Aluminium alloys are essential to a variety of industry sectors, particularly transport, where they are used in the production of cars and aeroplanes. However, aluminium alloys are susceptible to degradation through corrosion which can compromise the integrity of components manufactured from this material. Therefore research into the means by which these alloys degrade is important. This thesis aims to understand how one of the more potentially damaging types of corrosion, known as pitting corrosion, occurs in the important aluminium alloy 2024-T3 (AA2024-T3). In order to study this phenomenon, this thesis first characterises the alloy microstructure in detail, particularly the type and distribution of intermetallic particles since these play an important role in corrosion processes. The microstructure was studied using an electron microprobe analysis of a 5 mm x 5 mm area of AA2024-T3 and some 80,000 particles were characterised. This investigation was one of the most comprehensive studies to date of any aluminium alloy. Of the particles studied, it was found that the major types included the S and θ phases and a number of compositions based around AlCuFeMn and AlCuFeMnSi. Depletion zones were an integral feature of the alloy microstructure. Pair correlation functions were used to determine the degree of clustering and it was found that there was both inter particle as well as intra particle clustering. Inter particle clustering was observed at length scales well beyond 50 µm. A detailed study of corrosion on AA2024-T3 was undertaken by examining the surface after corrosion over a time period spanning 2.5 minutes to 120 minutes. From this investigation, a hierarchy of the localised corrosion was observed as it was very apparent that particles of particular elemental compositions were more susceptible to attack much sooner than other compositions. Larger corrosion attack sites on the surface, which were called co-operative corrosion, were attributed to intermetallic clustering affects and changes in chemical composition such as Cu-enrichment. These results were used to develop a detailed model of the initiation of stable pitting corrosion in AA2024-T3, which will lead to a better understanding on how to prevent pitting attack on commercially important aluminium alloys. AA2024-T3 is rarely used in the polished state, for real world applications is it generally finished by mechanical or chemical processing. In the final part of this thesis, the influence of clusters on metal finishing was examined using a standard aluminium chemical deoxidiser. It was found that the etch rate of this deoxidiser increased dramatically with the increase in temperature. Under certain processing conditions only the intermetallic particles are etched out and these retain the history of the spatial distribution of the clustering of the intermetallic particles. This leaves a cluster of 'holes' which could trap metal finishing solution and lead to severe subsurface attack

En réponse aux exigences de la réglementation REACH qui prévoit que les substances à base de chrome hexavalent devront être soumises à autorisation dès 2017, les industriels du secteur aéronautique mènent depuis plusieurs années des activités de recherche et développement de nouveaux procédés de traitement de surface moins néfastes pour l’environnement et la santé. Le travail prévu dans le programme de recherche NEPAL (NouvellEs Protections des ALuminiums) s’inscrit dans cette dynamique. Les traitements mettant en oeuvre des composés à base de Cr(VI) sont destinés à disparaître dans un proche avenir ; ainsi de nouvelles formulations ont été développées parmi lesquelles des procédés de conversion à base de chrome trivalent. Le projet de thèse vise à renforcer la robustesse de ces procédés en apportant des éléments de compréhension pour ce qui concerne la tenue à la corrosion des alliages d’aluminium revêtus de couches de conversion au chrome trivalent en relation avec leur état microstructural. Les travaux de thèse ont été développés sur une nuance d’alliage d’aluminium 2024, parmi celles les plus utilisées dans le secteur aéronautique. Plusieurs microstructures ont été considérées dans le but de modifier la distribution en éléments d’alliages majoritaires, le cuivre et le magnésium. La caractérisation fine de la microstructure et l’évaluation de la réactivité des échantillons dans les différents bains de traitement de conversion ont permis de mettre en évidence l’influence de la répartition du cuivre dans l’alliage, élément néfaste pour la croissance des couches de conversion lorsqu’il se trouve sous forme de précipités fins. Finalement, l’étude de deux tôles d’alliage d’aluminium d’épaisseurs différentes met en évidence l’influence de nouveaux paramètres microstructuraux tels que les joints de grains ou la précipitation grossière. Ce travail a été réalisé dans le cadre du projet FUI NEPAL. Le CIRIMAT a été financièrement soutenu par le Ministère de l’Économie et de l’Industrie français (BPI-France), la Région Occitanie Pyrénées-Méditerranée et l’Union Européenne (FEDER/ERDF).

A liga de alumínio AA2024-T3, amplamente utilizada na indústria aeroespacial, apresenta elevada resistência mecânica conferida pela adição de elementos de liga, principalmente o Cu, que formam precipitados finamente distribuídos e coerentes com a matriz, durante o processo de envelhecimento controlado. No entanto, durante este processo, também se formam intermetálicos (IMs) maiores com composição e atividade eletroquímica diferentes da matriz, tornando a liga altamente suscetível a processos de corrosão localizada. A literatura apresenta diversos trabalhos sobre o estudo da corrosão da AA2024-T3 associada à presença dos IMs, porém, o tema ainda é bastante controverso, e muitos aspectos precisam ser estudados com mais detalhes ou mesmo ainda desvendados. Nos últimos anos, técnicas de microscopia de alta resolução, como a microscopia eletrônica de transmissão (TEM) e microscopia eletrônica de varredura (MEV), associadas à espectroscopia por energia dispersiva de raios X (EDS) e a microscopia de força atômica (AFM) bem como técnicas eletroquímicas convencionais e localizadas têm auxiliado nestas pesquisas. Na presente tese serão apresentados os resultados dos estudos realizados sobre a corrosão da liga AA 2024-T3 em soluções de cloreto de baixa concentração. A investigação foi realizada aliando as técnicas de caracterização anteriormente descritas com técnicas eletroquímicas tradicionais: acompanhamento do potencial de circuito aberto (OCP) com o tempo, curvas de polarização (CP) e espectroscopia de impedância eletroquímica (EIS) e técnicas com elevado poder de resolução espacial: como a técnica do eletrodo vibratório de varredura (SVET). A caracterização microestrutural confirmou a heterogeneidade dos IMs, e como contribuições principais, evidenciou a existência de fases secundárias (cristalinas ou não) no interior dos próprios IMs e que a matriz da liga nas proximidades dos IMs apresenta microestrutura variável, todos estes aspectos podendo contribuir de maneira sinérgica para o complexo comportamento de corrosão exibido pelos IMs. Além do mais, mostrou-se que na corrosão os IMs estão submetidos a ataque eletroquímico e químico, este último ocasionado por aumento local do pH. Os resultados dos testes eletroquímicos convencionais mostraram que nas condições empregadas no presente estudo a corrosão da liga é dominada pelo comportamento de corrosão dos IMs, e que, dentro deste contexto, o transporte de oxigênio até os sítios catódicos formados pelos IMs, ao invés da concentração de cloretos, parece ser a etapa controladora da velocidade de corrosão. Por outro lado o tratamento da região em altas freqüências dos diagramas de EIS se mostrou um importante aliado na compreensão dos processos que ocorrem na interface quando da corrosão da liga. No que se refere às técnicas localizadas, a viabilidade do uso da técnica do eletrodo vibratório de varredura (SVET) para estudo do início da corrosão na AA2024-T3 foi avaliada e um sistema para aquisição de dados experimentais por espectroscopia de impedância localizada (LEIS) foi montado e validado. A SVET confirmou que a atividade eletroquímica da liga AA2024-T3 se inicia desde os primeiros instantes de imersão do material no eletrólito teste. / The AA2024-T3 aluminium alloy is largely used in the aerospace industry. It has high mechanical properties due to the presence of alloying elements, mainly copper, that form finely distributed precipitates coherent with the matrix, during controlled aging treatment. However, during this process, large intermetallics (IMs) are also formed with composition and electrochemical activity different from the matrix, making the alloy highly susceptible to localized corrosion. Many works have been published in literature on the corrosion behavior of the AA2024-T3 alloy associated to the presence of IMs, however, the subject is still controversial and many aspects need further detailed investigation. In the last years, high resolution microscopy techniques, such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) associated to X-ray energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM) simultaneously with conventional and localized electrochemical techniques have largely contributed to these studies. In the present work the results of studies carried out on the corrosion behavior of the AA 2024-T3 alloy in chloride solutions of low concentration are presented. The investigation was undertaken by combining the above characterization techniques with conventional electrochemical techniques: open circuit potential (OCP) measurements as a function of immersion time, polarization curves (PC) and electrochemical impedance spectroscopy (EIS) and also techniques of high spatial resolution such as scanning vibrating electrode technique (SVET). The microstructural characterization of the AA 2024-T3 alloy corroborated the heterogeneous nature of the IMs, and as the main contributions, evidenced the presence of secondary phases (either crystalline or amorphous) inside the IMs and the heterogenous microstructure of the matrix at the vicinity of the IMs. All these properties might sinergically contribute to the complex corrosion behavior showed by the IMs. Besides, it was also found that the IMs suffer electrochemical and chemical attack, this last type being caused by local pH increase. The results of the conventional electrochemical techniques showed that under the conditions adopted in the present study, the corrosion of the AA2024-T3 is ruled by the corrosion behavior of the IMs. In this perspective, the oxygen transport to the cathodic sites generated by the IMs rather than the chloride concentration, seems to be the controlling step of the corrosion rate. On the other hand, the analysis of the EIS high frequency data allowed to elucidate the the interfacial processes occurring during the corrosion of the aluminium alloy investigated. The viability of the use of SVET to study the corrosion initiation in the AA2024-T3 was evaluated and a system for experimental data acquisition by localized electrochemical impedance spectroscopy was installed and validated. The SVET showed that the electrochemical activity of the AA2024-T3 takes place since the first periods of immersion in the test electrolyte.

En réponse aux exigences de la réglementation REACH qui prévoit que les substances à base de chrome hexavalent devront être soumises à autorisation dès 2017, les industriels du secteur aéronautique mènent depuis plusieurs années des activités de recherche et développement de nouveaux procédés de traitement de surface moins néfastes pour l’environnement et la santé. Le travail prévu dans le programme de recherche NEPAL (NouvellEs Protections des ALuminiums) s’inscrit dans cette dynamique. Les traitements mettant en oeuvre des composés à base de Cr(VI) sont destinés à disparaître dans un proche avenir ; ainsi de nouvelles formulations ont été développées parmi lesquelles des procédés de conversion à base de chrome trivalent. Le projet de thèse vise à renforcer la robustesse de ces procédés en apportant des éléments de compréhension pour ce qui concerne la tenue à la corrosion des alliages d’aluminium revêtus de couches de conversion au chrome trivalent en relation avec leur état microstructural. Les travaux de thèse ont été développés sur une nuance d’alliage d’aluminium 2024, parmi celles les plus utilisées dans le secteur aéronautique. Plusieurs microstructures ont été considérées dans le but de modifier la distribution en éléments d’alliages majoritaires, le cuivre et le magnésium. La caractérisation fine de la microstructure et l’évaluation de la réactivité des échantillons dans les différents bains de traitement de conversion ont permis de mettre en évidence l’influence de la répartition du cuivre dans l’alliage, élément néfaste pour la croissance des couches de conversion lorsqu’il se trouve sous forme de précipités fins. Finalement, l’étude de deux tôles d’alliage d’aluminium d’épaisseurs différentes met en évidence l’influence de nouveaux paramètres microstructuraux tels que les joints de grains ou la précipitation grossière. Ce travail a été réalisé dans le cadre du projet FUI NEPAL. Le CIRIMAT a été financièrement soutenu par le Ministère de l’Économie et de l’Industrie français (BPI-France), la Région Occitanie Pyrénées-Méditerranée et l’Union Européenne (FEDER/ERDF). / In reply to REACH regulation, which stipulates that hexavalent chromium substance shall besubject to authorization in 2017, manufacturers of aeronautic industries have performed, forseveral years, research and development of new surface treatments less harmful for theenvironment and health. The work planned in NEPAL (NEw Protections for ALminium) researchprogram is part of this large program. The chromate-containing treatments, using Cr(VI)compounds, will disappear in the near future; new formulations have been developed includingtrivalent chromium conversion processes. The thesis project aims to reinforce robustness of theseprocesses by providing data useful for understanding the corrosion resistance of aluminium alloyscoated with trivalent chromium coatings, in relation to their microstructural state. The thesis workwas developed on a 2024 aluminium alloy, among those most used in the aeronautics sector.Several microstructures were considered in order to modify the distribution of major alloyselements, copper and magnesium. The fine characterization of the microstructure and theevaluation of the reactivity of the samples in the different conversion treatment baths highlight theinfluence of the copper distribution in the alloy, this alloying element being detrimental to thegrowth of the conversion coating when it is in the form of fine precipitates. Finally, the study of twosheets of aluminium alloy with different thicknesses reveals the influence of new microstructuralparameters such as grain boundaries or coarse precipitation. This work was performed in theframework of the NEPAL FUI project. CIRIMAT was financially supported by the French Ministry ofEconomy and industry (BPI-France), the Région Occitanie Pyrénées-Méditerranée and theEuropean Union (FEDER/ERDF).

A liga de alumínio AA2024-T3, amplamente utilizada na indústria aeroespacial, apresenta elevada resistência mecânica conferida pela adição de elementos de liga, principalmente o Cu, que formam precipitados finamente distribuídos e coerentes com a matriz, durante o processo de envelhecimento controlado. No entanto, durante este processo, também se formam intermetálicos (IMs) maiores com composição e atividade eletroquímica diferentes da matriz, tornando a liga altamente suscetível a processos de corrosão localizada. A literatura apresenta diversos trabalhos sobre o estudo da corrosão da AA2024-T3 associada à presença dos IMs, porém, o tema ainda é bastante controverso, e muitos aspectos precisam ser estudados com mais detalhes ou mesmo ainda desvendados. Nos últimos anos, técnicas de microscopia de alta resolução, como a microscopia eletrônica de transmissão (TEM) e microscopia eletrônica de varredura (MEV), associadas à espectroscopia por energia dispersiva de raios X (EDS) e a microscopia de força atômica (AFM) bem como técnicas eletroquímicas convencionais e localizadas têm auxiliado nestas pesquisas. Na presente tese serão apresentados os resultados dos estudos realizados sobre a corrosão da liga AA 2024-T3 em soluções de cloreto de baixa concentração. A investigação foi realizada aliando as técnicas de caracterização anteriormente descritas com técnicas eletroquímicas tradicionais: acompanhamento do potencial de circuito aberto (OCP) com o tempo, curvas de polarização (CP) e espectroscopia de impedância eletroquímica (EIS) e técnicas com elevado poder de resolução espacial: como a técnica do eletrodo vibratório de varredura (SVET). A caracterização microestrutural confirmou a heterogeneidade dos IMs, e como contribuições principais, evidenciou a existência de fases secundárias (cristalinas ou não) no interior dos próprios IMs e que a matriz da liga nas proximidades dos IMs apresenta microestrutura variável, todos estes aspectos podendo contribuir de maneira sinérgica para o complexo comportamento de corrosão exibido pelos IMs. Além do mais, mostrou-se que na corrosão os IMs estão submetidos a ataque eletroquímico e químico, este último ocasionado por aumento local do pH. Os resultados dos testes eletroquímicos convencionais mostraram que nas condições empregadas no presente estudo a corrosão da liga é dominada pelo comportamento de corrosão dos IMs, e que, dentro deste contexto, o transporte de oxigênio até os sítios catódicos formados pelos IMs, ao invés da concentração de cloretos, parece ser a etapa controladora da velocidade de corrosão. Por outro lado o tratamento da região em altas freqüências dos diagramas de EIS se mostrou um importante aliado na compreensão dos processos que ocorrem na interface quando da corrosão da liga. No que se refere às técnicas localizadas, a viabilidade do uso da técnica do eletrodo vibratório de varredura (SVET) para estudo do início da corrosão na AA2024-T3 foi avaliada e um sistema para aquisição de dados experimentais por espectroscopia de impedância localizada (LEIS) foi montado e validado. A SVET confirmou que a atividade eletroquímica da liga AA2024-T3 se inicia desde os primeiros instantes de imersão do material no eletrólito teste. / The AA2024-T3 aluminium alloy is largely used in the aerospace industry. It has high mechanical properties due to the presence of alloying elements, mainly copper, that form finely distributed precipitates coherent with the matrix, during controlled aging treatment. However, during this process, large intermetallics (IMs) are also formed with composition and electrochemical activity different from the matrix, making the alloy highly susceptible to localized corrosion. Many works have been published in literature on the corrosion behavior of the AA2024-T3 alloy associated to the presence of IMs, however, the subject is still controversial and many aspects need further detailed investigation. In the last years, high resolution microscopy techniques, such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) associated to X-ray energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM) simultaneously with conventional and localized electrochemical techniques have largely contributed to these studies. In the present work the results of studies carried out on the corrosion behavior of the AA 2024-T3 alloy in chloride solutions of low concentration are presented. The investigation was undertaken by combining the above characterization techniques with conventional electrochemical techniques: open circuit potential (OCP) measurements as a function of immersion time, polarization curves (PC) and electrochemical impedance spectroscopy (EIS) and also techniques of high spatial resolution such as scanning vibrating electrode technique (SVET). The microstructural characterization of the AA 2024-T3 alloy corroborated the heterogeneous nature of the IMs, and as the main contributions, evidenced the presence of secondary phases (either crystalline or amorphous) inside the IMs and the heterogenous microstructure of the matrix at the vicinity of the IMs. All these properties might sinergically contribute to the complex corrosion behavior showed by the IMs. Besides, it was also found that the IMs suffer electrochemical and chemical attack, this last type being caused by local pH increase. The results of the conventional electrochemical techniques showed that under the conditions adopted in the present study, the corrosion of the AA2024-T3 is ruled by the corrosion behavior of the IMs. In this perspective, the oxygen transport to the cathodic sites generated by the IMs rather than the chloride concentration, seems to be the controlling step of the corrosion rate. On the other hand, the analysis of the EIS high frequency data allowed to elucidate the the interfacial processes occurring during the corrosion of the aluminium alloy investigated. The viability of the use of SVET to study the corrosion initiation in the AA2024-T3 was evaluated and a system for experimental data acquisition by localized electrochemical impedance spectroscopy was installed and validated. The SVET showed that the electrochemical activity of the AA2024-T3 takes place since the first periods of immersion in the test electrolyte.

Laser surface melting (LSM) of aluminium alloys with high power continuous wave (CW) CO2 and Nd:YAG lasers has been shown to produce dendritic/cellular microstructures with refined second-phase particles distributed along the dendritic boundaries. Although refinement of the microstructure and extension of the solute solubility in the matrix can be achieved, the refined second-phase particles still act as preferential sites to initiate localized corrosion. In contrast to a CW laser, an excimer laser with a UV wavelength and pulses width in the range of nanoseconds, resulting in extremely high cooling rate up to 1011 K/s, is expected to generate a further refining of the near-surface microstructure and hence, improved corrosion performance. In this project, a Lumonics IPEX 848 KrF excimer laser, with a wavelength of 248 nm and pulse width of 13 ns, has been used for surface melting of an AA2024-T351 alloy. The aim is to investigate the microstructure and the resultant corrosion behaviour of the laser treated surface, and its contribution to alloy performance. The laser fluence was fixed at 7 J/cm2 and the number of pulses per unit area was varied as 10, 25 and 50 pulses respectively. Microstructural characterisation and compositional analysis have been performed by SEM/EDX, TEM/EDX and XRD to disclose solidification phenomena and phase transformations. The results show that the melted layers, with a melt depth from 3 to 7 µm, have been achieved, that is far more chemically uniform than the bulk alloy. In particular, the relatively fine precipitates and dispersoids in the matrix have been dissolved, while large constituent intermetallic particles at the melted layer/matrix interface have been partially melted. In addition, solute-rich bands, containing particularly copper, were formed within the melted layers, especially at the melted layer/matrix interface. SKPFM also reveals that the laser-melted layers exhibit a uniform surface potential distribution. The corrosion performance of AA2024-T351 alloy before and after LSM has been evaluated by anodic polarisation in deaerated and aerated 0.1 M NaCl solution, and immersion tests in 0.1 NaCl solutions. Exfoliation corrosion immersion test ASTM G34- 01 (EXCO test) was also carried out to evaluate the intergranular corrosion (IGC)/exfoliation resistance of the alloy. The results show that the untreated alloy exhibits severe pitting corrosion and IGC. After LSM, significant improvement of corrosion resistance has been achieved. However, delamination of the laser melted layer from the matrix was evident after an EXCO test for 6 hours. The absence of significant corrosion product may suggest a stress-related mechanism. In order to investigate the effect of LSM on anodising of AA2024-T351 alloy and its influence on the corrosion resistance, excimer LSM has been applied as a pre-treatment method prior to anodising in 0.46 M H2SO4 solution. The results show that LSM significantly improved the corrosion performance following anodising compared with the alloy anodised without LSM and LSM alone.

No description available.

No description available.

No description available.

Les alliages d’aluminium constituent environ 80 % des structures d’avions d’affaire Dassault Aviation. Ils sont choisis pour leur bonne tenue mécanique spécifique (rapportée à leur densité) et pour leur coût. Cependant, ils sont spécifiés pour des applications généralement limitées à la plage de températures comprises entre -55 et +85 °C. Qu'en est-il de leur emploi, volontaire ou accidentel, au delà de 85 °C ?L’objectif de cette thèse est de déterminer l’évolution des caractéristiques des alliages d’aluminium courants en fonction des facteurs temps/température/chargement imposés pour un large intervalle de températures (85 à 250 °C) et des durées comparables à celles en service (1 à 10 000 h). Pour cela deux nuances sont étudiées : le 2024 aux états T3 et T8 et le 5086 à l’état H111 et testées à différentes températures, avec ou sans vieillissement artificiel en corrélation avec leur évolution structurale afin d’appréhender les évolutions potentielles d’une pièce de structure aéronautique tout au long de sa vie.La finalité de l’étude est de générer une cartographie complète microstructure/propriétés dans un large domaine de température et pour de courtes et longues durées de vieillissement. Un couplage avec des méthodes de contrôle non destructives sera également établi afin de déceler les pièces surexposées en température et d’accéder à leurs propriétés rémanentes résultantes de l’évolution de leur microstructure. / Aluminum alloys possess low weight and cost and have a high strength which makes them one of the most used materials in aircraft industry. Their operating temperature is included in the range of – 55 to + 85~°C. Nevertheless, the literature data are lacking for aluminum alloys exposed at temperature significantly higher than 85~°C.The aim of this study is to characterize the microstructural and mechanical properties evolution of aluminum alloys as a function of the time, temperature and load after different times of exposure at temperatures above 85 °C.2024 T3 and T8 and 5086 H111 alumnium alloys were artificially aged at different temperatures in the range of 85 to 250~°C for different durations, comparable to service time, varying from 1 to 10~000~h. Then, the mechanical properties were investigated in regard of the microstructural evolutions.Finally, a microstrure/properties map was obtained in a wide range of temperatures and for short and long durations of aging. Non destructive tests were also carried out in order to detect parts exposed in temperature and assess to their remanent properties.