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Abstract: Predicting steel corrosion in porous media – e.g. soil, bentonite, concrete – remains challenging due to the limited understanding of the precipitation and evolution of corrosion products under realistic conditions. To shed light on these processes, we investigate the long-term temporal and spatial evolution of iron (hydr)oxides at the pore-scale, employing in-situ synchrotron-based chemical imaging including X-ray fluorescence, X-ray diffraction, X-ray absorption spectroscopy, and absorption contrast. A pore is simulated in a 300 μm diameter capillary, containing iron and stagnant electrolyte, and is aged for up to 2.7 years. The electrolyte is buffered at pH 8, with HCO3-, Cl-, and SO42- ions, both under aerobic and near-anaerobic conditions. Results show that, in electrolytes containing HCO3-, ferrihydrite initially formed remotely from the iron and transformed into goethite over time. In the long-term, mixed-valence green rust and magnetite formed near the iron. The spatial distribution and oxidation states of corrosion products can be rationalized using a reactive transport conceptual framework: Fe(II) and O2 release or consumption in electrochemical, diffusion, oxidation and additional reactions determine the concentration profiles and hence the location and time when corrosion products precipitate. Electrolyte composition – Cl- and SO42- – further influences which corrosion product is formed and stabilized over a certain time (e.g. green rust and lepidocrocite). Our findings demonstrate preferential precipitation on pore walls and far from the iron, with prolonged green rust stability, showcasing the relevance of investigating iron (hydr)oxides in realistic pore models. These insights improve predictions of corrosion product evolution, pore clogging, and stresses in porous media.

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Abstract: De-icing road salts are widely employed for snow and ice mitigation in cold climate regions, with sodium chloride (NaCl) being the most commonly used salt. The extensive application of NaCl has raised significant infrastructure, sustainability, and environmental concerns, and it has led to the emergence of various alternative de-icing salts, including other chloride-based and organic salts and compounds. In this study, the effect of zinc and acetate species on the corrosion behaviour of steels was systematically investigated using a combination of atmospheric corrosion testing, immersion testing, electrochemical measurements, cross-sectional microscopy, Zn K-edge X-ray absorption spectroscopy (XANES), and thermodynamic speciation modelling. The effect of eight chloride and non-chloride salts and their mixtures on the corrosion of structurally important galvanized steel, mild steel, and high-strength steel was studied. The chloride-based salts were found to be more detrimental than the organic salts to the corrosion of mild and high-strength steels, but all the salts were similarly corrosive to galvanized steel. It was found that the presence of both zinc and acetate species significantly enhanced corrosion and the Fe dissolution rate in steels. More than 40 wt.% of the 20 µm-thick galvanized zinc layer was dissolved after one week of immersion in 0.5 M sodium chloride or sodium acetate. After this one-week immersion, or the 10-week atmospheric field exposure, any remaining zinc was entirely in the form of zinc oxide. Our findings call for further investigation before using organic de-icing salts, alone or in mixtures with NaCl, on galvanized steel.

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Abstract: In this study we have investigated the nucleation mechanism of ‘sweet’, CO2 corrosion scale in situ using synchrotron scattering under industrially relevant conditions (CO2 saturated brine, ultra-low oxygen (8–32 ppb), 80 °C, pH 6.8, open-circuit-potential (OCP)). Simultaneous small and wide-angle X-Ray Scattering (SAXS-WAXS) measurement allows the phase and state of the nucleating corrosion scale to be characterised as a function of both immersion time and location with respect to the metal/solution interface. The results indicate that a precursor amorphous phase is formed prior to the emergence of a crystalline iron carbonate scale.

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Abstract: Waterside corrosion of zirconium (Zr) alloy nuclear fuel cladding occurs during normal operating conditions in pressurized water reactors. This corrosion yields zirconium oxide (ZrO2) and hydrogen, with a fraction of the hydrogen absorbed into the fuel cladding where it can redistribute through solid-state diffusion via concentration, temperature, and stress gradients. If the local hydrogen solubility in the metal is exceeded, then zirconium hydrides (ZrHx) precipitate in the zirconium matrix. Zirconium hydrides negatively impact the integrity of zirconium-based nuclear fuel cladding during both normal operation and extended dry storage through cladding embrittlement. With the desire to operate reactors with higher fuel enrichment and to higher fuel burnups, which will result in additional corrosion to the zirconium alloys, modeling hydrogen and hydride behavior is fundamental to ensuring safe reactor operations. Thus, this study aims to understand how applied tensile stress and irradiation-induced defects influence hydride precipitation and dissolution in zirconium alloys. In-situ synchrotron X-ray diffraction experiments were performed to study these influences in Zircaloy-4 under different thermo-mechanical conditions and levels of irradiation fluence. Additional analysis of Zircaloy-4 coupons under applied stress was also performed to explore the effect of stress on hydrogen migration and hydride dissolution. The applied tensile stress studies indicate that stress does not influence the dissolution solvus or the temperature hydrides completely dissolved at. Furthermore, the studies suggest that hydride nucleation is not noticeably changed by an applied tensile stress. However, hydride precipitation, defined by the Terminal Solid Solubility for Precipitation (TSSp), appears to slow under applied tensile stresses greater than or equal to 180 MPa. Furthermore, previous isothermal hydride growth experimental results were replicated and expanded on. Previous results were confirmed and indicate that applied tensile stresses slow hydride growth when the applied tensile stress is above the hydride reorientation threshold stress. Isothermal hydride growth did not result in the concentration of hydrogen in solid solution (Css) reaching the Terminal Solid Solubility for Dissolution (TSSd) during an isothermal hold, potentially reaching a quasi-equilibrium. It is hypothesized that the applied tensile stress impacts hydride growth rather than hydride nucleation. A potential explanation is that hydrides precipitating in different orientations due to the applied tensile stress have differences in hydride structure and dislocation networks. Studies on the influence of irradiation-induced defects on hydride precipitation and dissolution in as-irradiated and post-annealed materials were performed. Annealing the samples removed <a>-dislocations per the reduction in Full-Width at Half-Maximums (FWHMs) for alpha-Zr diffraction peaks. However, the FWHM of the alpha-Zr (002) peak did not change during the anneal, indicating that <c>-dislocation loops are still present and may still act as hydrogen traps. The delta (111) hydride diffraction peak integrated area was measured and found to increase from the as-irradiated to post-annealed state. This indicates hydrogen trapped by irradiation-induced defects in the material was released into solid solution and is available to form hydrides. The amount of hydrogen trapped by irradiation-induced defects was estimated at 12--63 wppm (8--47% of the total hydrogen content), which agrees with values reported in literature. TSSp and TSSd equations were fit to the data in both material states. Compared to the post-annealed data and literature, the as-irradiated TSSp and TSSd were lower. This is the opposite trend reported in literature for as-irradiated data. Little hydride growth during isothermal holds was observed in both the as-irradiated and post-annealed states.

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Abstract: Stainless steel is one of our most versatile materials. Its hygienic qualities ensure the safety of medical instruments and implants, and its corrosion-resistant properties make it indispensable in industries from construction to food processing. The corrosion resistance arises from the alloy's chromium content, as the chromium forms a passive film on the surface that can self-heal in the presence of oxygen, shielding the bulk of the material from corrosion. However, the stability of the passive film can be affected by hydrogen absorption, leading to microstructure embrittlement that lowers the stress required for cracks to occur and propagate in the metal. A challenge for the hydrogen energy industry is that high-performance metallic materials are highly susceptible to hydrogen embrittlement. One potential candidate for building a safe hydrogen economy infrastructure is super duplex stainless steel (SDSS). In work recently published in Applied Surface Science, an international team of researchers used in situ surface-sensitive synchrotron X-ray measurements to investigate the early stages of hydrogen-induced degradation of SDSS occurring at the near surface. Their results show that SDSS’s exceptional resistance to hydrogen embrittlement can be explained by the stability of the passive oxide film, and that the semiconducting property of the passive film plays an important role in hydrogen embrittlement. The authors also conclude that profound in situ experimental characterisation and computational calculation are needed to reveal the complex processes behind material degradation.