Martin Bojinov

Evidence of coupling between film growth and metal dissolution in passivation processes

Abstract 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.

Barrier oxide film vs. salt layer formation on bismuth in tartaric acid solutions

The anodic oxidation of bismuth metal in aqueous tartaric acid solutions is studied using electrochemical and photoelectrochemical techniques. The oxidation process comprises four stages: active anodic dissolution, primary passivation by a semiconductor oxide layer formation, secondary passivation due to salt layer formation at the oxide film/electrolyte interface and further growth of the oxide sublayer by a high-field transport mechanism. The chemical nature of the salt film was determined by ex-situ X-ray diffraction to be very close to normal bismuth tartrate Bi2(C4H4O6)3 · 6H2O. Electrophysical characteristics of the oxide sublayer point to its identification with Bi2O3.

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.

Role of surface reactions in the transpassive dissolution of ferrous alloys in concentrated H3PO4

Abstract The transpassive dissolution of a range of ferrous alloys (Fe–12% Cr, Fe–12% Cr–5% Mo, Fe–25% Cr, Fe–25% Cr–10% Mo) in concentrated H3PO4 was studied within the frames of an investigation of electropolishing mechanisms. Measurements by the contact electric resistance (CER) technique have demonstrated that in the transpassive region, the resistance of the anodic film first decreases with increasing potential due to enhanced conductivity of that film and then increases at higher potentials, indicating transpassive film growth. The release of soluble Cr(VI) as detected by rotating ring–disc measurements is the higher, the higher the Cr content in the alloy, and further increases upon addition of 10% Mo to the Fe–Cr alloys. Impedance spectra in the transpassive region have been found to include contributions both from the anodic film and a multistep transpassive dissolution reaction at the film/solution interface. The results were interpreted using a generalised model of transpassive dissolution. The kinetic parameters of the process were determined and the influence of alloy composition on their values is discussed in relation to the processes of anodic surface treatment.

Mechanism of the Transpassive Dissolution and Secondary Passivation of Chromium in Sulphuric Acid Solutions

The transpassive dissolution and secondary passivation of Cr in 1…10 M H 2 SO 4 solutions were studied by a combination of different electrochemical methods. The steady-state polarization curves for transpassive dissolution exhibited a Tafel behaviour with a slope being independent on the acid concentration. Lower dissolution rates were measured for higher acid concentrations. Ring-disk measurements showed a release of both soluble Cr(VI) and Cr(III) during transpassivity. Impedance spectra were qualitatively similar in all acid concentrations, comprising one capacitive and two inductive semicircles. A kinetic model comprising two parallel transpassive dissolution paths was consistent with the experimental results, Typical passivation diagrams were observed for Cr in 10 M H 2 SO 4 , and a secondary passive state was established at higher potentials. The formation of the secondary passive film was confirmed by contact electric resistance (CER) measurements. A renewed version of the surface charge approach was consistent with the experimental results in the region of the secondary passivation.

Transpassive Dissolution Mechanism of Ni-Based Alloys in a Simulated Bleaching Solution Effect of Alloying Elements

The transpassive dissolution of several nickel-based alloys has been studied in a simulated bleaching solution (0.014 M SO 2- 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.

Electrical Properties of Surface Films Formed on Copper

Electrochemical and electrical properties of oxide films on Cu OFP were characterised at 80°C temperature in 0.1 M Na 2 B 4 O 7 at 2 MPa pressure in a reducing environment. The formation of oxides on copper was characterised using the conventional electrochemical techniques and the film resistance data. The electrical properties of films on Cu OFP were characterised in situ using both the Mott-Schottky analysis and the CER technique.

Corrosion of Copper in 1 M NaCl under Strictly Anoxic Conditions

The corrosion of copper in 1 M NaCl has been investigated at room temperature and at 80°C by an on-line resistance probe, solution analysis of dissolved copper and weight loss measurements. At room temperature, corrosion of the copper as indicated by an increase of the probe resistance has been detected during the first 60–80 h of exposure. After the corrosion potential has reached the immunity region of Cu, a decrease in the resistance of the sensor is observed, probably due to redeposition of Cu from the solution. It can be concluded that the corrosion of copper at room temperature virtually stops after 60–80 h due to the anoxic conditions established in the experiments. The similarity in the trends of the corrosion and redox potential during exposure seems to indicate that Cu(II) contained in the native oxide on Cu and/or formed by homogeneous oxidation of Cu(I) during the initial oxic period of exposure acts as a redox-agent determining the chemical conditions in the closed system.

Modelling the anodic corrosion of alloys in acid solutions on the basis of AC impedance and photoelectrochemical measurements

A renewed version of a surface charge approach to describe the impedance response of anodic film growth on passive metals in acidic solutions is presented. It is based on the point defect chemistry, the fact that oxygen vacancies are the main charge carriers in a range of anodic oxides and the suggestion of a constant field strength in the bulk of the barrier layer. High-field transport equations are employed to the point defect motion. A negative surface charge due to accumulation of metal vacancies near the film/solution interface accelerates the oxygen vacancy transport in transient conditions, and a corresponding positive surface charge of oxygen vacancies at the opposite interface is shown to retard it, thus explaining either a capacitive or a pseudoinductive relaxation of the metal/film/electrolyte system under small amplitude ac perturbation. The model is consistent with the anodic passivity of Bi-Sb and Fe-Mo alloys in concentrated acid solutions. Kinetic and structural parameters of the alloy/oxide film/electrolyte system are thereby determined.