Impact of Microstructure, Stress, and Phase Composition on Hydrogen Permeation in Duplex Steel: A Synchrotron X-ray Diffraction Study

In the context of climate change and the global pursuit of sustainable energy solutions, hydrogen has re-emerged as a promising energy carrier. However, the transition toward hydrogen-based technologies poses serious challenges to the use of existing steel infrastructure due to hydrogen embrittlement. This phenomenon, which compromises critical mechanical properties, has been under scientific scrutiny for decades. Yet, experimental investigation of hydrogen uptake and its interactions with the microstructure under application-relevant conditions remains difficult. This cumulative thesis addresses this gap by developing and applying an in situ methodology based on high-energy X-ray diffraction, enabling spatially and temporally resolved analysis of hydrogen-induced lattice distortions and equivalent internal stresses. It provides, for the first time, detailed insights into the phase-specific behavior of duplex steel during electrolytic hydrogen charging. The results confirm that hydrogen is absorbed mainly in the austenite phase, while lattice parameters in ferrite remain largely unaffected. Nevertheless, this single-phase uptake leads to the development of compressive stresses in both phases. The employed method serves as a basis for further investigations in which microstructural effects on hydrogen transport were systematically analyzed through targeted grain refinement and the controlled introduction of lattice defects via deformation and heat treatment. Complementary techniques revealed that hydrogen uptake is primarily governed by defect structures rather than grain size or interface density. Building on this, the influence of externally applied tensile and compressive stresses on hydrogen absorption was examined using a four-point bending configuration. Contrary to current prevention strategies, the results showed symmetric lattice expansion and hydrogen-induced compressive stresses on both the tensile and compressive sides. Altogether, this cumulative work provides a significant contribution to the fundamental understanding of hydrogen-microstructure interactions in duplex steels. The presented methodology lays the experimental groundwork for future in-situ studies in complex material systems. The derived findings offer valuable perspectives for designing more resistant materials and surface treatments for hydrogen-exposed applications in the energy, mobility, and infrastructure sectors.