Structural Materials Characterisation for Hydrogen Systems

Due to increased demand for decarburization, the transport of hydrogen from low energy cost production locations to high energy cost locations of demand is rapidly moving into focus. Here, the costs are heavily influenced by the material costs for the required infrastructure. As a result, full utilisation of currently available structural materials under hydrogen and the development of new, cost effective materials is becoming increasingly important. Since all parts dealing with hydrogen under pressure or in cryogenic form are safety relevant, a deep understanding of the underlying mechanisms is highly desired. This demands the ability to recreate hydrogen loading and accelerated testing in the lab, characterisation of the effects of hydrogen on the material from the macro to the atomic scale and new theories on the failure mechanisms exhibited by hydrogen resistant materials.<br/><br/>In this talk, we will present the testing environment built up at the Friedrich-Alexander University Erlangen-Nuremberg (FAU) as well as a new development in the area of low-cost hydrogen resistant steels. In order to create a full materials development cycle for hydrogen resistant materials, we combine materials synthesis, materials testing and materials characterisation. At FAU, we are able to create new alloys through various powder and melt metallurgical means with associated hot deformation and heat treatment schedules. Materials testing under hydrogen is provided through electrochemical charging (in-situ and ex-situ) and through a high pressure autoclave station up to 1000 bar and 300°C. The latter is especially important for hydrogen resistant materials such as austenitic steels and Ni based superalloys due to their relatively low diffusion coefficients for hydrogen. Mechanical testing is then done ex-situ from the macro (tensile, fatigue) to the micro (pillars, cantilevers SEM-in-situ) scale. In order to understand the mechanical behaviour, characterisation is especially important. Here, hydrogen is especially challenging due to its relatively mobile nature.<br/><br/>At FAU, we have therefore built a special set of equipment to gain a deeper understanding of the microstructural response of structural materials under hydrogen. Besides standard materials characterisation equipment (SEM/EBSD, TEM, ...) this includes specialized equipment including a hydrogen capable titanium atom probe and a very high sensitivity titanium thermal desorption spectrometry (TDS) unit. Both of these instruments are UHV characterisation instruments built from titanium, in order to remove the usual hydrogen background found in UHV systems. For the atom probe, this enables the direct analysis of hydrogen at crystal defects without the use of tracers, as well as the analysis of hydrogen in materials using laser assisted atom probe tomography where typically even deuterium/tritium tracers are not suitable. The latter is especially important since analysis yields of hydrogen exposed materials are relatively low in voltage pulsed atom probe tomography. In the case of TDS, the drastically lower hydrogen background potentially allows for much smaller specimen sizes, increasing analysis dynamics and locality. The latter system is currently in the testing phase. Both systems are connected to a cryo-FIB system through a transfer suitcase in order to preserve hydrogen distributions.<br/><br/>In this talk we will also present the development of a novel, low cost class of hydrogen resistant steels. In these materials, we use high amounts of carbon to stabilise the austenitic phase. This has a major cost advantage over the use of Ni, Mn or N. These steels show no change of mechanical properties after exposure to hydrogen and provide good strength levels at a yield strength of 700 MPa and above. In the future, we hope to be able to develop these steels to mechanical properties under hydrogen of the highly cold worked Cr-Ni-Mo austenitic stainless steels currently used, at a fraction of the cost.