Thèse doctorat : Génie des Matériaux : Villeurbanne, INSA : 2005. Thèse doctorat : Génie des Matériaux : Université de Pitesti (Roumanie) : 2005. / Thèse soutenue en co-tutelle. Titre provenant de l'écran-titre. Bibliogr. , 6 p.

Extruded Al-Mn alloy are used in heat exchanger applications due to their light weight and good thermal conductivity. Depending on the application, the units may be subjected to external corrosion, which can lead to perforation of the tube. The industrial test most commonly used to assess heat exchanger alloys is the seawater acetic acid test (SWAAT). This is a cyclic fog at 40°C and pH 2.9. In the present study, it was found that pits developing in extruded Al-Mn tubes during the SWAAT test are purely crystallographic. Furthermore, a mechanistic understanding for crystallographic pitting has been developed. The SWAAT test can be of relatively long duration and, typically, does not yield information on the underlying corrosion initiation and propagation mechanisms. In the present study, alternate methods to assess pitting corrosion were elaborated. A drop testing procedure has been successfully implemented to study the mechanism of pit initiation. It was revealed that pits initiated within the aluminium matrix in the vicinity of grain boundaries. A close link between large second-phase particles and pit initiation was established. No preferred grain orientation for pit initiation was evident. Scanning electron microscopy and associated tomography were undertaken for the first time to clarify the mechanism of pit propagation. The pit walls were oriented {100}, while the fast-dissolving planes were {110} and {111}. The findings were in accordance with previous literature. Corrosion penetrated deeper into the alloy when the corrosion front was close to a grain boundary. Pit walls were cathodic to the aluminium matrix, possibly due to enrichment of alloying elements at pit walls. The effect of alloy additions on the corrosion behaviour of extruded aluminium alloys was investigated. Alloys with varying copper, iron and manganese contents were compared. Shot noise analysis and post-mortem analyses were undertaken. The increased amount of manganese in solid solution delayed the transition from micropits to stable pitting. This delay is attributable to second-phase particles that are less cathodic to the aluminium matrix in alloys with increased manganese content. Increasing copper decreased the size of the dissolved polyhedra during stable pitting. Furthermore, pits propagated faster in alloys rich in copper. This could be attributed to an increased level of copper enrichment at the pit walls. Finally, more second-phase particles were present in alloys with increased iron levels. Additionally, pits located in those alloys propagated deeper than pits located in alloys with low levels of iron. A competition between two different types of cathodes, enrichment layer and second-phase particles, is suggested. In conclusion, the effect of microstructure and alloy additions on the corrosion mechanism for crystallographic pitting developed during the project was clarified.

Anionic chromate (VI) compounds are inhibitive pigments and have been effectively incorporated into organic coatings to protect metal surfaces from aggressive ions, but their risk as a human carcinogen and being harmful to the environment has led to the search of suitable alternatives. Aluminium alloy, AA2024-T3, is the substrate metal alloy used in the experiments and can be found in aircraft fuselage structures due to their high strength-to-weight ratio. However, the presence of intermetallic particles increases susceptibility to localised corrosion. To investigate the protection mechanisms of primers on light alloys, many different factors must be taken into account; from aluminium alloy corrosion processes, the effects of intermetallic additions to coating chemistry, morphology and inhibitive pigments. The chemical environment in which the samples are tested in will also affect the corrosion mechanisms of the alloy as well as the performance of the coatings and release of pigments. It will be important to consider which factors are operating under particular conditions so that experimental results can then be best interpreted. As part of this project, potentiodynamic polarisation, electrochemical impedance spectroscopy and electrochemical noise analysis have been used to investigate the protective mechanisms in which chromate-based paints protect against corrosion and UV-Visible spectroscopy, scanning acoustic microscopy and optical microscopy have been used to investigate pigment release mechanism to identify what characteristics are important when developing new primers.

Sacrificial metallic coatings have been used in the past, for protecting steel in industrial and urban developments. It has been suggested that thermally sprayed aluminium coatings, immersed in marine environments, are also capable of protecting steel by providing galvanic protection. Researchers have also speculated that the growth of inert oxide products, on top of the coating as well as inside pores and cracks, could enhance even further its protective “shielding” properties. In addition, the self-healing abilities of the coatings, in case of mechanically induce damage, are still a matter of debate. This research project takes a critical look at these assumptions, focusing on the electrochemical response of aluminium metal sprayed (AMS) coatings immersed in 0.6 M NaCl and 0.6 M Na2SO4 solutions, in order to investigate how the presence of chlorides can alter the corrosion behaviour of these coatings. For such, a thin aluminium protective coating was deposited over two different panels (aluminium 1050 and low carbon steel), by the steel making company Fairfield-Mabey, using electric arc thermal spraying (TS). Subsequently, the coated panels were sectioned into smaller specimens, and with the aid of a SEM/EDS, and XRD, observations and analysis were conducted in their surfaces and cross sections, in order to determine the morphology, quality of deposition, and chemical composition of the coatings. Internal porosity/oxide growth after immersion, was studied using 3-D X-ray tomography scanning. Furthermore, the electrochemical behaviour of the coatings (intact and scribed) was also investigated, initially by analyzing the evolution of the open circuit potential in time, when immersed in chloride rich and chloride free environments. Additionally, the anodic polarization behaviour of the substrate and coatings were simultaneously analyzed, by connecting a zero resistance ammeter (ZRA) with the W.E. terminal of a potentiostat. To conclude, EIS and Rp vs. time plots were made in order to corroborate the data obtained from other tests. Results show that although the coating can generate superficial and internal corrosion products, these oxides are not isolating in nature nor will enhance its protective properties. On the other hand, the corrosion potential behaviour of the coatings revealed that AMS coatings have a more active than the substrate; regardless of the environment in which they were immersed or their substrate. Nevertheless, it was also observed that AMS coatings deposited onto steel will corrode faster than their substrates, and for that matter capable of offering corrosion protection, exclusively if chlorides are present in the solution.