A new study led by researchers from VTT Technical Research Centre of Finland investigates how different steels withstand long-term exposure to ammonia at high temperatures, providing new information for material selection in ammonia-fuelled combustion engines.

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Published in Fuel, the research examined three different commercial steels exposed to ammonia-containing atmospheres at 400 °C and 500 °C for up to thousand hours. The study explains the degradation mechanisms of the nitrided materials by combining extensive material characterization with thermodynamic modelling. Nitridation is a form of high-temperature corrosion caused by nitrogen uptake in the steel.

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Key findings:

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• All three steels underwent nitridation but the severity and corrosion mechanisms depended strongly on the steel composition and temperature.

• The low-alloy and alloy steels developed hard, brittle nitride layers with porosity and cracking. Nitridation was more severe at 500 °C than at 400 °C for those materials.

• The stainless steel showed markedly different nitridation behaviour: significant nitridation occurred at 400 °C whereas at 500 °C a protective chromium nitride layer formed.

• The results demonstrate that both alloy composition, particularly chromium content, as well as the temperature play a critical role in nitridation resistance in ammonia environments.

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The findings highlight the importance of careful material selection for ammonia-fuelled combustion engines. Understanding how different steels react with ammonia over long exposures can support the development of reliable materials for applications where ammonia is used as a carbon-free fuel, such as marine combustion engines.

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The research was funded through the MASCOT project, funded by Business Finland, participating companies and VTT and the JustH2Transit project, funded by the Strategic Research Council of Finland.

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Read the full open-access article:​ Ojasalo, S., Antikainen, A., Nandy, S., Nurmi, V., Bojinov, M., Karastoyanov, B., and Huttunen-Saarivirta, E (2026). High-temperature corrosion of steels by nitridation in ammonia: Degradation mechanisms and comparison between steel grades. Fuel, 419, 138872.