Browsing by Author "Pineda, Fabiola"

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    Alumoxane film for corrosion protection of 2024 aluminum alloy
    (2023) Vejar, Nelson; Rojas, Javier; Alvarado, Claudia; Solis, Roberto; Pineda, Fabiola; Sancy, Mamie; Munoz, Lisa; Paez, Maritza
    Alumoxane film on anodized 2024 was obtained using stearic acid to prevent corrosion in a chloride medium. A cleaning pretreatment was applied to the metal surface to improve the adhesion and formation of the alumoxane film using three different sprays, ethanol, water, and NaOH. Then a molten stearic acid was applied to form an alumoxane. The obtained films were characterized by X-ray photoelectron spectroscopy and glow discharge optical emission spectroscopy, and electrochemical techniques, such as linear sweep voltammetry and electrochemical impedance spectroscopy, evaluated the corrosion protection. The surface analyses suggested an interaction between the boehmite and stearic acid to form alumoxane, and the electrochemical results revealed that alumoxane film using ethanol significantly improved the protection against corrosion due to the formation of compact and homogeneous films with a hydrophobic characteristic for the 2024-T3 alloy. & COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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    Compatibility of alumina forming alloys with LiNO3-containing molten salts for solar thermal plants
    (2022) Fernandez, Angel G.; Pineda, Fabiola; Fuentealba, Edward; Jullian, Domingo; Mallco, Abdiel; Walczak, Magdalena
    The next generation of solar thermal plants will increase the operating temperatures; thus, new structural materials with better performance than the currently used should be required. Alumina forming alloys (AFA) are an alternative since they have been reported as highly resistant to corrosive environments, including molten salts. In this study, two AFA (OC4 and HR224) were exposed to a ternary lithium-containing nitrate molten salt mixture (57 wt.% KNO3-30 wt.% LiNO3-13 wt.% NaNO3) at 550 degrees C for 1000 h to determine their corrosion compatibility through gravimetric and complementary techniques. The mass gain results revealed a good performance of both alloys, allowing them to be recommended for use in solar thermal plants. However, HR224 showed lower weight change attributed to a thin layer of non-porous and continuous corrosion products composed of nickel oxide and aluminum-nickel spinel, which act as protective compounds. On the contrary, OC4 showed a thick multi-layer structure of highly porous, rough, and irregular corrosion products composed mainly of iron oxides and spinels.
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    Corrosion evaluation of alumina-forming alloys in carbonate molten salt for CSP plants
    (2019) Fernandez, Angel G.; Pineda, Fabiola; Walczak, Magdalena; Cabeza, Luisa F.
    The use of carbonate molten salts for the new generation of concentrated solar power (CSP) plants have been considered and analysed during the last years to improve the efficiency of energy generation. However, the high temperature and corrosivity of the salts pose a risk on safety and profitability of the technology requiring more resistant materials. In this study, two alumina-forming austenitic (AFA) alloys corresponding to modified OC4 and HR224 grades, were exposed to the eutectic ternary Li2CO3-K-2-CO3-Na2CO3 (32.1-34.5-33.4 wt%) salt mixture at 650 degrees C for 1000 h. The evolution of weight change along the exposure time and analysis of the resulting scales by means of scanning electron microscopy (SEM), X-Ray diffraction (XRD), and glow discharge optical emission spectroscopy (GDOES) revealed a good performance of both steel grades associated with the formation of multi-layered corrosion products. Whereas both alloys undergo external oxidation with the formation of NiO, internal oxidation with the formation of two spinels is the case of HR224. The rate of corrosion is significantly lower than those reported in molten carbonated in non-AFA alloys, allowing to recommend OC4 and HR224 for use in the carbonate-based CSP. (C) 2019 Elsevier Ltd. All rights reserved.
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    Evaluation of the surface fatigue behavior of amorphous carbon coatings through cyclic nanoindentation
    (2021) Weikert, Tim; Wartzack, Sandro; Baloglu, Maximiliano V.; Willner, Kai; Gabel, Stefan; Merle, Benoit; Pineda, Fabiola; Walczak, Magdalena; Marian, Max; Rosenkranz, Andreas; Tremmel, Stephan
    Diamond-like carbon (DLC) coatings, frequently used to reduce wear and friction in machine components as well as on forming tools, are often subjected to cyclic loading. Doping of DLC coatings with metals or metal carbides as well as the usage of multilayer architectures represent promising approaches to enhance toughness, which is beneficial for the coatings' behavior under cyclic loading. In this study, we utilized cyclic nanoindentation to characterize the tribologically induced surface fatigue behavior of single-layer tungsten-doped (a-C:H:W) and multilayer silicon oxide containing (a-C:H:Si:O/a-C:H)25 amorphous carbon coatings under cyclic loading. Columnar growth was observed for both coatings by focused ion beam microscopy and scanning electron microscopy, while the multilayer architecture of the (a-C:H:Si:O/a-C:H)25 coating was verified by the silicon content using glow-discharge optical emission spectroscopy. In cyclic nanoindentation of the (a-C:H:Si:O/a-C:H)25 multilayer coating, stepwise small changes in indentation depth were observed over several indentation cycles. The surface fatigue process of the single-layer a-C:H:W covered a smaller number of indentation cycles and was characterized by an early steep increase of the static displacement signal. Microscopical analyses hint at grain deformation, sliding at columnar boundaries, and grain detachment as underlying fatigue mechanisms of the a-C:H:W coating, while the (a-C:H:Si:O/a-C:H)25 multilayer coating showed transgranular crack propagation and gradual fracturing. In case of the (a-C:H:Si:O/a-C:H)25 multilayer coating, superior indentation hardness (HIT) and indentation modulus (EIT) as well as a higher HIT3/EIT2 ratio suggest a higher resistance to plastic deformation. A high HIT3/EIT2 ratio, being an indicator for hindered crack initiation, combined with the capability of stress relaxation in soft layers contributed to the favorable surface fatigue behavior of the (a-C:H:Si:O/a-C:H)25 multilayer coating observed in this cyclic nanoindentation studies
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    Evolution of corrosion products on ASTM A36 and AISI 304L steels formed in exposure to molten NaNO3-KNO3 eutectic salt: Electrochemical study
    (2022) Pineda, Fabiola; Walczak, Magdalena; Vilchez, Franco; Guerra, Carolina; Escobar, Rodrigo; Sancy, Mamie
    Thermal energy storage uses molten salt as a heat transfer fluid implies a high corrosion risk. In this work, ASTM A36 and AISI 304L steel, exposed to solar salt at 390 degrees C for 21 days, were studied by electrochemical impedance spectroscopy. The results were validated by mass gain and the characterization of corrosion products. Carbon steel revealed a porous behavior related to the formation of corrosion products based on iron oxides, as described the De Levies theory, whereas stainless steel showed the formation of a passive multilayer of iron and chromium oxides, which was adjusted to the Power-law model.
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    Influence of Bacillus safensis and Bacillus pumilus on the electrochemical behavior of 2024-T3 aluminum alloy
    (2022) Vejar, Nelson; Gutierrez, Sebastian; Tareelap, Napachat; Alvarado, Claudia; Solis, Roberto; Guerra, Carolina; Pineda, Fabiola; Sancy, Mamie; Paez, Maritza
    In this work, electrochemical techniques were employed to evaluate the contribution to the corrosion and corrosion inhibition of 2024-T3 aluminum alloy by two Gram-positive bacteria. In addition, polarized impedance was used to determine the microbial effect on the cathodic and anodic reactions. These microorganisms were collected from a tropical environment due to the favorable bacterial growth of this kind of climate. The alloy was exposed to the sterile medium and inoculated for up to 12 days evaluating the microbiological and electrochemical behavior. The results by linear scanning voltammetry showed that the B. safensis and B. pumilus caused a dual effect of increase and decrease currents, and through electrochemical impedance spectroscopy, showed in some cases, inductive loop, which could be associated with local corrosion and another case, an increasing impedance could be related to protection. In addition, a morphological characterization was performed by scanning electron microscopy before and after exposure, showing an increase in copper precipitation in the vicinity of the intermetallic phases by bacteria, attributed to local corrosion, but, in general, a significant effect of damages was not observed. (C) 2021 Elsevier B.V. All rights reserved.
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    Quaternary nitrate and chloride molten salts for the next concentrating solar power plants: Corrosion considerations for the use of AISI 304L steel
    (2024) Castro-Quijada, Matias; Jullian, Domingo; Walczak, Magdalena; Pineda, Fabiola; Videla, Alvaro
    The study addresses the advancement in concentrating solar power (CSP) plants by transitioning from binary nitrate salts to nitrate and chloride quaternary salts, focusing on the corrosivity of four salts as evaluated by immersion testing of AISI 304L stainless steel. Four compositions of salts were evaluated, from solar salt to equimolar NaNO3-KNO3-NaCl-KCl, up to 21 days (500 h) at 500 degrees C in an open atmosphere, determining the corrosion kinetics using gravimetry. The morphology, chemical composition, and microstructure of the corrosion products were characterized using XRD, FESEM-EDS, and GD-OES. Exposure to the molten salt with 0 mol% Cl(Solar Salt) resulted in negligible corrosion kinetics, consistent with previous studies. The salt with 14 mol% Clcaused a stable corrosion product with a corrosion rate 30 times higher than without chloride. All quaternary salts exhibited a multilayer structure of the top surface with selective chromium (Cr) removal. Specimens exposed to salts with more than 29 mol% Cl- displayed a similar structure with Cr and iron (Fe) removal, resulting in more brittle layers and corrosion rates 90 to 250 times higher than the salt without chloride. GD-OES analysis confirmed Cl diffusion into the oxide layer, highlighting the role of pCl2(g) and pO2(g) in driving corrosivity. Based on the results, using in salts with 29 mol% Cl- at 500 degrees C is discouraged, while using 304L with 14 mol% Cl- in a cold tank may be considered.