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

By Quiambao, Kathleen F.; McDonnell, Stephen J.; Schreiber, Daniel K.; Gerard, Angela Y.; Freedy, Keren M.; Lu, Pin; Saal, James E.; Frankel, Gerald S.; Scully, John R.
Published in Acta Materialia 2019

Abstract

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

Read » Back