Etude des mécanismes de colmatage de films anodiques sur alliage d'aluminium aéronautique 2024

Les alliages d'aluminium utilisés dans le secteur aéronautique sont sensibles à la corrosion. Une protection de ces pièces peut être assurée grâce à l'élaboration de films anodiques colmatés, le colmatage avec un sel de Cr(VI) garantissant jusqu'à présent les meilleures propriétés de tenue à la corrosion. Toutefois, les composés à base de chrome Cr(VI), classés CMR, sont en passe d'être totalement interdits par la directive environnementale REACH, ce qui conduit les industriels à développer de nouveaux procédés alternatifs. Or, une revue bibliographique a mis en évidence que les procédés existants sont empiriques et qu'il existe des lacunes dans la compréhension des mécanismes de colmatage. L'objectif de cette thèse a donc consisté à identifier les facteurs conditionnant le colmatage, notamment la pénétration du solvant et des sels dans les pores, en relation avec les caractéristiques chimiques et microstructurales des films anodiques non colmatés. Les caractéristiques du film anodique non colmaté ont été tout d'abord amplement étudiées, notamment par des techniques innovantes. La tortuosité a ainsi été évaluée pour la première fois quantitativement, par MEB-FEG et BET, en fonction de la nature de l'alliage. Des mesures dynamiques de mouillabilité ont révélé par ailleurs qu'une tortuosité importante peut constituer une limitation pour la pénétration du solvant lors du colmatage. La caractérisation des charges de surface du film anodique non colmaté a indiqué en outre que les interactions avec les cations du bain de colmatage sont défavorables (répulsions) en milieu acide, sauf à un pH donné, pour lequel la charge globale s'annule. Après différents colmatages, les modifications des revêtements ont été étudiées en termes de composition chimique, de charges superficielles et de morphologie, ce qui a permis de proposer des mécanismes réactionnels. En particulier, pour le nouveau procédé de colmatage, il a été montré que ce n'est qu'à l'issue de la seconde étape de colmatage que le revêtement constitue une barrière à la pénétration d'un électrolyte agressif. Enfin, l'impact des différentes étapes de colmatage a été analysé au regard du comportement en corrosion, en fatigue et en fatigue-corrosion.Aluminum alloys used in the aeronautic field are subject to corrosion. Sealed anodic films act as an efficient solution for the protection of aircraft parts, sealing using hexavalent chromium salts providing the best anticorrosion behavior so far. However, REACH environmental law leads to totally ban chromium (VI) based compounds which are CMR, thus forcing surface finishers to develop new alternative processes. A review has firstly shown that existing processes are mainly empirical and sealing mechanisms are far from being fully understood. So, this thesis aims at identifying sealing key factors that especially drive solvent and penetration into pores of the porous anodic film. Secondly, chemical and microstructural characteristics of unsealed anodic film have been widely studied, especially using innovative techniques. For instance, tortuosity was quantitatively evaluated for the first time, by FEG-SEM and BET analysis, as a function of the alloy nature, while wettability measurements have revealed that a high tortuosity limits water penetration during sealing. Surface charges characterization has also highlighted that working in acidic conditions has usually an adverse effect on interactions between film and cations, except for a special pH value for which global charge becomes zero. Thirdly chemical composition, superficial charges and morphology modifications of the coatings have been studied at each step of sealing, allowing to propose reactional mechanisms. In particular, it is only after the second step of the new sealing process, that the coating becomes an effective barrier against aggressive electrolyte. Finally, the impact of the different sealing treatments on corrosion, fatigue and fatigue-corrosion behaviors has been pointed out

Dynamic measurements and wettability phenomena in mesoporous anodic films prepared on 1050 and 2024T3 aluminium alloys

The wettability of mesoporous anodic films has until now been a parameter rarely studied although it remains crucial especially for sealing treatment following anodising. Wettability measurement is in fact complex as it depends on a number of process parameters relating to the three phases (mesoporous anodic film, deposited liquid and ambient gas). In the present study, an innovative experimental approach was first adopted involving measuring the characteristics of a deposited water drop (contact angle, chord length and height of the drop) in relation to time. Following this, a flow balance allowed the amount of water having infiltrated the mesopores to be determined for two types of anodic film, one tortuous (on an AA 2024T3 substrate), the other not (on an AA 1050 substrate). On the basis of models already available in the literature, a phenomenological mechanism (spreading, penetration and evaporation) was finally proposed and discussed especially with respect to the tortuosity and prior drying of mesoporous anodic films

Effect of new sealing treatments on corrosion fatigue lifetime of anodized 2024 aluminium alloy

The effect of two sealing processes, i.e. an usual hydrothermal sealing and an innovating sealing process called (B1 + B2), on fatigue behavior of anodized AA2024 was studied in air for as prepared and pre-corroded samples. Pre-corrosion exposure corresponded to salt-spray tests or continuous immersions. For salt-spray tests, the best corrosion resistance was related to the (B1+B2) sealing and, for continuous immersions, to the hydrothermal sealing. Fatigue life tests in air on pre-corroded samples revealed that anodized samples presented a decrease in fatigue life more pronounced than anodized and sealed samples in relation with a lower corrosion resistance; fatigue crack initiation was localized on pits issued from the degreasing and pickling steps. Independent of the sealing process, the fatigue behavior of the anodized and sealed samples depended on the pre-corrosion exposure. Corrosion fatigue tests induced an additional decrease in fatigue life for both sealing treatments. Crack initiation occurred preferentially on pits issued from degreasing and pickling but also on pits issued from interaction between cyclic loading and corrosive media, in relation with amechanical damage of the sealed anodic film. The differences in sealed layer morphology could explain the difference in fatigue resistance between the sealed anodic films

Accurate evaluations of both porosity and tortuosity of anodic films grown on rolled AA 1050 and on rolled or machined AA 2024 T3

The porosity of the anodic films grown on aluminium substrates depends on various operating conditions related to the anodization electrolyte and to the applied electrical parameters, as well as to the substrate itself. In the present study, three different aluminium substrates were studied and anodized: AA 1050 (rolled; thickness = 1 mm), AA 2024 T3 (rolled; 1 mm), and AA 2024 T3 (machined; 3 mm). For each type of anodic film, the porosity, as well as its changes during anodization, was accurately characterized using both field‐emission gun scanning electron microscopy (FEG‐SEM) and a reanodization technique. Moreover, for the first time, the corresponding tortuosity was quantified for all studied substrates. Results for rolled AA 2024 T3 and for machined AA 2024 T3 especially showed significant differences in tortuosity values, contributing towards clarifying, in part, their different wettability characteristics or anticorrosion behaviour, so far not clearly explained