Pursuing sustainable and efficient energy storage technologies has led to advancements in iron-air batteries. Understanding the intricate relationship between the microstructural features of iron electrodes and their oxidation and reduction behaviour is crucial for optimizing battery performance and lifespan. This thesis aims to investigate the impact of microstructural characteristics, such as phases, grain size, and defect density, on the formation of stable iron oxide/hydroxide compounds and the evolution of hydrogen gas (HER), using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments in 6M KOH. The influence of an electrolyte additive, sodium stannate trihydrate, and an iron foam functioning as alternative electrode material are also examined. Hot-rolled, pure iron samples were subjected to annealing heat treatments, resulting in different grain-sized specimens. A dual-phase steel, DP1000 steel composed of ferrite and martensite phases as well as hot-rolled and cold-rolled iron electrodes, completed the materials that formed this study’s basis. Initial surface identification via optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), atomic force microscopy (AFM) and X-ray diffraction (XRD) have been performed. Results indicate a relatively lower formation of FeOOH for a DP1000 steel anode compared to cold-rolled alpha iron over 12 cycles. A marginally larger decline in HER kinetics is observed for a hot-rolled small grained anode compared to a coarse grained anode, while a clear effect of grain size on the development of Fe3O4 and FeOOH could not be established. An iron foam electrode showcases greatly enhanced anodic and cathodic current densities in comparison to solid sheet iron electrodes, due to its cellular structure. The effect of 0.01M sodium stannate added to the electrolyte illustrates a significant reduction in HER intensity for both foam and solid iron samples.

The reliability of battery performance is crucial for the implementation of long-term large-scale energy storage. The promising Iron-Air battery is known for its suitable properties relying on its advantageous iron-based anode, characterized by its high volumetric energy density, robustness, material abundance, low cost and scalability. The anode does however present drawbacks, such as self-discharge, hydrogen evolution reaction (HER) and initial- and gradual surface passivation. In previous studies the effect of porosity and specific additives have been presented as important factors in tackling these drawbacks. This multidisciplinary research aimed to study porosity and additive effects on the electrochemical behaviour and performance of iron-based mechanically-stable anodes, manufactured at elevated temperature and pressure. Specifically, the effect of 10wt% pore former K2CO3 on the initial capacity and on the passivation behaviour was studied by galvanostatic charge-discharge, with prior investigation on pore formation. In addition, the effect of high-performance additives (Bi2S3, Bi2O3, ZnS and FeS) was investigated on HER kinetics and the passivation behaviour, by cyclic voltammetry (CV) and galvanostatic cycling, respectively. Surface characterization was performed by optical microscopy, scanning electron microscopy (SEM), and profilometry. Post-test analysis was carried out by optical microscopy and X-ray Photoelectron Spectroscopy (XPS). An intensification of surface cracks was observed for the K2CO3-rich sample before testing and its initial discharge capacity is 27 mAh/g less than a sample without pore former, but passivated later than the pristine sample in the full cell. Amongst the additives, the highest cycle of 100% capacity retention was marked at cycle 41 for FeS and for Bi2S3 at cycle 29 in the full cell, both showing 100% retention in the half cell up to 36 cycles. FeS might benefit from its readily available soluble reservoir of S2− ions. Bi2O3 showed the lowest capacity retention which might be explained by low conductivity, low solubility and/or lack of beneficial role of S2− ions. However, the additive Bi2O3 showed great reversibility of discharge products in the CV, confirmed by the lower O1s peaks and the lower respective Fe2O3 and FeOOH peak in XPS spectra. The pristine sample showed in the CV over the cycles increased current density and slope near the HER potential, with low reversibility. Apart from Bi2O3 and ZnS, the pristine sample showed lower capacity retention than FeS and Bi2S3, confirming the effective working of these additives. This systematic study portrays a good starting point for further studying porosity and additive effects on the electrochemical behaviour in hot-pressed anodes for the upcoming Iron-Air battery. Improvements can be assigned to cell design, electrolyte control and further detailed porosity characterization. Another type of current collector might withstand higher current densities and anode thickness reduction can lead to higher discharge capacities over its lifespan.

Microstructure-corrosion property correlation is an open question in the field of materials science. The microstructure of metals and alloys consists of several features with different individual corrosion response. The corrosion behavior of the macroscopic system is an outcome of the complex interaction of the components of microstructure. Hence, a definitive understanding of the corrosion response of the microstructural features is needed for improving the material durability and design.In the present work, the effect of grain size  on the active corrosion behavior of pure iron is investigated. Samples with different grain sizes are obtained by annealing heat-treatment. The microstructure of samples is characterized by optical microscopy, electron backscatter diffraction and X-ray diffraction. Electrochemical characterization using potentiodynamic polarization and electrochemical impedance spectroscopy are performed on the samples in deaerated 0.1 M and 0.01 M H2SO4 solutions. The surface topography of the corroded sample surface is characterized by atomic force microscopy. Following the experiments, a numerical corrosion model is attempted to replicate the observations. The results reveal an aggregate effect of grain size and crystallographic orientation of grains on the corrosion behavior of the samples.

The fuel consumption of cars has become an important issue in the development of new materials and. These developments resulted in the generation of new grades of steel for the automotive industry. In particular, new advanced high strength steels (AHSS) have been developed for the automotive industry to lower fuel consumption (with weight reduction) by combining strength with formability. Especially, quenching and partitioning (Q&P) steels from 3rd generation AHSS can exhibit significant strength and ductility balance by combining martensite with retained austenite. Recently, there has been a new interest in applying the Q&P treatment to stainless steels, in particular martensitic stainless steel. For the automotive industry, the development in Q&P treated martensitic stainless steel can be a game-changer. The mechanical properties of Q&P treated commercial martensitic stainless steels have been widely researched. Unlike mechanical behaviour, the corrosion behaviour of Q&P treated martensitic stainless steel has not been investigated deeply. The effect of environmental factors or the effect of microstructure on the corrosion performance of Q&P processed martensitic stainless steel needs to be studied. This master thesis aims to identify the effect of microstructural constituents (retained austenite, primary and fresh martensite) on the corrosion response of Q&P treated martensitic stainless steels. To this end, an experimental approach is taken for this project. This work aims to create a relationship between heat treatment, microstructure, and the resulting corrosion properties. As an experimental path, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and Mott-Schottky experiments were carried out in 3.5 wt.% NaCl solution to reveal the corrosion response and passive film properties of the Q&P microstructures. In addition, X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical composition and fractions of oxide layers in the passive layer. Results demonstrate a phase dependency for the corrosion performance of Q&P treated martensitic stainless steels.

Literature reports a great deal of contradictory results concerning the effect of microstructure on the corrosion and passivity behaviour of advanced high strength steels. The difficulty in identifying the controlling cause of corrosion results from the inability of disentangling the coupled effects of individual microstructural features in a scientifically rigorous manner, thereby attributing the core behaviour to the wrong sources. The aim of this thesis is to isolate and identify the effect of phases on the electrochemical response of high strength steels. To this end, a combined computational and experimental approach is taken. This work starts by analysing the connection between heat treatment, microstructure, and the resulting corrosion properties. After clarification of this interdependence, a finite element electrochemical model illuminates the corrosion behaviour of idealised two phase ferrite-martensite and ferrite-pearlite systems for different phase volume fraction combinations. The results from the simulations guide the microstructure creation for electrochemical experiments, where employed heat treatments result in ferrite-martensite and ferrite-pearlite microstructures with similar ferrite volume fractions. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) experiments in 0.1M and 0.01M H2SO4 solutions; potentiostatic polarisation, EIS and Mott-Schottky analysis in 0.1M NaOH solutions reveal the corrosion response and passive film barrier properties of the microstructures. Results demonstrate a clear phase dependency for both active and passive conditions, and are further discussed in light of microstructural features of secondary martensite and pearlite phases.