Unmasking corrosion to design better protective thin films for metals

Traditionally, these protective films were thought to be mere oxide compounds of long-awaited compounds, but new work from scientists at Northwestern University, the University of Virginia, and the University of Wisconsin-Madison reveal spectacular new information about these oxide films.

Using experimental techniques and state – of – the – art theoretical modeling, scientists were able to analyze oxide films at the atomic level, deciphering how atoms are arranged in oxides.

Their discoveries? Protective films develop novel structures and compositions that depend on the growth rate of the oxide film. The authors of the study say that their findings could provide clues on how to improve protection films, or even much better.

This is an advance that could have implications for everything from nuts and bolts to high-tech batteries and turbine engines.

"That changes a lot of things about how we understand these oxide films and opens the door to radically new methods of protecting metals," said Laurence Marks, professor of materials science and engineering at the McCormick School of Engineering Northwestern, who led the study. "We now know that there are ways to predict the chemical composition of these films, which we can exploit to make protective films last much longer."

The study was published today (October 3) by the journal Letters of physical examination.

"We now have more ways than ever to control and adjust oxides to protect materials," said John Scully, professor chaired by Charles Henderson and director of the department's materials science and engineering department. University of Virginia and one of the authors of the study.

"This provides essential information on how to design new materials that will corrode much less," said Peter Voorhees of Northwestern, another author of the study. Voorhees is a Frank C. Engelhart Professor of Materials Science and Engineering at Northwestern Engineering.

The team has studied in detail the oxides that form on alloys composed of nickel and chromium, widely used in various products, such as the heating elements of a domestic toaster or motors. ;plane.

These oxides are also used for applications in the presence of water, as in dental implants. It has long been known that these oxides work hot and resist corrosion in the mouth due to the formation of a chromium oxide. It was assumed that nickel formed a separate oxide or, in some cases, dissolved in the body. The team discovered something unexpected: the oxide was not only chromium and oxygen, but rather contained a very large number of atoms of nickel.

Why? Because the nickel atoms do not have time to escape the oxide and to do it. The captured fraction depends on the growth rate of the oxide. If it grows very slowly, the nickel atoms can escape. If it grows very fast, they can not.

This occurs both when metals react with oxygen in the air at high temperatures and when they react with water in ships or dental implants. According to the authors of the study, the atoms captured in the oxide modify many properties.

The results indicate that it is possible to deliberately trap atoms in these oxides in a new way, and thus modify their behavior.

"We are close to the limits of what we can do with aircraft engines, for example," said John Perepezko, IBM-Bascom Professor of Materials Science and Engineering at the University. from Wisconsin-Madison and other author of the study. "This new vision of protective oxide formation is leading to many new ways to build better engines."

The title of the article is "Capture of out of equilibrium solutes in passivating oxidation films".

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More information:
Non-equilibrium solute capture in passivating oxidation films, Letters of physical examination (2018). journals.aps.org/prl/accepted/… 67034c72cb55bff36027