Elzbieta Sikora

Nature of the passive film on nickel

Abstract The passive film formed anodically on nickel in borate buffer solution in both the passive and transpassive regions is found to be p-type in electronic character, corresponding to a preponderance of metal vacancies (over oxygen vacancies and nickel interstitials) in the barrier layer. However, at high anodic potentials, some n-type character was detected by Mott–Schottky analysis, which is probably due to the presence of free charge carriers (electrons) from the evolution of oxygen and/or the oxidative ejection of Ni 3+ at the barrier layer/outer layer interface. The p-type character of the film is consistent with the diagnostic criteria obtained from the Point Defect Model for a passive film, in which the majority defect in the NiO barrier layer is the metal vacancy. The transpassive state is postulated to comprise a thick, porous oxide film on the surface, with the current probably being due to the oxidative ejection of Ni 3+ species from the barrier layer and oxygen evolution within the pores, or both.

A new method for estimating the diffusivities of vacancies in passive films

Using Mott-Schottky (M-S) analysis in conjunction with the Point Defect Model (PDM) for passive films, we have determined the oxygen vacancy diffusion coefficient in the WO3−x passive film formed on tungsten in phosphoric acid (pH = 0.96) solution at ambient temperature. The value obtained for Do is in the range (3.7–5.3)× 10−15cm2s.This value was extracted from the exponential relationship between the donor density and the film formation voltage assuming that the depletion region in the film close to the metal/film interface dominates the semiconductor properties.

Characterization of the Passive State on Zinc

Despite intensive investigations, the nature of the passive state is one of the most complex and unresolved subjects in the electrochemistry of zinc in alkaline solutions. In this paper, we explore the electrochemistry of the passive state on zinc in 0.1 M sodium borate/1 M sodium hydroxide solution of pH 10.5. During the course of passivation, several characteristic features in the anodic region are observed, including a wide passive range extending over >2 V and a secondary passivation phenomenon that occurs at high anodic potentials. The steady-state current in the passive state is found to be independent of the applied voltage, which is consistent with the barrier layer being an interstitial zinc conductor or an oxygen vacancy conductor (or both) with interstitial zinc being the most likely defect. This model is also consistent with the well-known n-type character of the passive film on zinc.

The Passivity of Iron in the Presence of Ethylenediaminetetraacetic Acid I. General Electrochemical Behavior

The effect of EDTA (ethylenediaminetetraacetic acid, added as ) on the electrochemical behavior of iron and on the semiconducting properties of passive films formed on this metal in borate buffer solution (pH 8.4) was studied. Electrochemical studies, including potentiodynamic and galvanostatic reduction experiments and capacitance measurements coupled with Mott‐Schottky analysis, were carried out over the entire passive range in borate buffer solutions in the presence and absence of 0.01 M EDTA. Passive films formed in the presence of EDTA were thinner and showed higher donor concentrations. EDTA present in the borate buffer solution effectively inhibited the formation of the Fe(III) outer layer, thereby rendering the defective inner layer to direct examination. The barrier layer is found to be an n‐type semiconductor, which is consistent with the oxide being a defective magnetite in which the principal defects are oxygen vacancies and cation interstitials. EDTA effectively competes with chloride ion for adsorption into oxygen vacancies at the barrier layer/solution interface, thereby suggesting that the chelating agent might be an effective inhibitor of passivity breakdown.

The electronic structure of the passive film on tungsten

Abstract The capacitance of the passive film on tungsten in 1 M phosphoric acid solution has been measured as a function of frequency and voltage and Mott–Schottky analysis is used to derive dopant type and concentration data. The passive films were grown potentiodynamically to formation potentials within the range 3–10 V and were allowed to age until steady-state conditions were achieved. The capacitance was measured during a potential sweep from the formation voltage in the negative direction at a sufficiently high sweep rate that the thickness of the film remains constant. The data are interpreted in terms of a modified point defect model that takes into account the existence of an insulating outer layer on the defective WO3 barrier layer. The primary defect in the barrier layer was found to be the oxygen vacancy, which is concentrated in the region of the barrier layer adjacent to the metal, with the outer layer being postulated to form either by restructuring of the outer surface of the barrier layer or by hydrolysis of emerging cations or both. The data suggests that the space charge capacitance in the barrier layer is dominant at low formation voltages, but that at higher voltages the capacitance of the insulating outer layer makes an important contribution to the measured capacitance.

On the Kinetics of Growth of Anodic Oxide Films

Two theories, the high field model (HFM), and variants thereof, and the point defect model (PDM), have been advanced to account for the properties of thin anodic passive films (barrier layers). The HFM states that, under potentiostatic conditions, the electric field strength in a passive film decreases as the film thickens, while the PDM proposes that the field strength remains constant but that the potential drop across the metal/film interface decreases as the film grows. These different hypotheses lead to different kinetic laws for film growth. In this work, diagnostic criteria have been derived to determine the electric field-thickness relationship and hence to identify the appropriate kinetic law. Experiments on the growth of anodic oxide films on zirconium, tungsten, and tantalum under potentiostatic conditions in phosphate buffer solution show that the PDM provides a better account of the experimental data than does the HFM.

Characterizing electrochemical systems in the frequency domain

Abstract The use of various impedance spectroscopies, including electrochemical impedance spectroscopy (EIS), photoelectrochemical impedance spectroscopy (PEIS), and those defined using various electrochemical, hydrodynamical, and mechanical conjugate variables, for studying physico-electrochemical systems is briefly reviewed with emphasis on identifying and defining a common theoretical basis. In particular, we explore the concepts of “conjugate properties” and “reciprocity”, the applicability of the Kramers-Kronig transforms, and the distinction between an impedance and a transfer function. We argue that the interpretation of impedance data in terms of electrical analogs and in terms of classical linear systems theory (LST) is possibly too restrictive, and that a more general basis is required to accommodate the broad range of “impedance spectroscopies” that are now being developed.

Defining the passive state

The growth of passive film on tungsten in phosphate buffer solution has been described in terms of the Point Defect Model (PDM). The steady-state current and passive film thickness have been measured as a function of voltage, with the film thickness being obtained from an analysis of capacitance and reflectance data. The observed data cannot be accounted for by the High Field Model (HFM) in its classical form, but can be understood in terms of the PDM. Diagnostic criteria that have been derived from the PDM were used to identify the majority charge carriers in the passive film. The Point Defect Model was employed, together with Mott-Schottky analysis to explore the crystallographic defect structures of the passive films, whereas their electronic structures were studied using photoelectrochemical impedance spectroscopy (PEIS). The experimental results demonstrate that these structures are strongly coupled with the vacancies acting as the dopants.

The kinetics of growth of the passive film on tungsten in acidic phosphate solutions

The growth of the passive film on tungsten in phosphate buffer solutions (pH range from 1.5 to 5.1) has been described in terms of the High Field Model (HFM) and the Point Defect Model (PDM). The steady-state current and passive film thickness have been measured as a function of voltage, with the film thickness being obtained from an analysis of capacitance and reflectance data. The observed data cannot be accounted for by the HFM in its classical form, but can be understood in terms of the PDM. A modified HFM, taking into account film dissolution at the film/solution interface, has also been developed, but it, too, fails to fully account for the experimental observations.

Photo-inhibition of pitting corrosion on types 304 and 316 stainless steels in chloride-containing solutions

The influence of incident monochromatic uv light on the susceptibility of types 304 and 316 stainless steels to pitting corrosion in chloride-containing solutions is described. Illumination of the immersed surfaces, in neutral and acidic solutions, gave rise to an enhanced resistance to the onset of pitting corrosion that persisted for some 220 h after the irradiation had been removed. This was evident from an increase in the breakdown potential, longer induction periods, and notable changes in the current noise at constant potentials for specimens polarized under illumination. It was found that the degree of inhibition to pitting attack depended on the photon energy, the illumination period and the nature of the passive film. Higher breakdown potentials and longer induction periods were observed with high energy photons (λ < 375 nm); these photons having energies in excess of the bandgap energy. Also, the photo-inhibition effect increased with the period of illumination; the maximum effect being observed following a 5-h illumination period. A much reduced photo-inhibition effect was observed for specimens polarized in alkaline solutions. This was attributed to the formation of a precipitate layer at the film-solution interface, which hindered the interaction of the incident photons with the barrier layer.