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When Good Metals Go Bad

When Good Metals Go Bad

USC scientists explore the complex and costly problem of corrosion in metals

By Eric Mankin
October 2004

Air travel could become safer as a result of USC interdisciplinary research into corrosion-induced failure in high-performance metals used in aerospace, marine and other demanding applications.

Supercomputing specialists Priya Vashishta, Aiichiro Nakano and Rajiv K. Kalia, who all hold joint appointments in USC College and the USC Viterbi School of Engineering, will model the behavior of large numbers of individual atoms to gain greater understanding of how and why alloys of titanium and other metals suffer “stress-corrosion cracking.” Cracking from mechanical strain in chemically unfriendly environments can lead to catastrophic failures.

In addition to the obvious safety implications, passengers may reap another bonus: comfort.

“Anyone who has traveled on an airplane knows the air is extremely dry,” says Vashishta, the project’s principal investigator and a professor of materials science, biomedical engineering, computer science, and physics and astronomy. “And this is deliberate.”

“The purpose is to minimize corrosion of the airplane — which is accelerated by moisture — and extend its life,” he says. “But if we can we understand more precisely how corrosion takes place, we may be able to find ways that will deal with the problem with less discomfort for travelers, while still keeping planes airworthy.”

The trio will carry out their investigations as part of an Information Technology Research project funded by the National Science Foundation. The new $3.8 million grant will be shared by the USC team and colleagues at Caltech and Purdue, with $2 million supporting the USC work.

“Corrosion is an enormously complex technological and economic problem with an annual cost of about 3 percent of the U.S. gross domestic product,” says Kalia, a professor of physics and astronomy in the College with joint appointments in materials science and computer science in the Viterbi School.

“We will study how chemical bonds between atoms break in corrosive conditions. Understanding stress-corrosion cracking at the level of atoms will help us find ways to inhibit and even prevent this kind of failure,” Kalia says.

The long-time collaborators have earned international reputations for their extraordinary work using ever-more powerful supercomputers to model atomic and molecular behavior. The goal of their interdisciplinary research has been to find ways to achieve greater strength and toughness in materials and greater speeds in electronic devices.

Among its many accomplishments, the group has developed unique computer software that allows the visualization of billions of atoms of material at one time.

“To learn the microscopic properties of many things, from materials to chemicals to biological systems, you need simulations of billions of atoms,” says Nakano, a professor of computer science, materials science and physics. “The principles involved in our work can be applied to everything because all things are composed of atoms.”

In the current project, the team will use large-scale computer simulations and techniques of nanoscience to supplement the traditional structural engineering methods.

The traditional approach, called continuum mechanics, works well in providing reliable forecasts of how the material will behave when new, says Vashishta. But it offers little guidance into how and when materials may fail because of stress-corrosion cracking — damage from corrosion that starts when ordinary strain on the metal produces tiny cracks that allow the entrance of moisture and oxygen.

Nanoscientific analysis can supply such guidance, Vashishsta says. The idea is to go down to the basic atomic structure of the material and simulate the behavior of individual atoms at the point where cracks appear in the surface.

“We start by accurately modeling the behavior of collections of… a few hundred atoms at one point; proceed from there to modeling thousands of atoms along the surface, [and] going to millions of atoms over a larger area,” he explains.

The results of the nanoanalysis have to produce the same predictions for behavior as the continuum mechanics technique, Vashishta says.

“But by understanding exactly what is going on, in detail, at the point where the material is failing, we can find better ways to prevent damage and create more corrosion resistant materials,” Vashishta says.