Potential-pH diagrams considering complex oxide solution phases for understanding aqueous corrosion of multi-principal element alloys

Abstract

The potential-pH diagram, a graphical representation of the thermodynamically predominant reaction products in aqueous corrosion, is originally proposed for the corrosion of pure metals. The original approach only leads to stoichiometric oxides and hydroxides as the oxidation products. However, numerous experiments show that non-stoichiometric oxide scales are prevalent in the aqueous corrosion of alloys. In the present study, a room temperature potential-pH diagram considering oxide solid solutions, as a generalization of the traditional potential-pH diagram with stoichiometric oxides, is constructed for an FCC single-phase multi-principal element alloy (MPEA) based on the CALculation of PHAse Diagram method. The predominant reaction products, the ions in aqueous solution, and the cation distribution in oxides are predicted. The oxide solid solution is stabilized by the mixing free energy (or mixing entropy) and the stabilizing effect becomes more significant as the temperature increases. Consequently, solid solution oxides are stable in large regions of the potential-pH diagram and the mixing free energy mostly affects the equilibrium composition of the stable oxides, while the shape of stable regions for oxides is mostly determined by the structure of the stable oxides. Agreements are found for Ni2+, Fe2+, and Mn2+ between the atomic emission spectroelectrochemistry measurements and thermodynamic calculations, while deviations exist for Cr3+ and Co2+ possibly due to surface complexation with species such as Cl− and the oxide dissolution. By incorporating the solution models of oxides, the current work presents a general and more accurate way to analyze the reaction products during aqueous corrosion of MPEAs.

Thermodynamic descriptors to predict oxide formation in aqueous solutions

“…The thickness h Ni(OH) 2(s) starts at ≈ 5 nm, demonstrating fast initial growth, but plateaus during further oxidation potential change. This and other past literature utilized model Pourbaix diagrams and interpreted this phase evolution using ∆ f G values, but could not fully capture the pHdependent and depth-dependent scale formation [4,5,28].…”

Recent Advances in Corrosion Science Applicable To Disposal of High-Level Nuclear Waste

“…Many spinels (i.e., AB 2 O 4 structure, where A and B are two of the elements in the MPEA) can also form, but the availability of cations in the proper stoichiometric ratios is often limited. 391 Therefore, spinels do not seem to provide an explanation for good passivity. Other possibilities exist such as the formation of complex oxide solute solutions with atoms occupying the structure of a particular host atom or an entirely new oxide lattice structure containing multiple elements.…”

“…Other possibilities exist such as the formation of complex oxide solute solutions with atoms occupying the structure of a particular host atom or an entirely new oxide lattice structure containing multiple elements. 391 Lastly, there is the possibility of the formation of phase-separated stoichiometric oxides, perhaps in layers or islands. Phase separation is more likely when approaching equilibrium conditions and when the oxides formed are insoluble.…”

“…This cooperative effect can render an MPEA passive over a broad range of conditions, which is an improvement over a binary or ternary alloy with one or two oxides that are stable over a limited range in E and pH. Many spinels (i.e., AB 2 O 4 structure, where A and B are two of the elements in the MPEA) can also form, but the availability of cations in the proper stoichiometric ratios is often limited . Therefore, spinels do not seem to provide an explanation for good passivity.…”

“…Therefore, spinels do not seem to provide an explanation for good passivity. Other possibilities exist such as the formation of complex oxide solute solutions with atoms occupying the structure of a particular host atom or an entirely new oxide lattice structure containing multiple elements . Lastly, there is the possibility of the formation of phase-separated stoichiometric oxides, perhaps in layers or islands.…”