MARIANA XAVIER MILAGRE

MARIANA XAVIER MILAGRE

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Intergranular corrosion susceptibility of Al-Cu-Li alloys
2017 - MILAGRE, MARIANA X.; MACHADO, CARULINE S.C.; ARAUJO, JOAO V.; ASTARITA, ANTONELLO; PEBERE, NADINE; VIVIER, VINCENT; COSTA, ISOLDA
In the present study, the intergranular corrosion (IGC) susceptibility of commercial Al-Cu-Li alloys of the third generation (AA2098-T351, AA2198-T3 and AA2198-T851) was compared with that of the AA2024-T3 alloy according to ASTM G110 test. In addition, anodic and cathodic polarization curves were carried out in the ASTM G110 test solution at room temperature. The cross-section of the samples after the ASTM G110 test was observed by scanning electron microscopy (SEM) to evaluate the extension of the corrosion attack. The susceptibility to intergranular attack of the tested alloys was ranked according to the attack by IGC and from the anodic and cathodic polarization curves. The results showed that the AA2024-T3 and the AA2198-T3 alloys presented higher susceptibility to intergranular corrosion comparatively to the other alloys tested (AA2098-T351 and AA2198-T851). The AA2098-T351 alloy was associated to the highest resistance to intergranular attack among the alloys evaluated. A correlation was seemingly established between the stress relief treatment of the alloy and its corresponding susceptibility to intergranular or intragranular corrosion.
Macro and microgalvanic interactions in friction stir weldment of AA2198-T851 alloy
2019 - DONATUS, UYIME; SILVA, REJANE M.P. da; ARAUJO, JOAO V. de S.; MILAGRE, MARIANA X.; ABREU, CAIO P. de; MACHADO, CARULINE de S.C.; COSTA, ISOLDA
The galvanic interactions within and between the friction stir weld zones of the AA2198-T851alloy have been investigated using electrochemical and microscopy techniques. The parentmaterial (PM) was the most anodic region and exhibited pronounced severe localized corro-sion (SLC) both when coupled and isolated. The stir zone was the most resistant to corrosionand exhibited no SLC when coupled, but exhibited SLC when isolated. Profiles associatedwith dissolved oxygen consumption and hydrogen generation currents across the weldmentwere inversely related because the anodic (PM) region produced higher hydrogen bubblesand, interestingly, consumed more dissolved oxygen compared with the other regions.
Exfoliation corrosion susceptibility in the zones of friction stir welded AA2098-T351
2019 - MILAGRE, MARIANA X.; DONATUS, UYIME; MACHADO, CARULINE S.C.; ARAUJO, JOAO V.S.; FERREIRA, RAPHAEL O.; SILVA, REJANE M.P.; ANTUNES, RENATO A.; COSTA, ISOLDA
In the present study, the exfoliation susceptibility of the weld zones in friction stir weldedAA2098-T351 was compared with that of the base metal (BM) according to ASTM G34 stan-dard practice. Friction stir welding (FSW) had a significant effect on the microstructure of theAl alloy tested and the susceptibility to exfoliation was strongly affected by the microstruc-ture. Different features of corrosion attack and exfoliation susceptibility were observed whenthe zones affected by FSW were tested isolated or coupled. Also, the near-surface deformedlayer had an important effect on the Al alloy susceptibility to exfoliation. These are themain findings of this work. The corrosion features were correlated with the microstructuralmodifications related to the welding process and with the electrochemical response. TheT1 phase morphology, distribution and size were critical for exfoliation susceptibility. Thestir zone (SZ) was the zone most resistant to exfoliation. However, resistance to exfoliationvaried with the temperatures reached in the heat affected zones (HAZs). The HAZ exposedto the lowest temperatures during welding, HAZ (LT), was the most susceptible to exfoli-ation, whereas the HAZ exposed to the highest temperatures, HAZ (HT), presented highresistance to exfoliation, similarly to the SZ. The ASTM-G34 practice was an effective anduseful method in identifying the different exfoliation resistances of the BM and the vari-ous zones affected by FSW. The results of this practice were supported by electrochemicalimpedance spectroscopy (EIS) tests.
The local electrochemical behavior of the AA2098‐T351 and surface preparation effects investigated by scanning electrochemical microscopy
2019 - SILVA, REJANE M.P. da; MILAGRE, MARIANA X.; OLIVEIRA, LEANDRO A. de; DONATUS, UYIME; ANTUNES, RENATO A.; COSTA, ISOLDA
In this work, scanning electrochemical microscopy (SECM) measurements were employed to characterize the electrochemical activities on 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 scanning electron microscopy (SEM) were also employed to characterize the morphology of the surfaces. The surface chemistry was analyzed by X‐ray photoelectron spectroscopy (XPS). The surface 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 surfaces. The feedback mode of SECM was adopted to characterize the electrochemical activity of the polished surface that was previously corroded by immersion in a chloride‐containing solution, in order to investigate the influence of the products formed on the active/passive domains. The precorroded surface and as‐received surfaces revealed lower electrochemical activities compared with the polished surface showing that either the NSDL or corrosion products largely decreased the local electrochemical activities at the AA2098‐T351 surfaces.
Effect of surface treatments on the localized corrosion resistance of the AA2198‐T8 aluminum lithium alloy welded by FSW process
2019 - MACHADO, CARULINE de S.C.; KLUMPP, RAFAEL E.; AYUSSO, VICTOR H.; DONATUS, UYIME; MILAGRE, MARIANA X.; ARAUJO, JOAO V. de S.; MACHADO, GLAUSON A.F.; COSTA, ISOLDA
In this work, the effect of eight types of surface treatments on the corrosion resistance of friction stir welded samples of an AA2198‐T8 Al‐Cu‐Li alloy were tested and compared in an attempt to find suitable alternatives to toxic and carcinogenic hexavalent chromium treatments. All the samples were anodized and subjected to different post‐anodizing treatments. The post‐anodizing treatments were (1) hydrothermal treatment in Ce (NO3)3 6H2O solution; (2) hydrothermal treatment in Ce (NO3)3 6H2O solution with H2O2; (3) hydrothermal treatment in boiling water; (4) hexavalent chromium conversion coating; and (5) immersion in BTSE (bis‐1,2‐(triethoxysilyl) ethane. The corrosion resistance of the treated samples was evaluated by immersion tests in sodium chloride solution (0.1 mol L−1 NaCl) and electrochemical impedance spectroscopy (EIS) of the friction stir weldment. The results showed that among the alternative treatments, the Ce‐containing solutions presented the best corrosion resistance, especially when used without peroxide.
The effect of surface pretreatment on the corrosion behaviour of silanated AA2198‐T851 Al‐Cu‐Li alloy
2019 - DONATUS, UYIME; KLUMPP, RAFAEL E.; MOGILI, NAGA V.V.; ANTUNES, RENATO A.; MILAGRE, MARIANA X.; COSTA, ISOLDA
The corrosion behaviour of silanated AA2198‐T851 alloy substrates with and with no manufacturing‐process induced near‐surface deformed layer (MPI‐NSDL) has been investigated. Two methods (alkaline etching + desmutting and mechanical polishing) were employed in removing the MPI‐NSDL. Silanization was performed using 2‐bistriethoxysilylethane. Electrochemical impedance spectroscopy (EIS), salt spray test, and microscopy techniques were employed in the investigation of the corrosion behaviours. The studies revealed that polishing appeared to be the best silanating pre‐treatment (compared with degreasing and etching + desmutting) for the new generation AA2198‐T851 Al‐Cu‐Li alloy, and this was reflected in the EIS spectra. The etched + desmutted and the degreased surface with MPI‐NSDL did not respond well to silanization and presented more pitting sites per square millimeter. However, the severity of corrosion per pit was more on the polished sample compared with the other two. Also, the corrosion mechanisms were different for the three cases.
Exfoliation susceptibility of aluminum alloys joined by Friction Stir Welding (FSW)
2018 - MILAGRE, MARIANA X.; DONATUS, UYIME; MACHADO, CARULINE S.C.; ARAUJO, JOAO V. de S.; MOGILI, VISHNU; ASTARITA, ANTONELLO; COSTA, ISOLDA
In the present study, the exfoliation susceptibility of a commercial Al-Cu-Li alloy of the third generation, AA2098-T351, joined by FSW was investigated according to ASTM G34 standard practice and the results were compared with the results of the parent material. Susceptibility to exfoliation attack was classified by the depth of attack penetration. The cross-sections of the samples after test were observed by optical and scanning electron microscopy (SEM) to evaluate the penetration depth of corrosion attack. A comparison to conventional Al-Cu alloys (AA2024-T3/T351) was carried out and the results showed that the Al-Cu-Li alloy tested (AA2098-T351) was more susceptible to exfoliation. Besides, exfoliation susceptibility varied with each welded zone and the active zones remained active for long periods of time after removal from the test solution when the attack continuously propagated in the corrosion front.
Comparison of the corrosion resistance of AA2024 and AA2098 alloys in different solutions
2018 - MILAGRE, MARIANA X.; ARAUJO, JOAO V. de S.; GOMES, MAURILIO P.; DONATUS, UYIME; MACHADO, CARULINE S.C.; COSTA, ISOLDA
In this work the corrosion resistance of the AA2098-T351 and AA2024-T3 was evaluated and compared by electrochemical tests in 0.01 mol.L-1 of NaCl and 0.1 mol.L-1 of Na2SO4 with 0.001 mol.L-1 NaCl electrolytes. Monitoring of corrosion evolution in both alloys was carried out by electrochemical impedance spectroscopy (EIS) and polarization methods. The surface of the samples exposed to the test solutions was observed by microscopy after corrosion tests. The results showed different corrosion mechanisms in the two test solutions. In the sulfate containing electrolyte, the Al-Cu-Li alloy showed corrosion mechanism mainly associated with the micrometric particles at the surface, similarly to the AA2024-T3 alloy. However, in the chloride solution without sulfate, the AA2098-T351 alloy showed susceptibility to severe localized corrosion (SLC) besides the corrosion associated to the micrometric particles and the first type of corrosion was the main type of attack. The AA2024-T3 showed lower susceptibility to SLC than the AA2098- T351 alloy.
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.
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.