Elemental partitioning and corrosion resistance of Ni-Cr alloys revealed by accurate ab-initio thermodynamic and electrochemical calculations

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 (Delta 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 Delta fG values 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.