Exploring the Role of Alloying a Third Element as a Passivator and Chemical Short-Range Ordering Agent in Enhancing Aqueous Passivation of Compositionally Complex Alloys

Compositionally complex alloys (CCAs) are multi-component systems in which each principal element exceeds 5 at.%, often designed to stabilize a single solid solution. The high concentrations and diverse elemental interactions can yield unique synergies that surpass conventional solvent-solute binary alloy design strategies. Exploring this vast compositional and phase space requires efficient and impactful high-throughput experimental methods. In this work, Fe, Co, and Ni-based CCAs containing Al and Cr were investigated using a high-throughput to high-fidelity workflow integrating electrochemical and metallurgical screening techniques. The fate of alloying elements was tracked using novel methods. This approach identified previously unstudied alloy stoichiometries of Al-Cr transition metal alloys with excellent corrosion resistance, comparable to 304L stainless steel. The role of each alloying element towards corrosion resistance was investigated using atomic emission spectroelectrochemistry (AESEC) and x-ray photoelectron spectroscopy, highlighting the synergistic role of Al and Cr in passivation mechanisms. Specifically, in Cr-lean FeCoNi based FCC CCAs, where the challenge lies in attaining passivation below the threshold of 13 at.% Cr, alloying 3 at.% Al with a 10 at.% Cr-containing FeCoNi-based CCA lowered the critical current density required for passivation by an order of magnitude in sulfuric acid solution, where Fe, Co, and Ni do not passivate. The third element, Al, was classified as both a “facilitator” and “passivator", assisting the primary passivator (i.e., Cr) in CCAs by promoting a clustering-type chemical short-range ordering (CSRO) of Cr–Cr pairs in the single-phase alloy. Additionally, Al contributed to the formation of a more stable mixed Cr(III) and Al(III) oxide, which exhibited more protection than Al₂O₃ or Cr₂O₃ due to molecular mixing Al₂O₃-Cr₂O₃ observed after 10 ks. Extending this study to chloride-containing environments confirmed the benefit of alloying Al in improving resistance to localized film breakdown and enhancing passivation in both dilute and concentrated chloride anions. Beyond the Al-Cr synergy observed in FeCoNi-Cr-Al CCAs, the role of Ti as a third element in enhancing the aqueous passivation of BCC Fe-Cr-Al-Ti alloys was demonstrated using surface-sensitive characterization and electrochemical methods. Two solid-solution alloy series: Fe-8Cr-8Al-xTi and Fe-xCr-{16-x}Al-8Ti, where x was varied from 0 to 16 at.%, were investigated in acidified 0.1 M Na₂SO₄ solutions. Alloys with at least 4 at.% Ti with 8 at.% both Cr and Al, or 8 at.% Ti with a Cr/Al ratio greater than 0.5, exhibited excellent corrosion resistance in pH 1 sulfate solution, even comparable to 304L stainless steel containing 20 at.% Cr. A 4 at.% increase in Ti content significantly improved the passivation parameter by an order of magnitude. The improved passivation behavior was attributed to two underlying mechanisms: the promotion of clustering-type CSRO of Cr–Cr pairs from Ti addition and the enhanced stability of mixed oxide species in passive films. Finally, the role of Sn alloying as a third element was also investigated on the aqueous passivation behavior of Cu-Al alloys. Less than 3 at.% of Sn additions to Cu-Al alloys were found to be beneficial in decreasing corrosion rates in a deaerated 0.1 M Na₂SO₄ solution, adjusted to pH 4, by a passivation process of enriching oxides with Al and Sn in the passive film. One of the roles of Sn was categorized as a “facilitator” as it assists the primary passivator Al to passivate by introducing a clustering type CSRO of Al–Al pairs in an FCC lattice. Additionally, Sn may lead to forming an Al(III)-Sn(IV, II) mixed oxide that is more protective than Al(III) or Sn(IV, II) after 300 s. Further, Al and Sn enrichment was confirmed during passive film formation from in-operando AESEC experiments, indicating Sn can also be considered as a secondary minor “passivator”. Two possible governing mechanisms describing the enhanced passivation behaviors are discussed based on passive film stability and the promotion clustering-type CSRO of Al–Al pairs from Sn addition. In summary, the thesis investigates two mechanisms by determining the roles of certain alloying of third elements in enhancing the performance of the primary passivator (e.g. Cr) in corrosion resistant alloys: (i) the introduction of clustering type CSRO of primary passivating atom pairs in the alloy leading to faster passive film formation and/or (ii) the third element acting as a secondary passivator that stabilizes mixed oxides within the passive film. Both mechanisms reduce the classical threshold content of the primary passivator often observed in binary alloys, thereby reducing the dependency on its alloying content.