The role of chromium content in aqueous passivation of a non-equiatomic Ni38Fe20CrxMn21-0.5xCo21-0.5x multi-principal element alloy (x = 22, 14, 10, 6 at%) in acidic chloride solution
Angela Y. Gerard, Elizabeth J. Kautz, Daniel K. Schreiber, Junsoo Han, Stephen McDonnell, Kévin Ogle, Pin Lu, James E. Saal, G. S. Frankel, John R. Scully
Abstract
The effect of the Cr content on the corrosion behavior in a series of single-phase non-equiatomic Ni38Fe20CrxMn21-0.5xCo21-0.5x (6 < x < 22 at%) multi-principal element alloys (MPEAs) was investigated in acidified NaCl solutions. Comparisons were made with binary solid solution Co-Cr, Ni-Cr, and Fe-Cr alloys over a similar range of Cr contents. The corrosion behavior was evaluated using in-situ AC and DC electrochemical methods and ex-situ surface sensitive characterization techniques. Passivity and various levels of local corrosion resistance were obtained in the MPEA with 10 at% Cr or above. The binary Ni-Cr alloys with 12–30 at% Cr behaved similarly. The MPEA with 6 at% Cr and binary alloys with 5, 6, or 10 at% Cr were marginally passive or active and underwent localized corrosion during both linear sweep voltammetry and potentiostatic hold experiments. Passive films formed during potentiostatic hold experiments were characterized with X-ray photoelectron spectroscopy and atom probe tomography. Cr cation enrichment in the passive films was observed for all alloys and similar enrichment factors were obtained as a function of Cr content regardless of whether MPEA or binary alloy except for the Fe-Cr alloy. Moreover, passive current density was correlated with the Cr cation fraction in the passive film. The degree of Cr enrichment was attributed to a combination of thermodynamic and kinetic factors, such as selective chemical dissolution of alloying elements as well as limitation due to solute depletion at the metal oxide interface in alloys with a bulk Cr content ≤ 10 at%.