yield the same conclusion: the barrier-layer thickness increases linearly with the applied voltage. For 0.75 M oxalic acid electrolyte, the resulting regression line has a positive intercept. In this instance the field strengths can be enhanced by increasing the applied voltage. Conversely, for 0.1 M oxalic acid solution, the regression line has a negative intercept and the field strengths decrease with the applied voltage. There are different linear dependences between the barrier-layer thickness and the applied voltage that determine the change in the field strength during anodization. In the highly concentrated oxalic acid electrolyte, the high-field anodization of aluminum can be realized by enhancing the applied voltage, but cannot in the dilute acid solution. Moreover, the ordering qualities of PAA films increase with increasing field strength instead of the applied voltage. The present results may provide a decisive step towards a thorough understanding of the PAA film.

Bipolar Electrochemical Anodization Route for the Fabrication of Porous Anodic Alumina with Nanopore Gradients

“…As illustrated in Figure b, its pore diameter also has a gradient distribution over a range of locations from 0 to 5 mm, showing a continuous change from the largest pore diameter of ∼44 nm to a minimum of ∼22 nm. Unlike interpore distance, the pore diameter of the sample decreases exponentially with X though the pore diameter is also found to be linearly proportional to the anodization voltage. , …”

“…Unlike interpore distance, the pore diameter of the sample decreases exponentially with X though the pore diameter is also found to be linearly proportional to the anodization voltage. 10,34 From the SEM micrographs of the bottom porous patterns (Figure 3c−e), it is clearly seen that the interpore distance of the prepared PAA films decreases significantly with increasing X. Nevertheless, their pattern ordering is relatively poor compared with conventional PAA films with uniform pore size, suggesting that the obtained PAA films with nanopore gradients have a lower degree of spatial ordering of the pore channels.…”

“…On the other hand, as mentioned above, the interpore distance was observed to be directly proportional to the anodization voltage. 10,34 Hence, the interpore distance of the prepared PAA films should also decrease continuously with X, leading to a gradient of the interpore distances across the surface of the aluminum sheet. Moreover, the PAA formed at 250 V exhibits a much steeper gradient of interpore distance than that of the one formed at 200 V (Figure 2a), which is attributed to the greater ΔV change along the X-direction in the case of 250 V. This is because the greater the driving voltage, the steeper is the gradient of ΔV while the distance between the driving electrodes is fixed.…”

Influence of the Anodization Process on Zamak 5 Corrosion Resistance

“…The electrochemical oxidation of aluminum at the base of the pores is controlled by diffusion as a consequence of the rapid growth of the oxide in high field conditions. Because of this rapid growth, it quickly depletes anionic species containing oxygen in the bottom of pores and a concentration gradient of anions along the narrow pore channels of aluminum oxide should be established 22 . In the case of this work, the oxalic acid electrolyte would be responsible for the anodization of aluminum, resulting in the depletion of the oxygen necessary for the formation of oxide (due to the high field), and for the formation of Zn 2+ (due to the dissolution of Zn) which would combine with oxalate species to form zinc oxalate as a crystallite.…”

Electrochemical Oxidation of Ti15Mo Alloy—The Impact of Anodization Parameters on Surface Morphology of Nanostructured Oxide Layers

“…This indicates that in the present case, the observed outer part of the samples is still an initial layer formed at the early stages of the anodization process. It is also known that the higher the anodizing potential, the thicker barrier film is formed on the surface of metal subjected to anodic oxidation [51,52]. Therefore, although the field-assisted dissolution of oxide is enhanced at higher potentials, generation of wider pores within the initially formed film is more difficult, so, in consequence, layers with narrower outer pores are formed.…”