Abstract
In this work, aluminum (Al) anodization in malic acid electrolytes of different concentrations (0.15 M, 0.25 M, and 0.5 M) was studied. The close-packed hexagonal pore structure was obtained for the first time in this organic acid in a 0.5 M solution, at 250 V and temperature of 5 °C. Moreover, the process was investigated as a function of the number of cycles carried out in the same electrolyte. A repetition of anodization under seemingly the same external electrochemical parameters (applied voltage, temperature, etc.) induced serious changes in the electrolyte. The changes were reflected in the current density vs. time curves and were most evident in the higher concentrated electrolytes. This phenomenon was tentatively explained by a massive incorporation of malate anions into anodic alumina (AAO) framework. The impoverishment of the electrolyte of the malate anions changed internal electrochemical conditions making easier the attraction of the anions to the Al anode and thus the AAO formation. The electrolyte modification was advantageous in terms of pore organization: In a 0.25 M solution, already after the second anodization, the pore arrangement transformed from irregular towards regular, hexagonal close-packed structure. To the best of our knowledge, this is the first observation of this kind.
Highlights
Aluminium (Al) and its alloys are frequently used for lightweight structures such as aircrafts, automobiles, aerospace components and interiors, where good mechanical properties, corrosion resistance and economic considerations are of high importance
The results presented in this work can be important for anodization carried out in other organic acid electrolytes
In 0.25 M solution, the process was stable at 300 V and 5 ◦ C (Figure 1b): At the initial stage a current overshoot is observed, but the ia drops to a minimum value, and increases gradually to about 60 A/m2, passing through a plateau
Summary
Aluminium (Al) and its alloys are frequently used for lightweight structures such as aircrafts, automobiles, aerospace components and interiors, where good mechanical properties, corrosion resistance and economic considerations are of high importance. A naturally occurring thin layer of passive oxide on Al surface makes it resistant to corrosive environment. To increase the inherent corrosion resistance of Al, the metal (and its alloys) is subjected to anodization [1,2,3]. A thick and porous anodic alumina is formed on Al surface with a characteristic barrier layer that separated the metal from the corrosive environment. Apart from its excellent anti-corrosive properties, anodic aluminium oxide (AAO) is frequently used as a template for fabrication of various functional nanostructures applied in sensors, capacitors, optical devices, high density magnetic recording media, etc. There is Materials 2020, 13, 3899; doi:10.3390/ma13173899 www.mdpi.com/journal/materials
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