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Science

Can hydrogen content strengthen steel?

The picture above shows a microscopic fracture in stainless steel with a high level of hydrogen concentration that occurred when the material was subjected to repeated stress. The image below is of a crack produced with no hydrogen embedded into the material.   © Courtesy of Yukitaka Murakami

OSAKA -- Hydrogen is now considered a precious clean energy source, but has long been thought a cause of metals becoming weaker and brittle. In 2010, researchers at Kyushu University and the National Institute of Advanced Industrial Science and Technology discovered that adding a supersaturated level of the colorless, odorless gas into a metal made it stronger. The result ran counter to conventional wisdom. However, has this finding led researchers around the world to change their view of hydrogen's impact on metals?

     The phenomenon called hydrogen embrittlement occurs when the substance enters the metal crystal lattice, eventually causing cracking and degrading fracture resistance. The phenomenon has been studied for over 40 years. Brittle fractures in high-strength metals, notably stainless steel, are often caused by hydrogen within a year or so of manufacture, even though the material is supposed to last for 30 years or more. To counter this problem, the development of cracking-resistant metallic materials has become a central research interest of materials science.

     When two hydrogen atoms combine to form a hydrogen molecule, the new form is too large to diffuse into a metal's crystalline structure. However, singular hydrogen atoms can easily disperse throughout metals in high-pressure environments. Even if only a few parts per million of hydrogen are attracted to microscopic crystal defects and accumulate in these areas, the metal starts to crack, causing the entire material to become brittle and liable to crumble.

     One day, Yukitaka Murakami, professor emeritus of Kyushu University, said to his research team, "Let's look at what happens when we infiltrate the maximum possible amount of hydrogen into metals." He was aware that no one had conducted such an experiment. Since Kyushu University has experimental laboratories able to safely handle high-pressure hydrogen, researchers attempted to infiltrate over 70 ppm of the gas into metal, something that is not usually possible. They initially presumed that higher penetration of hydrogen would cause a greater susceptibility to cracking. A postdoctoral researcher reported to Murakami the unexpected result of the experiment -- that the material was able to withstand the hydrogen without fatigue failure. "It was a big surprise to me," Murakami said.

Institutional challenge

When Murakami submitted a research paper on the subject to an American academic journal, one of its two selectors rejected his work. They argued that the paper simply presented contradictory findings to established theories. After a yearlong discussion with the selector, Murakami got the paper published in the journal, but only after persuading the editor-in-chief that any refusal to accept work based on empirical analysis is unfair.

     Murakami assumes that excessive hydrogen penetration makes it possible for the gas to be compressed into small spaces in areas where the crystal has defects, helping to solidify the metallic crystalline structure.

     Following publication of the findings, a research group from Germany published a report showing that hydrogen has two contradictory effects. This paper presented findings that hydrogen can cause resistance to dislocation motion in crystalline metals, but can also enhance it. Several attempts are being made around the world to verify the experimental results achieved by Murakami and his team.

     There are several methods already in use for reducing the risk of hydrogen embrittlement. When metals are heated to a sufficiently high temperature, hydrogen in the crystal structure can decrease. It has also been known that use of aluminum-based coatings inside a hydrogen storage tank can improve hydrogen-entry resistance.

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