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

该项目的目的是通过一种新的同位素示踪实验策略，揭示水介质中材料降解过程中的元素迁移现象。直到今天，完全理解钝化合金表面的局部腐蚀仍然是腐蚀科学的一个关键挑战。由于其复杂性，即使在优化的合金中也可能发生的局部钝化击穿机制尚未完全理解，因此仍然是讨论和研究的主题。在高温下，反应物的流动性是另一个需要特别考虑的限制。为了阐明在气体环境中暴露期间的氧传输，存在复杂的实验装置。在将样品暴露于含有天然丰度的氧同位素的气氛之后，在显著富集18 O2的气氛中进行第二次处理。发展的氧化物相，表现出显着的18 O强度只能发生在第二步的曝光，标记的空间位置的持久氧化。18 O在鳞片中的富集为迁移机制提供了直接证据。为了表征水介质中两阶段交换实验后的氧示踪剂分布，将用原子探针层析成像（APT）分析内部氧化物。这种复杂的分析技术将飞行时间质谱仪与特定地点样品体积（~100 x 100 x 100 nm³）的3D、逐个原子重建相结合。高纯二元Ni-xCr（x = 5，20，30 at.%）合金代表了一个合适的模型系统，以探索所提出的研究策略的适用性，这将与相关商业合金600中更复杂的行为进行比较。为随后的表征表面分析技术的样品区域的仔细的特定地点的准备是至关重要的。由此产生的3D原子-原子重建可以揭示亚纳米空间分辨率的元素和同位素分布。拟议的实验策略将可视化氧气运输路径的氧气沿着限制（异质）接口的空间分辨率，不能达到任何其他质谱。此外，氧化物和相邻合金区域的3D元素映射将同时揭示关于活性物质的流动性的看不见的细节，这对于成功定制用于在高腐蚀性环境中持续操作的先进合金组合物至关重要。

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.