Abstract
The anodization of aluminum (Al) in three alpha-hydroxy acids (AHAs): glycolic (GC), malic (MC), and citric (CC), was analyzed. Highly ordered pores in GC were obtained for the first time. However, the hexagonal cells were characterized by a non-uniform size distribution. Although common features of current density behavior are visible, the anodization in AHAs demonstrates some peculiarities. The electric conductivity (σ) of 0.5 M GC, MC, and CC electrolytes was in the following order: σ(CC) > σ(MC) > σ(GC), in accordance with the acid strength pKa(CC) < pKa(MC) < pKa(GC). However, the anodization voltage, under which a self-organized pore formation in anodic alumina (AAO) was observed (Umax), decreased with increasing pKa: Umax(CC) > Umax(MC) ≥ Umax(GC). This unusual behavior is most probably linked with the facility of acid ions to complex Al and the active participation of the Al complexes in the AAO formation. Depending on the AHA, its tendency and different modes to coordinate Al ions, the contribution of stable Al complexes to the AAO growth is different. It can be concluded that the structure of Al complexes, their molecular mass, and the ability to lose electrons play more important roles in the AAO formation than pKa values of AHAs.
Highlights
The anodization of aluminum is one of the most studied electrochemical processes owing to its ability to produce a regular porous structure with tunable pore geometry [1,2]
Since relatively small variations of acid concentration have a negligible effect on anodization compared with the impact of the dissociation constant [18], the magnitude of the Umax usually applied during anodization in the three most studied electrolytes: 0.3 M sulfuric, 0.3 M oxalic, and 0.1–0.3 M phosphoric, increased from 25 [19] and 40 [20] to 195 V [10,21,22], respectively, owing to the following order of pKa: pKa(H2SO4) < pKa(H2C2O4) < pKa(H3PO4) (H2SO4 ionizes completely in aqueous solutions, pKa (H2C2O4) = 1.3, and pKa (H3PO4) = 2.1 at 25 ◦C [23])
Different molecular structures and chemical properties of citric, malate, and glycolate complexes may, in turn, modify to a different extent the field-assisted oxide dissolution process leading to the peculiar behavior of current flow during anodization, which is further reflected in the resulted anodic aluminum oxide (AAO) morphology
Summary
The anodization of aluminum is one of the most studied electrochemical processes owing to its ability to produce a regular porous structure with tunable pore geometry [1,2]. Since relatively small variations of acid concentration have a negligible effect on anodization compared with the impact of the dissociation constant [18], the magnitude of the Umax usually applied during anodization in the three most studied electrolytes: 0.3 M sulfuric, 0.3 M oxalic, and 0.1–0.3 M phosphoric, increased from 25 [19] and 40 [20] to 195 V [10,21,22], respectively, owing to the following order of pKa: pKa(H2SO4) < pKa(H2C2O4) < pKa(H3PO4) (H2SO4 ionizes completely in aqueous solutions, pKa (H2C2O4) = 1.3, and pKa (H3PO4) = 2.1 at 25 ◦C [23]) From this point of view, it can be deduced that to produce AAO with a Dc larger than that obtained in the H3PO4 solutions, the acids with pKa > pKa (H3PO4) should be selected. This unusual behavior was discussed, taking into consideration the possible participation of ionic species in AAO formation and their strong ability to form stable complexes with Al
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