Diamond Annual Review 2023/24

29 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 3 / 2 4 Surface secrets: Understanding stainless steel’s resistance to hydrogen embrittlement Stainless steel is one of our most versatile materials. 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 tomicrostructure 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). ​ A 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. On the I07 beamline, they used Gazing-Incidence X-Ray Diffraction (GIXRD) and X-Ray Reflectivity (XRR) measurements to probe the near-surface region to approximately 40 nm and 170 nm during in situ cathodic electrochemical polarisation. Synchrotron- based XRR is a very sensitive technique for measuring the thickness and density of thin surface films. Electrochemical Impedance Spectroscopy (EIS) measurements revealed the electrochemical properties of the surface oxide film. 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. ​ Örnek, C. et al. DOI: 10.1016/j.corsci.2020.109021 Laser-induced crystallisation offers a quicker route to smart windows Smart windows change their properties in response to external factors, with glazing that can switch between transparent and opaque depending on temperature, light levels or an applied voltage. Smart windows using thermochromic materials, for example, can change to block infrared transmission as temperatures rise, remaining transparent to visible light. The thermochromic properties of vanadium dioxide (VO 2 ) offer great potential for energy-saving smart windows. However, depositing VO 2 films and coatings through sputtering, chemical or physical vapour deposition can be time- consuming and requires complex and expensive equipment. Solution-based methods are a simpler option, but usually require using a furnace to heat the materials above around 400°C to achieve the necessary crystalline structure, limiting the materials that can be used as a substrate. A team of researchers crystallised VO 2 thin film using laser annealing, substantially reducing the annealing time and crystallisation temperature. On Diamond’s I09 beamline, the research team used hard X-ray photoelectron spectroscopy (HAXPES) to probe the oxidation state of the material beneath the surface. Their results showed that the thermochromic properties were comparable with those of furnace-treated samples and that pulsed laser annealing of VO 2 could be exploited for a range of applications, including smart windows, metamaterials and flexible electronics.​Using lasers to induce crystallisation is much quicker and more energy efficient, paving the way for smart windows to become more cost-effective and more readily available. ​ Basso, M. et al. DOI: 10.1016/j.apsusc.2023.157507​ Figure: Snapshots from the operando, real-time GIXRD measurements during galvanostatic hydrogen charging with a current density of −37.5 mA/ cm². The 2D-diffraction images on the left show the diffraction signals from the austenite (111) and ferrite (110) phase. The graphs on the right-hand side are data extracted from partial integration over the highlighted regions in the diffraction images. (a-c) show diffraction information at t = 0 s (uncharged condition), (d-f ) show the diffraction data for hydrogen charging at 1765 s, (g-i) show the real-time diffraction data 900 s after the termination of hydrogen charging (1800 s). Figure: Laser annealing for the sol–gel VO 2 thin films preparation and transmittance at 20°C and 90°C of the laser-annealed spots at fixed fluence of 100 mJ/cm2 and increasing values of pulse number (100–1200). Transmittance at 20°C and 90°C of the laser-annealed spots at fixed pulse number of 600 and increasing values of fluence (70–200 mJ/cm2). All the spectra are related to the 33 ± 5 nm lasered thin films.