Atomic maps provide new view of rust


Rebecca Pool

Wednesday, February 6, 2019 - 10:30
Image: Atom probe tomography reveals re-arrangement of different iron atoms in a small iron oxide crystal. [PNNL]
Using atom probe tomography with iron isotopes, researchers from Pacific Northwest National Laboratory, US, have created the first ever 3D atomic maps of iron atom re-arrangement in an iron oxide crystal.
Images reveal the distribution of the two different iron isotopes on individual goethite crystals during a redox reaction, with one isotope - 57Fe - gathering in specific defect sites at the surface.
3D image of geothite recrystallization using atom probe tomography. (The grey box is 5 nanometres square for scale. )
According to Dr Sandra Taylor from Geochemistry at PNNL, the maps of these oxidation-reduction reactions reveal a surprisingly dynamic iron cycle, showing the continual movement of iron joining and leaving the mineral surfaces.
"We saw that iron atoms in water specifically sought out and filled in tiny potholes, or defects, in the crystal surfaces," she explains. "Seeing these recrystallized regions at the atomic scale showed us that the reaction can effectively 'heal' damaged areas on the crystal surface, and growth is driven by perfection."
Lead researcher, Professor Kevin Rosso, says the results confirm that reactions with rust minerals in soils and steel corrosion products are more dynamic than typically thought.
What's more, the results also illustrate how rust persists on metal pipes under changing chemical conditions, enabling it to continually corrode and deteriorate over time.
The latest discovery follows a year-long effort to capture chemical composition measurements and images at the atomic scale in 3D using atom probe tomography.
As Rosso says: "This study sets a new precedent for characterizing this important redox interface."
The researchers will now use their latest results to better understand a broad range of processes, including how crystals grow and dissolve, and also the underlying causes of corrosion and how it creates rust on surfaces.
Knowing more about the chemical reactions that drive and sustain rust could hold clues for engineering more durable iron-based materials.
It could also lead to advances in fertilizers or soil conditioners that increase iron uptake for plant nutrition.
Research is published in PNAS.


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