Early-stage NiCrMo oxidation revealed by cryo-transmission electron microscopy

Hydroxide formation at the surface of corroded alloys is critical for understanding early-stage oxidation of many corrosion-resistant alloys. Many hydroxides are unstable in an ambient environment and are electron-beam sensitive, limiting the use of conventionally-prepared specimens for transmission electron microscopy characterization of these alloy-water interfaces. In order to avoid sample dehydration, NiCrMo alloys corroded in a Cl^--containing electrolyte solution were cryo-immobilized by plunge freezing. A cryo-focused ion beam microscope was used to thin the sample to electron transparency, while preserving the alloy-water interface, and the sample was then cryo-transferred to a transmission electron microscope for imaging and diffraction. The presence of rocksalt Ni_1-xCr_2x/3O and β-Ni_1-xCr_2x/3(OH)₂ phases and their orientational relationship to the underlying alloy were observed with electron diffraction, confirming the preservation of the surface structure through the fully-cryogenic sample preparation and analysis.