Visualization is used for stainless steel container wall and lid cross section characterization. Two specific types of containers are examined: 3013 and SAVY. The container wall examined is from a sample of the inner container of a 3013 container. The inner lid cross section examined is from a SAVY container. Laser confocal microscope data and photographic data are used to determine features of the surfaces. The surface features are then characterized by various feature statistics, such as, maximum depth, area, eccentricity, and others. The purpose of this pilot study is to demonstrate the effectiveness of using the methodology to detect potential corrosion events on the inner container surfaces. The features are used to quantify these corrosion events. An automatic image analysis system uses this methodology to classify images for possible further human analysis by flagging possible corrosion events. A manual image analysis methodology is used to determine the amount of MnS on the SAVY container lid cross section. Visualization is an integral component of the analysis methodology.

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Stress corrosion, SC, in some cases gives rise to stress corrosion cracking, SCC, which differs from purely stress intensity driven cracks in many aspects. They initiate and grow under the influence of an aggressive environment in a stressed substrate. They grow at low load and may branch. The phenomenon of SCC is very complex, both the initiation phase and crack extension itself of SCC is seemingly associated with arbitrariness due to the many unknown factors controlling the process. Such factors could be concentration of species in the environment, stress, stress concentration, electrical conditions, mass transport, and so on.In the present thesis, chemically assisted crack initiation and growth is studied with special focus on the initiation and branching of cracks. Polycarbonate plates are used as substrates subjected to an acetone environment. Experimental procedures for examining initiation and branching in polycarbonate are presented. An optical microscope is employed to study the substrate.The attack at initiation is quantified from pits found on the surface, and pits that act as origin for cracks is identified and the distribution is analysed. A growth criterion for surface cracks is formulated from the observations, and it is used to numerically simulate crack growth. The cracks are seen to coalesce, and this phenomenon is studied in detail. Branching sites of cracks growing in the bulk of polycarbonate are inspected at the sample surface. It is found that the total width of the crack branches are approximately the same as the width of the original crack. Also, angles of the branches are studied. Further, for comparison the crack growth in the bulk is simulated using a moving boundary problem based algorithm and similar behaviour of crack branching is found. / <p>Both papers in thesis as manuscript, paper II with title "Branching cracks in a layered material - Dissolution driven crack growth in polycarbonate"</p>

In this work, Slow Strain Rate Test (SSRT) testing, Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) were used to study the effect of microstructure, corrosive environments and cathodic polarisation on stress corrosion cracking (SCC) of two grades of high strength steels, Type A and Type B. Type A is manufactured by quench and tempered (Q&amp;T) method. Type B, a normalize steel was used as reference. This study also supports electrochemical polarisation resistance method as an effective testing technique for measuring the uniform corrosion rate. SSRT samples were chosen from base metal, weld metal and Heat Affected Zone (HAZ). SSRT tests were performed at room temperature under Open Circuit Potential (OCP) and cathodic polarisation using 4 mA/cm2 in 1 wt% and 3.5 wt% NaCl solutions. From the obtained corrosion rate measurements performed in 1 wt% and 3.5 wt% NaCl solutions it was observed that increased chloride concentration and dissolved oxygen content enhanced the uniform corrosion for all tested materials. Moreover, the obtained results from SSRT tests demonstrate that both Q&amp;T and normalized steels were not susceptible to SCC in certain strain rate (1×10-6 s-1) in 1 wt% and 3.5 wt% NaCl solutions under OCP condition. It was confirmed by a ductile fracture mode and high reduction in area. The weld metal of Type A with acicular ferrite (AF), pro-eutectoid (PF) and bainite microstructure showed higher susceptibility to hydrogen assisted stress corrosion cracking compared to base metal and HAZ. In addition, typical brittle intergranular cracking with small reduction in area was observed on the fracture surface of the Type A due to hydrogen charging.

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Les systèmes de maintien situés sur les structures assemblages-combustibles des réacteurs nucléaires à eau sous pression (REP) sont constitués d'un empilement de lames qui agissent à la fois, comme élément accommodant les incompatibilités thermiques résultant des différences de coefficients de dilatation Acier, alliages de zirconium et principalement, comme système permettant de limiter les effets hydrodynamiques induits par le passage du fluide caloporteur à travers les assemblages. Actuellement, l'alliage 718 est le matériau constitutif de ces ressorts à lames. Il présente les performances en service nécessaires et suffisantes pour répondre aux sollicitations de ces systèmes ainsi qu'aux exigences des autorités de sûreté (dans les conditions actuelles de fonctionnement des REP). Or, dans le cadre de l'augmentation des performances générales des assemblages combustibles, l'emploi d'autres matériaux, dont les propriétés de relaxation sous flux neutronique sont supérieures à celles du 718, est envisagé par AREVA. Les matériaux étudiés sont principalement des superalliages base Ni, tels que les nuances 625+ et 725 qui à l'instar de l'alliage 718 durcissent par précipitation de phases secondaires, ainsi que des nuances d'alliage 718 riche en Molybdène. Cependant, bien que ces nouveaux matériaux présentent une relaxation sous flux neutronique améliorée, ils doivent répondre également à un cahier des charges strict, propre à leur utilisation en centrale : des propriétés mécaniques équivalentes, une bonne résistance à la corrosion sous contrainte (CSC) et une bonne résistance à la fragilisation par l'hydrogène (FPH) en milieu primaire de REP. Chacune de ces propriétés a été étudiée avec attention dans le double but de comparer ces matériaux entre eux et afin de cerner les paramètres clés contrôlant leur différence de comportement aussi bien en CSC qu'en FPH / Hold-down systems used in the fuel assembly of Nuclear Pressurized Water Reactor (PWR) are constituted by stiff springs. The role of the hold-down springs is to ensure the bond between the fuel assembly and the lower plate of the intern structure of the core, thus holding down the assembly on the bottom plate of the reactor, during all the exploitation and maintenance periods. Nowadays, alloy 718 is the constitutive material of these hold-down springs. Its properties in terms of mechanical behaviour, corrosion resistance… fill in the specifications required for such application in the present service conditions. However, in order to improve the common efficiency of fuel assemblies, the upgrading of their design as well as the use of new materials are advocated by the nuclear power plant company, AREVA. Though other Ni-base superalloys known for their good behaviour under neutronic radiation can be proposed as new materials, those superalloys must fill in all the application specifications in order to substitute alloy 718. So, sufficient mechanical properties, good resistance to Stress Corrosion Cracking (SCC) and good resistance to Hydrogen Embrittlement (HE) are also required to allow the replacement. All of these properties are carefully studied with the double aim to characterize and compare different superalloys, and to determine key parameters governing the SCC and HE behaviours of such alloys in primary water of PWR

La corrosion sous contrainte (CSC) des alliages à base nickel est un des principaux phénomènes de dégradation des composants du circuit primaire des Réacteurs à Eau Pressurisée (REP). La compréhension de ce mécanisme de fissuration est un élément essentiel pour la prolongation de la durée d’exploitation des réacteurs.Des études antérieures ont permis d’établir un modèle de propagation de la CSC basé sur une oxydation sélective et dissymétrique du joint de grains en pointe de fissure qui s’accompagne d’une zone appauvrie en chrome. La cinétique de diffusion du chrome étant plus lente que celle de l’oxygène, il est supposé que la diffusion du chrome est une étape limitante de la propagation de la fissure. Si ces observations ont été validées dans la littérature, les hypothèses proposées sur l’origine de l’appauvrissement en chrome dans le grain sont encore sujettes à discussion. Comme la diffusion du chrome en volume dans les alliages base nickel à 350°C ne permet pas d’expliquer les ordres de grandeur des appauvrissements en chrome mesurés dans la littérature, il est supposé qu’il existerait un élément accélérateur de la diffusion du chrome dans l’alliage en pointe de fissure. Ainsi, deux hypothèses sont proposées dans ces travaux : la diffusion du chrome accélérée sous l’effet de la plasticité et la migration des joints de grains induite par la diffusion.L’objectif principal de la thèse a été de confronter les deux hypothèses énoncées au moyen d’essais expérimentaux et de modélisation afin de déterminer le mécanisme de formation de la zone appauvrie en chrome et d’identifier les paramètres favorisant cet appauvrissement.A cet effet, des essais de diffusion sous charge ont été réalisés dans le but d’étudier l’effet de la déformation plastique sur la diffusion du chrome. Les résultats ont permis d’établir une relation entre le coefficient de diffusion et la vitesse de déformation. Ainsi, une accélération de la diffusion en volume de l’ordre de 106 est observée à 350°C sous l’effet de plasticité. De même, des traitements thermiques visant à mettre en évidence la migration des joints de grains induite par la diffusion (DIGM) sont présentés dans ces travaux. Les caractérisations chimiques et microstructurales montrent que la DIGM est bien associée à la formation d’une zone appauvrie en chrome observée dans le sillage du joint de grains migrant. Pour finir, une discussion est proposée afin de relier ces hypothèses au modèle de propagation de la CSC. / Stress Corrosion Cracking (SCC) of nickel base alloys is one of the major degradation phenomena in the primary circuit of Pressurized Water Reactors (PWR). Understanding the SCC mechanism is a key issue for the extension of reactor lifetime.A SCC model based on a selective and asymmetrical oxidation of the grain boundary ahead of the crack tip has been proposed in previous studies. Adjacent to this oxide, a chromium-depleted area is observed exclusively in one of the two grains adjacent to the grain boundary. As oxygen transport is found to be faster than chromium diffusion in the alloy, the latter is assumed to be the rate-limiting step of crack propagation. Nevertheless, the mechanism responsible for chromium depletion is still under debate. Indeed, the lattice and the grain boundary diffusion coefficients of chromium in nickel-based alloys at 350°C are not high enough to explain the chromium depletion magnitudes measured in the literature. Accordingly, factors accelerating chromium diffusion in the alloy ahead of the SCC crack tip should exist. Thus, two assumptions have been proposed in this work: plasticity-enhanced chromium diffusion and diffusion-induced grain boundary migration (DIGM).The aim of this study is to confront these two assumptions by combining both experiments and modeling in order to explain chromium depleted areas observed at the SCC crack tip.Thus, diffusion tests under loading were performed in order to study the effect of plastic deformation on chromium diffusion. Plasticity-enhanced diffusion is evidenced. A relationship between the diffusion coefficient and strain rate has been established leading to a 106-fold increase of the diffusion coefficient at 350°C. In addition, thermal treatments and oxidation tests have shown that diffusion-induced grain boundary migration occurs in Ni-Cr alloys. DIGM leads to dissymmetric Cr-depleted areas, observed in the wake of the moving grain boundary.