Passivation Phenomena in Single Phase High Entropy Alloys: An Evaluation of the Role of Chromium Content in a Ni-Fe-Cr-Mn-Co Alloy
Authors: Gerard Angela Yu , Freedy Keren M , Wang Kang , 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), often 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]. Many unique properties have been reported pertaining to HEAs microstructure, passive films, and material properties[3-6]. With an increase in the degrees of freedom for HEA design, material properties can be regulated including corrosion behavior. In this study, a systematic scientific examination of the effect of Cr composition on passivity was undertaken.
A series of single solid solution phase Ni38Fe20Crx(MnCo)42-x HEAs was synthesized with varying chromium percent (x = 22, 14, 10, 6) and balanced by changes in the manganese and cobalt contents. All Ni38Fe20Crx(MnCo)42-x HEAs formed a single solid solution phase in a solutionized and quenched condition with a FCC crystal structure. The HEAs were compared to Fe-Cr and Ni-Cr binary alloys.
The E-log(i) behavior of all alloys was obtained in a chloride environment. Potentiodynamic polarization curves under deaerated conditions enabled the identification of the passive current density, primary passive potential, pitting potential, and repassivation potential. Passive films were electrochemically grown on sample surfaces utilizing a potential hold for 10ks within the passive region (0 VSCE). During passive film growth a single frequency sinusoidal AC potential perturbation of 20mV-rms at 1 Hz was superimposed on the DC potential hold scan to provide single frequency electrochemical impedance spectroscopy (SF-EIS) data that provides an in-situ method to monitor oxide thickness[7]. X-ray photoelectron spectroscopy was utilized to characterize the composition of the electrochemically grown passive film and the composition of alloy just below the oxide. Oxide characteristics were elucidated and XPS data was compared to thermodynamic predictions. Both thermodynamic and kinetic factors where considered to develop a better understanding about the origins of excellent corrosion resistance as a function of Cr content.

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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