by Lo, Chi Ho Chester. / Title also in Chinese characters. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves 159-163). / Acknowledgements --- p.i / Abstract --- p.ii / Table of Contents --- p.iv / List of Figures --- p.vii / List of Tables --- p.xii / Chapter Chapter One --- Introduction --- p.1 / Chapter Chapter Two --- Domain Theory --- p.8 / Chapter 2.1 --- Energies in Magnetic Domain Structure --- p.8 / Chapter 2.2 --- Domains in Iron and Nickel --- p.13 / Chapter 2.3 --- Magnetization Process --- p.15 / Chapter 2.4 --- Effect of Applied Stress --- p.19 / Chapter Chapter Three --- Magnetoacoustic Emission --- p.25 / Chapter 3.1 --- Models of MAE --- p.25 / Chapter 3.1.1 --- Discontinuous Wall Motion --- p.25 / Chapter 3.1.2 --- Displacement Model --- p.28 / Chapter 3.1.3 --- DW Creation and Annihilation --- p.30 / Chapter 3.1.4 --- Combined Model of MAE --- p.31 / Chapter 3.2 --- MAE and Magnetic Induction --- p.34 / Chapter 3.2.1 --- Eddy Current Shielding --- p.34 / Chapter 3.2.2 --- Magnetic Reluctance Calculation --- p.35 / Chapter Chapter Four --- Experiments / Chapter 4.1 --- Instrumentation --- p.40 / Chapter 4.1.1 --- Introduction --- p.40 / Chapter 4.1.2 --- Basic Setup --- p.41 / Chapter 4.1.3 --- Arrangement for Stress Measurement --- p.46 / Chapter 4.1.4 --- Specimen Preparation --- p.43 / Chapter 4.2 --- Methodology / Chapter 4.2.1 --- The Fundamental Study of MAE --- p.53 / Chapter 4.2.1.1 --- Effects of Demagnetizing and Stray Fields on MAE --- p.53 / Chapter 4.2.1.2 --- Dependence of MAE on Frequency of Applied Field --- p.55 / Chapter 4.2.1.3 --- Dependence of MAE on Specimen Thickness and Width --- p.55 / Chapter 4.2.2 --- Stress Measurement --- p.58 / Chapter 4.2.2.1 --- Effect of Uniaxial Stress on MAE --- p.58 / Chapter 4.2.2.2 --- Effect of Biaxial Stresses on MAE --- p.58 / Chapter 4.2.3 --- Defect Detection --- p.60 / Chapter 4.2.3.1 --- Nickel --- p.60 / Chapter 4.2.3.2 --- Mild Steel --- p.61 / Chapter Chapter Five --- Results and Discussion / Chapter 5.1 --- Effects of Demagnetizing and Stray Fields on MAE --- p.63 / Chapter 5.1.1 --- MAE Profiles --- p.63 / Chapter 5.1.2 --- Magnetic Reluctance Calculation --- p.68 / Chapter 5.1.3 --- Effect of Annealing --- p.74 / Chapter 5.1.3.1 --- Experimental Results --- p.74 / Chapter 5.1.3.2 --- Discussion --- p.77 / Chapter 5.1.3.3 --- Magnetic Reluctance Calculation --- p.78 / Chapter 5.2 --- Dependence of MAE on Frequency of Applied Field --- p.83 / Chapter 5.2.1 --- Experimental Results --- p.83 / Chapter 5.2.2 --- Theoretical Consideration --- p.88 / Chapter 5.3 --- Dependence of MAE on Specimen Thickness and Width --- p.96 / Chapter 5.3.1 --- Experimental Results --- p.96 / Chapter 5.3.2 --- Theoretical Consideration --- p.99 / Chapter 5.4 --- Effects of Uniaxial and Biaxial Stresses on MAE --- p.107 / Chapter 5.4.1 --- Effect of Uniaxial Stress --- p.107 / Chapter 5.4.1.1 --- Experimental Results --- p.107 / Chapter 5.4.1.2 --- Discussion --- p.116 / Chapter 5.4.2 --- Effect of Biaxial Stresses --- p.120 / Chapter 5.4.2.1 --- Study on Mild Steel Specimen --- p.120 / Chapter 5.4.2.2 --- Study on Nickel Specimen --- p.132 / Chapter 5.5 --- Defect Detection by MAE --- p.137 / Chapter 5.5.1 --- Study on Nickel Specimen --- p.137 / Chapter 5.5.1.1 --- Experimental Results --- p.137 / Chapter 5.5.1.2 --- Discussion --- p.140 / Chapter 5.5.2 --- Study on Mild Steel Specimen --- p.142 / Chapter 5.5.2.1 --- Experimental Results --- p.142 / Chapter 5.5.2.2 --- Discussion --- p.151 / Chapter Chapter Six --- Conclusions and Suggestions for Further Studies --- p.153 / References --- p.159

De nombreux secteurs industriels ont besoin de matériaux métalliques avec des propriétés de résistance à l'usure et de résistance à la corrosion dans des environnements agressifs ou à des températures élevées très élevées. Ces propriétés spécifiques, notamment mécaniques, électriques, magnétiques et optiques, peuvent être obtenues par la production de couches métalliques. Les revêtements d’alliage ou de métal nanocristallin offrent les caractéristiques de performance souhaitées.L'électrodéposition est un procédé largement utilisé pour fabriquer des revêtements sur des substrats métalliques. La structure et les propriétés des revêtements nanocristallins Co-Mo en font des matériaux prometteurs pour diverses applications. De nos jours, les alliages à base de cobalt sont utilisés dans les avions, l’automobile et l’électronique en raison de leur facilité d’utilisation. En outre, les alliages de cobalt sont largement étudiés en raison de leur utilisation prometteuse pour remplacer les alliages nocifs pour l'environnement dans le corps humain. Les alliages nanocristallins sont utilisés dans de nombreux secteurs industriels, tels que la biologie, l'énergie, la nanotechnologie, l'aviation et bien d'autres.Le but de ce travail est de produire des revêtements nanocristallins de Co-Mo électrodéposés et d'étudier l'effet de la microstructure sur leur résistance à la corrosion dans des solutions physiologiques simulées.Le travail est divisé en cinq chapitres:• Chapitre I. Bibliographie. Aspects métallurgiques de l’ingénierie des surfaces métalliques pour les revêtements nanocristallins de Co-Mo.Dans ce chapitre, les aspects métallurgiques de l’ingénierie de surface des revêtements nanocristallins de Co-Mo sont présentés sur la base des données de la littérature. Différentes méthodes utilisées pour le dépôt de revêtements métalliques sont discutées.• Chapitre II. Echantillons et méthodes et techniques expérimentales.Ce chapitre présente les conditions expérimentales d'électrodéposition pour les revêtements nanocristallins de Co-Mo et les techniques utilisées pour les caractériser.• Chapitre III. Structure, propriétés mécaniques et comportement électrochimique des revêtements nanocristallins Co-Mo.Ce chapitre présente la structure, les propriétés mécaniques et le comportement électrochimique des revêtements nanocristallins Co-Mo déposés sur du cobalt pur.• Chapitre IV. Co-Mo / TiO2 revêtements nano-composites.Ce chapitre présente la structure, les propriétés mécaniques et le comportement à la corrosion des revêtements nano-composites Co-Mo / TiO2 galvanisés sur du cobalt pur.• Chapitre V. Mécanismes de croissance des revêtements nano-composites Co-Mo / TiO2.Ce chapitre étudie la structure des revêtements nanocomposites Co-Mo / TiO2 électrodéposés pour des temps d'électrodéposition courts et longs dans des conditions potentiostatiques sur des électrodes ayant la forme d’un film ou d’un disque.En résumé, des revêtements nanocristallins Co-Mo et des revêtements nanocomposites Co-Mo / TiO2 ont été obtenus par dépôt électrochimique. Leur structure et l'influence de différentes conditions d'électrodéposition sur leur structure ont été étudiées. Leur résistance à la corrosion a été testée dans des solutions physiologiques simulées. De plus, le mécanisme de croissance des couches a été déterminé sur les deux types d'électrodes.Cette thèse a été réalisée sous la forme d'une coopération en Cotutelle entre l'Université des sciences et technologies AGH Stanisław Staszic à Cracovie et l'Université de Bourgogne à Dijon pour promouvoir les relations scientifiques franco-polonaises. Ce travail a été soutenu financièrement par le gouvernement français et l'ambassade de France en Pologne. / Currently, various methods are used in surface engineering for the production of surface layers and coatings on many metal substrates to improve their performance. Different industry sectors are oriented to increase the wear resistance and corrosion resistance of materials that are used in aggressive environments or work at high temperatures. Specific properties, including mechanical, electrical, magnetic and optical, can be achieved by the production of metallic layers. Nanocrystalline and amorphous metal or alloy coatings are unique and offer the desired performance characteristics. They are often characterized by very high strength, associated with the effect of strengthening the edge grain.Electrodeposition is a widely used method of producing coatings on metallic substrates. The structure and properties of Co-Mo nanocrystalline coatings make them promising materials for various applications. Nowadays, cobalt based alloys are used in aircraft, automotive and electronics due to their good usability. In addition, cobalt alloys are widely studied due to their promising use in replacing alloys harmful to the environment or the human body. Nanocrystalline alloys are used in many industrial sectors, such as biology, energy, nanotechnology, aviation and many others.The aim of this work is to obtain electrodeposited Co-Mo nanocrystalline coatings and to investigate the effect of microstructure on their corrosion resistance in simulated physiological solutions.The work is divided into five chapters:• Chapter I. Bibliography. Metallurgical aspects of metal surface engineering for Co-Mo nanocrystalline coatings.In this chapter, metallurgical aspects of surface engineering of Co-Mo nanocrystalline coatings are presented on the basis of literature data. Various methods used for deposition of metallic coatings are discussed.• Chapter II. Samples and experimental methods and techniques.This chapter presents experimental electrodeposition conditions for Co-Mo nanocrystalline coatings and techniques used to characterize them.• Chapter III. Structure, mechanical properties and electrochemical behavior of Co-Mo nanocrystalline coatings.This chapter presents the structure, mechanical properties and electrochemical behavior of nanocrystalline Co-Mo coatings deposited on pure cobalt.• Chapter IV. Co-Mo / TiO2 nano-composite coatings.This chapter presents the structure, mechanical properties and corrosion behavior of Co-Mo / TiO2 nano-composite coatings electroplated on pure cobalt.• Chapter V. Growth Mechanisms of Co-Mo / TiO2 nano-composite coatings.This chapter investigates the structure of Co-Mo / TiO2 nanocomposite coatings electrodeposited for short and long electrodeposition times under potentiostatic conditions on the electrode from pure cobalt-shaped cobalt and pure cobalt electrode in the form of a wire.In summary, nanocrystalline Co-Mo coatings were obtained by electrochemical deposition as well as Co-Mo / TiO2 nano-composite coatings. Their structure and the influence of different electrodeposition conditions on the structure were investigated and their corrosion resistance was tested in simulated physiological solutions. In addition, the mechanism of layer growth was determined on two types of electrodes: a disk-shaped electrode and a wire-shaped electrode.This thesis was carried out in the form of Cotutelle cooperation between the AGH University of Science and Technology Stanisław Staszic in Krakow and the University of Burgundy in Dijon to promote Polish-French scientific relations. This work was financially supported by the French government and the French Embassy in Poland.

Durant ce travail de thèse, nous nous sommes intéressés au comportement hydromécanique d’un éco-géo-matériau à base de terre crue. Différents aspects ont été traités visant l’optimisation de sa formulation et aussi ses interactions avec les renforts. Nous avons tenté de lever trois verrous scientifiques : l’optimisation des interactions physico-chimiques argiles-liants, l’identification des mécanismes de fissuration en relation avec la succion, et les interactions électrochimiques sol-armatures en acier. L’ajout de la chaux impose le comportement du sol à court terme et favorise le gain en résistance en rendant le milieu plus alcalin. Nous avons montré qu’à l’aide d’une mesure macroscopique, la conductivité électrique, nous pouvons suivre les interactions sol-liants. Lors de la dessiccation d’un sol argileux, le retrait global est homogène. Une déformation locale hétérogène induit des contraintes de traction responsables de l’apparition des fissures. L’ajout des renforts réduit le pourcentage de fissuration d’un facteur 10. La succion générée lors du séchage est interprétée en termes de contraintes de traction. Le contact entre la terre et l’acier entraine des réactions d’oxydoréduction dont le produit est la corrosion. Trois familles d’argiles ont été testées. La vitesse de corrosion obtenue est de l’ordre de 3μm/an. / Throughout this thesis, we are interested in hydro-mechanical behavior of an eco-geo-material based on raw earth. Different aspects have been treated viewing the optimization of his formulation as well as its interactions with the reinforcements. We have tried to resolve the following three scientific locks: the optimization of the physical-chemical clay-binders’ interactions, the identification of the mechanisms of cracking in relation with the suction and the electrochemical soil-iron interactions. Adding lime imposes the behavior of the soil and improves its resistance by making the environment more alkaline. We showed that we can follow soil-binders’ interactions by using the electrical conductivity technique. During desiccation of a clayey soil, global shrinkage is homogenous. A heterogonous local deformation induces tensile stress responsible of cracking. Adding reinforcement reduces crack ratio by a factor 10. Suction generated during drying is interpreted in terms of tensile stresses. Interaction between earth and metal allows oxydo-reduction reactions producing corrosion. Three type of clay were tested. Corrosion speed is about 3μm/year.

No presente trabalho analisa-se o comportamento do aço de reforço ante à corrosão, com o uso dos inibidores: nitrito de sódio, fosfato de sódio e etalonamina, na água de poros contaminada com cloreto, e no concreto com a finalidade de analisar seus resultados e seus mecanismos diferenciados. Para cumprir este objetivo o presente trabalho divide-se em duas etapas: uma primeira etapa baseada em simular sinteticamente a água de poro de um concreto, cuja solução é KOH 28g/l+NaOH 4g/l. Essa água de poro é simulada em ambiente marinho, cuja solução é KOH 28g/l + NaOH 4g/l+NaCl 35g/l, e a esta solução referência incorporamse os inibidores (20g/l da cada um). Realizaram-se ensaios de espectroscopia de impedância eletroquímica (EIE) (após 3 e 72 horas de imersão) e curvas de polarização (após 72 horas de imersão) com vistas a obter respostas da cinética da corrosão ante a cada solução. Obteve-se o melhor comportamento para a água de poros. No caso da água de poro contaminada por cloretos, o melhor comportamento se obteve para o inibidor nitrito de sódio. Na segunda etapa adotou-se apenas o inibidor nitrito de sódio, pois estatisticamente as eficiências dos três inibidores foram muito similares. Analisou-se o nitrito de sódio em amostras reais de concreto armado contaminado com cloreto de sódio. Para isso se elegeram dois tipos de cimentos (CP IV e CP V) e três relações água-cimento (a/c-0.4, a/c-0.5, a/c- 0.65). Para simular o ambiente marinho, realizaram-se ensaios acelerados de cloretos. Comparam-se métodos de análises simuladas sinteticamente e reais, concluindo-se em ambos meios, embora fossem um solido e outro líquido o inibidor Nitrito de Sódio aumento a sua eficiência com os ciclos de exposição. / In this paper it is analyzed the behavior of reinforcing steel against corrosion using inhibitors: sodium nitrate, sodium phosphate and ethanolamine in water contaminated with chlorides pore and concrete, in order to analyzing the results and different mechanisms. To meet the objective of this work, it was divided into two stages, a first stage based on synthetically simulate the pore water of a concrete, through the following solution KOH 28g/l+NaOH 4g/l, this same solution simulated pore water to a marine environment it would be KOH 28g/l + NaOH 4g/l+NaCl 35g/l, it is then incorporated into both reference solutions inhibitors in a proportion, (20g/l de cada um). Assays were performed electrochemical impedance spectroscopy (EIE) (last 3 hours and 72 hours of immersion) and polarization curves (last 72 hours of immersion) in order to obtain responses corrosion kinetics in each solution. the best performance was obtained in the pore water. In the case of water contaminated with chlorides pore, the best performance was obtained in the presence of sodium nitrite inhibitor. In the second step was performed only with the inhibitor sodium nitrate, as statistically efficiencies of the three inhibitors were similar. Sodium nitrate was analyzed in real samples of reinforced concrete contaminated with chlorides of sodium. So they were chosen two types of cement CP- IV and CP-V, cement water three relationships 0.4, a/c-0.5, a/c- 0.65. In this case to simulate the marine environment, accelerated tests were performed chloride. They were compared the methods of analysis, simulated synthetically and simulated in real concrete.

La France prévoit de construire des réacteurs nucléaires de 4e génération. Ils utiliseraient du sodium liquide comme fluide caloporteur et seraient construits en acier austénitique 316L(N). Afin de garantir un fonctionnement optimal, la tenue de cet acier doit être vérifiée. Pour cela, la corrosion de l’acier 316L(N) par le sodium liquide doit être bien connue.La littérature montre que plusieurs phénomènes de corrosion sont possibles. Pour chacun de ces phénomènes, l’influence de la présence d’oxygène dans le sodium est grande. Nous avons donc étudié la corrosion des aciers austénitiques par le sodium liquide en présence d’oxygène.Les données thermodynamiques permettent de montrer que la formation de chromite de sodium est possible sur les aciers austénitiques immergés dans le sodium contenant de l’ordre de 10 μg.g-1 d’oxygène pour des températures inférieures à 650 °C (conditions réacteurs).L’étude expérimentale montre que la chromite de sodium se forme à 650 °C dans le sodium contenant 200 μg.g-1 d’oxygène. À cette même concentration et à 550 °C, la chromite de sodium est observée avec certitude uniquement pour les longues durées d’immersion (> 5000 h). Les résultats à 450 °C sont moins évidents. Par ailleurs, l’acier est appauvri en chrome dans toutes les conditions étudiées.Ces résultats suggèrent que la chromite de sodium se dissout dans le sodium au fur et à mesure de sa formation. Des modèles de formation de la chromite de sodium, approchéelimitée par la diffusion du chrome dans l’acier (en volume et aux joints de grains) et de dissolution, supposée limitée par le transport dans le métal liquide ont permis de montrer que la formation et la dissolution simultanée de la chromite de sodium est un mécanisme possible pour expliquer nos résultats. / France is planning to construct the 4th generation of nuclear reactors. They will use liquid sodium as heat transfer fluid and will be made of 316L(N) austenitic steel as structural materials. To guarantee optimal operation on the long term, the behavior of this steel must be verified. This is why corrosion phenomena of 316L(N) steel by liquid sodium have to be well-understood.Literature points out that several corrosion phenomena are possible. Dissolved oxygen in sodium definitely influences each of the corrosion phenomenon. Therefore, the austenitic steel corrosion in oxygen-containing sodium is proposed in this study.Thermodynamics data point out that sodium chromite formation on 316L(N) steel is possible in sodium containing roughly 10 μg.g-1 of oxygen for temperature lower than 650 °C (reactor operating conditions).The experimental study shows that sodium chromite is formed at 650 °C in the sodium containing 200 μg.g-1 of oxygen. At the same concentration and at 550 °C, sodium chromite is clearly observed only for long immersion time (> 5000 h). Results at 450 °C are more difficult to interpret. Furthermore, the steel is depleted in chromium in all cases.The results suggest the sodium chromite is dissolved in the sodium at the same time it is formed. Modelling of sodium chromite formation - approached by chromium diffusion in steel (in grain and grain boundaries -, and dissolution - assessed by transport in liquid metal - show that simultaneous formation and dissolution of sodium chromite is a possible mechanism able to explain our results.

Dissimilar metal joining was performed with the main goal being maximization of the strength of the joined samples, but because of some potential applications of the dissimilar joints, analyzing their corrosion behavior also becomes crucial. Starting with materials that initially have suitable corrosion resistance, ensuring that the laser processing does not diminish this property is necessary. Conversely, the laser shock peening processing was implemented with a complete focus on improving the corrosion behavior of the workpiece. Thus, many commonalities occur between these two manufacturing processes, and this thesis goes on to analyze the thermal and mechanical influence of laser processing on materials’ corrosion resistances. Brittle intermetallic phases can form at the interfaces of dissimilar metal joints. A process called autogenous laser brazing has been explored as a method to minimize the brittle intermetallic formation and therefore increase the fracture strength of joints. In particular, joining of nickel titanium to stainless steel wires is performed with a cup/cone interfacial geometry. This geometry provides beneficial mechanical effects at the interface to increase the fracture strength and also enables high-speed rotation of the wires during irradiation, providing temperature uniformity throughout the depth of the wires. Energy dispersive X-ray spectroscopy, tensile testing, and a numerical thermal modelling are used for the analysis. The material pair of nickel titanium and stainless steel have many applications in implantable medical devices, owing to nickel titanium’s special properties of shape memory and superelasticity. In order for an implantable medical device to be used in the body, it must be ensured that upon exposure to body fluid it does not corrode in harmful ways. The effect that laser autogenous brazing has on the biocompatibility of dissimilar joined nickel titanium to stainless steel samples is thus explored. While initially both of these materials are considered to be biocompatible on their own, the laser treatment may change much of the behavior. Thermally induced changes in the oxide layers, grain refinement, and galvanic effects all influence the biocompatibility. Nickel release rate, polarization, hemolysis, and cytotoxicity tests are used to help quantify the changes and ascertain the biocompatibility of the joints. To directly exert a beneficial influence on materials’ corrosion properties laser shock peening (LSP) is performed, with a particular focus on the stress corrosion cracking (SCC) behavior. Resulting from the combination of an applied load on a susceptible material exposed to a corrosive environment, SCC can cause sudden material failure. Stainless steel, high strength steel, and brass are subjected to LSP and their differing corrosion responses are determined via cathodic charging, hardness, mechanical U-bend, Kelvin Probe Force Microscopy, and SEM imaging. A description accounting for the differing behavior of each material is provided as well as considerations for improving the effectiveness of the process. SCC can occur by several different physical processes, and to fully encapsulate the ways in which LSP provides mitigation, the interaction of microstructure changes induced by LSP on SCC mechanisms is determined. Hydrogen absorbed from the corrosive environment can cause phase changes to the material. Cathodic charging and subsequent X-ray diffraction is used to analyze the phase change of sample with and without LSP processing. Lattice dislocations play an important role, and transmission electron microscopy helps to aid in the analysis. A finite element model providing spatially resolved dislocation densities from LSP processing is performed.

Le metal dusting est un type de corrosion catastrophique des alliages à base de fer, de nickel ou de cobalt. Il se caractérise par une dégradation de ces matériaux en une fine poussière de particules métalliques et de carbone graphitique, appelée « coke », pouvant également contenir des carbures et des oxydes. Ce phénomène a lieu lorsque le mélange gazeux est sursaturé en carbone (ac>>1), à des températures comprises entre 400°C et 800°C. Cinq alliages commerciaux austénitiques (800HT, HR120, Inconel 625, Inconel 690 et Inconel 693) et deux matériaux modèles fabriqués par SPS (NiFeCr et NiFeCr+Cu) sont testés dans deux environnements de metal dusting à 570°C. Le premier test est effectué sous pression atmosphérique dans un mélange CO-H2-H2O, le second dans une atmosphère CO-H2-CO2-CH4-H2O sous 21 bars de pression. La première composition est ajustée pour obtenir une activité en carbone et une pression partielle en dioxygène proches de celles de l’environnement sous haute pression. Après plus de 14 000 h heures d’exposition, l’alliage 625 n’est pas dégradé. Il présente une précipitation d’aiguilles de γ’’-Ni3Nb, le niobium migrant vers la surface suite à l’appauvrissement en chrome par oxydation. Le matériau NiFeCr+Cu présente une évolution microstructurale proche, le cuivre formant une couche continue à l’interface métal/oxyde. Le cuivre étant non-catalytique pour la formation de carbone, sa ségrégation en surface améliore la résistance du matériau. L’alliage 690 présente une carburation homogène sur toute la surface qui n’évolue pas dans le temps. L’alliage 693 présente au contraire une carburation très importante, de plus en plus profonde avec la durée d’exposition. Celle-ci s’explique par la formation d’une couche continue d’alumine de transition, métastable. Sa transformation en alumine α, stable, s’accompagne d’une contraction de la maille, fissurant la couche d’oxyde. L’atmosphère accède alors directement à la surface métallique, carburant l’alliage. La bonne tenue de cet alliage, malgré la fissuration de l’oxyde, s’explique par sa forte teneur en chrome et par la faible cinétique de la transformation allotropique à 570°C. Les alliages 800HT, HR120 et NiFeCr sont corrodés par piqûration. Pour l’alliage 800HT, celle-ci est simulée en surface par un modèle de germination-croissance dépendant du temps d’incubation des piqûres, de leur croissance et de la densité de piqûres. La prise en compte du volume des piqûres pour reproduire les pertes de masses enregistrées est concluante sous haute pression mais pas à pression atmosphérique. Cela met en exergue l’influence de la géométrie de l’échantillon (les échantillons testés à pression atmosphériques étant très attaqués sur les bords), et donc l’intérêt d’étudier la piqûration. Sous pression atmosphérique, la croissance latérale des piqûres se fait par oxydation des carbures tandis que la croissance en profondeur se fait par un mécanisme de graphitisation accélérée, lorsque le flux de carbone est suffisamment grand devant le flux d’oxygène. La graphitisation accélérée n’a lieu qu’en fond de fissure du fait du faible renouvellement de l’atmosphère. Les fissures se forment lors du cyclage thermique effectué toutes les 500 h pour caractériser les échantillons. Cela conduit à un faciès de corrosion constitué d’une oxydation interne fine et discontinue exposant directement à l’atmosphère l’alliage carburé, qui est alors graphitisé. Il en résulte l’apparition d’une succession d’anneaux de corrosion, un sur deux croissant en profondeur. La morphologie issue du mécanisme de graphitisation favorisée est visible sur toute la circonférence des piqûres formées sous haute pression. Le même mécanisme a donc lieu, globalement cette fois, le flux de carbone étant suffisamment grand devant le flux d’oxygène dès l’introduction dans le banc de corrosion. Les morphologies observées sont donc liées aux conditions expérimentales (température, atmosphère, débit) et à la procédure de suivi (retraits). / “Metal dusting” is a catastrophic corrosion phenomenon of Fe-, Ni- and Co-based alloys. It is characterised by the degraded of these materials into a dust of fine metallic particles and graphitic carbon, named “coke”, which can also contain oxides and carbides, depending on the alloy. This phenomenon occurs when the gas mixture is oversaturated in carbon (ac>>1), for temperatures between 400°C and 800°C. Five commercial austenitic alloys (800HT, HR120, Inconel 625, Inconel 690 and Inconel 693) and two model alloys fabricated by SPS (NiFeCr et NiFeCr+Cu) are tested under two metal dusting environments at 570°C. The first test is carried out under atmospheric pressure in a CO-H2-H2O gas mixture, while the second is performed at 21 bar in a CO-H2-CO2-CH4-H2O atmosphere. The first environment is adjusted to obtain a carbon activity and a dioxygen partial pressure similar to the ones of the second environment. After 14 000 h of exposure, 625 alloy is not degraded. Chromium depletion stemmed from oxide scale formation induces niobium diffusion to the surface and precipitation of needle-like γ’’-Ni3Nb precipitates below the alloy surface. NiFeCr+Cu alloy presents a close microstructural evolution, as copper forms a continuous scale at the metal/oxide interface. Its segregation induces an improved resistance of the material, copper being non-catalytic to carbon formation. 690 alloy presents an homogeneous carburation under its whole surface which does not deepen during further exposure. 693 alloy, however, presents an important carburisation, which increases with the exposure time. This can be explained by the formation of a continuous oxide scale composed of a metastable transient alumina. Allotropic transformation of this alumina in its stable form, α-alumina, induces lattice contraction. The oxide scale undergoes tensile stress, and cracks form. The atmosphere can then accede directly to the catalytic surface and carburise the material. Despite this, the macroscopically good behaviour of 693 alloy can be explained by its high chromium level and the low kinetics of the allotropic transformation at 570°C. 800HT, HR120 and NiFeCr alloys are degraded by pitting. Pitting on 800HT is modelled using a nucleation-growth model. It depends on pit incubation time, pit growth kinetics and pit density. Taking into account the volume of the pits to model the mass losses undergone by the alloys is concluding for the specimens tested under high pressure but not for those tested at atmospheric pressure. This shows the influence of the sample geometry (samples tested at atmospheric pressure are more attacked on the edges), hence the interest to study corrosion via pitting. For tests at atmospheric pressure, pit lateral growth occurs by oxidation of internal carbides while pit inward growth stems from an enhanced graphitisation mechanism, when the carbon flux is high enough compared to the oxygen flux. Enhanced graphitisation takes place at the bottom of cracks formed through the internal oxidation zone due to the sample cooling performed every 500h for characterisation. The atmosphere is hardly renewed at the bottom of the crack. It leads to a thin, discontinuous oxidation layer exposing directly to the atmosphere the carburised alloy, which is then graphitised. This results in a succession of corrosion rings, one from two being deep, due to the combination of cracking and enhanced graphitisation. The morphology observed under atmospheric pressure due to enhanced graphitisation is also visible under high pressure, but on the entire pit circumference. It reveals that the same mechanisms takes place but on the entire pit, the carbon flux being high enough compared to the oxygen flux, right from its introduction in the corrosion rig. The two observed pit morphologies are then strongly linked to experimental conditions (temperature, gas mixture, gas flow) and the experimental procedure (thermal cycling induced by regular withdrawals).

La présente étude porte sur les problématiques d’entreposage de colis de déchets nucléaires. En France, via le projet Cigéo, il est envisagé d’entreposer les déchets radioactifs de moyenne et haute activité à vie longue à 500 mètres de profondeur dans un conteneur en acier API 5L X65 (chemisage) déposé dans des galeries creusées au sein d’une formation argileuse (argilites). Une température maximum de 90°C est attendue à la surface de l’acier en raison de l’intense radioactivité. Finalement, un coulis cimentaire (matériau de remplissage) sera injecté entre le chemisage et les argilites. La synthèse de couches de produits de corrosion susceptibles d’être rencontrées à la surface de l’acier dans les conditions de stockage a constitué le premier objectif de ce travail. Les conditions pour l’obtention de sidérite (FeCO3), de mackinawite (FeS) et de magnétite (Fe3O4) ont été déterminées. Par la suite, des essais de couplage impliquant deux électrodes recouvertes de produits différents ont été réalisés dans le but de simuler le comportement d’une surface d’acier recouverte d’une couche hétérogène de produits de corrosion. Il est apparu que l’acier recouvert d’une couche de mackinawite se comportait dans chaque cas comme une cathode. Ce phénomène est associé au caractère plutôt protecteur de la couche de mackinawite. Parallèlement, les essais de formation de la mackinawite par polarisation anodique ont montré que la formation d’une couche de produits de corrosion composée de magnétite/mackinawite était associée à un phénomène de corrosion localisée. Le comportement de l’acier au contact du matériau de remplissage envisagé pour combler l’espace entre le chemisage et les argilites a ensuite été étudié. Ce deuxième volet de l’étude a montré que la couche de produits de corrosion se formant à la surface de l’acier était principalement composée de magnétite, associée à des sulfures de fer tels que la mackinawite. L’hétérogénéité du matériau de remplissage, la présence de sulfures en son sein et la présence d’oxygène dissous dans les solutions de test sont les principales causes de l’apparition d’un phénomène de corrosion localisée. Ce résultat traduit également le caractère imparfaitement protecteur des couches d’oxyde se formant sur l’acier au contact du matériau cimentaire. Enfin, la dernière partie de ce travail de recherche a porté sur l’étude d’éventuels effets galvaniques entre une zone recouverte d’argilites et une zone recouverte de matériau de remplissage impliquant la présence de magnétite et de mackinawite et pouvant faire office de cathode. De tels effets n’ont pas été mis en évidence, ce qui est attribué à l’absence d’un véritable état passif de l’acier en contact avec le matériau cimentaire. / The present study relates to the problem of long-term disposal of nuclear waste. In France, with the Cigéo project, it is envisaged to store high and intermediate level long lived radioactive waste at a depth of 500m inside a carbon steel (API 5L X65) casing in a deep geological disposal, drilled in a very stiff clay formation. A maximum temperature of 90°C is expected at the carbon steel surface, because of the intense radioactivity. Finally, a specific cement grout will be injected between the carbon steel casing and the argilites. The synthesis of corrosion product layers likely to form on the steel surface was the first objective of this work. The different parameters to obtain siderite (FeCO3), mackinawite (FeS) and magnetite (Fe3O4) have been determined. Subsequently, coupling tests were carried out with two steel electrodes covered with different corrosion products in order to simulate the behaviour of a steel surface covered with a heterogeneous corrosion product layer. It appeared that the steel electrode covered with mackinawite was in each case the cathode. This phenomenon is associated with the somewhat protective properties of the mackinawite layer. Besides, anodic polarization experiments conducted to prepare mackinawite layers showed that the formation of a corrosion product layer composed of magnetite/mackinawite was associated with localized corrosion. The behaviour of carbon steel in contact with cement grout envisaged to fill the gap between casing and argillites was studied next. This second part of the study showed that the corrosion product layer forming on the steel surface was mainly composed of magnetite, associated with iron sulphides such as mackinawite. Heterogeneity of the cement grout, presence of sulphide within it, and presence of dissolved oxygen in the test solutions are the main causes of the appearance of localized corrosion processes. This result also shows that the oxide layers forming on the steel surface in the specific cement grout only provides an imperfect protection. Finally, the last part of this research work focused on the study of possible galvanic effects between a zone covered with argilites, and a zone covered with cement grout implying the presence of magnetite and mackinawite and thus able to act as cathode. Such effects could not be evidenced which is attributed to the absence of a real passive state of the steel in contact with the cement grout.

The research reported in this doctoral thesis involves constructing physiochemical models that reproduce the transport behaviour of aqueous chemical species present in environments relevant to the oil and gas industry to gain an improved insight into the local electrochemistry near the electroactive surface (uniform corrosion) or inside the pit (pitting corrosion). The first part of the project involved constructing physiochemical models with one dimensional geometry with aqueous chemical species and chemical and electrochemical processes observed in oxygen (O2) containing brine environments to determine the changes in the local electrolyte composition and the potential within an initiated pit for a variety of external physical and chemical conditions. It was determined that the bottom of the pit suffers greatly from the effects of iR drop (Ohmic drop) if the pit geometry is taken to be macroscopic. The model was extended to include additional aqueous chemical species in conjunction with the chemical and electrochemical processes observed in carbon dioxide (CO2) rich environment to investigate the effects of CO2 on the local electrolyte chemistry at the bottom of the pit. It was found that the proton reduction electrochemical process on its own was incapable of supplying the high currents experimentally measured in CO2 environments via the buffering effect. The second part of the project was to investigate the influence of different experimental conditions on the polarisation behaviour of near static carbon steels in CO2 saturated brine electrolyte via multiple electrochemical measurement techniques. The key observation from this study was the presence of two distinct mass transport limited regions on the cathodic polarisation curve at natural pH (3.775). From the physiochemical model fitted to the experimental cathodic curve, the first mass transport limited region, occurring at lower cathodic potentials, was identified to be the direct reduction of carbonic acid while the second wave, occurring at slightly higher cathodic potentials, was shown to be the direct reduction of aqueous carbon dioxide. Based on the polarisation scans under forced convection, the rate of the direct reduction of carbon dioxide was determined to be under neither potential nor mass transport control. The third part of the project involved extending the existing one dimensional models to include the precipitation of salt films (iron chloride – FeCl2(s) and iron carbonate – FeCO3(s)) in O2 and CO2 saturated brine electrolyte respectively along with the capability to track their respective thickness. Furthermore, the ability of the underlying metal to undergo a change in its state from active to passive is implemented in the model via a set of rules based on the Pourbaix diagram. It was determined that the precipitation of salt films is greatly influenced by the mass transport with no or minimal thickness observed under even natural convection conditions. Furthermore the successful precipitation of salt film was determined to be a precursor step to the metal attaining passivation.

La présente étude porte sur les interactions entre deux types de matériaux susceptibles d'être mis en jeu au sein d'un stockage géologique de déchets radioactifs : les matériaux métalliques, constituant notamment le surconteneur de déchets de haute activité, et les matériaux argileux, telle que la roche hôte argileuse. Les interactions entre ces deux matériaux en conditions de stockage pourraient en effet modifier leurs propriétés de confinement initiales. Les travaux présentés ont visé à déterminer l'influence d'hétérogénéités (vides technologiques et fractures) et d'activités bactériennes sur ces interactions, notamment en termes d'évolution de propriétés chimiques et hydrauliques de l'argile. Dans cet objectif, deux expériences intégrées en cellules de percolation ont été mises en œuvre, à 60 °C, pendant 13 mois : la première en présence de deux souches bactériennes (BSR, BFR), la seconde constituant un témoin abiotique. Ces expérimentations ont permis la circulation d'une eau synthétique dont la composition est représentative de celle de l'eau porale de Tournemire au travers d'un compact de fer pulvérulent puis d'une carotte d'argilite du Toarcien de Tournemire, artificiellement fissurée. L'une des deux demi-carottes d'argile contenait également un cylindre de fer massif. Les caractérisations post-mortem (MEB, MEB/EDS, Raman, DRX, tomographie aux rayons X) ont permis d'étudier deux interfaces : l'interface fer pulvérulent/argilite et l'interface fer massif/argilite.Concernant le déroulement des expérimentations, il convient de noter en premier lieu que la fissure a probablement été circulante durant la totalité de l'essai, ce que tendent à confirmer les modélisations couplées chimie/transport avec le code HYTEC. Toutefois, aucune phase néoformée n'a été mise en évidence à son niveau. Par ailleurs, la survie bactérienne dans la cellule biotique a été confirmée au cours de l'expérimentation par le suivi régulier de la population et par une analyse de la diversité génétique à la fin des essais. Une nette diminution de la concentration en sulfates en sortie de cellule confirme l'activité des BSR.Les caractérisations solides en fin d'essais ont mis en évidence dans la zone de fer pulvérulent, avec et sans bactéries, de la magnétite et de la chukanovite, cette dernière étant localisée majoritairement à l'interface avec l'argilite. Une zone enrichie en fer (10 µm) a été identifié à l'interface dans l'argilite. La vitesse moyenne de corrosion du fer pulvérulent a été estimée à 0,2 µm/an (valeur basse). A l'interface fer massif/argilite, deux faciès ont été mis en évidence. Le premier faciès, identifié dans les deux dispositifs, est composé de chukanovite en couche interne et de sidérite en couche externe. L'extension de la perturbation de l'argilite est de l'ordre de 30 µm. Le second faciès, mis en évidence seulement en présence de bactéries, montre la présence de sulfure de fer (mackinawite) et d'avancées locales de corrosion. Enfin, les simulations HYTEC réalisées ont permis de mieux comprendre les mécanismes biogéochimiques observés expérimentalement, notamment l'effet du pH, et de mieux quantifier certains paramètres cinétiques clefs. / This study focuses on the interactions between two materials that may be introduced in a geological disposal of radioactive waste: metallic materials such as the high-level waste overpack, and clay materials such as the clay host rock. Indeed, the interactions between these two materials in such conditions could induce a change of their initial confinement properties. This work aimed at determining the influence of heterogeneities (technological gaps and fractures) and bacterial activities on these interactions, in terms of evolution of chemical and hydraulic properties of clayey materials. To this end, two percolation cells have been conducted during 13 months: the first one with two bacteria (SRB, IRB), the second one without bacteria. These experiments, carried out at 60°C, involved circulating synthetic water representative of the Tournemire pore water through iron powder and through Toarcian artificially cracked argillite from Tournemire. An iron rod was also placed into the argillite. Thus, solid characterizations (SEM, SEM/EDS, Raman, XRD, X-ray tomography) allowed the study of both interfaces: the iron powder/argillite interface and the iron rod/argillite interface.The water probably circulated into the crack during the entire test, which was confirmed by reactive transport modeling with the HYTEC reactive transport code. However, no secondary phase was identified in the crack. In addition, bacteria survival in the biotic cell was confirmed during the experiment by monitoring their population and by analyzing their genetic diversity at the end of the experiment. A strong decrease in sulfate concentration was measured in the output, which confirms the SRB activity.Solid characterization conducted at the end of the experiments have highlighted, with and without bacteria, the occurrence of magnetite and chukanovite in the iron powder, the latter being mainly located close to the argillite interface. In the argillite, a Fe-enriched zone (10 µm) was identified. The mean corrosion rate was estimated at 0.2 µm/y (lower bound). At the iron rod/argillite interface, two corrosion facies were observed. The first, identified in both cells, is mainly constituted of chukanovite in the inner layer and siderite in the outer layer. Extent of the argillite perturbation reaches about 30 µm. The second, only observed with bacteria, highlights the presence of iron sulfide precipitation (mackinawite) and localized corrosion patterns. Finally, HYTEC simulations have enabled us to better understand the observed biogeochemical processes, such as the pH effect, and to better quantify some key kinetic parameters.