Networks of Metal Nanoparticles Are Culprits in Alloy Corrosion

Schematic representation of a network of metallic nanoparticles present in oxide scale recently discovered by Argonne researchers. This network provides pathways for carbon diffusion into the underlying alloy, which eventually leads to corrosion.

Oxide scales develop on the outer surface of alloys at high temperatures, creating a protective barrier that keeps destructive carbon-bearing molecules from slipping into the alloy. The diffusion of carbon into oxide scales should be negligible, but studies have shown that carbon can sneak through the oxide line of defense leading to brittleness and corrosion.

Common belief held that carbon-containing molecules escaped into cracks or pores in the oxide scales, but the use of three separate techniques at Argonne user facilities told another story: magnetic force microscopy at the Center for Nanoscale Materials, X-ray diffraction at the Advanced Photon Source, and scanning electron microscopy at the Electron Microscopy Center. Argonne scientists Ken Natesan, Zuotao Zeng, Seth Darling, and Zhonghou Caidiscovered networks of iron and nickel nanoparticles embedded within the oxide scales. Carbon can easily diffuse through the metals and create a path for carbon atom transport that does not involve defects in the scale.

"By examining the oxide scale, we find the metal nanoparticles," Zeng, a CNM user, said. "If they are eliminated we can create a more corrosion-resistant and longer lasting alloy." The findings may have broad influence on not only metal dusting and carburization, but also in other research areas such as alloy development and surface coatings for high-temperature fuel cell applications.

A paper based on this work was published recently in Nature Materials.

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