Passivation of a corrosion resistant high entropy alloy in non-oxidizing sulfate solutions

The passivation behavior of a novel high entropy alloy (HEA) of composition 38Ni-21Cr-20Fe-13Ru-6Mo-2W at.% (33Ni-16Cr-17Fe-19Ru-9Mo-6W wt.%) was investigated in sulfate solutions of various pH levels. The HEA was compared to a commercially available Ni-Cr-Mo-W-Fe alloy, C-22. Experiments were conducted using in-situ electrochemical and ex-situ surface-sensitive methods to probe the growth and dissolution of the passive layer. The HEA exhibited excellent corrosion resistance in highly acidic and highly alkaline solutions, maintaining passivity through a broad range of potentials below the Cr transpassivity range. Extremely rapid and efficient oxide formation and lower quasi-steady passive current densities were observed during oxide growth in the passive range compared to commercial alloy C-22. Ex-situ characterization of the passive film by atom probe tomography and X-ray photoelectron spectroscopy provided insight into the oxide composition, thickness, and elemental valence states of potentiostatically-grown and air-formed oxides. All elements in the alloy were oxidized following potentiostatic oxide growth. Instead of distinct stoichiometric compounds (i.e. with integer cation ratios) and oxide phases, a non-stoichiometric oxide solid solution was observed with significant enrichment in Cr and Ru, as well as depletion of Fe and Ni. The behavior of Cr was modeled with a modified surface enrichment model. The oxide thickness was estimated to be 3-4 nm thick. The connection between the solid solution oxide, enrichment of Cr in the passive layer, the presence of Mo and W, and excellent passivity is discussed. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.