Formation Of Anodic Films Research Articles

X-ray Photoelectron Spectroscopic Study on the Anodic Passivity of Sputter-deposited W-Nb Alloys in 12 M HCl Solution

The effect of a simultaneous additions of tungsten and niobium on the anodic passivity of the surface films of the sputter-deposited amorphous/nanocrystalline W-xNb alloys was studied using corrosion tests, electrochemical measurements and X-ray photoelectron spectroscopic (XPS) analyses. The formation of spontaneous passive film on the alloys, which is composed of double oxyhydroxides of Wox and Nb5+ ions, is responsible for their higher corrosion resistance than those of the alloy-constituting elements in 12 M HCl solution open to air at 30°C. The quantitative surface analysis by XPS clarifies that the improved anodic passivity of the alloys than those of alloy-constituting elements is based on the formation of new double oxyhydroxide anodic films composed of W6+ and Nb5+ ions. These anodic passive films have higher protectiveness and stability than those of the anodic passive oxyhydroxide films of alloy-constituting elements, that is, oxyhydroxides of hexavalent tungsten and pentavalent niobium after potentiostatic polarization for 1 h in 12 M HCl at 30°C.Keywords: W-Nb alloys; Sputter deposition; Polarization curve; Passivity; Surface analysis.© 2011 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi:10.3329/jsr.v3i3.7207 J. Sci. Res. 3 (3), 467-480 (2011)

Morphological Instability Leading to Formation of Porous Anodic Oxide Films

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A Mechanism of Anodic Film Formation in Molten Melts Using Preformed Porous Film Formed in 0.6 M Oxalic Acid Electrolyte

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Area Selective Formation of Porous Type Aluminum Anodic Oxide Film by a Solution Flow-Type Micro Droplet Cell

A novel solution flow-type micro droplet cell with co-axial dual capillary tubes (Sf-MDC) was applied to form porous type aluminum anodic oxide film locally. This technique makes it possible to form porous type anodic oxide film on aluminum locally. When the droplet cell remains stationary, the oxide film was growing both vertically and laterally. The growth rate in the lateral direction can be controlled by moving the droplet cell. By moving the droplet cell, a porous type anodic oxide film line was successfully formed. The thickness of the oxide film increases with decreasing scanning speed of the cell. The oxide film growth rate was greatly affected by specimen temperature.

Formation of nanoporous anodic films on pure aluminum in sulfuric acid

The effect of the duration of electrochemical anodic oxidation on the formation of nanoporous anodic oxide films on high-purity aluminum (99.99%) in a sulfuric acid solution is studied. The resulting oxide film is analyzed using an atomic force microscope. The method of photoelectric polarization (PEP) and X-ray analysis revealed that these films are sesquioxides of aluminum with variable compositions, nonstoichiometry due to a metal deficiency, and p-type conductivity. The mechanism of the formation of a regular nanoporous oxide structure that results from the nonstoichiometry relation and the intrinsic formation of defects in the oxide is proposed.

Kinetics and Mechanism of the Electrochemical Formation of Iron Oxidation Products on Steel Immersed in Sour Acid Media

From electrochemical techniques (cyclic voltammetry, potential steps, and EIS), XRD, and SEM-EDX, the kinetics and mechanism of anodic film formation applying anodic potential steps on steel immersed in sour acid media was determined. It was found, from a thermodynamic analysis, based on equilibrium phase diagrams of the system considered in this work, that iron oxidation may produce different new solid phases, depending on the applied potential, the first being the iron oxidation associated with formation of FeS((c)) species, which in turn can be reoxidized to FeS(2(c)) or even to Fe(2)O(3(c)) at higher potential values. From analysis of the corresponding experimental potentiostatic current density transients, it was concluded that the electrochemical anodic film formation involves an E(1)CE(2) mechanism, whereby the first of the two simultaneous processes were the Fe electrochemical oxidation (E(1)) followed by FeS precipitation (C) that occurs by 3D nucleation and growth limited by mass transfer reaction and FeS oxidation (E(2)) forming a mix of different stoichiometry iron sulphides and oxides. From EIS measurements, it was revealed that the anodic film's charge transfer resistance diminishes as the potential applied for its formation becomes more anodic, thus behaving poorly against corrosion.

A rotating tungsten disc electrode in concentrated strong alkaline solutions: An electroanalytical aspect

Electrochemical behavior of tungsten in concentrated strong alkaline solutions (10−3–10−1M KOH, 11<pH<13) was studied by means of rotating disc electrode technique. Contrary to the case of less concentrated alkaline solutions, the formation of solid anodic passive film was not evidenced. Chronoamperometric measurements evidenced a fast establishment of steady state conditions in the surface region. In spite of the complexity of overall surface electrochemistry, a linear relationship between the limiting anodic currents and the concentration of OH− ions was evidenced, which may be used for easy and accurate determination of OH− ion concentration at high pH values where the glass electrode loses its high reliability.

AZ91 Magnesium Alloys: Anodizing of Using Environmental Friendly Electrolytes

An anodizing process, based on environmental friendly electrolyte solutions has been studied on AZ 91 magnesium alloys by using three types of electrolytes: the first is based on sodium silicate, the second on sodium hydroxide-boric acid-borax and the third on sodium silicate- potassium hydroxide-sodium carbonate-sodium tetra borate. A pretreatment including fluoride activation was applied before the anodizing process. It was found that the anodic film thickness increases as current density or anodizing voltage increases. It is also increased with deposition time until the deposition stops due to the formation of a thick anodic film. Optimization of the anodizing conditions - current density and deposition time- was made for each electrolyte. Characterization of anodizing layer was achieved by determination of surface morphology, microstructure, phase analysis, coat thickness, adhesion and corrosion resistance. In all cases, excellent adhesion and corrosion resistance was obtained. A corrosion efficiency ranging from 94% to 97% was reached; the highest value corresponding to the third electrolyte.

Anodic Film Formation on AA 2099-T8 Aluminum Alloy in Tartaric–Sulfuric Acid

The anodizing behavior of a lithium-containing aluminum alloy (AA 2099-T8) in an environmentally friendly electrolyte, namely tartaric–sulfuric acid (TSA), has been examined under potentiodynamic and potentiostatic conditions. Specifically, the dependence of the anodic film morphology and composition on the anodizing voltage was investigated. It is revealed that porous anodic films with well-defined cells were formed at relatively low voltages while porous anodic films with pores of increased dimensions and lateral porosity were formed at increased voltages. In addition, it is indicated that copper in the alloy matrix can be occluded in the anodic film material as copper-rich nanoparticles or it can be oxidized and incorporated into the film material as copper ions, depending on the anodizing voltage. In the latter case, the process is accompanied by oxygen gas generation within the film material, resulting in the lateral porosity in the anodic film. Further, the structures of the copper-rich nanoparticles have been determined and the mechanism of the formation of such nanoparticles has been discussed.

Analysis of the impact of alloy microstructural properties on passive films formed on low-C 13CrNiMo martensitic stainless steels

Thermal cycles during welding lead to inhomogeneous conditions in the chemical composition and microstructure of both weld metal and parent metal. These properties are directly related to pitting corrosion susceptibility of low-C 13CrNiMo martensitic stainless steels via the stability of their passive films. This work describes how passive film properties vary according to the substrate microstructure and allows interpreting the way in which these changes impact on pitting resistance. Thickness and structure of the anodic films on low-C 13CrNiMo martensitic stainless steels are markedly influenced by heat treatments previously applied to the alloys. Higher retained austenite amounts preserve higher Cr contents and lead to thicker anodic films. This beneficial microstructure is also related to higher rates of anodic film formation.

Formation and dielectric properties of anodic oxide films on Zr–Al alloys

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm−3 ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO2 with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO2 phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.

Incorporation and Migration of Phosphorus Species in Anodic Alumina Films Containing Tungsten Tracer Layers

Al–W nanolayers have been used previously to investigate the mechanism of formation of porous anodic films on aluminum in phosphoric acid. The present study considers the interactions of the tungsten tracer species and phosphorus species in the films and the consequence to the transport of the tracer. The findings showed that phosphorus species migrate inward and pass through the nanolayer of outwardly migrating tungsten species. Further, the migration behaviors indicated negligible influence of either species on the migration of the other.

ChemInform Abstract: A Photocurrent Spectroscopic Study of the Initial Stages of Anodic Oxide Film Formation on Niobium.

Abstract An extensive electrochemical and photoelectrochemical investigation has been carried out with very thin anodic oxide layers grown on niobium metal in sulphuric acid solutions. From the experiments the presence of an initial suboxide layer on the metal surface was inferred. Upon anodic polarization a strongly non-stoichiometric pentoxide film is formed, which is subsequently oxidized to the normal amorphous pentoxide phase (a-Nb 2 O 5 ). The use of photocurrent spectroscopy (PCS) allowed this oxidation process to be followed at various potentials and polarization times. Moreover, from the experimental results we obtain information both on the composition and the solid-state structure of the different phases grown on the metal surface. It is shown that the strong non-stoichiometry introduces a continuous defect band near to the conduction band edge of the initial film. Evidence for the existence of localized states inside the mobility gap of the films is also reported. The nature of the defects in anodic a-Nb 2 O 5 is briefly discussed.

Formation of barrier-type anodic films on sputtering-deposited Al–Ti alloys

The formation of amorphous anodic films at constant current is investigated for sputtering-deposited Al–Ti alloys containing from 3–30 at.% Ti. The films were grown at high efficiency in a borate electrolyte and comprised a main region containing units of Al 2O 3 and TiO 2, with a thin surface region enriched in titanium species. The formation ratios of the films increased with increase of titanium content of the alloys. The presence of the outer region is explained by the faster migration of Ti 4+ ions relative to that of Al 3+ ions through the films.

Area Selective Formation of Porous Type Aluminum Anodic Oxide Film by Sf-MDC

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Formation of porous anodic titanium oxide films in hot phosphate/glycerol electrolyte

Abstract The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm −3 K 2 HPO 4 + 0.2 mol dm −3 K 3 PO 4 /glycerol electrolyte containing only 0.04% water at 433 K. The growth efficiency is reduced by increasing the formation voltage to 20 V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20 V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m −2 . The pore diameter is not significantly dependent upon the formation voltage, being ∼10 nm.

Formation of Anodic Oxidation Films on As-Cast Surface of AC2A Casting Al Alloy

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Anodic film formation on binary AlMg and AlTi alloys in nitric acid

AbstractNitric acid is commonly used for surface treatments of aluminium alloys. It is used to clean the surfaces after alkaline etching; it has application in chemical polishing and is also used for electrograining. The majority of these treatments undergo the application of anodic polarisation that results in formation of anodic oxide film. However, little is known about the behaviour of aluminium containing magnesium or titanium in solid solution under such conditions. To reveal the effects of magnesium and titanium alloying additions on anodic film formation in nitric acid, Al‐1800 ppm Mg and Al‐800 ppm Ti alloys were investigated. It was found that porous alumina film developed on the surfaces with reduced efficiency of 40%, due to the reactive nature of nitric acid to alumina. The presence of magnesium and titanium in aluminium had little influence on the efficiency of film growth, as confirmed by the relatively similar thicknesses of oxide formed on binary alloys and aluminium. However, incorporation of magnesium ions into the alumina film led to development of a high‐population density of localised voids near the alloy/film interface. An increased titanium content was found in the film regions close to the alloy/film interface, indicating its oxidation. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Evidence and analysis of parallel growth mechanisms in Cu 2O films prepared by Cu anodization

We have studied the preparation of Cu 2O films by copper anodization in a 0.1 M NaOH electrolyte. We identified the potential range at which Cu + dissolution takes place then we prepared films with different times of exposure to this potential. The morphology, crystalline structure, band gap, Urbach energy and thickness of the films were studied. Films prepared with the electrode unexposed to the dissolution potential have a pyramidal growth typical of potential driven processes, while samples prepared at increasing exposure times to dissolution potential present continuous nucleation, growth and grain coalescence. We observed a discrepancy in the respective film thicknesses calculated by coulometry, atomic force microscopy and optical reflectance. We propose that anodic Cu 2O film formation involves three parallel mechanisms (i) Cu 2O nucleation at the surface, (ii) Cu + dissolution followed by heterogeneous nucleation and (iii) Cu + and OH − diffusion through the forming oxide and subsequent reaction in the solid state.

Transient film formation on chalcopyrite in acidic solutions

The transient time-dependent region characteristic of the anodic passive film formation on chalcopyrite in sulphuric acid solutions was studied by galvanostatic measurements. The occurrence of two passivation subregions was observed. Both followed the Sato–Cohen (logarithmic) model for the growth of anodic passive films. The experimental electric field for the two passivated steps was approximately 105 V cm−1. The electric field for the first step, at lower potentials, decreased with increasing ionic strength, but was pH independent. In the second step, the electric field decreased with increasing pH. The Tafel relationship was followed for constant electrical charge passed. The transition points between first and second step had almost the same electrical charge for different current densities and the electrical charge of the transition point decreased with increasing pH. The effects of temperature and iron/copper ion additions were also studied.