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Iskalni niz: "avtor" (Andraž Legat) .

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Elucidating nucleation stages of transgranular stress corrosion cracking in austenitic stainless steel by in situ electrochemical and optical methods
Sarmiento Klapper Helmuth, Bojan Zajec, Andreas Heyn, Andraž Legat, 2019, izvirni znanstveni članek

Povzetek: The pitting and environmentally assisted cracking resistance of austenitic stainless steels (SS) is challenged in several industrial applications particularly those involving hot chloride-concentrated streams. Directional drilling used in the oil and gas exploration is one of these applications. Indeed, high strength CrMn-SS commonly used in drilling technology have a high tendency to fail by stress corrosion cracking (SCC) preceded by localized corrosion once subjected to highly chloride-concentrated drilling fluids at elevated temperatures. A comprehensive understanding regarding the mechanisms governing the transition from pitting into SCCis not currently available, though. Therefore, mechanistic aspects such as the effect of loading conditions on pit nucleation and repassivation as well as the synergistic effect between pit stabilization and the nucleation of a stress corrosion crack are of great practical significance. To investigate this an electrochemical-, optical- and mechanical- monitored SCC test was conducted on a CrMn-SS in an alkaline brine at elevated temperature. The transition from metastable to stable pitting and subsequently to SCC in this system was documented in-situ for the first time. Results supported H.S. Isaacs postulates regarding the interpretation of electrochemical signals and demonstrated that loading conditions affect pit nucleation and repassivation leading to a higher susceptibility of the material to pitting, which preceded SCC.
Ključne besede: pitting corrosion, stress corrosion cracking, monitoring, elektrochemical noise, austenitic stainless steel
Objavljeno v DiRROS: 23.11.2023; Ogledov: 30; Prenosov: 22
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Hydrothermal synthesis of rare-earth modified titania : influence on phase composition, optical properties, and photocatalytic activity
Nejc Rozman, David Maria Tobaldi, Uroš Cvelbar, Harinarayanan Puliyalil, Joao Antonio Labrincha, Andraž Legat, Andrijana Sever Škapin, 2019, izvirni znanstveni članek

Povzetek: In order to expand the use of titania indoor as well as to increase its overall performance, narrowing the band gap is one of the possibilities to achieve this. Modifying with rare earths (REs) has been relatively unexplored, especially the modification of rutile with rare earth cations. The aim of this study was to find the influence of the modification of TiO2 with rare earths on its structural, optical, morphological, and photocatalytic properties. Titania was synthesized using TiOSO4 as the source of titanium via hydrothermal synthesis procedure at low temperature (200 °C) and modified with selected rare earth elements, namely, Ce, La, and Gd. Structural properties of samples were determined by X-ray powder diffraction (XRD), and the phase ratio was calculated using the Rietveld method. Optical properties were analyzed by ultraviolet and visible light (UV-Vis) spectroscopy. Field emission scanning electron microscope (FE-SEM) was used to determine the morphological properties of samples and to estimate the size of primary crystals. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical bonding properties of samples. Photocatalytic activity of the prepared photocatalysts as well as the titania available on the market (P25) was measured in three different setups, assessing volatile organic compound (VOC) degradation, NOx abatement, and water purification. It was found out that modification with rare earth elements slows down the transformation of anatase and brookite to rutile. Whereas the unmodified sample was composed of only rutile, La- and Gd-modified samples contained anatase and rutile, and Ce-modified samples consisted of anatase, brookite, and rutile. Modification with rare earth metals has turned out to be detrimental to photocatalytic activity. In all cases, pure TiO2 outperformed the modified samples. Cerium-modified TiO2 was the least active sample, despite having a light absorption tail up to 585 nm wavelength. La- and Gd-modified samples did not show a significant shift in light absorption when compared to the pure TiO2 sample. The reason for the lower activity of modified samples was attributed to a greater Ti3+/Ti4+ ratio and a large amount of hydroxyl oxygen found in pure TiO2. All the modified samples had a smaller Ti3+/Ti4+ ratio and less hydroxyl oxygen
Ključne besede: TiO2, photocatalytic activity, rare earths, modification, visible light activity
Objavljeno v DiRROS: 22.11.2023; Ogledov: 47; Prenosov: 22
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Activation and repassivation of stainless steels in artificial brines as a function of pH
Emir Mujanović, Bojan Zajec, Tadeja Kosec, Andraž Legat, Stefan Hönig, Gerald Zehethofer, Gregor Mori, 2019, izvirni znanstveni članek

Povzetek: When planning oil wells with stainless steel components, two possible reasons for depassivation have to be considered-chemical depassivation caused by acidizing jobs and mechanical depassivation caused by various tools and hard particles. The study explores conditions causing chemical activation of investigated steels and circumstances under which repassivation occurs after activation. The main focus of the study is to determine, how quickly various steels can repassivate under different conditions and to find pH values where repassivation will occur after depassivation. The investigated steels were ferritic (martensitic or bainitic) in the cases of 13Cr, 13Cr6Ni2Mo, and 17Cr4Ni2Mo, austenitic in the case of 17Cr12Ni2Mo, and duplex (austenitic and ferritic) in the case of 22Cr5Ni3Mo. Potentiodynamic experiments were employed to obtain electrochemical properties of investigated steels, followed by immersion tests to find ultimate conditions, where the steels still retain their passivity. After obtaining this information, scratch tests were performed to study the repassivation kinetics. It was found that repassivation times are similar for nearly all investigated steels independent of their chemical composition and microstructure.
Ključne besede: stainless steels, activation, repassivation
Objavljeno v DiRROS: 21.11.2023; Ogledov: 56; Prenosov: 25
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The corrosion properties of steel in pore solutions obtained from alkali‐activated mortars
Nina Gartner, Tadeja Kosec, Stéphane Poyet, Andraž Legat, 2023, izvirni znanstveni članek

Povzetek: Alkali‐activated materials (AAMs) are considered a promising alternative to materials made from ordinary Portland cement (OPC). Other than considering the durability of the material itself, the use of AAMs for reinforced concrete elements also raises the question of steel corrosion processes in these materials, which are still relatively unknown. Three different alkali‐activated mortars were prepared for this study, based on either fly ash, slag, or metakaolin. Pore solutions were then extracted from each mortar and chemically analyzed. Electrochemical techniques were used to study the corrosion of steel in synthetic pore solutions containing varying concentrations of chlorides. In parallel, the same corrosion tests were performed in a generic pore solution representing OPC mortar. It was shown that the chemical composition differed in each pore solution tested, thus affecting the corrosion properties of the steel. The addition of chloride also had a varying effect on the corrosion properties of the steel in each type of pore solution tested. This study provided a basic overview of the corrosion behavior and mechanisms of the various AAM environments in comparison to that of OPC.
Ključne besede: alkali-aktivated materials, corrosion, electrochemical corrosion techniques, extracted pore solution
Objavljeno v DiRROS: 20.11.2023; Ogledov: 67; Prenosov: 10
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Monitoring the galvanic corrosion of copper–steel coupling in bentonite slurry during the early oxic phase using coupled multielectrode arrays
Tadeja Kosec, Miha Hren, Klara Prijatelj, Bojan Zajec, Nina Gartner, Andraž Legat, 2023, izvirni znanstveni članek

Povzetek: In the case of a two-part container for spent nuclear fuel, consisting of an iron-based inner structure with a copper coating, the potential perforation of copper through minor damage may result in intensive galvanic corrosion between copper and steel. The present work focuses on the corrosion of steel galvanically coupled to copper and exposed to a slightly saline environment under oxic conditions. The electrochemical processes on individual electrodes were monitored by coupled multielectrode arrays (CMEAs). The CMEAs were either in contact with groundwater saturated with bentonite or immersed in groundwater only. Very high galvanic corrosion currents were detected between carbon steel and pure copper in the early oxic phase. Additionally, the use of CMEAs further made it possible to monitor the distribution of cathodic currents around the steel electrode, which behaved anodically. Various microscopy and spectroscopy techniques were applied to identify the modes of corrosion and the type of corrosion products present at the end of the period of exposure.
Ključne besede: copper, steel, bentonite, Aspö groundwater, coupled multi electrode array, electrochemical properties, Raman analysis, corrosion
Objavljeno v DiRROS: 16.11.2023; Ogledov: 75; Prenosov: 9
.pdf Celotno besedilo (1,66 MB)
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Influence of laser colour marking on the corrosion properties of low alloyed Ti
Tadeja Kosec, Andraž Legat, Janez Kovač, Damjan Klobčar, 2019, izvirni znanstveni članek

Povzetek: In the field of surface treatment, laser colour marking can be used to produce coloured marks on the surfaces of metals. Laser colour markings can be applied to various materials, but on titanium alloys a wide spectra of vivid colours can be achieved. This study presents an analysis of the corrosion properties of laser treated surfaces that were exposed to aggressive environments. Different samples were prepared with laser light of various power intensities and processing speeds. The samples were prepared on low alloyed Ti. Electrochemical, spectroscopic and microstructural analyses were conducted in order to study the properties of the laser treated surfaces. Corrosion testing showed different effects of laser power and production speed on the properties of the laser treated surfaces. It was shown that a high intensity and slow processing rate affect the surfaces by forming oxides that are relatively stable in a corrosive environment of 0.1 M NaCl. Spectroscopic investigations including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses showed the differences in chemical structure of the surface layer formed after laser treatment. Similarly, microstructural investigations showed different effects on the surface and sub-surface layer of the laser treated samples.
Ključne besede: Ti alloy, laser treatment, XPS, corrosion
Objavljeno v DiRROS: 25.10.2023; Ogledov: 192; Prenosov: 69
.pdf Celotno besedilo (3,16 MB)
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Development of an electrical resistance sensor from high strength steel for automotive applications
Tadeja Kosec, Viljem Kuhar, Andrej Kranjc, Vili Malnarič, Branko Belingar, Andraž Legat, 2019, izvirni znanstveni članek

Povzetek: This work focuses on a demonstration of the monitoring of corrosion processes taking place in high strength steel in automotive applications. This is performed by means of a corrosion sensor, which operates as an electrical resistance sensor. It was developed from the same type of material that is used for the high-strength steel parts produced in the automotive industry. Using the sensor, real time corrosion processes can be measured. It is attached to a location inside the vehicle’s engine and is equipped with a data logger, which enables wireless transfer of the measured data. In this study the development, operation, and evaluation of the monitoring process are presented. Corrosion estimation is verified by means of electrochemical methods. A metallographic investigation was included in order to verify the similarity between the microstructural properties of the sensor and those of the as-received high-strength steel sheet.
Ključne besede: high strenghth steel, automotive, electrical resistance sensor, corrosion
Objavljeno v DiRROS: 24.10.2023; Ogledov: 195; Prenosov: 88
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Effects of specific parameters on simulations of energy use and air temperatures in offices equipped with radiant heating/cooling panels
Sabina Jordan, Jože Hafner, Martina Zbašnik-Senegačnik, Andraž Legat, 2019, izvirni znanstveni članek

Povzetek: When creating a simulation model to assess the performance of buildings, there is usually a lack of feedback information. Only in the case of measurements of a real building is a direct comparison of the measured values and simulated results possible. Parameter data related to users’ behavior or other events can also be obtained. Their evaluated frequency, magnitude and duration, along with boundary conditions, are crucial for the results. It is clear that none of them can be predicted very accurately. Most of them, however, are needed for computer modeling. In this paper we analyzed the well-defined TRNSYS simulation model of offices equipped with radiant ceiling panels for heating and cooling. The model was based on real case offices and was validated based on measurements for 1 year. The analysis included simulations in order to define what effect the parameters related mainly to users have on the energy use and the indoor air temperatures. The study confirmed that specific human activities influence the annual energy use to a relatively small degree and that their effects often counteract. It also confirmed the even more important fact that although small, these activities can influence the thermal comfort of users. It is believed that despite the fact that this research was based on an analysis of offices equipped with radiant ceiling panels, most of the results could be applied generally.
Ključne besede: measurements, modeling, simulation, validation, analysis, energy use, temperature
Objavljeno v DiRROS: 15.09.2023; Ogledov: 122; Prenosov: 65
.pdf Celotno besedilo (1,83 MB)
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Corrosion performance of steel in blended cement pore solutions
Miha Hren, Tadeja Kosec, Andraž Legat, Violeta Bokan-Bosiljkov, 2019, izvirni znanstveni članek

Povzetek: Blended cements might change the chemistry of the pore solution and subsequently affect the corrosion of steel in concrete. Pore solutions were extracted, analyzed and compared from mortars made of CEM I, CEM II, CEM III and CEM IV cements. Three combinations of carbonation and chloride states were studied, i.e., non-carbonated without chlorides, non-carbonated with chlorides and carbonated with chlorides. Different electrochemical and spectroscopic techniques were used to study the electrochemical properties, the type and the extent of the corrosion products, as well as the type and the extent of the corrosion damage. It was confirmed that the most corrosive environments were pore solutions extracted from the carbonated mortars with chlorides. In this environment the highest corrosion rate was observed for the CEM III pore solution, and the lowest for the CEM I. The extent and the type of corrosion products and the corrosion damage varied according to the environment.
Ključne besede: corrosion, blended cements, pore solution, mortar
Objavljeno v DiRROS: 14.09.2023; Ogledov: 135; Prenosov: 55
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Depassivation and repassivation of stainless steels by stepwise pH change
Emir Mujanović, Bojan Zajec, Andraž Legat, Tadeja Kosec, Janez Kovač, Gregor Mori, Stefan Hönig, Gerald Zehethofer, 2020, izvirni znanstveni članek

Povzetek: Immersion tests with different stainless steels have been performed, while the pH was stepwise decreased and then increased again. During 8.5-day exposure, the depassivation and repassivation pH values as a function of pitting resistance equivalent number were determined. There is always a gap between both pH values (depassivation and repassivation), indicating that for every steel, there are conditions where an existing passive layer can be maintained but cannot be rebuilt after depassivation. In such environments, the passive layer is thicker, consisting mainly of molybdenum and iron rich oxides, while chromium is dissolved. Usually, depending on conditions, the passive layer is more chromium-rich, especially the inner layer. This is relevant, for example, for acidizing jobs in oil and gas industry, proving that repassivation after acidizing will happen promptly, when the pH is increased again.
Ključne besede: stress corrosion cracking, surface finish impact, tapered tensile specimen, autoclave, accelerated testing, light water nuclear reactor, critical threshold stress
Objavljeno v DiRROS: 30.08.2023; Ogledov: 138; Prenosov: 65
.pdf Celotno besedilo (4,01 MB)
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