In an ideal world, materials scientists and engineers could make alloys that resist corrosion organically, by design. In reality, achieving corrosion protection as a material property remains a hard problem.

Petra Reinke, professor of materials science and engineering at the University of Virginia’s School of Engineering and Applied Science, conducts research to explain how surface chemistry factors into corrosion protection. She shared results of her nanomaterials and surface science research group’s experiments in a Dec. 16, 2021 e-talk for fellow members of the American Vacuum Society, which seeks to advance the science and technology of materials, interfaces and processing.

Reinke’s experiments help explain why minor alloying elements such as tungsten and molybdenum lead to a better protective oxide layer in nickel-based superalloys.

“We knew that sprinkling molybdenum or tungsten suppresses a pitting event, which is a catastrophic breakdown. But we couldn’t explain how to consistently get that beneficial reaction,” Reinke said.

Reinke led viewers through her group’s process of growing clean, well designed samples and how they tested corrosion response as a function of alloy composition. More specifically, they compared nickel-based alloys with different percentages of chromium at temperatures ranging between 200 and 600 degrees Celsius.

“We believe that crystallographic orientation is a significant and understudied aspect of corrosion resistance,” Reinke said. “We want to know what changes in the oxidation process if we add tungsten or molybdenum.”

Scanning tunneling microscopy experiments conducted at UVA and ambient pressure XPS experiments conducted at Brookhaven National Laboratory revealed how the competition between nickel and chromium in a given alloy creates a preferred thermodynamic or kinetic pathway toward corrosion resistance, and how the addition of tungsten causes a massive diffusion of chromium and chromium-oxide nucleation. Initial results are published and cited in the talk.

This research narrows a knowledge gap in surface science, linking the first steps of oxidation on a clean alloy surface to traditional oxide growth regimes and recording the cascading effects.

“The initial oxidation reaction prejudices the oxide layer growth and subsequent performance,” Reinke said. “A minor alloying element impacts the initial oxidation step and is not confined to a role within the oxide layer.”

Reinke credited the research teams’ external sponsors including the Office of Naval Research Corrosion in 4D Multi-University Research Initiative led by Laurence Marks, professor of materials science and engineering at Northwestern University; National Science Foundation XSEDE support for computational work; and U.S. Department of Energy funding for the XPS study conducted at the National Synchrotron Light Source II, beamline 23-ID-2. Continuation of this work is supported by the National Science Foundation Metals and Metallic Nanostructures program.

These experimental results inform computational models that help materials scientists and engineers manipulate an alloy’s mechanical properties to perform well in whatever chemical environment the material experiences. In this interplay of experimentation and computational modeling, Reinke sees potential to develop rules for designing more corrosion-resistant alloys and protective coatings across different material systems.