Understanding Microstructural Phase Evolution, Compositional Partitioning, Passivation, and Corrosion Resistance of Dual-Phase Complex Concentrated Alloys

Complex Concentrated Alloys (CCAs), alloys which contain four or more elements at concentrations above 5 at. % are evaluated for their possible contributions to corrosion-resistant alloy design. Possible benefits of the alloy class including unique distributions of passivating elements in protective oxides as well as exploiting elemental synergies in protective passivating oxides not necessarily available in conventional alloys. Such oxides regulate corrosion processes. However, the formation of additional phases in CCAs may cause passivating elements to partition within the microstructure changing their impact on passivation, and create interfaces limiting their benefit on corrosion resistance. In this work, the passivity and overall corrosion resistance of CCAs within the multi-phase Al-Cr-Fe-Mn-Mo-Ni-Ti system were evaluated. Compositions were targeted to reduce density to the range of 7-7.75 g.cm-3, below those of conventional stainless steels, while also ensuring mechanical ductility due to the presence of an FCC matrix. Phases were synthesized and/or dual and three phase structures were interrogated with high resolution methods to evaluate the contributions of individual phases to corrosion behavior. Corrosion resistance was evaluated with combinations of AC and DC methods which determined attributes and marker of oxide protectiveness in dilute chloride solution at neutral or slight acidic pH. Additionally, corrosion behavior was evaluated across a range of pH, chloride concentrations, and in sulfate environments. Combinations of X-ray photoelectron spectroscopy and atomic emission spectroelectrochemistry were used to determine the roles or fates (e.g. passivation in an oxide film, dissolution into the electrolyte) of individual elements during the passivation process. The findings are used in an iterative design process to inform new optimal compositions. Optimal CCA compositions showed corrosion resistance comparable to, and in the case of best-performing compositions such as Al0.3Cr0.5Fe2Mn0.25Mo0.15Ni1.5Ti0.3, superior to conventional corrosion resistant alloys such as 316L.
Increasing both Al and Ti content in CCA series promoted the formation of an L21 secondary phase enriched in Al, Ni, and Ti along with a third phase enriched in Cr, Fe, and Mo. All the evaluated CCAs demonstrated passivity, with Al, Cr, and Ti generally all suggested to promote passive film formation and stability in a variety of relative compositions. Increasing Al concentrations has minimal effect on parameters representative of passive film strength (e.g., passive current density, polarization resistance) while increasing Ti concentrations are shown to improve such parameters. However, high concentrations of both Al and Ti harm resistance to localized corrosion as evidenced by decreasing pitting and repassivation potentials. The effects of Mn and Mo on the microstructure and corrosion behavior are also considered. Mn decreases the degree of microstructural partitioning for Al and Ti, allowing for improved corrosion resistance when added at low concentration despite the instability of Mn passive species itself. Decreasing Mo concentrations were similarly shown to decrease the degree of microstructural partitioning and potentially improve passivity, but also significantly decreased the resistance to localized corrosion.
Finally, to elucidate the relationships between the microstructure, passivity, and corrosion resistance for each individual phase, the passive film composition of a dual-phase FCC+L21 CCA was characterized with high-resolution techniques capable of differentiating the composition over individual phases in the bulk microstructure. Two distinct phases within the passive film were observed, with the interface acting as a preferential pitting site. The passive film formed over the FCC phase had comparatively higher concentrations of Fe, Cr, and Mo while the film formed over the L21 phase had higher concentrations of Al, Ni, and Ti, mirroring concentrations in the bulk microstructure. Thus, the passive film, and therefore the corrosion behavior, of the dual-phase was suggested to be representative of a mixture of each constituent phase. The application of composite theory to corrosion was evaluated by synthesizing single-phase CCAs representative of the constituent phase compositions. Passivity of the dual-phase CCA was attributed to the ability of both constituent phases to passivate. The dual-phase CCA showed similar corrosion resistance to each constituent phase, but the resistance to localized breakdown was suggested to be lower for both the dual-phase and single-phase L21 CCA than for the isolated FCC CCA phase. The findings indicate the importance of ensuring the ability of all individual phases within a multi-phase CCA microstructure to passivate for overall corrosion resistance. Furthermore, strategies to utilize CCA composition to affect phase structure, composition, and ability to passivate are addressed within the context of corrosion resistant alloy design.