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).     

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.     

Determining some structure-sensitive characteristics of nano-sized anodic Ag(I) oxide from photopotential spectroscopy

A relatively simple device involving a set of LEDs and a digital processing of the signal is developed for in situ photopotential and photocurrent spectroscopy of thin semiconductors including oxide films under conditions of discretely changed light wavelength. Decreases in the electrode potential and photopotential of Ag(I) oxide with time upon the anodic effect are shown to obey different kinetic regularities, namely, logarithmic and exponential respectively. Photopotential measurements revealed n-type conductivity in thin (6 to 22 nm) semiconductor Ag₂O films potentiostatically formed on either poly or monocrystalline silver (111) with a band gap of 3.1 eV. The structural disordering of the oxide is minimum at a potential of 0.490 V, which corresponds to nearly a midpoint of the anodic peak shoulder of the voltammogram. The concentration of donor defects in an Ag₂O film grown on a “smooth” polycrystalline silver is lower than that on an ultrafine-dispersed silver, but higher than on Ag(111) monocrystal. A linear dependence of the photoresponse on the light beam power is found, and the spectral dependence of the photopotential is discussed. The photopotential dependence on the film thickness, as well as crystalline and microstructural state of the silver surface means that the photoresponse is determined by the bulk rather than superficial electronic states, and the film thickness is smaller than the Debye shielding length. Original Russian Text ? D.A. Kudryashov, S.N. Grushevskaya, A.V. Vvedenskii, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 652–661.     

Determining the partial currents of silver ionization, its oxide formation, and chemical dissolution by multicycle chronoammetry with an RRDE

A multicycle chronoammetry with a rotating disc electrode with a ring (RRDE) enables one to experimentally discriminate between the partial currents of the substrate metal ionization, anodic formation of the oxide, and chemical dissolution of the oxide in the summary polarization current of the disc. The technique is approved by an example of Ag|Ag₂O|OH⁻(H₂O) system. In a range of relatively small anodic potentials of the Ag disc (0.48 to 0.51 V), the active dissolution of silver at the open surface sites and via pores in the surface film dominates; the phase formation current and, accordingly, the current efficiency of the process rapidly drop. At the potentials of the voltammogram maximum (0.52 to 0.53 V) when the silver active dissolution current is suppressed, the phase formation currents prevail and substantially exceed the chemical dissolution rate of the oxide. The thickness of an Ag₂O film rapidly increases under these conditions, and the current efficiency of the oxide formation is close to 100% for the whole polarization period. The rate constant of the chemical dissolution of an Ag(I) oxide is practically independent of the anodic phase-formation potential, but slightly depends on the oxide film thickness, reflecting changes in the film structure and, possibly, in its composition, from AgOH to Ag₂O. Original Russian Text ? D.A. Kudryashov, S.N. Grushevskaya, A.V. Vvedenskii, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 3, pp. 321–330.     

Anodic Oxidation of Silver and Ag–Au Alloys in Alkaline Medium

Anodic oxidation of Ag and Ag–Au alloys containing from 0.1 to 30 at. % Au under the conditions of Ag(I) oxide formation at t 0.5 s is determined by the peculiarities of migration transfer of charge carriers through phase oxide layer. The Ag₂O film is two-layer; this is reflected in the rise of additional current peaks in cyclic ivs. E(t) curves and also in the variation in the slopes of linear segments of chronoammograms presented in the i vs. t ^–1/2 coordinates. The conductivity of Ag₂O film slightly varies with its thickness; it is virtually independent of small gold additions to silver, but steeply decreases at 15 and 30 at. % Au. The formation and reduction of AgO occur via single-electron conversions of Ag₂O proceeding in the oxide phase. At all stages of oxidation of Ag and Ag–Au alloys in alkaline medium, the charge-transition stage remains reversible.     

Environmental factors affecting the corrosion behaviour of reinforcing steel: I. The early stage of passive film formation in Ca(OH)2 solutions

Oxide film thickening on reinforcement steel at early stage of formation is followed in naturally aerated Ca(OH)₂ solutions, recalling the natural behaviour in concrete, by measuring the open-circuit potential, E, with time up to 4 h. The final potentials, E_fin, are reached from negative values indicating oxide film growth. E varies with the Ca(OH)₂ concentration according to a straight line relationship. Oxide film thickening, at early stage of immersion, follows a direct logarithmic growth law as evident from the linear relationship between E and log t. The rate of oxide film thickening deceases by increasing the concentration and pH of the solution and by raising the temperature. The free activation energy of oxide film thickening is determined and found to be 29.28 kJ/mole, indicating that the process of oxide film growth is under diffusion control.     

Multicycle chronoammetry of RRDE in the investigation of the anodic oxide formation

The method of multicycle chronoammetry of RRDE makes it possible to obtain separately the partial currents of metal electrode ionization, anodic oxide formation and chemical oxide dissolution. The method is tested for Ag∣Ag₂O∣OH⁻(H₂O) system. In the range of low anodic potentials (0.48 ÷ 0.51 V) the process of active silver dissolution prevails; the phase formation current rapidly drops. At higher potentials (0.52 ÷ 0.53 V) the phase formation current prevails and noticeably exceeds the rate of the chemical oxide dissolution. The thickness of Ag₂O film rapidly increases; and the net phase formation current is close to 100%.     

Methodological Aspects of the Partial Charge Transfer in the Adsorption of Anions. Part II

The partial charge transfer coefficient and the electrosorption valence of anions on metals, as well as the role of electron tunneling in the formation of contact electric resistance (CER) signal are discussed. The CER value is shown to depend on the potential (E), the surface coverage with adsorbate particles, and their apparent charge. Bell-shaped CER–E curves of copper, silver, and gold in solutions containing halide ions are obtained. The maxima of these curves (E _max) correspond to the beginning of a substantial charge transfer and depend on the metal, anion, and its concentration. At E < E _max, halides adsorb without a noticeable charge transfer; while in a range of E _max + 0.1 V, the transfer is almost complete. On a chosen metal, E _max increases in a series of anions: I^– < Br^– Cl^–. For a chosen anion (e.g., I^–), it increases in a series of metals: Cu Ag Au. A correlation between the charge transfer of the adsorbed anions and the adsorption of the solvent, as well as the surface reconstruction of monocrystal electrodes, is discussed. The results are compared to the literature data.     

Anodic oxidation of Al–Ag alloys

The anodic oxidation of solution-treated and quenched Al–Ag alloys containing 0.3, 0.6, 0.9 and 1.2 at.%Ag, is examined in ammonium pentaborate electrolyte, which leads to growth of barrier-type anodic films. Enrichments of silver, 3.1×10¹⁵ Ag atoms cm⁻², are developed in the alloys immediately beneath the amorphous alumina films, with the level of enrichment not depending significantly upon the composition of the bulk alloy. The enrichment is relatively low due to clustering of silver atoms in the bulk alloy, which reduces the concentration of silver that is available to enrich from solid solution. Silver species are incorporated into the anodic film, where they migrate outward faster than Al³⁺ ions.     

Formation of oxides on copper in alkaline solution and their photoelectrochemical properties

The anodic formation of Cu(I) and Cu(II) oxides on polycrystalline copper in a deaerated alkaline solution is studied using the technique of the synchronous recording of transients of the photocurrent and polarization current. The oxide formation in a currentless mode is analyzed via the synchronous recording of photopotential and corrosion potential. It is found that copper is susceptible to corrosion oxidation due to traces of dissolved oxygen with the formation of a Cu(I) oxide. The preliminary formation of the underlayer of anodic Cu(I) oxide on copper hinders its further corrosion oxidation. It is confirmed that copper oxides Cu₂O and CuO, which appear on copper in both anodic and corrosion modes of formation, are p-type semiconductors. The initial stage of anodic oxidation of copper is characterized by the formation of an intermediate compound of Cu(I), possibly CuOH, which exhibits n-type conductivity. A film of Cu(I) oxide is thin and has a band gap of 2.2 eV for indirect optical transitions. Anodic polarization in the range of potentials of CuO formation leads to the formation of a thicker oxide film, which is a mixture of Cu(I) and Cu(II) oxides.     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

Critical potential of the surface development of Ag-Au alloys

Based on the model taking into account the appearance of a morphological instability and the evolution of a surface with time during the selective anodic dissolution of a homogeneous A-B alloy with the predominant content of the electrochemically negative metal A, equations for the critical potential E _cr and overpotential η_cr of the surface development are derived. Criteria of the nature of chemical hindrances of the selective dissolution in the over-critical range are formulated. The E _cr and η_cr values corresponding to the breakage of the morphological stability of the A-B alloy surface (with the atomic silver part X _Ag = 0.65 to 0.95) in an acidic nitrate solution containing diverse amounts of silver ions (10⁻⁴ to 10⁻² M) are experimentally found. The character of the effect of the atomic gold content in the alloy and the concentration of silver ions in the solution on η_cr and E _cr values of the alloys is determined. The nature of the kinetic limitations of the selective dissolution in the over-critical potential range is clarified. Original Russian Text ? A.V. Vvedenskii, O.V. Koroleva, O.A. Kozaderov, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 1, pp. 28–37.     

Anodic dissolution of iron silicides in alkaline electrolyte

Anodic dissolution of iron and its silicides (FeSi, FeSi₂, as well as the eutectic alloy FeSi₂-Si) and pure Si, in 0.1 to 5.0 N NaOH solutions is studied by cyclic voltammetry and x-ray photoelectron spectroscopy. Principal characteristic features of the silicide anodic dissolution are revealed and the composition of surface films investigated. It is shown that, despite an increase in Si solubility at higher pHs, the iron silicides are highly resistant to anodic dissolution due to especial protective properties of the complex oxide surface film. Original Russian Text ? A.B. Shein, I.L. Rakityanskaya, S.F. Lomaeva, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 59–63.     

A study of the chemical composition of the passive film on a Ti-Mo alloy in HCI and H2SO4

The composition of the passive film on Ti-15Mo alloy, formed in 1 mol/L and 4mol/L HCI and 2 mol/L H₂SO₄ solutions at 70 °C (160 °F) under anodic polarization is investigated by XPS and electrochemical techniques. Anodic polarization potential is found to have an obvious influence on the content of Mo in the film. At lower anodic polarization potentials, the surface of the passive film is enriched in Mo. However, at higher anodic polarization potentials, the surface is diluted in Mo. The anions of the electrolytes influence the composition of the passive film. In HCI solution, chloride ions are incorporated with the passive film during its formation. The passive film consists of a compound containing chloride and oxide ions. While in H₂SO₄ solution, the passive film only consists of titanium-molybdenum oxide. Sulfide ions and other sulfur are not incorporated. Ti-Mo alloys have a better passivity than pure Ti in HCI and H₂SO₄ solutions. This passivity is related to the enrichment of Mo in the surface of the passive film.     

Photoacoustic study on cathodic reduction of anodic oxide films formed on copper in borate solution

An in-situ photoacoustic (PAS) technique, using a piezoelectric detector with high sensitivity was applied to the study on duplex oxide films anodically formed on copper in pH 8.4 borate solution. The PAS signals from the copper electrode were produced by an irradiation of light beam with a wavelength of 514.5 nm. The PAS amplitude during cathodic reduction of the outer oxide layer to Cu₂O changed in the opposite direction, depending on the anodic potential of film formation and oxidation time. Assuming that the change in PAS amplitude is proportional to both optical absorption coefficient and film thickness, it was deduced from comparison of the estimated absorption coefficients for Cu (OH)₂, CuO and CuO_0.67 films that dehydration of the outer layer having an average composition of CuO_x (OH)_2?2x proceeded with increasing anodic potential of film formation and oxidation time during growth of the duplex oxide film. Moreover, it was found that the change in PAS amplitude during cathodic reduction of the total Cu₂O film involving the inner layer to metallic copper was proportional to the electric charge required for cathodic reduction, i.e., the film thickness, irrespective of anodic potential of film formation and oxidation time, which proved the validity of the above assumption.     

Galvanically coupled process for the conversion of Ag2O to AgI

The conversion of Ag₂O, grown-on Ag substrates, to AgI has in been investigated, using open-circuit potential, linear polarization, and potentiostatic control measurements. Three distinct reaction stages were clearly identified from the time-dependent behaviour of the open-circuit potential, E_OC, and the corrosion currents measured by linear polarization. In the early stages, when the silver surface was covered with a coherent Ag₂O film with low porosity, E_OC∼(E^e)_Ag2O/Ag and only chemical conversion of Ag₂O to AgI was observed. As Ag₂O was consumed and the area of Ag exposed to I⁻ solution increased, galvanic coupling of Ag₂O reduction to Ag and the oxidation of the Ag substrate to AgI also occurred. Once all the Ag₂O has been reduced, AgI formation stopped and E_OC fell to (E^e)_AgI/Ag. Chemical conversion was shown to produce fine particulate AgI whereas that formed anodically via galvanic coupling was in the form of large crystals. Since few larger crystals were observed, conversion via galvanic coupling appeared to be a minor process.     

Optical Interference During rf-GDOES Depth Profiling of Anodized Aluminum-Tantalum Alloy Films

The optical interference effect observed in radio frequency glow discharge optical emission spectroscopy (rf-GDOES) depth profiles of anodic oxide films formed on several aluminum-tantalum alloys is investigated and then used to determine refractive index, thickness, and composition of the anodized Ta-Al alloy layer. Refractive indices at the tantalum wavelength (1.4-2.4) were found to increase approximately linearly with the Ta₂O₅/Al₂O₃ ratio in the inner layer of the anodic oxide film. The optical interference effect was used to determine the composition of the inner film layer from the intensity/time rf-GDOES depth profile with excellent agreement with RBS-derived values. Good accuracy is obtained for constant sputtering rates and hence good interfacial depth resolution and for intensity signals oscillating uniformly across the film. A procedure is also proposed for calculating the thickness of the transparent layer of the anodic oxide film using the refractive index value derived from calibration plots of refractive index versus Ta₂O₅/Al₂O₃ ratio. The quantified depth profile of anodized Ta-24at.%Al alloy is presented. There is excellent accuracy of the quantified composition. The accuracy of the film depth, however, has a strong correlation with calculated density.     

Enhanced corrosion resistance of magnesium and its alloys through the formation of cerium (and aluminium) oxide surface films

Cerium (and aluminium) oxide layers were formed on magnesium and its alloys (AZ91) by chemical surface treatment with or without subsequent annealing. The corrosion behaviour modifications provided by the formation of these surface films were studied by means of different electrochemical and surface analysis techniques. The electrochemical behaviour, studied in sodium sulphate (Na₂SO₄) solution, showed (i) a marked shift of the corrosion potential towards more positive values, (ii) a slight inhibition of the cathodic reaction and (iii) a significant decrease of the anodic dissolution current. X‐ray photoelectron spectroscopy (XPS) was used for the characterisation of the composition of the deposited films and of the changes in the film composition during the electrochemical corrosion tests. The components of some oxide films are cerium dioxide (CeO₂), aluminium oxide (Al₂O₃) and aluminium hydroxide (Al(OH)₃). Other metallic mixed oxide films were obtained as a function of the solution composition. Very little (or no) change in the oxide film composition during the cathodic and anodic polarization experiments was observed from XPS measurements. Chemical treatment provides thick and moderately adherent protective oxide films. Annealing under oxygen further improves the beneficial effect of the chemical treatment.     

Reproducibility of corrosion parameters for the acidic dissolution of pure iron: potentiostatic polarisation

Polarisation curves were determined potentiostatically for pure polycrystalline iron corroding in oxygen-free 0.5 M H₂SO₄. Four different working electrode pre-treatments (abrasion/polishing, pre-polarisation and time to establish E_corr) were employed and the reproducibility of E_corr and calculated corrosion parameters (i_corr, Tafel slopes and i₀) for each treatment was determined. Electrode pre-treatment effects changes in working electrode catalytic activity with subsequent variation in the reproducibility of polarisation curves and measured and calculated corrosion parameters. A method incorporating abrasion/polishing followed by anodic/cathodic pre-polarisation resulted in general in improved parameter reproducibility and cathodic and anodic Tafel slopes close to those predicted by the reaction mechanisms.     

Kinetics and mechanism of the nickel electrode—I. Acid solutions containing a high concentration of chloride and nickel ions

The kinetics of the Ni electrode in acid solutions with a high chloride ion concentration has been investigated in the range of 25–75°C. Dissolution occurs uniformly only at low anodic potentials. When the latter exceeds a critical value, E_crit, net localized metal corrosion takes place. The plot at E < E_crit corresponds to a curve approaching two limiting slopes, namely, at (

Full-size image (1K)

. The cathodic Tafel slope is . These results, including their pH dependence, are explained with a reaction mechanism involving the participation of adsorbed hydroxo-species.