In this work, I aim to clarify the mechanism that allows steel to attain higher chloride threshold as it is cathodically polarized. Specifically, I seek to provide empirical information on whether an intrinsic (predominantly interfacial effects of polarization) or an extrinsic (predominantly concentration changes due to polarization) mechanism may be dominant in the beneficial effect of polarization. I carried out this experiment with 12 identical concrete specimens, each with a cast-in steel plate, constantly exposed them to high-chloride environment. The specimens were divided into 4 triplicates and polarized at 4 different level from OCP, -200, -300 to -400 mVSCE The specimens were closely monitored for signs of corrosion. When corrosion was detected in a specimen, it was demolished to gain access to steel-concrete interface. Measurements of pH using a novel procedure and chloride ion concentration were done on the interface using an adapted in-situ pH measurement and a Florida Department of Transportation procedure respectively. The pH and chloride ion concentrations obtained in this study favor to some extent a dominant intrinsic mechanism interpretation, while the evidence in support of a dominant extrinsic mechanism interpretation remains elusive.

Lors de l'extraction de l'uranium par lixiviation acide, AREVA fait face à des phénomènes transitoires de précipitation de phases mal cristallisées entrainant un colmatage des installations. Les analyses de ces précipités ont mis en évidence la formation d'un solide jaune minoritaire et d'un blanc majoritaire composé de sulfate de calcium et d'hydroxysulfate d'aluminium. Afin de comprendre les phénomènes menant à la précipitation et pour prévoir, éviter ou limiter leur apparition, les travaux de thèse ont consisté en l'étude expérimentale des équilibres solides-liquide du système complexe Al–Fe–K–P–S–H2O (± Ca et Mg) en milieu acide à 25°C, avec pour objectif l'identification des solides susceptibles d'apparaître en conditions industrielles. Les résultats permettront ainsi de compléter les bases de données thermodynamiques utilisées pour modéliser le comportement global du milieu lors des opérations. Après la mise au point de techniques analytiques appropriées, le travail s'est porté sur la délimitation des équilibres liquide-solides du système relatif au précipité blanc et particulièrement sur l'analyse des ternaires impliquant les hydroxysulfates d'aluminium. Le colmatage se produisant durant la phase d'acidification, l'évolution de la précipitation de ces hydroxysulfates en fonction du pH a été étudiée, afin de caractériser les solides formés en présence de cations minoritaires (Na+, K+, Mg2+, Ca2+) et de définir leurs constantes de solubilités à partir d'un modèle de calcul des coefficients d'activités des ions en solution. Des tests dynamiques sur colonne à partir de minerai naturel ont également été effectués et comparés aux résultats statiques / During the uranium extraction by acid leaching, AREVA observed the precipitation of not well crystallized solid phases, leading to the clogging of various equipment. The analyses of these precipitates highlighted the formation of a minor yellow precipitate and a major white precipitate containing calcium sulphate and aluminium hydroxisulphate. To understand the phenomena leading to the precipitation and to predict, prevent or at least limit their formation, this thesis work consisted of the solid-liquid equilibria experimental study of the Al–Fe–K–P–S–H2O (± Ca and Mg) system in an acidic environment at 25°C, with the identification of solid phases which may appear in natural ore as the major goal. The results could complete the thermodynamic database used to model global behaviour of the environment during acid mining. Based on the observed precipitates, two quaternary systems were defined, such as the H2O-Al3+, Ca2+ // O2-, SO42- system. After the development of appropriate analytical techniques, the work focused on the delimitation of the liquid-solid equilibria of the system based on the white precipitate and particularly on the analysis of the ternaries involving aluminium hydroxisulphates. As clogging occurs during acidification, the hydroxisulphate precipitation depending on the pH was studied in the presence of minority cations (Na+, K+, Mg2+, Ca2+), in order to characterise the solid phases which were formed, and to define their solubility constants. For the solubility constant determination, a calculation model of the ion activity coefficients in solution was used. A dynamic survey on a column containing natural ore was done and compared to static results