High corrosion resistance duplex fcc plus hcp cobalt based entropic alloys: An experimental and theoretical investigation

A series of duplex fcc + hcp Co-based entropic alloys are being discovered as a new category of entropic alloys with outstanding mechanical properties, especially to overcome a typical mechanical trade-off between strength and ductility. In this work, CALPHAD-based (CALculation of PHAse Diagram) thermo-dynamic calculations were performed to facilitate alloy design and to understand corrosion behaviors. The kinetics of the electrochemical corrosion for designed alloys in typical aggressive anion Cl-was inves-tigated by electrochemical tests, including open circuit potential (OCP), polarization and cyclic polariza-tion curves, and electrochemical impedance spectroscopy (EIS). The valence state and the surface morphologies of the passive films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). High corrosion resistance materials with high strength and ductility per-formances were discovered in the present work. Except for Ni-oxides, various spinel compounds and many other oxides including Co2O3, Cr2O3, Fe2O3, MnO, MoO3, CoCr2O4, FeCr2O4, CoFe2O4, and CoMoO4 were observed in the passive films. The adsorbed and penetrated corrosive anion Cl-will be prone to breakdown the passive films with less Cr2O3, CoCr2O4 and MoO3 to form pitting corrosion (also include other localized corrosion, such as intergranular corrosion and crevice corrosion). The microstructure of the hcp martensite with the fcc matrix has played an important role in the propagation of the localized anodic dissolution in the form of cleavage and quasi-cleavage. The theoretical calculations are in good agreement with the experimental observations. This paper paves a way for the future devel-opment of high-performance Co-based entropic alloys served in some harsh environments.(c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

A series of duplex fcc + hcp Co-based entropic alloys with outstanding mechanical properties have been discovered. CALPHAD-based thermo-dynamic calculations and electrochemical tests were conducted to understand the alloy design and corrosion behaviors. High corrosion resistance materials with high strength and ductility were found, and various spinel compounds and oxides were observed in the passive films. The breakdown of passive films due to corrosive anion Cl- could lead to pitting corrosion. The microstructure of the alloys played a crucial role in localized anodic dissolution. The theoretical calculations were consistent with experimental observations. This study paves the way for future development of high-performance Co-based entropic alloys in harsh environments.