As a result of climate change, low-carbon power sources are being studied. Most of the green energy sources are volatile, relying on external conditions, making them hard to fully rely on. Storing the excess energy generated and using it when it’s needed is still a great problem with today’s energy storage technology. To try to solve this problem, hydrogen gas produced from water is under consideration as a form of energy storage.
Hydrogen gas that can be used in both the industry, to generate heat, or to produce electricity. There is already infrastructure in place to transport natural gas used for similar purposes. A reimagination of today's infrastructure to be repurposed to best optimize the transition, could make it cheaper, faster and with less waste.
The repurpose of current infrastructure is beeing studied. Hydrogen can be difficult to work with for most materials. It can permeate through most metals and change the mechanical properties, causing losses in ductility and toughness.
For this work, the effect of a hydrogen atmosphere is studied for an X70 pipeline steel, as well as how different mechanical and gas charging parameters influence the material. The material was collected from a pipeline used for the transport of natural gas. Smooth and notched mini-specimens are tested. The specimens are charged with a hydrogen gas of 6.0 quality inside a pressure chamber and tested in-situ. The extensometry measures are made with Edge Tracing (ET) techniques to ensure accurate measurements without the need for mechanical devices inside the gas chamber.
The tensile testing was done for a gas pressure of 240 bar and compared to tests in air. During the tensile test different strain rates (10-4 and 10-5 s-1) were applied and different mini-specimen geometries were compared to see the effect of triaxiality in the results. The hydrogen gas has a strong effect leading to the loss in radial contraction, showing the material embrittlement.
The strain rates also significantly impact not only the loss of ductility but also the reproducibility of the experiments. The displacement of 10-4 s-1 results shows a dispersion in the radial contraction data, going from 18 to 29%, while for the tests in air the radial contraction surpasses 40%. For the displacement of 10-5 s-1 the tensile tests were more repeatable, ranging from 17 to 19% radial reduction before failure.
The specimens were analyzed with SEM microscopy and synchrotron microtomography to better understand the fracture. Whilst the material tested under air showed classical ductile damage mechanisms with the highest void volume fraction in the specimen center, as expected, hydrogen charged samples showed surface cracks. These were identified as brittle by fractography of the broken surface and by the microtomography results.
The analysis of the brokens samples shows that there is a difference between the strains rates of 10-4 s-1 and 10-5s-1. While for the samples at 10-4 s-1 the crack growth is mostly initiated at the surface, with the center of the sample saying ductile. The samples at 10-5s-1 have the whole surface embrittled.
