Hydrogen Power and the Materials Problem: Is It Time to Upgrade Our Alloys?

We believe it’s fair to say that the energy transition has a clear materials problem. As countries move towards hydrogen as a lower-carbon alternative for energy production, the engineering community is faced with an important question: will the alloys we are using today meet the requirements imposed by hydrogen combustion? The straightforward answer is – no – but to understand why, we first need to examine the challenges presented by hydrogen.

Why Is Hydrogen Different?

Natural gas turbines are the product of decades of fine engineering and innovation. The materials used for their construction, especially the hot sections of those turbines (mainly different grades of nickel superalloys) have been selected, qualified and improved in response to a well-known combination of stresses and operating conditions. Hydrogen breaks several of those assumptions.

Hydrogen burns hotter than natural gas and generates a higher adiabatic flame temperature. The conditions of operation for hydrogen-powered turbines therefore exceed those within the design boundaries of many alloys currently in use. At the same time, the combustion products of hydrogen are water vapour and not carbon dioxide. This changes the environment within the turbine and consequently the performance of the oxide layers that protect nickel superalloys against further oxidation. Water vapour at high temperatures is particularly aggressive towards chromia and alumina scales.

In addition, hydrogen turbines are required to be flexible and cycle up and down in output as part of the strategy of integrating more intermittent renewable energy sources into the grid. This adds another dimension of demand to alloys in terms of their ability to withstand thermal fatigue that steady-state operations do not.

The Embrittlement Factor

One of the biggest problems associated with hydrogen is probably not to be found in the hot parts of the turbine but rather in the parts responsible for handling, storing and distributing the hydrogen fuel before it reaches the combustion chambers. Hydrogen embrittlement – the reduction of fracture toughness and ductility of metals due to hydrogen atoms diffusing into the microstructure of a metal – is a well-documented phenomenon that can affect many kinds of structural alloys, particularly high-strength steels.

The exact cause is complex and far from obvious – but the effect is very real. A piece of equipment that works flawlessly under normal operating conditions starts experiencing premature cracking due to exposure to high-pressure hydrogen atmosphere. As a rule, the higher the strength of the steel, the more vulnerable the alloy tends to be to this effect – which means there is an inverse relationship between material properties and equipment weight and size constraints. Lower-strength alloys that are more resistant to embrittlement lead to bigger and heavier designs while selecting higher-strength ones requires thorough qualification testing and careful monitoring of surface quality, potential stress concentration points and operating pressures.

On the other hand, nickel alloys and austenitic stainless steels tend to show better resistance to hydrogen embrittlement than martensitic and ferritic steels. This is one of the reasons for the prominent role of these materials in hydrogen infrastructure-related materials specification conversations.

What Are the Alloy Developers Up To?

Materials science is not standing still. There is active research in Europe, Japan and North America regarding the possibilities of improving alloy performance and behaviour in hydrogen environment through modification of existing alloys or creating new alloy chemistries, developing new coatings or refining existing coating systems and even changing the manufacturing processes. Some of the research is quite fundamental in nature – and may result in totally novel alloy concepts to be commercialised at some point in future – while some of it is incremental and deals with optimising existing alloys.

Thermal barrier coating systems used in modern gas turbines must be modified to deal with hydrogen combustion and the alloys used in the bond coats also must adapt to this new challenge.

The Specification Challenge Today

When trying to solve material specification issues for hydrogen power-related projects, one faces a problem of insufficient qualification data for many of the alloys in pure hydrogen or hydrogen blends service. Current standards are evolving; OEM guidance is not yet fully consolidated – so following traditional practices for material specification in gas turbine applications is not the best course of action to pursue.

The materials choices made in coming years will impact the durability and lifetime of hydrogen power infrastructures for decades ahead – so making sure these choices are the right ones and based on accurate information about the alloys' performance under given conditions – is critical. Dynamic Metals' technical team is ready to help with the specification issues for your hydrogen power-related projects – whether they are just starting or already under way.

Dynamic Metals supplies a range of nickel alloys, stainless steels and high-strength alloy steels suitable for the energy sector. If you are working on a hydrogen power project and have issues with material specification, feel free to contact us.