Corrosion of single-phase Ni-Fe-Cr-Mo-W-X non-equimolar multi-principal element alloys

The unique corrosion characteristics of a multi-principal element alloy with the composition Ni38Fe20Cr21Mo6W2Ru13 (referred to as MPEA1) have been reported. Ruthenium, present in the alloy ~20% by weight, is believed to play a key role arresting localized corrosion of alloy in harsh Cl-containing solutions. In this research, Ru in MPEA1 was substituted with commodity elements such as Mn, Al and Cu with a view to lower the cost of the alloy while causing a minimal drop in the corrosion resistance. Based on thermodynamic predictions, five candidate compositions analogous to MPEA1 were conceptualized and fabricated using vacuum arc melting. The solutionized, single-phase versions of all five candidate alloys were found to resist localized corrosion in 0.6 M NaCl at ambient temperature. While all five alloys show minimal hysteresis during cyclic potentiodynamic polarization experiments, one alloy that contains Mn exhibited metastable current transients but did not pit. Despite forming oxide films of similar thickness and quality as evaluated using single-frequency impedance methods, X-ray photoelectron spectroscopy (XPS) revealed constituent elements to dissolve in a non-congruent fashion. A clear Cr enrichment was not evident after potentiostatic polarization intended to passivate the alloys. The role of constituent elements in influencing the oxide formation process was indicated by a quantitative treatment of the XPS data.