Formation Of Anodic Oxide Films Research Articles

Periodic Oscillations of Luminescence Intensity at the Formation of Anodic Oxides on Aluminum in Oxalic Acid

The electroluminescence phenomenon observed during the formation of porous anodic oxide films on aluminum in oxalic acid solutions is studied. Periodic changes in the luminescence intensity arise during the transition from the barrier-film formation to the steady-state growth of porous oxide. The period and number of oscillations are determined by the oxidation voltage.

A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys

Transmission electron microscopy (TEM) has been employed to examine anodic oxide film formation on 99.8 wt.% aluminium, Al–10 wt.%Si and Al–10 wt.%Si–3 wt.%Cu alloys under conditions relevant to hard anodizing. In particular, anodic oxidation of silicon particles proceeded at a significantly reduced rate compared with that of the adjacent aluminium matrix. This gave rise to alumina film encroachment beneath the particles with development of tortuous porosity and, eventually, occlusion of partially anodized particle in the anodic film. Additional effects included the presence of gas-filled cavities above the silicon particles, associated with oxygen generation above the anodizing particle. The presence of such particles and the corresponding gas-filled voids across the anodic film thickness and at the alloy/film interface is considered responsible for the continuous voltage rise during anodizing of the Al–10 wt.%Si alloy, effectively blocking electrolyte access to the pore base and providing local region of high resistance at the alloy/film interface. A direct consequence of the voltage rise was a thickening of the barrier layer at the base of the porous anodic film. For the ternary alloy, with the additional presence of copper and the CuAl2 particles, the latter appear to have undergone complete oxidation, with copper detected in local film regions.

A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys

Transmission electron microscopy (TEM) has been employed to examine anodic oxide film formation on 99.8 wt.% aluminium, Al–10 wt.%Si and Al–10 wt.%Si–3 wt.%Cu alloys under conditions relevant to hard anodizing. In particular, anodic oxidation of silicon particles proceeded at a significantly reduced rate compared with that of the adjacent aluminium matrix. This gave rise to alumina film encroachment beneath the particles with development of tortuous porosity and, eventually, occlusion of partially anodized particle in the anodic film. Additional effects included the presence of gas-filled cavities above the silicon particles, associated with oxygen generation above the anodizing particle. The presence of such particles and the corresponding gas-filled voids across the anodic film thickness and at the alloy/film interface is considered responsible for the continuous voltage rise during anodizing of the Al–10 wt.%Si alloy, effectively blocking electrolyte access to the pore base and providing local region of high resistance at the alloy/film interface. A direct consequence of the voltage rise was a thickening of the barrier layer at the base of the porous anodic film. For the ternary alloy, with the additional presence of copper and the CuAl 2 particles, the latter appear to have undergone complete oxidation, with copper detected in local film regions.

Anodic oxide film formation on antimony and its currentless dissolution in sulphuric acid containing some monocarboxylic acids

Galvanostatic anodization of antimony was investigated in 0.05 M sulphuric acid containing some monocarboxylic acids such as formic, acetic and propionic acid. The results show that the formation rate of the anodic oxide film on antimony is a function of the monocarboxylic acid concentration and increases in the order formic<acetic<propionic acid. The dissolution mechanism of the anodic oxide film in sulphuric acid containing the above mentioned monocarboxylic acids is a first order reaction. The dissolution rate of the oxide film decreases with increase of the concentration and chain length of the monocarboxylic acid and increases with increase of the dissolution temperature. The activation energy for the dissolution of the oxide film is found to be 27±2 kJ mol −1.

The Electrochemical Properties and Mechanism of Formation of Anodic Oxide Films on Mg-Al Alloys

The electrochemical properties and the mechanism of formation of anodic oxide films on Mg alloys containing 0-15 mass% Al, when anodized in NaOH solution, were investigated by focusing on the effects of anodizing potential, Al content, and anodizing time. The intensity ratio of Mg(OH)₂ in the XRD analysis decreased with increasing applied potential, while that of MgO increased. Mg(OH)₂ was barely detected at 80 V, while MgO was readily detected. The anti-corrosion properties of anodized specimens at each constant potential were better than those of non-anodized specimens. The specimen anodized at an applied potential of 3 V had the best anti-corrosion property. The intensity ratio of the β phase increased with aluminum content in Mg-Al alloys. During anodizing, the active dissolution reaction occurred preferentially in β phase until about 4 min, and then the current density increased gradually until 7 min. The dissolution reaction progressed in α phase, which had a lower Al content. In the anodic polarization test in 0.017 mol·<TEX>$dm^{-3}$</TEX> NaCl and 0.1 mol·<TEX>$dm^{-3}$</TEX> Na₂SO₄ at 298 K, the current density of Mg-15 mass% Al alloy anodized for 10 min increased, since the anodic film that forms on the α phase is a non-compacted film. The anodic film on the α phase at 30 min was a compact film as compared with that at 10 min.

Three-dimensional Microstructure Fabrication with Aluminum Anodizing, Laser Irradiation, and Electrodeposition

Three-dimensional microstructures made of Ni metal or acrylic resin were fabricated by five sequential processes: porous anodic oxide film formation, pore sealing, laser irradiation, Ni electroplating or electrophoretic deposition of acrylic resin, and removal of the aluminum substrate and anodic oxide films. Cylindrical and prismatic aluminum rods were anodized in an oxalic acid solution to form porous-type anodic oxide films, and then immersed in boiling distilled water for pore sealing. The anodized and pore-sealed specimens were irradiated with a pulsed neodymium-doped yttrium aluminum garnet (Nd-YAG) laser beam in a Ni plating solution or doubly distilled water to remove anodic oxide film locally by rotating and moving up/down with an XYZθ stage. Nickel or acrylic resin was deposited at the area where film had been removed by cathodic or anodic polarization in the solution before removing the aluminum substrate and anodic oxide films in NaOH solutions. Cylindrical and prismatic network cages, rings, springs, and bellows made of Ni metal or acrylic acid resin were fabricated successfully. © 2003 The Electrochemical Society. All rights reserved.

Effect of hydrogen on stresses in anodic oxide film on titanium

Stresses in anodic oxide film on titanium thin film/glass electrode in pH 8.4 borate solution were investigated by a bending beam method. The increases in compressive stress observed with cathodic potential sweeps after formation of anodic oxide film were attributed to the volume expansion due to the compositional change of anodic oxide film from TiO 2 to TiO 2− x (OH) x . The instantaneous responses of changes in stress, Δ σ, in the anodic oxide film to potential steps demonstrated the reversible characteristic of the TiO 2− x (OH) x formation reaction. In contrast, the transient feature of Δσ for the titanium without anodic oxide film represented the irreversible formation of TiH x at the metal/oxide interphase. The large difference in stress between with and without the oxide film, has suggested that most of stresses generated during the hydrogen absorption/desorption reside in the anodic oxide film. A linear relationship between changes in stress, Δ(Δ σ) des, and electric charge, Δ Q des, during hydrogen desorption was found from the current and stress transients, manifesting that the stress changes were crucially determined by the amount of hydrogen desorbed from the oxide film. The increasing tendency of −Δ(Δ σ) des with increasing number of potential steps and film formation potential were discussed in connection with the increase in desorption amount of hydrogen in the oxide film with increasing absorption/desorption cycles and oxide film thickness.

ゾル・ゲル法によりＳｉＯ２を被覆したアルミニウムのアノード酸化 新規な高耐電圧アノード酸化皮膜の作製

Aluminum specimens were covered with SiO2 film by a sol-gel coating and then anodized galvanostatically in a boric acid solution. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of anodic oxide films were examined by TEM-EDX, and electrochemical impedance measurements. It was found that anodizing of aluminum coated with SiO2 films leads to the formation of anodic oxide films that consist of an outer Al-Si composite oxide layer and an inner Al2O3 layer at the interface between the SiO2 film and the metal substrate. The breakdown potential of anodic oxide films formed on specimens with SiO2-coating was about 100 V higher than that without SiO2 coating. The capacitance of specimens after sol-gel coating and anodizing was slightly higher than that without sol-gel coating. In the film formation mechanism, the conversion of Al2O3 into Al-Si composite oxide at the interface between the inner and outer layers is discussed in terms of inward transport of Si-bearing anions across the outer layer.

Anodic oxide films on nickel electrode in borate solutions

The effect of different concentrations of sodium borate, current densities, pH and temperature on the anodic oxidation of nickel was studied using galvanostatic polarization technique. The anodic potential-time curves showed four regions: the charging of the anodic double layer, an arrest corresponding to anodic dissolution and/or oxide film formation, linear rise in the potential due to the formation of barrier oxide film and finally deviation from linearity to reach steady-state potential value attributed to oxygen evolution reaction. The duration time of the arrest decreases with increasing current density and concentration of borate anion while the rate of oxide film formation increases. On the other hand, the duration time of the arrest increases with increasing pH and temperature of the solution while the rate of oxide film formation decreases. Increasing the borate concentration and pH of the solution shifted the starting potential of the arrest towards more negative values. Linear relationships were obtained between the starting potential of the arrest and both borate concentration and pH.

Kinetics of formation of complex anodic oxide films on aluminium

The kinetics of formation of “complex” anodic films on aluminium obtained by an electrochemical filling the porous alumina films in an aqueous borate electrolytic solution has been investigated. Complex anodic oxide films with entirely or partially filled pores depending on the porous matrix thickness were obtained. The slopes of the kinetic curves (∂ U f/∂ Q f) during a re-anodization as a function of the thickness of porous films formed in H 2SO 4 and (COOH) 2 are presented. The electric field strength values through the filling porous films were calculated. Three types of equations were used to calculate the ionic current as a function of the electrical field strength. The values of two important kinetic characteristics of the filling process (the electrical charge passed for the whole filling the porous film, and the re-anodization voltage reached at the moment of an entire filling the film) were determined. The values of these characteristics are plotted vs. the thickness of porous alumina films.

Anodizing of Aluminum Coated with Silicon Oxide by a Sol-Gel Method

Aluminum specimens were covered with film by a sol-gel coating and then anodized galvanostatically in a neutral borate solution. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of the anodic oxide films were examined by transmission electron microscopy, Rutherford backscattering spectroscopy, and electrochemical impedance measurements. It was found that anodizing of aluminum coated with films leads to the formation of anodic oxide films, which consist of an outer Al-Si composite oxide layer and an inner layer, at the interface between the film and the metal substrate. The capacitance of anodic oxide films formed on specimens with a coating was about 20% larger than without a coating. In the film formation mechanism, the conversion of to Al-Si composite oxide at the interface between the inner and outer layers is discussed in terms of inward transport of Si-bearing anions across the outer layer. © 2001 The Electrochemical Society. All rights reserved.

Formation and Electric Properties of Anodic Oxide Films on Al-Nd Sputtered Films in Nonaqueous Electrolyte Solutions

Sputtered Al-Nd alloy films were anodized in nonaqueous electrolyte solutions at various current densities, on which the amounts of incorporated carbon species into the oxide films depend. The resultant oxide films were characterized by Fourier transform infrared (FTIR), cross-sectional transmission electron microscopy (XTEM), and secondary-ion mass spectroscopy (SIMS). Electrical properties such as relative permittivity and dielectric strength were also measured for these films. The FTIR peak position associated with Al-O stretching vibration was correlated with the relative permittivity and dielectric strength of the anodic oxide film. Since the peak position is dependent on the condition of anodization, FTIR measurement will be useful in developing high insulating oxide films for microelectronic devices. © 2001 The Electrochemical Society. All rights reserved.

Formation of Composite Oxide Films on Aluminum by Sol-Gel Coating and Anodizing —For the Development of High Performance Aluminum Electrolytic Capacitors—

Aluminum coated with ZrO2, SiO2, and BaTiO3 films by a sol-gel dip coating was anodized to examine the structure and dielectric properties of anodic oxide films. Anodizing leads to the formation of anodic oxide films, which consist of an outer Al- (Zr, Si, BaTiO3) composite oxide layer and an inner Al2O3 layer, at the interface between the coated-oxide layer and Al substrate. The composite oxide converted into Al2O3 at the interface between the outer and inner layers during anodizing for the ZrO2- and BaTiO3-coated specimens, whereas Al2O3 converted into Al-Si composite oxide for the SiO2-coated specimens. The capacitance of the anodic oxide films formed on ZrO2- and SiO2-coated specimens was about 20% larger than that of anodic oxide films on aluminum without coating, and the capacitance for BaTiO3-coated specimens was almost identical to that without coating. Film formation mechanisms are discussed in terms of inward transport of Si-bearing anions and outward transport of Zr- and Ti-bearing cations across the composite oxide layer.

Photoelectrochemical Study of Oxide Films on Copper-Cobalt Alloys

The formation of anodic oxide films on Cu–Co (5 to 40 wt %) alloys in borate buffer solutions is studied by voltammetry and photocurrent measurements. It is shown that cobalt oxide Co2O3 is photosensitive; the formation of oxide films on the alloy surface and their composition depend on the alloy composition and the solution temperature.

Oxidizing valve metals: effect of electronic properties of the oxide films

Photocurrents emerging during the formation of anodic oxide films (AOF) on such valve metals as W, Ti, Zr, Nb, Ta are measured during an increase (direct run) and a decrease (reverse run of voltammetric curves) in the anodic potential. Capacitances of AOF formed at certain potentials are measured at potentials below the AOF formation potential. Effect of semiconductor properties on the AOF growth is considered through the formation of a Schottky barrier at the oxide/electrolyte interface. Calculated thicknesses of AOF and the depleted layer are compared. The donor-concentration drop in AOF with the distance from the metal/oxide interface is a condition for the growth of thick semiconductor oxide films. The measured potential dependence of the semiconductor-film capacitance is used to plot the donor concentration drop as a function of the distance from the Nb2O5/Nb interface in the oxide layer on a niobium electrode.

The formation and dissolution of anodic oxide films on pure aluminium in alkaline solution

The mechanisms of formation and dissolution of anodic oxide films on pure aluminium rotating disc specimens in alkaline solution have been investigated using galvanostatic anodizing, potentiostatic anodizing, potential drop experiments and impedance spectroscopy. It was found that in alkaline solution the strength of the applied electric field accelerates the formation of the anodic oxide film, while it retards its dissolution, indicating electrochemical formation of the film and a complete lack of its field-assisted dissolution. The rate of growth and the thickness of the anodic oxide film were observed to decrease with an increasing rate of specimen rotation as a result of accelerated chemical film dissolution due to rapid removal of aluminate ions from and fast supply of hydroxide ions towards the film/solution interface.

Effect of ferrous ion in solution on the formation of anodic oxide film on iron

The effect of Fe 2+ ion in the solution on the anodic formation of the passive oxide film onto iron was investigated by the ellipsometric trace of the film growth in a borate solution at pH 8.4. In addition to a compact inner passive film, formed by initial direct oxidation, the Fe 2+ ion in the solution is oxidized to build up an outer hydrous layer at the latter stage. The inner layer behaves similarly as the usual passive film, in which the film growth follows the ionic migration mechanism under high electric field. The outer layer growth may, however, be determined by a surface reaction between the film and the electrolyte solution, i.e., the growth rate is independent of potential and a linear function of the Fe 2+ ion concentration in the solution.

Inter–relationships between ionic transport and composition in amorphous anodic oxides

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

ChemInform Abstract: Formation and Breakdown of Anodic Oxide Films on Aluminum in Boric Acid/Borate Solutions.

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Formation of amorphous anodic oxide films of controlled composition on aluminium alloys

Binary, non-equilibrium Al-29at%Nb, Al-44at%Ta, Al-19at%Ti, Al-25at%Ti and Al-32at%Zr alloys were prepared by magnetron sputtering and subsequently anodized at high Faradaic efficiency to grow barrier-type anodic films. Examination in the transmission electron microscope revealed amorphous anodic films of relatively uniform compositions across the film thicknesses, except for a layer of relatively pure alumina, of about 5% of the film thickness, present at the film/electrolyte interface of the Al-Ta alloy. The film compositions, from Rutherford backscattering spectroscopy, indicate that the alloy constituents are oxidized in their alloy proportions to form films comprising intimately mixed units of the various oxides, namely alumina, niobia, titania, tantala and zirconia. The films grow by co-operative transport of metal and oxygen ions under the electric field with formation of film material by both migration of metal ions outwards and of oxygen, and possibly hydroxyl, ions inwards. The average migration rates of Al 3+, Nb 5+, Ti 4+ and Zr 4+ ions are similar, to within 10%, but Ta 5+ ions migrate more slowly than Al 3+ ions. The results of the study show that a wide range of compositions of amorphous oxide films can be readily formed by anodic oxidation of appropriate alloys, including compositions containing units of normally crystalline anodic oxides, namely TiO 2 and ZrO 2.