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Wednesday | 13 March, 2013 | 9:33 am

Brittle be gone

By Caitlin Tucker

New research predicts when a metal will suffer hydrogen embrittlement 

March 2013 - The term hydrogen embrittlement often is used by metallurgists, engineers and scientists, but there is little information on the phenomenon. A new study conducted by Jun Song, assistant professor in Materials Engineering at McGill University, Montreal, and William Curtin, director of the Institute of Mechanical Engineering at École polytechnique fédérale de Lausanne, Switzerland, aims to understand and predict the occurrence of hydrogen embrittlement and eventually develop a design framework for the next generation of hydrogen embrittlement-resistent materials.

Understanding hydrogen embrittlement

Hydrogen embrittlement occurs in ductile metals, usually high-strength steel, nickel, aluminum and titanium alloys. The gas is common in earth’s atmosphere, and when it dissolves “into metals, the metals become more brittle,” says Song. “So it’s like turning steel into ceramics.” Hydrogen modifies a metal’s plastic deformation, which Song defines as a metal’s ability to be bent, like soft plastic. 

“Within metals you have cracks,” says Song. “If the metal is ductile, the cracks will generate a lot of dislocations,” which are movements of atoms that serve to relieve stress in the material. “Those dislocations act as vehicles to carry the plastic deformation.”

Song explains, “The new finding is that when you have hydrogen in the material, it basically makes the metal lose the ability to generate dislocation. Then, if it cannot generate dislocation, then it cannot generate plastic deformation.” Think of a licorice stick becoming stale. It starts as a a bendable stick, then becomes harder and harder until it eventually shatters with force.

The current issue

Everyone knows a greater concentration of hydrogen causes metal embrittlement, says Song. “That’s what they know, but when and how, they don’t know either.” Companies currently use a type of trial-and-error approach to testing hydrogen embrittlement. “That’s an issue in the industry,” says Song, noting that engineers use a lot of empirical tests, which are not accurate and only provide a rough estimate. 

The ASTM 1459-06(2012) tests the susceptibility of metallic materials to hydrogen embrittlement but only provides an estimate. The ASTM G142 -98(2011) is similar, but states, “this test method may not be suitable for the evaluation of high temperature hydrogen attack in steels unless suitable exposure time at the test conditions has taken place prior to the initiation of tensile testing.”

“As a result, you don’t have actual design criteria, you have a rough range. You always need to be more conservative and that costs you more money,” he says. “But if you have some model then you’ll know precisely when the hydrogen embrittlement is going to happen then you can design directly according to that number.” And that’s exactly what this new study has unveiled. 

Future prevention

With this new knowledge, Song and other researchers can determine when hydrogen embrittlement will occur. “The model to predict it would depend on loading condition, temperature and the hydrogen concentration. So at a higher temperature apparently hydrogen embrittlement is less of a problem because the temperature would drive hydrogen to go out.”

In many experiments, researchers mimic a hydrogen-rich environment by charging hydrogen directly into the material, like a battery. Computer simulations revealed how hydrogen atoms move within metals and how they interact with metal atoms. This simulation was followed by kinetic analysis to link the nanoscale details with macroscopic experimental conditions, according to a McGill press release.

“At this moment, the main point of this finding is that we have precise predictive criterion to design the material,” says Song. “The physics say that hydrogen can inhibit materials’ ability to generate dislocations, so then we need to introduce some alloy elements to prevent this from happening. 

“Now we are trying to use this idea to design new materials,” he continues. “We now have a large hydrogen embrittlement consortium involving several groups and industry partners working together, so in the future we can generate more knowledge about the material design and coating design to prevent hydrogen embrittlement.”

Today, the industry uses rough estimates, “but they are also more conservative,” says Song. “For the oil industry they increase the thickness, for the airplane they replace the component more frequently. So basically, you are taking out those metal components before they even fail.”

He says those in the oil pipeline industry use thicker pipelines to prevent embrittlement, but if “you’re building an oil pipeline from Alberta, Canada to California, that’s a lot of material. Then you waste a lot of money in thickening the materials.” Song hopes that his research will help businesses use materials in a more economically conservative and environmentally stable way. MM

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