Atomic origins of water-vapour-promoted alloy oxidation

Abstract

The presence of water vapour, intentional or unavoidable, is crucial to many materials applications, such as in steam generators, turbine engines, fuel cells, catalysts and corrosion. Phenomenologically, water vapour has been noted to accelerate oxidation of metals and alloys. However, the atomistic mechanisms behind such oxidation remain elusive. Through direct in situ atomic-scale transmission electron microscopy observations and density functional theory calculations, we reveal that water-vapour-enhanced oxidation of a nickel-chromium alloy is associated with proton-dissolution-promoted formation, migration, and clustering of both cation and anion vacancies. Protons derived from water dissociation can occupy interstitial positions in the oxide lattice, consequently lowering vacancy formation energy and decreasing the diffusion barrier of both cations and anions, which leads to enhanced oxidation in moist environments at elevated temperatures. This work provides insights into water-vapour-enhanced alloy oxidation and has significant implications in other material and chemical processes involving water vapour, such as corrosion, heterogeneous catalysis and ionic conduction.

Surface-reaction induced structural oscillations in the subsurface

“…Transmission electron microscopy (TEM) is not subject to such limitations and offers the opportunity to study the oxide by providing precise information on the atomic scale for both the surface and subsurface. Particularly, TEM has dramatically evolved in recent years and now allows for temperature-resolved, pressureresolved, and time-resolved study of the reaction dynamics by flowing a reactive gas in the sample area while simultaneously probing atomic structural evolution from the outermost surface layer to deeper atomic layers [11][12][13][14][15][16][17][18][19] . Such an ability to capture the dynamics of structural evolution under reaction conditions is particularly important for understanding catalytic reactions because the state of the surface and subsurface of a catalyst is highly dynamic during its interaction with the surrounding.…”

Deciphering atomistic mechanisms of the gas-solid interfacial reaction during alloy oxidation

“…2D). It is well known that H can easily diffuse through oxides via interstitial sites with a small diffusion barrier (29,30); hence, we usually do not expect H to aggregate at specific sites. As shown by the simulation results, the interfacial enrichment of H indicates that the oxide-alloy interface acts as an energetically favorable "sink" for H ions.…”

“…Pure oxygen (~99.999%) was introduced into the TEM column through a leak valve to oxidize the thin films at a given temperature and pressure. For oxidation in water vapor, the water vapor was generated and delivered to the ETEM column by a water vapor system (29) developed in-house. The gas pressure can be adjusted from 1 × 10 −8 to 15 mbar.…”

Oxidative Originators of Graphene Barrier Coating Grown on Surfaces

“…Which has important implications for the graphene application as a protective coating. [50][51][52][53][54][55][56] Figure 2e depicts the long-term corrosion mechanisms of Cu with graphene coating. [50] As illustrated, the negative polarity of graphene to copper induces to the electrochemical corrosion of the Cu substrate over time, while the bare Cu would not form such a corrosion of galvanic circulate.…”