•A chemical-resistant anodic aluminum oxide (AAO) was formed by anodizing in sodium tetraborate.•The AAOs were obtained in five typical acidic electrolytes and also in sodium tetraborate.•These ...anodized specimens were immersed in acidic H3PO4 and alkaline NaOH aqueous solutions.•The AAO formed in sodium tetraborate exhibited the highest chemical resistance.•This can be attributed to their higher anodizing ratio and the formation of high-purity alumina.
A porous anodic aluminum oxide (AAO) possessing higher chemical resistance in both acidic and alkaline solutions was fabricated by anodizing Al in an alkaline sodium tetraborate solution. The 5 N Al plates were anodized in five types of major acidic solutions (sulfuric, oxalic, phosphoric, chromic, and etidronic acids) and in alkaline sodium tetraborate solution to form AAOs. The anodized specimens were then immersed in a 0.52 M phosphoric acid solution (pH = 1.3) and a 2.5 M sodium hydroxide solution (pH = 13.8). Subsequently, the electrochemical impedance measurements and morphological characterizations of the AAOs before and after immersion were investigated. The time required for the complete dissolution of the AAOs formed in typical acidic electrolytes, including sulfuric, oxalic, phosphoric, and etidronic acids, increased linearly with the anodizing voltage. The AAO formed in chromic acid exhibited a higher chemical resistance than this linear relationship owing to the formation of high-purity alumina without incorporated anions. Moreover, the AAO formed in sodium tetraborate exhibited the highest chemical resistance in both acidic and alkaline solutions – approximately two times higher than that formed in etidronic and chromic acids. This can be attributed to their higher anodizing ratio and the formation of a high-purity alumina layer.
High-aspect ratio ordered nanomaterial arrays exhibit several unique physicochemical and optical properties. Porous anodic aluminum oxide (AAO) is one of the most typical ordered porous structures ...and can be easily fabricated by applying an electrochemical anodizing process to Al. However, the dimensional and structural controllability of conventional porous AAOs is limited to a narrow range because there are only a few electrolytes that work in this process. Here, we provide a novel anodizing method using an alkaline electrolyte, sodium tetraborate (Na
B
O
), for the fabrication of a high-aspect ratio, self-ordered nanospike porous AAO structure. This self-ordered porous AAO structure possesses a wide range of the interpore distance under a new anodizing regime, and highly ordered porous AAO structures can be fabricated using pre-nanotexturing of Al. The vertical pore walls of porous AAOs have unique nanospikes measuring several tens of nanometers in periodicity, and we demonstrate that AAO can be used as a template for the fabrication of nanomaterials with a large surface area. We also reveal that stable anodizing without the occurrence of oxide burning and the subsequent formation of uniform self-ordered AAO structures can be achieved on complicated three-dimensional substrates.
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•Aluminum plates were anodized in phosphoric acid and six phosphate solutions.•The current density increased with pH getting farther from neutral.•A typical porous alumina film was ...formed in acidic solutions.•Flat alumina films with and without a narrow porous layer were obtained in near-neutral solutions.•Alkaline solutions caused feathering of the pore wall and flattening of the barrier layer.
High-purity aluminum plates were anodized in phosphoric acid and six phosphate solutions over a wide pH range from acidic to alkaline. The current density corresponding to the growth rate of the anodic oxide was minimum when anodizing in near-neutral solutions and increased when the pH value increased or decreased. A typical porous anodic aluminum oxide (PAAO) film consisting of an outer porous layer and inner hemispherical barrier layer, which is based on the Keller–Hunter–Robinson (KHR) model, was formed in acidic solutions. At the pH values closer to the neutral region, a uniform barrier oxide film or a PAAO film with a narrow porous layer was produced owing to the low solubility of aluminum oxide. On the other hand, two types of characteristic PAAO films were formed during anodizing in alkaline phosphate solutions: one was a PAAO film consisting of an outer porous layer with feathered pore walls and inner hemispherical barrier layer, and the second was a PAAO film consisting of an outer porous layer with elongated pores and inner flat barrier layer. We showed that novel PAAO films, which are not based on the KHR model, can be prepared by anodizing aluminum in various alkaline phosphate solutions.
A rapid electrochemical separation of an anodic porous alumina (APA) film formed by anodizing an aluminum substrate was achieved using a highly safe sodium chloride (NaCl)/ethylene glycol (EG) ...mixture. A thick APA film (thickness: 20–80 µm) with an ordered pore structure was formed on the aluminum surface by anodizing the substrate in a 0.3 M sulfuric acid solution at 25 V, and the specimens were subsequently polarized anodically in a 1.0 M NaCl/EG solution. A large current density was measured when a voltage higher than the anodizing voltage of 25 V was applied, and the APA film was completely separated from the aluminum substrate. This may be due to the formation of extremely small through-holes at the bottom barrier oxide layer of the APA film and the subsequent dissolution of aluminum from the substrate. The separation time of the APA film decreased with increasing applied voltage in the NaCl/EG solution, and rapid separation within 0.5 s was successfully achieved via anodic polarization at 36 V. We demonstrate that large-area and thick APA films can also be isolated from the aluminum surface by anodic polarization in the benign NaCl/EG solution. Moreover, an APA through-hole membrane can be successfully fabricated by subsequent immersion in a 0.52 M H3PO4 solution.
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Alkaline corrosion-resistant porous alumina was fabricated by anodizing aluminum in etidronic acid, and its hydration behavior during pore sealing in boiling water was investigated. High-purity ...aluminum plates were anodized in sulfuric, oxalic, citric, and etidronic acid solutions. Anodizing with etidronic acid caused stable growth of a uniform, porous alumina layer at a high voltage of approximately 200 V. This porous alumina possessed a thick barrier layer with an inner layer of pure aluminum oxide and exhibited a 10-fold increase in the corrosion resistance in a 2.5 M sodium hydroxide solution. When the porous alumina film formed by sulfuric acid anodizing was immersed in boiling water, plate-like hydroxide scales rapidly formed on the whole surface, and the pores were sealed within 10 min. In the case of etidronic acid, the hydroxides formed at the bottom of the pores in the initial stage of immersion, and the thickness of the hydroxide layer gradually increased with the immersion time. The porous layer was completely sealed by long-term immersion. Although the barrier layer was reduced to approximately 80% of its original size due to hydration, a thick barrier layer was still maintained at the bottom of the pores after immersion.
Advanced hard anodic alumina coatings measuring Hv=610–769 on the Vickers hardness scale were obtained on an aluminum surface via aluminum anodizing using a new electrolyte, etidronic acid. The ...ordered porous alumina was fabricated by two-step etidronic acid anodizing at 260V under self-ordering conditions, and pore-widening was carried out to control the porosity of the porous alumina. The Vickers hardness of the ordered porous alumina increased with decreasing diameter of the pores and porosity. Aluminum specimens were also anodized by the constant-current method under various concentrations, temperatures, and current densities. The Vickers hardness increased with decreasing concentration and temperature because chemical dissolution of the anodic oxide during anodizing was suppressed. A hard porous alumina measuring Hv=610 was obtained by anodizing in a 0.05M etidronic acid solution at 278K and 5Am−2. Subsequent thermal treatment caused the dehydration and corresponding hardening of the porous alumina, and a higher porous alumina hardness of Hv=769 was successfully achieved by thermal treatment at 873K for 12h.
•Advanced hard porous alumina was successfully fabricated by etidronic acid anodizing.•The Vickers hardness increased with decreasing pore diameter and porosity.•Hard porous alumina measuring Hv=610 was fabricated by constant-current anodizing.•Subsequent thermal treatment caused the formation of harder porous alumina with Hv=769.
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