Gunilla Fabricius

Coupling between ionic defect structure and electronic conduction in passive films on iron, chromium and iron–chromium alloys

Abstract A quantitative kinetic model is presented for the steady-state passive films on Fe, Cr and Fe–Cr alloys. It emphasises the coupling between the ionic defect structure and electronic conduction. According to the model, the passive film can be represented as a heavily doped n-type semiconductor–insulator-p-type semiconductor junction. At low potentials in the passive state, the positive defects injected at the metal/film interface play the role of electron donors. At high positive potentials, the negative defects injected at the film/solution interface play the role of electron acceptors. At sufficiently high positive potentials the concentration of these ionic defects and corresponding electron holes reaches high enough values for the film to transform into a conductor. This enables transpassive dissolution of Cr and oxygen evolution on the film surface. Equations for the electronic conductivity of the passive film, as depending on the concentration of point defects, are derived. The proposed model is compared with experimental data obtained for pure Fe, pure Cr, Fe–12%Cr alloy and Fe–25%Cr alloy passivated in 0.1 M borate solution (pH 9.2) using rotating ring-disk voltammetry, photocurrent and impedance spectroscopy and in situ dc resistance measurements by the contact electric resistance (CER) technique.

The stability of the passive state of iron-chromium alloys in sulphuric acid solution

Abstract The passivation and the transpassive dissolution of Fe–Cr alloys (12% and 25% Cr) was studied with a combination of electrochemical techniques—conventional and rotating ring–disk voltammetry, impedance spectroscopy and the contact electric resistance (CER) technique developed to measure the dc resistance of surface films. Rotating ring–disk studies indicated that both soluble Cr (VI) and Fe (III) are released from the alloys in the transpassive region. The electronic resistance of the transpassive anodic film was found to decrease as Cr (VI) is released from the outermost layers adjacent to the interface and to increase subsequently due to the formation of a Fe (III) rich secondary passive film. Impedance spectra of the Fe–25% Cr alloy were found to include contributions from both the film growth and transpassive dissolution reactions, whereas the corresponding spectra of the Fe–12% Cr alloy reflected mainly the contribution of the film. On the basis of the experimental results, a generalized model of the transpassivity of Fe–Cr alloys is proposed. The model represents the anodic film as a highly doped n-type semiconductor–insulator–p-type semiconductor (n–i–p) structure. Injection of negative defects at the film/solution interface results in their accumulation as a negative surface charge. It alters the non-stationary film growth rate controlled by the transport of positive defects (oxygen vacancies). The transpassive dissolution reaction is assumed to be a two-stage process featuring a Cr (IV) intermediate. The relaxation of the Fe fraction in the outermost cation layer of the film is taken into account as well. Fitting of the experimental data on the basis of equations derived for the steady state and impedance response enable the determination of the kinetic parameters of transpassive dissolution.

Electrochemical study of the passive behaviour of Ni–Cr alloys in a borate solution—a mixed-conduction model approach

This paper discusses the application of the recently introduced mixed-conduction model (MCM) to describe the electrochemical behaviour of anodic films formed on Ni–Cr alloys. The MCM emphasises the coupling between the spatial distribution of ionic point defects in the film and the electronic conductivity of the film. The basic concepts of the MCM are introduced, and equations for the concentration profiles of ionic defects, for the conductivity and for the ac response of the film are derived. The experimental part of this work consist of characterising the anodic behaviour of Ni–Cr alloys, pure Ni and pure Cr in 0.1 M Na2B4O7 solution (pH 9.3) at ambient temperature using rotating disc and cyclic voltammetry, contact electric resistance (CER) and electrochemical impedance spectroscopic (EIS) techniques. The experiments indicate that the film on a Ni–20%Cr alloy resembles that on pure Cr, whereas similarities can be found between the film on Ni–10%Cr alloy and pure Ni. The conductivity of the passive film formed on Ni–Cr alloys can be concluded to be based on the spatial and potential distribution of the valence states of Ni and Cr in the film. In spite of several simplifying assumptions used in the simulations, a qualitative agreement between the theory and experimental data was achieved.

The mechanism of transpassive dissolution of Ni–Cr alloys in sulphate solutions

Abstract The processes in the potential region corresponding to transpassive dissolution of Ni–Cr alloys (10 and 20 wt.% Cr), pure Ni and Cr were studied by electrochemical techniques — ring-disk voltammetry, impedance spectroscopy and dc resistance measurements — in 1 M sulphate solutions (pH values 0 and 5). Both the electronic and the ionic conductivity of the anodic film formed on Ni and Ni–Cr alloys were found to be considerably higher than those of the passive film on pure Cr. The rate of transpassive dissolution of Cr from Ni–Cr alloys was found to be somewhat higher than that from pure Cr, probably due to the higher electronic conductivity of the film in the former case. A model for the transpassive dissolution of Ni–Cr alloys is proposed on the basis of an earlier model for transpassive dissolution of Cr and the present results. The effect of the transpassive dissolution of Ni from the oxide film is taken into account as well. The kinetic parameters of the transpassive dissolution process are thereby determined.

Conduction mechanism of the anodic film on chromium in acidic sulphate solutions

Abstract The passive state of Cr in 1 M sulphate solutions (pH 0 and 5) was studied with a combination of electrochemical techniques — impedance spectroscopy, photoelectrochemistry and dc resistance measurements by the contact electric resistance (CER) technique. Passive film growth was found to be associated with an exponential increase of the film resistance probably due to the simultaneous dehydration/oxidation of Cr(II) to Cr(III) via a solid state electrochemical reaction. A description of the steady state passive film as a thin insulator layer with small intrinsic conductivity was consistent with the experimental results. Polarization of Cr, on which a steady state film had been formed, to positive and negative potentials led to substantial increase of the conductivity of the film. These changes can be attributed to the generation of lower and higher valency Cr species in solid state electrochemical reactions: At low potentials lower valency species are formed in the first layers adjacent to the metal/film interface, while at high potentials higher-valency species are formed in the first layers adjacent to the film/electrolyte interface. At sufficiently high positive (or low negative) potentials the film was concluded to be transformed into a conductor allowing transpassive (or active) dissolution to take place.

Inhibitive Effect of Benzotriazole on Copper Surfaces Studied by SECM

The inhibitive effect of benzotriazole on a commercial copper surface phosphorus-deoxidized copper was studied by a scanning electrochemical microscope SECM in an aqueous sodium sulfate solution using ferrocenemethanol FcMeOH as the redox mediator. The formation of the inhibitive film was followed as a function of time and as a function of the potential of the copper substrate. The results were analyzed using the existing models in the literature. The results show that the potential has a crucial effect on the growth of the CuI-BTA film. At a potential close to the dissolution range of copper, the surface changes gradually from almost ideally conductive to almost ideally insulating surface in the presence of benzotriazole, but at more negative potentials the effect is diminished and finally deceased. Quartz crystal microbalance experiments showed that the mass of an adsorbed layer must correspond to a multilayer.

Influence of thiourea on the nucleation of copper from acid sulphate solutions

The influence of thiourea on the nucleation and growth of copper onto glassy carbon from acid sulphate solutions was studied using potentiostatic steps and cyclic voltammetry. The electrolyte was a 0.07 M CuSO4–1.8 M H2SO4 solution containing 0, 1, 10, 100 or 1000 mg l−1 thiourea. In solutions without thiourea an instantaneous nucleation takes place at the surface. Addition of thiourea changes the initial nucleation to a progressive one. After the initial period of progressive nucleation, the nucleation process is terminated and a transition from progressive to instantaneous nucleation takes place. Thiourea increases the nucleation rate and the number density of nuclei at the surface. Since the deposit is smooth and planar at high nuclear number densities, it can be concluded that thiourea effectively promotes smoothness of the surface. The highest nuclear number densities are obtained at 10 mg l−1 thiourea. At higher concentrations of thiourea the nucleation process is disturbed by the high amount of thiourea molecules adsorbed. A convenient diagnostic criterion for thiourea concentrations too high to produce a smooth and homogeneous surface was found in the cyclic voltammograms. An extra anodic stripping peak, possibly due to dissolution of copper—thiourea complexes adsorbed on the surface, appears at too high thiourea concentrations.

Influence of molybdenum on the conduction mechanism in passive films on iron–chromium alloys in sulphuric acid solution

Abstract This paper describes an extension of the mixed-conduction model to predict quantitatively the electrochemical behaviour and transport properties of anodic films on pure Cr, Fe–Cr alloys and Fe–Cr–Mo alloys in 1 M sulphuric acid solution. The anodic films on Fe–Cr–Mo alloys (12 and 25% Cr; 0, 5 and 10% Mo) were studied using rotating ring-disk voltammetry, impedance spectroscopy and resistance measurements. The addition of Mo to the Fe–Cr alloys was found to decrease the resistance of the film both in the passive and transpassive region. During the re-activation of the Fe–12%Cr–x%Mo alloys at negative potentials, soluble products were found to be released at a higher potential than during the re-activation of the Fe–12%Cr alloy. Re-activation proceeds to a lesser extent for the Fe–25%Cr alloy, and was not observed for the Fe–25%Cr–x%Mo alloys. The addition of Mo was also found to lead to a marked increase of the transpassive dissolution rate of the alloys. The impedance spectroscopic results indicated that the addition of Mo increases the rate of the interfacial generation of positive defects and especially annihilation of negative defects. The present extension of the mixed-conduction model for anodic passive films was found to describe quantitatively the resistance and impedance spectroscopic data for Fe–25%Cr and Fe–25%Cr–10%Mo alloy, as well as earlier data on pure Cr. The same kinetic model can be used to describe the behaviour of the films on all these materials. It is thus most likely that a Cr-oxide-based film is formed which determines the behaviour of the Fe–25%Cr and Fe–25%Cr–10%Mo alloys as well.

Influence of thiourea and thiourea ageing on the electrodeposition of copper from acid sulfate solutions studied by the ring-disc technique

The influence of the additive thiourea in freshly made and aged solutions on copper deposition from acid sulfate solutions was studied using a Pt/Pt rotating ring-disc electrode. In fresh thiourea solutions copper deposition is retarded, but the most distinct effect of thiourea is seen during dissolution of the deposits. The intermediate in copper dissolution, Cu+, is partly complexed with adsorbed thiourea. Thus Cu+ is adsorbed at the electrode surface and the current peak at the ring due to dissolving Cu+ becomes very small. The small peak indicates that active additive is present in the deposit. Thiourea containing solutions aged for only a few days have a distinct effect on the electrode reactions. At low concentrations of thiourea (1 mg dm−3 ) a correlation between ageing and growth of the Cu+ peak at the ring is clearly seen. At high thiourea concentrations (100 mg dm−3) a small growth of the Cu+ peak is seen during the first few days but upon further ageing of the thiourea solution copper deposition becomes strongly inhibited. The ring current can be used as a qualitative diagnostic criterion for the concentration and state of thiourea in copper sulfate solutions.

Conduction Mechanism of the Anodic Film on Fe‐Cr Alloys in Sulfate Solutions

The passive state of Fe-12% Cr and Fe-25% Cr alloys in 1 M sulfate solutions of pH 0 and 5 was studied with a combination of electrochemical techniques: impedance spectroscopy, photoelectrochemistry, and dc resistance measurements by the contact electric resistance technique. The investigations were supported by studies on pure metal constituents (Fe, Cr) in the same solutions. As a result, the steady-state passive film on the alloys (and on pure Cr) can be described as a thin, essentially insulating layer. Polarization of the steady-state metal/anodic film/electrolyte system to negative and positive potentials away from the potential region corresponding to the highest electronic resistance was concluded to lead to the generation of lower or higher valency defects at the interfaces via solid-state electrochemical reactions. These reactions result in a substantial increase of the conductivity in the first layers adjacent to either the metal/film or the film/electrolyte interface. At very negative (or very positive) potentials the film is transformed into a conductor allowing active (or transpassive) dissolution to take place. A quantitative physical model of the conduction mechanism on the basis of the surface charge approach is proposed and compared with experimental data.