Thermodynamics of solute capture during the oxidation of multicomponent metals

In the classical theories of oxidation of metals it is assumed that the interface between the oxide and metal is in thermodynamic equilibrium. However, in many cases this is not true, the oxide grows too fast or the fluxes through the interface are too large for local interfacial equilibrium to exist, leading to nonequilibrium solute capture. We present a thermodynamic analysis using both an available database as well as density functional theory calculations of the thermodynamic conditions for this during the oxidation of Ni–Cr alloys. The analysis indicates that nickel atoms can be captured in the rocksalt or corundum crystallographies for a very wide range of compositions, consistent with recent experimental observations. The density functional theory analysis also provides information about the electronic structure of these oxides which is important to understand their properties, and also indicates that interpretation of spectroscopic data is not simple as mixed valence states as well as Cr⁴⁺ can occur under oxidizing conditions. We point out that across at least the first transition row of elements the thermodynamic conditions for nonequilibrium solute capture can easily be met.