The Valve Metal Character of Zirconium as inferred from anode potential measurements in mineral acid solutions

A study on the value metal character of Zr in 0.1 M solutions of H₂SO₄, HNO₃, and H₃PO₄ has been performed using the anode potential as the primary variable in galvanostatic, potentiostatic, and capacity measurements. A method of surface pre-treatment, which suppresses both O₂ evolution and metal dissolution, has been described. Kinetic parameters of oxide growth have been calculated. The results indicate that:

• (i) the high field approximation is applicable following an exponential law, and
• (ii) the height and activation distance of the energy barrier for ion transport through the oxide phase (Verwey model) are the same three acids.

Measurements have been also made on the dielectric breakdown of oxide, and this occurs at potentials above 200 V. Direct capacity measurements give similar results as those based on reciprocal capacity calculated from galvanostatic experiments. It is concluded that the dominant anodic oxide species is ZrO₂ having a dielectric constant of 25. Open circuit potential measurements show that Zr is spontaneously oxidized in the three acids.     

Evidence for the activation-controlled galvanostatic growth of thin Anode Films on Tin in some neutral media

Growth of thin transparent anode films on Sn in neutral media has been studied by measuring galvanostatic anodic charging curves in phosphate buffer, 0.1M KCl and 0.1M Na₂SO₄ solutions (pH 6.6–6.7) at low current densities. The experimental technique is essentially the same as that used in previous investigations on valve metals. Although the rise in the anode potential of Sn does not exceed 2.0 V, the shape of the anodic charging curve is identical to that observed on valve metals: being thus composed of a linear and a non-linear region. Application of the kinetics of galvanostatic anodization to the results on Sn show that: (i) the oxide formation rate is linearly related to the ionic current density i by a double logarithmic plot, (ii) the reciprocal capacity is linearly related to log i, and (iii) the Tafel behaviour is exhibited at constant charge. These relations indicate that the anode film growth occurs by an activation-controlled ion conduction under the influence of the electric field across the film phase according to an exponential law. Treatment of the results allows the estimation of some kinetic parameters of film growth, e.g.: (i) the constants a and b of the empirical relation between oxide formation rate and i, (ii) the constants A and B of the exponential law, (iii) the electric field which is of the order of 10⁶ V/cm in phosphate, and 10⁷ V/cm in both chloride and sulphate solutions, and (iv) the effective activation distance for the ionic jump over the energy barrier associated with cation transport within the film, whereupon relative (and not absolute) values can only be obtained. Comparison between the present results and previous ones (also on Sn) taken by potentiodynamic technique shows that while diffusion kinetics play an important role in the formation of thick anode films by the potentiodynamic technique, activation-controlled kinetics explain the present results on the galvanostatic formation of thin transparent anode films.     

Nanopore formation on the surface oxide of commercially pure titanium grade 4 using a pulsed anodization method in sulfuric acid

Titanium and its alloys form a thin amorphous protective surface oxide when exposed to an oxygen environment. The properties of this oxide layer are thought to be responsible for titanium and its alloys biocompatibility, chemical inertness, and corrosion resistance. Surface oxide crystallinity and pore size are regarded to be two of the more important properties in establishing successful osseointegration. Anodization is an electrochemical method of surface modification used for colorization marking and improved bioactivity on orthopedic and dental titanium implants. Research on titanium anodization using sulphuric acid has been reported in the literature as being primarily conducted in molarity levels 3 M and less using either galvanostatic or potentiostatic methods. A wide range of pore diameters ranging from a few nanometers up to 10 μm have been shown to form in sulfuric acid electrolytes using the potentiostatic and galvanostatic methods. Nano sized pores have been shown to be beneficial for bone cell attachment and proliferation. The purpose of the present research was to investigate oxide crystallinity and pore formation during titanium anodization using a pulsed DC waveform in a series of sulfuric acid electrolytes ranging from 0.5 to 12 M. Anodizing titanium in increasing sulfuric acid molarities showed a trend of increasing transformations of the amorphous natural forming oxide to the crystalline phases of anatase and rutile. The pulsed DC waveform was shown to produce pores with a size range from ≤0.01 to 1 μm². The pore size distributions produced may be beneficial for bone cell attachment and proliferation.     

High temperature kinetics of metal-oxygen reactions by solid oxide electrolyte cells: An application to nickel

Chronopotentiometric curves, generated by galvanostatic single steps, applied to a solid electrolyte cell, have been analysed on the basis of a dimensionless equation derived on the assumption that a scale of oxide grows at one of the electrode-electrolyte interfaces. This process is rate limiting for developing the charge transfer-diffusion overvoltage; Wagner's theory on tarnishing, under retarding electric field conditions, has been assumed for treating the kinetics of the growing scale. The kinetic model developed has been tested for the reaction of solid nickel (positive electrode) at 1268 K with oxygen partial pressures ranging from 3.9 × 10^?11 Atm [equilibrium pressure of Ni(s)-NiO(s) system] to 1.7 × 10^?10 Atm.Both the rate and the rate constant of the scale formation go through the maximum but at different times, while the rate constant dependence of the oxygen partial pressure agrees with the theory. The maximum rate constant value was found to be 9.8 × 10⁸ molecules cm^?1 s^?1. The calculated scale thicknesses were between 6 and 24 nm.     

Galvanostatic formation of barrier-type Anodic oxides

Experimental Factors Affecting Oxide Growth Results of galvanostatic anodization of various metals with varying valve-metal behaviour including Nb, Ta, Zr, Ti, Al, W, Te, Sb, and Bi are compiled and compared with respect to the effect of various experimental factors such as surface pretreatment, stirring, solution composition, temperature of the bath, on the efficiency of oxide growth relative to that for other anodic processes. It is concluded that all of the above factors play an important role in the anodization of the non-typical valve metals, particularly Sb, Bi and W. Typical anodic charging curves associated with the process of oxide growth are given for several valve metals. Experimental data relevant to the phenomena of oxide breakdown are recorded and compared. The results indicate that the mechanism of breakdown varies for the same metal with variations in the experimental conditions.     

An approach to the mechanism of Corrosion of tungsten in alkaline solutions

The mechanism of anodic dissolution and hence of corrosion of W in alkaline solutions (NaOH, Na₃PO₄) has been investigated by measuring anodic Tafel lines by the galvanostatic technique, and calculating both the Tafel slope b and the effect of pH on anode potential as diagnostic criteria for the mechanism. The quasi-equilibrium principle of Kabanov, Burstein and Frumkin for all reactions preceding the rate-determining (rd) step has been used to calculate the concentration of intermediates as a function of potential. Two mechanisms have been tested with this principle:

• (i) the first involving the formation and dissolution of nonstoichiometric and stoichiometric surface oxides, and
• (ii) the second deals with the formation and dissolution of hydroxide species.

     

Investigations on Stress corrosion cracking of sensitized austenitic stainless steels and nickel-base alloys in boiling aqueous solutions by potentiostatically controlled constant-load experiments

The susceptibility of high alloyed materials to intergranular SCC is markedly influenced not only by material and environmental parameters, but also by the potential. The present paper describes in detail the influence of the potential on the susceptibility to intergranular SCC of these materials in aqueous solutions boiling under normal pressure by means of potentiostatically controlled constant load experiments. The following parameters have been varied:

• I Material
  • (i) alloy composition: 18/10 non-stabilized stainless steel; Incoloy 800; Inconel 600;
  • (ii) degree of sensitization;
  • (iii) pre-damage by intergranular attack, caused in sulfuric acid/copper sulphate-solution;
  • (iv) level of mechanical stresses.
• II Environment
  • (i) sodium chloride concentration;
  • (ii) sodium sulphate concentration;
  • (iii) pH (buffer solutions of pH 2.2 and 4.5).
• III Potential

The test-duration was up to several thousand hours. The type of attack – intergranular corrosion, intergranular and/or transgranular cracking – has been established by metallografic investigations. The experimental results are presented in form of time-to-failure/potential curves with the above mentioned parameters. Conclusions for the in-service behaviour of the materials investigated are derived.     

Macroscopic Studies on the Properties of the Anodic Oxide Films on Titanium

The effect of fluoride ions on the formation and dissolution behaviour of anodic oxide films on Ti has been investigated in acidic fluoride media (pH=1) using impedance and galvanostatic techniques. A5 the fluoride ion concentration and temperature increase the rate of oxide film formation decreases while the dissolution process increases. oxide film formed at high tem-perature and formation voltage was found to contain more defect sites in the film than that formed at a lower one. Activation energies are calculated during the oxide film formation and dissolution and found to be 20.76 and 28.72 kJ/mol, respectively. Formation rate and reciprocal capacitance data are reported as a function of polarizing current density. Values are recorded for the electrolytic parameters A and B. Potentiostatic curves are derived from the galvanostatic results.     

Schallemissionsmessungen an beschichteten Stahlblechen

Acoustic Emission Measurements of Coated Sheet Steel Acoustic emission measurements of differently coated steel sheets have shown that the damaging behaviour of this group of materials can be investigated with the aid of this procedure. With electroplated sheets, the acoustic emission at the same basical material depends on:

• (a) the deposit metal
• (b) the composition of the electrolytes
• (c) the deposite current density
• (d) the thickness of the electrodeposit.

Investigations carried out on phosphatized and/or painted steel sheets have shown a dependently of the acoustic emission on:

• (a) the kind of phosphatization
• (b) the chemical composition of the paint
• (c) the level of the stoving temperature
• (d) the weathering
• (e) the thickness of the paint.

On the hitherto investigations the acoustic emission was registered only as impuls rate. In order to obtain more aimed statements on the failure of adhesions and on the formations of micro-cracks, and in order to elucidate the mechanisms of adhesion and failure of adhesions, the analysis of the amplitudes and frequencies of the acoustic emission of coated metals was started.     

On the determination of Oxide Formation Rate for tantalum in chromate bath from cell voltage

Anodic oxide growth has been followed on Ta in chromate solutions by measuring the cell voltage/time relations at current densities between 200 and 700 m?A/cm². It is demonstrated here that the reference electrode can be dispensed with for technical studies, and estimates for the parameters of oxide growth can be calculated from cell voltage/time relations. Results are given for the formation rate, electric field strength, reciprocal capacity and increase of oxide thickness. The formation rate has been found to depend on current density following the empirical relation previously established for valve metals from the results of anodic charging curves. The empirical constants calculated here agree with the values in the literature, thus showing that cell voltage measurements can be used successfully to estimate growth parameters.     

High-Entropy Doping Boosts Ion/Electronic Transport of Na4Fe3(PO4)2(P2O7)/C Cathode for Superior Performance Sodium-Ion Batteries

Fe-based mixed phosphate cathodes for Na-ion batteries usually possess weak rate capacity and cycle stability challenges resulting from sluggish diffusion kinetics and poor conductivity under the relatively low preparation temperature. Here, the excellent sodium storage capability of this system is obtained by introducing the high-entropy doping to enhance the electronic and ionic conductivity. As designed high-entropy doping Na₄Fe_2.85(Ni,Co,Mn,Cu,Mg)_0.03(PO₄)₂P₂O₇ (NFPP-HE) cathode can release 122 mAh g⁻¹ at 0.1 C, even 85 mAh g⁻¹ at ultrahigh rate of 50 C, and keep a high retention of 82.3% after 1500 cycles at 10 C. Besides, the cathode also exhibits outstanding fast charge capacity in terms of the cyclability and capacity with 105 mAh g⁻¹ at 5 C/1 C, corresponding 94.3% retention after 500 cycles. The combination of in situ X-ray diffraction, density functional theory, conductive-atomic force microscopy, and galvanostatic intermittent titration technique tests reveal that the reversible structure evolution with optimized Na⁺ migration path and energy barrier boost the Na⁺ kinetics and improve the interfacial electronic transfer, thus improving performance.     

Electrochemical preparation of potassium gold cyanide

The essential requirements for the industrial preparation of potassium gold cyanide (pgc) are: (a) high rate of dissolution and (b) smooth and uniform dissolution. Employing galvanostatic and potentiostatic polarisation data and observations on the surface topography of anodes dissolved by both the techniques, it is shown that potentiostatic dissolution of gold in potassium cyanide at +0·345 V satisfies the above requirements.     

Compositional profiles of anodized aluminum on InP

Previously it has been shown that aluminum films on InP can be converted to Al₂O₃ by wet anodization and that a native InP oxide can be grown beneath the Al₂O₃ layer if the anodization is continued after all of the aluminum has been oxidized. This paper presents an investigation of the location of this oxide layer and the distribution of substrate elements within the Al₂O₃. The native oxide is found to be localized at the interface although oxidized and unoxidized indium is distributed throughout the Al₂O₃ after prolonged anodization. The results reported here differ from those previously reported for plasma anodization of aluminum films on InP.     

The nanoporous structure of anodic aluminum oxide fabricated on the Au/Nb/Si substrate

We have studied the anodization behavior of an Al film evaporated on the Au/Nb/Si substrate and demonstrated an effective approach to fabricate the through-hole anodic aluminum oxide (AAO) template on the conducting substrate. The smoothness of the initial metal films and an appropriate wet etching of the oxide film anodized in the first step were found to be critical factors for successfully anodizing the Al film on Au surface. The barrier layer of the obtained AAO structure presented a convex and thinner characteristic, and the underlying Au surface became porous after the anodization. This phenomenon was similar to the case of anodizing the Al film on an ITO glass substrate and could be explained reasonably by the effect of high pressure O₂ gas impelling and H⁺ etching at the interface of the barrier oxide and the Au layer.     

Deposition of thin cobalt films onto silicon by galvanostatic and potentiostatic techniques

In this study, thin cobalt films were electrodeposited directly onto n-Si (100) using two different electrodeposition techniques: galvanostatic and potentiostatic. The morphological difference between galvanostatic and potentiostatic deposits was observed by atomic force microscopy (AFM) and X-ray diffraction (XRD). Analysis of the deposits by an alternating gradient field magnetometer (AGFM) showed the influence of the electrodeposition process on the magnetic properties of the film.     

Plasma anodization of titanium and molybdenum

The plasma anodization of Mo and Ti has been studied using in situ ellipsometry. The oxides were formed in a d.c. oxygen glow discharge at constant anodization current. Mo was found to give a reproducible oxide layer with an index of refraction $\text{n}\text{?}\text{(}\text{Mo oxide}\text{) = 2.10 –}\text{i}\text{0.008}$. In contrast, the refractive index of the Ti oxide layer varied from sample to sample, with a real part between 2.34 and 2.10 and an imaginary part of approximately 0.04. The growth efficiency and the thickness dependence of the sample voltage were determined and found to be similar to those found in our previous work on Nb and Ta.     

On the kinetics of Cu(I) transfer through the solid CuI-Cu interface

The Cu(I) transfer through the Cu/γ,β, α-CuI interface was studied by a galvanostatic transient technique. From the experimental activation energies obtained for both the anodic and cathodic processes, two different steps are postulated as rate controlling according to the temperature range:

• 1.a) the ion incorporation into the CuI solid electrolyte for the Cu/γ,β-CuI interface
• 2.b) the ion transfer from the metallic side to the electrolyte surface at the Cu/γ -CuI interface. A theoretical calculation of the energy barrier for the ion transfer is also presented.

     

Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization

Many conventional anodic aluminum oxide (AAO) templates were performed using two-step direct current anodization (DCA) at low temperature (0–5 °C) to avoid dissolution effects. This process is relatively complex. Pulse anodization (PA) by switching between high and low voltages has been used to improve wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum or hard anodization for current nanotechnology. However, there are only few investigations of AAO by hybrid pulse anodization (HPA) with normal-positive and small-negative voltages, especially for the one-step anodization, to shorten the running time. In this article, the effects of temperature and voltage modes (DCA vs. HPA) on one-step anodization have been investigated. The porous AAO films were fabricated using one-step anodization in 0.5 M oxalic acid in different voltage modes including the HPA and DCA and the environment temperature were varied at 5–15 °C. The morphology, pore size and oxide thickness of AAO films were characterized by high resolution field emission scanning electron microscope. The pore size distribution and circularity of AAO films can be quantitatively analyzed by image processing of SEM. The pore distribution uniformity and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperatures. On the other hand, increasing environment temperature can increase the growth rate and enlarge the pore size of AAO films. The results of one-step anodization by hybrid pulse could promote the AAO quality and provide a simple and convenient fabrication compared to DCA.     

Anodization: a promising nano-modification technique of titanium implants for orthopedic applications

Anodization is a well-established surface modification technique that produces protective oxide layers on valve metals such as titanium. Many studies have used anodization to produce micro-porous titanium oxide films on implant surfaces for orthopedic applications. An additional hydrothermal treatment has also been used in conjunction with anodization to deposit hydroxyapatite on titanium surfaces; this is in contrast to using traditional plasma spray deposition techniques. Recently, the ability to create nanometer surface structures (e.g., nano-tubular) via anodization of titanium implants in fluorine solutions have intrigued investigators to fabricate nano-scale surface features that mimic the natural bone environment. This paper will present an overview of anodization techniques used to produce micro-porous titanium oxide structures and nano-tubular oxide structures, subsequent properties of these anodized titanium surfaces, and ultimately their in vitro as well as in vivo biological responses pertinent for orthopedic applications. Lastly, this review will emphasize why anodized titanium structures that have nanometer surface features enhance bone forming cell functions.     

Growth of dielectric films on semiconductors and metals using a multipole plasma

A multipole plasma source (a hot electron emitter associated with magnetic confinement by permanent magnets) is very suitable for plasma deposition and anodization because it can create a high density (10¹⁰-10¹¹ cm^-3) homogeneous plasma which is free from energetic particles. The anodization kinetics of metals and semiconductors as well as technological applications of the oxide layers were investigated. Space charge effects in the oxide are shown to control the transport of negative oxygen ions and positive substrate ions during growth. Anodization through a thin CaO-stabilized ZrO₂ (CSZ) film results in strong enhancement in the anodization rates of aluminium, tantalum and silicon, probably because of an alteration in the surface chemistry between the plasma and the oxide. The applications of this process are very attractive: the room temperature plasma anodization of silicon resulting in good quality SiO₂, and the protective filter effect of the CSZ layer.A combination of a multipole source and an ultrahigh vacuum system is described and will be used to study the first steps in the interactions of a surface (mostly GaAs) with a plasma.