Browsing by Author "Bojinov M."
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Item An electrochemical and surface analytical study of the formation of nanoporous oxides on niobium(2007-10-10) Tzvetkov B.; Bojinov M.; Girginov A.; Pébère N.In the present paper, the anodization of Nb in mixed sulphate + fluoride electrolytes resulting in the formation of a nanoporous oxide film has been studied. Chronoamperometry and electrochemical impedance spectroscopy have been employed to characterise in situ the kinetics of the oxidation process. In addition, the evolution of the layer structure and morphology has been followed by ex situ scanning electron microscopy. Particularly, local electrochemical impedance spectroscopy has been used to discern between the mesoscopic 2D and 3D distributions of time constants at the electrode surface. The similarity between local and global impedance spectra during anodic oxidation of Nb demonstrates the presence of an inherent 3D distribution of the high-frequency time constant, which is interpreted as in-depth variation of the steady state conductivity of the passive film. The experimental and calculational results are discussed in relation to the micro- and nanoscopic structure of the formed oxide. © 2007 Elsevier Ltd. All rights reserved.Item Anodic Oxidation of Tungsten under Illumination-Multi-Method Characterization and Modeling at the Molecular Level(2023-11-01) Bojinov M.; Penkova Y.; Betova I.; Karastoyanov V.Tungsten oxide has received considerable attention as photo-anode in photo-assisted water splitting due to its considerable advantages such as significant light absorption in the visible region, good catalytic properties, and stability in acidic and oxidative conditions. The present paper is a first step in a detailed study of the mechanism of porous WO3 growth via anodic oxidation. In-situ electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS) during oxidation of W illuminated with UV and visible light are employed to study the ionic and electronic processes in slightly acidic sulfate-fluoride electrolytes and a range of potentials 4–10 V. The respective responses are discussed in terms of the influence of fluoride addition on ionic and electronic process rates. A kinetic model is proposed and parameterized via regression of experimental data to the EIS and IMPS transfer functions.Item Composition, structure, and properties of corrosion layers on ferritic and austenitic steels in ultrasupercritical water(2006-10-17) Betova I.; Bojinov M.; Kinnunen P.; Lehtovuori V.; Peltonen S.; Penttilä S.; Saario T.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. © 2006 The Electrochemical Society.Item Conduction Mechanism of the Passive Film on Iron Based on Contact Electric Impedance and Resistance Measurements(2001-06-01) Bojinov M.; Laitinen T.; Mäkelä K.; Saario T.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 predominantly 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. © 2001 The Electrochemical Society. [DOI: 10.1149/1.1371476] All rights reserved.Item Corrosion behavior of laser powder bed fusion manufactured nickel-free stainless steels in high-temperature water(2024-10-01) Goel S.; Bojinov M.; Capek J.; Saario T.; Polatidis E.; Kantonen T.; Salminen A.; Blankenburg M.; Ganvir A.; Que Z.The paper presents a comprehensive investigation of electrochemical behavior of laser powder bed fusion (PBF-LB/M) nickel-free stainless steels in high-temperature water. The PBF-LB/M nickel-free steels processed with various laser parameters and heat treatments exhibit tunable microstructures. Electrochemical impedance spectroscopy (EIS) indicates a 2–3 times lower oxidation rate for the nickel-free steels compared to wrought 316 L. The outer layer crystallites on nickel-free steels are close to pure Fe oxides. The significantly lower concentration of Ni in the inner oxide and at the alloy/oxide interface account for the important variations of the parameters of water reduction/hydrogen oxidation reactions than wrought 316 L.Item Corrosion Mechanism and Electrochemical Reactions on Alloy 690 in Simulated Primary Coolant of Water–Water Energy Reactors(2024-04-01) Bojinov M.; Betova I.; Karastoyanov V.During the power operation of the primary loop of a water cooled–water moderated energy reactor (WWER), the water chemistry evolves from a high-boron high-potassium composition to significantly lower concentrations of both constituents at the end of a campaign, and the Li concentration reaches ca. 0.7–0.9 ppm. In the present paper, the effect of primary water chemistry evolution during operation on the corrosion rate and conduction mechanism of oxides on Alloy 690 is studied by in situ impedance spectroscopy at 300 °C/9 MPa during 1-week exposures in an autoclave connected to a re-circulation loop. At the end of exposure, the samples were anodically polarized at potentials −0.8 to −0.1 V vs. SHE to evaluate the stability of the passive oxide. Simultaneously exposed samples of Alloy 690 were subsequently analyzed by XPS to estimate the thickness and in-depth composition of oxides. Impedance data were quantitatively interpreted using the mixed-conduction model (MCM) for oxide films. The effect of water chemistry evolution on the corrosion rate and conduction mechanism in the oxide on Alloy 690 in a primary coolant is discussed based on the obtained parameters.Item Corrosion of 316L stainless steel produced by laser powder bed fusion and powder metallurgy in pressurized water reactor primary coolant(2024-05-01) Bojinov M.; Chang L.; Saario T.; Que Z.Corrosion mechanism of 316 L stainless steel produced by laser powder bed fusion-hot isostatic pressing (LPBF-HIP) and powder metallurgy-hot isostatic pressing (PM-HIP) is studied with in-situ electrochemical impedance measurements coupled to detailed oxide film characterization. Quantitative analysis of impedance spectra using the Mixed-Conduction Model and estimation of local kinetic and transport parameters by interpretation of in-depth elemental composition profiles indicated lower corrosion and oxidation rates of LPBF-HIP and PM-HIP materials in comparison to conventional wrought 316 L. This owes to a higher fraction of low-angle grain boundaries, smaller grain size, the presence of nano-sized oxide particles and elevated Cr and Ni contents.Item Corrosion of Alloy 690 in Simulated Steam Generator Crevices: Effect of Applied Potential, pH and Pb Addition(2022-01-01) Betova I.; Bojinov M.; Ikäläinen T.; Saario T.; Sipilä K.The mechanism of general corrosion of Alloy 690 in simulated steam generator crevices is investigated by voltammetry, electrochemical impedance spectroscopy and slow strain rate tests, combined with ex-situ characterization of the oxides using Xray photoelectron and Glow-discharge optical emission spectroscopies. Quantitative interpretation of impedance spectra with the Mixed-Conduction Model furnishes estimates of kinetic and transport parameters of the corrosion and anodic oxidation processes. The results indicate a beneficial effect of Pb addition on general corrosion regardless of crevice solution composition.Item Corrosion of Stainless Steel in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling(2024-03-01) Bojinov M.; Betova I.; Karastoyanov V.; Avdeev G.In the present paper, the effect of the evolution of primary water chemistry during power operation on the corrosion rate and conduction mechanism of oxide films on stainless steel is studied by in situ impedance spectroscopy at 300 °C/9 MPa during 1-week exposure periods in an autoclave connected to a recirculation loop. At the end of the exposure period, the samples were anodically polarized in a wide range of potentials to evaluate the stability of the passive oxide. Separate samples of the same steel were simultaneously exposed to the coolant and subsequently analyzed by glow discharge optical emission spectroscopy (GDOES) in order to estimate the thickness and the in-depth composition of the formed oxides. Impedance data were quantitatively interpreted using the mixed-conduction model for oxide films (MCM) to estimate the rates of metal oxidation at the alloy/oxide interface, oxide dissolution and restructuring at the film/coolant interface, and ion transport in the protective corrosion layer.Item Decomposition products of oxygen scavengers and their effect on corrosion of steam generator materials – I. Diethyl-hydroxylamine and carbohydrazide(2024-11-01) Sipilä K.; Ferreirós P.; Ikäläinen T.; Mikkelson A.; Betova I.; Bojinov M.Hydrazine used as oxygen scavenger in the secondary circuit of pressurized water reactors is hazardous to the environment and potentially carcinogenic, thus, suitable replacement chemicals for it are actively sought. In the present paper, decomposition products of two potential replacements – carbohydrazide and diethyl-hydroxylamine – are analyzed, and their effect on secondary water chemistry and corrosion of the main steam generator materials – carbon steel 22 K, stainless steel 0X18H10T and Alloy 690 – is studied by in-situ electrochemical techniques complemented by ex-situ analyses of the formed oxides by spectroscopic and microscopic methods. Quantitative interpretation of the electrochemical impedance data with the Mixed-Conduction Model allowed for the estimation of oxidation and corrosion release rates depending on scavenger formulation, alloy type and temperature. Conclusions on the extent of interaction of decomposition products with construction materials are drawn based on the experimental and calculational results.Item Deposition of Colloidal Magnetite on Stainless Steel in Simulated Steam Generator Conditions—Experiments and Modeling(2022-12-01) Betova I.; Bojinov M.; Karastoyanov V.Sludge formation via colloidal magnetite deposition in steam generators is an important phenomenon that significantly influences the thermohydraulic properties and corrosion of structural materials. This paper aims to verify a model of sludge deposition and consolidation with emphasis on its most significant parameters and their experimental estimation. In-situ electrochemical impedance spectroscopic (EIS) measurements are employed for quantitative evaluation of magnetite deposition kinetics on stainless steel in ammonia-ethanolamine (AMETA) secondary coolant at different temperatures. Parameterization of the model by quantitative comparison of the mixed-conduction model (MCM) with experimental data is discussed. Model predictions are compared with literature data from laboratory experiments and plant operation. Conclusions are drawn about the applicability of the model for quantitative assessment of sludge deposition and consolidation rates.Item Effect of hydrogen on electrochemical behavior of additively manufactured 316L in pressurized water reactor primary water(2023-11-01) Bojinov M.; Saario T.; Ge Y.; Chang L.; Que Z.The electrochemical behavior of laser powder bed fusion (LPBF) 316 L stainless steel subject to different heat-treatments (solution annealing and hot isostatic pressing) is compared to nuclear-grade wrought 316 L in pressurized water reactor primary water at 288 °C (with and without dissolved hydrogen) using current-time transients, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Analysis of spectra by the Mixed-Conduction Model revealed slower corrosion rates of LPBF 316 L than wrought 316 L, the effect being more pronounced in the presence of dissolved hydrogen. The characteristics of the barrier layer and the oxide film/coolant interface were irreversibly altered upon removal of dissolved hydrogen.Item Effect of Sulfide Addition on the Corrosion Mechanism of Copper in Saline Groundwater Solution(2024-04-01) Bojinov M.; Goel S.; Ikäläinen T.; Saario T.The interaction of copper with sulfide-containing simulated saline groundwater is studied by electrochemical techniques (voltammetry, current-time transients, and electrochemical impedance spectroscopy, EIS) complemented with surface and cross-sectional microscopic observations and energy dispersive spectroscopic analyses. Both experimental findings and quantitative interpretation of EIS data using previously published kinetic models indicate that no continuous adherent barrier-type layer is formed on Cu in sulfide-containing groundwater, thus precluding the possibility of the development of localized corrosion modes.Item Flow-Assisted Corrosion of Carbon Steel in Simulated Nuclear Plant Steam Generator Conditions(2023-07-01) Betova I.; Bojinov M.; Karastoyanov V.Flow-assisted corrosion occurs via increased dissolution and/or mechanical degradation of protective oxide formed on the surface of construction materials in direct contact with coolant liquids. In the present paper, this phenomenon is studied on carbon steel in an ammonia-ethanolamine-hydrazine electrolyte by in situ electrochemical impedance spectroscopy in conditions that closely simulate those that prevail in nuclear plant steam generators. Based on the obtained results, a quantitative kinetic model of the process is proposed and parameterized by nonlinear regression of experimental data to the respective transfer function. On the basis of the experimental and calculational results, it is concluded that flow-assisted corrosion of carbon steel is limited by oxide dissolution and cation ejection processes and the protective layer–coolant interface. Expressions for the film growth and corrosion release processes are proposed and successfully compared to operational data.Item Influence of ionic strength on hydrogen generation during interaction of copper with deoxygenated neutral solution(2021-08-01) Betova I.; Bojinov M.; Lilja C.Interaction of copper with deoxygenated solutions is key to quantify corrosion during long-term exposure in a final high-level nuclear waste repository. To evaluate the effect of ionic strength on such interaction, electrochemical measurements in 0.001−0.1 mol dm−3 NaClO4 were performed at 22, 50 and 70 °C. An extended model including a homogeneous reaction is proposed to account for the extra features in impedance spectra, and the effect of perchlorate concentration (i. e. ionic strength) on its kinetic parameters is discussed. Using the model predictions, the rates of hydrogen production in pure water are estimated and compared to literature data.Item Localised electrochemical processes on laser powder bed fused 316 stainless steel with various heat treatments in high-temperature water(2022-12-01) Que Z.; Chang L.; Saario T.; Bojinov M.Laser powder bed fusion (LPBF) is an advanced additive manufacturing technology for stainless steel components fabrication, and a comprehensive understanding of electrochemical behaviour of the LPBF stainless steels is critical for expanding their applications in nuclear and other high-temperature water environments. In the present study, comparison of microstructure and high-temperature electrochemical behaviours between LPBF 316 and wrought 316 in simulated pressurized water reactor environment was made. Three heat treatments, stress relieving (SR), solution annealing (SA) and hot isostatic pressing (HIP), were used to heat treat the LPBF 316. The results showed that both the yield strength and impact energy of the SA and HIP treated LPBF 316 were lower than that of the SR treated sample, whilst the tensile elongation of the SA and HIP treated samples was higher than SR treated sample. These changes were found to be due to the disappearance of the cellular structures, decrease in the dislocation density and the occurrence of recrystallization during the treatments. Electrochemical impedance spectroscopy measurements at 288 °C and their interpretation with the Mixed-Conduction Model indicated that corrosion rate of the heat-treated LPBF 316 variants is significantly lower than that of the wrought 316. In addition, SR material exhibits marginally lower corrosion rates than SA and HIP ones. Microstructure examination after high-temperature water exposure revealed an inhomogeneous inner oxide layer on LPBF 316 in contrast to the thicker and more uniform inner oxide layer on wrought 316. The localised nature of electrochemical processes is suggested to be induced by the nano-precipitates in the LPBF samples.Item Long-Term Oxidation of Zirconium Alloy in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling(2023-04-01) Betova I.; Bojinov M.; Karastoyanov V.Oxidation of Zr-1%Nb fuel cladding alloy in simulated primary coolant of a pressurized water nuclear reactor is followed by in-situ electrochemical impedance spectroscopy. In-depth composition and thickness of the oxide are estimated by ex-situ analytical techniques. A kinetic model of the oxidation process featuring interfacial reactions of metal oxidation and water reduction, as well as electron and ion transport through the oxide governed by diffusion-migration, is parameterized by quantitative comparison to impedance data. The effects of compressive stress on diffusion and ionic space charge on migration of ionic point defects are introduced to rationalize the dependence of transport parameters on thickness (or oxidation time). The influence of ex-situ and in-situ hydrogen charging on kinetic and transport parameters is also studied.Item Mechanism of Anodic Dissolution of Tungsten in Sulfate–Fluoride Solutions(2024-09-01) Bojinov M.; Penkova Y.; Betova I.; Karastoyanov V.Thin passive films on tungsten play an important role during the surface levelling of the metal for various applications and during the initial stages of electrochemical synthesis of thick, nanoporous layers that perform well as photo-absorbers and photo-catalysts for light-assisted water splitting. In the present work, the passivation of tungsten featuring metal dissolution and thin oxide film formation is studied by a combination of in situ electrochemical (voltammetry and impedance spectroscopy) and spectro-electrochemical methods coupled with ex situ surface oxide characterization by XPS. Voltametric and impedance data are successfully reproduced by a kinetic model featuring oxide growth and dissolution coupled with the recombination of point defects, as well as a multistep tungsten dissolution reaction at the oxide/electrolyte interface. The model is in good agreement with the spectro-electrochemical data on soluble oxidation products and the surface chemical composition of the passive oxide.Item Mechanistic understanding of the localized corrosion behavior of laser powder bed fused 316L stainless steel in pressurized water reactor primary water(2024-01-01) Ge Y.; Chang L.; Bojinov M.; Saario T.; Que Z.The laser powder bed fused (LPBFed) stainless steels showed anomalous and localized corrosion behavior in the nuclear reactor high-temperature water compared to their wrought counterparts, which affects their performance during plant operation. In this study, advanced microstructural characterization was performed on LPBFed 316 L sample along with wrought 316 L sample after corrosion tests to understand the underlying mechanisms. The results showed that an inhomogeneous/discontinuous inner oxide layer formed on LPBFed 316 L, in contrast to the continuous inner oxide layer on the wrought 316 L specimen. This discontinuous inner oxide layer was identified to consist of Cr-enriched nano-sized spinel oxide and the barrier layer features a Ni-enriched hexagonal close-packed Laves phase. Localized/preferential oxidation was found to occur along the cellular walls which were tangled with high density dislocations and decorated with Mn and Si-enriched nano-sized precipitates, and the nano-precipitates were observed in the core of dispersed Cr-enriched inner oxide crystals.Item Oxidation model for construction materials in supercritical water-Estimation of kinetic and transport parameters(2015-11-01) Penttilä S.; Betova I.; Bojinov M.; Kinnunen P.; Toivonen A.An upgraded model was employed to obtain estimates of kinetic parameters of oxidation of stainless steel Sanicro 28 (UNS N08028) and nickel-based alloy 690 (UNS N06690) in supercritical water at 650. °C/25. MPa up to 2000. h. In the present approach, it is assumed that the growth of the outer layer is governed by the transport of cations through the inner layer via an interstitialcy mechanism. The model is able to reproduce quantitatively the depth profiles of constituent elements oxide layer, as well as in the transition layer situated between the inner layer and the bulk alloy.