S B Inman

Junsoo Han

M A Wischhusen

J Qi

S R Agnew

Kevin Ogle

J R Scully

Passivation and Localized Corrosion Resistance of Al 0.3 Cr 0.5 Fe 2 Mo x Ni 1.5 Ti 0.3 Compositionally Complex Alloys: Effect of Mo Content

Keywords: 5Ti0.3 Compositionally Complex Alloys Passivation, Compositionally complex alloys, High entropy alloys, Molybdenum, Atomic emission spectroelectrochemistry, X-ray photoelectron spectroscopy

Introduction

Compositionally complex alloys (CCAs), alloys which contain four or more elements at concentrations above 5 at. %, can have a range of beneficial properties including mechanical strength and corrosion resistance [1][START_REF] Qiu | Corrosion of high entropy alloys[END_REF][START_REF] Scully | Controlling the corrosion resistance of multi-principal element alloys[END_REF][START_REF] Tsai | High-Entropy Alloys: A Critical Review[END_REF][START_REF] George | High-entropy alloys[END_REF]. Improved homogeneity of passivating elements is often suggested to limit localized solute depletion that may function as preferential sites for localized corrosion [START_REF] Qiu | Corrosion of high entropy alloys[END_REF][START_REF] Scully | Controlling the corrosion resistance of multi-principal element alloys[END_REF].

Light-weight elements, particularly Al and Ti, can impart both corrosion resistance and reduced density when added to CCAs [1,[START_REF] Feng | Design of Light-Weight High-Entropy Alloys[END_REF]. In particular, passivity may be enhanced by simultaneous presence with Cr in the passive film [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF][START_REF] Qiu | Real-time dissolution of a compositionally complex alloy using inline ICP and correlation with XPS[END_REF]. However, increased Al and/or Ti concentrations in face centered cubic (FCC) CCAs have been shown to lead to the formation of BCC [START_REF] Feng | Design of Light-Weight High-Entropy Alloys[END_REF][START_REF] Lee | Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments[END_REF][START_REF] Li | Structure and properties of FeCoNiCrCu0.5Alx high-entropy alloy[END_REF][START_REF] Qiu | Microstructural evolution, electrochemical and corrosion properties of AlxCoCrFeNiTiy high entropy alloys[END_REF][START_REF] Tang | Alloying and Processing Effects on the Aqueous Corrosion Behavior of High-Entropy Alloys[END_REF][START_REF] Xiao | Microstructure, mechanical and corrosion behaviors of AlCoCuFeNi-(Cr,Ti) high entropy alloys[END_REF][START_REF] Yang | A novel FeCrNiAlTi-based high entropy alloy strengthened by refined grains[END_REF][START_REF] Guo | Effects of heat treatment on the microstructure, mechanical properties and corrosion resistance of AlCoCrFeNiTi0.5 high-entropy alloy[END_REF], B2 [START_REF] Qiu | Microstructural evolution, electrochemical and corrosion properties of AlxCoCrFeNiTiy high entropy alloys[END_REF][START_REF] Yang | A novel FeCrNiAlTi-based high entropy alloy strengthened by refined grains[END_REF][START_REF] Shi | Homogenization of AlxCoCrFeNi high-entropy alloys with improved corrosion resistance[END_REF], and/or Heusler [START_REF] Feng | Design of Light-Weight High-Entropy Alloys[END_REF][START_REF] Choudhuri | Formation of a Huesler-like L21 phase in a CoCrCuFeNiAlTi high-entropy alloy[END_REF][START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF][START_REF] Ishikawa | Phase equilibria and stability of Heusler-type aluminides in the NiAl-Ni2AlTi-Ni2AlY (Y: V, Cr or Mn) systems[END_REF] phases, where the second phase regions are enriched in Al, Ti, and often Ni [START_REF] Xiao | Microstructure, mechanical and corrosion behaviors of AlCoCuFeNi-(Cr,Ti) high entropy alloys[END_REF][START_REF] Yang | A novel FeCrNiAlTi-based high entropy alloy strengthened by refined grains[END_REF][START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF][START_REF] Wang | Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)(100-x)Mo(x) High-Entropy Alloys[END_REF].

Interfaces created by such secondary phase formation have been suggested to function as initiation sites for localized corrosion, decreasing overall corrosion resistance despite the thermodynamic stability of Ti and Al oxides 1 [START_REF] Guo | Effects of heat treatment on the microstructure, mechanical properties and corrosion resistance of AlCoCrFeNiTi0.5 high-entropy alloy[END_REF][START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF][START_REF] Shi | In-situ electrochemical-AFM study of localized corrosion of AlxCoCrFeNi high-entropy alloys in chloride solution[END_REF]. Thus, phase stability issues limit Al and/or Ti concentration, necessitating further alloying additions to limit the effects of microstructural segregation-driven localized corrosion.

Mo has been long established as improving corrosion resistance, most commonly in stainless steels [START_REF] Klimmeck | A study of the kinetics of passive layer formation on Cr-Mo alloys[END_REF][START_REF] Sugimoto | The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions[END_REF]. Although there is no universal agreement for the mechanism behind the contribution of Mo to the corrosion resistance of stainless steels, it is generally accepted as effective in preventing localized corrosion mechanisms such as pitting. Newman suggested Mo, in its non-oxidized state, acts as a 1 High temperature annealing treatments may limit localized solute depletion, however, two-phase equilibrium microstructures allow individual phases to be depleted of individual passivating elements, increasing the risk of localized corrosion.

J o u r n a l P r e -p r o o f 3 corrosion inhibitor in stainless steels [START_REF] Newman | The dissolution and passivation kinetics of stainless alloys containing molybdenum-1. Coulometric studies of Fe-Cr and Fe-Cr-Mo alloys[END_REF] while Hashimoto et al. proposed inhibition occurs via a Mo oxyhydroxide layer [START_REF] Hashimoto | An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HC1[END_REF]. Alternatively, Mo has been suggested to be present in the passive films across multiple valences and species [START_REF] Sugimoto | The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions[END_REF][START_REF] Shi | Corrosion-Resistant High-Entropy Alloys: A Review[END_REF][START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF]. Mo has also been shown to improve repassivation ability and rates in stainless steels and Ni-Cr-Mo alloys [START_REF] Ebrahimi | The influence of chromium and molybdenum on the repassivation of nickel-chromium-molybdenum alloys in saline solutions[END_REF][START_REF] Marshall | Effects of alloyed molybdenum on the kinetics of repassivation on austenitic stainless steels[END_REF], potentially due to redeposition after dissolution [START_REF] Henderson | Investigating the Role of Mo and Cr during the Activation and Passivation of Ni-Based Alloys in Acidic Chloride Solution[END_REF]. Tranchida et al. suggested Mo slows transpassive dissolution rates by doping the Cr oxide film formed on stainless steels and altering the band gap to limit the formation of unstable Cr species [START_REF] Tranchida | Role of Molybdenum on the Electronic Properties of Passive Films on Stainless Steels[END_REF].

Lutton et al. evaluated alloying Ni-Cr binary alloys with Mo, finding Mo to improve the corrosion resistance and increase the enrichment of Cr in the passive film in acidic and basic NaCl solutions [START_REF] Cwalina | In Operando Analysis of Passive Film Growth on Ni-Cr and Ni-Cr-Mo Alloys in Chloride Solutions[END_REF]. The phenomena were attributed to a favorable Cr-Mo synergy promoting O adsorption and low concentrations of oxidized Mo acting as a doping agent in the passive film. Lloyd et al. suggested Mo addition to Ni-Cr alloys to be most significant at high potentials where Cr(VI) is stable, attributing the beneficial effects of Mo to limitations of Cr-induced transpassivity [START_REF] Lloyd | Cr, Mo and W alloying additions in Ni and their effect on passivity[END_REF]. Additionally, Mo addition allows for stable oxide formation over grain facets at orientations for which it does not occur on Ni-Cr binaries of the same Cr content [START_REF] Gusieva | Repassivation Behavior of Individual Grain Facets on Dilute Ni-Cr and Ni-Cr-Mo Alloys in Acidified Chloride Solution[END_REF]. Such Ni-Cr-Mo alloys have been utilized to form a baseline survey of potential mechanisms for the beneficial contributions of Mo to the corrosion resistance of FCC steels and similar alloys [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF][START_REF] Larsson | The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys[END_REF]. While Mo addition has been suggested to be beneficial for both Fe-Cr and Ni-Cr based alloys, similar effects are not suggested in Cr-free counterparts [START_REF] Sugimoto | The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions[END_REF][START_REF] Stout | An Investigation of Pitting Behavior of Iron-Molybdenum Binary Alloys[END_REF][START_REF] Wang | Effects of Chloride Ions on Passive Oxide Films Formed on Cr-Fe-Co-Ni(-Mo) Multi-Principal Element Alloy Surfaces[END_REF]. For this reason, Mo is considered to have synergetic behavior with Cr.

Alloying benefits of Mo have been shown to extend into CCA microstructures. For example, when alloyed to the Co-Cr-Fe-Ni system, Mo may enhance mechanical strength by acting as a solid solution strengthener [START_REF] Liu | Effect of Mo Element on the Mechanical Properties and Tribological Responses of CoCrFeNiMox High-Entropy Alloys[END_REF]. Dai et al. evaluated varying annealing procedures to optimize the mechanical and corrosion resistance of FeCoCrNiMo 0.1 CCAs [START_REF] Dai | Microstructure and mechanical properties of FeCoCrNiMo0.1 high-entropy alloy with various annealing treatments[END_REF]. At high Mo concentrations, second phase strengthening is possible though second phase formation in FCC CCAs, in part due to the well-established ferritic stabilization of Mo in conventional Fe alloys. In the AlCoCrFeNiMo x system, higher Mo concentrations J o u r n a l P r e -p r o o f increase strength by forming an eutectic structure with a second phase enriched in Cr, Fe, and Mo [START_REF] Zhu | Microstructures and compressive properties of multicomponent AlCoCrFeNiMox alloys[END_REF].

Mo frequently combines with Cr by forming σ or μ intermetallic phases [START_REF] Wang | Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)(100-x)Mo(x) High-Entropy Alloys[END_REF][START_REF] Zhu | Microstructures and compressive properties of multicomponent AlCoCrFeNiMox alloys[END_REF][START_REF] Chou | The effect of molybdenum on the corrosion behaviour of the highentropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments[END_REF][START_REF] Rodriguez | Effect of Molybdenum on the Corrosion Behavior of High-Entropy Alloys CoCrFeNi2 and CoCrFeNi2Mo0.25 under Sodium Chloride Aqueous Conditions[END_REF][START_REF] Shang | Effect of Mo Addition on Corrosion Behavior of High-Entropy Alloys CoCrFeNiMox in Aqueous Environments[END_REF][START_REF] Ren | Effect of Mo addition on the microstructural evolution and mechanical properties of Fe-Ni-Cr-Mn-Al-Ti high entropy alloys[END_REF][START_REF] Linder | Effect of Mo Content on the Corrosion Resistance of CoCrFeMoxNi Thin Films in Sulfuric Acid[END_REF][START_REF] Niu | Microstructural evolution, mechanical and corrosion behaviors of as-annealed CoCrFeNiMox (x = 0, 0.2, 0.5, 0.8, 1) high entropy alloys[END_REF].

Mo, and its influence on second phase formation, has also been shown to affect the corrosion resistance of CCAs such as those in the Co-Cr-Fe-Mn-Ni system [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF][START_REF] Wang | Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)(100-x)Mo(x) High-Entropy Alloys[END_REF][START_REF] Wang | Effects of Chloride Ions on Passive Oxide Films Formed on Cr-Fe-Co-Ni(-Mo) Multi-Principal Element Alloy Surfaces[END_REF][START_REF] Dai | Microstructure and mechanical properties of FeCoCrNiMo0.1 high-entropy alloy with various annealing treatments[END_REF][START_REF] Chou | The effect of molybdenum on the corrosion behaviour of the highentropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments[END_REF][START_REF] Rodriguez | Effect of Molybdenum on the Corrosion Behavior of High-Entropy Alloys CoCrFeNi2 and CoCrFeNi2Mo0.25 under Sodium Chloride Aqueous Conditions[END_REF][START_REF] Linder | Effect of Mo Content on the Corrosion Resistance of CoCrFeMoxNi Thin Films in Sulfuric Acid[END_REF][START_REF] Niu | Microstructural evolution, mechanical and corrosion behaviors of as-annealed CoCrFeNiMox (x = 0, 0.2, 0.5, 0.8, 1) high entropy alloys[END_REF][START_REF] Wang | Origin of enhanced passivity of Cr-Fe-Co-Ni-Mo multi-principal element alloy surfaces[END_REF] pioneered by Cantor et al. [START_REF] Cantor | Microstructural development in equiatomic multicomponent alloys[END_REF]. Niu et al. showed increasing Mo content improved corrosion resistance in the CoCrFeNiMo x system in addition to improving mechanical strength due to the formation of σ and μ intermetallic phases ranging from nm to μm size scale. A Mo concentration of 11.1 at. % had the highest corrosion potential of all tested compositions and formed few stable pits [START_REF] Niu | Microstructural evolution, mechanical and corrosion behaviors of as-annealed CoCrFeNiMox (x = 0, 0.2, 0.5, 0.8, 1) high entropy alloys[END_REF]. Similarly, Wang et al. observed more positive pitting potentials in chloride environments with increased Mo concentration added to the (CoCrFeNi) 1-x Mo x system. However, more frequent pitting, higher current densities, and a more negative corrosion potential were observed with 3 wt. % Mo added due to the formation of a σ phase. Such decreases were not observed during polarization in sulfuric acid [START_REF] Wang | Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)(100-x)Mo(x) High-Entropy Alloys[END_REF]. Unlike most cases, where σ phases are likely at higher Mo concentrations, Linder et behavior with the addition of 7.64 at. % Mo to the CoCrFeNi 2 CCA [START_REF] Rodriguez | Effect of Molybdenum on the Corrosion Behavior of High-Entropy Alloys CoCrFeNi2 and CoCrFeNi2Mo0.25 under Sodium Chloride Aqueous Conditions[END_REF]. Improved overall corrosion resistance was speculated to occur due to the formation of an improved protective layer at grain boundaries and other preferential sites for localized corrosion. Shang et al. evaluated the microstructure and corrosion resistance of a series of CoCrFeMnNiMo x CCAs [START_REF] Shang | Effect of Mo Addition on Corrosion Behavior of High-Entropy Alloys CoCrFeNiMox in Aqueous Environments[END_REF]. Increased Mo concentration was suggested to improve the corrosion resistance. However, concentrations which were high enough to promote the formation of a σ phase were suggested to harm corrosion resistance. Mo was observed in the oxide film with X-ray photoelectron spectroscopy (XPS) surface analysis, suggesting it may play a role in the passivation process. The existing body of work highlights how balancing the beneficial pitting and passivity contributions with phase partitioning, intermetallic formation, cost, and density consequences adds considerable complexity to optimization of Mo concentration in CCAs.

Although Mo has well established benefits to corrosion resistance within traditional binary alloys as well as the CCA field, the addition to Al and Ti-containing CCAs is notably less explored. Mo has been shown to improve corrosion resistance in the Al-Ti-Mo system, although it also promoted chemical segregation leading to microgalvanic corrosion [START_REF] Sun | Effects of phase selection and microsegregation on corrosion behaviors of Ti-Al-Mo alloys[END_REF]. Limited work has suggested increased Mo concentrations in Al and Ti containing CCAs with modest Cr concentrations also promote the formation of separate phases [START_REF] Zhu | Microstructures and compressive properties of multicomponent AlCoCrFeNiMox alloys[END_REF][START_REF] Chou | The effect of molybdenum on the corrosion behaviour of the highentropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments[END_REF]. Furthermore, the increase in alloy density from Mo additions runs contrary to the light-weight applications for which Al and Ti CCAs are often targeted [START_REF] Feng | Design of Light-Weight High-Entropy Alloys[END_REF]. Despite potential adverse effects on microstructural stability, Mo may be beneficial to Cr-lean, Al and Ti containing CCA design space, particularly as such alloys have often been shown to have multi-phase microstructures vulnerable to J o u r n a l P r e -p r o o f localized corrosion at phase interfaces [START_REF] Guo | Effects of heat treatment on the microstructure, mechanical properties and corrosion resistance of AlCoCrFeNiTi0.5 high-entropy alloy[END_REF][START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF][START_REF] Shi | In-situ electrochemical-AFM study of localized corrosion of AlxCoCrFeNi high-entropy alloys in chloride solution[END_REF][START_REF] Shi | Corrosion of Al xCoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior[END_REF]. Thus, it is necessary to evaluate the effect of Mo on the corrosion and passivity of such CCAs to optimize compositions. This work investigates three Al 0.3 Cr 0.5 Fe 2 Mo x Ni 1.5 Ti 0.3 (x = 0, 0.05, 0.15) containing CCAs of FCC + L2 1 microstructure from a CCA alloy system designed and optimized for mechanical strength, ductility, and toughness [START_REF] Bhattacharyya | Low Cost Compositionally Complex Multiphase Alloys with Optimized Strength[END_REF]. The concentration of Mo is adjusted to evaluate the optimal concentrations for passivation and corrosion resistance in an alloy system that is dependent on three elements (Al, Cr, and Ti) for passivity [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF], as opposed to traditional alloys such as stainless steels with high Cr content and Crdominated passive films [START_REF] Olsson | Passive films on stainless steels-chemistry, structure and growth[END_REF]. The effects on phase content, passive film chemical composition, and elemental dissolution are observed with X-ray diffraction (XRD) and SEM, XPS and atomic emission spectroelectrochemistry (AESEC) respectively. While minimizing Mo concentration may decrease alloy cost and density, as well as improve compositional homogeneity, concentrations below a critical threshold may leave the alloys vulnerable to localized corrosion. The determining factors for compositional optimization and the role of Mo in the passivation and corrosion process will be discussed.

Experimental Methods

Three CCAs with the compositions Al 0.3 Cr 0.5 Fe 2 Mo x Ni 1.5 Ti 0.3 (x = 0, 0.05, 0.15) listed in Table I, henceforth referred to as Mo-0, Mo-1.1, and Mo-3.2 with regards to Mo concentrations in at. %, were synthesized via arc melting into a water-cooled copper hearth under Ar cover gas from high purity (greater than 99.9%, Cr greater than 99.2 %) elements. Compositions were selected based on pervious work in similar systems targeting light-weight, low-cost, corrosion resistance CCAs [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF][START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF][START_REF] Bhattacharyya | Low Cost Compositionally Complex Multiphase Alloys with Optimized Strength[END_REF] and were predicted to have an FCC matrix with Heusler (L2 1 ) second phases with computational methods discussed elsewhere [START_REF] Bhattacharyya | Low Cost Compositionally Complex Multiphase Alloys with Optimized Strength[END_REF][START_REF] Qi | High Entropy Alloys Mined From Binary Phase Diagrams[END_REF]. Samples were remelted and flipped five times to ensure chemical homogeneity before being suction cast in a copper mold to produce disk-shape samples with 5 mm thickness and 10 mm diameter.

Cast samples were encapsulated under Ar into quartz tubes and annealed for five hours at 1070 °C before J o u r n a l P r e -p r o o f quenching in water. Samples were mechanically ground with SiC paper up to 1200 grit and, in the case of microstructural and surface chemistry analysis, polished with polycrystalline suspension through 0.25 microns before cleaning with acetone and isopropyl alcohol. Surfaces for each alloy were then imaged with SEM in BSE mode.

Second, to observe the passivation behavior, the air-formed oxide was first exposed to a cathodic reduction treatment by applying a -1.3 V SCE potential for 600 s, which has been shown in previous work to adequately minimize the air-formed oxide in similar CCAs via in-situ impedance measurements [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF][START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF][START_REF] Gerard | Aqueous passivation of multi-principal element alloy Ni38Fe20Cr22Mn10Co10: Unexpected high Cr enrichment within the passive film[END_REF].

The sample was then potentiodynamically polarized in the forward direction from -1.3 to +0.8 V SCE at a scan rate of 0.5 mV/s followed by a reverse scan (+0.8 to -1.3 V SCE , 0.5 mV/s). To analyze the polarization behavior in the absence of localized corrosion, the procedure was repeated in 0.1 M Na 2 SO 4 with the pH adjusted to 4 and 10 with concentrated H 2 SO 4 and NaOH respectively. Finally, an oxide film was grown in the passive range by first exposing the sample to the -1.3 V SCE cathodic treatment and then applying a potential step to -0.2 V SCE for 10 ks. The potential was determined to be in the passive range by initial potentiodynamic polarization testing. Following the potentiostatic hold, a potentiostatic EIS experiment XPS was utilized to characterize the passive film composition and valence. Each sample was exposed to a 600 s cathodic treatment at -1.3 V SCE followed by a 10 ks potentiostatic hold at -0.2 V SCE , the potential and time used for the potentiostatic oxide growth procedure described above. Samples were transported under N 2(g) directly to a PHI VersaProbe III TM XPS system, bypassing any EIS and OCP measurement. Al Kα X-rays (1468.7 eV) at a 26 eV pass energy, 45° take off angle, and with a 100 μm spot for alloys for which good fit agreement is obtained when fit to both core series with the peak intensity constrained by fixing cation fractions for each species to stoichiometric ratios between each element.

Results

Characterization of Alloy Microstructure

The CCAs are suggested by XRD patterns shown in Figure 1 to have a predominantly FCC microstructure with a second phase also detected. The second phase has been suggested to be L2 1 using methods discussed elsewhere [START_REF] Bhattacharyya | Low Cost Compositionally Complex Multiphase Alloys with Optimized Strength[END_REF]. Figure 2 indicates second-phase regions are of single-micrometer size.

The phase volume and morphology is similar across all three CCAs with Mo-1.1 and Mo-3.2 having slightly higher L2 1 volume fractions than Mo-0. II) and mapping for the Mo-3.2 alloy (Figure 3) both suggest the matrix to be enriched in Fe, Cr, and Mo, while the second phase is enriched in Al, Ti, and Ni with each element present in both phases. Generally, the degree of Al and Ti partitioning increases with Mo content, with Mo-0 exhibiting the most chemically homogenous microstructure with regards to the ratios between the concentrations of each element in the FCC matrix compared to L2 1 EDS point scans. EDS mapping of additional CCAs is shown in the supplementary information section. J o u r n a l P r e -p r o o f Critical current densities and potentials are further obscured by changes in sign of the current density prior to the formation of a stable passive film for Mo-0 in pH 4 and Mo-1.1 in pH 10. In both environments, Mo-1.1 has the highest passive current density (i pass ). i pass of Mo-0 is comparable to that of 316L in the pH 4 solution, but 316L has a lower i pass than all CCAs at pH 10. Increasing current densities representative of transpassive dissolution of the passive film are not observed within the evaluated potential range for any of the CCAs. However, such behavior is suggested for the Al and Ti-free 316L at potentials above 0.6 V SCE .

No trends in corrosion potential (E corr ) are observable.

J o u r n a l P r e -p r o o f VI). Both Mo-0 and Mo-3.2 had lower mass loss rates by over an order of magnitude, indicating there were no clear trends with Mo concentration. Figure 8 shows film breakdown is attributable to pitting, with pits frequently observed at or near second phase interfaces. Neither phase is suggested to preferentially dissolve, nor do pits appear do penetrate into either phase at significantly higher rates.

Growth and Characterization of Passive Films by Impedance Spectroscopy

Furthermore, Figure 9 and Table VII show both the OCP and R P values of Mo-1.1 and Mo-3.2 decrease during long-term immersion. Decreasing OCP values suggest the passive film is not improved by the extended exposure, while both values increase during the immersion for Mo-0 and 316L, indicating growth of a more protective passive film. A film grown in the passive region after cathodic treatment via applying a -0.2 V SCE potential, indicated as part of the passive range of each CCA by initial potentiodynamic polarization (Figure 5), was also characterized. Figure 10 shows current density measurements throughout the passive film growth procedure. In pH 4 solution, the current density is generally higher in CCAs with less Mo beyond 100 s. A steady increase in the current density of Mo-0 suggests the formation of a weaker and/or dissolving passive film, although no pits or other features of localized corrosion were visible following exposure. In Following potentiostatic passive film growth, the films were characterized with EIS (Figure 12,

J o u r n a l P r e -p r o o f

Table VIII) and fit with the Randles circuit described above (Figure 7e). The high current density observed J o u r n a l P r e -p r o o f during film growth (Figure 10a) is characteristic of a less corrosion resistant passive film. Unlike the case of the air-formed solution-exposed films, R P increases with Mo content in both environments. α is above 0.85 for all alloys in both environments, indicating characteristic capacitive behaviour of the passive films.

The R P magnitude values vary less with Mo concentrations under pH 10 conditions than in pH 4 conditions, with much of the variability attributable to the low R P of the film formed on Mo-0 in pH 4 conditions. R P of all the CCA passive films is lower than that of 316L, regardless of Mo concentration or pH.

Characterization of Passive Film Cation Fractions

XPS was utilized to characterize the passive films grown at -0.2 V SCE with cation fractions of constituent elements in the passive films shown in Figure 13 and Table IX. Ni dissolution rates were below the detection limit while Mo dissolution was observed at pH 10 (Figure 11b). At pH 4, only Fe and Ni dissolution is observed (Figure 11a), which is in agreement with the comparatively lower Fe(II)/Fe(III) and Ni(II) cation fractions in the passive film. In summary, while the films are consistently enriched in passive species predicted to be thermodynamically stable, Mo concentration plays a prominent role in altering the amount of each species.

J o u r n a l P r e -p r o o f

Discussion

The selected alloys evaluate the effects of Mo additions from 0-3.2 at. % within the Al-Cr-Fe-Ni-Ti CCA systems. The alloying strategy preserves the ratios between passivating elements Al, Cr, and Ti and the composition of each of the three elements varies by less than 0.5 at. % between the CCAs (Table I).

Thus, effects on corrosion resistance are attributable to Mo content. Furthermore, the Cr concentration of the investigated alloys was kept below critical thresholds for passivation in Fe-Cr (~13 at. % Cr [START_REF] Uhlig | Anodic Polarization of Passive and Non-passive Chromium-Iron Alloys[END_REF]) and Ni-Cr (~12 at. % Cr [START_REF] Uhlig | Electron configuration in alloys and passivity[END_REF]) alloys with Al and Ti also maintained below traditional Fe binary thresholds (~15 at. % Al [START_REF] Peng | Influence of Al content and pre-oxidation on the aqueous corrosion resistance of binary Fe-Al alloys in sulphuric acid[END_REF], >47 at. % Ti [START_REF] Lim | ESCA studies of Fe-Ti alloys[END_REF]). Passivity below such Fe-Cr thresholds has been shown to be obtainable due to the addition of Al and Ti [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF] with the optimal Al-Cr ratio informed by work in similar CCA systems [START_REF] Blades | Tuning Chemical Short-range Order for Stainless Behavior at Reduced Chromium Concentrations in Multi-principal Element Alloys[END_REF].

The present study probes the effect of Mo alloying in the passive film (i.e., in Na 2 SO 4 ), pit stabilization, and passive film growth in NaCl solutions at pH 4 and 10. Mo is present in the alloys mainly J o u r n a l P r e -p r o o f 29 in the FCC matrix (Table II) and is suggested by XPS to be present in Mo(0), Mo(IV), and Mo(VI) valence states (Figure 14). Utilization of pH 4 and 10 solutions further compares the effect of Mo with different thermodynamically expected fates (solid state stability versus dissolved aqueous anions) [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF]. Stable or metastable Mo oxides are suggested in the oxide at anodic potentials near pH 4 whilst at pH 10, E-pH diagrams suggest Mo to be thermodynamically stable as MoO 4 2- [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF]. This implies that Mo may operate to inhibit pitting in its traditional function as a dissolution suppressor. However, the presence of multiple valence states introduces the possibility of aliovalent Mo doping in the passive film that can create more ionic defects which may affect passive film dissolution separate from pitting phenomena [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF]. Corrosion resistant passive film formation at OCP is found to exhibit the best attributes for the solution-exposed airformed Mo-0 passive film (Figure 7, Table V). For instance, R P of the Mo-0 solution-exposed air-formed oxide is higher following 20-day immersion (Figure 9a, Tables V, VII) and the mean i pass in chloride is generally the lowest of the evaluated CCAs in this work (Figure 5, Table III). In contrast, Mo-3.2 is suggested to be the most corrosion resistant with regards to pitting susceptibility indicated by the most positive pitting potential at pH 4 (Figure 5, Table III).

Effect of Mo on Microstructural Partitioning and Localized Breakdown

Both the average E pit and probabilistic distribution suggest improved resistance to film breakdown under acidic conditions (Figure 5b). More positive E rep values for Mo-3.2 than Mo-0 were observed in both environments with less variability than changes in E pit , indicating that Mo may enhance repassivation (Figures 5c,5f). Such findings suggest that Mo plays a more prominent role in pit propagation and stability than initiation. Mo enrichment suggested by decreased dissolution rates at transpassive potentials relative to bulk composition (Figure 6) may be attributable to the local decrease in pH leading to the deposition of Mo-rich corrosion products [START_REF] Henderson | Molybdenum surface enrichment and release during transpassive dissolution of Ni-based alloys[END_REF]. Mo has been reported to help reform a stable passive film in mild acids after surface activation caused either by the applied cathodic potential [START_REF] Li | The contribution of Cr and Mo to the passivation of Ni22Cr and Ni22Cr10Mo alloys in sulfuric acid[END_REF] or by mechanical scratching [START_REF] Newman | The dissolution and passivation kinetics of stainless alloys containing molybdenum-1. Coulometric studies of Fe-Cr and Fe-Cr-Mo alloys[END_REF]. In this case, the Mo-containing passive film may serve as a catalyst for the hydrogen J o u r n a l P r e -p r o o f evolution reaction [START_REF] Park | A review of nickel-molybdenum based hydrogen evolution electrocatalysts from theory to experiment[END_REF] resulting in a high cathodic current density in this potential domain. High Mo valence within the passive film may add charge carriers which in turn support charge transfer reactions such as the oxygen evolution reaction [START_REF] Larsson | The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys[END_REF]. For instance, Mo(IV)/(VI) doping in Cr(III) oxide leads to cation vacancies and/or excess free electrons [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF].

Frequent pitting at the interface between FCC and L2 1 (Figure 8) is observed, identifying a microstructural feature responsible for pit initiation. This location suggests a role of grain boundaries or phase interfaces as possible contributors to the weakening of localized corrosion resistance, potentially due to the formation of defects in the passive film [START_REF] Marcus | Atomic level characterization in corrosion studies[END_REF]. While increasing Mo concentration decreases pit initiation frequency in single-phase stainless steels and Ni-Cr-Mo alloys [START_REF] Ilevbare | The role of alloyed molybdenum in the inhibition of pitting corrosion in stainless steels[END_REF][START_REF] Kehler | Role of Metastable Pitting in Crevices on Crevice Corrosion Stabilizationin Alloys 625 and 22[END_REF], interface pitting may hinder the effect of Mo. Interfaces likely have less prominent effects on repassivation than initiation as the pit is grown well beyond the interface. The beneficial effects of Mo on pit repassivation (Figure 5)

suggest Mo slows propagation or enhances repassivation unhindered by the interface. Furthermore, the increase in compositional partitioning in the alloy at higher Mo concentrations (Table II) may indicate localized depletion of passivating elements (e.g. Ti, Al) that may harm resistance to localized corrosion, particularly at or near the bulk metal phase interfaces (and resultant oxide film interfaces) where the film may be vulnerable to breakdown [START_REF] Shi | In-situ electrochemical-AFM study of localized corrosion of AlxCoCrFeNi high-entropy alloys in chloride solution[END_REF][START_REF] Marcus | Localized corrosion (pitting): A model of passivity breakdown including the role of the oxide layer nanostructure[END_REF]. Further evaluation with high resolution Auger electron spectroscopy of the variation in both bulk metal and passive film chemistry across the phase interface, including the position and local enrichment of Mo, will be discussed in a forthcoming publication. Adverse effects of partitioning may limit the otherwise beneficial effects of Mo. This is shown by lowering of E pit distributions with increasing Mo concentration in basic environments, where Mo dissolution is likely and adverse microstructural effects may be more prominent (Figures 5b,5e). Despite clear effects of Mo on microstructural partitioning, neither imaging of corroded surfaces (Figure 8) nor transpassive elemental dissolution rates (Table IV) suggest preferential dissolution of either phase.

J o u r n a l P r e -p r o o f

Higher concentrations of Mo in the CCA correspond to slightly increased levels of partitioning,

where the concentration of an element in the second phase differs from that in the matrix to a greater extent than in the case of CCAs with less Mo, illustrating a secondary, but still important, microstructural effect of Mo. The σ phase that has been previously shown to harm corrosion resistance with increasing Mo in CCAs [START_REF] Wang | Effect of Molybdenum Additives on Corrosion Behavior of (CoCrFeNi)(100-x)Mo(x) High-Entropy Alloys[END_REF][START_REF] Chou | The effect of molybdenum on the corrosion behaviour of the highentropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments[END_REF][START_REF] Shang | Effect of Mo Addition on Corrosion Behavior of High-Entropy Alloys CoCrFeNiMox in Aqueous Environments[END_REF] was not formed, likely due to the comparatively low concentration of both Cr and Mo. Mo is enriched in the FCC matrix phase, despite its prominent role as a ferritic stabilizer. Such lateral variation of Mo content in the passive film chemistry has been shown to promote selective dissolution in CoCrFeMoNi [START_REF] Wang | Selective corrosion mechanism of CoCrFeMoNi highentropy alloy in the transpassive region based on the passive film characterization by ToF-SIMS[END_REF], but such behavior is not observed in the evaluated system (Figure 8).

Although Mo is not enriched in the L2 1 second phase, Al, Ni, and Ti generally increase in concentration in the second phase with increasing Mo concentrations (Table II). For example, the Ti content in the L2 1 phase of Mo-3.2 is over three times the Ti content in the matrix, whereas the Ti content in the L2 1 phase of Mo-0 is approximately twice that of the matrix. Similarly, Al concentrations in the L2 1 phase of Mo-0 are 2.5 times those of the matrix whereas the ratio between the same phases of Mo-3.2 is over four. This follows previous trends of Mo enrichment in phases differing from those enriched in Ti [START_REF] Chou | The effect of molybdenum on the corrosion behaviour of the highentropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments[END_REF] and Al [START_REF] Zhu | Microstructures and compressive properties of multicomponent AlCoCrFeNiMox alloys[END_REF] in similar CCAs, with both elements possibly being driven away from the Mo-enriched FCC phase by the unfavorable enthalpies of mixing [START_REF] Takeuchi | Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element[END_REF]. The enrichment of Ti and Al in phases depleted of Mo, rather than the FCC phase which is enriched Cr and Mo, may limit the ability for synergetic behavior in the passive film, particularly given high importance of Ti in the passivation process suggested by surface cation fractions (Figure 13, Table IX).

The beneficial contributions of Mo must be balanced with potential effects of phase partitioning of second phase-enriched elements on localized corrosion, adding a layer of complexity to previously established trends in the single-phase Fe-Cr-Mo and Ni-Cr-Mo systems. Unlike the proposed beneficial effects of low Mn concentrations on elemental degree of partitioning, poor passivation, and overall corrosion resistance observed in a similar CCA system [START_REF] Inman | Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0[END_REF], Mo concentrations must be limited to avoid J o u r n a l P r e -p r o o f 32 detrimental levels of partitioning. The addition of Mn or other FCC stabilizing elements could help increase the maximum allowable Mo content without harmful chemical segregation in the microstructure, but corrosion resistance may also be harmed by poor Mn passivation [START_REF] Torbati-Sarraf | The influence of incorporation of Mn on the pitting corrosion performance of CrFeCoNi High Entropy Alloy at different temperatures[END_REF][START_REF] Yang | Effects of Mn on the electrochemical corrosion and passivation behavior of CoFeNiMnCr high-entropy alloy system in H2SO4 solution[END_REF]. Although Ni passivation is suppressed, low transpassive Ni dissolution rates (Table IV) may suggest alternative fates such as enrichment at the metal-oxide interface [START_REF] Gerard | Aqueous passivation of multi-principal element alloy Ni38Fe20Cr22Mn10Co10: Unexpected high Cr enrichment within the passive film[END_REF][START_REF] Wang | Study of the surface oxides and corrosion behaviour of an equiatomic CoCrFeMnNi high entropy alloy by XPS and ToF-SIMS[END_REF].

The effect of Mo on Ni partitioning is noteworthy as the binary enthalpy of mixing between Mo and Ni is higher than that between Mo and other system elements [START_REF] Takeuchi | Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element[END_REF]. The unfavorable interaction between Mo and Ni may preferentially cause Ni to enrich in the Mo-depleted second phase. Unlike the case of Al and Ti, increased Mo concentration does not consistently increase the degree of Ni partitioning, although the uncharacteristically low Ni composition in the FCC phase of Mo-0 highlights uncertainty in the EDS point scans. In addition to microstructural factors, increasing Mo concentrations also reduced the concentration of Ni(OH) 2 , following trends in the Ni-Cr-Mo system proposed by Lutton et al. [START_REF] Cwalina | In Operando Analysis of Passive Film Growth on Ni-Cr and Ni-Cr-Mo Alloys in Chloride Solutions[END_REF] where

Ni was also less present in the passive film due to the enhancement of other passivating elements such as Cr.

Effect of Mo on Passive Film Composition

Mo is also considered with respect to its effects on global passive film chemistry. Cr is the primary passivating species in all three CCAs and in both pH values. However, the passive film concentration varies across the three alloys. Mo is not a primary passivator in any of the evaluated CCAs, although in both Mocontaining CCAs, the passive film is enriched in Mo(IV)/Mo(VI) at the film-electrolyte interface relative to bulk composition at similar ratios (Figure 13, Table IX). Notably, Mo content may be lower at film depths approaching the metal-film interface, given the tendency of Mo to enrich within outer layers of passive films formed on similar alloys [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF][START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF][START_REF] Wang | Origin of enhanced passivity of Cr-Fe-Co-Ni-Mo multi-principal element alloy surfaces[END_REF][START_REF] Wang | Selective corrosion mechanism of CoCrFeMoNi highentropy alloy in the transpassive region based on the passive film characterization by ToF-SIMS[END_REF][START_REF] Quiambao | Passivation of a corrosion resistant high entropy alloy in non-oxidizing sulfate solutions[END_REF][START_REF] Lynch | Passivation-Induced Cr and Mo Enrichments of 316L Stainless Steel Surfaces and Effects of Controlled Pre-Oxidation[END_REF][START_REF] Wang | Mechanisms of Cr and Mo Enrichments in the Passive Oxide Film on 316L Austenitic Stainless Steel[END_REF][START_REF] Eidhagen | Synchrotron XPS and Electrochemical Study of Aging Effect on Passive Film of Ni Alloys[END_REF]. Significant presence of Mo in the passive film is unsurprising given the thermodynamic stability of Mo oxides in a pure Mo system at pH 4 near -0.2 V SCE J o u r n a l P r e -p r o o f IX).

Increased Mo concentration promoted Cr(III) passivation in the Ni-Cr-Mo system explored by Lutton et al. [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF][START_REF] Lutton | Passivation of Ni-Cr and Ni-Cr-Mo Alloys in Low and High pH Sulfate Solutions[END_REF], particularly in Cl -environments where Ni dissolved. In contrast, the CCAs with the highest Mo concentrations have the lowest surface cation fraction of Cr in the passive film formed in the pH 4 environment despite still being enriched relative to bulk alloy composition (Figure 13, Table IX). While Ti preferentially partitions to the L2 1 phase, enough is still present in the FCC phase to affect global passive film chemistry. Thus, the limitation of the Cr(III) content in passive film of the CCA system may also be attributed to the high stability of Ti(IV) in the passive film. In the Ni-Cr-Mo system, Cr 2 O 3 is the most stable oxide (ΔG f ° = -1050 kJ/mol) [START_REF] Chase | JANAF Thermochemical Tables, 4th Edition[END_REF] whereas in the CCAs evaluated, Ti may be a preferential site for oxidation given the lower free energy of formation on a per cation basis of TiO 2 (ΔG f ° = -889 kJ/mol) [START_REF] Chase | JANAF Thermochemical Tables, 4th Edition[END_REF]. Likewise, very little Al 2 O 3 (ΔG f ° = -1580 kJ/mol) [START_REF] Chase | JANAF Thermochemical Tables, 4th Edition[END_REF] was observed in Mo-0 while the film grown on Mo-3.2 was enriched in Al(III). Additionally, Mo-3.2 did not form any observable Ni(OH) 2 species in the passive film (ΔG f ° = -447 kJ/mol) [START_REF] Archer | Thermodynamic Properties of Import to Environmental Processes and Remediation. II. Previous Thermodynamic Property Values for Nickel and Some of its Compounds[END_REF] and had the highest combined concentration of Al(III), Cr(III), and Ti(IV) species.

Thus, across both systems, Mo may be suggested to promote the presence of more stable oxide species.

Mo may affect the passive film composition by acting as an aliovalent dopant. XPS fitting (Figure 14) suggests that while oxidized Mo is dominated by Mo(VI) cations, Mo(IV) is also present. Mo(V) may have also been present given frequent observation in Ni-Cr-Mo alloys [START_REF] Ebrahimi | The influence of chromium and molybdenum on the repassivation of nickel-chromium-molybdenum alloys in saline solutions[END_REF][START_REF] Larsson | The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys[END_REF][START_REF] Lutton | Passivation of Ni-Cr and Ni-Cr-Mo Alloys in Low and High pH Sulfate Solutions[END_REF][START_REF] Larsson | Thickness and composition of native oxides and near-surface regions of Ni superalloys[END_REF], but was not identified, possibly due to overlap with other valence states. In the Ni-Cr-Mo system, Mo(IV) appears to stabilize Cr passivation while Mo(VI) cations are considered more likely to dissolve [START_REF] Li | The contribution of Cr and Mo to the passivation of Ni22Cr and Ni22Cr10Mo alloys in sulfuric acid[END_REF], however J o u r n a l P r e -p r o o f conflicting results have been suggested in CoCrFeMnNiMo x CCAs [START_REF] Shang | Effect of Mo Addition on Corrosion Behavior of High-Entropy Alloys CoCrFeNiMox in Aqueous Environments[END_REF]. The higher valence state Mo cations may be reduced to lower valence states to locally stabilize the higher valence Ti(IV) cations relative to Cr(III) and Al(III), potentially through affecting the local stability of O vacancies [START_REF] Kumaravel | Mo doped TiO2: impact on oxygen vacancies, anatase phase stability and photocatalytic activity[END_REF]. During the growth of the oxide in 0.1 M NaCl pH 4 (Figure 10), the current density was lowest for Mo-3.2, suggesting slower passivation kinetics along with the well-known properties of aliovalent Mo cations [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF].

A significant local enrichment of Mo(0) might support Mo acting as an non-oxidized dissolution blocker [START_REF] Newman | The dissolution and passivation kinetics of stainless alloys containing molybdenum-1. Coulometric studies of Fe-Cr and Fe-Cr-Mo alloys[END_REF], whereas preservation or local enrichment of the oxidized states may favor mechanisms dependent on passivated Mo [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF][START_REF] Henderson | Investigating the Role of Mo and Cr during the Activation and Passivation of Ni-Based Alloys in Acidic Chloride Solution[END_REF][START_REF] Tranchida | Role of Molybdenum on the Electronic Properties of Passive Films on Stainless Steels[END_REF][START_REF] Wang | Origin of enhanced passivity of Cr-Fe-Co-Ni-Mo multi-principal element alloy surfaces[END_REF]. Both valence states are suggested to be present within or near the passive film by XPS (Figure 14). However, under both Mo(0)-based mechanisms, Mo depletion in the L2 1 phase would limit the ability of the CCAs to resist localized corrosion within the L2 1 phase or at the FCC-L2 1 interface and mechanisms dependent on Mo dissolution must also be considered. Mo dissolution occurred across of a range of potentials, most noticeably in the cathodic region due to the well-known stability of MoO 4 2-in alkaline solutions [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF]. However, this was attributed to dissolution of the air-formed oxide, which has been shown to have high concentrations of Mo in similar CCAs [START_REF] Inman | Corrosion Behavior of a Compositionally Complex Alloy Utilizing Simultaneous Al, Cr, and Ti Passivation[END_REF]. Additionally, dissolution of air-formed oxides has yielded similar cathodic dissolution rates in the Ni-Cr-Mo system [START_REF] Li | The contribution of Cr and Mo to the passivation of Ni22Cr and Ni22Cr10Mo alloys in sulfuric acid[END_REF].

The likely proposed mechanism relating Mo dissolution to corrosion resistance is that MoO 4 2-ions act as an inhibitor to limit pit stability [START_REF] Sugimoto | The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions[END_REF][START_REF] Lillard | Crevice Corrosion of Alloy 625 in Chlorinated ASTM Artificial Ocean Water[END_REF], a possibility indicated by significant Mo dissolution being observed via AESEC (Figures 6,[START_REF] Qiu | Microstructural evolution, electrochemical and corrosion properties of AlxCoCrFeNiTiy high entropy alloys[END_REF], especially in pH 10 solution. In this case, Mo does not have to originate in the L2 1 phase or near the interface, but can be supplied from the FCC phase and transported via comparatively more rapid liquid phase diffusion.

Comparative Effects of Mo under Acidic and Basic Conditions

Elemental dissolution trends measured with AESEC generally follow those predicted by the electrochemical stability of pure constituent elements for a given solution pH [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF]. Pure Fe and Ni are J o u r n a l P r e -p r o o f predicted to dissolve at pH 4 and form insoluble species at pH 10 [63] at -0.2 V SCE . E-pH diagrams have predicted stability of both Ni oxide [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF] and Ni-Cr spinel [START_REF] Lu | Computational materials design of a corrosion resistant high entropy alloy for harsh environments[END_REF][START_REF] Wang | Potential-pH diagrams considering complex oxide solution phases for understanding aqueous corrosion of multi-principal element alloys[END_REF] is more favorable under basic conditions.

Therefore, XPS intensity attributable to Ni-containing complex oxide and single-cation passive species significantly increased (Figure 13, Table IX) while corresponding Ni dissolution rates decreased in basic conditions (Figure 11). Mo dissolves at much higher rates in basic solutions (Figure 10), likely due to the stability of the MoO 4 2-ion [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF]. The increased stability of Mo oxides in acids corresponds with an increase in the surface cation fractions attributable to oxidized Mo in the Mo-3.2 film grown in pH 4 relative to when grown in pH 10 solution (Figure 13, Table IX). The decreased Mo enrichment in the passive film with increasing pH follows previously established trends in 316L [START_REF] Wang | Effect of pH on the Electrochemical Behaviour and Passive Film Composition of 316L Stainless Steel[END_REF], Ni-Cr-Mo [START_REF] Cwalina | In Operando Analysis of Passive Film Growth on Ni-Cr and Ni-Cr-Mo Alloys in Chloride Solutions[END_REF][START_REF] Lutton | Passivation of Ni-Cr and Ni-Cr-Mo Alloys in Low and High pH Sulfate Solutions[END_REF], and the CoCrFeMoNi CCA [START_REF] Wang | Corrosion behavior and surface characterization of an equiatomic CoCrFeMoNi high-entropy alloy under various pH conditions[END_REF].

Furthermore, Mo metal is stable at higher potentials in acids than bases [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solutions[END_REF], potentially explaining the cathodic dissolution of Mo observed in the basic environment that was not observed in the acid (Figures

6, S.3).

During potentiostatic passive film growth, the current density is highest for the Mo-free sample in pH 4 solution (Figure 10a), indicating that Mo may result in an enhanced passive film formation.

Alternatively, current density shows inconsistent trends with Mo concentration in pH 10 solution (Figure 10b). Potentiostatic EIS further verifies that Mo-0 had the lowest R P at pH 4 and that of Mo-3.2 was the lowest at pH 10 (Figure 12).

J o u r n a l P r e -p r o o f

Increasing Mo concentration improves pitting resistance more prominently in acidic environments (Figure 5, Table III), similar to the case of the Mo 0.6 CoCrFe medium entropy alloy developed by Shuang et al. [START_REF] Shuang | Unusually high corrosion resistance in MoxCrNiCo medium entropy alloy enhanced by acidity in aqueous solution[END_REF]. However, the improvement in corrosion resistance with decreasing pH observed is not present at a statistically significant level in the presently evaluated CCA series, potentially due to the lower Mo concentrations in the tested CCAs (0-3. • An FCC matrix enriched in Fe, Cr, and Mo (when added as alloying element) with L2 1 regions enriched in Al, Ti, and Ni was present for all CCAs. Mo is found to have indirect effects on corrosion through its impact on elemental partitioning after heat treatment at 1070 ˚C. The microstructural partitioning affects the nature of films grown on CCAs with increased Mo concentration, particularly with regards to the concentration of elements that preferentially partition to the second phase such as Ti. Such changes in element partitioning are shown to alter passive film chemistry, and thus have a significant effect on the corrosion behavior that must be accounted for within the alloy design process.

J o u r n a l P r e -p r o o f

• The resistance to localized corrosion was improved with increased Mo concentration both during potentiodynamic polarization and long-term immersion, most noticeably through improvements in the repassivation potential. Pitting often occurred at the FCC-L2 1 interfaces, particularly in low-Mo CCAs, but neither phase was vulnerable to selective dissolution.

• Mo also affects passivity and passive film chemistry. Decreased passive current densities and more positive repassivation potential with Mo content may suggest Mo slows corrosion reaction kinetics through solid state effects in the passive film. Species found in the passive films of CCAs with higher Mo concentration generally had more negative standard free energies, and did not appear to be limited in passivation by local enrichments in the microstructure. The overall corrosion resistance was generally worse than 316L in both acidic and basic environments, which may be attributable to the lean Cr contents compared to the high Cr content 316L as well as the microstructure containing two phases with corresponding interfaces.

• Trends in corrosion behavior with Mo content are less prominent in Al and Ti containing CCAs than single-phase alloy dependent on only Cr for passivity. The CCA design space introduces a variety of effects of alloying elements such as Mo with both metallurgical and passivity roles that have secondary effects on passivation that are not observed in single-phase or less complex alloys.

to the electrochemical characterization. S. Joseph Poon contributed to funding acquisition and sample synthesis.

  al. observed σ phase only at low Mo concentrations in magnetron sputtered CoCrFeMo x Ni thin films. Little effect was observed on room temperature corrosion behaviour in acidic sulfate solutions, however, corrosion resistance improved with Mo concentration at 80 ˚C. Additionally, increased surface cation fractions of both Cr and Mo were observed in the passive film of the CCAs with increasing Mo concentration [45]. Chou et al. evaluated the corrosion resistance of the Co 1.5 CrFeNi 1.5 Ti 0.5 Mo x CCA system [41]. Increasing Mo concentrations marked a transition from single phase FCC to an FCC with σ phase dendritic microstructure. The corrosion resistance decreased with increased Mo concentration in both acidic (H 2 SO 4 ) and basic (NaOH) solutions. In neutral NaCl solutions, the Mo-free CCA had a distinct pitting potential well below the potentials of similar current increases for all evaluated Mo containing CCAs, which dissolved in a transpassive manner following formation of a stable passive film. The breakdown behavior of the Mo-containing CCAs was identified by a negative hysteresis and verified by scanning J o u r n a l P r e -p r o o f electron microscopy (SEM) imaging to indicate a lack of pitting. Improved resistance to general corrosion at lower Mo concentrations and to pitting corrosion at higher concentration suggests that the optimal Mo concentration for passivity and localized corrosion resistance may differ. Similarly, Rodriguez et al. observed an increased breakdown potential and improved repassivation

J o u r

  n a l P r e -p r o o f was carried out at an applied potential of -0.2 V SCE (1 MHz to 5 mHz, 8 points/decade, 20 mV rms , modified Randles circuit fitting) before OCP was recorded for 1800 s. All electrochemical experiments were repeated threefold to ensure reproducibility. The pitting potential (E pit ) in NaCl was defined at the potential for which anodic current densities exceeded 10 -4 A.cm -2 with abrupt change in slope. Statistically distributed properties obtained during potentiodynamic polarization testing in chloride solutions such as pitting and repassivation potentials necessitated additional replication. Cumulative probability plots were developed in accordance with ASTM G16-95 2[START_REF] Astm | Standard Guide for Applying Statistics to Analysis of Corrosion Data[END_REF] with methods previously adapted to characterize localized corrosion in Ni-based Mo-containing alloys[START_REF] Kehler | Crevice Corrosion Stabilization and Repassivation Behavior of Alloy 625 and Alloy 22[END_REF][START_REF] Kehler | Role of Metastable Pitting in Crevices on Crevice Corrosion Stabilization in Alloys 625 and 22[END_REF].To identify the fate of individual elements in passivation and dissolution, AESEC was utilized on Mo-3.2 and pure Mo samples during the polarization and potentiostatic film growth procedures described above. The electrolyte solution containing dissolved elements was transported to a Horiba Jobin Yvon Ultima 2C TM inductively coupled plasma atomic emission spectrometer (ICP-AES) via a flow cell. Elemental dissolution rates were evaluated by monitoring emission intensity at a characteristic wavelength for each element. A polychromator at a focal length of 0.5 m was utilized alongside a monochromator used to obtain a better signal resolution of Mo, Cr or Al depending on the electrolyte condition. To compare to electron current density (j e ), elemental dissolution rates were converted to an equivalent current density (j M ) with Faraday's law. A more thorough scientific basis and methodology for AESEC analysis is available elsewhere[START_REF] Ogle | Atomic Emission Spectroelectrochemistry: Real-Time Rate Measurements of Dissolution, Corrosion, and Passivation[END_REF]. Dissolution rates in Ar-deaerated 0.1 M NaCl pH 4 and 0.1 M NaCl pH 10 were monitored during potentiodynamic polarization from -1.3 to +1.0 V SCE (0.5 mV/s) following a 600 s exposure to OCP and a 600 s cathodic treatment at -1.3 V SCE . Additionally, dissolution rates were monitored during potentiostatic passive film formation (-0.2 V SCE , 4 ks) directly after exposure to OCP, bypassing any cathodic reduction. The test time was decreased relative to previous electrochemical tests to ensure plasma stability.

  size3 were used. High resolution spectra were collected over the Al 2p, Cr 2p 3/2 , Fe 2p 1/2 , Mo 3d, Ni 2p 3/2 , and Ti 2p 3/2 core series with spectra bounds defined by an initial survey scan. Spectra were calibrated with C 1s set to 284.8 eV and deconvoluted with a Shirley background correction, Doniach-Sunjic peaks for metallic features, and Voigt functions for oxidized features with KOLXPD™ analysis software. Peak deconvolutions were constrained by position, full-width half maximum, and multiple splitting utilizing spectra of known reference compounds obtained elsewhere[START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn[END_REF][START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn[END_REF][START_REF] Baltrusaitis | Generalized molybdenum oxide surface chemical state XPS determination via informed amorphous sample model[END_REF][START_REF] Obrosov | Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings[END_REF]. As Ni Auger peaks overlap the Fe 2p 3/2 core series, a constant position shift between the experimentally obtained Fe 2p 1/2 series and reference spectra in the Fe 2p 3/2 series was assumed to identify chemical species. The intensity of the Cr 3s series, which overlaps the Al 2p series, was fixed based on the intensity of the Cr 2p 3/2 series adjusted with relative sensitivity factors. Cation fractions were evaluated as the proportion of the intensities of the oxidized states for each element normalized with relative sensitivity factors with methods discussed further in the supplementary information section. Multi-cation species (e.g. spinels) are only suggested

Figure 1 :

 1 Figure 1: XRD patterns for synthesized and homogenized CCAs. Square and star indexes identify corresponding diffraction peaks for FCC and L2 1 phases respectively.

Figure 2 :

 2 Figure 2: BSE micrographs of a) Mo-0, b) Mo-1.1, and c) Mo-3.2 microstructures

Figure 3 :

 3 Figure 3: EDS mapping of representative second phase regions within the Mo-3.2 microstructure Characterization of Passivity and Transpassive Breakdown by Potentiodynamic Polarization in Cl -Free Electrolytes E-log(i) curves obtained in acidic and basic Na 2 SO 4 solutions, which allow for characterization of the passivation process without Cl -induced breakdown, are shown in Figure 4. Extracted parameters are

Figure 4 :Figure 5

 45 Figure 4: Potentiodynamic polarization of CCAs and 316L in N 2(g) bubbled a) 0.1 M Na 2 SO 4 pH 4 and b) 0.1 M Na 2 SO 4 pH 10 following cathodic reduction pre-treatment (600 s, -1.3 V SCE ). Characterization of Passivity and Localized Breakdown by Potentiodynamic Polarization in Cl -Containing

Figure 5 :

 5 Figure 5: a,d) E-log(i) and cumulative probability plots for b, e) pitting potential and c, f) repassivation potential summarizing potentiodynamic polarization of CCAs, pure Mo, and 316L in N 2(g) bubbled a,b,c) 0.1 M NaCl pH 4 and d,e,f) 0.1 M NaCl pH 10 following cathodic reduction pre-treatment (600 s, -1.3 V SCE ).

  . The repassivation potentials (E rep ) obtained from downward scans, such as those shown in Figure S.2, indicate the presence of positive trends with increasing Mo concentration. Although the distribution of E rep increases from Mo-0 to Mo-3.2 in both environments, Mo-1.1 has a similar distribution to Mo-0 in the pH 4 environment whilst it is similar to that of Mo-3.2 in the pH 10 environment. Similar trends are shown for the E pit in the pH 4 environment, with the E pit increasing by nearly 0.15 V from Mo-0 to Mo 3.2. The increase suggests Mo increases the resistance of the passive film to localized breakdown or that the stabilization and propagation stages are affected. However, E pit is higher for Mo-0 in the pH 10 environment, deviating from the trends established for E rep and E pit in the pH 4 environment. Neither E pit nor E rep are affected to a statistically significant level by pH, with the notable exception of the increased E pit of Mo-0 in the pH 10 solution. Intermediate spikes before breakdown in the E-log(i) curves, most noticeably those of Mo-3.2 and 316L, suggest breakdown and repassivation due to metastable pitting. Neither the passivation nor breakdown behavior of any of the CCAs resembled that of pure Mo.J o u r n a l P r e -p r o o fAESEC was utilized to track individual elemental dissolution rates during upward polarization of the Mo-3.2 alloy (Figure6). The elemental dissolution rates are presented in equivalent current densities with the oxidation states used to calculate them indicated in the parenthesis. At both pH 4 and pH 10, elemental dissolution rates are mostly below the detection level at potentials in the passive regime (approximately -0.45 to -0.20 V SCE for pH 4 and -0.8 to -0.15 V SCE for pH 10). For example, at pH 4, Ni and Fe begin to dissolve at potentials near E pit (-0.07 V SCE ) while other elements dissolve at more positive potentials (0.07 V SCE for Cr, 0.17 V SCE for Mo, Al, and Ti). At higher potentials (above 0.0 V SCE ), the elemental dissolution rates are not proportional to the composition of either phase.

Figure 6 :

 6 Figure 6: Equivalent current densities of in-situ elemental dissolution rates monitored via AESEC during upward polarization of Mo-3.2 in N 2(g) bubbled a) 0.1 M NaCl pH 4 and b) 0.1 M NaCl pH 10. Dashed lines indicate element detection limits.The Mo-3.2 elemental dissolution rates within the transpassive region calculated from the

  2 and pure Mo (Figure S.3) for both environments with significantly higher Mo dissolution rates in pH 10. The higher Mo dissolution in the cathodic potential range at pH 10 may be attributed to the dissolution of the pre-existing Mo-based corrosion products formed after mechanical grinding. A significantly high cathodic current density at the level of -1*10 -4 A.cm -2 is observed for both Mo-3.2 (Figure 6a) and pure Mo (Figure S.3) at pH 4 with relatively low Mo dissolution rate compared to that at pH 10. Notably, Mo dissolution shows an increased Tafel slope relative to other elements for both pH 4 and pH 10.

Figure 7 shows

 7 Figure7shows EIS characterization utilized to provide a more comprehensive electrochemical

Figure 7 :

 7 Figure 7: a,c) Bode and b,d) Nyquist plots obtained following 30 minutes exposure of the air-formed oxides to open circuit corrosion in N 2(g) bubbled a,b) 0.1 M NaCl pH 4 and c,d) 0.1 M NaCl pH 10. The final OCP values and potentials at which EIS was measured are indicated in the figure legends. e) Equivalent circuit model utilized for EIS fits J o u r n a l P r e -p r o o f

Figure 8 :

 8 Figure 8: BSE micrographs showing representative pit morphologies for a) Mo-0, b) Mo-1.1, c) Mo-3.2, and d) 316L. High magnification micrographs show pits formed on e) Mo-0, f) Mo-1.1, and g) Mo-3.2 with regards to second phase morphology.

Figure 9 :

 9 Figure 9: Bode plots following 30 minutes and 20-day exposure to open circuit corrosion in 0.1 M NaCl pH 4 of the air-formed oxides formed on a) Mo-0, b) Mo-1.1, c) Mo-3.2, and d) 316L. Spectra were fit with the equivalent circuit model shown in Figure 7e. The final OCP values and potentials at which EIS was measured are indicated in the figure legends.

  pH 10 solution, all current densities decrease with time and trends with Mo content are less prominent, with Mo-1.1 having the highest current density at the conclusion of the film growth procedure. Sharp decreases in current densities approaching zero occur on most alloys as anodic passivation approaches the residual oxygen reduction rate, indicating a transition from the anodic to cathodic regime. For both environments, 316L has lower current density magnitudes than all evaluated CCAs.

Figure 10 :

 10 Figure 10: Current densities of CCAs and 316L during exposure to a -0.2 V SCE potential in N 2(g) bubbled a) 0.1 M NaCl pH 4 and b) 0.1 M NaCl pH 10 following cathodic reduction pre-treatment (600 s, -1.3 V SCE ).

Figure 11 :

 11 Figure 11: Equivalent current densities of in-situ elemental dissolution rates monitored via AESEC during exposure of Mo-3.2 to a -0.2 V SCE potential in N 2(g) bubbled a) 0.1 M NaCl pH 4 and b) 0.1 M NaCl pH 10. Dashed lines indicate element detection limits. Al dissolution rates are not shown in pH 4 due to high amounts of noise.

Figure 12 :

 12 Figure 12: a,c) Bode and b,d) Nyquist plots obtained following cathodic reduction pre-treatment (600 s, -1.3 V SCE ) and 10 ks exposure to -0.2 V SCE in N 2(g) bubbled a,b) 0.1 M NaCl pH 4 and c,d) 0.1 M NaCl pH 10. Spectra were fit with the equivalent circuit model shown in Figure 7e. J o u r n a l P r e -p r o o f

  For all CCAs and growth conditions, Cr(III), Ti(IV), and (in the case of Mo-1.1 and Mo-3.2) Mo(IV)/Mo(VI) are present in the passive film at higher concentrations than their respective bulk compositions. Fe(II)/Fe(III) and Ni(II) are depleted in the passive film relative to alloy content for all cases. Al(III) is present for all CCAs, but depleted in the film compared to the bulk in the Mo-0 alloy at pH 4 and the Mo-3.2 alloy at pH 10. For all CCAs, Cr(III) is the most prominent constituent in the passive film. However, the concentration of Cr(III) in the passive film decreases both with Mo concentration and pH. Although Fe(II)/Fe(III) and Ni(II) remained depleted in the film grown on Mo-3.2 at pH 10 relative to bulk composition, the surface cation fractions increased relative to the film grown at pH 4. Such increases were compensated by decreases in Al(III), Cr(III), J o u r n a l P r e -p r o o f Mo(IV)/Mo(VI), and Ti(IV), suggesting the film more closely resembles bulk alloy composition. However, such decreases do not necessarily suggest instability of such elements in basic conditions.

Figure 13 :

 13 Figure 13: Surface cation fractions calculated from XPS characterization of CCA passive films formed during 10 ks exposure to -0.2 V SCE in 0.1 M NaCl pH 4 and 0.1 M NaCl pH 10 following cathodic reduction pre-treatment (600 s, -1.3 V SCE ).

Figure 14

 14 Figure14shows fit XPS spectra for Cr, Mo, and Ni. Spectra for Al, Fe, and Ti along with precise fit

Figure 14 :

 14 Figure 14: XPS spectra of CCA passive films formed during 10 ks exposure to -0.2 V SCE in 0.1 M NaCl pH 4 and 0.1 M NaCl pH 10 following cathodic reduction pre-treatment (600 s, -1.3 V SCE ).

[ 63 ]

 63 . Mo(IV)/Mo(VI) cations are still suggested to be present in the passive film, although at lower concentrations in the Mo-3.2 film formed in pH 10 solution even though Mo oxides are not thermodynamically predicted to be stable. Despite low concentration in the bulk alloy, the presence of Mo in the alloy affects the passive film chemistry and possibly ionic defect status. As Mo concentration increases in the evaluated CCAs, Cr(III), Fe(II)/Fe(III), and Ni(II) content decrease while Al(III), Mo(VI), and Ti(VI) increase in the electrochemically formed passive films (Figure13, Table

  Fe and Ni dissolve at pH 4 while they are below the detection limit at pH 10 at this potential for CCAs investigated in this work (Figures 11).

  Fe and Ni dissolve at a lower potentials than the other elements at both pH (Figure6), and are depleted in the passive film at pH 4, as observed via XPS (Figure13, TableIX). Despite thermodynamic instability, limited presence of Ni and Fe in the passive films formed at pH 4 indicate metastable oxide presence and/or stability enhanced by solubility with additional cations.

  2 at. %) than in Mo 0.6 CoCrFe (16.7 at. % Mo). The same effect is not observed with E rep , where Mo resulted in higher R P regardless of pH. Notably, the E rep distribution of Mo-1.1 resembles that of Mo-3.2 in the pH 10 environment, which shows good repassivation behavior, and that of Mo-0 in pH 4, which shows poor repassivation behavior. This may indicate a lower Mo concentration is necessary for significant repassivation benefits in basic conditions.ConclusionsThree Al 0.3 Cr 0.5 Fe 2 Mo x Ni 1.5 Ti 0.3 (x = 0, 0.05, 0.15) CCAs with Mo concentrations of 0, 1.1, and 3.2 at. % were synthesized and characterized to elucidate connections between microstructure and corrosion resistance with the main motivation to investigate the benefits of Mo in Cr-lean CCAs containing Al and Ti as passivators. The following conclusions were obtained:

Table I :

 I Nominal

	hereafter are reported. An area of 0.1 cm 2 was exposed via a rubber O-ring to electrolyte solutions of 0.1
	M NaCl adjusted to both pH 4 and 10 with concentrated HCl and NaOH respectively to evaluate corrosion
	behavior over a range of environments with differing stability levels of Mo. The electrolyte was continually
	CCA and 316L compositions in atomic percent. Pure elements were massed within 1% bubbled with N 2(g) throughout testing to reduce the activity of dissolved oxygen. All samples were
	error of the tabulated compositions prior to arc melting. The 316L compositions are defined as the compared to a commercially procured (North American Steel) 316L stainless steel (UNS S31600) control
	median of the acceptable range and also include minor amounts of C, N, P, S, and/or Si.
	of a comparable Mo concentration (2-3 at. %). Three procedures were utilized to provide a comprehensive
	Alloy Mo-0 Mo-1.1 Mo-3.2 316L Phases present in the microstructure were identified with XRD using Cu Kα X-rays (1468.7 eV) at Al Cr Fe Mo Ni Ti 6.5% 10.9% 43.5% 0.0% 32.6% 6.5% 6.5% 10.8% 43.2% 1.1% 32.3% 6.5% 6.3% 10.5% 42.1% 3.2% 31.6% 6.3% 0.0% 17.0% 68.5% 2.5% 12.0% 0.0% a scan rate of 0.15 °/s on a PANalytical Empyrean Diffractometer TM using Bragg-Brentano focusing geometry. The optics used were a 4 mm mask, 0.25° divergence slit, 1.52 mm anti-scatter slit, and 1.15° Soller slit on the incident beam side with a 16.8 mm anti-scatter slit and 2.3° Soller slit on the diffracted beam side. The microstructure was imaged with SEM on an FEI Quanta 650 TM in backscattered electron (BSE) imaging mode at an accelerating voltage of 15 keV, a probe size of approximately 4 nm, and a working distance of approximately 10 mm. Energy dispersive spectroscopy (EDS) was utilized in both point scan and mapping mode to evaluate phase composition and was analyzed with Oxford Instruments analysis of the passivity and corrosion resistance of the alloys. The solution-exposed air-formed oxide film was electrochemically characterized by measuring the open circuit potential (OCP) for 1800 s directly after grinding. OCP measurement was followed by electrochemical impedance spectroscopy (EIS) at the final OCP measured between frequencies of 1 MHz and 5 mHz with 8 points/decade and an applied AC potential of 20 mV rms . EIS spectra were fit to a Randles circuit modified to include a constant phase element (CPE) with Gamry Echem Analyst software TM . The J o u r n a l P r e -p r o o f procedure was repeated after 20-day immersion in 0.1 M NaCl pH 4 alongside mass loss measurements.
	Aztec TM software. Phase fractions were approximated from brightness threshold analysis of the BSE
	micrographs via ImageJ.
	A conventional three-electrode cell connected to a Gamry Instrument Reference 600+™
	potentiostat was utilized for electrochemical testing with the sample as the working electrode, a platinum
	mesh counter electrode, and a saturated calomel reference electrode (SCE) relative to which all potentials

Table II :

 II Elemental fractions obtained from EDS point scans over matrix and second phase regions of each CCA and area fractions obtained via ImageJ analysis of a series of BSE micrographs.

	Phase	Al	Cr	Fe	Mo	Ni	Ti	Area
				Mo-0
	Matrix	7.8	13.5	52.8	-	18.6	7.3	96.9%
	2 nd Phase	18.7	4.3	18.9	-	42.5	15.6	3.1%
				Mo-1.1
	Matrix	6.1	11.5	44.1	1.3	31.1	5.9	95.6%
	2 nd Phase	20.5	2.3	13.4	Trace	45.1	18.7	4.4%
				Mo-3.2
	Matrix	3.7	11.6	44.7	3.2	30.7	6.1	96.0%
	2 nd Phase	15.3	2.5	13.9	0.5	47.3	20.5	4.0%

Table III :

 III Selected

	b)		0.1 M NaCl pH 10
	Alloy	E corr (V SCE ) E pit (V SCE ) E rep (V SCE )	i pass
					(μA.cm -2 )
	Mo-0	-0.627 +/-	0.435 +/-	-0.131 +/-	5.38 +/-
		0.237	0.285	0.094	2.96
	Mo-1.1 -0.675 +/-	0.11 +/-	0.016 +/-	4.86 +/-
		0.26	0.157	0.049	3.93
	Mo-3.2 -0.852 +/-	0.338 +/-	0.028 +/-	6.59 +/-
		0.153	0.094	0.084	5.31
	316L	-0.307 +/-	0.431 +/-	-0.019 +/-	0.130 +/-
		0.004	0.111	0.004	0.028
	a)		0.1 M NaCl pH 4
	Alloy	E corr (V SCE ) E pit (V SCE ) E rep (V SCE )	i pass
					(μA.cm -2 )
	Mo-0	-0.827 +/-	0.256 +/-	-0.086 +/-	6.88 +/-
		0.194	0.231	0.129	2.13
	Mo-1.1 -0.767 +/-	0.222 +/-	-0.115 +/-	16.78 +/-
		0.238	0.218	0.106	15.55
	Mo-3.2 -0.501 +/-	0.399 +/-	0.091 +/-	13.76 +/-
		0.285	0.204	0.053	13.98
	316L	-0.372 +/-	0.465 +/-	-0.072 +/-	0.18 +/-
		0.062	0.008	0.079	0.05

corrosion parameters for potentiodynamic polarization shown in Figure 5 obtained in a) 0.1 M NaCl pH 4 and b) 0.1 M NaCl pH 10. Each term includes the mean value bounded by a one standard deviation range. J o u r n a l P r e -p r o o f All three CCAs generally have lower localized corrosion resistance than 316L in the Cl --containing solutions, with passive films breaking down by pitting and limited crevice corrosion

Table IV :

 IV Proportion of transpassive elemental dissolution attributable to each element in at. %. Values are determined from the ratios of equivalent current densities obtained via AESEC at E = 1.0 V SCE during LSV in 0.1 M NaCl at pH 4 and 10 shown in Figure6. Bolded terms indicate dissolution rates higher than the bulk alloy composition suggesting depletion.

	Electrolyte	Al	Cr	Fe	Mo	Ni	Ti
	0.1 M NaCl pH 4	4.1%	7.6%	53.6%	2.5%	28.6%	3.6%
	0.1 M NaCl pH 10	4.0%	7.7%	58.5%	2.8%	23.8%	3.2%

Dissolution of Mo in the cathodic region is observed for both Mo-3.

Table VI :

 VI Mass loss and average area penetration rates obtained during 20-day immersion of CCAs and 316L in 0.1 M NaCl pH4. The mass gain observed for Mo-3.2 approached the tolerance of the scale andmay be interpreted as negligible.

	Alloy	Mass Loss Rate	Area Average
		(mg.cm -2 .y -1 )	Penetration Rate (μm.y -1 )
	Mo-0	1.3	1.7
	Mo-1.1	13.2	17.3
	Mo-3.2	-0.1 (Trace)	-0.1 (Trace)
	316L	0.8	1.0

Table VII :

 VII EIS

	J o u r n a l P r e -p r o o f
			0.1 M NaCl pH 4: Day 20
	Alloy	R S (Ω.cm 2 )	R P (kΩ.cm 2 )	Y (μS.s α .cm -2 )	α f
	Mo-0	72.8	1206.0	28.4	0.877
	Mo-1.1	69.2	27.1	97.4	0.811
	Mo-3.2	84.2	45.8	68.8	0.895
	316L	71.6	1679.0	27.1	0.895

fit parameters for the spectra shown in Figure 9 obtained following 20-day immersion in 0.1 M NaCl pH 4. Each term is defined by the Randles circuit shown in Figure 7e.

Table IX :

 IX Surface

	Alloy	Al	Cr	Fe	Mo	Ni	Ti
	Mo-0 pH 4	0.9%	51.4%	8.9%	0.0%	17.8%	21.0%
	Mo-1.1 pH 4	9.1%	52.6%	9.9%	5.9%	1.9%	20.6%
	Mo-3.2 pH 4	6.9%	44.6%	1.7%	13.9%	0.0%	32.9%
	Mo-3.2 pH 10	1.7%	30.4%	25.1%	3.8%	21.6%	17.4%

cation fractions obtained from the XPS fits shown in Figure

14

. Bolded terms indicate surface cation fractions higher than the bulk alloy composition suggesting enrichment.

For the passive current density probability plots, the procedure was modified to take the linear regression from the logarithm of the current densities.J o u r n a l P r e -p r o o f

The XPS spot size is significantly larger than any second phase regions, limiting the ability to evaluate the lateral variation in passive film chemistry with regards to the bulk phase over which the film was grown. XPS measurements are obtained over a large enough area that they may be reasonably assumed to be representative of the entirety of the microstructure. High-resolution evaluation of the passive film chemistry with regards to phase interfaces in the microstructure will form the basis for a future study.

J o u r n a l P r e -p r o o f

Acknowledgements

This work was supported by the United States Office of Naval Research under grant # N00014-19-1-2420. Use of the Empyrean TM Diffractometer, Quanta 650 TM SEM, and PHI VersaProbe III TM XPS was made possible by the University of Virginia Nanomaterials Characterization Facility. The PHI VersaProbe III was procured under NSF award # 162601. Author K. Ogle and the AESEC equipment were supported by Agence Nationale de Recherche, award # ANR-20-CE08-0031 (TAPAS 2020). Kaitlyn Anderson contributed J o u r n a l P r e -p r o o f

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

J o u r n a l P r e -p r o o f

Highlights

• The passivation behavior and corrosion resistance in chloride and sulfate of a dual-phase compositionally complex alloy series with varying Mo content is characterized.

• Pitting and repassivation potentials increase with Mo concentration in acidic solutions where Mo is stable, indicating improved resistance to resistance to localized corrosion that often occurs at phase interfaces.

• Elemental fates are tracked with X-ray Photoelectron Spectroscopy and Atomic Emission Spectroelectrochemistry.

• Mo concentration affects the passive film chemistry, with Ni passivation suppressed in favor of more stable species such as TiO 2 .

• The addition of Mo increases the degree of elemental partition within the FCC+L2 1 microstructure, potentially limiting interaction between Al, Cr, Ti, and/or Mo in the passive film.

J o u r n a l P r e -p r o o f