Barrier-type Anodic Film Research Articles

The Role of Hydrated Alumina Layer on the Incorporation of Electrolyte Species in Anodizing of Aluminum

Hydration treatment of aluminum is often used for corrosion protection of aluminum alloys and for the formation of dielectric alumina layer in aluminum electrolytic capacitors. Hydrated alumina layer is usually formed by immersing aluminum with and without a porous anodic alumina layer in hot water. The hydrated alumina layer plays an important role in enhancing the corrosion resistance and forming crystalline γ’-alumina in subsequent barrier-type anodic film growth by anodizing. However, the properties of the hydrated layer are not yet well understood. In this study, we examined the influence of anion incorporation properties into the barrier-type anodic alumina films through the hydrated alumina layer, which was formed in hot water.High purity (99.99%) aluminum plate was used in this study. After electropolishing in a HClO4–ethanol mixed solution below 283 K, a hydrated alumina layer was formed by immersing the electropolished aluminum in hot water (95ºC) for 10 min. The resultant hydrated layer consists of two layers, comprising a flake-like outer porous layer and a relatively dense inner layer. The thickness of the hydrated layer was ~430 nm. Then the aluminum with the hydrated layer was anodized in boric acid and tungstate electrolytes to grow the barrier-type anodic films. When the boric acid electrolyte was used, boron species were incorporated into the outer part of the barrier-type anodic films through the hydrated alumina layer. In contrast, no incorporation of tungsten species into the barrier-type anodic film through the hydrated layer occurred in the tungstate electrolyte. Phosphate anions are known to be incorporated into the 70% of the film thickness in the anodic amorphous alumina film, but in the present study, phosphate anions were not incorporated into the barrier-type anodic films through the hydrated layer. It is likely from the GDOES elemental depth profile analysis suggests that the inner hydrated layer becomes a barrier for the incorporation of tungsten and phosphorus species. Thus, apparently, the inner hydrated alumina layer has cation-selective permeation properties so that anion incorporation is limited. In boric acid electrolyte, the acid dissociation is limited, and non-charged boric acid species can be penetrated through the hydrated layer.

Formation of quasi-spherical Au48-198 clusters in anodic titania nanotubes grown on Ti-Au alloys

The effect of alloying sputter-deposited Ti with 2 at.% of Au on the growth of anodic nanotubes was studied in monoethylene glycol electrolyte containing 1.0 mol dm−3 of water and 0.1 mol dm−3 of ammonium fluoride. The classic shape of nanotubes modified with quasi-spherical clusters of Au48-198 was obtained on Ti-Au (2 at.%). The results were compared to the formation of a barrier-type anodic film which suggested that gold located at the alloy/anodic film interface is enriched as a consequence of the preferential oxidation of titanium during the prior anodizing period and transported to the cell boundary region of the nanotubular film in the form of quasi-spherical clusters of Au48-198. A consequence of the inclusion of Au48-198 in the structure of the nanotubes was a reduction in the rate of nanotubular film growth due to the generation of oxygen catalysed on the clusters. A further increase in gold content up to 8 at.% in the alloy resulted in the formation of sponge-like or nanoporous anodic layers, with the structure depending on electrolyte composition.

The correlation between the Na2SiO3·9H2O concentrations and the characteristics of plasma electrolytic oxidation ceramic coatings

Abstract Various plasma electrolytic oxidation (PEO) ceramic coatings were fabricated on aluminum in the electrolytes with different concentrations of Na2SiO3·9H2O. The morphology of fractured cross-section and coating/substrate (C/S) interface, phase compositions, and corrosion behavior of these ceramic coatings were characterized employing field emission scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer (XRD), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The results showed that adding Na2SiO3·9H2O in the electrolyte could reduce the decrease amplitude of the voltage-time curve, and it can affect the types of the anodic film before the beginning of plasma discharges. A novel phenomenon was recorded as PEO treatment was carried out in the electrolyte with 1.85 g l−1 Na2SiO3·9H2O that the substrate was first covered by a barrier-type anodic film, and then a porous-type anodic film appeared after the barrier-type anodic film was broken down. The anodic film continued to grow for a long duration in the PEO process after being broken down. Characteristics of PEO ceramic coatings with a similar thickness acquired in various electrolytes were also evaluated. XRD results indicated that these ceramic coatings of similar thickness were mainly composed of α-Al2O3 and γ-Al2O3, and the α-Al2O3 proportion decreased with increasing Na2SiO3·9H2O concentration. The electrochemical data showed that the corrosion resistance of these PEO ceramic coatings of similar thickness decreased by adding Na2SiO3·9H2O in the electrolyte.

Highly increased breakdown potential of anodic films on aluminum using a sealed porous layer

The growth of a uniform barrier-type anodic film on aluminum is usually terminated by electric breakdown, which is controlled by the resistance of electrolyte or anion concentration. In this study, highly resistive porous layers have been introduced by anodizing aluminum in sulfuric acid electrolyte followed by boiling water treatment to examine their influence on the electric breakdown potential. The pores of the porous alumina film are sealed by forming hydrated alumina (pseudo-boehmite) after the boiling water treatment. The breakdown potential increases to over 1500 V for the pore-sealed aluminum specimens on anodizing in sodium tungstate electrolyte. The electrochemical impedance spectroscopy measurements revealed an increased resistance of the porous layer after the pore-sealing treatment. GDOES depth profile analysis disclosed that the sealed porous layer impedes the incorporation of tungsten species into the barrier layer. The introduction of a highly resistive layer that also suppresses the anion incorporation on aluminum is effective in increasing the breakdown potential of anodic films.

Growth of Barrier Type Anodic Film on Magnesium in Ethylene Glycol-Water Mixed Electrolytes Containing Fluoride and Phosphate

In this study, we report the formation of barrier-type anodic films on magnetron-sputtered magnesium films at a constant current density of 10 A m−2 in ethylene glycol (EG)-H2O electrolytes containing 0.1 mol dm−3 ammonium fluoride and 0.1 mol dm−3 dipotassium hydrogen phosphate. The growth efficiency is close to 100% up to 10 vol% H2O, but decreases to 52% in the EG-free aqueous electrolyte. Even at such a low efficiency in the aqueous electrolyte a uniform barrier-type anodic film with flat and parallel metal/film and film/electrolyte interfaces is developed over 100 V. This is contrast to the non-uniform film growth and low breakdown voltage in the phosphate-free aqueous electrolyte containing ammonium fluoride. The anodic films appear to be amorphous regardless of H2O concentration in the phosphate-containing electrolytes, and consist of phosphate-incorporated oxyfluoride. The phosphate incorporation is suppressed by an increase in H2O concentration. In addition, the anodic films consist of two layers with an inner layer containing less amount of phosphate. The outer layer is probably formed at the film/electrolyte interface by the migration of Mg2+ ions outwards, while the inner layer is formed at the metal/film interface. The film formation at the former interface even in the aqueous electrolyte at low efficiency is likely to contribute to the formation of barrier films, not porous anodic films.

Formation and field-assisted dissolution of anodic films on iron in fluoride-containing organic electrolyte

Magnetron-sputtered iron films were potentiodynamically anodized at two different sweep rates to 50 V in an ethylene glycol electrolyte containing ammonium fluoride and water. At a high sweep rate of 1.0 V s−1, a barrier-type anodic film was formed even though the current efficiency was as low as ∼50%. In contrast, a nanoporous anodic film developed at a low sweep rate of 0.05 V s−1, and the film-formation efficiency reduced to 37%. The main part of the anodic films consists of iron (III) hydroxyfluoride with a thin inner layer composed of FeF3. The inner fluoride layer is formed owing to the faster inward migration of fluoride ions compared to that of the oxygen species. During immersion or re-anodizing of the iron specimen with an approximately 100-nm-thick, barrier-type anodic film at and below 15 V, thinning of the anodic film proceeded uniformly and film dissolution was enhanced by applying an electric field. The impact of the electric field on film formation and dissolution is discussed.

Anodic film growth on Al–Li–Cu alloy AA2099-T8

AA2099-T8 aluminium alloy was anodized at a current density of 5mA/cm2 to selected voltages in 0.1M ammonium pentaborate electrolyte at 293K. It was found that the growth of barrier-type anodic film on the alloy was accompanied by oxidation of intermetallics, formation and rupture of oxygen gas-filled voids in the anodic film and healing of the anodic film at the sites of rupture. It was revealed that the formation of oxygen gas-filled voids was related to the oxidation of copper-rich nanoparticles in the copper-enriched layer at the film/alloy interface, and that the oxidation process of copper-rich nanoparticles depended on grain orientation. Further, the significantly reduced Pilling–Bedworth ratio for formation of anodic lithium oxide compared with that for formation of anodic alumina resulted in the formation of fine voids at the alloy/film interface and sequentially, detachment of the anodic film from the alloy surface.

Effect of grain orientation on the morphology, dielectric breakdown and optical behaviour of anodic film formed on Al–2wt%Cu binary alloy

Barrier-type anodic film growth on an Al–2wt%Cu alloy is shown to be dependent upon the crystallographic orientation of the substrate grains. Generally, on grains of orientations near [1 0 0], featureless amorphous films, with parallel film/electrolyte and alloy/film interfaces, are formed. Conversely, on grains of orientations near [1 1 0] and [1 1 1], anodic films with occluded oxygen-filled voids of different shape, size and population density, and with undulating film/electrolyte and alloy/film interfaces, are generated. It is also revealed that the population density of dielectric breakdown craters of the anodic film is dependent on the crystallographic orientation of substrate grains. Further, the anodized Al–Cu alloy shows a grain orientation dependent coloured appearance. The colouring arises from interference and unbalanced reflection of certain wavelengths of light caused by the thin anodic films of different morphologies.

Incorporation of oxide nanoparticles into barrier-type alumina film via anodic oxidation combined with electrophoretic deposition

The incorporation of metal oxide nanoparticles into a barrier-type anodic alumina film on an Al substrate was investigated using a one-step anodic oxidation combined with electrophoretic deposition. Basically, the electrolysis was carried out in an oxide nanoparticle dispersed ammonium pentaborate solution using an Al substrate as the anode. As a result, the TiO2 and SiO2 nanoparticles with a negative surface charge could be introduced into the alumina film grown on the Al anode, and the film thickness increased by the dispersion of the nanoparticles inside the alumina layer. The measurement of the capacitance and corrosion potential demonstrated that the present method was valid for enhancing the characteristics of the barrier-type alumina film.

Assessment of Proton Transport in Amorphous Aluminum Oxide by Cathodic Polarization : Time of Flight-Elastic Recoil Detection Analysis Method

A study has been made of proton migration in an anodic aluminum oxide film during cathodic polarization in 1 M hydrochloric acid solution at 30°C. The charge transported by proton migration in the oxide film during the polarization was quantitatively determined using time-of-flight-elastic recoil detection analysis. An exponential increase in ionic current with increasing cathodic potential indicated that proton conduction within the barrier-type anodic oxide film follows the high field conduction model. The corresponding parameters were also determined. © 2003 The Electrochemical Society. All rights reserved.

Electroluminescence from Barrier-Type Anodic Oxide Alumina Films Doped with Rare-Earth and Transition Metals by Ion-Implantation

Electroluminescence (EL) during reanodization of a barrier-type anodic oxide alumina film (barrier alumina film) doped with rare-earth and transition metals was observed. To dope the rare-earth and transition metals into the barrier alumina films, an ion-implantation technique was employed. The sharp emissions from the barrier alumina films doped with rare-earth metals were assigned to the 4f–4f transitions of the trivalent elements, with the exception of Ce which was assigned to the 5d–4f transitions. With the doping of multiple elements, the emission spectrum was a mixture of the spectra of all elements, that is, the emission color of each element was mixed to exhibit a certain color.

Anodic Oxidation of Aluminium and Aluminium Alloys. Barrier-Type Anodic Film Growth.

The growth of barrier-type anodic films on aluminium and its alloys is reviewed, with particular consideration of ionic transport processes in the growing anodic films. The films formed are generally amorphous, and grow both at the film/electrolyte interface by outwards migration of cations and at the metal/film interface by inwards migration of ani-ons. Various types of foreign species are incorporated into the anodic films from electrolytes or alloy substrates. The in-corporated species are mobile inwards, or outwards or immobile under the high electric field of 106-107 V cm-2. This provides us with some important information on the complex ionic transport processes in amorphous anodic films. The anodic films formed on uniform Al-valve metal alloys are generally of two layers, due to the different migration rates of A13+ ions and valve metal ions. The properties and growth behavior of the films vary almost linearly with film composi-tion.

Barrier-type anodic film formation on an Al-3.5 wt% Cu alloy

Barrier-type anodic film formation on a bulk Al-3.5 wt% Cu alloy has been examined by analytical transmission electron microscopy. Anodic alumina film formation proceeds in the usual way, with Al 3+ egress and O 2−ingress across the pre-existing film under the high electric field. Copper species are incorporated into the alumina film at the alloy/film interface and have a greater mobility under the field than outwardly mobile aluminium cations; the absence of any enrichment of copper species at the film/electrolyte interface also suggests their direct ejection into the electrolyte. The incorporation of copper species into the alumina film is related to clustering processes at the alloy/film interface which lead to a significant interfacial enrichment of copper and its local oxidation in the form of relatively short-lived copper oxide fingers. The generation of fine clusters and their preferential oxidation, with subsequent reformation as a result of the anodic oxidation of the adjacent aluminium matrix, explain the relatively narrow region of copper enrichment at the alloy/film interface.

Electron-Beam-Induced crystallization of anodic barrier films on aluminium

A study was made of electron-beam-induced crystallization of sections of a detached barrier-type anodic film formed on aluminium in ammonium pentaborate solution. On exposure to the electron beam, telling changes are evident in the section, proceeding more rapidly in a region of the film originally adjacent to the metal-film interface but eventually spreading throughout the film section. The results suggest that the virtually pure alumina region, separated from the electrolyte by the borate-containing region, crystallizes initially and hence this technique becomes a rapid method for direct assessment of apparent transport numbers.

Barrier-type anodic film formation on aluminium in borate solutions

Barrier-type anodic film formation on aluminium in borate solutions

An electronoptical study of the conversion coating formed on aluminium in a chromate/fluoride solution

The development of conversion coatings during the immersion of ‘bare’ aluminium, and aluminium covered by a barrier-type anodic film in a chromate/fluoride solution has been studied by transmission electron microscopy of stripped films and ultramicrotomed sections. The conversion coating produced on aluminium is always relatively uniform in thickness and grows at a rate that decreases rapidly with time. Fine randomly sited pathways through the coating apparently have an important role during film growth. During immersion of the barrier film non-uniform dissolution exposes the substrate metal and at sites where the metal is bared conversion coating formation occurs. Consequently, metal reaction and rapid conversion coating growth occur at these sites before all the adjacent barrier film is dissolved. This leads to a roughening of the surface.