PNNL

The Science                                

Alloys used in energy production are often exposed to hostile environmental conditions that cause corrosion and oxidation, degrading the alloy’s structural stability. To function safely, these materials must resist corrosion which is often achieved by slowing the atomic-scale transport of atoms between the environment and bare alloy. In corrosion resistant alloys, nanoscale oxide layers act as an effective barrier to oxidation and atom movement but still have defects that can enable ongoing corrosion. In this study, researchers used a two-stage oxidation process with different oxygen isotopes (¹⁶O and then ¹⁸O) and advanced imaging to identify the specific defects that enable this atomic movement, proving that oxide/metal interfaces are the main defect.

The Impact

Highly stable corrosion and oxidation resistant alloys are necessary to create effective and long-lasting energy generation systems. This work identifies a major mechanism of alloys oxidation that contributes to their degradation, highlighting the key weakness of current materials. With this additional information, researchers can begin identifying mitigation strategies during synthesis and material design.

Summary

Corrosion and oxidation resistant alloys are critical to maintaining the structural stability of components in high-temperature, aggressive environments that are common to energy generating systems. Nanoscale oxide layers that effectively block further movement of oxygen atoms into the metal or metal atoms toward the environment make this corrosion resistance possible. However, all passivating oxide layers contain structural flaws that can facilitate oxygen and metal transport to continue corrosion. Researchers used two different isotopes of oxygen (¹⁶O and ¹⁸O) in combination with advanced microscopy to understand how oxygen penetrates an otherwise highly protective oxide layer in model Ni-Cr alloys. The model alloys were first oxidized in a controlled CO/CO₂ environment at 600°C using ¹⁶O-rich CO₂, forming a protective oxide layer with ~99.8% ¹⁶O. The same material was then further oxidized in the same CO/CO₂ environment, but highly ¹⁸O enriched (~98% ¹⁸O). Using the isotopic sensitivity of atom probe tomography (APT), 3D imaging delineated oxides formed in the ¹⁶O- and ¹⁸O-rich conditions. Analyses proved that very little ¹⁸O diffused into the ¹⁶O-rich oxides; rather the ¹⁸O enriched and diffused along oxide/metal interfaces to form new ¹⁸O-rich oxides. Although this mechanism has long been postulated, these observations are the first to definitively reveal the 3D nature and role of these structural defects in ongoing oxidation through protective oxide layers. This opens the door to new alloy designs that could potentially seek to block these fast diffusion pathways in the future.

PNNL Contact

Daniel Schreiber, Pacific Northwest National Laboratory, Daniel.Schreiber@pnnl.gov

Funding

This work was supported by the Department of Energy (DOE) Office of Science, Basic Energy Sciences Materials Science and Engineering Division, Mechanical Behavior and Radiation Effects program (FWP 56909). MW acknowledges support from the Deutsche Forschungsgemeinschaft for a one-year research fellowship at Pacific Northwest National Laboratory, and support from the EAM, an FAU Competence Center, through Starting Grant EAM-SG21-3 for supplemental data analysis and manuscript preparation. Access to APT instrumentation was provided via a user proposal (10.46936/cpcy.proj.2019.51178/60006716) from EMSL, the Environmental and Molecular Sciences Laboratory, a DOE Office of Science user facility at Pacific Northwest National Laboratory.