Corrosion is an age-old problem that is estimated to cost about $1 trillion a year, or about 5% of the US gross domestic product. Corrosion of metals can be particularly troublesome, but fortunately metals are normally protected from catastrophic damage by naturally forming, super-thin oxide films.
Traditionally, these protective films have been viewed as simple oxides of well-anticipated compounds, but new work from scientists at Northwestern University, the University of Virginia and the University of Wisconsin-Madison has revealed dramatic new insights.
Using state-of-the-art experimental techniques and theoretical modeling, the scientists were able to analyze oxide films at the atomic level, deciphering how the atoms are arranged in the oxides. This revealed that the protective films develop new structures and compositions that depend on how fast the oxide film grows.
The scientists say their findings could provide clues about how to make the protective films better – perhaps much, much better. It's a breakthrough that could have implications for everything from nuts and bolts to high-technology batteries and turbine engines.
"This changes many things about how we understand these oxide films and opens the door to drastically new ways of protecting metals," said Laurence Marks, professor of materials science and engineering at Northwestern's McCormick School of Engineering, who led the study. "We now know that there are ways to predict the chemical composition of these films, something we can exploit so the protective films last much longer." The scientists report their findings in a paper in Physical Review Letters.
"We now have more routes than ever to control and tune oxides to protect materials," said John Scully, professor and chair of the Department of Materials Science and Engineering at the University of Virginia and one of the paper's authors.
"This provides key information about how to design new materials that will corrode far less," said Peter Voorhees, professor of materials science and engineering at Northwestern Engineering and another of the paper's authors.
"This changes many things about how we understand these oxide films and opens the door to drastically new ways of protecting metals."Laurence Marks, Northwestern University
The team studied, in detail, the oxides that form on alloys composed of nickel and chromium, which are widely used in a variety of products, from the heating elements of a household toaster to aircraft engines.
These oxides are also used for applications when there is water present, such as in dental implants. It has long been known that these oxides both work when hot and resist corrosion in the mouth because of the formation of an oxide of chromium. It was assumed that the nickel formed a separate oxide, or in some cases dissolved away in the body. But the team found something unexpected – the oxide didn’t just comprise chromium and oxygen, but also contained a very large number of nickel atoms.
It appears that the nickel atoms do not have time to escape from the oxide, becoming captured inside it. The fraction that is captured depends upon how fast the oxide grows. If it grows very slowly, the nickel atoms can escape. If it grows very fast, they cannot.
This occurs both when the metals are reacting with oxygen from the air at high temperatures, as well as when they are reacting with water in ships or in dental implants. The atoms that are captured in the oxide influence many of the film’s properties, the scientists say.
These findings suggest it may be possible to deliberately trap atoms in these oxides in new ways, and thus change how they behave.
"We are close to the limits of what we can do with aircraft engines, as one example," said John Perepezko, professor of materials science and engineering at the University of Wisconsin-Madison and another of the paper's authors. "This new vision of protective oxide formation leads to many new ways one could build better engines."
This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.