How hydrogen damages metals
In situ SEM set-up to study hydrogen embrittlement [Tasan et al]
In a breakthrough for hydrogen storage, MIT researchers have used in-situ SEM to observe hydrogen embrittlement at a metal surface.
By developing an in situ hydrogen-charging setup for scanning electron microscopes and other vacuum systems, Professor C Cem Tasan and colleagues, analysed microstructural changes as hydrogen penetrated a titanium alloy and stainless steel, at high resolution.
“It's definitely a cool tool,” says Chris San Marchi, Sandia National Laboratories researcher, who was not involved in this research.
“This new imaging platform has the potential to address some interesting questions about hydrogen transport and trapping in materials, and potentially about the role of crystallography and microstructural constituents on the embrittlement process," he adds.
While hydrogen fuel is considered to be key to limiting global climate change, expensive and heavy high-pressure tanks are needed to contain it.
Storing the fuel within the crystal lattice of the metal itself could provide a cheaper, lighter, and safer route to storgae, but first the process of how hydrogen enters and leaves the metal must be better understood.
“Hydrogen can diffuse at relatively high rates in the metal, because it’s so small,” explains Tasan. “If you take a metal and put it in a hydrogen-rich environment, it will uptake the hydrogen, and this causes hydrogen embrittlement as the hydrogen atoms tend to segregate in certain parts of the metal crystal lattice, weakening its chemical bonds."
Electron microscope images show the buildup of hydrogen within the crystal structure of a titanium alloy. The images reveal the way hydrogen, depicted in blue, preferentially migrates into the interfaces between crystal grains in the metal. [Tasan et al]
To better understand embrittlement, with a view to slowing or avoiding the process, Tasan and colleagues developed a novel imaging method that allowed them to expose metal surfaces to a hydrogen environment while inside the sEM vacuum chamber.
As part of this they used a liquid electrolyte as the hydrogen source; this was sealed within a chamber and then exposed to the underside of a thin sheet of metal.
The topside of the metal was left open to the SEM electron beam, so the researchers could characterise the metal structure as the hydrogen atoms from the electrolyte migrated into it.
Schematic of SEM: Metal layer in centre with blue electrolyte used as hydrogen source. Top probe can be used to test mechanical properties. [Tasan et al]
According to Tasan, devising a leakproof system was crucial to making the process work, as the electrolyte needed to charge the metal with hydrogen.
On trying out the set-up, he says: "We were excited, but also really nervous. It was unlikely that failure was going to take place, but there’s always that fear.”
"But this is a unique setup. As far as we know, the only one in the world that can realize something like this,” he adds.
The researchers have now used their systems to analyse embrittlement in stainless steels and also study the formation and growth process of a nanoscale hydride phase in a titanium alloy, at room temperature and in real time.
"[This] is very challenging because an acid solution for hydrogen cathodic charging is circulating into an SEM chamber. It is one of the most dangerous measurements for the machine," says Professor Kaneaki Tsuzaki, from chemical engineering at Kyushu University in Japan, and not involved in this research. "If the circulation joints leak, a very expensive SEM would be broken due to the acid solution. A very careful design and a very high-skill setup are necessary for making this measurement equipment.”
"[However the set-up] could be a key technique to solve how hydrogen affects dislocation motion...as it can give in-situ observations under a well-controlled hydrogen atmosphere," he adds. [I believe that Tasan and Kim] will obtain new findings of hydrogen-assisted dislocation motion by this new method, solve the mechanism of hydrogen-induced mechanical degradation, and develop new hydrogen-resistant materials.”
Research is published in the International Journal of Hydrogen Energy.