Rusting at the atomic level

Rust is an unwelcome yet inevitable occurrence that weakens the structural integrity of our infrastructure and electronics over time. People have studied the effects of rusting since ancient times, but we have yet to understand what exactly happens at the atomic-scale. Parkinson et al. report their latest findings on the intermediary processes in dissolution and corrosion of iron at the atomic level and hope to help improve iron-based materials for future industrial applications.

“Since magnetite is already oxidized iron, we expected that exposure to pure water at room temperature wouldn’t do all that much,” said author Gareth Parkinson. “Instead, we found an entirely new surface phase forming slowly at room temperature, which will have very different reactivity to what we have always studied before in our vacuum experiments.”

The researchers exposed a single crystal of magnetite (Fe₃O₄) to an ultrapure drop of water under ultrahigh vacuum conditions. Using X-ray photoelectron spectroscopy and low energy electron diffraction, they studied the crystal’s surface and found evidence for significant hydroxylation and structural changes.

They then delved even deeper by using scanning tunneling microscopy to show a complex scenario unfolding — the hydroxyl groups were extracting iron atoms from the underlying surface and forming a new oxyhydroxide phase on the surface, while other hydroxyl groups bind to the oxygen atoms and dissolve the remaining magnetite surface. When the oxygen lattice became saturated, the dissociation of hydroxyl groups from water stopped.

While this work is a step in an exciting direction, Parkinson cautions that a complete atomic model of how new structures form along the metal’s surface is necessary to explore the topic thoroughly.

Source: “Self-limited growth of an oxyhydroxide phase at the Fe₃O₄ (001) surface in liquid and ambient pressure water,” by Florian Kraushofer, Francesca Mirabella, Jian Xu, Jiří Pavelec, Jan Balajka, Matthias Müllner, Nikolaus Resch, Zdenék Jakub, Jan Hulva, Matthias Meier, Michael Schmid, Ulrike Diebold, and Gareth S. Parkinson The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5116652 .