Common testing practices for evaluating the behavior of materials in hydrogen atmosphere
A complete analysis of the various test methods to determine the performance of materials in hydrogen, i.e. to measure their effects on the mechanical properties of metal alloys, was performed. A literature review was also performed, reporting available data and work carried out from research projects .
There are currently numerous standards and technical reports that address, at different levels, the subject of characterization of materials in pressurized hydrogen gas. The effect of hydrogen on mechanical properties can be roughly considered to be of two types: quasi-static and dynamic. The first type considers a constant or slowly varying load, which allows for a general equilibrium of hydrogen distribution and is often relevant for components exposed to high-pressure gas. The second type is associated with dynamic components, such as vibrations in static equipment or fluctuations in gas pressure. Due to the temporal nature of the hydrogen embrittlement phenomenon, the susceptibility of materials is often studied through tests performed under quasi-static conditions. To evaluate the performance and integrity of materials and components operating in hydrogen, it is necessary to quantify mainly the tensile strength properties, fracture toughness properties and the rate of fatigue crack growth under cyclic loading in a relevant hydrogenated operating environment.
Slow Strain Rate (SSR) tests are commonly used to test the tensile properties of materials. Hydrogen reduces the ductility of a metal which in this type of test is seen as a change in area reduction and plastic elongation compared to tests performed in air. The results of SSR tests performed on steel show that the loss of area reduction for smooth and notched specimens varies from 20% to 50% and 80%, respectively, compared to the values measured in air. The results also showed that hydrogen embrittlement increases as the pressure increases up to a threshold pressure of about 5 MPa.
Fracture toughness tests are used to quantify fracture toughness properties and have shown that fracture toughness can be significantly reduced when a material is exposed to hydrogen gas. Data showed that the fracture toughness of pipe steels operating in a hydrogen environment can be between 48% to 60% of the fracture toughness measured in air, but is still high enough for most engineering applications (greater than 100 MPa m1/2 for steel grades up to X70). For higher grade X80 steel, hydrogen has a greater effect on fracture toughness of up to 92%; however, for the quasi-static fracture toughness test performed on the same grade of X80 steel, no effect was observed.
The Fatigue Crack Growth Rate (FCGR) test quantifies the rate of crack growth in terms of the rate of advance per load cycle. Experimental studies carried out in the literature show that hydrogen can have a negative influence on this aspect; in fact, the growth rate can increase by one or two orders of magnitude if the applied stress intensity range is higher than the threshold value. It has been observed that even the threshold value is reduced by 10-25% in hydrogen environment compared to the values in air. The results have also shown that the partial pressure of hydrogen has an influence on the phenomenon, in fact there is no impact of hydrogen on the crack growth behavior for a partial pressure of hydrogen lower than or equal to 1 bar. The effect of hydrogen also depends on the amount of hydrogen and the reported data indicated that X70 and X52 steels with H₂ respectively up to 25% and 50% does not have effect on the fatigue performance.
In light of the above, it is important to test the performance of materials in an environment that closely simulates actual operating conditions. At the same time, to do this, it is necessary to select an appropriate test specimen that reflects the likely shape of the component and the characteristics such as welding, deformation or surface condition of the component under test. The selection of test parameters is also crucial; for example, increasing the pressure and purity of the hydrogen gas and reducing the dynamic loading rates generally increase the severity of hydrogen embrittlement.
Screening tests, such as tensile tests and disk pressure tests, provide qualitative methods for assessing the relative behavior of metals and alloys in hydrogen gas environments. However, these tests do not provide data that allow a quantitative assessment of the performance of structural metals in service. For the latter purpose, tests that explicitly address crack initiation and growth under quasi-static or cyclic loading are more relevant. In particular, fracture mechanics test methods provide quantitative measurements of hydrogen-assisted crack propagation, and similarity concepts allow these data to be used in structural life assessments of components.
There are also several non-standardized high-pressure gaseous hydrogen metal characterization tests, such as hydrogen preloading and the drilled specimen method. Non-standardized tests generally provide less information, due to imposed geometric limitations or different environmental conditions from the real ones, however the costs are lower, making them attractive for preliminary characterization tests.
The performed study will provide information on the experimental methodologies necessary for the appropriate selection and qualification of the materials and components used in the construction of the HYDRA project.
