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Ferritic stainless steels are the most commonly used materials for solid oxide fuel cell interconnect application. Although these alloys may meet the criteria for interconnect application for short periods of service, their application is limited for long-term use (i.e., 40,000 h) due to poor oxidation behaviour that results in a rapid increase in contact resistance. In addition, volatile Cr species migrating from the chromia scale can poison the cathode resulting in a considerable drop in performance of the cell. Coatings and surface modifications have been developed in order to mitigate the abovementioned problems. In this study, composite electrodeposition of reactive element containing particles in a metal matrix was considered as a solution to the interconnect problems. Nickel and Co were used as the metal matrix and LaCrO3 particles as the reactive element containing particles. The role of the particles was to improve the oxidation resistance and oxide scale adhesion, while the role of Ni or Co was to provide a matrix for embedding of the particles. Also, oxidation of the Ni or Co matrix led to the formation of conductive oxides. Moreover, as another part of this study, the effect of substrate composition on performance of steel interconnects was investigated. Numerous experimental techniques were used to study and characterise the oxidation behaviour of the composite coatings, as well as the metal-oxide scale interface properties. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), as well as surface analysis techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS), were used for the purpose of characterization. The substrate used for coating was AISI-SAE 430 stainless steel that is considered as a typical, formerly used interconnect material. Also, for the purpose of the metal-oxide scale interfacial study, ZMG232 stainless steel that is a specially designed interconnect alloy was used. It is shown that the composite coatings greatly reduce the contact resistance and effectively inhibit Cr outward migration. In addition, it was determined that the presence of impurities in the steel, especially Si, and the absence of reactive elements drastically contribute to interconnect degradation. / Materials Science and Engineering

Industrial and power generation processes employ units like boilers and gasifiers to burn sulfur containing fuels to produce steam and syn gas (H2 and CO), which can generate electricity using turbines and fuel cells. These units often operate under environments containing gases such as H2S, SO2, O2 etc, which can attack the metallic structure and impose serious problems of corrosion. Corrosion control in high temperature sulfur bearing environments is a challenging problem requiring information on local gaseous species at the surface of alloy and mechanisms of degradation in these environments. Coatings have proved to be a better alternative for improving corrosion resistance without compromising the bulk mechanical properties. Changes in process conditions may result in thermal and/or environment cycling between oxidizing and sulfidizing environments at the alloy surface, which can damage the protective scale formed on the alloy surface, leading to increase in corrosion rates. Objective of this study was to understand the effect of fluctuating environments on corrosion kinetics of carbon steels and develop diffusion based coatings to mitigate the high temperatures corrosion under these conditions. More specifically, the focus was : (1) to characterize the local gaseous environments at the surface of alloys in boilers; (2) optimizing diffusion coatings parameters for carbon steel; (3)understand the underlying failure mechanisms in cyclic environments; (4) to improve aluminide coating behavior by co-deposition of reactive elements such as Yttrium and Hafnium; (5) to formulate a plausible mechanism of coating growth and effects of alloying elements on corrosion; and (6) to understand the spallation behavior of scale by measuring stresses in the scales. The understanding of coating mechanism and effects of fluctuating gaseous environments provides information for designing materials with more reliable performance. The study also investigates the mechanism behind the effect of REs on scale adhesion and sulfidation behavior. Thus, the present work will have a broad impact on the field of materials and coatings selection for high temperature industrial environments such as boilers and gasifiers, and provides information on RE-modified aluminized coatings on carbon steel as an alternative for the use of bulk superalloys under high temperature sulfur bearing environments.

When a metallic alloy is placed in an oxygen environment, oxide scale may be formed on the metal surface. The continuous growth of such oxide scale is enabled by the diffusion of various ionic species in the scale layer primarily driven by the gradient of chemical potentials of these ionic species. In addition, the molar volume of oxide is typically greater than that of the base metal. Consequently, mechanical stresses are generated in the oxide scale. Such mechanical stress, in return, may affect the diffusion of ionic species resulting in different oxide growth kinetics. Such interaction between ionic diffusion and mechanical stresses and its effect on oxide scale growth have not been studied. The goal of this thesis is to develop a systematic model for oxide scale growth that takes into account the diffusion-stress interaction. To achieve this goal, the coupled equations based on continuum formulas for diffusion and stresses are developed in first part of this study. The chemical potentials are defined as a stress dependent function. The variation of stress can therefore change the diffusion force, which is the gradient of chemical potentials, to affect the ionic species distribution and consequently have effects on the oxidation kinetics. The model is used to investigate several important aspects of oxidation including scale growth kinetics, stress distribution in the oxide scale, void formation near the metal/oxide interface, and initiation of oxide scale spallation. The reactive element effect (REE) during oxidation of reactive element doped alloy is extensively studied in this study using the developed stress-diffusion interaction model. The key information, such as the modification effects of reactive element upon the diffusion properties of ionic species in oxide scale are quantitatively accessed for yttrium doped Cr alloy. Finite element method was used through a User Element subroutine for ABAQUS to solve the fully coupled stress-diffusion equations in 2D domains with accounting for both elastic and inelastic deformations. The REEs are comprehensively investigated by studying the effects of yttrium on interfacial delamination driving force, energy release rate (G), oxide-alloy interface morphology, and defect diffusion. The outcomes of this study give (1) a deeper understanding of how stresses affect the oxidation, (2) a model to simulate oxide scale growth, and (3) design guidelines on rare earth element doping for improving oxidation resistance. The results of this work elucidate the impact and importance of stress-diffusion coupling on oxidation kinetics and mechanical reliability.

L’intégrité des couches protectrices d’oxyde thermiques se développant à haute température à la surface des matériaux métalliques dépend des niveaux de contraintes générées et de leurs mécanismes de relaxation. Le comportement des films de chromine formés sur les alliages modèles NiCr et Fe47Cr a ici été étudié. Les contraintes résiduelles générées après oxydation des substrats à différentes températures (700°C-1000°C) et durées d’oxydation (3h et 18h) ont été déterminées par spectroscopie Raman. Ces contraintes évoluent avec les conditions d’oxydation de manière monotone (système Ni30Cr/Cr2O3) ou non (système Fe47Cr/Cr2O3). Les variations de déformation à l’échelle du grain ont aussi été suivies par AFM. Pour le premier système, relaxation non destructive par fluage de l’oxyde et délamination sont plus ou moins activées en bon accord avec l’évolution des contraintes résiduelles. Dans le second cas, un mode supplémentaire de relaxation des contraintes par fissuration doit également entrer en jeu. Les propriétés viscoplastiques des couches de chromine formées sur Ni30Cr et Ni28Cr ont pu être caractérisées par diffraction in situ à haute température sur rayonnement Synchrotron, en découplant les effets liés à l’activation thermique de ceux liés à la taille de grain. Le type de fluage intervenant dans la relaxation des contraintes générées dans les couches de chromine a pu être mis en évidence en confrontant les résultats obtenus à un modèle théorique de fluage diffusion. La valeur de l’énergie d’activation associée (130 kJ/mol) a montré, par comparaison avec les données de la littérature, que ce mode non destructeur de relaxation est gouverné par le transport des anions d’oxygène aux joints de grains de l’oxyde. L’ajout d’un élément réactif (Y ou Zr) au substrat Ni28Cr provoque, avec l’augmentation de la quantité introduite, un ralentissement croissant de la cinétique de formation des films de chromine. Cependant, cette quantité introduite ne semble exercer aucun effet significatif sur les niveaux de contraintes résiduelles. A l’échelle microscopique, on constate en général une diminution de la taille des grains avec la présence d’éléments réactifs, une double distribution ayant aussi été observée pour les quantités élevées. Les résultats issus des mesures par diffraction in situ et ceux obtenus par AFM (glissement aux joints de grains de l’oxyde) ont montré que la capacité des films de chromine à relaxer les contraintes grâce à leur comportement viscoplastique pourrait être retardée et/ou diminuée en présence des éléments yttrium et zirconium. Toutefois, ce mécanisme pourrait opérer pour des épaisseurs plus faibles des films de chromine. / Integrity of protective oxide scales developing at the metallic alloys surface at high temperature depends on the stress generation and their relaxation mechanisms. In this work, the behaviour of chromia scales formed on NiCr and Fe47Cr model alloys has been investigated. Raman spectroscopy was used to determine the residual stress level in chromia thin films after oxidation at different temperatures (700°C-1000°C) for 3 h and 18 h. A monotonous evolution of residual stresses with oxidation conditions was noted for the Ni30Cr/Cr2O3 system but not for the Fe47Cr/Cr2O3 one. The strain variations at microscopic scale was also determined by using atomic force microscopy. For the first studied system, non destructive relaxation by creep of the oxide and buckling can be more or less activated, in agreement with the residual stresses evolution. And a third additional stress release mode by intra film cracking could take place for the second system. In situ high temperature oxidation coupled with Synchrotron X-rays diffraction was also used to characterize the viscoplastic properties of chromia scales grown on Ni30Cr and Ni28Cr, with dissociating the effects related to thermal activation and grain size. The creep mechanism responsible of stress release in chromia scales has been evidenced by comparing experimental results with a diffusional creep model. Confrontation of the obtained activation energy (130 kJ.mol-1) with literature results has shown that this non destructive relaxation mode was likely governed by grain boundary transport of oxygen species. When a reactive element (Y or Zr) was added to the metallic substrate Ni28Cr, an important decrease of the oxidation rate was noted when increasing the amount of introduced element. No significant effect of this quantity on the residual stress level was however observed. At microscopic scale, a reduction of grain size has been also noted and two distinguished grains distribution appeared for the higher quantities. Results of in situ Synchrotron diffraction measurements and those of atomic force microscopy (grain boundary sliding) showed that the ability of chromia films to release stress thanks to their viscoplastic properties could be delayed and/or decreased with the presence of an active element. However, this mechanism could operate for smaller chromia films thicknesses.