MAURILIO PEREIRA GOMES

MAURILIO PEREIRA GOMES

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Improved experimental setup to reach a broad frequency domain in local electrochemical impedance spectroscopy measurements
2022 - GOMES, MAURILIO P.; GATEMAN, SAMANTHA M.; COSTA, ISOLDA; GHARBI, OUMAIMA; NGO, KIEU; ROSSI, JESUALDO L.; TURMINE, MIREILLE; VIVIER, VINCENT
We report a new experimental setup enabling local electrochemical impedance spectroscopy (LEIS) measurements over a wide frequency range, including the low frequency domain. First, an improved signal treatment was performed using a potential shifter to minimize the DC component of the measured potential, allowing for the local AC potential to be probed using an ideal electrochemical system. Then, a unique, three-electrode LEIS probe was implemented to study the influence of radial and lateral contributions on the LEIS response. The radial LEIS response is highest when the LEIS probe is positioned in the middle of the gold electrode substrate, whereas the lateral LEIS response is similar in both measured regions. Both experimental modifications were implemented to study the LEIS response over a corroding aluminum substrate, for which reliable results are obtained in the low frequencies. These tools could be used in future LEIS studies in other fields that require time-sensitive measurements.
A new look on the corrosion mechanism of magnesium
2022 - BENBOUZID, ABDELMOHEIMAN Z.; GOMES, MAURILIO P.; COSTA, ISOLDA; GHARBI, OUMAIMA; PEBERE, NADINE; ROSSI, JESUALDO L.; TRAN, MAI T.T.; TRIBOLLET, BERNARD; TURMINE, MIREILLE; VIVIER, VINCENT
This study investigates the electrochemical impedance diagrams obtained for a Mg electrode in a sodium sulphate solution at different pHs. A comprehensive model, independent of the pH range, with the presence of two adsorbed intermediates accounting for the anodic dissolution and the cathodic reaction was proposed to explain the singular behaviour of Mg at low pH values. It was also shown that, in acidic media, the contribution of the double layer as well as the cathodic partial reaction must be considered, whereas at higher pH values the oxide/hydroxide layer plays a significant role and slows down the magnesium corrosion rate.
Investigation on the corrosion mechanisms of pure magnesium and the effect of friction stir welding (FSW) on the corrosion resistance of aluminum alloy 2524-T3
2021 - GOMES, MAURILIO P.
Friction stir welding (FSW) is a process that has proven to be quite efficient when it comes to joining high-strength aluminum alloys, for instance AA2524-T3. This can be justified by the fact that welding aluminum alloys by FSW technique allows (i) reduction of aircraft weight by eliminating the rivets commonly used and (ii) the use of different aluminum alloys that have low specific density and high mechanical strength. However, even though FSW allows the joining of metallic parts without their effective fusion, which theoretically would result in a defect-free weld bead, the heat resulting from the friction of the welding tool causes significant microstructural changes. In consequence, it results in variations of mechanical properties and corrosion resistance in the welded region. It was evaluated the FSW welding process influence on the corrosion resistance of the joined 2524-T3 aluminum alloy. Different tools have been used, such as: optical and scanning electron microscopy; open circuit potential and electrochemical impedance spectroscopy measurements; and corrosion tests: agar-agar test, intergranular corrosion test, and exfoliation corrosion test. It was proven by different techniques that the thermomechanically/thermal affect zone interface on the retreating side was the most susceptible to corrosion among all investigated zones. A parallel study was developed regarding the corrosion mechanism of pure magnesium. It has been the subject of a considerable amount of work, and despite its ubiquity and history, it remains controversial. This is mainly due to the presence of the negative difference effect (NDE), which increases hydrogen formation when the magnesium is biased on the anodic domain. We was performed a detailed analysis of the electrochemical impedance spectra obtained for the Mg electrode during immersion in a sodium sulfate solution. A model was proposed which took into account the presence of: (i) a thin oxide film (MgO) which progressively covered the Mg electrode surface, (ii) film-free areas where the Mg dissolution occurs in two consecutive steps, (iii) a thick layer of corrosion products (Mg(OH)2), (iv) an adsorbed intermediate Mg+ads which is responsible for the chemical reaction allowing the NDE to be explained. From the impedance data analyses, various parameters were extracted such as the thin oxide film thickness, the resistivity at the metal/oxide film interface and at the oxide film/electrolyte interface, the active surface area as a function of the exposure time to the electrolyte, the thickness of the thick Mg(OH)2 layer and the kinetic constants of the electrochemical reactions.
Simulação computacional de recristalização de grãos
2020 - GOMES, MAURILIO P.; REIS, LUIS A.M. dos; BETINI, EVANDRO G.
Um modelo de Monte Carlo bidimensional foi desenvolvido para simular a recristalização de grãos na presença de uma partícula circular imóvel para diferentes valores de energias armazenadas. Para energias armazenadas maiores que um valor crítico (H/J = 2,01) a propagação da recristalização pôde facilmente transpor a partícula imóvel, levando a uma densidade aleatória dos grãos e a uma influência insignificante da partícula imóvel na cinética da recristalização. Para energias armazenadas mais baixas (H/J = 0,1), a partícula imóvel limita a propagação da recristalização, levando à uma recristalização incompleta.
Study of recycling process viability of zirconium alloys chips for melting in VAR furnace
2017 - REIS, L.A.M.; ALENCAR, M.; GOMES, M.P.; PEREIRA, L.A.T.; MUCSI, C.S.; ROSSI, J.L.
Cavacos de ligas de zircônio (M5, Zirlo, Zircaloy) são gerados em elevadas quantidades na confecção de tubos e tampões das varetas que compõem o elemento combustível de reatores de potência refrigerados a água pressurizada (PWR). Essas são ligas importadas e por isso é mostrado interesse pela indústria de reciclagem na sua reutilização. Este trabalho apresenta os estudos de um processo de reciclagem e a obtenção de eletrodos prensados para serem fundidos em um forno VAR (Vacuum Arc Remelting). O processo se inicia com uma separação magnética, lavagem do fluido de corte que é solúvel em água, utilização de um desengraxante industrial, seguido por um enxágue com fluxo contínuo de água em alta pressão e secagem por fluxo de ar quente. Para a obtenção de eletrodos, os cavacos foram prensados em uma matriz de seção quadrada 40x40 mm² com 500 mm de comprimento, resultando num eletrodo com 20% da densidade aparente da liga. A fusão foi feita um forno VAR de laboratório no CCTM-IPEN, gerando um lingote maciço de 0,8 kg. A fusão dos cavacos é possível e viável em um forno VAR o que reduz em até 40 vezes o volume de armazenamento desse material.
Microstructural characterization of air quenched valve seat inserts obtained with AISI D2 tool steel
2017 - GOMES, M.P.; SANTOS, I.P.; COUTO, C.P.; BETINI, E.G.; COLOSIO, M.A.; ROSSI, J.L.
The obtention of valve seat insert (VSI) by powder metallurgy using a mixture of iron powder and tool steel is fulfilling the requirements for substitution of cobalt and lead in commercial parts, aiming cost reduction and avoid the lead toxicity. The aim of this work consisted of evaluate the influence of heat treatment on VSI obtained with AISI D2 tool steel powder. The AISI D2 tool steel powder was mixed with iron powder and additives such as manganese sulphide, zinc stearate, graphite and carbides. The heat treatment of the VSI consisted of air quenching followed by double tempering it at 500 °C. The microstructural characterization was performed using scanning electron microscopy and energy dispersive spectroscopy.
Influence of air quenching on apparent hardness of valve seat inserts obtained with AISI M2 high-speed steel powder
2017 - GOMES, M.P.; SANTOS, I.P.; REIS, L.A.M.; CIONE, F.C.; COLOSIO, M.A.; ROSSI, J.L.
The aim of this work consisted of evaluate the influence of heat treatment on sintered valve seat inserts (VSI) obtained with the AISI M2 high-speed steel powder. The AISI M2 high-speed steel powder was mixed with iron powder and additives such as manganese sulphide, zinc stearate, graphite and carbides. The heat treatment of the VSI consisted of air quenching followed by double tempering it in seven different equidistant temperatures, from 100 °C until 700 °C. The mechanical property was measured regarding the VSI apparent hardness. The results indicated that the VSI obtained with the AISI M2 high-speed steel powders mixtures showed the best results regarding its apparent hardness after air quenching and double tempering it at 600 °C.
Effect of the cooling rates on heat-affected zone of UNS S32304 duplex stainless steel welded by pulsed GTAW process
2017 - BETINI, E.G.; GOMES, M.P.; REIS, L.A.; MUCSI, C.S.; ALENCAR, M.C.; ORLANDO, M.T.D.; LUZ, T.S.; AVETTAND-FENOEL, M-N.; ROSSI, J.L.
Two thin plates of UNS 32304 of duplex stainless steel were joined by pulsed gas tungsten arc welding (GTAW) using a mixture 98% of argon plus 2% of nitrogen as shielding gas or pure argon as purging gas at the bottom of the plate without filler metal. The temperature profile close and away from the weld pool was measured using K type (Ni-Cr) thermocouples, connected to a digital data acquisition system. The thermal cycles was recorded in the heat-affected zone (HAZ) at a distance in between 1 and 3 mm from the joint line. The cooling rate and holding time were studied with respect to the used purging gas. Experimental thermal profiles are in good agreement with literature values. The joints welded without any purging gas revealed high peak temperature and cooling rates. The present investigation showed that temperature peaks are comprised in the phase transformation temperature ranges for the welded duplex stainless steel.
Influence of heat treatment on corrosion resistance of press hardened steel coated with AlSi and ZnNi
2017 - COUTO, CAMILA P.; COSTA, ISOLDA; COLOSIO, MARCO; GOMES, MAURILIO; ROSSI, JESUALDO; BOLSANELLO, MARILIA
The ultrahigh strength boron manganese steels, also known as PHS (press hardened steels), are strategic materials for the automotive industry, satisfying safety needs and fuel reduction requirements by means of the lightweight design concept, a current trend for new vehicles. It is possible by the hot stamping process which consists in heating a steel blank to the austenitization temperature, transferring it to press tooling, forming and quenching to fully martensitic transformation. At the end of process, the steel achieves tensile strength up to 1,500 MPa. The transference step from furnace to press is a critical stage because it might promote deleterious steel oxidation. The use of metallic coatings avoids this outcome. AlSi coatings have been the most applied on PHS. Alternative coatings such as electroplated ZnNi are under investigation to evaluate their potential for replacement of AlSi coatings, besides to keep up with the high projected world demand. Zinc based coatings are advantageous comparatively to AlSi ones because they provide cathodic protection to steel substrates. In this study the influence of hot stamping heat treatment on corrosion resistance of 25MnB5 steel coated by AlSi and electroplated ZnNi has been investigated. The tests include open circuit potential measurement, electrochemical impedance spectroscopy and anodic polarization curves. The samples have been tested as received and after the heat treatment which consists in heating the sample at 900°C by 10 minutes and then, fast cooling in water. It is expect that the heat treatment changes the corrosion resistance due to the intermetallics formation at the coating layer as a consequence of diffusion.
Characterization of sintered valve seat inserts obtained with AISI M2 high-speed steel after air quenching
2017 - GOMES, MAURILIO P.; SANTOS, IGOR P. dos; REIS, LUIS A.M. dos; COUTO, CAMILA P.; MUCSI, CRISTIANO S.; COLOSIO, MARCO A.; ROSSI, JESUALDO L.
The aim of this work was to heat treat and characterize sintered valve seat inserts (VSI). The powder metallurgy route was the only way found to substitute cobalt and lead, used in the VSI original alloy, due to their high cost and toxicological effect, respectively. The studied VSI was obtained with AISI M2 high-speed steel powder mixed with iron powder and other additives such as manganese sulphide, graphite, zinc stearate, carbides and copper, which was added by metallic infiltration. All the VSI were air quenched and double tempered, for one hour each, at seven different equidistantly temperatures, ranging from 100 °C up to 700 °C. The physical and mechanical properties were evaluated by means of the VSI apparent density, apparent hardness and crush radial strength. The chemical composition was determined through gas analysis, for the light elements such as carbon and sulfur, and energy dispersive X-ray fluorescence spectrometry for other elements. Microstructural characterization was performed with the support of scanning electron microscopy and energy dispersive spectroscopy. Regarding the VSI final application, the best results were achieved with the inserts air quenched and double tempered at 600 °C.