Influence of oxidation rate and alloy composition on alloy enrichments of anodized Al–W alloys

The effect of oxidation rate on enrichment of tungsten in metastable, solid-solution Al–W alloys during anodizing has been investigated by Rutherford backscattering spectroscopy and medium energy ion scattering. The oxidation rate has negligible influence upon enrichment for current densities in the range 0.01–100 mA cm⁻², with levels between 1.3 × 10¹⁵ and 1.5 × 10¹⁵ W atoms cm⁻² for an Al–0.7at.%W alloy. Further, enrichment factors, expressing the ratio of the enrichment of tungsten in the alloy, in units of 10¹⁵ atoms cm⁻², to the bulk composition of the alloy, in units of at.%, for alloys of compositions in the range 0.1–30 at.%W, decrease progressively in the approximate range 2.5–0.2.     

Kinetic peculiarities of anodic dissolution of silver and Ag–Au alloys under the conditions of oxide formation

The kinetics of anodic dissolution of silver and Ag–Au alloys (X_Au = 0.1–30 at.% Au) in aqueous alkaline solution under the conditions of the formation of silver oxides has been examined. The techniques of cyclic voltammetry, chronoammetry, and photopotential measurements have been used. It was established that the anodic formation and cathodic reduction of Ag₂O on silver and alloys are controlled by migration in the oxide layer. Ag₂O oxide is an n-type semiconductor with an excess of silver atoms. Oxide layers formed on monocrystalline Ag(1 1 1) and Ag(1 1 0) are more stoichiometric than the layer formed on polycrystalline Ag.     

Effect of orientation of crystal face of silver and its alloying with gold on properties of thin anodic Ag(I) oxide films: II. Photopotential

The current efficiency of the formation of anodic oxide on polycrystalline silver is shown to decrease with an increase in the concentration of KOH solutions, while the rate-limiting stage remains the solid-phase mass transfer. Photopotential in nano-size Ag(I) oxide films anodically formed on polycrystalline silver is independent of the OH⁻ ion concentration, which means that a photoresponse is generated in the bulk oxide. The n-type conductivity of oxide films on silver, Ag-Au alloys, and low-index silver crystal faces, which was determined previously when measuring photocurrent, is confirmed. Replacing polycrystalline silver with its monocrystals results in a substantial decrease in the photopotential amplitude due to the decrease in the deviation from a stoichiometric composition. The electron mobility and the partial electronic photoconductivity in the anodic Ag(I) oxide depend on the orientatinon of the crystal face in silver and the gold content. At E = 0.56 V, a series of changes in these characteristics correlates to the changes in other structure-dependent parameters of Ag₂O oxide (the optical absorption coefficient α, the concentration of donor defects N _D, the width of the spatial charge region W, and the Debye screening length L _D).     

Incorporation of transition metal ions and oxygen generation during anodizing of aluminium alloys

Enrichment of nickel at the alloy/film interface and incorporation of nickel species into the anodic film have been examined for a sputtering-deposited Al-1.2at.%Ni alloy in order to assist understanding of oxygen generation in barrier anodic alumina films. Anodizing of the alloy proceeds in two stages similarly to other dilute aluminium alloys, for example Al-Cr and Al-Cu alloys, where the Gibbs free energies per equivalent for formation of alloying element oxide exceeds the value for alumina. In the first stage, a nickel-free alumina film is formed, with nickel enriching in an alloy layer, 2 nm thick, immediately beneath the anodic oxide film. In the second stage, nickel atoms are oxidized together with aluminium, with oxygen generation forming gas bubbles within the anodic oxide film. This stage commences after accumulation of about 5.4 × 10¹⁵ nickel atoms cm⁻² in the enriched alloy layer. Oxygen generation also occurs when a thin layer of the alloy, containing about 2.0 × 10¹⁹ nickel atoms m⁻², on electropolished aluminium, is completely anodized, contrasting with thin Al-Cr and Al-Cu alloy layers on electropolished aluminium, for which oxygen generation is essentially absent. A mechanism of oxygen generation, based on electron impurity levels of amorphous alumina and local oxide compositions, is discussed in order to explain the observations.     

Effect of the crystal face orientation and alloying with gold on the properties of thin anodic films of Ag(I) oxide: I. Photocurrent

The anodic formation of nano-size anodic films of Ag(I) oxide on polycrystalline silver and Ag-Au alloys, as well as on low-index monocrystalline silver faces in 0.1 M KOH, is studied. Based on the data of photocurrent measurements, the n-kind of the oxide film conductivity is determined. Replacing the polycrystalline silver with monocrystals, as well as adding a small amount of gold atoms to its lattice (X _[Au] <= 4 at. %), results in substantial structure ordering of the oxide due to the decrease in the deviation from the stoichiometric composition. The structure-dependent parameters of Ag(I) oxide (the optical absorption coefficient α, the concentration of donor defects N _D, the width of the spatial charge region W, and the Debye screening length L _D) are determined by the silver crystal face orientation, the gold content, and the film formation potential. At E = 0.52 V, the series of changes in these characteristics correlate with the series of reticular density. An increase in E breaks the series and increases the deviation of the composition from the stoichiometric Ag₂O composition.     

Influence of current density in anodizing of an Al–W alloy

The influences of current density in development of amorphous, barrier anodic films on a metastable, solid solution, Al–6.5at.%W alloy is examined, with focus on enrichments in the alloy, oxidation processes at the alloy/film interface and migration of species in the film. The films were formed at current densities between 0.01–100 mA cm⁻² in sodium tungstate electrolyte and examined by transmission electron microscopy and Rutherford backscattering spectroscopy. Enrichments of tungsten in the alloy develop at each of the selected current densities, with enrichments in the range 2–4×10¹⁵ W atoms cm⁻². Incorporation of tungsten species into the films at the alloy/film interface proceeds non-uniformly, with cycles of local enrichment of the alloy, oxidation of the enriched tungsten and subsequent depletion of the alloy. The tungsten species migrate more slowly than Al³⁺ ions, leading to layering of the film composition. The cyclic incorporation of tungsten species into the film and the migration of tungsten species within the film are similar for the selected current densities, at the resolution of the experiments.     

Effect of Chloride and Fluoride on the Rate of Corrosion of dental amalgam alloys

The anodic behaviour of massive and dental amalgam alloys in artificial saliva containing different concentrations of Cl^? or F^? ions has been studied. Quantitative relations could be established between concentrations of these ions and the height of the peaks corresponding to anodic oxidation of Sn or Ag. Contrary to f^?, Cl^? ions from Ag complexes via AgCl which excess cl^?ions. The massive alloy is stable below 0.15 V. Calcium fluoride can be safely admixed with dental amalgam alloy without enhancing its corrosion.     

Enrichment factors for copper in aluminium alloys following chemical and electrochemical surface treatments

Surface treatments, including chemical polishing, alkaline etching, acid pickling, and electropolishing, of aluminium and copper-containing aluminium alloys lead to enrichment of solid solution copper in the metal just beneath the residual oxide films of the treatment processes. The paper presents the enrichment factor for copper as a function of the copper content of the bulk matrix material. The factor is defined as the ratio of the copper enrichment, measured in units of 10¹⁵ atoms cm⁻², to the copper content of the matrix in at.%. Although absolute levels of enrichment increase with increase in copper content of the alloy, the enrichment factor increases in the opposite sense.     

The corrosion behavior of Cu–Al and Cu–Al–Be shape-memory alloys in 0.5 M H2SO4 solution

The corrosion behavior of Cu–Al and Cu–Al–Be (0.55–1.0 wt%) shape-memory alloys in 0.5 M H₂SO₄ solution at 25 °C was studied by means of anodic polarization, cyclic voltammetry, and alternative current impedance measurements. The results of anodic polarization test show that anodic dissolution rates of alloys decreased slightly with increasing the concentrations of aluminum or beryllium. Severe intergranular corrosion of Cu–Al alloy was observed after alternative current impedance measurement performed at the anodic potential of 0.6 V. However, the addition of a small amount of beryllium was effective to prevent the intergranular corrosion. The effect of beryllium addition on the prevention of intergranular corrosion is possibly attributed to the diffusion of beryllium atoms into grain boundaries, which in turn deactivates the grain boundaries.     

Influence of pre-treatment on the surface composition of Al-Zn alloys

The influences of pre-treatments on the composition of Al-Zn alloys, containing 0.6-1.9 at.%Zn, are investigated using scanning and transmission electron microscopies, glow discharge optical emission spectroscopy and ion beam analysis. Alkaline etching, anodic alkaline etching and electropolishing result in zinc enrichments, just beneath the oxide/hydroxide films on the alloy surfaces. The enriched alloy, about 4-5 nm thick, contains zinc in solid-solution with aluminium, up to about 18 at.%. The highest and lowest enrichments are produced by alkaline etching and electropolishing respectively. For a given pre-treatment, the enrichment increases with increase in zinc content of the bulk alloy.     

Voltamperometry of selective dissolution of Ag-Au alloys under conditions of solid phase-liquid phase mass transfer

The problem of anodic kinetically reversible selective dissolution of a homogenous metal A-B alloy at comparable rates of the mass transfer of ions of the metal being dissolved (in the liquid phase) and atoms A (in the solid phase) is solved and the conditions for the occurrence of purely solid phase diffusion transport during potentiodynamic ionization are found. A comparison of the results of the numerical modeling of the voltamperometric process with the experimental data on the selective dissolution of Ag-Au alloys shows that an increase in the ionic concentration of silver, a decrease in the scan rate of the anodic potential, and a diminution in the atomic fraction of silver in the alloy favor the occurrence of the purely solid-phase diffusion mode of the transition of Ag to the solution.     

Stress corrosion cracking of copper and silver, specific effect of the metal cations

Based on stress corrosion cracking (SCC) studies on brass, and on Ag-Cd alloys (a model alloy used to reproduce the behaviour of brass), it was found that both pure copper and pure silver are susceptible to SCC in 1 M copper(II) nitrate and in 1 M silver nitrate aqueous solutions, at the equilibrium potentials for the reactions: Cu²⁺+2e⁻↔Cu and Ag⁺+e⁻↔Ag, respectively. The results were analysed under the light of recent developments in surface science. It was concluded that the same SCC mechanism that operates in brass and in Ag-Cd alloys should be operating during SCC of pure copper and pure silver, under equivalent experimental conditions.     

Dealloying behaviour of Cu-20Zr alloy in hydrochloric acid solution

The dealloying of the duplex-phase Cu-20Zr cast alloys and corresponding sputtered Cu-20Zr films (wt.%) in hydrochloric acid solution was investigated using electrochemical, chemical and surface analysis techniques. Results show that the dissolution mechanisms for the cast alloy and the films were strongly depended on the electrochemical activities of Zr and Cu in each phase in 0.1 mol/l HCl solution. When the potential was between the redox potentials of Cu/Cl⁻/CuCl_s and Zr/Zr⁴⁺, the dissolution is attributed to the selective removal of Zr atoms in the cast alloy and the sputtered films; when the potential was higher than the redox potential of Cu/Cl⁻/CuCl_s, Cu and Zr atoms in the samples dissolved simultaneously. In duplex-phase cast alloy, of Zr in the different phases in the alloy, Zr in the zirconium-rich phase was in preference to dissolve owing to its high electrochemical activities. Namely the selective phase attach firstly happens in the cast alloy. The study of the corrosion characteristic of sputtered films showed that Cu atoms reacted with Cl⁻ ions in the solution and formed the corrosion product of Cu(I) and Cu(II) complex ions, which restrained the dissolution of Cu atoms.     

Enrichment behaviour of gallium in heat and surface treatments of Al–Ga foils

The behaviour of gallium is examined after heat and surface treatments of aluminium foils containing either 120 or 1300 ppm gallium, with low enrichments of gallium at the surfaces arising from foil manufacture. Vacuum heat treatment at 823 K for 20 ks caused negligible additional enrichment, probably associated with the high solubility of gallium in aluminium. Subsequent alkaline etching in 0.25 M sodium hydroxide for 60 s at 348 K increased significantly enrichments, to about 3.2×10¹⁴ and 3.6×10¹⁵ Ga atoms cm⁻² for the foils containing 120 and 1300 ppm gallium respectively. The enrichments were probably located in both the metal and the overlying etching products. In contrast, electropolishing in perchloric acid/ethanol eliminated pre-existing enrichments, probably due to activation of the foil, with no enrichment developing during the electropolishing procedure. Barrier-type anodic films formed on the alkaline-etched foils contained gallium species, with increased concentrations at the film surface. Gallium enriched the metal region immediately beneath the anodic film during anodizing and led eventually to detachment of the growing film from the etched foil, followed by growth of a second film beneath the detached film. The combined amounts of gallium in the anodic film and enriched metal layer were similar to the levels at the surface regions of the etched foils. The anodic films formed on the electropolished foils, with no pre-existing enrichment, were free of gallium species and remained attached to the foils, since insufficient gallium could enrich in the metal beneath the anodic film during the relatively short period of anodizing.     

The effect of Au/Ag ratios on surface composition and optical properties of co-sputtered alloy nanoparticles in Au–Ag:SiO2 thin films

Gold–silver alloy nanoparticles with various Au concentrations in sputtered SiO₂ thin films were synthesized by using RF reactive magnetron co-sputtering and then heat-treated in reducing Ar + H₂ atmosphere at different temperatures. The UV–visible absorption spectra of the bimetallic systems confirmed the formation of alloy nanoparticles. The optical absorption of the Au–Ag alloy nanoparticles exhibited only one plasmon resonance absorption peak located at 450 nm between the absorption bands of pure Au and Ag nanoparticles at 400 and 520 nm, respectively, for the thin films annealed at 800 °C. The maximum absorption wavelength of the surface plasmon band showed a red shift with increasing Au content. XPS results indicated that the alloys were in metallic state, and they had a greater tendency to lose electrons as compared to their corresponding monometallic state. Moreover, the positive and negative shift of the Au(4f) core-level binding energies was observed for low and high Au concentration, respectively. Also a negative shift of the Ag(3d) binding energies was increased by increasing Au concentration. Diffusion of the particles toward the surface by increasing the temperature has also been illustrated by AFM images. Based on AFM observations, we have found that the particle size reduced from 35 to 20 nm by increasing the annealing temperature from 600 to 800 °C, while particle size increased by increasing Au concentration in films. In addition, lateral force microscopy (LFM) analysis showed that the alloy particles were uniformly distributed on the surface.     

掺V和Ag的TiAl合金中缺陷和电子密度的正电子湮没研究

测量了Ti50Al50，Ti50Al48V2，Ti50Al48Ag2合金和充分退火的Ti，Al，Ag，V金属的正电子寿命谱，利用正电子寿命参数分别计算了合金基体和缺陷态的自由电子密度。TiAl合金的脆性与其基体和晶界缺陷处的自由电子密度较低有关。在富Ti的TiAl合金中加入V，V原子比Al和Ti原子能提供较多的自由电子参与形成金属键，因而提高了合金基体和晶界缺陷处的自由电子密度；在TiAl合金中加入Ag也有类似的效应。在TiAl合金中加入V和Ag，有利于提高合金的韧性。     

Oxygen evolution within barrier oxide films

Using residual gas analysis, with scraping of oxide films, it is demonstrated that oxygen gas can be generated within anodic films on Al-Cu alloys and InP. The oxygen is produced by oxidation of O²⁻ ions of the film material, and is present in numerous bubbles in the bulk film. The generation of oxygen is suggested to be related to the electron energy levels of the film material, which in the case of alumina films are modified by the incorporation of copper species and, from previous work, other non-valve, transition metal species. Oxygen is also present in low amounts in alumina films nominally free of the latter species; the oxygen may be located either in the bulk oxide or at flaws.     

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni₅₀Mn₃₀Ga₂₀ and austenite Ni₄₈Mn₃₀Ga₂₂ alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5-11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga₂O₃ and MnO₂, passive films formed in near neutral media (pH 5-8.4) are composed of Ni(OH)₂, NiOOH and Ga₂O₃ in the outer region and of NiO, MnO₂ and MnO in the metal-near region.     

Generation of copper nanoparticles during alkaline etching of an Al-30 at.%Cu alloy

A mechanism of formation of copper nanoparticles is proposed for alkaline etching of a sputtering-deposited Al-30 at.%Cu alloy, simulating the equilibrium θ phase of 2000 series aluminium alloys. Their formation involves enrichment of copper in the alloy beneath a thin alumina film, clustering of copper atoms, and occlusion of the clusters, due to growth of alumina around the clusters, to form nanoparticles. The proposed mechanism is supported by medium energy ion scattering, Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy of the alloy following immersion in the sodium hydroxide solution, which disclose the enrichment of copper and the generation of the nanoparticles in the etching product of hydrated alumina. The generation of the nanoparticles is dependent upon the enrichment of copper in the alloy in a layer of a few nanometres thickness, with no requirement for bulk de-alloying of the alloy.     

The behaviour of chromium during anodizing of Al–Cr alloys

The anodic oxidation of dilute Al–Cr alloys, containing 0.8 and 1.7 at% Cr, has been investigated in order to understand the oxidation behaviour of the alloying element and its influences on the film composition and morphology. The alloys reveal two stages of oxidation: an initial stage, in which only aluminium atoms are oxidized to form a chromium-free anodic alumina film, and a subsequent stage, in which both aluminium and chromium are oxidized, in their approximate alloy proportions, with generation of a chromium-contaminated anodic alumina film. In the first stage, chromium is enriched in a thin layer of alloy, immediately beneath the anodic film, to an amount corresponding to a layer of average thickness 1.5 nm and of average composition, Al–20 at% Cr. Following the oxidation of chromium, oxygen is produced electrochemically within the film at or near the alloy/film interface, probably associated with the development of chromium-rich clusters in the enriched alloy layer and, subsequently, formation of semiconducting chromium-rich oxide. Thus, the film material formed at the alloy/film interface by inward migration of O^2- ions contains many oxygen-filled bubbles with associated high pressures. The chromium species present in the film migrate outward more slowly than Al³⁺ ions. Hence, a layer of chromium-free anodic alumina, which thickens as the film grows, is maintained adjacent to the film/electrolyte interface.