Engenharia de Materiais
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Navegando Engenharia de Materiais por Autor "Alonso-Falleiros N."
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Artigo Effect of Niobium on Phase Transformations, Mechanical Properties and Corrosion of Supermartensitic Stainless Steel(2017) de Oliveira M.P.; Calderon-Hernandez J.W.; Magnabosco R.; Hincapie-Ladino D.; Alonso-Falleiros N.© 2017, ASM International.The influence of niobium addition in a supermartensitic stainless steel with 13Cr-5Ni-2Mo has been studied. The steel with Nb tempered at 600 °C for 2 h showed improved mechanical resistance properties and lower degree of sensitization, without compromising elongation and pitting corrosion resistance, when compared to the reference steel. In order to understand the Nb effect in such steel, mainly regarding phase transformation, different tempering time intervals have been studied. The better performance of the SM2MoNb is attributed to the hindering effect that Nb has in the kinetics of the phase transformations during tempering, delaying the precipitation start and coarsening stages of the present phases.Artigo Effect of sulfate on the pitting potential of austenitic stainless steels 18Cr8Ni and 17Cr6Mn5Ni in chloride media(2019) Calderon-Hernandez J.W.; Hincapie-Ladino D.; Magnabosco R.; Alonso-Falleiros N.© 2019, Escola de Minas. All rights reserved.Pitting corrosion resistance of two austenitic stainless steels, 18Cr8Ni (Type 304L or UNS S30403 stainless steel) and 17Cr6Mn5Ni, was evaluated in three electrolytic solutions containing a fixed concentration of 0.6M sodium chloride and different additions of sodium sulfate, 0.06M, 0.15M or 0.6M. Cyclic potentiodynamic polarization tests were performed on as-received samples and on specimens annealed at 1010°C, 1040°C, 1070°C or 1100°C. The best pitting corrosion resistance was presented by the 17Cr6Mn5Ni steel annealed at 1070°C or 1100°C, with lower delta ferrite contents, in the electrolyte with the highest sulfate concentration. It can be found that the addition of sodium sulfate inhibits pitting corrosion and has a better inhibitor effect in the high manganese steel.Artigo Pit morphology and its relation to microstructure of 850°C aged duplex stainless steel(2005) Magnabosco R.; Alonso-Falleiros N.The relation between pit morphology and microstructure formed during 850°C isothermal aging of UNS S31803 duplex stainless steel (DSS) could be reached through the explanation of pit nucleation and growth during potentiodynamic polarization in 3.5 wt% sodium chloride (NaCl) aqueous solution. Aging times up to 10 min at 850°C led to sigma formation by direct precipitation from ferrite, resulting in Cr- and Mo-depleted ferrite, or secondary ferrite. Between 30 min and 5 h of aging, the ferrite suffered eutectoid decomposition, resulting in sigma phase and secondary austenite, also impoverished in Cr and Mo. Those Cr- and Mo-depleted zones around the sigma phase were preferential sites for the nucleation of pits. At those sites, the passive film breakdown was easier, as shown by the low pitting potential values found in aged samples. Pit growth occurred as selective corrosion of the metallic matrix surrounding the sigma phase, as a consequence of the lower corrosion resistance of the secondary ferrite and austenite phases found. The low passivation potential values indicated the difficulty in repassivation of the pits formed, probably related to the Cr and Mo depletion of the regions surrounding the sigma phase. © 2005, NACE International.Artigo Relation between pitting potential, degree of sensitization, and reversed austenite in a supermartensitic stainless steel(2017) Calderon-Hernandez J.W.; Hincapie-Ladino D.; Filho E.B.M.; Magnabosco R.; Alonso-Falleiros N.© 2017, NACE InternationalPitting potential (Ep) and degree of sensitization (DOS) of a supermartensitic stainless steel (SMSS) were determined on specimens heat-treated at different tempering temperatures (550°C, 575°C, 600°C, 625°C, 650°C, or 700°C). The effect of pH (pH 6 and pH 8) on the Ep obtained in a 3.5 wt% NaCl solution, as well as the correlation between Ep and DOS obtained in 0.5 M H2SO4 + 0.01 M KSCN solution, was studied. The results showed correlation between Ep and DOS values, the samples with the higher DOS values being the ones with the lower Ep. A mechanism was proposed that would explain the correlation of the presence of reversed austenite formed during tempering, Ep, and DOS.Artigo Selective corrosion in sodium chloride aqueous solution of cupronickel alloys with aluminum and iron additions(2007) Liberto R.C.N.; Magnabosco R.; Alonso-Falleiros N.Cupronickel pipes are widely used in cooling systems. Under this application, localized corrosion is observed, particularly associated with selective corrosion of nickel (denickelification). This work studied the denickelification process in solution-treated cupronickel alloys 90-10 (Cu-10wt%Ni), with aluminum and iron additions, in 0.01 M, 0.1 M, or 0.6 M sodium chloride (NaCl) aqueous solutions. Polished surfaces of the studied materials were submitted to potentiodynamic polarization, and after that, the polarized surfaces were studied using optical microscopy (OM) and scanning electron microscopy (SEM) assisted by energy-dispersive spectroscopy (EDS). A passivity break potential (Epb) was determined in all solutions, related to the initiation of selective corrosion of nickel. When submitted to cyclic potentiodynamic polarization, alloys with higher Epb showed the higher nickel losses in localized spots. However, potentiodynamic polarization tests conducted in 0.01 M NaCl until current densities of 10-1 A/cm2 were reached showed that the selective corrosion process was stopped by the formation of copper-rich corrosion products over the surface. © 2007, NACE International.Artigo Selective corrosion of 550°C aged Cu10Ni-3Al-1.3Fe alloy in NaCl aqueous solution(2011) Liberto R.C.N.; Magnabosco R.; Alonso-Falleiros N.This work studied the electrochemical behavior of a solution treated or 550°C aged Cu10Ni-3Al-1.3Fe alloy, in 0.01M NaCl aqueous solution, through potentiodynamic polarization in both stagnant condition or under erosion process. Results showed the occurrence of a passivity break potential (Epb), related to the beginning of the denickelification process, which occurred as a localized attack under stagnant electrolyte. Under erosion conditions localized denickelification was not observed, despite of the presence of Epb. This could indicate that selective corrosion of Ni, which caused the observed Epb, occurred as a dissolution-redeposition process, with removal of the Cu deposits during erosion process. © 2011 Elsevier Ltd.Artigo Sensitization and pitting corrosion resistance of ferritic stainless steel aged at 800°C(2006) Paroni A.S.M.; Alonso-Falleiros N.; Magnabosco R.This work investigates the influence of sensitization conditions on pitting corrosion resistance of ferritic stainless steel UNS S43000 aged at 800°C between 10 min and 320 min. UNS S43000 steel is susceptible to intergranular corrosion after solution treatment at 1,160°C followed by water-quenching, as a consequence of the chromium depletion of the grain boundaries adjacent areas, due to chromium carbide precipitation during cooling. Isothermal treatments of 10 min at 800°C decrease the sensitization degree, and intergranular corrosion resistance is recovered after 20 min at 800°C. The solution-treated specimens have a high pitting potential in 3.5% sodium chloride (NaCl) solution compared to the same material treated at 800°C for 10 min. The resistance to pitting corrosion is reduced by chromium depletion associated with an increase of inter- and intragranular precipitation of chromium carbide. For treatments longer than 20 min at 800°C, the pitting corrosion resistance returns to the high level observed for the solution-treated condition, as a consequence of chromium redistribution that occurs after 20 min at 800°C. The scan rate used in potentiodynamic tests plays a minor role on the pitting potential, while a rougher surface leads to lower pitting potentials in all studied conditions. © 2006, NACE International.Artigo Sigma phase formation and polarization response of UNS S31803 in sulfuric acid(2005) Magnabosco R.; Alonso-Falleiros N.For a better understanding of the relationship between the microstructure of UNS S31803 duplex stainless steel (DSS) and the shape of the polarization curves, this study evaluated the influence of the microstructure on the potentiodynamic polarization of the 850°C isothermal-aged UNS S31803 DSS in 0.5 M sulfuric acid (H2SO4). In the transpassive region, selective corrosion of chromium- and molybdenum-rich phases occurred. In the solution-treated sample, ferrite was selectively corroded, and in all aged samples, the sigma phase was the selectively corroded phase. Five current density maxima in the passive region were found during potentiodynamic polarization, and they can be related to the microstructures formed. The current density maximum at 564 mV us. saturated calomel electrode (SCE) can be related to secondary ferrite, impoverished in chromium and molybdenum, that was formed during direct precipitation of sigma phase from the original ferrite. Secondary austenite, impoverished in chromium and molybdenum and formed together with the sigma phase during eutectoid decomposition of the original ferrite, can be related to the current density maxima at -85 mVSCE and -40 mVSCE. The austenite phase, present in all heat-treatment conditions, can be related to the current density maxima at -155 mVSCE and 111 mVSCE. © 2005, NACE International.