High-density DLC films prepared using a magnetron sputter type negative ion source

Diamond-like carbon (DLC) films have been deposited at room temperature using a magnetron sputter type negative ion source (MSNIS). The film characteristics such as density, surface roughness, film structure and thicknesses are obtained and analyzed using X-ray reflectivity (XRR), XPS and AFM methods. XRR results showed that the density of the film varies from 1.9 to 3.0 g/cm³ with respect to the different deposition parameters. At average negative carbon ion energy 700 eV and Cs injector temperature 150 °C, the densest film (3.0 g/cm³) was observed at 1 nm/s deposition rate. The surface roughness of the film was less than 1.0 nm in most cases for the 10-nm thickness film. XPS peak showed no Cs content on the film. The films prepared at different negative ion production yields and negative ion energies were compared. The result suggests that the density is the strong function of negative ion energy and the negative ion production yield.Higher cathode voltage induced denser film, while Cs flow rate had optimum temperature condition around 150 °C.     

Oxide film growth on Al-In alloys in a borate buffer solution in conditions of galvanostatic anodising

Anodic oxide films have been formed galvanostatically on Al-In alloys (containing up to 0.074% In) in a borate buffer solution (pH 7.8) at different current densities (20-100 μA cm⁻²). The mechanism, kinetics of growth and properties of formed oxide films have been investigated. The study of charge curves suggests that the growth of oxide films on Al-In alloys occurs by an activation-controlled ionic conduction under the influence of the high electric field through the oxide film according to an exponential law, like on valve metals. The following parameters have been calculated: the constants of the exponential law, ionic conductivity through the film, the effective activation distance for ion movement and the corresponding field strength. The values for the field strength, of the order of magnitude of 10⁶ V cm⁻¹, justify the application of the high field migration mechanism. Properties of anodic oxide films have been determined by means of electrochemical impedance spectroscopy; the resistance and thickness of the oxide film have been found to increase with the increase in the indium content in the alloy and with increased anodic current density. It has been established that the current efficiency in oxide films formation on Al-In alloys is lower than 100%: the increase of the indium content in the alloy, as well as the increase in anodic current density, increases the value of current efficiency.     

Detection of hydrogen profiles through anodic films on aluminium by secondary ion mass spectrometry

A Cameca Ion Analyser has been used to examine barrier-type anodic films grown on aluminium in ammonium tartrate solution, pH 2.2–10.6, at 293 K. Films formed in alkaline solution are thicker than their formation voltages would suggest, with an inner compact region and an outer, probably porous or heavily hydrated region. The degree of film hydration increases with pH, particularly in the acidic neutral range. The outer regions of films are more hydrated than the inner, particularly for formation in the neutral to alkaline range. The results are discussed in terms of current theories of anodic film formation involving the lattice transport of OH⁻ ions and protons, or dissolution-precipitation phenomena, and are shown to be consistent with the theories but unable to differentiate between them.     

The formation and properties of Al/Mg scales on Al heat- exchange surfaces in desalination plant

Three types of scales containing Al and Mg have been shown to form during the corrosion of Al in seawater in the pH range 6.3–8.5. The type most frequently occurring has a structure similar to that of the natural mineral hydrotalcite. It can be formed from the Al(OH)₃ produced by the corrosion of Al and Mg²⁺ ions present above pH 6.3. The Al:Mg ratio in this compound increases with pH, resulting in a form of buffering action. In the presence of these scales anodic breakdown of the corrosion film takes place at a potential 100–200 mV more anodic than in their absence. The structures, mechanisms of formation, and effects of these scales in the operation of distillation plants with Al surfaces are discussed, as well as their effects on the operation and interpretation of laboratory corrosion tests.     

The kinetics of silver iodide film formation on the silver anode

The potentiodynamic formation of AgI films by reaction (1) on an Ag rotating disc electrode was studied and the results were compared to previous work regarding the formation of anodic AgBr and Ag₂S films,

In dilute iodide solutions, a very porous AgI film is formed at a rate controlled by the diffusion of I^? to the electrode surface. At higher iodide concentrations, and particularly in high conductivity supporting electrolytes, a porous AgI film forms at a rate initially limited by ionic migration in solution followed by diffusion of I^? in the pores of the AgI film.In 1 M NaI solutions, when AgI becomes rather soluble, thick granular AgI films form at a rate limited by the solid-state migration of ions in the bulk of the film. On the basis of the low-field model of film growth, the ionic conductivity of these AgI films has been determined to be 8.7 × 10^?5 Scm^?1     

Evidence for the rate determining step at the solution—oxide film interface in the anodic growth of thin oxide films at platinum and nickel electrodes in aqueous solutions

The rates of growth of anodic oxide films at Pt in acid and alkaline solutions and Ni in alkaline solutions are compared. In acid solutions, the rates are pH independent. In alkaline solutions, they are affected by pH. The exchange current densities, i₀, in alkaline solutions increase one decade as pH increases one unit. The dependence of i₀ on pH is not expected for the model of high field assisted formation of films with either the step at the metal—oxide film interface or a step within the film as rate determining. It is suggested that a process at the oxide film—solution interface is rate determining with OH^? as reaction species. In acid solutions, too, this step is rate determining but with H₂O as reacting species. The possibility that this step can be rate determining is usually overlooked in analyses of growth of anodic films.     

Cathodic reduction of the anodically formed zinc species as a contribution to the study of the potentiodynamic passivation of zinc in alkaline media

The cathodic part of the potentiodynamic curves obtained for upward-facing horizontal 99.9% zinc electrodes in KOH solutions 0.4, 1.0, 2.0 and 3.0 M and sweep rates in the range 1–100 mV s⁻¹ have been systematically analyzed in order to assign the possible species formed and contribute to the study of the potentiodynamic passivation of Zn in alkaline media. The anodic limit of the potentiodynamic cycles was changed and set for significant points of the total curve (between hydrogen and oxygen evolution). Also, the anodic sweep was interrupted at the potentials corresponding to the anodic limit and the cathodic sweep applied immediately from a potential near that of zero current of the cathodic half-cycle. Only two cathodic peaks have been found for the non-interrupted cycles. The assignation of peaks according to the equilibrium potentials of the reduction of the possible species implied, ie Zn(OH)²⁻₄, Zn(OH)₂ and ZnO, is not possible because local pH changes are expected and the zincate concentration near the electrode is unknown. The peak placed at more positive potentials for KOH concentrations 0.4 and 1.0 M is assigned to zincate and that at more negative potentials, to the reduction of the film. Just the opposite assignation has been found for 2.0 and 3.0 M KOH solutions. The experimental results can be interpreted assuming that the product formed at the passivation potential consists of the same chemical species as those corresponding to the first anodic peak, probably Zn(OH)₂ or hydrated ZnO. From calculating the maximum film thickness according to the charge passed and taking into account the recent theoretical analysis made by Chang and Prentice, it is concluded that the direct formation of ZnO on the electrode at the passivation potential as a consequence of local pH changes is not probable.     

Voltammetric and galvanostatic studies of hydrous and anhydrous iridium oxide films

Electrolytically grown hydrous oxide films on iridium wire electrodes have been thermally treated from 473 to 773 K. Anhydrous oxide films formed by this treatment have been subjected to cathodic polarization at the potential of the hydrogen evolution reaction, square-wave pulsing of potential from –0.25 to +1.25 V with respoect to SCE and to anodic galvanostatic polarization in 0.5 mol dm^–3 H₂SO₄. Cathodic pretreatment caused an increase of the voltammetric charge in the oxide formation region while the square-wave pulsing formed a hydrous oxide film whose voltammetric charge was superimposed on the charge of the anhydrous oxide film. Both procedures restored the hydrophilic nature of the electrode/solution interface. Potential-time curves during anodic galvanostatic polarization served as a diagnostic criterion for the stability and the state of the oxide film.     

Concentration effects of hydrochloric acid on the anodic behaviour of lead sulphide

The anodic dissolution of lead sulphide is studied at various chloride concentrations and at different pH values. At 25° C it is found that in hydrochloric acid the dissolution rate reaches a maximum around 3.0 mol dm^–3. It has also been observed that at concentrations between 0.7 and 1.2 mol dm^–3, a crystalline sulphur deposit formed during the dissolution process leads to an independent peak on theI-E curve whereas at higher concentrations it merges with the PbCl₂ peak formation. pH has no significant effect on the dissolution rate. The results of a systematic study on the kinetics of the dissolution process as a function of concentration, temperature and pH are discussed.     

Kinetics and mechanism of urea-formaldehyde reaction

The various initial reactions in the urea-formaldehyde reaction have been isolated and the course of the reaction investigated using a quantitative thin-layer chromatographic technique developed for the purpose. The rate constants for the formation of monomethylol urea pass through a minimum in the pH range 4.5–8, thereby proving catalysis by H⁺ and OH^?. Whereas the reaction under alkaline conditions leads to methylol formation, acidic conditions favour formation of methylene bridges. It was found that the higher homologues were formed through the methylolation of methylene urea followed by its condensation with free urea and not by the reaction of methylene urea with methylol urea.     

Formation of Dy-Fe alloy films by molten salt electrochemical process

The electrochemical formation of Dy-Fe alloy films was investigated in a molten LiCl-KCl-DyCl₃ (0.50 mol%) system at 773 K. The deposition potential of Dy metal was 0.47 V (vs. Li⁺/Li) at a Mo electrode. Repetition of the potential sweep treatment at a fresh Fe electrode was effective in increasing the rate of formation of Dy-Fe alloys. Using an Fe electrode activated by repetition of the potential sweep treatment, a DyFe₂ film was formed by potentiostatic electrolysis at 0.55 V. However, the alloy film was thin and not adhesive. An adhesive DyFe₂ film was formed at the activated Fe electrode by potentiostatic anodic electrolysis at 0.55 V after cathodically electrodepositing Dy meal at 0.40 V. By using a similar procedure, Dy₆Fe₂₃ was formed at 0.68 V. The equilibrium potential for (2/11)Dy₆Fe₂₃+Dy(III)+3e⁻?(23/11)DyFe₂ was estimated as 0.62 V.     

Corrosion resistant TiO2 film formed on magnesium by liquid phase deposition treatment

Liquid phase deposition treatment (LPD) was applied to form a corrosion protective titanium dioxide (TiO₂) film on commercially available pure magnesium. Changing the solution pH, from acidic to highly alkaline, and with the addition of sucrose, it becomes possible to form a highly adhesive and thin TiO₂ film on commercially available pure magnesium without any heat treatment. The role of the sucrose may be attributed to the formation of tetrafluoroboric acid (BF₄)⁻ in the solution reducing the homogeneous nucleation of TiO₂ in the LPD solution. The film formed in the weak alkaline environment shows better corrosion resistance than at other LPD conditions, while the average rest potential is the same as that of as-polished specimens. This low rest potential may be due to micro-cracks in the formed film and high activity of the magnesium substrate.     

Reactivity of disulfide peptide l-cystine with iodido (diethylenetriamine)platinum(II) and synthesis of products in the presence of an iodide ion

Substitution reactions of complex [PtI(dien)]⁺ (where, dien=diethylenetriamine ) with sulfur-containing peptide l-cystine has been studied in 1.0×10^?1 M aqueous perchlorate or acetate medium between 298≤T (K)≤323 and 2.30≤pH≤4.45 using a UV–visible spectrophotometer. Products obtained have characterized from their physico-chemical and spectroscopic methods at various pH and temperatures. From this characterization, products have indicated that [PtI(dien)]⁺ has formed a complex with l-cystine and acts as a bidentate ligand, through Pt–S bond at 2.30≤pH≤3.30 and through Pt–N and Pt–S bond of cystine in 3.95≤pH≤4.45. At 2.30≤pH≤3.30, ring opening and closing of dien have occurred at 308 and 323 K, respectively, and the same has happened at pH≥3.95. All reactions have followed the rate law – d[mixture]/dt=(k ₁+k ₂ [cystine]) [Pt(II)], where k ₂ denotes the second-order rate constant. Activation parameters E _a , Δ H ^# and Δ S ^# have been determined. Product formation and reversible and forward reaction rate constants have also been evaluated.     

Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH 8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1–1 m NaHCO₃ solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (–0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe²⁺ ions into the solution is hindered by the formation of a Cr₂O₃ layer. As the potential reaches 0.4V, the oxidation-dissolution of Cr(iii) oxide/hydroxide to CrO₄ ² ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment of iron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive film properties. At high potentials, the presence of a passive film of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(vi) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A film of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.     

Electrodeposition of lead sulphide

The electrodeposition of lead sulphide on polycrystalline lead from aqueous sulphide solutions has been investigated. At low deposition potentials, a unit cell of galena oriented on the (110) plane is deposited through a two-dimensional nucleation and growth mechanism. Non-linear two-dimensional growth rates have been observed, possibly caused by impedements originated in the grain structure of the underlying substrate. High field growth of a thicker lead sulphide film occurs at higher potentials, and dielectric breakdown was observed at fields of ca 4 × 10⁺⁶ V cm^±1. The electrochemical reactions attending the film formation process are discussed on the basis of variations in the voltammetric peak potentials with sulphide concentration and pH.     

Photoelectrochemical analysis on the passive film formed on Fe-20Cr in pH 8.5 buffer solution

The electronic properties of passive film formed on Fe-20Cr stainless steel in pH 8.5 buffer solution were examined by the photocurrent measurements of the film. The passive film on Fe-20Cr exhibited an n-type semiconductor, which was confirmed by anodic photocurrent. The photocurrent spectrum for the passive films formed on Fe-20Cr was almost same in shape to that for the passive film on Fe except for the large difference in photocurrent intensity, which demonstrated that the passive film on Fe-20Cr is composed of Cr-substituted γ-Fe₂O₃ involving the d-d and p-d electron transitions. However, the large difference in photocurrent intensity for passive film between Fe and Fe-20Cr was due presumably to the fact that Cr³⁺ ions in passive film act as effective recombination sites of electron-hole pairs. The abrupt increase in the photocurrent intensity for the passive film formed on Fe-20Cr at a transpassive potential is due presumably to the decrease in the recombination site resulting from the reduction of Cr³⁺ content accompanying the oxidation of Cr³⁺ to Cr⁶⁺ in transpassive region.     

Electropolymerized poly(2-vinylpyridine) coatings as ion-exchange polymer modified electrodes

The effects of pH and of the nature and concentration of the electrolyte on the electrochemical behaviour of the Fe(CN)^3–/4– ₆ charge-transfer reaction at a poly(2-vinylpyridine)-coated electrode formed by electropolymerization have been studied. Cyclic voltammetry during the Fe(CN)^3– ₆ incorporation process was combined with measurement of the saturated concentration of the Fe(CN)^3– ₆ confined in the films to investigate the electrochemical behaviour and the fundamental nature of the ion-exchange polymers. The poly(2-vinylpyridine) films formed by electropolymerization were found to have better properties (i.e., larger amount of Fe(CN)^3– ₆ can be incorporated at various pH values and films are more chemically stable under acidic conditions) as polymer-modified electrodes than those formed by solvent evaporation. Of the various anions studied, ClO^– ₄ was found to be distinct from the others (Cl^–, NO^– ₃, Br^– and SO^2– ₄). On the one hand, the polymer films exposed to ClO^– ₄ are more dense and rigid than those exposed to other anions and show relatively little electroactivity. On the other hand, when the films are exposed to increasing concentrations of Cl^–, the films become more swollen, thereby reducing the resistance within the film and enhancing the rate of charge-transfer from the outer film surface to the electrode surface.     

The electrochemical behaviour of silver sulphide in sulphuric acid solutions

The electrochemical behaviour of synthetic silver sulphide (acanthite) electrodes in sulphuric acid solutions has been investigated using several techniques including cyclic voltammetry, anodic polarization and constant potential experiments. Under anodic polarization the dissolution has been attributed to the reaction Ag₂S=2Ag⁺+S+2e which occurs in two sequential, single electron transfer steps. A kinetic model for this stepwise anodic dissolution process at lower overpotential, where the current is a function of potential, is provided. At high dissolution rates (i.e. high currents) the slightly soluble silver sulphate salt is formed on the surface due to the saturation of the electrolyte near the Ag₂S interface. This observation is supported by the influence of electrolyte composition on the cyclic voltammetry and the polarization curve. A paralinear film growth model has been found to describe the formation and growth of the silver sulphate product layer indicating an initial region of parabolic kinetics which gradually changes to linear kinetics as the rate of film dissolution approaches that of film formation.     

Ellipsometric response of titanium electrodes in sulphate acid solutions under different potential perturbations

Optical characteristics and thickness of thin oxide films potentiodynamically formed on a titanium electrode in acidic sulphate solutions at pH 4.0 were investigated by dynamic and steady ellipsometric measurements. The dynamic ellipsometric response of an electrode polarized up to 1.0 V vs rhe depends on the charge involved in the massive oxide electroformation. Cycling the electrode at a constant potential sweep rate between −0.65 and 0 V produces a decrease on film thickness and the electrode reactivation. This electrode treatment becomes less effective as oxide electroformation is repeated several times. Experiments performed by holding the potential at the negative limit were not so effective in reducing film thickness and an appreciable change on optical constants was found. Results were interpreted as hydrogenated species formation within the oxide film.     

Effect of surface pretreatment on the galvanostatic oxidation of nickel

The open-circuit, cathodic reduction and anodic charging characteristics of oxide films on both electropolished and electropolished + HNO₃ etched nickel electrodes have been studied in aqueous Na₂SO₄ solutions. While the oxide film thicknesses are approximately the same, the film on etched nickel dissolves much more rapidly than that on electropolished nickel. To galvanostatically passivate etched nickel at charging rates in the range 20–200 μA cm^?2 in pH 2.8 Na₂SO₄, the rate of oxide chemical dissolution has to be substantially decreased and this is achieved by lowering the electrolyte temperature from 25°C to 5°C. The transient anodic passivation charge thus observed for etched nickel is ～ 2x larger than that for electropolished nickel. With either pretreatment, the current efficiency for oxide formation is low (～ 20%), the majority of the transient charge (～ 80%) accounted for by Ni²⁺ in solution. Chemical dissolution of the oxide alone cannot explain the observed charging differences between etched and electropolished nickel since the individual charges and oxide formation current efficiencies are not influenced by anodic charging rate or solution aggressiveness. The results are best explained in terms of a continual breakdown and repair of the oxide film at defect areas during anodic charging, with most of the charge consumed by inefficient film repair. The influence of the oxide defects and the reason for the differences in behaviour between electropolished and etched nickel are discussed.