Growth Of Anodic Films Research Articles

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.

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.

Electro-chemo-mechanical properties of anodic oxide (passive) films formed on Cu, Ni and Fe

Abstract Electro-chemo-mechanical properties of anodic oxide (passive) films formed on metals have been reviewed focusing on the results of stress variations caused by anodic oxidation of Cu, Ni, and Fe thin film electrodes in deaerated pH 8.4 borate buffer solution at 25 °C. The surface stress varies toward compressive direction due to adsorption of OH on Cu from aqueous solution as well as adsorption of oxygen on metals from gas phase. The stresses are generated with the growth of three-dimensional anodic oxide films on metals. The magnitude and sign (tensile or compressive) of the intrinsic film stress were determined by taking the residual stress of the substrate and the dielectrostriction into consideration. The tensile or compressive intrinsic film stress depends on p-type or n-type semiconductive properties of the anodic oxide films, which is explained in terms of the void formation or oxide formation in the metal side at the metal/film interface. Furthermore, the stress variation toward compressive direction during cathodic reduction of the anodic oxide films is explained in terms of the volume expansion due to the formation of intermediate species.

Effect of Oxalic Acid Additives on Aluminum Anodizing in Formic Acid Containing Ammonium Heptamolybdate

This paper reports a systematic study of the role of oxalic acid additives in aluminum anodizing in formic acid containing ammonium heptamolybdate. Adding oxalic acid in a concentration range of 5–20 mM to the 0.4 M formic acid solution containing 0.03 M ammonium heptamolybdate improves anodic film growth, increasing the film thickness and smoothing strongly wavy interface between the film and aluminum, and adding 100 mM of oxalic acid results in an almost complete block of the regular anodic film formation. In the case of aluminum anodizing in formic acid, the ammonium heptamolybdate additive prevents aluminum dissolution more effectively than only oxalic acid. The role of oxalic acid in this process is only to improve film growth and morphology. However, ammonium heptamolybdate improves film growth by increasing its thickness. Linear sweep voltammetry studies combined with SEM investigations of alumina growth show that in heptamolybdate-containing electrolytes, a thin porous alumina film is formed at the beginning of the process. Then, when the electrolyte oxidation potential is reached, the thin film on the surface breaks, resulting in a significant increase in the anodizing surface, and anodic oxide begins to grow rapidly.

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.

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.

Микро/наноструктурированные анодно-оксидные покрытия на монокристаллах ниобия

The paper discusses the results of studying of the growth of anodic oxide films (AOFs) by anodizing of niobium single crystals in a fluorine-containing electrolyte (aqueous solution of 10 % H2SO4 + 1 % HF), as well as the effect of saturation of the niobium surface with oxygen on the structure and properties of AOF.

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.

(Europe Section Heinz Gerischer Award) The Long Reach of Electrochemistry: Semiconductors, Metallization and Energy Storage

This talk will attempt to illustrate the long reach of electrochemistry by briefly reviewing our work in a number of different areas including oxygen electrochemistry of platinum metals, various aspects of compound semiconductors and their electrochemistry, electrodeposition of copper metallization and electrochemical kinetics of vanadium redox reactions. The discussion will briefly touch on early work on oxygen evolution on ruthenium and iridium and electrochromism in anodic iridium oxides.1-3 Compound semiconductors will be discussed in a number of aspects from epitaxial crystal growth to electrochemistry. Specific topics will include chemical vapor deposition technology4,5 and electrochemistry at the semiconductor-solution interface including film growth6, oscillatory reactions7 and pore formation on indium phosphide8,9 and photoelectrochemical etching of gallium nitride.10 Aspects of electrodeposited copper metallization will be discussed including in-situ stress measurements and in-situ atomic force microscopy during deposition11 and spontaneous morphology changes during room-temperature aging12. Electrochemical kinetics of vanadium redox reactions on carbon electrodes13 will be discussed as well as other aspects of vanadium flow batteries.14,15 References D. N. Buckley, L. D. Burke, “Lowering of Overvoltage for Oxygen Evolution at Noble Metal Electrodes in the Presence of Ruthenium Salts”, J. Electroanal. Chem. 52, 433 (1974).D. N. Buckley, L. D. Burke, “Oxygen Evolution and Corrosion at Iridium Anodes”, J. Chem. Soc. Faraday Trans. 1, 72, 2431 (1976).D. N. Buckley, L. D. Burke, “Enhancement of Charge Capacity of an Iridium Surface in the Anodic Region”, J. Chem. Soc. Faraday Trans. 1, 71, 1447 (1975).D. N. Buckley, J. R. C. Filipe, F. R. Lineman, K. W. Wang, K. M. Lee, A. A. Westphal, S. M. McEwan and M. A. DiGiuseppe, “Spatial Variations in the Epitaxial Growth of InP and InGaAs by Trichloride VPE and their Physical and Chemical Origins,” J. Electrochem. Soc., 139, 1185 (1992).D. N. Buckley C. W. Seabury, J. L. Valdes, G. Cadet, J. W. Mitchell, M. A. DiGiuseppe, R. C. Smith, J. R. C. Filipe, R. B. Bylsma, U. K. Chakrabarti and K. W. Wang, “Growth of InGaAs Structures using In Situ Electrochemically Generated Arsine”, Appl. Phys. Lett. 57, 1684 (1990).D. N. Buckley, E. Harveyand S. N. G. Chu, “Growth of Anodic Films on Compound Semiconductor Electrodes: InP in Aqueous (NH4)2S”, Monatshefte fur Chemie , 133, 785 (2002)E. Harvey, D. N. Buckley, and S. N. G. Chu, “Oscillatory Behavior during the Anodization of InP”, Electrochem. Solid State Lett, 5 , G22 (2002)C. O'Dwyer, D. Sutton, M. Serantoni, S. B. Newcomb and D. N. Buckley, "An Investigation by AFM and TEM of the Mechanism of Anodic Formation of Nanoporosity in n-InP in KOH", J. Electrochem. Soc. 154, H78-H85 (2007).Robert P. Lynch, Nathan Quill, Colm O’Dwyer, Shohei Nakahara and D. Noel Buckley, "Propagation of Nanopores during Anodic Etching of n-InP in KOH," Phys. Chem. Chem. Phys., 15, 15135-15145 (2013).C. Heffernan, R. P. Lynch and D. N. Buckley, “A Study of the Photoelectrochemical Etching of n-GaN in H3PO4 and KOH Electrolytes”, ECS Journal of Solid State Science and Technology, 9, 015003 (2020).S. Ahmed, T. T. Ahmed, M. O’Grady, S. Nakahara and D. N. Buckley, "Investigation of Stress and Morphology in Electrodeposited Copper Nanofilms by Cantilever Beam Method and In-Situ Electrochemical Atomic Force Microscopy," J. Appl. Phys. 103, 073506 (2008).S. Ahmed, D. N. Buckley, S. Nakahara, T. T. Ahmed, and Y. Kuo, "An Isothermal Annealing Study of Spontaneous Morphology Change in Electrodeposited Copper Metallization," J. Electrochem. Soc., 154, D103 (2007).A. Bourke, M. A. Miller, R. P. Lynch, X. Gao, J. Landon, J. S. Wainright, R. F. Savinell, and D. N. Buckley, “Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of VII-VIII and VIV-VV to Electrochemical Pretreatment”, J. Electrochem. Soc., 163(1), A5097-A5105 (2016); doi:10.1149/2.0131601jesC. Petchsingh, N. Quill, J. T. Joyce, D. Ní Eidhin, D. Oboroceanu, C. Lenihan, X. Gao, R. P. Lynch, and D. N. Buckley, “Spectroscopic Measurement of State of Charge in Vanadium Flow Batteries with an Analytical Model of VIV-VV Absorbance” J. Electrochem. Soc. 163, A5068-A5083 (2016) doi:10.1149/2.0091601jes.D. N. Buckley, D. Oboroceanu, N Quill, C. Lenihan1 and R. P. Lynch, “Modelling and Accelerated Testing of Catholyte Stability in Vanadium Flow Batteries,” J. Electrochem. Soc., 168, 030530 (2021). Figure 1

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.

The effect of argon ion etching treatments on anodic oxide film growth and the electrochemical properties of tantalum

The aim of present paper was to improve the anticorrosive properties of tantalum. Herein, the formation of tantalum oxide film via an argon ion etching pretreatment step and anodic oxidation is reported. The surface morphologies of etched tantalum investigated using scanning electron microscopy. The composition and corrosion resistance of the tantalum oxide film was characterized using X-ray photoelectron spectroscopy and an electrochemical workstation, respectively. The effects of vacancy defects and O atom adsorption on the on Ta (110), (200) and (211) surfaces have been investigated based on first-principles calculations to simulate the etched surface. The experimental results showed that the etched tantalum had a relatively clean surface with fine pores, which promoted the formation of an anodization film to 131.53 nm. The corrosion resistance of tantalum was obviously improved by etching followed by anodization, and the highest corrosion potential of 0.133 V and relatively low corrosion current density of 1.016 μA cm–2 were observed. Furthermore, first principle calculations revealed that vacancy defects increased the activity of the tantalum surface and enhanced the adsorption of oxygen, thereby promoting the formation of oxide film. Among all these defective surfaces, O atom adsorption on the (110) surface exhibited relatively a high adsorption energy of −519.66 kJ/mol and charge transfer of −1.310 e, followed by O atoms on the (200) and (211) surfaces showing an enhanced adsorption energy of −487.33 and −494.27 kJ/mol, and charge transfer of 1.255 and 1.194 |e |, respectively. This work demonstrated an effective argon ion etching pretreatment step to promote the formation of oxide film, which improved the corrosion behavior of tantalum.

Growth of mixed anodic films on combinatorial Al-Gd alloys and their superimposed potential-pH diagrams

• Ultra-thin anodic films are grown on the surface of an Al-Gd combinatorial library. • Thermodynamic calculations identify electrolytes for the growth of anodic films. • Different compositions and pH electrolytes can be used for efficient anodization. • Electrolyte species incorporation in anodic films extend their applications range. • Impedance analysis shows uniform growth of anodic films for all electrolytes. Growth of ultra-thin anodic films on alloys of aluminium and gadolinium having mixed chemistries is demonstrated in aqueous phosphate and borate electrolytes using scanning droplet cell microscopy. The present study reveals that electrolytes having different compositions and pH can be used for the efficient growth of anodic films. Consequently, the applications of anodic films can be extended owing to the modification of film properties by incorporation of various electrolyte species. Dielectric characteristics of films are examined by using electrochemical impedance measurements, which disclose a linear increase in inverse capacitance with formation voltage suggesting the uniform growth of anodic films irrespective of electrolyte selection. Oxide formation factor, k , greater than unity was obtained in selected electrolytes.

Passivity of Holmium Studied Theoretically by Potential-pH Diagrams for Selection of Electrolytes and Experimental Proof of the Formation of Ultra-Thin Anodic Films

We present an efficient strategy to identify anodizing electrolytes by coupling conventional Eh–pH diagrams with first-principles density functional theory calculations. Herein, the growth of ultra-thin films having a thickness of 10–20 nm is successfully demonstrated on thermally evaporated holmium. Considering only thermodynamic basis, simulated Eh-pH diagrams, and solubility analysis, electrolytes having different compositions and pH values are suggested for the efficient growth of anodic films. The Eh–pH diagrams are modified by incorporation of oxide–forming species in such a way to appreciably extend the stability domain. The predicted diagrams showed a strong agreement with the experimental observations and provide a better understanding of Ho–H2O based aqueous systems and can serve as a guide in other rare-earth elements anodizing. Grown film properties are investigated by using electrochemical impedance studies which disclose a linear increase in inverse capacitance with formation voltage indicating a uniform growth of anodic films irrespective of electrolyte selection. Formation factor, k, of anodic films grown up to 10 V varies from 1.16 to 1.95 nm V −1 in selected electrolytes. The k values greater than unity may contribute to the uniform film growth at different pH values.

Mixed oxide growth on combinatorial aluminium\u2013gadolinium alloys \u2014 a thermodynamic and first-principles approach

Metal surfaces covered with oxides have attracted considerable scientific attention in various applications. In particular, anodic films fabricated by cost-effective anodizing have been widely used in nano-structured engineering to provide various surface functionalities. However, understanding of alloy film stability, having individual elements with widely varying structures and morphologies, is very limited due to lack of thermodynamic information and effects of electrolyte chemistry. This requires many tedious efforts on a trial and error basis in selecting suitable electrolytes that can produce the protective film at high efficiency on alloys having mixed chemistries. It is, therefore, crucial to develop a combination of high throughput theoretical analysis and automated rapid localized electrochemical probing that provides a fast and simple solution for electrolyte choice and paves the way to the remarkable expansion of industrial applications of oxides. Herein, we demonstrate that combinatorial Al–Gd alloys covering 1.0 to 10.0 at.% Gd can be oxidized into ultra-thin anodic films of controlled thickness through a selection of electrolyte based on thermodynamics (phosphate buffer with a pH of 8.20). We propose that growth of anodic films on alloys at high efficiency is possible if Gibbs free energy minimization criteria would be systematically contemplate.Graphical abstract

Anodization of titanium in reduced graphene oxide-citric acid electrolyte

Crystalline oxide film coated pure titanium has been fabricated by anodizing in citric acid (C6H8O7) solution containing reduced graphene oxide (RGO) electrolyte. The voltages ranging from 10 to 90 V were used during the anodization process. The results demonstrate that the growth rate and thickness of barrier layer decrease with the increase of applied voltage. The anodic oxide film mainly consists of anatase TiO2. The sample anodized at 50 V has the highest crystallinity, which is caused by the high lattice oxygen (OL2−) content on the sample surface. When the applied voltages are 10–50 V, the samples exhibit excellent corrosion resistances.Combiningmicrostructure characteristics, it is concluded that the growth of anodic oxide film in the uniformly mixed C6H8O7 and RGO electrolyte is more favorable under low voltages 10–50 V.

Similarity and Difference in the Plating Pretreatment for Mg and Al Alloys

Mg and Al are light-weight less-noble metals and are expected as a key material for potential applications in the fields of transportation to save energy. Especially for Mg alloys, the addition of corrosion resistance is essentially important for outside use. Plating has been used to provide various functionalities to metals such as good appearance, wear resistance, sliding performance, wettability or repellency, and corrosion resistance. Both Mg and Al have similar difficulty in the plating process because the surface of metals is always covered with the passive film (Al) or corrosion film (Mg) that prevents the plating process as the electron transfer and the metal-to-metal bonding. In the plating process, therefore, such surface films should be removed before or during the treatment process. The zincate pretreatment has been applied to these metals in which the surface was replaced with Zn layer that provides a good substrate for the subsequent plating process. In the zincate process, Al or Mg surface is dissolved, and Zn is deposited simultaneously by the substitution reaction in the zincate bath containing Zn(II) ions to cover the substrate. In the case of Al, this substitution reaction proceeds rapidly, and the substrate surface is almost completely covered with Zn layer. In the case of Mg, however, the coverage with Zn is more important than Al because Mg can be rapidly dissolved in the zincating bath. Excess dissolution of Mg substrate causes the undesired poor adhesion of the plating film to the substrate. Therefore, the zincating condition for Mg alloys should be optimized to take a balance between the Mg dissolution and the substitutional deposition of Zn. On the other hand, a complicated structure composed of the area where dense pillars are formed and a bank surrounding this area is sometimes found on AZ31 alloy after the zincate treatment, as shown in Fig. 1. Such a structure is formed by the combination of the continuous dissolution of Mg substrate and the local deposition of Zn that protects Mg substrate from the dissolution. A bank is formed on the grain boundary of AZ31 surface, where initial Zn deposition preferentially proceeded. Such micro-structure can be compared with a well-known ordered porous alumina film formed on Al by the dynamic process of the continuous growth of anodic oxide film and mass transportation of ions in the growing film. Figure 1

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.

The influence of electrolyte usage on the growth of nanostructured anodic films on copper in sodium carbonate aqueous solution

Anodization of copper was performed in 0.1 M K2CO3 at 25 and 50 V. In order to study the influence of the electrolyte usage, samples were anodized in fresh electrolyte and electrolyte that was applied for anodizing for 2, 4 and 6 h. It was found that during anodization, the current oscillates with time. It means that there are cycles of adsorptions and delaminations of the anodic film of the samples. Moreover, the longer the electrolyte is used, the faster steady state of current densities without oscillations is achieved and the greater the current densities are. According to the Pourbaix diagram, the greater concentration of the water soluble copper species, the greater the CuO passivity field, thus the anodic oxides are formed much easier. For this reason, X-ray diffraction patterns revealed increasing content of tenorite phase (CuO) over cuprite (Cu2O) when the electrolyte is being re-used. For electrolytes pre-used for 6 h, no cuprite was found in the samples. Moreover, the longer the pre-usage of the electrolyte, the smaller the diameter of the grown nanowires. For anodizing performed at 25 V, nanowire diameter decreases from 55 ± 12 nm to 36 ± 7 nm (electrolyte pre-used for 6 h) in a linear manner.

Hard anodization of copper in potassium carbonate aqueous solution

Anodization of copper was performed in potassium carbonate aqueous solution. Registered current curves allowed to claim that the anodic oxide film growth occurs according to the hard anodization mechanism. Detailed investigation of the anodization products revealed formation of nanowires made of crystalline phases including various oxides and complex phases containing hydroxyl anions and carbonates. It was found that the diameter of the nanowires grows linearly with the anodizing voltage between 30 (10 V) and 47 nm (50 V). The major, crystalline components of the anodically grown nanowires were: paramelaconite (Cu4O3), cuprite (Cu2O) and tenorite (CuO). The presence of malachite, Cu2CO3(OH)2 was also reported and meant effective incorporation of the electrolyte anions into the growing anodic film.

Determining the electric-field strength in a passive film via photo-induced electric fields

The nature of the electric-field strength in the passive film on tungsten is explored using photo-electrochemical techniques. A theoretical expression for the photo-stimulated growth of the film has been derived. Rotating ring-disk electrode (RRDE) experiments indicate that photo-corrosion of a tungsten electrode is negligible under super-band gap light illumination. XPS results show that tungsten is in the maximum possible oxidation state (VI) in the film and hence no higher oxidation state is available. The photo-stimulated transient film growth of tungsten in pH 8.5 ± 0.1 boric-borax buffer was recorded as a function of film formation potential (1 VSCE, 2 VSCE, 3 VSCE, 4 VSCE, 5 VSCE, 6 VSCE) and light intensities (50, 200, 1000 mW/cm2). Steady state passive film thickness measurements of the passive film on tungsten indicate that super band gap, UV light suppresses the electric field in the barrier layer, and hence stimulates anodic oxide film growth. The data obtained in this study demonstrate that the electric field strength in the steady state is independent of the applied potential and film thickness, as postulated in the Point Defect Model.