Film Formation Potential Research Articles

Determination of the Diffusivity of Cation Vacancy in the Passive Film on Ni

The diffusivity of the cation vacancy (DM) in the passive film formed on Ni in pH 8.5 buffer solution at ambient temperature was estimated using the Mott-Schottky analysis based on the Point Defect Model combined with the high field migration equations. The steady state behaviors of the passive film on Ni, such as current density, film thickness, and acceptor density, agreed with the postulations of the Point Defect Model. From the dependence of the acceptor density and steady state current density on film formation potential, DM for the passive film on Ni was determined to be 3.7×10-18 cm2s-1 by the high-field migration equations combined with the Point Defect Model.

Semiconducting properties of passive films formed on Fe–Cr alloys using capacitiance measurements and cyclic voltammetry techniques

Cyclic voltammetry, capacitance measurements, and AES analysis have been carried out in order to study semiconducting properties of passive films formed on ferritic Fe–Cr alloys in aqueous environments. The influence of Cr content and film formation potentials on their semiconducting properties was investigated in pH 9.2 borated buffer solutions. Cyclic voltammetry results showed that peak current densities of Fe ion species decreased with the increase in Cr content while the reduction peak of Cr 6+ to Cr 3+ increased. Capacitance measurement results exhibited that passive films characterized n-type and p-type semiconductors, depending upon potentials applied above and below the flat-band potential. Doping densities of passive films decreased with increasing the Cr content and/or the film formation potential. AES results showed that as Cr content and the film formation potential increased, the highest peak of Cr shifted from the outside to the inside of a passive film.

Effects of fluid composition and temperature on the salt film formation potential

Elevated activating-ion concentrations in solution favor anionic activation of the metal and lead to the salt film formation potential becoming less noble. Addition of inhibiting ions makes the potential more noble in the series sulfate, nitrate, and hydroxyl on account of changes in the strength of the Fe-[A−] bonds in the surface complex.

Semiconductive behavior of passive films formed on Fe-Cr alloy

Semiconductive properties of passive films formed on Fe-18Cr alloy in a borate buffer solution were investigated using photoelectrochemical response and electrochemical impedance spectroscopy. Band gap energy of the passive films, E g , was estimated from a photoelectrochemical spectrum that could consist of two components. Egs were recognized as approximately 2.4 eV and 3.4 eV, which were mainly derived from Cr hydroxide layer and Cr oxide layer, respectively. The values were almost constant independent of passivation time, film formation potential and additive element. Positive photocurrents generated from the outer part hydroxide and the inner oxide in the passive film increased as applied potential increased, which indicates that the passive films behaved as an n-type semiconductor. Mott-Schottky plot of capacitance showed positive slope. This means that the passive films behaved as an n-type semiconductor. Donor densities of the passive films estimated by the Mott-Schottky plots depended on passivation time, film formation potential and additives. Corrosion resistance of Fe-18Cr alloy was discussed in terms of the semiconductive properties.

Semiconductive properties of titanium anodic oxide films in McIlvaine buffer solution

The semiconductive properties of anodic oxide films grown on titanium surface at different formation potentials: 1.0, 2.0, 3.0 and 4.0 V were investigated by means of electrochemical impedance spectroscopy and cyclic voltammetry in McIlvaine buffers at pH 2, 4 and 5. The passive films show a Mott–Schottky behavior typical for an n-type semiconductor at the studied potential range with a high concentration of donors. On increasing the film formation potential, the flat band potential and the donor density decrease. Further, lower flat band potentials were obtained for higher pH buffer. These experimental results were related to the film thickness and composition. The influence of film thickness on the oxidation reactions taking place at the titanium electrodes covered by oxide film was evaluated.

Diffusivity of point defects in the passive film on Fe

Diffusivity of point defects ( D 0) in the passive film formed on Fe in deaerated pH 8.5 buffer solution at ambient temperature was estimated by the Mott-Schottky analysis based on the Point Defect Model and surface charge approach assuming that donors are oxygen vacancies and/or iron interstitials. From the exponential decay of the concentration of donors with film formation potential, D 0 was calculated to be 1.69 × 10 −20 cm 2 s −1.

Semiconductor properties of passive films formed on sputter-deposited Fe–18Cr alloy thin films with various additive elements

The semiconductor properties of passive films formed on an Fe–18Cr alloy with various additive elements in a borate buffer solution were studied using electrochemical impedance spectroscopy and photoelectrochemical response. The photocurrent was plotted as a photoelectrochemical action spectrum that could be separated into two components, which were mainly derived from Cr oxide (Eg = 3.4 eV) and Cr hydroxide (Eg = 2.4 eV). The band gap energy, Eg; of each component was almost constant, independent of the species and amount of additive elements. The photoelectrochemical response showed positive photocurrent for most potential in the passive region, which indicates that the passive film behaved as an n-type semiconductor. In addition, the Mott–Schottky plot of the capacitance showed positive slope, which also means that the passive films behaved as an n-type semiconductor. The donor density of the passive films, which can be estimated by the Mott–Schottky plots, changes depending on the film formation potentials and a variety of additives.

Electronic band structure of passive film on X70 pipeline steel

Mott-Schottky analyses and photoelectrochemical measurements were used to explore the effects of film formation potentials, time, and chloride ions on the electronic properties of the passive film on X70 pipeline steel in 0.5 M NaHCO 3. Mott-Schottky analyses showed that with increasing film formation potentials, the capacitance and donor density of the passive film decrease, and the flat band potential and thickness of the space charge layer increases. The addition of chloride ions increases the capacitance and donor density of the film and results in a more negative flat band potential and a thinner space charge layer. Photoelectrochemical measurements imply that photo-generated carriers with a low mobility exist in the passive film and the photocurrents of the film increase when the film formation potential becomes more positive. The bandgap energy, E g, of the passive film decreases with increasing film formation potentials. The extension of film formation time increases the photocurrents and leads to a decrease in the bandgap energy of the passive film. The effect of film formation time on the photocurrents of the film formed at 0.6 V SCE was less remarkable than that formed at 0.2 V SCE. The existence of chloride ions in 0.5 M NaHCO 3 decreases the photocurrents and increases the bandgap energy of the passive film.

Effect of hydrogen on stresses in anodic oxide film on titanium

Stresses in anodic oxide film on titanium thin film/glass electrode in pH 8.4 borate solution were investigated by a bending beam method. The increases in compressive stress observed with cathodic potential sweeps after formation of anodic oxide film were attributed to the volume expansion due to the compositional change of anodic oxide film from TiO 2 to TiO 2− x (OH) x . The instantaneous responses of changes in stress, Δ σ, in the anodic oxide film to potential steps demonstrated the reversible characteristic of the TiO 2− x (OH) x formation reaction. In contrast, the transient feature of Δσ for the titanium without anodic oxide film represented the irreversible formation of TiH x at the metal/oxide interphase. The large difference in stress between with and without the oxide film, has suggested that most of stresses generated during the hydrogen absorption/desorption reside in the anodic oxide film. A linear relationship between changes in stress, Δ(Δ σ) des, and electric charge, Δ Q des, during hydrogen desorption was found from the current and stress transients, manifesting that the stress changes were crucially determined by the amount of hydrogen desorbed from the oxide film. The increasing tendency of −Δ(Δ σ) des with increasing number of potential steps and film formation potential were discussed in connection with the increase in desorption amount of hydrogen in the oxide film with increasing absorption/desorption cycles and oxide film thickness.

Dependence of Semiconductive Properties of Titanium Oxide Film on the Growth Rate and Formation Potential

Semiconductive properties of a titanium anodic oxide film are found to depend on the film formation rate and potential. Both donor density and flat-band potential obey a power dependence on the formation potential with the coefficients depending linearly on the logarithm of the growth rate. The donor density increases with the temperature following an Arrhenius-like equation.

A study of the passive films on chromium by capacitance measurement

Using capacitance measurement and Mott–Schottky analysis, the semiconducting properties of passive films formed on chromium within the passive potential range under different conditions were investigated. The study reveals a p-type behavior of the passive layers. Two semiconductive parameters, i.e., the acceptor density ( N A) and the flatband potential ( E FB), which are mainly related to composition and surface charges of the passive films, have been measured. The effect of film formation potential, passivation time, pH and composition of solutions on the parameters are discussed. N A increases either with lowering film formation potentials or with prolonging passivation times. This is attributed to the transformation of a less hydrated oxide film into a more hydrated form. The changes of E FB are discussed as a function of adsorptive anions and pH values of electrolyte solution.

Determination of the diffusivity of point defects in passive films on carbon steel

Mott–Schottky analysis is performed for passive films formed on carbon steel in borate solution. It is shown that the calculated donor density decreases exponentially with increasing film formation potential. The experimental data are interpreted in terms of the point defect model for passivity of carbon steel, assuming that the donors are oxygen vacancies and cation interstitials. The diffusivity of the donors D 0 is calculated to be in the range of 10 −16–10 −15 cm 2 s −1.

Electrochemical investigation of the effects of hydrogen on the stability of the passive film on iron

The effects of hydrogen on the stability of passive films on iron were investigated by electrochemical methods: open circuit potential decay, cathodic galvanostatic reduction, electrochemical impedance spectroscopy, and breakdown potential measurements. The results show that hydrogen decreases the final static open circuit potential, the cathodic charge for reduction and the charge transfer resistance of the passive film, indicating that hydrogen decreases the stability of the passive film. The charge transfer resistance of the passive film formed on the charged specimen does not change with increasing the film formation potentials, suggesting that increasing film formation potentials under hydrogen charging conditions cannot improve the stability of the passive film. Hydrogen decreases the breakdown potential of the passive film, especially at lower chloride ion concentrations, confirming that hydrogen promotes the susceptibility of the passive film on iron to pitting corrosion. The reasons why hydrogen decreases the stability of the passive film were discussed.

Photo-electrochemical analysis of passive film formed on Cr in pH 8.5 buffer solution

The structure and composition of a passive film formed on Cr in pH 8.5 buffer solution were explored through the photo-electrochemical and impedance analysis of the film. The passive film on Cr was confirmed to be a single layer or duplex layers depending on the film formation potential. At low film formation potentials such as −300 mV versus saturated calomel electrode (SCE), a single Cr(OH) 3 layer was formed on Cr. In contrast, the passive film formed on Cr at potentials noble to 0 V SCE was composed of duplex layers; inner CrOOH and outer Cr(OH) 3 layers. Specifically, photocurrent spectra for the passive film could be resolved into two spectral components responsible, respectively, for the inner and the outer layers. The outer Cr(OH) 3 layer, exhibiting an n-type semiconductor with a band gap energy ( E g ) of 2.75 eV at 300 mV SCE , changed to a p-type semiconductor by decreasing the applied potential to 0 V SCE or lower. In contrast, the inner CrOOH layer revealed an n-type semiconductor with E g of 3.0 eV, irrespective of the applied potential. However, the Mott–Schottky behavior for the passive film on Cr revealed a p-type semiconductor above −100 mV SCE . An electronic band structure model for the passive film on Cr was proposed to explain the photocurrent spectral responses and the Mott–Schottky behavior of the film.

The Passivity of Iron in the Presence of Ethylenediaminetetraacetic Acid. II. The Defect and Electronic Structures of the Barrier Layer

The passive state on iron in EDTA (ethylenediaminetetraacetic acid, disodium salt) containing borate buffer solutions of pH 8.4 to 10.0 at ambient temperature has been explored using potentiodynamic polarization and steady-state techniques, including Mott-Schottky analysis. EDTA effectively suppresses the formation of the outer layer of the passive film thereby rendering the barrier layer amenable to direct examination. It is shown that the barrier layer on iron is a highly disordered, n-type semiconductor. The barrier layer thickness varies linearly with applied potential, whereas the steady-state current density does not depend on the formation voltage. EDTA present in the solution renders the barrier layer of the passive film more defective, as indicated by Mott-Schottky analysis, and the donor density decreases with increasing film formation potential. These findings suggest that the dominant defects in the barrier layer are oxygen vacancies or cation interstitials, or both. No evidence of cation vacancies, which would render the barrier layer p-type and would impart a voltage dependence to the steady-state current was obtained. Thus, if cation vacancies are present, they are not the dominant defect. © 2001 The Electrochemical Society. All rights reserved.

Effects of hydrogen on the electronic properties and stability of the passive films on iron

The effects of hydrogen on the electronic properties and stability of passive films on iron formed at various potentials in 0.3 M H 3 BO 3+0.075 M Na 2B 4O 7·10H 2O were investigated by polarization curves, ac impedance, and induction period measurements. It was found that the polarization behavior of iron changes from self-passivation to active/passivation when the hydrogen charging current density reaches a critical value; the charge transfer resistance decreases with increasing hydrogen charging current density and finally reaches a relatively stable value. Mott–Schottky analyses of capacitance data show that hydrogen results in an increase in donor density of the passive films formed in the range of investigated film formation potentials and the increased amount decreases with increasing film formation potentials. The effect of hydrogen charging current density on the donor density of a passive film formed at 0.2 V is stronger than that of a film formed at 0.6 V, resulting in a much larger decrease in pitting corrosion resistance for a film formed at 0.2 V than for one formed at 0.6 V. These results are explained by the reduction of iron oxide by hydrogen.

Photo-electrochemical analysis of the passive film on zircaloy-4

The structure and composition of passive film formed on Zircaloy-4 in pH 8.5 buffer solution was examined qualitatively through the photo-electrochemical analysis of the film. The passive film was found to be an n-type semi-conductor composed of single or duplex layers depending on the film formation potential, polarization time and applied potential. The passive film formed at potentials active to 0 V versus saturated calomel electrode (SCE) was a single layer of hydrous ZrO 2 with a band-gap energy of 2.98±0.29 eV. In contrast, the passive film formed at potentials noble to 0.25 V SCE was composed of duplex layers with an outer hydrous ZrO 2 layer and an inner anhydrous ZrO 2 layer with band-gap energy of 4.30±0.15 eV that is close to that (4.77 eV) of crystalline ZrO 2 film formed in air at 400°C. The inner anhydrous ZrO 2 layer grew as film formation potential or polarization time increased. By examining the effects of applied potential on the photo-current spectrum for each layer of the passive film formed at 1 V SCE, the flat-band potential for the outer hydrous oxide layer was determined to be −0.6 V SCE while that for the inner anhydrous oxide layer was −1.2 V SCE.

Photoelectrochemical study on the passive film on Fe

Electronic properties of the passive film formed on Fe in pH 8.5 buffer solution were examined by the photoelectrochemical spectroscopy. Photocurrent responses for the passive film on Fe with film formation potential are explained by the modified band structure model developed on the basis of that for crystalline spinel (γ-Fe 2O 3) involving two types of electronic excitation processes. i.e., the p–d and the d–d transition. The photocurrent spectra for the passive film on Fe measured in a wide range of photon energy could be resolved into two components of photocurrent spectrum reflecting effects of each electronic transition on the photocurrent spectra. The band gap energy of each electronic transition process was determined from the resolved photocurrent spectrum; 2.87 eV for the d–d transition, and 3.67 eV for the p–d transition. A small photocurrent peak at 2.7 eV appeared at film formation potentials higher than 400 mV was associated with the formation of γ-FeOOH layer on γ-Fe 2O 3 film.

Estimating steel pitting resistance in flowing media from the salt film formation potential

Estimating steel pitting resistance in flowing media from the salt film formation potential

Biodegradable films from isolate of sunflower (Helianthus annuus) proteins.

The film-forming potential of isolate of sunflower proteins (ISFP) was investigated. Homogeneous films were obtained by dissolution of ISFP in alkaline water (pH 12), addition of a plasticizer, casting, and drying. Maximum protein solubilization and unfolding led to films with the highest elasticity. The effects of five dissolving bases and five plasticizers on the mechanical properties were studied. The use of ionic bases (LiOH, NaOH) capable of interfering with the interproteic noncovalent bonds resulted in the greatest tensile strength (sigma(max)) and elongation at break (epsilon(max)) values (3.9 MPa and 215-251%, respectively). Plasticizers conferred diverse tensile properties to the films: the use of 1,3-propanediol resulted in the highest sigma(max) (27.1 MPa), and glycerol resulted in the greatest epsilon(max) (251%). Different mechanical properties were obtained by using mixtures of these plasticizers.