Reliable hydrogen storage tanks depend on American-made stainless steel

The US hydrogen economy is now starting to grow fast, with estimates suggesting that by 2050 it will achieve revenues of $750 billion per year in revenue, supporting 3.4 million jobs. Providing the safe, efficient production and transportation of hydrogen essential to achieve this growth will require the industry to construct large numbers of tanks to store both liquid hydrogen and gas at high pressures.

Stainless steel is particularly useful for the construction of storage tanks because it combines corrosion resistance, high strength, durability, low maintenance and sustainability.  This contrasts with carbon steel that will require the application of a protective coating, needing regular maintenance over the typical 25-year life of the tank.

The stainless steel products that Outokumpu produces can be divided into five main groups that are differentiated by their microstructure: ferritic, martensitic and precipitation hardening, duplex, and austenitic. It is the last of these – austenitic stainless steel, that is particularly suitable for hydrogen storage tanks.

Our Calvert mill in Alabama produces two important austenitic grades targeted at hydrogen tank applications. The first is Core 304L/4307, a low-carbon alternative to Core 304. The lower carbon content minimizes carbide precipitation following heat input, for example during welding, giving improved resistance against intergranular corrosion. Second, for when a higher level of corrosion is required - such as for tanks that will be installed in the salt-laden environment along the Gulf Coast - we offer Supra 316L. This is a low-carbon alternative to Supra 316.

Overcoming cryogenic challenges to store liquid hydrogen

The challenge with storing hydrogen as a liquid is that its boiling point at atmospheric pressure is -253 °C/-423F. This calls for tanks to operate in the cryogenic range that starts when temperatures fall below -150°C/-238F. Therefore, they must be constructed from materials with the low-temperature ductility to avoid brittle fractures that will harm the structural integrity of the tank.

Storage tanks for liquid hydrogen are generally comprised inner and outer shells, with a vacuum or a layer of insulation between them. In terms of construction, the inner tanks are required to withstand the low temperatures, while the outer shells need to resist the external environment, especially in terms of corrosion.

A further storage option is underground tanks. In these tanks, the walls are built from  gas-tight stainless steel, and the mechanical support is provided by the rock walls of the underground chamber. The chamber could be purpose-made or possibly a disused mine.

Several stainless steel grades are well-proven in this type of environment. They are listed in technical standards such as the ASME Boiler and Pressure Vessel Code, and EN 13445-2, which include our Core 304L and Core 316L. What these grades have in common is their austenitic microstructure that enables them to be used safely at temperatures as low as -273 °C/-459°F.

Gas storage and avoiding the risk of hydrogen embrittlement

One of the main considerations when specifying a material for storing hydrogen gas under pressure is avoiding the potential risk of hydrogen embrittlement (HE). This is a process that results in the reduction in the fracture toughness or ductility of a metal due to the absorption of hydrogen atoms. Because hydrogen atoms are small, they can permeate solid metals. These atoms can then lower the stress at which cracks in the metal will initiate and propagate; the result is embrittlement.

HE can occur in a variety of metals - high-strength steel, titanium and aluminium alloys are all vulnerable. The sensitivity to HE also varies between different types of stainless steel.

The risk of HE increases with the pressure; it does not occur at low pressures. Therefore, most stainless steels are suitable for this case.

However, when the storage pressure rises it starts to force hydrogen atoms into the internal surface of the tank, increasing the risk of them diffusing into the steel matrix of the tank wall. Austenitic stainless steel has a higher resistance to HE than ferritic and martensitic grades. This is due to its particular crystal structure that reduces the rate at which hydrogen atoms can diffuse through the metal. This is why austenitic stainless steel is the preferred material for many storage applications that operate in the range of 200 bar, with some sites working at up to 800 bar.

Together with their low-temperature ductility, resistance to HE makes Core 304L or Core 316L a good option for hydrogen storage tanks.

Comprehensive technical support for designing hydrogen tanks

We recognize that finding the right material to construct a hydrogen tank can seem like a complex process with many factors to consider. We can help customers in the US cut through this by calling on the wealth of expertise from our local mill in Calvert, together with technical support from Outokumpu’s global organization. Whatever the hydrogen storage challenge, stainless steel offers a safe, durable, cost-effective and sustainable solution.