Timo Saario

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 transpassive dissolution mechanism of highly alloyed stainless steels. I. Experimental results and modelling procedure

The transpassive dissolution of austenitic stainless steels (AISI 316L, AISI 904L, 254SMO and 654SMO) in a 0.5 M sulphate solution with pH 2 was studied by conventional and rotating ring–disc voltammetry, as well as electrochemical impedance spectroscopy. The main process in the transpassive potential region was found to be the release of soluble Cr(VI), while small amounts of lower-valency Cr or Mo species are released as well. Secondary passivation readily occurs for AISI 316L, whereas the remaining highly alloyed steels dissolve at high current densities in the whole potential range studied. The dissolution rate was found to increase in the order AISI 904L < 254SMO < 654SMO. Thus it can be correlated to the increase in the Cr and especially Mo content of the steel substrate. The impedance spectra contain contributions from the transpassive dissolution of Cr and secondary passivation, probably due to enrichment of Fe in the outermost layer of the surface film. A kinetic model of the process is proposed, including a two-step transpassive dissolution of Cr via a Cr(VI) intermediate and the dissolution of Fe(III) through the anodic film. The model was found to be in quantitative agreement with steady state current vs. potential curves and electrochemical impedance spectra. The kinetic parameters of transpassive dissolution were determined and the relevance of their values is discussed. � 2002 Elsevier Science Ltd. All rights reserved.

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.

A mixed-conduction model for oxide films on Fe, Cr and Fe–Cr alloys in high-temperature aqueous electrolytes—II. Adaptation and justification of the model

The aim of this two-part work is to propose a model for the corrosion mechanism of ferrous alloys in high-temperature aqueous environments. In this second part, the modifications to the mixed-conduction model (MCM) are discussed on the basis of experimental data presented in the first part for Fe, Cr and two Fe–Cr alloys (12 and 25 wt% Cr) in an aqueous solution at 200 °C. Application of the MCM to fit and predict experimental behaviour both at room temperature and at 200 °C is demonstrated. The major difference between the behaviour of films at room temperature and at 200 °C is that the mobility of ionic defects is much higher at the higher temperature. Estimates show that the ratio of the electronic and ionic diffusion coefficients (De/Di) is of the order of 105 at room temperature and ≈30 at 200 °C for pure Fe. Such a large difference explains the higher growth rate and thickness of films formed on Fe at the higher temperature. It is also in agreement with the higher defect content and lower field strengths in high-temperature films. The application of the MCM to Fe–Cr alloys indicates that the diffusion coefficient of major ionic current carriers is smaller for the alloys than for pure Fe. Alloying with Cr thus lowers the ionic mobility in the passive film on a ferrous alloy also at 200 °C.

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.

A mixed-conduction model for oxide films on Fe, Cr and Fe–Cr alloys in high-temperature aqueous electrolytes––I. Comparison of the electrochemical behaviour at room temperature and at 200 °C

Abstract The aim of this two-part work is to propose a model for the corrosion mechanism of ferrous alloys in high-temperature aqueous environments. In the first part of the work, experimental results of the electrochemical behaviour of pure Fe, pure Cr, Fe–12%Cr alloy and Fe–25%Cr alloy during the initial stage of oxide film formation at 200 °C are compared to those obtained at room temperature. The results have been obtained by using voltammetry, electrochemical impedance spectroscopy (EIS), contact electric resistance (CER) and contact electric impedance (CEI) techniques. An increase of temperature from room temperature up to 200 °C has been found to result in higher currents in the passive region for all the materials. The CER, EIS and CEI results indicate that the film especially on Fe is considerably thicker at the higher temperature. In addition, the EIS and CEI results give information of an ionic transport process at 200 °C, which has not been observed in the EIS response at room temperature. The dependence of the electrical and transport properties of the film on potential suggests that the films at 200 °C can also be described by a mixed-conduction model (MCM) introduced recently for room temperature. However, the faster rate of ionic defect transport has to be emphasised at the higher temperature. The adaptation of the MCM to high-temperature oxide films is discussed in more detail in the second part of this work.

The transpassive dissolution mechanism of highly alloyed stainless steels: II. Effect of pH and solution anion on the kinetics

The effect of pH and solution anion on the kinetics of transpassive dissolution of highly alloyed austenitic stainless steels (AISI 904L, 254SMO and 654SMO) was studied by a combination of electrochemical techniques. The experiments were performed in 0.5 M sulphate and 0.5 M chloride solutions, and in an equimolar mixture of the two. The transpassive dissolution was found to start at higher potentials in solutions with higher pH. The rate of transpassive dissolution was shown to decrease with increasing pH and to be the lowest in chloride solutions and the highest in sulphate electrolytes. The steady-state current vs. potential curves and the impedance spectra of the studied materials in the transpassive potential region were found to be consistent with a proposed kinetic model. The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the anodic oxide film via parallel reaction paths. The kinetic parameters of the model in solutions with different pH values and different anions were determined. The role of pH and solution anion in the transpassive dissolution process is discussed in relation to changes induced by these parameters in the composition of the anodic passive film. The factors determining the efficiency of Fe as a secondary passivating agent are also considered.

Conduction Mechanism of the Passive Film on Iron Based on Contact Electric Impedance and Resistance Measurements

The application of a mixed conduction model and a new contact electric impedance (CEI) technique to predict quantitatively the electronic and ionic transport properties of oxide films on iron in a nearly neutral tetraborate solution is discussed. The mixed-conduction model emphasizes the coupling between the ionic defect structure and the electronic conductivity in an anodic film. Conventional electrochemical techniques have not been sufficient to characterize properly the electronic and ionic properties of anodic films on metals. The CEI technique makes it possible to distinguish between processes taking place at different rates within oxide films. Using this technique together with the contact electric resistance technique, we have found that the diffusion coefficient for the electronic conduction in the anodic film on iron is several orders of magnitude higher than that for the ionic transport. This shows that the passive film on iron is predomiantly an electronic conductor. The fitting of the experimental results to the mixed conduction model gives a good agreement and thus supports the validity of this model in the present case.