Stainless steel for the hydrogen economy

Until now, stainless steel has been susceptible to corrosion and embrittlement during the transport and storage of hydrogen. An international research team has achieved a breakthrough by atomically passivating grain boundaries with nitrogen: a new alloy design overcomes corrosion and hydrogen embrittlement.

Hydrogen is a key component of future climate-neutral energy systems. However, the safe storage and transport of hydrogen continue to pose a major challenge in materials science. Stainless steels are regarded as promising materials as they are robust, cost-effective and widely available. However, even modern grades of stainless steel are susceptible to corrosion and hydrogen embrittlement – a process in which hydrogen penetrates the metal, weakens internal bonds and, in extreme cases, leads to sudden material failure.
In a new study, an international research team led by the University of Science and Technology Beijing and the Max Planck Institute for Sustainable Materials (MPI-SusMat) has developed a novel austenitic stainless steel that addresses both problems simultaneously. The findings have now been published in the journal *Science Advances*.

Nitrogen protects the grain boundaries

Grain boundaries are among the most vulnerable defects in metals. They act as fast diffusion pathways for hydrogen and are preferred sites for electrochemical corrosion reactions. Hydrogen embrittlement occurs when mobile hydrogen accumulates at these interfaces, causing local stresses that can lead to the weakening of the microstructure and the formation of cracks. Corrosion, on the other hand, is the result of electrochemical interactions between the material’s microstructure and its environment.
“The challenge was to develop a new, industrially viable stainless steel that remains mechanically reliable under hydrogen stress whilst also exhibiting high corrosion resistance,” explains Professor Dierk Raabe, Director at MPI-SusMat and corresponding author of the study. “Furthermore, the material had to be cost-effective and suitable for processing using established industrial manufacturing methods. As grain boundaries – that is, planar atomic defects in metals – are the most critical weak points for hydrogen embrittlement, we targeted them specifically to prevent the penetration of hydrogen by occupying these regions with finely distributed atomic nitrogen. So this involves the use of an atomic protective layer on the grain boundaries – that is engineering at the atomic level.”

Atomic passivation ensures lasting protection

Instead of relying solely on a conventional oxide layer on the surface, the research team integrated nitrogen atoms directly into the grain boundaries of the steel. In this way, the penetration of hydrogen is blocked before any damage can occur. ##The newly developed alloy (Fe-20Cr-9Ni-2.5Mn-1.6Mo-1Cu-0.2N) thus exhibits 3.8 times higher corrosion resistance and 1.35 times greater resistance to hydrogen embrittlement compared to commercial 316L stainless steel.

A scalable and sustainable solution

Unlike approaches in which hydrogen is bound in precipitates that quickly become saturated, the passivation of grain boundaries offers long-term protection. ##The new material is cost-effective, compatible with established industrial processes and has a lower carbon footprint than many high-performance alternatives. By combining corrosion resistance, hydrogen tolerance and cost-effectiveness, the developed stainless steel opens up realistic prospects for safer pipelines, tanks and components in hydrogen infrastructure. ##The next step is to apply this atomic design strategy to other alloys and create new possibilities for durable materials in energy, chemical and infrastructure applications.
Source: Max Planck Institute for Sustainable Materials