Aqueous Passivation of Multi-Principal Element Alloy Ni38Fe20Cr22Mn10Co10: Unexpected High Cr Enrichment within the Passive Film

Abstract The aqueous passivation of a non-equiatomic Ni38Fe20Cr22Mn10Co10 - at. % (Ni40Fe20Cr20Mn10Co10 - wt. %) multi-principal element alloy (MPEA) was investigated in 0.1 M NaCl at pH 4 and compared to a conventional binary Ni76Cr24 – at. % (Ni78Cr22 – wt. %) alloy. The electrochemical behavior and oxide film characteristics were explored utilizing in-situ electrochemical and ex-situ surface-sensitive techniques. The passive film composition, thickness, and elemental valence states, and fate of each element were studied by in-situ atomic emission spectro-electrochemistry, ex-situ X-ray photoelectron spectroscopy, and atom probe tomography. The Ni38Fe20Cr22Mn10Co10 MPEA demonstrated slightly better corrosion resistance compared to the binary Ni76Cr24, alloy. Passive films on the MPEA contained primarily Cr, and small amounts of Ni, Fe and Mn, while dissolution of Ni, Fe, Co was observed. Ni0 was enriched at the oxide/metal interface while Cr was depleted. Enrichment of Cr in the passive film occurred to a greater extent in the MPEA than for the Ni-Cr binary alloy. Enrichment factors were determined and the origins of enrichment are discussed.