- Metallic materials for safe hydrogen technologies
- Plastics and composites for safe hydrogen technologies
- Materials and lubricants for friction systems in hydrogen technology
In this field of competence, BAM's expertise lies in answering questions regarding the suitability of metallic materials, plastics, composites and lubricants for new and existing piping systems, aggregates and other main components for future hydrogen technologies. For the majority of materials, this information is not available and must be tested through innovative mechanical testing equipment. The focus is on the use of materials for pure hydrogen, for example in H2 refueling stations, as well as the injection of hydrogen into existing natural gas systems through the development of new and rapid testing methods.
Metallic materials for safe hydrogen technologies
Production, storage and transport of hydrogen as a future energy carrier place high demands on the materials used and require careful processing.
The useful application of hydrogen as an energy carrier should not be opposed by negative effects on some materials. Phenomenologically, these effects depend on the local mechanical stress, the microstructure present and the level of hydrogen concentration. BAM is engaged in science-based research on both the phenomena of material damage by hydrogen and the corresponding mechanisms in order to identify possibilities and limits of use for the respective components. In particular, unexpected and brittle hydrogen-assisted cracking and associated safety risks are to be excluded as far as possible.
Whether the crack-critical hydrogen concentration is reached for a particular microstructure depends on how much is absorbed under the various manufacturing or operating conditions and how the hydrogen is distributed in the often very heterogeneous microstructure. In several projects, BAM is continuously determining values for hydrogen uptake and hydrogen transport in a wide variety of metallic microstructures, including materials for hydrogen technologies. Among other things, these values are used in numerical simulations of hydrogen distribution and cracking in heterogeneous microstructures.
The analyses are based on carrier gas hot extraction (CGHE) which are available for the determination of the hydrogen concentration. This technique has its origin in welding technology, where it has found its way into standardization (ISO 3690). The addition of a mass spectrometer enables even more precise determination of hydrogen diffusion and concentrations in the ppb and ppm range in the temperature interval from 20 °C to 950 °C. In addition to the chemical composition, the microstructure also plays a decisive role. With the aid of thermal microstructure simulation, this can be mapped in a targeted manner. In this way, different microstructures, such as those found in weld seams, can be investigated.
The combination of thermal microstructure simulation and carrier gas hot extraction enables the investigation of hydrogen transport behavior in a wide variety of metallic materials. These investigations, based on existing experience on hydrogen-assisted crack corrosion and cold cracking, are gaining importance for the new hydrogen technologies, especially in the field of high-pressure applications and power-to-gas.
Degradation of mechanical material properties by hydrogen manifests itself predominantly in reduced ductility, which is sometimes imprecisely referred to as hydrogen embrittlement. At very high concentrations, the strength of the materials may also be affected. Depending on the type and direction of mechanical stress, brittle material separations can occur, which are referred to as hydrogen-assisted cracking.
Through the development of the hollow specimen technique, in-situ tensile tests at low strain rates can be used to investigate the corresponding materials or microstructures under the influence of different environmental media. For example, the structure-specific sensitivity in the form of decreasing ductility can be determined. In addition to pure hydrogen at various pressure levels (up to 1000 bar), gas mixtures of hydrogen and natural gas or methane can also be used as ambient media. In addition, the test spectrum is supplemented by the possibility of specimen temperature control, so that a wide range of possible operating conditions in the field can be represented by the existing test infrastructure. This allows hydrogen-dependent crack criteria to be determined, which can be used in corresponding simulations of crack initiation and crack growth.
Plastics and composites for safe hydrogen technologies
In the future, hydrogen will not only be of great importance as an energy carrier, but also as a storage medium for renewable electricity from wind, sun and geothermal energy. A prerequisite for this is the safe storage of the gas, which must take place under high pressure. Polymer materials, some of which are already being used for hydrogen, are becoming the focus of interest here. The more intensive use of these materials also places greater safety requirements on them.
Components such as seals, coupling systems and surfaces subject to friction are critical parts that require special attention. Does hydrogen or gas-hydrogen mixtures affect the serviceability of polymer materials or polymer-matrix composites?
Metallic materials can be permanently loaded with hydrogen, depending on the alloy type, and embrittlement of some alloys is evident. In contrast, polymer materials absorb hydrogen to a high degree due to their free volume and swelling capacity, but this diffuses out again quickly even after weeks of H2 autoclave storage at up to 1000bar under normal pressure. After releasing the pressure, cavities and cracks appear as a result of "explosive decompression". Lasting material changes have been observed, especially in elastomers. Therefore, their properties under H2 pressure stress have to be investigated in situ for irreversible aging phenomena in tensile test experiments. For this purpose, an H2 autoclave was designed, fabricated and integrated into a tensile testing machine by a BAM team. This allows material testing of unreinforced polymers under typical pressure and temperatures of urban gas supply technologies. An in-situ testing system is currently being procured/designed for H2 high-pressure storage systems made of continuous fiber-reinforced plastics (e.g. CFRP) and for metallic materials. The plan here is to be able to realize H2 pressures of up to 1000bar and forces of up to 150kN.
Materials and lubricants for friction systems in hydrogen technology
The generic term "tribology" covers the entire field of friction, wear and lubrication. Examples of components subject to tribological stress are bearings, piston rings, seals and joints. Hydrogen environments place special demands on such components. BAM has special test equipment to determine friction and wear parameters in liquid and gaseous hydrogen. Currently, mainly polymer composites as well as friction-reducing, wear-resistant coatings are investigated. For practical applications, however, components such as ball bearings can also be tested in the same apparatus.
Results to date show that some materials exhibit more favorable properties in hydrogen, even in cryogenic liquefied form, than in air. These include some high-performance plastics in pure form or as components of composite materials.
In the case of elastomers, the frictional properties are being investigated, particularly in the presence of high-pressure hydrogen, and significant changes have already been shown which must be taken into account, particularly for use in seals.
From the field of metals, austenitic stainless steels were investigated, and are commonly used for vessels and pipelines, as they are considered to be less sensitive to hydrogen-induced degradation. However, microstructural transformations were detected in friction systems, which then again resulted in cracking. For this purpose, BAM is investigating and testing solid lubricants for their suitability in hydrogen application. The qualification of oils and greases is conducted by a test apparatus which is similar to an instrument that is utilized in industry for such investigations.
Moreover, BAM is studying the use of alternative coatings which show in some cases friction coefficients and a service life in the range of grease- or oil-lubricated systems. The Goal is to find a compromise, to eliminate damage to the systems during evacuation of hydrogen for regular maintenance of the devices.