Modeling of mechanisms affecting the growth and breakdown of iron sulfide films

[摘要] Modeling the behavior of ionic materials in electrochemical environments is a topic of great scientific interest for important systems like like fuel cells, batteries, and catalysts and corrosion passive films. However, existing analytical and numerical models of such electrochemical systems like passive films are derived from mathematical expressions for the growth and degradation of the ionic material that are not derived from the rates of underlying physical processes and are instead merely benchmarked to experimental data. These models are intrinsically empirical and fall short of providing mechanistic insights into the physical processes occurring in the passive film that are required to design better-performing electrochemical materials. In this thesis, based on our understanding that overall film behavior is a collective outcome of several atomic-scale phenomena like defect formation and ionic diffusion that contribute to the growth and/or breakdown of the passive film, we construct a construct a novel multiscale modeling framework that explicitly models atomic scale phenomena and couples it to higher-length- scale description of the microstructure to estimate passive film behavior in a mechanistic way and we further demonstrate this capability on an ionic system of FeSx phases on a steel substrate. The primary contributions of the thesis are two-fold: We identified the atomistic mechanisms responsible for the growth and breakdown of iron sulfide phases, and also quantified non-empirically the kinetic parameters that govern the rate of these unit processes. We constructed a novel multiscale modeling framework that couples accurate modeling of atomic-scale unit processes to efficient modeling of the micron-scale behavior of the passive film that captures phenomena at multiple length scales ranging from point defect dynamics to microstructure formation and evolution. The thesis is divided into three sections, the first of which is devoted to investigating two unit processes (surface sulfidation and ionic diffusion) that contribute to the growth of iron sulfide phases and is primarily focused on quantifying the kinetic parameters involved in the sulfidation (adsorption energies and dissociation barriers) and ionic diffusion (diffusivities and migration barriers) processes. In this section, we use a combination of ab initio density functional theory and kinetic Monte Carlo to quantify the surface-defect-induced changes in the electronic structure (0.4 eV reduction in the band gap and the introduction of mid-gap states) that increase the reactivity of and the rate of sulfidation on FeS₂ (100) surfaces. We also use the d-band theory to explain the effect of surface coverage and surface charge (or electric potential) on the reactivity of the reactivity of the surface, and provide guidelines for more accurate calculation of reactivity metrics in realistic electrochemical conditions. Finally, through the use of high-accuracy hybrid DFT and NEB methods, coupled with kinetic Monte Carlo calculations, we resolve the influence of magnetic and vacancy ordering transitions on ionic migration barriers and diffusivity of pyrrhotite, Fe₁-xS. Specifically, we identify that the ionic migration barrier is unaffected by the local vacancy configuration in different pyrrhotite polytypes, but is strongly influenced by the local magnetic order imposed by strong antiferromagnetic superexchange interactions that exist in pyrrhotite, which is a finding that is also important for other scientifically important material systems like NiO. The next section of the thesis discusses the mechanistic pathways and kinetic parameters of two processes involved in the local degradation of iron sulfide passive films (vacancy agglomeration-induced pit initiation and hydrogen-evolution-assisted debonding). For the first process, we use first-principles modeling to explain experimental observations of non-Arrhenius vacancy concentrations and nanoscale pitting on the FeS₂ surface. We identify a mechanistic pathway comprising of concerted vacancy formation and diffusion unit processes that leads to the formation of vacancy agglomerates that serve as sites for nanopit initiation. We then use the same set of computational tools to identify another mode of failure in the layered iron sulfide phase, mackinawite. We identify through computational stress-strain curves that cathodically generated H2 molecules are detrimental to the mechanical stability of the mackinawite phase and cause localized degradation of the passive film and act as a precursor to pitting corrosion. The final section of the thesis describes the formulation of the multiscale modeling framework that incorporates the kinetics of film growth and breakdown mechanisms identified in previous sections. This novel coupled kMC and phase-field model is capable of describing the macroscale morphology, passivity and protectiveness of the passive film while also possessing sufficient resolution to simulate atomic scale dynamics occurring at the film interfaces, and represents a significant improvement in terms of mechanistic detail over existing analytical corrosion models especially for sour systems. We demonstrate the capability of the model in constructing kinetic stability diagrams, which extend the information contained in phase and Pourbaix diagrams by accounting for the relative rates for formation and dissolution of different iron sulfide phases. We also demonstrate the calculation of degradation maps, that help identify the dominant mechanisms behind the localized failure of the passive film at different environmental conditions (temperature, partial pressure of H₂S, electrode potential, pH etc) and thus identify environmental conditions where the passive film is stable against localized degradation. Finally, the direct coupling between atomic scale processes and overall film passivity allows us to identify the impact of different material properties (like vacancy formation energy and diffusivity) on the resistance of the passive film to pitting and localized corrosion.

[发布机构] Massachusetts Institute of Technology