Researchers at Oxford University are investigating how hydrogen and metals interact at the atomic level in an effort to develop high-strength materials that do not become brittle and weaken when exposed to hydrogen.

First observed before the turn of the 20th century, hydrogen embrittlement is a phenomenon in which some materials—particularly steel, but also metals like zirconium and titanium—become weaker when exposed to hydrogen. Some steels can suffer a decrease in strength by as much as a factor of 10 when exposed to the element. Researchers say this means that they could fail when subjected to just a fraction of the maximum stress they can usually withstand.

Some steels can weaken by a factor of 10 when exposed to hydrogen. Image credit: Oxford University."It seems like it should be a straightforward problem to solve," says Professor Alan Cocks of the university's Department of Engineering Science. "But there's immense controversy about why it happens and, despite lots of experimental studies and theoretical modeling, there's still no real solution."

Broadly, two schools of thought exist on the subject. One suggests that hydrogen atoms build up around tiny defects called dislocations in the metal's crystal structure, affecting the way the material irrevocably deforms. A second proposes that the hydrogen gathers at boundaries between chunks of metallic crystal known as grains, making it easier to pull them apart.

"We suspect it could be a combination of both," says Cocks.

At the same time, the engineers are proposing new ways to help steels cope in the presence of hydrogen. Past efforts have focused on using coatings to protect against hydrogen's absorption by metals. He views that approach as inadequate because if a crack develops in the coating, the material underneath is exposed. "And anyway, because of its size, hydrogen can ultimately diffuse through any material, so a coating will only ever slow down the process," he says.

Instead, the researchers are seeking to develop a series of internal traps that can capture the hydrogen as it enters the metal, which he likens to "little sponges in the steel" that mop up all the hydrogen so that it's not detrimental to the material properties. So far the team has studied how carbides—a type of carbon-metal compound—can be used within the microstructure of metals to such ends, because hydrogen appears to gravitate toward boundaries between them and grains of iron. The hope is that once they move to this boundary, the hydrogen will rest benignly.

Many questions that need to be answered before such techniques calm the minds of engineers eager to use these kinds of materials, researchers say. Not least among them is what happens to the hydrogen stored within those traps: it may affect the material properties in an unexpected way, he says

Ultimately, if a way can be found to overcome metals' weakness in the presence of hydrogen, it could allow engineers to use steels and other metals in a range of uses—for example, titanium in aerospace applications and zirconium sheathing of nuclear power plant control rods.