Petri Kinnunen

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.

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.

A Mixed-Conduction Model for the Oxidation of Stainless Steel in a High-Temperature Electrolyte Estimation of Kinetic Parameters of Oxide Layer Growth and Restructuring

The oxide films formed on AISI 316L(NG) in the temperature range 150-300°C have been characterized by impedance spectroscopy and ex situ analysis using Auger electron spectroscopy. Relatively thick films containing a highconcentration of mobile defects form on stainless steel in a high-temperature borate electrolyte, but their impedance response is most probably controlled by the properties of a thin barrier sublayer. The ability of the mixed conduction model for passive films to reproduce the experimental impedance data in both alloy/oxide/electrolyte and alloy/oxide/inert metal configurations has been tested. A procedure for the calculation of the kinetic constants of the interfacial reactions of point defect generation/consumption, as well as those characterizing the transport rates of ionic/electronic defects in the oxide, has been developed. The effect of temperature on the kinetic and transport parameters has been assessed, and the relevance of these parameters for the corrosion behavior of stainless steel in a high-temperature electrolyte is discussed. The results show that the nature of the barrier layer does not change drastically with temperature, although the growth mechanism of the oxide film is different at 150-300°C than at room temperature.

Mixed-Conduction Model for Stainless Steel in a High-Temperature Electrolyte: Estimation of Kinetic Parameters of Inner Layer Constituents

A quantitative procedure of determination of kinetic and transport parameters for individual alloy constituents during anodic film growth on stainless steels in light reactor water is developed. It is based on in-depth compositional data for oxides obtained from ex situ analyses using Auger electron spectroscopy and X-ray photoelectron spectroscopy. The growth of the inner compact layer is described as a sequence of interfacial reactions and transport driven by homogeneous diffusion-migration mechanism. Based on the mixed-conduction model for oxide films, a fitting procedure for the calculation of the in-depth distribution of the individual alloy constituents in the inner layer is put forward. The effects of temperature and applied potential on the kinetic and transport parameters in the inner layer are assessed. In addition, the growth of an outer layer consisting of crystallites with pores filled with electrolyte in-between is described formally as a diffusion process and the transport parameters characterizing this process are estimated. The estimates of the kinetic and transport parameters obtained are discussed in relation to the corrosion mechanism of the steel and the incorporation of electrolyte-originating species in the bilayer oxide film. Using the proposed quantitative procedure, the kinetic and transport parameters of long-term (up to 10,000 h) film growth and restructuring on AISI 304 stainless steel in simulated pressurized water ractor (PWR) with or without zinc addition are also estimated on the basis of a quantitative comparison of the model predictions and literature data on the in-depth concentration profiles of the constituent elements. The obtained values are discussed in terms of the effect of Zn on the growth rate of the inner and outer layers of the corrosion film.

Corrosion of copper in simulated nuclear waste repository conditions

The corrosion of copper in simulated deep repository conditions has been studied using thermodynamic calculations and short-term corrosion rate measurements. Thermodynamic calculations have been used to predict the corrosion behavior of copper in 0.0015, 0.4, and 1.5 m chloride solutions at 80°C. The results of the calculations were summarized in Pourbaix diagrams. A combination of experimental techniques (weight loss, solution analysis, and electrochemical impedance spectroscopy) was used to investigate the extent of corrosion in simulated ground waters with different salinity. The results were found to be in accordance with thermodynamic predictions. The corrosion rates in simulated saline ground waters were significant, whereas they were at the limit of detection in simulated fresh groundwater. The corrosion rates have been shown to correlate well with the levels of residual oxygen in the solutions. This means that studies in conditions as close as possible to anoxic are required in order to estimate the possibility of copper corrosion in simulated deep repository conditions. In addition, corrosion current densities determined from impedance spectroscopy were several times too high compared with weight loss and solution analysis data, indicating the inaccessibility of the true polarization resistance from the impedance data.

Transpassive dissolution of Ni-Cr alloys in sulphate solutions: comparison between a model alloy and two industrial alloys

Abstract The application of a general model for the transpassive dissolution mechanism of binary Ni-based alloys to industrial alloys, Alloy 600 and Alloy C276, containing Ni, Cr, Fe and Mo, in 1 M sulphate solutions at pH 0 and 5 is described. A comparison of the electrochemical behaviour of these two alloys to a binary Ni–15%Cr alloy is also included. The techniques used were ring-disc voltammetry, impedance spectroscopy and resistance measurements. Soluble high-valency products were found to be released in a considerable amount from all the materials. The presence of Mo in Alloy C276 was found to increase the transpassive oxidation rate in comparison to alloys 600 and Ni–15%Cr at pH 0, but the same effect of Mo is not so well pronounced at pH 5. The mechanism of transpassive dissolution was found to be similar on every material at pH 0. At pH 5 the mechanism of the transpassive dissolution on Alloy C276 at high overpotentials is different from that at low overpotentials or from that at pH 0. This change is concluded to be due to the increased effect of adsorbed intermediates at the film/solution interface. The model was found to reproduce the steady state current and the impedance spectra satisfactorily.

Photocurrent response of the passive film on iron in a high-temperature aqueous electrolyte

Abstract An experimental setup for in situ photoelectrochemical measurements of passive films in high-temperature, high-pressure electrolytes are described. The setup enables photovoltammetric and photocurrent spectroscopic measurements at temperatures up to 300 °C and at pressures up to 10 MPa. The first results on the photocurrent response of the passive film on Fe in 0.05 M Na 2 B 4 O 7 (pH 9.3) are presented. The dependences of the photocurrent on photon energy and electrode potential are explored. The results are discussed in terms of the effect of temperature and potential on the nature of the photoactive layer in the passive film on Fe. It can be concluded that despite the increase in the overall thickness of the film on Fe with increasing temperature, the thickness of the photoactive layer seems to remain constant.

Mechanism of transpassive dissolution of nickel-based alloys studied by impedance spectroscopy and rotating ring-disc voltammetry

A generalised kinetic model of transpassive dissolution of industrial nickel-based alloys is presented in this paper. It is based on electrochemical impedance spectroscopic (EIS) and rotating ring-disk (RRDE) measurements on several nickel-based alloys in 0.5 M sulphate and chloride solutions with pH ranging from 2 to 7. The model assumes that the dissolution of Cr from the transpassive film occurs via a Cr(VI) surface intermediate species. The dissolution of Ni as Ni(II) is proposed to occur via two parallel paths featuring two reaction intermediates. The role of Fe as a secondary passivating agent and Mo as an accelerator of transpassive dissolution is also taken into account. The kinetic model has been found to reproduce quantitatively the current vs. potential curves and the impedance spectra for the studied nickel-based alloys in the transpassive region. The proposed mechanism is sensitive to the impact of alloy and electrolyte composition on the dissolution process. Thus it can serve as a starting point to describe the transpassive behaviour of engineering alloys.

Composition, Structure, and Properties of Corrosion Layers on Ferritic and Austenitic Steels in Ultrasupercritical Water

In situ electrical and electrochemical measurements during oxidation of ferritic steel P91 and austenitic steel AISI 316L(NG), as well as of their main constituents (Fe, Cr, and Ni) in ultrasupercritical water (500-700°C, 30 MPa) have been reproducibly performed. Features observed in those measurements were substantiated by ex situ results on the thickness, composition, and morphology of the formed oxide layers from weight gain measurements, scanning electron microscopic observations, and in-depth glow-discharge optical emission spectroscopic analyses of corrosion layers. The oxidation process was followed in situ by using the contact electric resistance and contact electric impedance techniques. Impedance spectra of the Ni-Ni and Cr-Cr contacts during oxidation have been reproducibly measured. They could be quantitatively interpreted by using general considerations of the corrosion process and the mixed-conduction model for oxide films. Preliminary estimates of the diffusion coefficients of principal ionic and electronic current carriers have been obtained and their relevance with respect to available data on Ni and Cr oxidation is discussed.

Technical Note: Detection of Soluble Species Released during Metal Corrosion in High-Temperature Aqueous Solutions

Abstract This work demonstrates the use of a novel wall-jet ring-disc (WJRD) configuration to detect soluble species released as end products or as reaction intermediates in metal corrosion in high-temperature aqueous environments. This wall-jet configuration is based on a recent controlled-distance electrochemistry (CDE) arrangement. It is shown to give qualitatively similar results at room temperature to those obtained using a conventional rotating ring-disc electrode (RRDE) technique. Measurements also showed that the transpassive dissolution of Cr from pure Cr, Fe-Cr, and Fe-Cr-Mo alloys can be detected at elevated temperatures (up to 288°C) by using the present WJRD configuration. The transpassive dissolution of Cr increased with increasing Mo content of the alloy. In addition, the flow rate of the electrolyte was found to influence the stability of oxide films formed on the studied material as well as the collection efficiency for the soluble species. To improve the quantitativeness of the results, ...