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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.

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Abstract: Advanced applications of hardmetal (HM) in chemically aggressive ambients call for the improvement of their corrosion resistance, combined with optimal abrasion and toughness properties. This goal is chiefly achieved with binder alloying. In particular, Cr and Mo additions to Co-Ni matrices have proved effective in many cases, but the physico-chemical mechanisms underlying this functional improvement are poorly understood. This situation, on the one hand does not enable the fine-tuning of the composition, and, on the other hand, does not allow to reliably extrapolate the performance of these grades to new ambients. The aim of this work is to propose a molecular-level approach to the understanding of the electrochemical corrosion of HM, based on space-resolved compositional and chemical-state analysis of the surface of HM subjected to controlled electrochemical corrosion. This type of analysis is enabled by synchrotron-based scanning photoelectron microscopy (SPEM) and photoelectron microspectroscopy (μ-XPS). Specifically, we studied a WC-based grade with 15 w% binder with 4.67 Co/Ni ratio, alloyed with 0.78 w% Cr and 0.94 w% Mo, corroded potentiostatically in neutral sulphate aqueous solution in the pseudopassive and transpassive ranges. SPEM and μ-XPS allowed to locate and distinguish the chemical state of the binder and carbide elements in the respective phases and to distinguish the surface stoichiometry brought about by corrosion in different conditions. In particular, we found that pseudopassive conditions lead to the formation of surface films, enriched in W(VI) and Cr(III). Transpassive conditions cause notable surface enrichment in Cr(III) as well as to the formation of mixed Cr-W oxides in the WC regions. Mo is leached from the binder at all potentials investigated, while Mo(0) is present in the WC region under pseudopassive conditions, while transpassivity leads to its oxidative removal. Chloride in the electrolyte favours dissolution of oxidized metals, in particular causing the leaching of Cr(III), while Mo(III) is found at the surface of WC grains.

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Abstract: The CO2 corrosion-induced scale developed in the surface region of the low-grade carbon steel is thoroughly investigated to understand the corrosion mechanisms involved during exposure to CO2-saturated aqueous environments. In this work, in addition to the electron microscopy and lab-source X-ray diffraction (XRD) methods, ex-situ depth-resolved phase identification of the corrosion scales developed on steel with different initial microstructures is performed using synchrotron grazing incidence XRD at different incidence angles. The CO2 corrosion mechanism is discussed considering the observed distribution of the corrosion products formed at a different depth relative to the time of electrochemical exposure of the steels.