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.
Highlights
Degradation by aqueous corrosion is ubiquitous for materials under various environments, causing significant damage to structural and functional engineered components in most industrial sectors[1,2]
To construct the E-pH diagrams by calculation, independent reactions involving solid and aqueous products are listed with their reaction thermodynamics, and the equilibrium of reactions correspond to the boundaries between predominant species
We investigated the predominant reactions in heterogeneous interaction systems between 10−2 mol of FCC
Summary
Degradation by aqueous corrosion is ubiquitous for materials under various environments, causing significant damage to structural and functional engineered components in most industrial sectors[1,2]. To construct the E-pH diagrams by calculation, independent reactions involving solid and aqueous products are listed with their reaction thermodynamics, and the equilibrium of reactions correspond to the boundaries between predominant species Such diagrams cannot be applied to predict the dissolution of an alloy composed of multiple metallic elements[6]. We investigated the predominant reactions in heterogeneous interaction systems between 10−2 mol of FCC transformations from experiments This enables optimization of the corresponding thermodynamic parameters so that an single-phase MPEA Ni38Fe20Cr22Mn10Co10 (at.%, denoted as Cr22 hereafter)[37] and 1 kg of water containing 0.6 mol NaCl at 25 °C and analytical expression of the Gibbs energy as a function of temperature and composition can be obtained[18].
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