Stainless steel for hydrogen storage tanks

The hydrogen economy is evolving fast as the world strives to meet ambitious Net Zero targets. A report published by the IEA (International Energy Agency) in September 2023 predicts that hydrogen output will have to reach over 150 megatonne (Mt) by 2030 to be on track for net zero emissions by 2050. This can be compared to the global demand in 2022 of 95 Mt. Most of this increase will come from new applications, particularly in heavy industry, power generation and the production of hydrogen-based fuels.

As the hydrogen economy grows then safe and efficient production will require the strength and corrosion resistance of stainless steel at many stages in the value chain, especially for storage tanks.

Ductility at very low temperatures

Many storage and transportation applications will involve liquid hydrogen. The challenge is that hydrogen boils at -253°C under normal atmospheric pressure. That means operators will require special materials with the ductility at low temperatures essential to avoid brittle fractures that could impact the structural integrity of a storage vessel.

Multiple grades of stainless steel are already well proven for this type of environment and are listed in technical standards such as the ASME Boiler and Pressure Vessel Code and EN 13445-2. What they have in common is a stable austenitic microstructure that has a maintained low temperature toughness that enables them to operate at temperatures as low as -273°C. The table below shows stainless steel grades that are listed in the standards as useful at these low temperatures:

P: Hot rolled plate, H: Hot rolled coil, C: Cold rolled coil

Generally, tanks for liquid hydrogen storage will have inner and outer shells, with a vacuum or a layer of insulation between them. In terms of construction, the inner tanks will need to be able to manage the low temperatures, while in static applications the outer shells will need to resist the local environment. Since storage facilities are likely to be in coastal areas with a salt-laden environment, the corrosion resistance of stainless steel is an important additional factor. For mobile applications then a high level of crashworthiness is also essential.

Resisting hydrogen embrittlement under pressure

Another specific challenge for steels, and other materials, is the potential risk of hydrogen embrittlement (HE) when storing the gas under pressure.  Hydrogen embrittlement is a reduction in the ductility of a metal due to absorbed hydrogen. This is because hydrogen atoms are small and can permeate solid metals. Once absorbed, the hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement.

Sensitivity to HE varies, depending on the material. High-strength steel, and titanium alloys are all vulnerable. Sensitivity also varies between different types of stainless steel.

The risk of HE increases with the pressure, there is little risk at low pressures, therefore most stainless steels are suitable for storing or transporting hydrogen at pressures usually below 20 bar. However, as the pressure rises above this, so does the likelihood of the hydrogen atoms diffusing into the internal surface of a tank.

It is more difficult for hydrogen to diffuse into austenitic stainless steel than other stainless steel types such as ferritic and martensitic. That is why, along with its low-temperature ductility, austenitic stainless steel is selected for many hydrogen storage applications that operate in the range of 200 to 300 bar, such as when storing green hydrogen for later re-feed as energy to the grid or for later use in process industry such as for the production of fossil-free steel, as well as for liquid hydrogen storage.

The resistance of a particular grade of austenitic stainless steel against HE can be evaluated by its austenite stability. This is how well it resists the phase transformation to martensite under thermal or mechanical stress. Austenite stability is important as the martensite is considered to be susceptible to HE and therefore causing the loss of ductility.

Alloying elements in the stainless steel play a role in reducing susceptibility to HE. A general rule for the hydrogen industry has been that the most suitable materials are low-carbon austenitic stainless steels with around 12-13% nickel and 2-3% molybdenum, a combination that has been found to resist the diffusion of hydrogen. But more recently this is being replaced by focusing upon the austenite stability. Austenite stability is often determined by the M_d30 temperature and by the nickel equivalent (NiEq). M_d30 is defined as the temperature at which 50% of martensite is formed after a true tensile strain of 0.30 and is often translated to empirical formulas depending on the composition. NiEq is another empirical formula for austenite stability. Grades having low M_d30 temperature or a NiEq that is high, or rather in the range of 27 to 30, are preferred for hydrogen applications.  These grades have a high austenite stability and resist the formation of martensite during forming and or welding processes. Figure 1 shows Outokumpu 300-series austenitic stainless steel grades with corresponding calculated values of M_d30 acc. to Nohara and Figure 2 the calculated nickel equivalents acc to Sanga’s equation.

Figure 2 – Austenitic stainless steels ranked by their nickel equivalent.

Choosing the right grade

When designing and manufacturing hydrogen storage tanks it’s always important to find the material with the right set of properties. However, it can sometimes be a complex task to identify the perfect solution. This is where Outokumpu can make a real difference, as our years of practical experience combined with a wide portfolio of grades can offer a short cut to obtaining the performance and safety essential for the smooth operation of the hydrogen economy.