Porous Anodic Oxide Research Articles

Exploring the use of an AC-DC-AC technique for the accelerated evaluation of anticorrosion performance of anodic films on aluminium alloys

This work explores the use of an AC-DC-AC method for the faster evaluation of the anticorrosion performance provided by anodic films grown on an aerospace aluminium alloy. The test is based on the cyclic application of a cathodic potential step followed by a step of free corrosion and, finally, the measurement of the electrochemical impedance response of the treated surface. During the cathodic polarization step in each cycle, alkalinisation is induced on the specimen surface, which results in attack of the porous anodic oxide. The effects of the magnitude and the duration of the cathodic polarization step are considered, and the impedance responses measured during the various cycles are compared to the responses measured during natural immersion for longer times. The methodology is applied for two types of anodic oxides, which are similar for thicknesses but differ for pore morphology and corrosion behaviour. Qualitative correlation is found between the electrochemical testing method and longer term immersion tests at the free corrosion potential, suggesting that the methodology can provide a faster estimation of anticorrosion performance compared to immersion test.

The effect of different carboxylic acids on the sulfuric acid anodizing of AA2024

Porous anodic oxides on AA2024-T3, grown in Cr(VI)-free mixtures of H2SO4 with carboxylic acids (malic, tartaric, oxalic, malonic, citric) under identical experimental conditions were comparatively studied. The corrosion resistance determined by pit initiation times in 0. 1 M NaCl is improved by the addition of malic, tartaric, and citric acid and impaired by malonic and oxalic acid. It is shown that the current efficiency of oxide formation can be obtained from the ∂Ecell/∂t value verified during barrier layer formation. The ranking of the corrosion performance can be deduced from electrochemical parameters of anodic oxide formation, morphology, structure, and composition of the oxide films. Beneficial for the corrosion resistance are higher voltage slopes ∂Ecell/∂t during barrier layer formation, which also indicates higher formation efficiency, as well as smaller flaws, denser oxides, lower OH− and S-content, as well as higher Cu content of the anodic oxide.

Formation of nanostructures on the basis of porous anodic niobium oxide

Formation of nanostructures on the basis of porous anodic niobium oxide

Fabrication and formation mechanisms of ultra-thick porous anodic oxides film with controllable morphology on type-304 stainless steel

In this work, pore initialization and further growth of metal-oxides-film (MOsF) with nanoporous structure on stainless steel (SS) substrate by electrochemical anodization have been investigated for controlling morphology of the porous MOsF. The competition between electric field assisted dissolution of the oxidized metals and anodization of SS substrate are analyzed by current-time response combined with the thickness variation of the barrier layer. The current density-time (i-t) curves show a drastic increase-a drastic drop-a gradual increase-stable shape while the barrier layer thickness presents a drastic increase-a gradual decrease-stable trend. Furthermore, the morphology (pore size, pore shape and film thickness) of the porous MOsF on SS substrate is strongly dependent on voltage drop across the barrier layer, anodizing duration and chemical constituents of the SS substrate. An electric field-assisted selective chemical dissolution model has been proposed to explain the formation of porous MOsF with cross-connected nanotube-shaped pores. An anodic etching model and an equivalent circuit of anodization system are built to theoretically analyze the formation mechanisms of pore shrinking (pore size varies from 53 ± 7 nm at the top to 34 ± 4 nm at the bottom) across the porous MOsF. Based on the achieved results, we are able to design an anodization scheme by step-potential tailoring to fabricate thick, porous MOsF with uniform nanostructures. By applying the step-potential tailoring anodization to keep a quasi-stable electric field across the barrier layer, ultra-thick (>17 μm), uniform, porous nanostructured MOsF on the SS substrates have been reported.

Oriented porous anodic oxide layers on Ti-50Al with outstanding oxidation resistance at 800 °C

A fluoride-containing anodic oxide layer with oriented porous structure coated Ti-50Al intermetallic can be fabricated by anodizing in sulfuric acid (H2SO4) solution containing a small quantity of hydrofluoric acid (HF). X-ray photoelectron spectroscopy (XPS) analysis reveals that the porous anodic oxide layer consists mainly of TiO2, Al2O3, titanium fluoride and aluminum fluoride. Cyclic oxidation tests indicate that all Ti-50Al samples anodized in fluorine-containing electrolyte exhibit outstanding oxidation resistance with little or even no scale spallation at 800 °C. The improvement of oxidation resistance is mainly attributed to the fluorine effect and the oriented porous structure of anodic oxide layer.

The effect of anodizing potential and annealing conditions on the morphology, composition and photoelectrochemical activity of porous anodic tin oxide films

In this work, the effect of the thermal annealing on the composition of crack-free anodic tin oxide layers with different channel diameters, formed in 1 M NaOH at 2 and 4 V, was investigated in detail. STEM tomography confirmed that anodic oxides formed at 2 V have narrower channels, higher porosity, and higher surface area than those anodized at 4 V. Structural properties of oxides films were studied using different techniques such as X-ray diffraction, Raman spectroscopy, and UV-Vis diffuse reflectance spectroscopy. In addition, Raman imaging was employed to study local changes in the composition of anodic films. The photoelectrochemical performance of as-formed and annealed oxides was correlated with applied anodizing potential and annealing conditions. It was found that as-formed anodic oxides are composed of poorly crystalline SnO2-x with a significant amount of Sn2+ defects. When the oxide layers were annealed at 200 °C, crystallization of the SnO phase was observed, however, the amount of Sn2+ defects was not drastically reduced. A significant reduction of Sn2+ defects occurred at 400 °C due to a formation of intermediate Sn3O4 oxide and, in the final result, more stoichiometric and crystalline SnO2 was formed. We proved that appropriate design of anodizing and annealing procedures can lead to the formation of nanoporous tin oxide layers with controlled morphology, composition and, in consequence, photoelectrochemical properties.

Self‐Organized Arrays of SnO2 Microplates with Photocatalytic and Antimicrobial Properties

Porous anodic metal oxides have been widely used for the development of various functional nanostructures. So far, these self‐organized pore structures were prepared as hexagonal arrangements of nanopores, which have become a popular template system for the fabrication of functional nanostructures. Here we report a new oxalic acid‐based anodization process for long‐range ordered arrays of submicrometer tin dioxide plates at low applied voltages in buffered solution, which prevents the precipitation of SnC2O4 on the Sn anode surface. The chemical composition of the surface SnO2 was established by Mössbauer spectroscopy and X‐ray photoemission spectroscopy (XPS). The growth direction of the SnO2 nanoplates was shown to be related to the orientation of the domains of the underlying polycrystalline yet oriented β‐Sn foil. The structured SnO2 surface is highly photoactive and generates superoxide radicals (O2·–) upon UV exposure resulting in a strong degradation of organic contaminants and a strong antimicrobial activity. Upon illumination the SnO2 coating exhibits a switchable amphiphilic behavior, which – in combination with its photoactivity – creates a self‐cleaning surface.

ELECTROCHEMICAL SYNTHESIS OF POROUS CRYSTALLINE TANTAL OXIDE

Self-organization of nanoscale structures during electrochemical processing is most pronounced during the formation of porous anodic metal oxides (aluminum, titanium, tungsten, niobium, tantalum). These oxides contain arrays oriented perpendicular to the pore substrate. A distinctive feature of these films is a high degree of orderliness in the arrangement of pores and the possibility of controlled variation of the pore diameter in a wide range (from 10 to 150 nm). In this work, the features of the electrochemical formation of nanoporous oxide coatings on tantalum in acid-fluoride and organic electrolytes are investigated. Tantalum foil with a thickness of 0.1 mm and a purity of 99.99 % was used as a working electrode. For the formation of tantalum oxides, 1 M H2SO4 solutions with the addition of HF (0.1 M; 0.25 M; 0.5 M; 1 M) and an organic electrolyte were used: EG + 5.5 М H2O + 0.05 М H3PO4 + 0.8 М NH4F, EG + 5.5 М H2O + 0.05 М H3PO4 + 0.8 М NaF. Polarization studies were carried out on a P-45X potentiostat. in potentiodynamic mode. The reference electrode is saturated silver chloride. The magnitudes of the potentials are given relative to the normal hydrogen electrode. The morphology of the obtained coatings was studied using scanning electron microscopy using a JSM-7001F microscope. It is shown that the use of fluoride ion activator and electrolytes of different nature allows at the initial stage of anodizing to provide conditions for the formation of a crystalline anodic oxide film with different surface morphology. By varying the anodizing regime on tantalum, it is possible to synthesize porous films of amorphous or crystalline types. To obtain a crystalline porous oxide of tantalum with a developed surface, it is advisable to use an organic aprotic electrolyte composition: EG + 5.5 М H2O + 0.05 М H3PO4 + 0.8 М NH4F.

Rapid growth of TiO2 nanotubes under the compact oxide layer: Evidence against the digging manner of dissolution reaction

The growth mechanism of porous anodic oxides still remains controversial. The field-assisted dissolution theory suggests that the growth of nanotubes is driven by a “top-down” digging manner. Here, a three-tier structure (nanotubes + compact layer + nanotubes) was prepared by three steps anodization. The anodizing conditions of the first-anodized nanotubes and the third-anodized nanotubes were exactly the same, but the length and diameter of the third-anodized nanotubes were larger than those of the first-anodized nanotubes. These are all unexplainable by the digging manner of the field-assisted dissolution. Based on the theoretical expression of voltage–time curves under constant current anodization, the total anodizing current can be divided into ionic current and electronic current. Through fitting results, it is found that the electronic current of the third anodization appears as a large value at the beginning and the ionic current of the third anodization is higher than that of the first anodization, which is consistent with the experimental phenomena. In other words, the reason why the length and diameter of the third-anodized nanotubes are larger than those of the first-anodized nanotubes is due to the large ionic current during the third anodization.

Titania Photonic Crystals with Precise Photonic Band Gap Position via Anodizing with Voltage versus Optical Path Length Modulation.

Photonic crystals based on titanium oxide are promising for optoelectronic applications, for example as components of solar cells and photodetectors. These materials attract great research attention because of the high refractive index of TiO2. One of the promising routes to prepare photonic crystals based on titanium oxide is titanium anodizing at periodically changing voltage or current. However, precise control of the photonic band gap position in anodic titania films is a challenge. To solve this problem, systematic data on the effective refractive index of the porous anodic titanium oxide are required. In this research, we determine quantitatively the dependence of the effective refractive index of porous anodic titanium oxide on the anodizing regime and develop a model which allows one to predict and, therefore, control photonic band gap position in the visible spectrum range with an accuracy better than 98.5%. The prospects of anodic titania photonic crystals implementation as refractive index sensors are demonstrated.

Morphology of nanoporous anodic films formed on tin during anodic oxidation in less commonly used acidic and alkaline electrolytes

Porous anodic tin oxide layers were obtained by simple one-step potentiostatic anodic oxidation (anodization) of Sn foil in aqueous ammonia and citric acid in a transpassive region. Different anodizing conditions including electrolyte concentration, applied potential, and duration of the electrolysis have been examined in order to find the effect of process parameters on the morphology of anodic films. Layers with completely clogged surface and porous interior have been obtained at all studied conditions. Moreover, the synthesized samples exhibit a typical stacked morphology with a lot of internal cracks and discontinuities within the anodic film being a result of local accumulation and trapping of gaseous oxygen inside the pores. Diameters of nanochannels were found to be almost independent on the applied potential, and their formation was also attributed to the O2 co-evolution at the positive electrode during anodization. Increasing electrolyte concentration and anodizing potential usually result in higher rates of the oxide formation. However, no significant effect of anodizing conditions on the nanochannel diameter, and morphology of the compact outer layer was found. Anodic films with the almost identical morphology have been successfully obtained in other electrolytes such as tartaric acid and phosphoric acid solutions. It was proven that the as-obtained layers were amorphous in nature and composed mainly of tin and oxygen with a surface O/Sn ratio close to stoichiometric SnO2. Incorporation of phosphorous species to the anodic film during anodization in H3PO4 was also confirmed. Band gaps of all materials were found to be significantly lower (2.94–3.05 eV) than that typically observed for pure SnO2 (3.6 eV).

Porous Anodic Oxide Films on Aluminum

Porous Anodic Oxide Films on Aluminum

Cross-Sectional Examination of Porous Anodic Oxide Films Grown on Al-Li-Cu Alloys

Cross-Sectional Examination of Porous Anodic Oxide Films Grown on Al-Li-Cu Alloys

Porous anodic oxide film with self-healing ability for corrosion protection of aluminum

Nanopores in aluminum oxide film served as vessels for corrosion inhibitor to add self-healing ability. Aluminum oxide films were immersed into a solution containing corrosion inhibitor to fill the nanopores. To monitor polarization resistance, the aluminum oxide film was coated with polymer, which was then scratched with a knife-edge. Uniform and large nanopores demonstrated a higher level of polarization resistance. The corrosion inhibitor was released in two steps: a large release due to dissolution was followed by a more sustained release due to corrosion inhibitor desorption.

Conditions for self-ordering of porous structure of anodic aluminum oxide in weak and strong acids

The model, which under certain assumptions enables one to predict the conditions for self-ordering of porous anodic oxide film (AOF) on aluminum in the weak and strong acids, is proposed. The work is a development of the theory of concentration changes in the growing oxide pores proposed by Patermarakis et al. The most important difference from the theory is the consideration of the second-stage dissociation of strong acids. The problem of the conditions of self-organization of porous AOF on aluminum is solved analytically. The critical porosity Ps and the corresponding anodizing voltage Vr that provides the optimal self-organization of porous structure of AOF are interpreted. Close values of Vr and the breakdown voltage VBP are explained. Using the model, the critical porosity Ps for solutions of weak monobasic acids and strong dibasic acids with low second-stage dissociation constant is calculated and the effect of various factors on the critical porosity Ps is determined. The obtained results give an answer to the question why all activating anions, which cause pitting dissolution, are the anions of strong monobasic acids.

Mechanistic study of thin film sulfuric acid anodizing rate difference between Al2024 T3 and Al6061 T6

Why different aluminum alloys anodizing at very different rate has been a puzzle for more than half a century. In this study, two high strength aluminum alloys (Al2024 and Al6061) were quantitatively compared for thin film sulfuric acid anodizing (TFSAA) process, on anodizing current density, anodized coating thickness, coating weight, and anodizing efficiency, to understand the underlying mechanism for the different anodizing rates. In-situ electrochemical impedance (EIS) measurement was carried out to deconvolute the different factors that affect the anodizing process. Anodized coating thickness and weight growth were correlated to different charge transfer resistances from the equivalent circuit model. The higher Cu content in Al2024 led to slower anodizing rate due to enhanced charge transfer resistance, and to lower anodizing efficiency through its catalytic effect for O2 evolution side reaction. A new schematic mechanism of the anodizing process was proposed taking into consideration of the electrolyte chemistry. O2 generation from water oxidation in the porous anodic oxide layer during anodizing was elaborated as the main factor for anodizing efficiency loss.

Label‐free immunosensor based on interdigitated electrodes with Ni–Co nanowires

This work presents a label-free immunosensor consisting of interdigitated electrodes and Ni–Co nanowires (NWs) to determine the relative amount of analyte, using electroanalytical measurement. Interdigitated electrodes were fabricated by the conventional printed circuit board treatment and Ni–Co NWs were synthesised by the electrodeposition method using a porous anodic alumina oxide membrane as the template. The experimental results showed that Ni–Co NWs can provide specific binding with histidine-tagged capture agent based on the method of immobilised metal affinity chromatography. Due to the large surface-to-volume ratio of NWs, this immunosensor had 2.6-fold the immobilisation ability of flat Ni–Co film to absorb histidine-tagged capture agent. Moreover, this immunosensor exhibited sensitivity in detecting low amounts of analyte. The electrical responses of the electroanalytical measurement of analyte, from 1000 to 625 ng/mL, could be discriminated; however, the fluorescence intensities became indiscriminative. This label-free immunosensor has the potential to immobilise various capture agents through the use of histidine for different analyte analyses.

Comparison with the osteoconductivity and bone-bonding ability of the iodine supported titanium, titanium with porous oxide layer and the titanium alloy in the rabbit model.

One of the serious postoperative complications associated with joint replacement is bacterial infection. In our recent investigations, iodine supported titanium implants demonstrated antibacterial activity in both invitro and invivo studies. The surfaces of the implants have porous anodic oxide layer with the antiseptic properties of iodine. According to the literature the titanium with porous anodic oxide have good osteoconductivity. But it is not clear whether the properties of iodine influence bone bonding of implants. The aim of this study is to evaluate the influence of the properties of iodine and porous anodic oxide layer in the bone bonding ability of titanium implants. Titanium rods were implanted in intramedullary rabbit femur models, in regard to the cementless hip stem. The implant rods were 5mm in diameter and 25mm in length. Three types of titanium rods were implanted.One was untreated titanium (control group (CL)), another was titanium with oxide layer without iodine (oxide layer group (OL)), and the other was Iodine treated Titanium (iodine group (ID)). The rods were inserted into the distal femur. We assessed the bonding strength by a measuring pull-out test at 4, 8, and 12 weeks after implantation. The bone-implant interfaces were evaluated at 4 weeks after implantation. Pull-out test results of the ID implants were 202, 355, and 344N, and those of the OL implants were 220, 310, 329N at 4, 8, and 12 weeks, significantly higher than those of the CL implants (102, 216, and 227N). But there were no significant difference in ID implants and OL implants. Histological examination revealed that new bone formed on the surface of each types of implants, but significantly more bone made direct contact with the surfaces of the ID implants and OL implants. This research showed that new type of coating, iodine coated titanium has low toxicity and good osteoconductivity.

Self-Assembly of Metal Oxide Nanoparticles in Liquid Metal toward Nucleation Control for Graphene Single-Crystal Arrays

Summary We have achieved the spontaneous assembly of alumina nanoparticles on a liquid metal surface and further synthesized a very-large-scale graphene single-crystal array. The effective electrical field established as a result of the electrical charge of the alumina nanoparticles in the liquid metal ensures the ordered self-assembly of the alumina islands and lays the foundation for precise spatial control of graphene nucleation, leading to the formation of graphene single-crystal arrays. In addition, controlling the density of the alumina in the liquid metal can precisely tune the periodicity of the arrays. This simple strategy, which is unlike the usual solid/aqueous solution system, opens up new territory for nanoparticle assembly. Focusing on the origin of the nanoparticle interaction that drives the self-assembly of ordered structure in the liquid metal system is a promising avenue for triggering more interesting applications.

Morphology evolution of porous anodic alumina in mixed H3PO4/NH4F electrolytes

Most anodizations of Al to obtain porous anodic alumina (PAA) are conducted in acid solution including sulfuric acid, oxalic acid and phosphoric acid solutions. It is well known that fluoride-containing electrolyte is generally used to fabricate anodic TiO2 nanotubes (ATNTs). Nevertheless, there are few articles reporting anodization processes of Al in NH4F electrolyte and mixed H3PO4/NH4F/ethylene glycol electrolyte. Herein, in order to explore different morphology of anodic alumina film during a series of constant current anodization processes, NH4F solutions, H3PO4 solutions of different concentrations (2wt%, 4wt% and 6wt%), and their mixed electrolytes containing different concentrations of NH4F and H3PO4 were employed. It is the first time for Al to be performed anodization in NH4F solution under constant current. PAA with thick pore wall, thick barrier oxide layer and narrow pores were obtained. The results show that three stages of voltage-time curve are not the requirement of obtaining PAA. Which anodic oxide (porous anodic oxide or compact anodic oxide) occurs only depends on the size of the electronic current. As long as the electronic current is high enough, porous anodic oxide is easily obtained. While the concentration of NH4F increases to 0.6wt%, the barrier oxide is much thicker than that in H3PO4 electrolyte due to the high ionic current and low electronic current.