Carmelo Sunseri

Photocurrent Spectroscopic Investigations of Passive Films on Chromium

A detailed photoelectrochemical study was carried out in order to identify the nature and the properties of the passive layers on chromium under different experimental conditions. Different compositions of the films, related to various hydration degrees of the Cr(III) oxides, have been inferred on the basis of the bandgap values measured at changing pH, electrode potential, and polarization time. By comparison with air‐grown films, it has been concluded that anhydrous is never formed in acidic solution. The water content in the passive film increases at decreasing potentials and for long polarization times. In some cases, especially at neutral pH values, passive films possess a duplex structure, consisting of two layers with different hydration degrees. The photoelectrochemical study suggests an insulating or a slightly p‐type behavior of the passive layers.

Semiconducting properties of anodic WO3 amorphous films

Abstract The semiconducting properties of amorphous WO3 anodic films grown in different solutions and at different current densities have been investigated. The Mott—Schottky plots have shown that the donor density, Nd, of the grown films is strongly dependent on the films thickness and is not influenced by the nature of the anodizing solutions. The possible influence of the kinetics of anodization on Nd is discussed. The intersection voltages at 1/C2sc = 0 in the Mott—Schottky plots show a complex dependence on the film thickness. The possibility of obtaining the flat band potential from these plots is also discussed. A linear relationship between the square of the photocurrent and the electrode potential has been observed as previously reported for single crystals. The dissolution rate of anodic WO3 films is increased under illumination and strongly decreased in presence of Fe 2+ 3+ couple. The kinetics of electron transfer between the amorphous WO3 anodic films and the Fe 2+ 3+ redox couple in the electrolyte seems to occur in accordance with the theory developed for single crystal semiconductors.

A photoelectrochemical characterization of passive films on stainless steels

Abstract Passive films on stainless steels for sea water service have been studied by means of photocurrent measurements. Four experimental ELI (Extra Low Interstitital) ferritic stainless steels (types 25Cr-4Ni-4Mo) and a commercial superaustenitic stainless steel (type 20Cr-18Ni-6Mo-N) have been investigated. Differences in the photoelectrochemical behaviour have been attributed to the different nature of the passive films grown on ferritic or superaustenitic stainless steels. An interpretation is provided to explain the complex dependence of the photocurrent transients on the applied potential. Manuscript received 27 February 1986; in amended form 19 June 1986.

A Photocurrent Spectroscopic Investigation of Passive Films on Ferritic Stainless Steels

Etude experimentale de la nature des films passifs croissant sur trois aciers inoxydables ferritiques de compositions differentes immerges dans une solution a 3,5% NaCl a la temperature ambiante

Amorphous semiconductor—electrolyte junction. Impedance study on the a-Nb2 O5—electrolyte junction☆

A systematic study of the impedance behaviour of the anodic niobium oxide film/aqueous electrolyte interface was carried out using the lock-in technique at different signal frequencies. The dependence of both components of the impedance on the electrode potential and on frequency is analysed by taking into account the amorphous nature of the films. The lack of long-range order in these oxide layers modifies the physical picture in respect to the case of single crystal semiconductors. A new equivalent circuit has been assumed, based on recent theory of an amorphous semiconductor Schottky barrier. Such a new approach allows the characterization of the interface and the determination of the electronic states distribution within the oxide gap. Possible mistakes arising from the use of models valid for crystalline semiconductors to study these kinds of interfaces are pointed out. The effect of annealing of the films under argon atmosphere as well as the possible nature of the defects in Nb2O5 anodic films are discussed.

Kinetics of Growth of Amorphous WO 3 Anodic Films on Tungsten

The kinetics of growth of anodic films have been studied in 0.01Nsolutions of , , , and at room temperature. The validity of the relationship was verified in the current density range 0.2–16 mAcm−2, Nearly constant B values were found in all investigated solutions. A lower value of dielectric constant was measured for the anodic films grown in phosphoric acid solutions. The effect of phosphate ions incorporation on the breakdown voltage is also discussed. The hypothesis of growth in presence of high space charge is carefully considered.

Anodic oxide films on tungsten—I. The influence of anodizing parameters on charging curves and film composition

Abstract The formation of anodic films on tungsten has been studied in 1N solutions of H 2 PO 4 , H 2 SO 4 , HNO 3 , HClO 4 and HCl at different temperatures. The shape of the anodic charging curves depends on the electrolyte composition and on the temperature. The chemical composition of the films is strongly influenced by the working temperature. The first step of the anodization is the formation of a barrier film of amorphous WO 3 . The thickening of the oxide layer occurs on account of the WO 3 dissolution and following precipitation of sparingly soluble oxides on the electrode surface.

Photoelectrochemical study of the corrosion product layers on copper in weakly acidic solutions

Abstract The nature of the surface layers grown on copper electrodes in free corrosion conditions in unbuffered aerated Na 2 SO 4 solutions (pH = 3.0–6.0) was investigated by photoelectrochemical methods. The formation of cuprous oxide has been revealed by means of a careful study of the photocurrent spectra obtained in situ . The growth of n -Cu 2 O has been observed to occur with or without Cu 2+ ions in solution in a narrow range of pH around five. More acidic solutions seem to favour the formation of p -Cu 2 O both in absence as well as in presence of Cu 2+ ions in solution ([Cu 2+ ] = 0.05 M). The addition of chloride ions in variable amounts ([Cl − ] ⩽ 0.1 M) seems to affect mainly the corrosion rate of copper electrodes at low concentrations ([Cl − ] ⩽ 10 −2 M). At higher chloride concentrations changes in the semiconducting properties as well as in the nature of the corrosion layers can occur. The presence of complex transients in the photocurrent vs time plots has been related to the presence of chloride ions in solution.

In situ characterization of passive films on Al-Ti alloy by photocurrent and impedance spectroscopy

Abstract The anodic behaviour of an Al-Ti alloy (Ti-48Al-1V, atomic %) was investigated in different aqueous electrolytes. In all cases the alloy was passive owing to the growth of a barrier-like oxide film according to the high field mechanism. The study of the growth curves suggests partial dissolution of the film during the formation process in acidic solution. The kinetic parameters for film formation have been estimated in neutral solutions and the dielectric constant of the passive layer was roughly estimated. The in-situ characterization of the passive film revealed a n-type behaviour only for very low thicknesses, whilst thicker films showed insulator-like characteristics. The analysis of the photocurrent spectra suggests that the composition of the surface oxide does not change in a large thickness interval since the very thin initial layers formed by simple immersion of the alloy in solution. From the values of the threshold energy derived from both anodic and cathodic spectra recorded in different conditions it is postulated that this passive film consists of a mixed oxide of the main components of the alloy. A more complex behaviour was observed at higher film thicknesses, which reveals some modification in the properties of the surface layer.

The photoelectrochemistry of thin passive layers. Investigation of anodic oxide films on titanium metal

A photoelectrochemical investigation has been performed on thin TiO2 films grown anodically in 0.5 M H2SO4 solution at high growth rates. The shape of the photocurrent vs. potential curves under monochromatic irradiation (photocharacteristics) depends on the photon energy of the incident light at energies above the optical band gap of the films (3.25 ± 0.05 eV). This finding has been explained by considering the presence of geminate recombination of the photogenerated electron-hole pairs. In order to fit the experimental photocharacteristics, an expression for the photocurrent is proposed which takes into account the low drift range of photocarriers and possible recombination in the space-charge region. Moreover, an exponential potential distribution in the film was assumed from the theory of the amorphous semiconductor (a-SC) Schottky barrier. The results of theoretical simulations suggest that the behaviour of TiO2 passive films can be explained in the frame of the theory of a-SC, taking into account the influence of the photon energy on the thermalization length of the photocarriers.