Origin of enhanced passivity of Cr-Fe-Co-Ni-Mo multi-principal element alloy surfaces

The study addresses how ultrathin passive oxide films confer enhanced corrosion resistance to Cr-Fe-Co-Ni-Mo multi-principal element alloys (MPEAs), particularly against chloride-induced breakdown. While passivity on conventional stainless and Ni-based alloys is known to arise from a stratified oxide enriched in Cr (and often Mo), the exact roles, distribution, and chemical states of Mo in MPEAs, as well as the origins of their superior passivity, remain insufficiently resolved. The purpose is to elucidate the chemical structure, stratification, thickness, and depth distribution of key species—especially Mo valence states—in native and electrochemically passivated films on single-phase fcc MPEAs. Understanding these features is important for rational alloy design to maximize resistance to localized corrosion.

Prior surface-analytical studies (XPS, ToF-SIMS, AES) on stainless and Ni-based alloys established a bilayer passive film: an inner Cr(III)-oxide barrier and an outer layer containing Cr hydroxides and Fe/Ni/Mo oxides/hydroxides. Chromium confers stability via a slowly dissolving Cr2O3 inner layer, while Mo has been proposed to hinder aggressive ion penetration or promote self-repair. Passivation generally enriches Cr (and Mo in Mo-bearing alloys) through selective dissolution of Fe, and can dehydroxylate films. Passive film thicknesses typically range 1–3 nm and alloy beneath the oxide often shows Ni enrichment due to preferential oxidation of Cr/Fe. The bilayer model and possible nonequilibrium solute capture of species like Ni2+, Fe2+, and Mo4+ within the Cr-oxide matrix have been reported. For Mo-containing MPEAs, studies (e.g., FeCoCrNiMox; Ni38Cr21Fe20Ru13Mo6W2) observed similar bilayer films and hinted at solute capture, but comprehensive depth-resolved chemistry and Mo speciation/location in single-phase fcc MPEAs remained limited. These gaps motivate a detailed analysis to clarify Mo’s role in enhanced passivity and localized corrosion resistance in chloride environments.

Materials: Three single-phase fcc MPEAs with similar grain sizes were studied: Cr35Fe20Co5Ni40 (MPEA-35Cr), Cr15Fe10Co5Ni60Mo10 (MPEA-15Cr10Mo), and Cr25Fe25Co5Ni40Mo5 (MPEA-25Cr5Mo) in at.%. Surfaces were mechanically polished (to 0.25 µm diamond), ultrasonically cleaned (acetone, ethanol, deionized water), and dried to yield native air-formed oxides. Electrochemical passivation: Conducted in a 3-electrode cell with MSE reference (ESHE = EMSE + 0.64 V) and Pt counter. Electrolyte: 0.05 mol L−1 H2SO4, deaerated with Ar for 30 min. After 15 min OCP, potential was stepped to 0 VMSE for 1 h at room temperature. The selected potential corresponds to minimum passive current density previously determined. Surface analysis: ToF-SIMS depth profiling using IONTOF ToF-SIMS 5 in dual-beam mode: analysis with pulsed 25 keV Bi+ (1.2 pA) over 100 × 100 µm2, sputtering with 0.5 keV Cs+ (17 nA) over 300 × 300 µm2, both at 45° incidence; negative ions recorded. The Ni2− signal (115.8707 amu) was used to locate the metal/oxide interface (80% of maximum). Representative ions included CrO−, CrO3H2−, FeO2−, NiO2H−, MoO−, MoO3−, indicating Cr(III) oxide/hydroxide, Fe(II/III) oxides/hydroxides, Ni(II) hydroxide, and Mo(IV/VI) oxides. XPS: Thermo ESCALAB 250 with monochromatic Al Kα (hν = 1486.6 eV); base pressure ~1e−8 mbar; high-resolution spectra (Cr 2p, Fe 2p, Co 2p, Ni 2p, Mo 3d, O 1s, S 2p, C 1s) at pass energy 20 eV, step 0.1 eV. Angle-resolved XPS at 90° and 40° take-off angles for depth sensitivity. Curve fitting used CasaXPS with Shirley background (metals) and linear background (S 2p). Extensive handling of overlapping Auger contributions (Ni L3M23M45 and Co L2M23M45) was implemented via reference lineshapes measured on oxide-free pure metals. Metallic components were fitted with asymmetric LF lineshapes; oxides/hydroxides with GL mixes. Fixed BE/FWHM parameters were derived from literature and in-house references. Quantification/modeling: A bilayer model (outer hydroxide/oxide layer; inner Cr2O3 barrier) with sharp interfaces and uniform composition/thickness per layer was applied to XPS intensities to estimate equivalent layer thicknesses and compositions; metallic signals were assigned to a modified alloy region beneath the oxide. A trilayer adaptation was used to evaluate the presence and equivalent thickness of a Mo(IV)-rich interfacial feature. Assumptions neglect surface roughness and gradual gradients; results represent average compositions and equivalent thicknesses.

• Bilayer passive films on all alloys: inner barrier predominantly Cr(III) oxide (Cr2O3), outer layer enriched in Cr(III) hydroxide with Fe/Ni oxides-hydroxides and Mo oxides on Mo-bearing alloys.
• Thicknesses: For Mo-bearing quinary alloys (MPEA-25Cr5Mo and MPEA-15Cr10Mo), native films were 1.4–1.5 nm; passive films increased to ~1.8–2.1 nm. For Mo-free MPEA-35Cr, native film was thicker (~2.1 nm) and thinned upon passivation to ~1.8 nm.
• Layer-specific growth upon passivation depends on bulk composition: MPEA-25Cr5Mo (higher Cr) showed pronounced inner-layer thickening (inner layer to ~0.6 nm, consistent with a continuous barrier), whereas MPEA-15Cr10Mo (higher Mo) showed more outer-layer growth.
• Outer-layer composition enrichment after passivation: • MPEA-25Cr5Mo: Cr from 38 to 48 at.%; Mo from 15 to 21 at.% (Fe and Ni decreased accordingly). • MPEA-15Cr10Mo: Cr from 17 to 45 at.%; Mo from 21 to 31 at.% (Fe and Ni decreased). • MPEA-35Cr: outer-layer Cr increased to 74 at.% after passivation; Fe and Ni decreased.
• Modified alloy region beneath oxide: Ni enrichment and Cr/Fe depletion in the native state; Ni enrichment attenuated after passivation, consistent with selective dissolution of Ni into the electrolyte; Mo in the modified alloy did not further deplete despite increased Mo in the oxide.
• Depth distribution of Mo species (AR-XPS and ToF-SIMS): Mo(VI) is most outward in the outer layer; Mo(IV) is located deeper within the outer layer near the outer/inner interface; an intermediate Mo species lies beneath Mo(IV), near/within the outermost part of the inner layer.
• Discontinuous Mo(IV)-rich interfacial feature: Trilayer modeling indicates a very thin (0.04–0.12 nm) Mo(IV) layer at/near the outer–inner interface, below a monolayer, implying discontinuity and suggesting Mo(IV) terminations at Cr-oxide inner layer.
• Intermediate Mo species location: inferred to reside primarily at the outermost inner layer close to the interface, possibly reflecting nonequilibrium solute capture or substitution at local Cr(III)-depleted sites.
• Mo(IV) content within inner layer (from XPS assignments): ~8% (native) and ~6% (passive) for MPEA-25Cr5Mo; ~18% (native) and ~19% (passive) for MPEA-15Cr10Mo.
• Evidence of nanoscale heterogeneity: The inner barrier equivalent thickness on native films is extremely low (0.2–0.3 nm) and likely heterogeneous with Cr(III)-depleted zones that act as weak sites for chloride attack; Mo(IV) concentrated near the interface may reinforce these weak sites.
• Ni hydroxide peaks at the topmost surface of the oxide (ToF-SIMS NiO2H− peak preceding others), consistent with surface segregation of Ni(II) hydroxide.

The findings delineate a stratified, bilayer passive film structure on single-phase fcc Cr-Fe-Co-Ni-(Mo) MPEAs and map the depth distribution of Mo species, directly addressing the question of how Mo enhances passivity and resistance to chloride-induced breakdown. The inner Cr2O3 barrier underpins corrosion resistance, but its ultrathin, potentially heterogeneous nature suggests localized weak sites depleted in Cr(III). The detection of a discontinuous Mo(IV)-rich feature near the outer/inner interface and an intermediate Mo species at the outermost inner layer indicates that Mo partitions to regions where it can reinforce the barrier—either by sealing or substituting at Cr-depleted, Fe-richer weak sites—while Mo(VI) enriches the outer surface, potentially contributing to suppressing aggressive ion ingress and facilitating self-repair dynamics. Passivation-driven increases in outer-layer Cr and Mo, coupled with selective dissolution of Fe and Ni, further densify and enrich the protective outer layer. Composition-dependent growth (inner-layer growth with higher bulk Cr; outer-layer growth with higher bulk Mo) links alloy design to passive film architecture. Together, the depth-resolved chemistry provides a mechanistic rationale for the observed superior resistance to localized corrosion in chloride media for Mo-bearing MPEAs.

This work combines ToF-SIMS, XPS, and AR-XPS with tailored spectral decomposition and bilayer/trilayer modeling to resolve the stratified passive films on Cr-Fe-Co-Ni-(Mo) MPEAs. It establishes that: (i) a Cr2O3-rich inner barrier underlies passivity; (ii) the outer layer is enriched in Cr(III) hydroxide and Mo oxides, with Mo(VI) toward the surface and Mo(IV) concentrated near the outer/inner interface; (iii) passivation increases film thickness and Cr/Mo enrichment, with layer growth governed by bulk alloy composition; and (iv) Mo(IV) forms a discontinuous interfacial feature that likely seals Cr-depleted weak sites in the inner barrier, explaining enhanced resistance to chloride-induced localized corrosion. These insights inform alloy design strategies that balance Cr (to ensure a continuous inner barrier) and Mo (to reinforce interfacial weak sites and enrich the outer layer). Potential future work includes direct nanoscale imaging/quantification of Cr-depleted heterogeneities and Mo distributions, definitive identification of the intermediate Mo species, and correlating spatial chemistries with localized corrosion initiation under chloride exposure.

• The bilayer model assumes uniform, dense layers with sharp interfaces and neglects surface roughness and compositional gradients; results are equivalent thicknesses and average compositions.
• The inner barrier layer shows extremely low equivalent thickness, and inferred nanoscale heterogeneity is indirect; local variations are not explicitly imaged in this study.
• Co oxide/hydroxide speciation could not be reliably resolved due to low Co content and overlapping Auger features, limiting discussion of Co’s role.
• Assignments of the intermediate Mo species are based on indirect spectroscopic evidence (binding energies, AR-XPS depth sensitivity, ToF-SIMS profiles); its exact chemical nature remains uncertain.
• Sputter depth profiling can introduce artifacts; while care was taken, absolute depth scales and interfacial sharpness may be affected.