Abstract
The realization of superior corrosion-resistant alloys requires a detailed understanding of surface oxidation to mitigate the impact of corrosion. To this end, we have studied the nanoscale evolution of surface oxides prior to the formation of a complete layer. The oxidation of Ni(100), Ni(111), and Ni-Cr(100), Ni-Cr(111) surfaces was captured by sequential oxidization and measured with scanning tunneling microscopy/spectroscopy (STM/STS) in situ. Alloy thin films (8-18 wt.% Cr) were prepared on MgO(100/111) and exposed to oxygen up to 400 L at 773 K. NiO was found to undergo anisotropic growth and drive step-edge faceting on Ni-Cr(100), while NiO initiated across the terraces on the Ni-Cr(111) samples. On both surfaces the chromia initially nucleated as flat disks and eventually coalesced as large nodules. Several novel Cr(100)-O surface reconstructions were observed on the FCC Ni-Cr(100) surface, indicating surface segregation and phase separation of BCC Cr. Each aforementioned surface oxide presented a unique electronic signature, and STS maps were used to spatially resolve and assess local fluctuations in the DOS.
To understand how the presence of grain boundaries and crystallography affect the initial stages of oxidation, a polycrystalline Ni-22wt.%Cr alloy was oxidized under UHV conditions. The surface was exposed to O2 and studied as a function of time and crystallographic orientation in operando with a Spectroscopic Photoelectron and Low Energy Electron Microscope, which can also perform XAS and XPS measurements in the valence band and core level regime. The oxidation across (104) and (212) alloy grains was simultaneously acquired with 25×25 nm2 resolution by monitoring the XAS Cr-L-edge peak during a cumulative 65 L O2 exposure at 773 K. Initial changes in image contrast showed anisotropic surface oxide growth, and as oxidation continued each grain became saturated with oxide at different rates. The grain boundary influenced the oxide growth on the (104) grain, while the (212) grain was unperturbed and oxidation proceeded more uniformly. After further oxidation, chromia nodules appeared across the (104) grain, their area growing linearly with time. These experimental results, when combined with computational level-set image characterization and DFT, underscore the importance of electronic heterogeneity and crystallographic orientation in driving kinetic behavior during surface oxide growth within the pre-Cabrera-Mott regime.
The corrosion of binary Ni-Cr and ternary Ni-Cr-Mo alloys was considered under a variety of different electrochemical conditions and their surfaces measured ex-situ with AFM. The data analysis was informed by in operando single-frequency electrochemical impedance spectroscopy (SF-EIS) measurements, taken by the Scully group (UVA), on the electrochemically grown passive film in either a chloride or sulfate solution as a function of time. Our results found that oxide nucleation occurs in the form of islands, which form on newly passivating surfaces with seconds of the establishment of an electrochemical potential driving force favoring oxidation of both Cr and Ni. Oxides tend to grow first as islands and coalesce to form nm-scale coverages on metallic surfaces consistent with the sharp decrease in oxidation rates commensurate with thin film-field driven passivation. At the same time, it is found that chemical driving forces favor simultaneous oxide dissolution, especially in the case of Ni2+ oxides. The balance between these two processes is argued to govern morphologies and thickness evolution. Roughness and topographical heterogeneity evolve with time and depend critically on anion identity. Cl- is shown to enhance the spread in oxide island radii, heights, and Cr/Ni ratio compared to SO42-. The possible impacts of morphology and chemical composition on oxide instability and breakdown are discussed.
