A better understanding of hydrogen embrittlement enables the design of alloys for gas turbines for hydrogen combustion
To the point:
- Transformation to hydrogen economy: It was unknown so far how hydrogen, with which gas turbines can be operated CO2-free, affects the material of turbines, at elevated temperatures.
- Accelerated corrosion: Hydrogen-induced embrittlement in Nickel-base superalloys is twice as severe at elevated temperatures as at ambient temperatures. It is caused by the decomposition of carbides and localized formation of methane through chemical reactions between hydrogen and carbon, which can lead to potentially catastrophic failure.
- Alternative alloys: Carbides increase the strength of alloys. In a future hydrogen economy alloys need to be strengthened by different mechanisms.
Xizhen Dong characterized how hydrogen affects Nickel-base superalloys at elevated temperatures.
© Max-Planck-Institut für Nachhaltige Materialien GmbH
Can we fuel gas turbines with hydrogen instead of fossil fuels and cut 15 percent of global carbon dioxide (CO2) emissions? Gas turbines generate around 22 percent of the world's electricity. Replacing fossil fuels is a key step towards more sustainable power generation. Hydrogen is widely considered a promising alternative fuel for gas turbines in both power generation and aviation. However, before hydrogen can be used safely on a large scale, researchers need to better understand how it affects the materials exposed to the extreme operating conditions inside turbines.
While the interaction between hydrogen and metallic materials has been extensively studied at ambient temperatures, far less is known about its effects at elevated temperatures found in gas turbines. Gas turbines are operated at temperatures above 1000 centigrade, but during ramp-up and cool-down they go through a temperatre range of several hundred centiograde. An international team of researchers has now investigated how hydrogen affects Nickel-base superalloys - the material of choice for gas turbines - at elevated temperatures. Their results indicate that hydrogen-induced embrittlement can be at least twice as severe, posing a significant challenge for components that must meet the highest standards of safety and reliability. Researchers of the Max Planck Institute for Sustainable Materials and their collaborators published new findings in the journal Nature Materials.
Why hydrogen trapping at elevated temperatures fails
"When hydrogen enters a Nickel-base superalloy at ambient temperatures, it is usually trapped at interfaces and dislocations. At elevated temperatures, hydrogen atoms migrate to carbon vacancies within carbides, causing their partial decomposition. Moreover, hydrogen and carbon atoms react and form methane. This highly pressurized methane exerts a high local internal pressure that weakens the interfaces, and promotes damage", explains Xizhen Dong, postdoctoral researcher at the Max Planck Institite for Sustainable Materials.
Dong and her colleagues tested how hydrogen affects Nickel-base superalloys in a temperature range from 400 to 1000 centigrade. By combining atom probe tomography and density functional theory calculations, the researchers were able to show that the degradation mechanisms in Nickel-base superalloys exposed to hydrogen fundamentally change depending on the operating temperatures. The degradation only takes place at around 400 degree Celsius, while no methane formation was seen in measurements at 600 centigrade. "What we discovered here is essential especially for gas turbines and flying turbines, which, unlike stationary steam turbines, are frequently switched on and off and therefore experience a larger temperature and load spectrum where embrittlement effects can occur", explains Dierk Raabe, director at the institute.
Designing hydrogen-resistant Nickel-base superalloys
Having shown that carbides are the entry gate for hydrogen-induced cracking, a future alloy design would aim to tailor the alloy's microstructure by replacing carbides. Since carbides are widely used to strengthen high-performance alloys, new strengthening strategies will be required. Alternatively, a balance has to be found between mechanical strength gained from carbides and hydrogen embrittlement resistance.
These findings underscore the need for temperature-specific mechanistic frameworks and predictive models to describe hydrogen-induced damage not only in Nickel-base superalloys, but also in other carbide-containing alloys, including steels and metal-ceramic composites and pave the way for future hydrogen-fuelled gas turbine energy and air traffic systems.
The research was led by scientists from the East China University of Science and Technology (China), Max Planck Institute for Sustainable Materials (Germany), and the Hunan University (China).
YAS
