Corrosion of Ni-Fe-Cr-Mo-W-X Multi-Principal Element Alloys

Abstract

An extraordinarily corrosion-resistant non-equimolar high entropy alloy with the composition Ni 38 Fe 20 Cr 21 Mo 6 W 2 Ru 13 (referred to here as MPEA1) has been reported. Ruthenium is one important reason why the alloy exhibits excellent passivity and resistance to localized corrosion in a variety of environments. The goal of this study was to replace Ru in MPEA1 with commodity elements such as Mn, Al and Cu while minimizing the penalty on corrosion performance. Thermodynamic calculations were performed to survey the phase diagram for a stable disordered face centered cubic (fcc) phase at elevated temperatures. Five test compositions analogous to MPEA1 were conceptualized based on this design strategy. In two of these candidate alloys, the Ru content was substituted wholly by additional Fe or Ni. The other three contain Mn, Al, or Cu, respectively. Corrosion performance and passive behavior of these alloys in 0.6 M NaCl and 6 M HCl were evaluated using cyclic potentiodynamic polarization and single-frequency impedance experiments. Solutionized forms of all five alloys were found to resist localized corrosion in 0.6 M NaCl. Despite forming oxide films of similar thickness in 0.6 M NaCl solutions, X-ray photoelectron spectroscopy revealed non-congruent dissolution of constituent elements. The role of alloying elements in influencing the oxide formation process was indicated.