From Alloy to Oxide: Capturing the Early Stages of Oxidation on Ni–Cr(100) Alloys

Abstract

The interaction of oxygen with Ni−Cr(100) alloy surfaces is studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to observe the initial steps of oxidation and formation of the alloy−oxide interface. The progression of oxidation was observed for Ni(100) and Ni−Cr(100) thin films including Ni−8 wt % Cr(100) and Ni−12 wt % Cr(100), which were grown on MgO(100) in situ. These surfaces were exposed to between 1 and 150 L O 2 at 500 °C, and additional annealing steps were performed at 500 and 600 °C. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and band gap distributions. Initial NiO nucleation and growth begins along the step edges of the Ni−Cr alloy accompanied by the formation of small oxide particles on the terraces. The incubation period known in oxidation of Ni( 100) is absent on Ni−Cr alloy surfaces illustrating the significant changes in surface chemistry triggered by Cr-alloying.Step edge faceting is initiated by oxide decoration along the step edges and is expressed as moirépatterns in the STM images. The surface oxide can be characterized by NiONi(6 × 7) and NiO−Ni(7 × 8) coincidence lattices, which have a cube-on-cube epitaxial relationship. Small patches of NiO are susceptible to reduction during annealing; however, additional oxide coverage stabilizes the NiO. NiO regions are interspersed with areas covered predominantly with a novel cross-type reconstruction, which is interpreted tentatively as a Cr-rich, phase-separated region. Statistical analysis of the geometric features of the surface oxide including step edge heights, and NiO wedge angles illustrates the layer-by-layer growth mode of NiO in this pre-Cabrera−Mott regime, and the restructuring of the alloy−oxide interface during the oxidation process. This experimental approach has offered greater insight into the progression of oxide growth in Ni−Cr thin films and underscores the dramatic impact of alloying on oxidation process in the pre-Cabrera−Mott regime.

Identifying heating rate dependent oxidation reactions on a nickel-based superalloy using synchrotron diffraction

“…3. This was in agreement with studies on the early stages of oxidation on binary alloys where it was shown that the first oxides formed were nickel, nucleated as islands distributed across the surface of the specimen [4][5][6][7]. It has also been shown that a metastable chromium-oxygen phase formed on the surface of binary alloys, surrounding the NiO islands [5][6][7][8][9].…”

“…These processes have been studied for some pure metals and simple alloy systems using exacting experimentation techniques; e.g. [1][2][3][4][5][6][7][8][9]. These studies involved monitoring the changes occurring to clean metallic surfaces when exposed to oxidising environments, using a number of high spatial resolution techniques.…”

“…Ni -Cr binary alloys [5][6][7], demonstrated the formation of NiO islands interspersed with a Cr complex; when at very early stages of Cr 2 O 3 formation. The studies all show that the nucleation and growth rates depend on a number of factors including crystal orientation at the exposed surface of the metal [2,3,4,5,8] and alloying additions [5][6][7][8][9]. These processes may occur rapidly in time periods of a few minutes or may extend to 90 min90 minutes.…”

Rapid assessment of structural and compositional changes during early stages of zirconium alloy oxidation

“…Hence, a comprehensive understanding of oxidation mechanisms is necessary to evaluate applicability of a material in a desired environment. Characterising one or more oxides formed during the initial stages of oxidation is a significant challenge because oxidation of alloying elements is simultaneous with the nucleation and growth of metastable phases 10,11 . Further, the composition across buried oxide/metal interfaces can also change, often heterogeneously, depending on the crystallographic orientation of the base metal grains below 6 , requiring a spatially resolved analysis of such changes.…”

The role of Cr atom in the early steam oxidation of Fe‐based alloys: An atomistic simulation

“…This is due to the experimental difficulties connected to the timescale of the early oxidation of clean alloy surfaces. [30,31] In addition, with the increase of the main steam parameters, the corrosion of superheater and reheater becomes more serious. [2] To mitigate the overall corrosion of the power plant, water chemistry-related species, such as oxygen, have been added to feedwater.…”