Stopping hydrogen embrittlement and preventing disaster
Monday, November 18, 2013
Water vapor makes up between 1-5 percent of air on average, delivering a one-two-three punch of oxidation, hydrogen blistering and embrittlement.
Hydrogen embrittlement has proven itself particularly dangerous. Catastrophic structural failures that result from it have been at the root of the majority of the nautical and aerospace disasters over the past few decades.
The smallest of atoms is one of the biggest threats to our national infrastructure, attacking the structural integrity of buildings, bridges, ships and planes. But embrittlement is reversible in some cases and is becoming more preventable as we come closer to understanding the mechanism that causes it.
No one can predict where and when hydrogen embrittlement will strike because no one really understands the mechanism of microfracturing. What we know for certain is that hydrogen is not only present in water vapor, but is also a byproduct of corrosion, creating a feedback loop.
Sometimes, hydrogen is released in attempts to protect the metal from corrosion. Hydrogen embrittlement often results from cathodic protection, which is a treatment designed to make coated metal more resistant to corrosion.
The release of hydrogen atoms into a metal causes them to pool in microscopic flaws and tears. Individual atoms bond to form molecular hydrogen, which is then under pressure to escape and drives greater cracks into the metal. Eventually, the metal will blister and become brittle.
Certain metals that are more prone to hydrogen embrittlement are magnetic steel, nickel and titanium, which are often used as load-bearing materials for their strength under pressure. In this case, their own strength works against them.
Although a great deal about the embrittlement process remains unknown, there has been progress in stopping it and, under special circumstances, reversing its effects through baking. According to Omega Research, nearly 71 percent of aircraft embrittlement failures since the 1960s have been attributed to failures of the baking process, including a bake that has been missed, a long delay between plating to baking or not enough time spent in the baking processes.
Baking has also shown to be remarkably effective in reversing hydrogen embrittlement by dissolving hydrogen and releasing the pressure built up in fractures on a microscopic scale.
Last November, researchers from Canada and Switzerland teamed up to tackle the mystery of hydrogen embrittlement with a new theory that they hope will be able to predict the reaction. Jun Song, an assistant professor of materials engineering at McGill University, and William Curtin, director of the Institute of Mechanical Engineering at École Polytechnique Fédérale de Lausanne in Switzerland, published their findings in the journal Nature Materials.
Their premise was that hydrogen atoms behave in a different way on the molecular level when they interact with material under pressure. The strength of metal lies in its ability to deform itself under force. Plasticity results from microtears that create "dislocations" within the metal that allow the atoms to shuttle around and relieve and accommodate stress across the material. Song explained their theory about how the seeds of hydrogen embrittlement get started.
"Dislocations can be viewed as vehicles to carry plastic deformation, while the nano- and microsized cracks can be viewed as hubs to dispatch those vehicles," Song explains. "The desirable properties of metals, such as ductility and toughness, rely on the hubs functioning well.
"Unfortunately, those hubs also attract hydrogen atoms. The way hydrogen atoms embrittle metals is by causing a kind of traffic jam: They crowd around the hub and block all possible routes for vehicle dispatch. This eventually leads to the material breaking down.”
Their experiments have shown impressive results in predicting embrittlement thresholds in ferric iron-based steels. With this data, next-generation materials and treatments can be created to stop hydrogen embrittlement before it gets started.
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