Passivation Phenomena in Single Phase High Entropy Alloys: The Evolution of Oxide Composition in Chloride
Authors: Gerard Angela Yu , Freedy Keren M , Wang Kang , HAN Junsoo , Zhou Bi-Cheng , McDonnell Stephen , Schreiber Dan , Saal James , Lu Pin , Sahu Sarita , Li Tianshu , Windl Wolfgang , Frankel Gerald S. , Scully John R. 
Published: February 2020

High entropy alloys (HEAs), sometimes known more broadly as compositionally complex alloys (CCAs) are emerging materials that have recently attracted great interest due to their potentially attractive properties. This class of alloys deviates from traditional alloys by containing five or more alloying elements which results in a high entropy of mixing promoting the formation of a solid solution alloy [1-3]. Initial electrochemical testing and characterization of HEAs revealed excellent passivity and resistance to local corrosion[3-6]. For some HEAs, a non-stoichiometric solid solution oxide forms during fast passivation. In this case, all alloying elements may be oxidized if the passivation potential is far from equilibrium. Also, some elements will dissolve preferentially at the oxide/electrolyte interface leading to an enrichment of other alloying elements in the passivating oxide[4].
The focus of this study is to understand the evolution of the electrochemically grown oxide formed on solid solution HEAs: Ni38Fe20Cr21Ru13Mo6W2 and Ni38Fe20Cr22Mn10Co10 within a slightly acidic Cl- environment. Results are compared to a conventional Ni-20Cr binary alloy. This study attempts to establish the fate of the alloying elements with respect to the outer and inner layers of the oxide, whether enriched or depleted at the oxide metal interface, or preferentially dissolved into solution.
The E-log(i) behavior was first characterized on the HEA within a chloride environment enabling the identification of the passive current density, pitting potential, and repassivation potential. Passive films were electrochemically grown on alloy surfaces utilizing a potential hold for times ranging from 102 to 105 seconds within the passive region. In-situ analysis was conducted using AESEC, AC as well as DC electrochemistry methods to monitor passive current density, oxide thickness, and cation dissolution into solution. Ex-situ analysis was utilized to characterize the passive film and consisted of the following: 3D-atom probe tomography and X-ray photoelectron spectroscopy. Results are compared against theories such as non-equilibrium solute capture compared to thermodynamic expectations as well as enrichment models such as those proposed by Grimal and Marcus, Kirchheim, and others[7-9].

Acknowledgements

This work was supported as part of the Center of Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584

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