Nonequilibrium Solute Capture in Passivating Oxide Films

Abstract

If all humans vanished tomorrow, almost every metal structure would collapse within a century or less, the metal converting to an oxide. In applications ranging from the mature technology of nuts and bolts to high technology batteries, nuclear fuels and turbine engines, protective oxide films are critical to limiting oxidation. To date models of these oxide films have assumed that they form thermodynamic equilibrium stable or metastable phases doped within thermodynamic solubility limits. Here we demonstrate experimentally and theoretically the formation of unusual nonequilibrium oxide phases, that can be predicted using a scientific framework for solute capture at a moving oxide/substrate interface. The theory shows that solute capture is likely a generic process for many electrochemical processes, and suggests that similar phenomena yielding nonequilibrium phases can occur and be predicted for a wide range of other processes involving solidfluid and solid-solid chemical reactions.

In Situ Observations of Early Stage Oxidation of Ni-Cr and Ni-Cr-Mo Alloys

“…With intermediate velocities as in the experiments herein, solute atoms are captured in the oxide with compositions far in excess of the solubility limits. Further details can be found in Yu, et al 31 Several points merit further discussion and analysis. The first is the presence of Kirkendall voids which are less probable in the presence of Mo.…”

“…Because of technical limitations in detecting the distribution of this low of a concentration of Mo (∼3 at%) in the samples by electron microcopy, atom probe tomography was used and it was confirmed elsewhere that Mo was also captured in the initial oxides formed during the early-stage oxidation/corrosion process. 31 To briefly clarify why solute atoms are present far beyond conventional solubility limits, the authors have an alloy that is oxidizing with a moving oxidation front that combines point defect migration and physical motion of the interface. The interface therefore has an effective velocity proportional to the rate of incorporation of alloy atoms as cations in the oxide.…”

“…This indicates that the kinetic configuration/composition dominates during early-stage oxidation where the solute atoms are captured in the initial oxide. 31 From the line-scan profile of the Ni-Cr-Mo alloy,…”

“…The metastable phase is a consequence of non-equilibrium solute capture during formation of the oxide as discussed elsewhere in more detail. 31 The experimental evidence indicates that Mo stabilizes individual cation vacancies by promoting the formation of a corundum phase, which is analyzed in terms of valence accommodation by the Mo and with specific density functional theory (DFT) calculations.…”

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

“…In alloys with sufficient aluminium NiAl 2 O 4 spinels have been shown to form [21][22][23][24][25][26]. At lower temperatures, and on alloys with low concentrations of aluminium, NiCr 2 O 4 would be the preferred composition [27][28][29][30][31][32][33]. Spinels can form by a solid state reaction between adjacent oxides or oxide and alloy [27][28][29][30][31].…”