Exploring Lightweight Element Roles in Aqueous Passivation Through the Study of (Fe, Co, Ni)90-x-Cr10-(Al, Si, Ti)x Ternary Alloy Families

Compositionally complex alloy (CCA) design allows for the co-utilization of many elements with differing properties within a single-phase microstructure through entropic stabilization. Such a design philosophy has opened the door toward the multi-objective optimization of beneficial properties such as corrosion resistance and lightweighting by including different elements (e.g., Cr and Al, respectively) which influence seemingly disparate properties simultaneously. However, despite evidence being presented that such a design strategy is effective, a dearth of studies on simpler systems have limited CCA designers’ ability to ascertain relationships between elemental composition and reported performance benefits due to the inherent complexity of CCA systems. To address this lack in the current literature base, this work reports the phase stability and electrochemical behavior of ternary combinations of common matrix elements (M) Fe, Co, and Ni, the primarily passivating Cr, and relevant lightweighting elements (LWE) Al, Si, and Ti, within a harsh sulfuric acid environment. The resulting alloy set (stylized as M90-xCr10LWEx) indicates that relative to relevant M90Cr10 binary alloys, increasing LWE content improves passivation efficiency (represented by the h-value) and the low-frequency impedance of potentiostatically-grown passive films. Among LWE species, Ti is noted to have the greatest per-at.% effect, with Fe86Cr10Ti4 reflecting an approximately 1000x improvement in passive film impedance relative to Fe90Cr10, for instance. Furthermore, a percolation theory analysis on variable Cr Fe100-xCrx and Fe96-xCrxTi4 alloys indicate that Ti’s incorporation reduces the critical bulk Cr concentration by over 3 at.%, offering a science-based path toward the optimization of electrochemical properties. Finally, novel "effective Cr" and previously established h-value calculations are presented as robust and consistent ways of quantitatively the benefits of ranking third-element additions and overall corrosion resistance, respectively.