Hydrogen embrittlement occurs when atomic hydrogen (I.e. hydrogen ions – h+) from dissociated hydrogen gas or from other sources (e.g. H2S) finds its way into the metal lattice of a pipeline.
This can occur in several conditions such as high-pressure hydrogen systems, corrosion, cathodic protection, extreme heat and many more. Once the hydrogen ions have worked their way into the metal lattice, they act via a multitude of different mechanisms to lower the mechanical stress required for cracks to initiate and propagate. This results in embrittlement of the metal.
While the size of hydrogen and its ability to permeate different materials presents numerous challenges, there are a number of established solutions. There are different metal alloys which are more resistant to hydrogen embrittlement such as austenitic stainless steels, aluminium alloys, low-alloy ferritic steels, copper alloys and many more. For liquid hydrogen systems metals should be able to handle cryogenic conditions, such as austenitic stainless steel and aluminium alloys. The risk of embrittlement can also be reduced by avoiding vulnerable metals that such as high-strength ferritic alloys, cast iron, titanium alloys and nickel alloys.
Research has also been conducted on different type of coatings to combat leaks, such as ceramic-based barriers like oxides, Nitrides and Carbides. Additionally, different materials can be used for the actual pipe where polymers with metallic lining or epoxy coatings can be used.
Sour gas possesses the same ability in to cause hydrogen embrittlement as pure hydrogen gas. Therefore, lines which have been assessed and built to operate with sour gas will also be ready for hydrogen gas to flow through it. However, if more rigorous methods are required to pass regulations, coatings, pip-in-pipe or gaseous inhibitors can be used.
Currently the only regulatory body who has standards for repurposing lines for hydrogen is American School of Mechanical Engineers (ASME), where ASME B21.12 (ASME, 2023) states two options. The first is option 1 where you drop the operating pressure of the hydrogen system far below what the operating pressure for the natural gas system was, this essentially mitigates crack propagation as lower pressures cause less stress on the pipe. Option 2 is to cut out a piece of the pipeline and send it to the lab for testing, this is more costly and for subsea systems may not be feasible or for systems that are still operating.
Currently in the Netherlands (Zeeland province) there is a 12 km high pressure (40 bar) natural gas pipeline which has been repurposed for hydrogen. The project was completed in Q3 of 2018 and travels from DOW chemical plant to the customer (YARA). This line being repurposed and operating for half a decade shows that hydrogen embrittlement in repurposed lines can be mitigated by choosing the right lines to repurpose and/or implementing mitigation procedures.
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