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Efficient exploitation and conservation of the oceans poses great technological challenges for scientists and engineers who must develop materials, structures and equipment for use in harsh environment of the oceans. For the applications of materials in marine environment, knowledge of the corrosion properties is essential for selection purposes. Presently, effort is being devoted to exploit deep-sea mineral resources. Deterioration of materials in the deep sea is due to the cumulative effect hydrostatic pressure, temperature, pH, dissolved oxygen, salinity and sea current. For the first time, in-situ corrosion measurements on the effect of deep sea environment on some metallic and composite materials were carried out at depths of 500,1200,3500, and 5100 m for 168,174 and 174 days of exposure in the Indian Ocean. Corrosion rate was obtained from weight loss measurements (mm/year) and surface morphology of as-exposed and cleaned specimens of the above materials was studied under scanning electron microscope and ED AX. Galvanic coupling of steel with zinc, magnesium and aluminium were also studied.. Tensile on metal and alloys and tensile, compressive, flexure and ILSS tests on carbon fibre reinforced composite specimen were performed on exposed specimens. XRD studies were conducted on the corrosion product of materials. In order to correlate the performance of materials in deep-sea environment, seawater current and temperature data were also collected at same period Results reveal that the corrosion behaviour of steels is controlled by dissolved oxygen prevailing and corrosion rate corresponds to dissolved oxygen available at these depth levels. This is due to the fact that oxygen acts as a cathodic deploarizer during corrosion reaction of steels in seawater. Corrosion rate of aluminium increases as the depth increases. This is due to the effect of hydrostatic pressure, which reduces the ionic radii of chlorine ions and facilitates easy penetration of these ions into surface layer. Titanium, titanium alloy (Ti-6A1-4V) and stainless steels did not show any deterioration at all depths studied. Morphology of as exposed and corroded coupons reveal different features. EDS analyses on exposed specimens are analyzed in light of seawater parameters. Carbon fibre reinforced composite did not show any change in properties like tensile, compression flexural and ILSS compared to control (unexposed) specimens. The deposition of calcium carbonate on galvanically coupled mild steel with zinc, aluminium and magnesium corresponds to availability of calcium in the deep ocean. EDS analyses on exposed coupons did not reveal calcium element below the calcium carbonate compensation depth (CCD) at 3800 m in Indian Ocean. Potentiodynamic polarization studies on some metals and alloys indicate that the behaviour of materials in deep-sea environment is a cumulative effect of all oceanographic parameters. Tensile test results on stainless steels SS-304 & SS-316L), titanium and titanium alloy (exposed) specimens did not show any significant change in their tensile properties and is again attributed to the passive film formed on its surface and nearly zero corrosion rate observed. Microbiological investigations on the exposed materials indicate that except carbon fibre reinforced composite all other metals and alloys harboured bacterial colonies. Results have been used to recommend structural materials suitable for the deep-sea applications.

Under normal conditions, cathodically protected mild steel in seawater is protected by a precipitated film of calcium carbonate and magnesium hydroxide, the so-called calcareous film. This study has attempted to investigate the dynamics of calcareous deposit formation during cathodic protection and the composition of calcareous deposits formed under different applied current densities, and also the role played by the initial current density in forming a good quality calcareous deposit. In addition, an under protection situation can occur where current demand values are under estimated, or where structures are approaching the end of their design lives. In these conditions, a calcareous film might well occur but complete protection is probably not possible. These situations have also been studied. At low insufficient current densities where steel corrosion is still occurring, a clear correlation exists between the iron containing corrosion product and the overlaying magnesium hydroxide layer. Such effects have also been investigated using pH titration analysis, where the effect of co-precipitation of the iron and magnesium oxides/hydroxides has been shown. At higher current densities a layered precipitate has been shown to occur consisting of an inner magnesium containing layer and an outer calcium containing layer. At obvious overprotection current densities, the mechanical stresses involved in hydrogen evolution are assumed to give rise to film cracking. To augment and compliment the study on calcareous calcium/magnesium films formed during cathodic protection, a calcium-magnesium containing pigment has also been investigated in aqueous solutions at open circuit as a possible corrosion inhibitor. Another study looked at the same inhibitor in conjunction with a sacrificial zinc anode. Very effective inhibition has been shown with the film containing not only magnesium, calcium and phosphorous but also zinc. In all the investigations electrochemical methods have been used together with various surface analytical techniques.

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Le principe de la projection dynamique par gaz froid ou « cold spray » repose sur la projection de particules de poudres convoyées par un gaz à des vitesses supersoniques vers un substrat. La déformation des particules à l’impact avec ce dernier permet la construction d’un dépôt. Ce procédé permet de conserver la microstructure des particules de poudre et de produire des dépôts peu poreux. Cette dernière caractéristique est essentielle dans le cadre d’applications anticorrosion. L’ambition de la thèse est de comprendre le rôle de la microstructure sur les mécanismes de corrosion marine d’un dépôt à base d’aluminium élaboré par cold spray. Pour atteindre cet objectif sont projetées des poudres à base d’aluminium (aluminium pur, alliages d’aluminium et mélanges avec ajout d’alumine) sur un substrat en acier. Les microstructures des dépôts sont étudiées jusqu’à l’échelle nanométrique (MET). L’adhérence des dépôts est mesurée par l’essai de plot collé. A partir des microstructures sont proposés plusieurs mécanismes de formation de la porosité dans un dépôt cold spray à différentes échelles. Une étude numérique par éléments finis complète cette analyse microstructurale. Grâce aux mesures de la vitesse (DPV-2000) et de la température (caméra thermique) d’impact des particules, les paramètres de nouveaux modèles matériau sont optimisés pour simuler le comportement de l’aluminium et de l’alumine à l’impact. De plus, plusieurs essais de corrosion marine (immersion et brouillard salin) sont conduits. L’étude des microstructures corrodées permettent d’établir différents mécanismes de corrosion du dépôt cold spray. Un lien entre la porosité du dépôt et son comportement en corrosion est notamment montré. Enfin, une première approche du transfert de technologie du procédé à l’échelle industrielle est décrite. / Cold spray process is based on spraying particles carried by a gas at a supersonic speed onto a substrate. Particle deformation during impact with the substrate creates a coating. This spraying process can retain particle microstructure and produce very dense coating. This property is crucial for anticorrosion applications. The aim of this work is to understand the effect of cold spray aluminum coating microstructure on marine corrosion mechanisms. To achieve this goal, several aluminum powders (including pure aluminum, aluminum alloys and mixtures with alumina) are sprayed onto a steel substrate. Coating microstructure is studied down to a nanoscale (TEM). The coating-substrate bond strength is determined using pull-off testing. From a thorough microstructure study, various mechanisms are proposed to explain multiscale porosity formation in coatings. A numerical study using finite elements modeling complements this microstructure analysis. From particle speed (DPV-2000) and temperature (thermal camera) measurements during impact, new material models are optimized to model aluminum and alumina behavior at particle impact. Moreover, corrosion tests are conducted (including immersion and salt spray tests). The study of corroded coating microstructures is used to identify corrosion mechanisms which occur in the coating. A relationship between coating porosity and its corrosion behavior is particularly brought into light. Finally, a first approach to a technological transfer of this process to an industrial application is proposed.