• Energy Research

In the present paper, in situ electrical and electrochemical measurements during oxidation of pure Ni in ultrasupercritical water (500–600 °C) have been performed and analysed. The onset of oxidation was followed using contact electric resistance measurements. The impedance spectra of the Ni–Ni contact during oxidation could be quantitatively interpreted 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 oxidation is discussed.

In the anodisation of Nb in acidic fluoride solutions or warm aqueous alkali, and in that of Mo in concentrated phosphoric acid, impedance spectroscopy provides evidence that 3D film growth proceeds already during the active–passive transition. Indeed, linear potential dependences of both the inverse of the high-frequency capacitance and of the product of the high-frequency resistance times the steady-state current are observed already below the peak potential. Simultaneously, a pseudo-inductive loop at intermediate frequencies is detected in the impedance spectra. In the present paper, we propose a kinetic model for the interpretation of these data. The model assumes that the metal is covered with a non-stoichiometric oxide containing at least two oxidation states of the cation. The processes of oxidative dissolution of the lower valence cations and their transformation to cations of higher valence, leading to passivation, explain the shape of the I–E curve. These processes are limited by both charge transfer at the film–solution (F/S) interface and transport of cation vacancies through the film. Thickening of the oxide film is assumed to proceed simultaneously, and is limited by transport of oxygen vacancies accelerated by an interfacial charge of cation vacancies.

A selective review of the anodic and cathodic processes associated with electroextraction, electrorefining and plating of refractory metals (Ti, Zr, Nb and Ta) from molten salt-based electrolytes is presented with regard to the results obtained during the last two decades. Special attention is paid to the mechanism of cathodic reduction and its rate-limiting steps, as well as to the parallel chemical reactions of disproportionation and complexing. The anodic processes in corrosion and electrodissolution of Ti, Zr, Nb and Ta in molten electrolytes are also discussed. As a conclusion, a few contradictory subject-related questions are outlined and some new trends of investigation are suggested.

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 of several nickel-based alloys has been studied in a simulated bleaching solution (0.014 M SO2−4, 0.017 M Cl−, and 0.0025 M diethylenetriaminopentaacetic acid, DTPA, pH 2.8) by electrochemical techniques. The alloys have been prepared by welding superaustenitic stainless steels (UNS S31254 and S32654) with nickel-based filler metals (UNS N06625 and N06059). According to the rotating ring-disk voltammetric studies, the release of soluble high-valency products from the alloys plays a significant role in the transpassive potential region. Electrochemical impedance spectroscopic data point to a competition between transpassive dissolution and secondary passivation reactions. The experimental data have been compared to a previously proposed model for the transpassive dissolution of nickel-based alloys. The model treats the transpassive dissolution of Cr as a two-step reaction featuring a Cr(VI) intermediate. It assumes that the dissolution of Ni follows two parallel reaction paths, one of which involves a self-catalytic step. The kinetic parameters of the process for the studied alloys have been determined, and a discussion of the role of alloying elements such as Fe and Mo on the transpassive dissolution mechanism is given

Anodic oxidation has proven to be a promising route for the growth of self-ordering oxide nanotubes on Ti, the best results being obtained in ethylene glycol (EG)-based electrolytes with the addition of fluoride and small amounts of water. In the present paper, emphasis is put on the investigation of barrier film growth and dissolution on Ti in EG electrolytes with the addition of H2O (0.3–2.4 M) and NH4F (0.015–0.17 M) using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential (−0.1/5.0 V vs. AgCl/Ag), water and fluoride content have been registered. In addition, the chemical composition of the surface of the oxides obtained at 0.1 and 1.0 V has been estimated by X-ray photoelectron spectroscopy (XPS). XPS analysis revealed the presence of a non-stoichiometric oxide containing mainly Ti4+ and a certain amount of Ti3+, with a certain degree of hydroxylation. Estimates of the total thickness of the oxide from the XPS data using a dual layer model are also presented. A kinetic model of the process is advanced to quantitatively interpret the electrochemical and surface analytical results.

The transpassive dissolution of Fe-12%Cr and Fe-25%Cr alloys in near-anhydrous phosphoric acid/acetic acid mixtures has been studied by conventional voltammetry and electrochemical impedance spectroscopy. Both steady-state and transient techniques point to two parallel pathways for the process involving oxidative dissolution of Cr as Cr(VI) and isovalent dissolution of Fe most probably mediated by an electrolyte-originating species. A simplified kinetic model of the process including only surface kinetic steps has been found to reproduce successfully both the steady-state and the small-amplitude AC response of the studied materials. The kinetic parameters of the model are determined and their relevance regarding the influence of the alloy and electrolyte composition on the relative importance of the two parallel pathways is discussed. The experimental data and model calculations indicate that the effect of acetic acid on the reaction steps related to dissolution of Fe is more significant.

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 high concentration 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.