MARIANA XAVIER MILAGRE

MARIANA XAVIER MILAGRE

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A homemade electrochemical hanging droplet cell to evaluate the corrosion resistance of friction stir weld zones of the AA2198-T8 Al-Cu-Li alloy
2021 - MACHADO, CARULINE de S.C.; HERNANDEZ, JOSE W.C.; MILAGRE, MARIANA X.; ARAUJO, JOAO V. de S.; DONATUS, UYIME; COSTA, ISOLDA
In this study, the corrosion resistance of the different zones of the AA2198-T8 alloy welded using friction stir welding (FSW) was investigated by immersion and electrochemical tests during exposure to 0.005 mol L-1 NaCl solution. After immersion tests, the welding zones were classified in two groups, according to their severe localized corrosion (SLC) morphology. Zones exposed to higher temperatures during welding presented intergranular attack, whereas those exposed to lower temperatures presented intragranular attack. Electrochemical measurements performed employing a homemade hanging droplet cell revealed potential galvanic coupling between the two groups. The base metal (BM) presented the most anodic potentials. The open circuit potential (OCP) increased from BM towards the stir zone (SZ). The results obtained by the hanging droplet cell were reproducible and coherent with the classification of the zones in the two proposed groups. The homemade electrochemical hanging droplet cell proved a very reliable tool to investigate the electrochemical behavior of the FSWed zones in Al-Cu-Li alloys.
Microelectrochemical evaluation of severe localized corrosion sites developing on third-generation aluminium alloys
2021 - IZQUIERDO, J.; SILVA, R.M.P. da; MILAGRE, M.X.; COSTA, I.; BETANCOR-ABREU, A.M.; SOUTO, R.M.
Third-generation aluminium-copper-lithium alloys exhibit attractive mechanical properties, in particular light-weight and significant strength. However, the developed microstructure often results in severe localized corrosion (SLC) sites with fast in-depth pit propagation accompanied by H2 evolution.1 Such phenomena stem from a strong galvanic coupling, mainly established between Fe- and Cu-rich particles and the surrounding matrix. As a result, strong concentration and pH gradients develop throughout the aluminium surface, determining the local breakdown of the passive regime eventually provided by aluminium oxides and corrosion products. Understanding the development of such distributions is key to outline appropriate strategies for the prevention of fast degradation and materials failure. Scanning Electrochemical Microscopy (SECM) and Scanning Vibrating Electrode Technique (SVET) are capable of providing local information on the distribution of active sites and the presence of reactive chemical species. SVET has previously demonstrated the formation of gas bubbles ascribed to hydrogen evolution (i.e., electro-reduction process) at the anodically-activated sites,2,3; whereas local hydrogen production, oxygen consumption and pH changes are readily accessible using SECM, although with some limitations with regards to the detection of evolving gas.4 The present contribution reports recent advances in the investigation of local degradation phenomena occurring at the surface of Al-Cu-Li alloy AA-2098, as bare material and after friction stir welding. Oxygen consumption over nobler particles acting as cathodic sites, and SLC accompanied with strong acidification and H2 production at the local anodes were observed. The determined pH and concentration gradients allow to progress in the knowledge of the mechanistic aspects involved in the degradation processes on these materials.
Influence of surface finishing on the electrochemical activity of the 2098-T351 aluminum alloy
2019 - SILVA, REJANE; MILAGRE, MARIANA; OLIVEIRA, LEANDRO; ANTUNES, RENATO; DONATUS, UYIME; COSTA, ISOLDA
In this work, scanning electrochemical microscopy (SECM) measurements were employed to characterize the electrochemical activities of polished and as-received surfaces of the 2098-T351 aluminum alloy (AA2098-T351). The effects of the near surface deformed layer (NSDL) and its removal by polishing on the electrochemical activities of the alloy surface were evaluated and compared by the use of different modes of SECM. Confocal Laser Scanning Microscopy (CLSM) and energy dispersive X-ray spectroscopy (EDS) were also employed to characterize the morphology of the surfaces. The surface chemistry was analyzed by X-ray Photoelectron Spectroscopy (XPS). The generation/tip collection (SG/TC) and competition modes of the SECM were used to study hydrogen gas (H2) evolution and oxygen reduction reactions, respectively. H2 evolution and oxygen reduction were more pronounced on the polished surface compared to the asreceived surface that revealed lower electrochemical activities showing that either the NSDL largely decreased the local electrochemical activities at the AA2098-T351 surface.
Effect of galvanic coupling on the corrosion susceptibility of friction stir weldment of AA2198-T851 alloy
2018 - COSTA, I.; DONATUS, U.; MILAGRE, M.X.; MACHADO, C.S.C.; ARAUJO, J.V.S.
The AA2198-T851 alloy is a new generation Al-Li alloy which finds application in the aerospace industry. Its preference over conventional Al alloys is premised on the fact that it exhibits better strength to weight ratio and improved fatigue resistance due to the addition of Li as a major alloying element. However, it is a relatively new alloy and its processing-structure-property-performance relationship is far from being established. This is very true for the relationship between its structure and corrosion resistance which is very scarce in the literature. Our recent findings have revealed that this alloy is highly susceptible to severe localized corrosion which is caused by the abundance of active hexagonal T1 (Al2CuLi) particles in its microstructure. The corrosion susceptibility is further promoted by the non-uniform precipitation of these particles which are often preferentially concentrated in bands aligned parallel to the {111}Al. Furthermore, bulk zonal heterogenities with pronounced non-uniform distribution of the active T1 particles are created when this alloy is welded using friction stir welding. The friction stir welding process is a solid state welding process that has been adjudged to be the best welding technique for Al alloys. Nonetheless, this process generates different zones in the weldments of Al alloys. Thus, zones exhibiting different electrochemical characteristics and severe galvanic interactions can occur when the weldment is exposed to a corrosive media. The galvanic interactions can lead to a faster propagation of attack in the most susceptible region of the weldment, which in this case is the parent material (with the highest volume fraction of the T1 particles). To establish this, the galvanic interactions between the zones of the weldment have been investigated by separately exposing the parent material and the stir zone of the weld and then by exposing the entire weldment using NaCl and EXCO solutions. Optical, scanning electron and transmission electron microscopes were also employed in the study. The results revealed that the parent material of the weldment was the most susceptible (as previously mentioned) and appeared to corrode at a faster rate when coupled to the more cathodic stir zone, thermomechanically affected zone and the heat affected zones of the weldment (with lower volume fraction of the T1 particles). The modes of corrosion in the zones of the weld were also observed to be different. However, the most susceptible region corroded intragranularly with the grain boundaries exhibiting more noble electrochemical characteristics.
The effect of FSW on microstructure and intergranular corrosion of the AA2198- T8 alloy
2018 - MACHADO, C.S.C.; DONATUS, U.; ARAUJO, J.V.S.; KLUMPP, R.E.; MILAGRE, M.X.; GIORJAO, R.A.R.; MOGILI, N.V.V.; COSTA, I.
In this investigation, the intergranular corrosion (IGC) resistance of the AA2198-T8 alloy welded by friction stir welding (FSW) was investigated by ASTM G110-97 test. The different zones resulting from FSW showed distinct resistances to intergranular attack depending on the heating and/or mechanical effects produced by welding. The base metal (BM) was the most susceptible to IGC whereas the thermomechanically affected zone (TMAZ) and the stir zone (SZ), presented the highest resistances to IGC comparatively to the other zones. The results of IGC tests were correlated with those of thermal simulation, microhardness measurements, transmission electron microscopy (TEM) and optical microscopy (OM). The dissimilarities in corrosion resistance of the various zones were associated to differences in hardening phase quantities between the various zones. In the BM and HAZ, T1 phase, the main hardening phase, was found at grain boundaries and it resulted in IGC susceptibility. The slight IGC observed in the SZ and TMAZ was not associated with T1 phase which was rarely found but to another precipitate (T2 phase) which was found preferentially located at the grain boundaries in these zones.
Efeito dos tratamentos termomecânicos T8 e T851 na susceptibilidade a exfoliação e corrosão intergranular da liga AA2198
2018 - ARAUJO, J.V.S.; MACHADO, C.S.C.; MILAGRE, M.X.; COSTA, I.; FERREIRA, R.O.; VIVEIROS, B.G. de
As ligas Al-Cu-Li são ligas avançadas de grande interesse para a indústria aeronáutica devido à baixa densidade e alta resistência mecânica destas. Apesar destas vantages, estas apresentam susceptibilidade a diferentes tipos de corrosão. Além disso, os tratamentos termomecânicos aos quais são submetidas durante o processo de fabricação, afetam a microestrutura destas ligas bem como a resistência à corrosão destas. Existem alguns trabalhos publicados sobre a resistência à corrosão destas ligas, porém nestes trabalhos não se faz um comparação entre os efeitos dos diferentes tratamentos termomecânicos. Neste estudo foi investigado os efeitos dos tratamentos termomecânicos T8 e T851 na susceptibilidade à corrosão por exfoliação e corrosão intergranular (IGC) da liga AA2198, segundo as normas ASTM G34 e ASTM G110, respectivamente. Os resultados mostraram significativas diferenças na microestrutura das duas condições, com liga submetida ao tratamento T8 apresentando grãos alongados enquanto a exposta ao tratamento T851, grãos equaxiais. Ambas as ligas apresentaram suscetibilidade a exfoliação. Entreanto, enquanto a liga T8 apresentou susceptibilidade à corrosão intergranular e intragranular, a liga T851 não foi suscetível à corrosão intergranular, apresentando ataque de corrosão dentro dos grãos, ou seja, corrosão intragranular.
Application of scanning electrochemical microscopy (SECM) for local characterization of AA2098-T351 surface in different conditions
2018 - SILVA, R.M.P.; MILAGRE, M.X.; COSTA, I.; OLIVEIRA, L.A. de
Aluminum alloys are commonly used in automotive and aerospace applications because of their good mechanical properties and low density. The aluminum-lithium (Al-Li) alloys are a class of Al-alloys which have some superior properties, such as lower densities than conventional ones and, for this reason, used as structural components in aircrafts. The addition of lithium aims to reduce density of the material with gains in the mechanical properties and in the specific modules, which is important for the aerostructures. However, the addition of this element usually increases localized corrosion susceptibility. The scanning electrochemical microscopy (SECM) technique can provide a good understanding about the corrosion behavior and the local reactions of this class of materials. In this work, different modes of SECM measurements were employed to characterize the AA2098-T351 surface in various conditions. The surface generation/tip collection (SG/TC) mode and competition mode were employed to study the hydrogen gas (H2) evolution and oxygen reduction, respectively, in the AA2098-T351 surface in the conditions: polished and unpolished. It was found that the main domains of H2 evolution and oxygen reduction are associated with the alloy surface in the polished condition.The feedback mode of SECM was employed to characterize the electrochemical activity of the surface after immersion in a solution containing ions chloride, in order to verify the influence of the oxide layer formation over the surface in the active/passive domains due the corrosion process. The previously attacked surface revealed a much lower electrochemical activity compared with the surface in the polished condition due to the protective character of the oxide formed during immersion test.
Comparação da resistência a corrosão localizada de ligas de alumínio pela técnica de varredura com eletrodo vibratório (SVET)
2018 - MILAGRE, M.X.; ARAUJO, J.S.; MACHADO, C.S.C.; DONATUS, U.; COSTA, I.
As ligas Alumínio-Cobre-Lítio (Al-Cu-Li) foram desenvolvidas como substitutas para as ligas convencionais de Al-Cu. Apesar de apresentarem melhor módulo específico também são suscetíveis à corrosão localizada. As técnicas eletroquímicas convencionais são ferramentas úteis para entender o comportamento de corrosão das ligas de Al, no entanto elas dão uma resposta global dos fenômenos de corrosão. Neste trabalho, a técnica de varredura com eletrodo vibratório (SVET) foi utilizada para comparar os mecanismos de corrosão de algumas ligas de Al. Os ensaios com SVET foram realizados em solução de NaCl 0,005 mol L-1 durante 24 h e os mapas foram obtidos a cada 2 h para as ligas AA2024-T3, AA2098-T351 e AA2198-T851. Os resultados mostraram diferentes mecanismos para cada liga sendo que as ligas Al- Cu-Li maiores densidades de corrente associadas a corrosão severa localizada em relação a liga AA2024-T3.