Porous Anodic Films Research Articles

Nature of instability in flow-driven porous anodic oxide.

Self-organized porous anodic oxide films are formed by the electrochemical oxidation of reactive metal aluminum in acidic solutions in which the oxide is soluble. Recently, viscous flow models have shown using linear stability analysis that the instability results from a trade-off between the destabilizing effect of viscous flow of oxide and the stabilizing effect of oxide formation, which provides the wavelength selection mechanism for pattern formation. Anion adsorption on surface growth sites causes nonuniform compressive stress at the oxide-solution interface, which drives the flow. This anodic instability is analogous to the classical Marangoni instability induced by surface tension gradients. In this work, nature of the instability beyond the stability threshold is determined using a weakly nonlinear analysis. For the growth of well-developed pores beyond the threshold, a subcritical nature of the instability is essential. However, our weakly nonlinear analysis shows that the solutions emerging from neutral stability are supercritical in nature at all wavenumbers for the practical range of anodizing control parameters investigated. We also determine the region where the model is Hadamard stable, a necessary condition for well-posedness.

A multi-strategy coating for Al alloy based on orientationally arranged nanosheets doped SiO2 coating and corrosion inhibitor loaded porous anodic alumina film

Although aluminum alloys are widely used in various industries, their wear and corrosion resistance are still needed to be enhanced. In this study, a robust anti-corrosion composite coating was prepared on the surface of Al alloy based on multiple reinforcement strategies. The composite coating is consisted of horizontally arranged and functionally modified h-BN nanosheets doped SiO2 coating (SiO2-hBN(NH3+) coating) and corrosion inhibitor MBT loaded porous anodic alumina (PAA-MBT) film. Results show that the composite coating prepared in this work has excellent corrosion resistance due to the effect of the extended penetration path of corrosive medium by h-BN(NH3+) nanosheets, the full-thickness hydrophobicity of the SiO2 coating, the corrosion inhibition of the MBT released from the PAA film and the physical barrier of the entire coating system. In addition, the composite coating also exhibits good mechanical properties including high hardness, high bonding strength and superior wear resistance. This work provides a novel design of protective coating with excellent anti-corrosion resistance and mechanical properties, which has a promising prospect in the development of superior long-term anti-corrosion coatings for aluminum alloy.

Nanocontrolled thinning of the barrier layer thickness of porous anodic films using galvanodynamic polarization of aluminum alloys

Thinning the barrier layer thickness of anodic films prepared on refined aluminum substrates (4 N or 5 N) was usually performed to elaborate -after the usual two-step anodizing method- anodic aluminum oxide (AAO) templates with pores opened at both sides, allowing the subsequent nanodevices synthesis. By contrast, there is a lack of studies concerning similar barrier layer thinning for anodic films grown -by a single-step anodizing - on commercial low alloyed aluminum substrates (1XXX series) or other more complex aluminum alloys (AA 2XXX-8XXX). Yet such a need is now emerging, particularly for future applications requiring transverse electrical conductivity of usual anodic films toward the aluminum alloy substrate.This work aims to study and to master a simple and cheap method for thinning the barrier layer thickness of such anodic films on commercial aluminum alloys. Here, the thinning is studied using a current density decrease (at a constant rate) after the anodic film growth in phosphoric acid-based electrolyte, using the conjunction of in-situ accurate electrochemical follow-up and ex-situ FE-SEM morphological observations. Variation of current density scan rate highlights that fast processes lead to no barrier layer thinning, and slow ones to the detachment of the whole oxide film. However, by choosing the right scan rate range, it is possible to successfully obtain, in a straightforward manner, anodic films still supported on aluminum alloy substrates, but with barrier layer thickness tuned at the nanoscale.

Lithium salt post-treatment of anodized 7075-T6 alloy for enhancing corrosion resistance in neutral and acidified NaCl environment

An anodized 7075-T6 aluminum alloy was post-treated in three different lithium salt solutions (Li2CO3, Li2C2O4 or LiNO3) under optimized conditions, respectively, in order to improve corrosion resistance of the alloy in both neutral NaCl (0.05 M, pH = 6.5) and acidified NaCl (0.6 M, pH = 3.5) solutions. The results indicate that the porous anodic film can be effectively sealed by the post-treatment and the sealing products are a mixture of boehmite, Li–Al amorphous hydroxides and Li–Al-LDH, with the relative proportion of each component varying with the sealing conditions. All lithium salt treated alloy specimens passed 1100 h of neutral salt spray test, and the Li2C2O4-based alloy specimen exhibited higher corrosion resistance than others in the neutral NaCl environment. Although the corrosion resistance of the coating deteriorated in the acidified NaCl solution, the lithium salt treated alloy specimens still showed distinct advantage over the hot water sealed counterpart, especially for the LiNO3-based process.

Isotopic Tracer Study of Initiation of Porosity in Anodic Alumina Formed in Chromic Acid.

In this paper, we focused on the initiation of porosity in the anodic alumina under galvanostatic conditions in chromic acid, using an 18O isotope tracer. The general concept of the initiation and growth of porous anodic oxide films on metals has undergone constant development over many years. A mechanism of viscous flow of the oxide from the barrier layer to the pore walls has recently been proposed. In this work, two types of pre-formed oxide films were analysed: pure Al2O3 formed in chromic acid, and a film containing As ions formed in a sodium arsenate solution. Both were anodized in chromic acid for several different time durations. Both pre-formed films contained the oxygen isotope 18O. The locations and quantities of 18O and As were analysed by means of ion accelerator-based methods supported by transmission electron microscopy. The significant difference observed between the two oxide films is in the 18O distribution following the second step of anodization, when compared with galvanostatic anodization in phosphoric or sulfuric acid reported in previous works. From the current experiment, it is evident that a small amount of As in the pre-formed barrier layer appears to alter the ionic conductivity of the film; thus, somehow, it inhibits the movement of oxygen ions ahead of advancing pores during anodization in chromic acid. However, anodising pure alumina film under these conditions does not enhance oxygen movement within the oxide layer. In addition, the tracer stays in the outer part of the growing porous oxide film. A lower-than-expected value for pure alumina enrichment in 18O in the pre-formed films suggests, indirectly, that the pre-formed film may contain hydrogen species, as well as trapped electrons, since no Cr is detected. This may lead to the presence of space charge distribution, which has a dual effect: it both retards the ejection of Al3+ ions and prevents O2- ions from migrating inward. Thus, the negative- and positive-charge distributions might play a role in the initiation of pores via a flow mechanism.

Weakly Nonlinear Analysis of Flow-Driven Porous Anodic Oxide

Porous anodic oxide (PAO) films are formed by the electrochemical oxidation of metals such as titanium, aluminum, and other metals in acidic solutions. The optimal selection of parameters leads to the formation of submicron-sized pores arranged in a parallel hexagonal array within these films. Porous anodic oxide has enormous industrial applications in the development of functional nanodevices. The present study builds upon the flow-driven porous anodic oxide model proposed by Mishra and Hebert. They have shown through a linear stability analysis of the model that the inter-pore distance in PAO is successfully determined by the competition between stabilizing oxide formation and destabilizing viscous flow. In this work, we investigate the weakly nonlinear analysis of the model to determine the nature of instability arising in flow-driven porous anodic films beyond the stability threshold. For formation of well- developed pores, the nature of instability must be subcritical. However, the results of weakly nonlinear analysis show that the solutions resulting from neutral stability exhibit a supercritical nature for the practical range of anodizing control parameters as shown in Figure 1. Thus, in order to make accurate nonlinear predictions about the formation of pores, it is imperative to incorporate additional physics into the viscous flow model. We also determine the Hadamard stability criteria by tracking the neutral wavenumber to ascertain the regions of well- posedness of the viscous flow model. Figure 1

A controllable fabrication strategy of anodic oxides film with dense, nano-porous and open-top ordered porous arrays morphology on 304 stainless steel in fluoride-based ethylene glycol electrolyte

A facile anodizing strategy without using perchloric acid was reported in this work to controllably produce three different types of self-organized structure anodic oxides films with dense, nano-porous and open-top ordered porous arrays structure morphology. These oxides were developed on 304 austenitic stainless steel (SS) substrate in fluoride-based ethylene glycol electrolyte by simply regulating the H2O concentration in the electrolyte and anodizing voltages. From our analysis, it was found that the above-mentioned anodizing parameters significantly affect the morphologies of the anodic oxides film rather than the chemical composition. The chemical compositions of the different types of films were α-Fe2O3 and Cr2O3 or (FexCr1-x)2O3 composite oxides. Moreover, the higher (70 V), middle (50 V), and lower voltages (30 V) were respectively suitable for the preparation of the dense, nano-porous, and open-top ordered porous arrays morphology structure anodic oxides film due to the different anodized film formation rate, respectively. Finally, a plausible formation-dissolution mechanism was provided by considering the dissolution effect with the assistance of the oxygen evolution derived from H2O, which is favourable to form the sparser anodized film on the SS substrate. In addition, the anodizing parameters selected principle for the corresponding anodic oxides layers with different morphologies onto the SS substrate. The controllable fabrication strategy of the various film will greatly broaden the application of the SS substrate in both anti-corrosion and decoration fields.

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

Self-Healing Coatings Consisting of an Outer Electrodeposited Epoxy Resin Layer and an Inner Porous Anodic Oxide Layer with Healing Agents for the Corrosion Protection of Al Alloys

Recently, new surface treatments for the corrosion protection of Al alloys by forming self-healing layers have attracted the attention of many researchers. The authors of this paper have previously developed self-healing polyurethane coatings with micro-capsules containing healing agents and porous anodic oxide films filled with healing agents. In this study, self-healing coatings consisting of an outer electrodeposited epoxy resin layer and an inner porous anodic oxide layer with healing agents were developed for the corrosion protection of Al alloys. The corrosion protection abilities of the self-healing coating were shown in Cu2+/Cl− solutions after damaging with indenters and were affected by freezing treatments and the tip angles of the indenter.

Optical Fiber Temperature and Humidity Dual Parameter Sensing Based on Fiber Bragg Gratings and Porous Film.

A porous anodic alumina film is proposed to construct an optical fiber temperature and humidity sensor. In the sensor structure, a fiber Bragg grating is used to detect the environment temperature, and the porous film is used to detect the environment humidity. The proposed porous anodic alumina film was fabricated by anodic oxidation reaction, and it is suitable for the use of humidity detection due to its porous structure. Experimental results show the temperature sensitivity of the proposed sensor was 10.4 pm/°C and the humidity sensitivity of the proposed sensor was 185 pm/%RH.

Composition and growth mechanism of nanoporous anodic fluoride films on stainless steel

Anodizing of 304L stainless steel performed in ethylene glycol solution containing 0.1 M NH4F and 0.1 M H2O at constant voltage under static conditions at 5 °C results in the formation of porous anodic films. Several analysis techniques revealed a rather complex composition of the anodic layer for stainless steel compared to that reported in the literature for iron in the same anodizing conditions. Contrary to what might be expected, the anodic layers consist mainly of iron and chromium fluorides rather than oxides. Furthermore, the multilayer fitting of the Rutherford Backscattered spectroscopy shows a decreasing content of chromium and nickel fluorides from the outermost layer to the innermost layer at the metal/film interface, which is composed only of iron fluoride. Film-assisted dissolution mechanisms and the Gibbs-free energy appear to be responsible for the cation distribution and compounds formed throughout the anodic film. In addition, the thickness and final composition of the anodic layer appear to be dependent on the cleaning process carried out after the anodizing.Graphical abstract

Element Distribution in Porous Ga Oxide Obtained by Anodizing Ga in Phosphoric Acid

A STEM/EDS study of a porous Ga oxide film formed by an anodization process was conducted in this study to examine the crystalline structure of the film and the elemental distribution in the oxide film before and after heat treatment. The as-formed anodic film with a morphology resembling the well-known porous anodic Al oxide film was amorphous, crystallizing after heat treatment at 600 °C without changing the morphology and elemental distribution. The EDS elemental maps disclosed the duplex nature of the pore wall oxide; the phosphate anion was contaminated in the outer oxide layer next to the pores, and the inner layer consisted of relatively pure Ga oxide, practically free from phosphate. The similarity of morphology and elemental distributions between the porous anodic Al and Ga oxides suggests that the growth of both anodic oxide films proceeds under the same mechanism. In addition, crystallized porous Ga oxides are expected to be applied to fabricate various functional devices requiring geometrically controlled semiconductor nanohole arrays, such as devices for hydrogen formation.

Oxygen Vacancy-Induced Ferromagnetism and Resistive-Magnetization Switching Characteristics in In2O3 Films

Ordered porous indium oxide thin films (In2O3) with higher ferromagnetism were prepared by the direct current magnetron sputtering method using porous anodic aluminum oxide thin films as the template. The experiment results show that the saturation magnetization reaches a maximum of about 42.03 emu/cm3 at a sputtering pressure of 3.2 Pa. The room-temperature ferromagnetism of In2O3 films mainly originates from the oxygen vacancies in the films. In addition, the spin can generally only be regulated by a magnetic field, which makes it challenging to realize miniaturization and integration of spin devices, while the regulation by an electric field can solve this contradiction. Based on this, the In2O3 composite thin-film resistive-magnetization switching device (Ag/In2O3/PAA/Al) was successfully prepared, showing bipolar resistive-magnetization switching characteristics and stable electrical and magnetic control characteristics. This research provides a way to develop a new multifunctional memory combining magnetic and resistive-magnetization storage and lays the foundation for the application of In2O3 in spintronic devices.

The effect of electrolyte temperature on the growth, morphology, and properties of porous anodic tin oxide films

The effect of electrolyte temperature on the anodic formation of nanoporous tin oxide layers, their morphology and properties was studied in detail. Electrochemical synthesis of SnOx films was carried out in both acidic (H2C2O4) and alkaline (NaOH) electrolytes at various potentials. Differences in morphological features of as-generated films and the kinetics of their formation were carefully investigated from FE-SEM images. The effect of anodizing temperature was successfully verified revealing its undeniable importance when designing the anodization procedure. For instance, both in acidic and alkaline electrolytes, applying too low temperatures results in formation of cracked oxide layers with the least open pores or compact surface, while too high temperatures (≥40 °C) leads to obtaining films with clogged pores due to the partial etching of the material by the electrolyte. Anodizing temperature was also found to be a crucial parameter responsible for the type of morphology of the synthesized materials (compact or porous layers or even crater-like hierarchical structures) when the process is carried out at higher potentials (here 9 V). Finally, we have proved that the impact of electrolyte temperature on the crystallinity of the anodic tin oxide layers is negligible, whereas the photoelectrochemical activity is significantly influenced due to the tight relation with the morphology of as-synthesized materials.

Changes in the Structure and Corrosion Protection Ability of Porous Anodic Oxide Films on Pure Al and Al Alloys by Pore Sealing Treatment

It is well known that corrosion protection of pure Al is enormously improved by the formation of porous anodic oxide films and by pore sealing treatment. However, the effects of anodizing and pore sealing on corrosion protection for Al alloys are unclear, because the alloying elements included in Al alloys affect the structure of anodic oxide films. In the present study, porous anodic oxide films are formed on pure Al, 1050-, 3003- and 5052-Al alloys, and pore sealing was carried out in boiling water. Changes in the structure and corrosion protection ability of porous anodic oxide films on pure Al and the Al alloys by pore sealing, were examined by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). SEM observation showed that anodic oxide films formed on pure Al have a smooth surface after pore sealing, and that cracks are formed in anodic oxide films on 1050-, 3003- and 5052-aluminum alloys, after pore sealing. Corrosion protection after pore sealing increased with anodizing time on pure Al, but only slightly increased with anodizing time on the Al alloys.

The Mechanism Regulating Film Thickness During Porous Anodic Oxide Growth on Aluminium: Transition from Homogeneously Thick Films During Soft Anodizing to Nodule Formation During Hard Anodizing

The growth of porous anodic film during sulphuric acid anodizing has been studied with the aim of investigating the mechanism resulting in uniform film thickness under conventional anodizing conditions and localized film growth under hard anodizing conditions. Based on the field-assisted flow model, it is proposed that the film comprises a rigid part, the cell walls, and a barrier layer plasticized by the combined effect of the electric field, the electrostriction stresses and the ionic migration process. Under conventional anodizing conditions, the film is uniform in thickness because the rigid part regulates the growth rate at the plasticized barrier layer by providing a mechanical connection between neighbouring pores, and hence a mechanical constraint. Vice versa, under hard anodizing conditions, cracking of the rigid part of the film due to rapid film growth rate leads to loss of the regulation mechanism provided by the mechanical constraint and hence localized film growth and formation of nodules.

Energy consumption, wear and corrosion of PEO coatings on preanodized Al alloy: the influence of current and frequency

The influence of current and frequency on energy consumption of plasma electrolytic oxidation (PEO) of a commercial aluminium alloy pre-treated by conventional anodizing in sulphuric acid has been investigated. The combination of a 20-μm thick precursor anodic porous film, high current (500 mA cm−2) and frequency (400 Hz) during PEO enables up to 76% energy savings compared to direct PEO treatment (50 Hz). The time needed to achieve a uniform coating thickness was also reduced from 3350 s to 750 s, for PEO_50 and A + PEO_400, respectively. The wear and corrosion performance of the optimised coatings (400 Hz) were also improved despite the lower thickness of the coatings. Such improvement was mainly attributed to the microstructural refinement associated with high frequency processing and early transaction to the “soft-sparking” regime. The modification of the frequency has a stronger influence on the corrosion response than the presence of the anodic precursor. The best corrosion response was obtained for PEO_400 followed closely by A + PEO_400.

Features of the porous morphology of anodic alumina films at the initial stage of disordered growth

A characteristic feature of disordered porous anodic film growth at the initial stage of aluminum anodizing was revealed by varying the electrolyte type and anodizing voltage. The samples were obtained by the electrochemical oxidation of thin aluminum films (100 nm thick) on SiO2/Si substrates in a 0.3 M oxalic acid at 10–50 V and were studied by SEM. The ImageJ analysis of the images revealed the simultaneous development of two large groups of pores: major pores with a large diameter and minor pores with a smaller diameter. When anodizing in oxalic acid at 10–50 V, it has been shown that the ratio of the diameters of the major and minor pores remains constant and is about 1.17. Using a geometric model, we demonstrated that the centers of the minor pores are located inside the elementary hexagonal cell formed by the centers of the major pores. Moreover, our results are very close to the theoretical value of 2/√3. At the initial stage of disordered pore growth, the development of minor pores rather than major pores is not a random process and is determined by energy-efficient conditions for the development of pores inside the hexagonal cells formed by the major pores. The increase in compressive mechanical stress in the anodic film leads to an interruption in the development of such pores.

Anodising of aluminium in formate solutions with formation of porous alumina arrays

ABSTRACT New applications of porous anodic aluminium oxide (alumina) structures for the fabrication of photonic devices, etching masks, and periodic pore templates with prospective magnetic, photoluminescent, anti-reflecting properties recently are of high interest. In this respect, new anodising processes for controllable formation of periodic structures in unconventional solutions at low voltages might be challenging. In this study, the anodic behaviour of aluminium in aqueous formate solutions was studied within a wide range of concentrations, pH, electrolyte temperatures, anodising times, and voltages. It was established that the thickness of porous anodic films depended on the concentration of solution applied (c), pH, bath temperature (T), and voltage (U a) increasing with increase in c, U a, and T. The thickest films were formed in the solution containing 0.2 mol L−1 formic acid at a pH of 2.5. These films contained up to 5.8 at% of entrapped carbon. The transition of porous anodising to the ‘burning’ type occurred at 22–25 V. Under appropriate anodising conditions ordered hexagonal cell arrays can be formed.

The effect of dissolution by periodic reverse current on the microstructure of bamboo-shaped porous anodic alumina

• Periodic reverse current with small amplitude can promote the growth of the porous layer. • Periodic reverse current has the effect of pore expansion. • The water reduction speed increases during the reverse voltage stage. • The ions driven by the electric field change with the direction of current. • H+ (H2O) and OH− are the main ions that move in the reverse voltage stage. In this paper, the effects of a cyclic anodizing method with the introduction of periodic reverse current are reported. The effects of the periodic reverse current on the microstructure of bamboo-shaped porous anodic alumina (PAA) film are investigated using positive and negative current triangular waveforms. The results of the experiment showed that when the reverse current in each current cycle was I D ' = -5 mA, the growth velocity was the fastest. The analysis found that the introduction of the periodic reverse current was beneficial to increasing the growth velocity of the porous anodic alumina film and had the additional effect of pore expansion. However, with any further increase of the periodic reverse current, the growth velocity of the porous anodic alumina film gradually decreased, and the period length showed a decreasing trend. The main reason for this phenomenon is that in a larger reverse voltage phase, the ions driven by the electric field are mainly H + (H 2 O) and OH − . Then the reaction that occurs is mainly dissolution. With large pore size and adjustable period length, this porous anodic alumina film can be used as a template for the synthesis of nanomaterials, which have practical significance for the synthesis of nanodevices.