Browsing by Author "Saario T."
Now showing 1 - 9 of 9
Results Per Page
Sort Options
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 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 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 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 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.