Structure Of Passive Film Research Articles

Chemical composition and electronic structure of passive films formed on Alloy 600 in acidic solution

The chemical composition and the semiconducting properties of passive films formed on nickel based alloy (Alloy 600) in acidic sulphate solution, pH 2.0 at room temperature were studied using Auger analysis, voltammetric techniques and the Mott–Schottky approach. The results obtained revealed that the presence of both chromium and mixed nickel–iron oxides in the films leads to the development of a p–n heterojunction, which controls their electronic structure, similarly manner to the case of stainless steels and Alloy 600 in borate buffer solution. This behavior has been interpreted as representing of an oxide system, which has a duplex character, with an inner p-type semiconducting region, mainly formed by chromium oxide and an outer n-type semiconducting region, containing iron oxide. It could also be observed that the nickel oxide present in the films acts as a barrier layer conferring improved protection.

Potential dependence of surface crystal structure of iron passive films in borate buffer solution

The effect of passivation potential on surface crystal structure, apparent thickness and passivity of oxide films formed on pure iron prepared by plasma sputter deposition was investigated. The crystallinity was improved with passivation potential and the width of atomically flat terraces was expanded to 6 nm when passivating at 750 mV for 15 min, as observed by ex situ scanning tunneling microscopy (STM) after aging in air (<30% RH). Apparent thickness and passivity are linearly dependent on passivation potential. The former weakly depends on passivation duration, the latter strongly depends on passivation duration. This is well explained by the correlation between crystal structure and passivity.

In situ study on the effects of Ni and Mo on the passive film formed on Fe–20Cr alloys by photoelectrochemical and Mott–Schottky techniques

Abstract Effects of alloying elements (Ni and Mo) on the structure of passive film formed on Fe–20Cr alloys in pH 8.5 buffer solution were explored by analyzing the in situ electronic properties measured using the photoelectrochemical technique and Mott–Schottky analysis. The passive film formed on Fe–20Cr–10Ni was found to be mainly composed of Cr-substituted γ-Fe2O3 from similarities in photocurrent response for the passive films formed on the alloy and Fe–20Cr. On the other hand, the photocurrent spectra for the passive films of Fe–20Cr–15Ni–(0, 4)Mo alloys exhibited the spectral components associated with NiO and Mo oxide (MoO2 and/or MoO3) in addition to that induced by Cr-substituted γ-Fe2O3. Mott–Schottky plots for the passive films formed on Fe–20Cr–(10, 15)Ni and Fe–20Cr–15Ni–4Mo confirmed that the passive films on Fe–20Cr–(10, 15)Ni–(0, 4)Mo alloys have a base structure of Cr-substituted γ-Fe2O3 with variation of densities of shallow and deep donors depending on the Ni and Mo contents in the alloys. We suggest that the passive film formed on Fe–20Cr–(10, 15)Ni and Fe–20Cr–15Ni–4Mo alloys are composed of (Cr, Ni, Mo)-substituted γ-Fe2O3 when the concentrations of Ni and Mo are below critical values. However, NiO and Mo oxide (MoO2 and/or MoO3) would be precipitated in the passive films when the concentrations of Ni and Mo exceed critical values.

Photoelectrochemical analysis on the passive film formed on Ti in pH 8.5 buffer solution

The structure of passive film formed on Ti in pH 8.5 buffer solution was examined by comparing the photocurrent for both the passive film and the thermally grown oxide on Ti in air at 400 °C. The passive films formed on Ti in pH 8.5 buffer solution showed optical band gap energy of 3.36 eV, while the thermally grown oxide film on Ti was found to have band gap energy of 3.1 eV. The higher value of band gap energy of passive film was attributed to the less crystalline or more disordered structure of passive film compared to that of the thermal oxide. This fact was supported by the higher disorder energy of passive film, determined from the absorption tail of photocurrent spectrum, than that of thermal oxide.

Electrochemical characterization of the passive films formed on niobium surfaces in H2SO4 solutions

The electrochemical formation and characteristics of passive films on niobium surfaces in aqueous H2SO4 solutions were studied using open circuit potential and cyclic voltammetry. In the potential region between -1.0 and 1.2 V(NHE), the cyclic voltammetry data showed that the active/passive transition involves slow metal dissolution followed by the formation of semiconducting passive oxide films. The possible electrochemical reactions and the change of the oxidation steps of some niobium oxides occurring in the passive film during the polarization are proposed. A strong influence of the natural air-formed oxide film on the chemical composition of the passive film was shown. This influence makes chemical structure of thin passive films more complicated than that of thick anodic films. It is shown that the passive films consists of more or less stable oxides, such as NbO, NbO2 and Nb2O5. The Raman spectra revealed that the thin passive films were amorphous, while the films formed at higher voltages consist, primarily, of well-crystallized Nb2O5.

Erosion — corrosion of duplex stainless steel under Kuwait marine condition

Erosion — corrosion of stable passive metals like Duplex Stainless Steels (DSS) proceeds by the repeated removal and repair of the oxide film. When sand particles are entrained it can be a problem in marine pumps, piping systems, heat exchangers, and units handling seawater in the Arabian Gulf for use as part of desalination plants. More specifically, sand particles in the presence of seawater enhanced the erosion corrosion problem. If the passive film structure is stable, it will reform spontaneously when it becomes damaged. The erosion-corrosion study is concerned with the application of a jet impingement apparatus to study the behavior of DSS steel of Cr/Ni/Mo/N grade in simulated Arabian Gulf seawater. An interference color imaging technique (ICI) for thickness measurement of the oxide film passivity, pitting, and transpassivity of the surface of DSS has been developed under the effect of well defined hydrodynamic condition and potentiodynamic polarization technique. Electrochemical imaging technique of surface corrosion potential mapping (SCM) was also introduced to measure the rate of repair of the passive film combined poteniodynamic polarization under flowing conditions, both with and without the addition of sand particles. The main aim is to obtain a better understanding of the electrochemistry of DSS alloys in marine erosion–corrosion condition and to examine the viability of SCM and ICI to be used as a diagnostic tool for DSS material evaluation. The results indicated that both techniques gave optimum imaging analysis of DSS steel. Water jet impingement showed higher current densities at the ferrite dissolution potential and austenite pitting potential during the introduction of sand particles.

Corrosion characterization of titanium alloys by electrochemical techniques

The electrochemical behavior of Ti–6Al–4V and Ti–6Al–7Nb alloys, commonly used implant materials, particularly for orthopaedic and osteosynthesis applications, was investigated together with that of Ti–13Nb–13Zr alloy in Hank's solution at 37 °C. The aim of present study was to evaluate their corrosion resistance in an artificial physiological solution. This evaluation was carried out through the analysis of the corrosion potential variation with time, potentiodynamic polarization curves, and electrochemical impedance spectroscopy (EIS) tests. Very low current densities were obtained (order of nA/cm 2) from the polarization curves, indicating a typical passive behavior for all investigated alloys. The EIS results exhibited capacitive behavior (high corrosion resistance) with phase angles close to −90° and high impedance values (order of 10 6 Ω cm 2) at low and medium frequencies, which are indicative of the formation of a highly stable film on these alloys in the test solution. The obtained EIS spectra indicated two relaxation time constants and their interpretation in terms of an “equivalent circuit” with the circuit elements representing the electrochemical properties of a two-layer oxide, composed of a porous outer layer and a dense inner layer, was in good agreement. The electrochemical impedance spectroscopy (EIS) technique, therefore, was able to provide reliable data for determination of the passive film structure.

Photoelectrochemical analysis on the passive film formed on Ni in pH 8.5 buffer solution

The structure and composition of passive film formed on Ni in pH 8.5 buffer solution were examined by comparing the photocurrent and Mott–Schottky responses for both the passive film and the thermally grown oxide on Ni in air at 400 °C. The photocurrent spectra for the passive film on Ni are resolved into two spectral components, each of which originated from inner NiO and outer Ni(OH) 2 layers, respectively. The band gap energy ( E g) for the inner NiO and the outer Ni(OH) 2 were measured to be 4.80 ± 0.1 and 3.05 ± 0.1 eV, respectively. E g (4.80 ± 0.1 eV) of the inner NiO of the passive film on Ni was close to that (4.97 eV) of the thermally grown oxide of Ni within an error range, suggesting that the inner NiO in the passive film is crystalline. The Mott–Schottky plots for both the passive film and the thermal oxide film on Ni demonstrated that the two films exhibited p-type semiconductivity with different values of flat band potential ( U fb); 0.3 V SCE for the passive film and 0 V SCE for the thermally grown NiO. An electronic energy band model of P–p heterojunction composed of inner NiO and outer Ni(OH) 2 layers was proposed to explain the photocurrent and Mott–Schottky responses for the passive film on Ni.

Effects of EDTA on the electronic properties of passive film on Fe–20Cr in pH 8.5 buffer solution

Effects of EDTA (ethylene diammine tetraacetic acid) on the electronic properties of passive film formed on Fe–20Cr alloy in pH 8.5 buffer solution were examined by the photo-electrochemical and impedance techniques to elucidate the structure and composition of passive film on the alloy. XPS composition profile in depth for the passive film revealed that Cr content in the outermost layer of the passive film was higher for the passive film formed in the solution containing EDTA than that in solution without EDTA, due presumably to the selective dissolution of Fe oxide by EDTA. In spite of the selective dissolution of Fe oxide in the outer layer of the passive film, the photocurrent spectra and the Mott–Schottky plots for the passive film formed on Fe–20Cr in EDTA containing solution were very close to those in EDTA-free solution, which strongly supports that the passive film on Fe–20Cr in pH 8.5 buffer solution is a single layered Cr-substituted γ-Fe 2O 3 oxide rather than a duplex layered oxide composed of inner Cr oxide and outer Fe oxide.

Effect of niobium on the electronic properties of passive films on zirconium alloys

The effects of niobium on the structure and electronic properties of passive films formed on zirconium alloys in pH 8.5 buffer solution were examined by photo-electrochemical analysis. For Zr– xNb alloys ( x = 0, 0.45, 1.5, 2.5 wt%), the photocurrent began to increase at an incident energy of 3.5–3.7 eV and exhibited the first peak at 4.3 eV and the second peak at 5.7 eV. From the ( I ph hν) 1/2 vs. hν plot, indirect band gap energies of E g 1 = 3.01 – 3.47 eV and E g 2 = 4.44 – 4.91 eV were obtained. With increasing Nb content, the relative photocurrent intensity of the first peak increased significantly. Compared with the photocurrent spectrum of the thermal oxide of Zr–2.5Nb, it was revealed that the first peak in the photocurrent spectrum for the passive film formed on Zr–Nb alloy was generated by two types of electron transitions; one caused by hydrous ZrO 2 and the other created by Nb. Two electron transition sources were overlapped over the same range of incident photon energy. In the photocurrent spectrum for passive film formed on Zr–2.5Nb alloy in which Nb is dissolved into the matrix by quenching, the relative photocurrent intensity of the first peak increased, implying that dissolved Nb acts as another electron transition source.

Electrochemical and XPS studies and the potential scan rate dependent pitting corrosion behavior of Zircaloy-2 in 5% NaCl solution

Electrochemical studies of Zircaloy-2 in 5% NaCl solution were carried out using polarization and electrochemical impedance spectroscopy (EIS). Scan rate dependent pitting behavior of Zircaloy-2 was observed when polarization experiments were carried out at different scan rates (5 mV/min, 50 mV/min and 500 mV/min). Polarization resistance, double layer capacitance, and the thickness of passive film were calculated from the EIS study. The slope of the Bode plot in the capacitive region showed non-ideal behaviour, suggesting formation of pits. A distribution of chloride concentration was seen at the surface of the electrode when the test solution was sampled from different parts of the sample surface and analyzed for chloride. Inclusion of chloride ions in the passive film was observed from the XPS study and the chloride concentration increases with the higher anodic polarization. The outermost layer of the passive film was rich in hydroxide and oxy-hydroxide. A bi-layer passive film structure on the alloy surface is proposed.

XPS and STM study of the growth and structure of passive films in high temperature water on a nickel-base alloy

The early stages of passivation in high temperature water of a nickel–chromium–iron alloy (Alloy 600) have been investigated by X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). The samples (polycrystal Ni−16Cr−9Fe (wt. %) and single crystal Ni−17Cr−7Fe (1 1 1)) have been exposed for short time periods (0.4–8.2 min) to high temperature (325 °C) and high pressure water, under controlled hydrogen pressure, in a microautoclave designed to transfer the samples from and to the XPS spectrometer without air exposure. In the early stages of oxidation of the alloy (0.4–4 min), an ultra-thin oxide layer (about 1 nm) is formed, which consists of chromium oxide (Cr 2O 3), according to the Cr 2p 3/2 core level spectrum. An outer layer of Cr(OH) 3 with a very small amount of Ni(OH) 2 is also revealed by the Cr 2p 3/2, Ni 2p 3/2, and O 1s core level spectra. At this early stage, there is a temporary blocking of the growth of Cr 2O 3. For longer exposures (4–8 min), the Cr 2O 3 inner layer becomes thicker, at the expense of the outer Cr(OH) 3 layer. This implies the transport of Cr and Ni through the oxide layer, and release of Ni 2+ in the solution. The structure of the ultra-thin oxide film formed on a single crystal Ni−17Cr−7Fe(1 1 1) alloy was analysed by STM in the constant current mode; STM images reveal that, in the early stages of oxidation, the oxide is crystalline, and the observed structure is consistent with the hexagonal structure of the oxygen sub-lattice in the basal plane (0 0 0 1) of α-Cr 2O 3.

New Interpretation of the Effect of Hydrogen on the Ion Distributions and Structure of Passive Films on Microalloyed Steel

It has been reported that hydrogen decreases the stability of the passive films on iron and stainless steel and promotes their localized corrosion. However, the mechanism of hydrogen-promoted corrosion is still not clear. Potentiostatic, electrical impedance spectroscopy measurements, and Mott-Schottky analysis were used to investigate the effect of hydrogen on the properties of the passive film on X70 microalloyed steel. In the range of passive potentials, hydrogen retards the passive film formation and decreases its stability. Impedance spectra of passive film on X70 steel show that hydrogen decreases the resistance towards charge transfer and ion diffusion and increases the capacitance of the passive film. Mott-Schottky analyses show that hydrogen increases the capacitance and donor density, and decreases the flatband potential and the space-charge layer thickness of the passive film. A new explanation is proposed for the effects of hydrogen on the properties of the passive films. According to this explanation, hydrogen creates an additional electric repulsion on or and oxygen vacancies, and enhances the diffusion rate of cations and anion vacancies. At steady state, hydrogen will not only decrease the concentrations of and but also increase the concentration of in the passive films, resulting in changes in the structure and a decrease in the thickness of the passive films. © 2004 The Electrochemical Society. All rights reserved.

Influence of the Chemical Composition of Stainless Steels on the Electronic Structure of Passive Films Formed in Artificial Sea Water

Influence of the Chemical Composition of Stainless Steels on the Electronic Structure of Passive Films Formed in Artificial Sea Water

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.

Recent advances in photocurrent spectroscopy of passive films

The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-metal alloys gives satisfactory agreement with the theoretical expectations. The experimental results evidence the role of the alloying metal in determining the crystalline or disordered structure of passive films grown on Al,d-metal alloys.

Electrochemical and XPS studies of sputter-deposited ternary Mn–Ta–Cr alloys in chloride-free and chloride-containing sulphuric acid solutions

Amorphous manganese-rich (67–80 at.%) Mn–Ta–Cr alloy films were prepared by DC magnetron sputtering method. The corrosion behavior and stability of the Mn–Ta–Cr alloy films were examined in chloride-free and -containing 1 M H 2SO 4. AC and DC electrochemical techniques in combination with XPS analysis were used. Mn–Ta–Cr alloys exhibited very high corrosion resistance and stability up to transpassive dissolution region of chromium. The high corrosion resistance of Mn–Ta–Cr sputtered films is based on the formation of double oxyhydroxide passive film composed mainly of chromium and tantalum. The partial substitution of tantalum with chromium improves the corrosion resistance of the sputter-deposited alloys via accelerating the preferential dissolution of manganese and stabilizing alloy/passive film interface. A change in the passive film structure was observed when the alloys were anodically polarized at potentials higher than 0.6 V (SCE).

Electronic structure study of ac modified passive film on stainless steel

The semiconductor properties, electronic structure and reduction susceptibility of passive films on AISI type 304 stainless steel in dilute sulphuric acid solutions have been studied by electrochemical impedance spectroscopy measurements, Mott-Schottky analysis and the photoelectrochemical technique. The passive film shows features of an amorphous or highly disordered n type semiconductor. The donor level is in the range of 1026-1027 m-3 and increases with the amplitude of ac potential. The passive film thickness increases linearly with the amplitude of the applied ac potential. The reduction susceptibility of the passive film is also decreased with the amplitude of the applied ac. The photoelectrochemical measurements also indicate that the passive film is n type semiconductor and the photcurrent is decreased after ac modulation. It might be related to the surface chromium accumulation.

Initiation of a Local Breakdown of Passive Film on Iron due to Chloride Ions Generated by a Liquid-Phase Ion Gun

A small amount of could be locally generated from a liquid-phase ion gun (LPIG), which consists of a silver microelectrode covered with silver chloride, by cathodic polarization. The LPIG was employed to induce the local breakdown of the passive films formed on iron in deaerated borate solutions. The concentration of generated locally on the passivated iron surface was estimated to be about 1 mol dm−3 from the measurement of the potential difference between the narrow space enriched with and bulk solution. The induction period, was required for initiation of local breakdown. A longer induction period was needed for initiation of local breakdown of the thicker passive films with the lower electric field. The structural changes of passive films due to before initiation of local breakdown are discussed. Moreover, the induction period depended on the substrate grain orientation, which is explained in terms of difference in thickness or defect structure of passive films on substrate grains. © 2001 The Electrochemical Society. All rights reserved.

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.