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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

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.