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The effect of capillarity on the chemical equilibrium between a dissolved metal cation and the corresponding metal oxide is determined within the framework of Gibbssian thermodynamics. We examined the equilibrium between Cr³⁺and Cr₂O₃and found that for 10⁻⁶M Cr³⁺in the electrolyte and a curvature of −2 × 10⁹m⁻¹, the equilibrium pH is −1.8. The corresponding potential-pH diagram shows that chromium passivates in strong acids. This analysis potentially resolves a long-standing issue in corrosion science. 

Abstract Elemental partitioning during thermal processing can significantly affect the corrosion resistance of bulk alloys operating in aggressive electrochemical environments, for which, despite decades of experimental and theoretical studies, the thermodynamic and electrochemical mechanisms still lack accurate quantitative descriptions. Here, we formulate an ab initio thermodynamic model to obtain the composition- and temperature-dependent free energies of formation (Δ_fG) for Ni–Cr alloys, a prototypical group of corrosion-resistant metals, and discover two equilibrium states that produce the driving forces for the elemental partitioning in Ni–Cr. The results are in quantitative agreement with the experimental studies on the thermodynamic stability of Ni–Cr. We further construct electrochemical (potential–pH) diagrams by obtaining the required Δ_fGvalues of native oxides and (oxy)hydroxides using high-fidelity ab-initio calculations that include exact electronic exchange and phononic contributions. We then analyze the passivation and electrochemical trends of Ni–Cr alloys, which closely explain various oxide-film growth and corrosion behaviors observed on alloy surfaces. We finally determine the optimal Cr content range of 14–34 at%, which provides the Ni–Cr alloys with both the preferred heat-treatment stability and superior corrosion resistance. We conclude by discussing the consequences of these findings on other Ni–Cr alloys with more complex additives, which can guide the further optimization of industrial Ni–Cr-based alloys. 

We present some general concepts and pose questions connected to the difference between the ambient temperature passive film formed on elemental aluminum in acid vs that which forms on iron-aluminum alloys containing less than about 35–40 at% Al. Data is presented which demonstrates that the non-protective oxide that forms on aluminum is not related to impurity effects, either in the matrix or in grain boundaries.We argue that the ability of aluminum to form a protective passive film in a single-phase solid solution alloy is connected to atomic-scale size effects that vanish once the aluminum concentration increases to about 60 at%.