Oxygen tracer exchange experiments combined with atom probe tomography to understand internal oxidation of advanced Ni-base alloys in high-temperature water

• Applicant: Dr. Martin Weiser
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: from 2019 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 435243707

The aim of the project is to reveal elementary transport phenomena during material degradation in aqueous media through a novel, isotopic tracer experimental strategy. Until today, complete understanding localized attack on passivity alloy surfaces remains a key challenge in corrosion science. Due to their complexity, mechanisms of local passivity breakdown that can occur even in optimized alloys are not fully understood and therefore remain a subject of discussion and research. At elevated temperatures, the mobility of reactants is another constraint that requires particular consideration. For the elucidation of oxygen transport during exposures in gaseous environments, a sophisticated experimental set-up exists. After exposing samples to atmospheres that contain the natural abundance of oxygen isotopes, a second treatment is conducted in an atmosphere that is considerably enriched in 18O2. The development of oxide phases that demonstrate significant 18O intensities could only occur during the second step of exposure, marking the spatial locations of persistent oxidation. Enrichment of 18O in the scale provides direct evidence for transport mechanisms. To characterize oxygen tracer distribution after two-stage exchange experiments in aqueous media, the internal oxides will be analyzed with atom probe tomography (APT). The sophisticated analysis technique combines a time-of-flight mass spectrometer with a 3 D, atom-by-atom reconstruction of site-specific sample volumes (~100 x 100 x 100 nm³). High-purity binary Ni-xCr (x = 5, 20, 30 at.%) alloys represent a suitable model system to explore the applicability of the proposed research strategy, which will be compared with more complex behaviors in the related commercial Alloy 600. The careful site-specific preparation of sample regions for subsequent characterization by surface analytical techniques is of crucial importance. The resulting 3 D atom-by-atom reconstruction can reveal both elemental and isotopic distributions with sub-nanometer spatial resolution. The proposed experimental strategy will visualize oxygen transport paths of oxygen along confined (heterogenous) interfaces with a spatial resolution that cannot be reached by any other mass spectrometry. Additionally, 3 D elemental mappings of oxides and adjacent alloy regions will simultaneously reveal unseen details on the mobility of reactive species that is of crucial importance for the successful tailoring of advanced alloy compositions for sustained operation in highly corrosive environments.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Daniel Schreiber