# Effects of Chloride Ions on Passive Oxide Films Formed on Cr-Fe-Co-Ni(-Mo) Multi-Principal Element Alloy Surfaces

Xueying Wang, Dimitri Mercier, Sandrine Zanna, Antoine Syeux, Loïc

Perriere, Mathilde Laurent-Brocq, Ivan Guillot, Vincent Maurice, Philippe
Marcus

► To cite this version:

Xueying Wang, Dimitri Mercier, Sandrine Zanna, Antoine Syeux, Loïc Perriere, et al.. Effects of Chloride Ions on Passive Oxide Films Formed on Cr-Fe-Co-Ni(-Mo) Multi-Principal Element Alloy Surfaces. Journal of The Electrochemical Society, 2023, 170 (4), pp.041506. 10.1149/1945-7111/accb10 . hal-04076058

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Effects of Chloride Ions on Passive Oxide Films
Formed on Cr-Fe-Co-Ni(-Mo) Multi-Principal
Element Alloy Surfaces

To cite this article: Xueying Wang *et al* 2023 *J. Electrochem. Soc.* **170** 041506

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# Effects of Chloride Ions on Passive Oxide Films Formed on Cr-Fe-Co-Ni(-Mo) Multi-Principal Element Alloy Surfaces

Xueying Wang,<fnref n="1" /> Dimitri Mercier,<fnref n="1" />,<fnref n="2" /> ID Sandrine Zanna,<fnref n="1" /> Antoine Syeux,<fnref n="1" /> ID Loïc Perriere,<fnref n="2" /> Mathilde Laurent-Brocq,<fnref n="2" /> Ivan Guillot,<fnref n="2" /> Vincent Maurice,<fnref n="1" /> ID and Philippe Marcus<fnref n="1" />,*,z ID

The composition and stratification of the passive oxide films formed on three Cr-Fe-Co-Ni(-Mo) multi-principal element alloys by electrochemical anodic passivation in sulfuric acid electrolyte containing 0.2 and 4.7 M NaCl were investigated, combining X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry surface analysis. The passive films show a bilayer structure similar to that formed in Cl⁻-free electrolyte with an inner layer mostly consisting of Cr oxide and an outer layer containing of Cr hydroxide, Ni hydroxide, Mo oxides, and Fe (hydr)oxide. The Mo-free alloy exhibits a thickening of the inner Cr oxide layer and the thinning of the outer layer in 0.2 M Cl⁻, whereas the two Mo-containing alloys do not show significant alteration even in 4.7 M Cl⁻ evidencing their higher stability in Cl⁻-containing solutions. The chloride penetration is limited to the external part of the outer oxide layer, except in the most severe tested conditions where traces reach the inner barrier layer, and the chloride entry into the layer is strongly reduced after pre-passivation in Cl⁻-free solution. The results allow us to discuss the beneficial effects of pre-passivation in Cl⁻-free conditions and Mo addition providing these alloys enhanced resistance to passivity breakdown.

© 2023 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/1945-7111/accb10]

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Manuscript submitted February 18, 2023; revised manuscript received April 3, 2023. Published April 18, 2023. This paper is part of the JES Focus Issue on Critical Factors in Localized Corrosion in Honor of Gerald Frankel.

Multi-principal element alloys (MPEAs) constitute a new generation of alloys stemming from the high entropy concept. Many definitions have been proposed to rationalize this concept and MPEAs can be considered as containing more than three alloying elements, among which at least two are principal elements, of single-phase structure or not, and without entropy effect.<fnref n="1" /> The concept of such alloys started in the 1980s and the first production dates back to 2004.<fnref n="2" />,³ This new alloying strategy strongly broadens the phase and compositional possibilities of alloy systems, allowing to seek after the optimization of desired properties via appropriate alloy design. Enhanced corrosion resistance is one focus of research.⁴⁻⁷

Many studies on traditional alloys have shown that self-protection via the formation of a passive oxide film is one of the effective means to endow alloys with high corrosion resistance. Either being thermodynamically stable or dissolving extremely slowly, a protective passive oxide film formed on the surface isolates the alloy substrate from the environment, thus protecting it from corrosion.⁸,⁹ Cr-containing alloys are typical examples of highly corrosion-resistant alloys thanks to passivity.⁹⁻¹¹ In acidic aqueous environment, their passivity results from the slow dissolution rate of Cr-rich surface oxide film.⁹ Given the important role of the surface oxide films in providing passivity, the nature of passive films has been extensively studied using multiple surface analytical techniques such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and scanning tunneling microscopy (STM).¹⁰⁻¹⁷ For Fe- or Ni-based Cr-containing alloys, it has been demonstrated that the passive oxide films formed in acidic solution at room temperature are only 1–3 nanometers thick and have an in-depth compositional stratification,¹¹,¹⁶,¹⁸⁻²² typically described by a bilayer model.¹⁸,²²,²³ The inner layer is mainly composed of Cr(III) oxide and considered as the barrier layer ensuring the high corrosion resistance, while the outer layer is reported to contain various species such as Cr(III) hydroxide, Fe(II), III) oxide/hydroxide, Mo(IV,VI) oxide and Ni(II) hydroxide depending on the alloy composition.¹⁵,¹⁸,²²⁻²⁴ It has been suggested

that depending on whether the diffusion process from the substrate is fast enough, the constituent species can be stoichiometric or not.²⁵⁻²⁷ During the oxide film formation, owing to the difference in oxygen affinity (Cr>Fe>Ni), preferential oxidation of Cr occurs and modifies the composition of the alloy close to the metal/oxide interface, usually resulting in depletion of metallic Cr and enrichment of metallic Ni.¹⁰,²¹,²²,²⁸ Due to the simultaneous nucleation in multiple sites at the early stage of oxide film formation,²⁹,³⁰ the oxide films are nanogranular, with crystalline oxide grains and non-ordered intergranular areas.¹²,¹⁷,¹⁹,³¹⁻³³ Extending the electrochemical passivation time is proven to promote the crystallization of the oxide film, resulting in the extension and coalescence of the nanograins.¹²,³¹ Apart from the electrolyte, the passivation duration and passivation potential are important factors influencing passivity.¹⁹ Besides, because of the local lack in metallic Cr supply from the substrate in the early stage of oxidation, the inner barrier layer may contain Cr-depleted oxide grains.³⁴

When passivity locally breaks down and cannot be self-repaired, localized corrosion initiates. Cl⁻ is one of the well-known aggressive ions promoting passivity breakdown and localized corrosion. Mechanisms such as acceleration of the oxide film dissolution and thinning via adsorption and complexation, promotion of the formation and accumulation of cation vacancies leading to oxide film collapse, or oxide film cracking due to stress accumulation have been proposed to explain Cl⁻-induced passivity breakdown.²⁹,³⁵⁻³⁹ Ab initio modeling suggests that these different routes may act concurrently in passivity breakdown, and may switch when conditions change.⁴⁰,⁴¹ Defective sites such as Cr-depleted sites and intergranular boundaries of the oxide film are viewed as more susceptible to local oxide film breakdown.²²,³²,⁴² Mo is an important alloying element improving corrosion resistance and its addition in minor amount in Cr-containing alloys can significantly increase their resistance to localized corrosion.⁴³⁻⁴⁵ Different mechanisms have been proposed for explaining the enhancing role of Mo in stabilizing passivity, and they remain under debate: (i) impediment to the penetration of aggressive ions,¹⁵,⁴⁶,⁴⁷ (ii) promotion of self-repair by suppressing active dissolution,²²,⁴³,⁴⁸,⁴⁹ and (iii) removal of adsorbed detrimental ions.⁵⁰,⁵¹

Such corrosion-related systematic investigations are being extended to MPEAs, leading to recent results. For oxide films formed

<footnote n="1">¹PSL University, CNRS - Chimie ParisTech, Institut de Recherche de Chimie Paris, Physical Chemistry of Surfaces Group, 75005 Paris, France</footnote>

<footnote n="2">²Université Paris Est Creteil, CNRS, ICMPE, UMR7182, 94320 Thiais, France</footnote>

<footnote>*Electrochemical Society Fellow.</footnote>

<footnote n="2">²E-mail: dimitri.mercier@chimieparistech.psl.eu; philippe.marcus@chimieparistech.psl.eu</footnote>
on the Cantor alloy (CrFeCoNiMn) and on CrFeCoNiMoₓ alloys,¹⁴,⁵² a stratified structure similar to that on traditional alloys has been confirmed. An oxide film composed of non-stoichiometric species in solid solution state and outside thermodynamic equilibrium is also reported for the NiCrFeRuMoW alloy.¹³ Several studies prove that the addition of less than ~5 at. % Mo leads to an enhanced corrosion resistance, whereas a higher Mo concentration limits the corrosion resistance performance due to the formation of secondary phases.⁵³⁻⁵⁶ However, in a previous work on the Cr-Fe-Co-Ni-Mo MPEA system, it has been shown, using a predictive computational thermodynamic approach, that the formation of detrimental secondary phases can be avoided while ensuring a high Cr and Mo content, thus allowing to produce Cr-Fe-Co-Ni-Mo MPEAs with very high resistance to Cl⁻-induced localized corrosion.⁵⁷ It has also been shown that Mo oxides have a refined valence-dependent in-depth distribution and may reinforce the Cr-depleted weak sites of the inner barrier layer.⁵⁸

The present work aimed at studying the relation between the composition and stratification of the passive films formed on Cr- and Mo-containing single-phase MPEAs in Cl⁻-containing environment and their high corrosion resistance. The passive oxide films formed in the presence of Cl⁻ in the electrolyte on three Cr-containing MPEAs of different compositions were investigated by using XPS and ToF-SIMS and compared, including with those formed in Cl⁻-free environment and previously studied.⁵⁸ The results allow us to discuss the influence of Cl⁻ in the electrolyte on the passive oxide films structure and composition, as well as the beneficial effects of pre-passivation in Cl⁻-free conditions and Mo addition on the resistance to passivity breakdown.

## Experimental

**Surface preparation.**—Single-phase fcc Cr₃Fe₂₀Co₅Ni₄₀, Cr₁₅Fe₁₀Co₅Ni₆₀Mo₁₀, and Cr₂Fe₂₅Co₅Ni₄₀Mo₅ (at. %) samples were used, denoted as MPEA-35Cr, MPEA-15Cr10Mo, and MPEA-25Cr5Mo, respectively. The conception and manufacturing of these alloys have been presented elsewhere.⁵⁷ The surface of the samples was mechanically polished with SiC paper down to 4000 grade and then with successive diamond suspensions of 3, 1 and 0.25 µm to obtain a mirror finish. After polishing, the samples were successively cleaned in ultrasonicated baths of acetone, ethanol and deionized water (Merck Millipore, resistivity of 18.2 MΩ.cm), and then dried in compressed air flow.

Electrochemical passivation was performed at room temperature in a 3-electrode cell consisting of a saturated Mercury sulfate reference electrode (MSE, E_SHE = E_MSE + 0.64 V), a Pt wire used as counter electrode and the alloy sample as working electrode. The exposed area to the electrolyte was 1.13 cm², delimited by a Viton O-ring. All electrochemical measurements were performed using a VersaSTAT4 potentiostat (AMETEK). Electrolytes were prepared from ultrapure chemicals (VWR) and deionized water. Prior to the experiments, the electrolyte was deaerated by bubbling Ar gas for 30 min in order to minimize the dissolved oxygen. Two different passivation procedures were applied for each alloy, and for two different Cl⁻ concentrations except MPEA-35Cr: Passivation procedure (1) was performed directly in the Cl⁻-containing electrolyte (0.05 M H₂SO₄ containing 0.2 or 4.7 M NaCl). After 15 min of stabilization at open circuit potential (OCP), the sample was passivated for 1 h by stepping the potential to 0 V_MSE, which corresponds to the same potential previously used for studying the passive films formed in absence of Cl⁻.⁵⁸ Passivation procedure (2) was performed in two steps: first in the absence of chlorides, and then after adding Cl⁻. After stabilization at OCP for 15 min in the initially Cl⁻-free 0.05 M H₂SO₄ solution, the sample was pre-passivated for 1 h by stepping the potential to 0 V_MSE. This stage is called pre-passivation for the sake of comparison with the passivation process performed in the presence of Cl⁻. After 1 h of pre-passivation, a 0.05 M H₂SO₄ solution containing 5 M NaCl was added in the cell to adjust the Cl⁻ concentration in the electrolyte to

0.2 or 4.7 M and the anodic polarization, at the same potential, was pursued for another 1 h.

The low Cl⁻ concentration of 0.2 M was chosen based on preliminary tests, enabling us to apply the two selected passivation procedures to all three MPEAs without initiating localized corrosion. For the two highly corrosion-resistant quinary alloys, the high concentration of Cl⁻ of 4.7 M, tested in previous work,⁵⁷ was additionally examined. In all cases, no sudden rise of the passive current characteristic of passivity breakdown followed by the initiation of localized corrosion was observed during the potentiostatic passivation experiments. After electrochemical passivation, no pitting or crevice corrosion was identified using optical microscopy. After being rinsed with ultrapure water and dried in a flow of compressed air, the passivated samples were analyzed by XPS and ToF-SIMS.

**Surface characterization.**—XPS analyses were performed in a Thermo Electron ESCALAB 250Xi spectrometer with a base pressure lower than 10⁻¹⁰ mbar, using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). High-resolution spectra of the Cr 2p, Fe 2p, Co 2p, Ni 2p, Mo 3d, Cl 2p, S 2p, O 1s, Na 1s and C 1s core levels were recorded at 90° take-off angle of the photoelectrons, with a pass energy of 20 eV and a step size of 0.1 eV. The Na 1s core level spectrum was measured to verify that there was no detectable residual electrolyte on the sample surface after rinsing and drying. Data processing (curve fitting) was performed with CasaXPS. Due to the non-negligible Auger peaks overlapping with the Fe 2p and Co 2p photoelectron peaks, the decomposition procedure previously developed for these MPEAs and reported in detail elsewhere was applied.⁵⁸ It allows us to subtract the overlapping Auger peaks according to reference area ratio and peak shape. For the Cr 2p, Fe 2p, Co 2p, Mo 3d, S 2p and Cl 2p core level spectra, the entire region containing both 2p₃/₂ and 2p₁/₂ (or 3d₅/₂ and 3d₃/₂ for Mo 3d) peaks was decomposed due to the overlap of the spin-orbit doublets. For Ni, only the Ni 2p₃/₂ region was decomposed since it is sufficiently separated from the Ni 2p₁/₂ region. Shirley type background and constraints on binding energy (BE), full width at half maximum (FWHM), intensity ratio of spin-orbit doublet, and lineshape were applied for curve fitting. The asymmetry of the metallic peaks and the multiplet splitting structure of the Cr(III) oxide peak were taken into account by using LF (α, β, w, m) lineshapes or in-house reference lineshapes obtained on corresponding pure metal or oxide samples.⁵⁸,⁵⁹ The in-house reference lineshapes were overall similar to published reference spectra,⁶⁰ and used as such without detailed decomposition in order to avoid complicating the spectral decomposition when taking into account the multiplet splitting structure.⁵⁸ The LF (α, β, w, m) lineshape represents an asymmetric Lorentzian curve convoluted by a Gaussian curve, with α and β defining the asymmetry, w the damping of the tail, and m the width of the Gaussian. Other non-metallic components peaks were fitted with symmetric Gaussian/Lorentzian lineshape products GL(x) with x ranging from 0 to 100 defining the percentage of Lorentzian character.

In-depth elemental analysis was performed using an IONTOF ToF-SIMS 5 spectrometer with a base pressure lower than 10⁻⁸ mbar. The depth profiles were obtained in dual-beam mode, with analysis performed in static SIMS conditions using a 25 keV Bi⁺ primary gun delivering 1.2 pA target current over a 100 × 100 µm² area interlaced with sputtering using a 0.5 keV Cs⁺ gun delivering 17 nA target current over a 300 × 300 µm² area. The two ion beams, at 45° incidence with respect to sample surface, were aligned in order to analyze the center of the sputtered crater. Negative ions were recorded since this polarity shows higher yields for ions coming from oxide matrices. For each measured sample, two depth profiles at different locations were carried out in order to ensure the reproducibility of the data. Data acquisition, processing and post-processing were done with the IONSPEC 6.5 software.
Results

Figure 1 shows the XPS Cr 2p, Ni 2p, Fe 2p, Mo 3d, Co 2p, Cl 2p and Na 1s core level spectra together with their spectral decomposition (except for Na 1s) for the MPEA-15Cr10Mo surface directly passivated in the electrolyte containing 0.2 M Cl⁻ (i.e. passivation procedure (1)). The fitting parameters are compiled in Table I. Peaks representing Ni L₃M₂₃M₄₅ and Co L₂M₂₃M₄₅ Auger peaks were added according to the reference Ni L₃M₂₃M₄₅/Ni 2p₃/2 (0.615) and Co L₂M₂₃M₄₅/Co 2p₃/2 (0.350) intensity ratios when fitting the Fe 2p spectrum in order to subtract the overlapping Auger

peaks. The Ni L₃M₂₃M₂₃ and Fe L₃M₄₅M₄₅ Auger peaks overlapping with the Co 2p spectrum were also subtracted. However, owing to the low content of Co (5 at.%) in the studied alloys, the oxidized Co signal in the Co 2p core level region had much lower intensity than the overlapping Auger peaks of Ni and Fe, precluding a reliable enough decomposition for quantitative discussion. The intensity of S 2s signal, resulting from adsorbed sulfate from the electrolyte and overlapping with the Mo 3d spectrum, was calibrated using the S 2s/S 2p intensity ratio (1.052). The spectral decomposition demonstrates that the oxide film is composed of Cr(III)

<figure><img src="image_7.png" /><figcaption>**Figure 1.** XPS spectra and peak fitting of the Cr 2p (a), Ni 2p (b), Fe 2p (c), Mo 3d (d), Co 2p (e), Cl 2p (f), and Na 1s (g) core level regions for the MPEA-15Cr10Mo surface passivated in 0.05 M H₂SO₄ + 0.2 M NaCl at 0 V*MSE* for 1 h.</figcaption></figure>
<table><caption>Table I. Parameters and assignments of the component peaks used for XPS spectral decomposition.</caption><thead><tr><th>Core level region</th><th>References</th><th>Peak assignment</th><th>BE (±0.1 eV)</th><th>FWHM (±0.1 eV)</th></tr></thead><tbody><tr><td rowspan="3">Cr 2p<sub>3/2</sub>|2p<sub>1/2</sub></td><td rowspan="3">12, 15, 59, 62, 71</td><td>Cr(0)<sub>met</sub></td><td>573.9|583.2</td><td>1.1|1.5</td></tr><tr><td>Cr(III)<sub>ox</sub></td><td>576.5<sup>a)</sup></td><td>—<sup>b)</sup></td></tr><tr><td>Cr(III)<sub>hyd</sub></td><td>577.2|587.0</td><td>2.5|2.5</td></tr><tr><td rowspan="7">Fe 2p<sub>3/2</sub>|2p<sub>1/2</sub></td><td rowspan="7">58, 60, 72–74</td><td>Fe(0)<sub>met</sub></td><td>706.9<sup>a)</sup></td><td>—<sup>b)</sup></td></tr><tr><td>Fe(II)<sub>ox/hyd</sub></td><td>709.3|722.3</td><td>2.0|2.0</td></tr><tr><td>Fe(II)<sub>ox/hyd</sub> satellite</td><td>712.8|725.8</td><td>3.7|3.7</td></tr><tr><td>Fe(III)<sub>ox/hyd</sub></td><td>711.0|724.3</td><td>2.5|2.5</td></tr><tr><td>Fe(III)<sub>ox/hyd</sub> satellite</td><td>714.1|727.4</td><td>4.3|4.3</td></tr><tr><td>Ni L<sub>3</sub>M<sub>23</sub>M<sub>45</sub> Auger</td><td>712.4<sup>a)</sup></td><td>—<sup>b)</sup></td></tr><tr><td>Co L<sub>2</sub>M<sub>23</sub>M<sub>45</sub> Auger</td><td>712.8<sup>a)</sup></td><td>—<sup>b)</sup></td></tr><tr><td rowspan="4">Co 2p<sub>3/2</sub>|2p<sub>1/2</sub></td><td rowspan="4">75, 76</td><td>Co(0)<sub>met</sub></td><td>778.2<sup>a)</sup></td><td>—<sup>b)</sup></td></tr><tr><td>Co(II)<sub>ox/hyd</sub></td><td>781.0|796.9</td><td>2.8|2.8</td></tr><tr><td>Co(II)<sub>ox/hyd</sub> satellite</td><td>786.8|802.7</td><td>3.4|3.4</td></tr><tr><td>Ni L<sub>3</sub>M<sub>23</sub>M<sub>23</sub> Auger</td><td>777.8</td><td>—<sup>b)</sup></td></tr><tr><td rowspan="4">Ni 2p<sub>3/2</sub></td><td rowspan="4">21, 22, 77–79</td><td>Fe L<sub>3</sub>M<sub>45</sub>M<sub>45</sub> Auger</td><td>783.8</td><td>—<sup>b)</sup></td></tr><tr><td>Ni(0)<sub>met</sub></td><td>852.7</td><td>0.9</td></tr><tr><td>Ni(0)<sub>met</sub> satellite</td><td>859.0</td><td>3.0</td></tr><tr><td>Ni(II)<sub>hyd</sub></td><td>856.0</td><td>2.1</td></tr><tr><td rowspan="4">Mo 3d<sub>5/2</sub>|3d<sub>3/2</sub></td><td rowspan="4">16, 62, 80–82</td><td>Ni(II)<sub>hyd</sub> satellite</td><td>861.5</td><td>3.3</td></tr><tr><td>Mo(0)<sub>met</sub></td><td>227.5|230.7</td><td>0.6|0.7</td></tr><tr><td>Mo(IV)<sub>ox</sub></td><td>228.9|232.1</td><td>1.7|1.9</td></tr><tr><td>Intermediate Mo</td><td>231.1|234.3</td><td>2.0|2.0</td></tr><tr><td rowspan="2">S 2p<sub>3/2</sub>|2p<sub>1/2</sub></td><td>15, 83</td><td>Mo(VI)<sub>ox</sub></td><td>232.3|235.5</td><td>1.2|1.3</td></tr><tr><td>15, 83</td><td>SO<sub>4</sub><sup>2-</sup></td><td>168.6|169.9</td><td>1.3|1.3</td></tr><tr><td>S 2 s</td><td>15, 83</td><td>SO<sub>4</sub><sup>2-</sup></td><td>232.6</td><td>2.9</td></tr><tr><td>Cl 2p<sub>3/2</sub>|2p<sub>1/2</sub></td><td>84</td><td>Cl<sup>-</sup></td><td>198.4|199.9</td><td>1.3|1.3</td></tr></tbody></table>

a) The reference lineshape measured on the pure substance was not decomposed in a doublet thus no parameter for 2p<sub>1/2</sub> component. b) The FWHM was fixed to that obtained for the reference lineshape.

oxide, Cr(III) hydroxide, Fe(II) and (III) (hydr)oxides, Mo (IV) and (VI) oxides, Ni(II) hydroxide, and Co(II) (hydr)oxides. Besides, Cl⁻ ions are also identified. No trace of Na was detected, as exemplified in Fig. 1g, demonstrating that the Cl⁻ contamination from residual electrolyte is negligible.

The same decomposition procedure and fitting parameters were applied to all spectra recorded in this work and the same species were identified for all studied surfaces. The identified oxide and hydroxide species are exactly the same as previously observed on the alloys passivated in the Cl-free 0.05 M H₂SO₄ electrolyte.⁵⁸ Thus, the presence of up to 4.7 M Cl⁻ in the electrolyte does not alter the nature of the species composing the passive oxide films, whatever the pre-passivation treatment.

Figure 2 shows the ToF-SIMS depth profiles obtained on the MPEA-15Cr10Mo surfaces passivated with 0.2 and 4.7 M Cl⁻ in the sulfuric acid solution, and without and with performing pre-passivation in the Cl⁻ free electrolyte. Similarly to previous work,⁵⁸ the monitored secondary ions are the following: CrO⁻ (67.9354 amu), CrO₃H₂⁻ (101.9409 amu), FeO₂⁻ (87.9248 amu), NiO₂H⁻ (88.9330 amu), MoO₂⁻ (129.8952 amu), MoO₃⁻ (145.8902 amu), and Ni₂⁻ (115.8707 amu). They are characteristic of the following species: Cr(III)ox, Cr(III)hyd, Fe(II,III)ox, Ni(II)hyd, Mo(IV)ox, Mo(VI)ox, and Ni(0)met, respectively.⁵⁸ ³⁷ Cl⁻ (36.9659 amu) is also plotted and represents Cl⁻.¹⁵ For the sake of comparison between samples, the intensities of the signals are normalized to the intensity of Ni₂⁻ signal in the bulk. For these passivated MPEA-15Cr10Mo surfaces, using the conventional rule of 80% of the maximum intensity of the CrO⁻ and Ni₂⁻ signals,⁵⁸ the interface between inner and outer layers of the oxide film and the interface between oxide film and metal are always located at around 18–19 s and 35–38 s of sputtering time, respectively, independently of Cl⁻ concentration and pre-passivation treatment.

The depth profiles in Fig. 2 show that, whatever the applied passivation procedure, a stratified oxide film is formed, which is evidenced by the clear shift in sputtering time between the intensity

maximum of the signal characteristic for chromium oxide (CrO⁻) and the intensity maxima of the signals of the other oxides/hydroxides (CrO₃H₂⁻, FeO₂⁻, NiO₂H⁻, MoO₂⁻, MoO₃⁻). This stratified structure is similar to that observed for the passive oxide film formed in the Cl⁻-free electrolyte.⁵⁸ It consists of an inner layer mostly composed of Cr oxide and an outer part composed of Ni and Cr hydroxides and Fe and Mo oxides. The intensity increase and decrease of each signal are gradual, indicating some intermixing between layers. This intermixing should result mainly from surface roughness and concentration gradient since the displacement mixing effect is not strong for alloy components⁶¹ and sputtering during measurements was homogeneous. Interestingly, a shoulder (designated by a red arrow) is observed on the FeO₂⁻, NiO₂H⁻ and MoO₂⁻ profiles in the Cr-rich inner oxide region, indicating that low concentrations of oxidized Fe, Ni and Mo are localized in the Cr-rich inner oxide film, probably concentrated in the Cr-depleted weak sites of the ultra-thin inner Cr oxide layer as previously discussed.³⁴ The Ni₂⁻ signal exhibits a maximum after reaching the metal/oxide interface position, indicating an enrichment of Ni thus a modification of the alloy composition beneath the oxide film.

Figure 3 highlights the ³⁷Cl⁻ signal, characteristic of Cl⁻ ions, for MPEA-15Cr10Mo and MPEA-25Cr5Mo passivated following procedure 1 or procedure 2 with 4.7 M Cl⁻. In all cases, the ³⁷Cl⁻ signal peaks in the outmost part of the oxide film region (as observed for 0.2 M Cl⁻ as shown in Fig. 2) whatever the passivation procedure. No Cl⁻ peak indicating Cl⁻ accumulation was observed at the metal/oxide interface, or in the inner barrier layer of the passive films. This shows that even in the presence of 4.7 M Cl⁻ in the electrolyte, the penetration of Cl⁻ into the oxide film is limited to its outmost part, in agreement with previous observations done on FeCrNiMo stainless steel at lower Cl⁻ concentration.¹¹,¹⁵

Although the in-depth locations of the Cl⁻ ion intensity maxima are similar, a clear difference between their maximum intensities can be noticed. As shown in Fig. 3, the surface directly passivated in 4.7 M Cl⁻ electrolyte exhibits a higher maximum Cl⁻ intensity than
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<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 2.** Normalized ToF-SIMS negative ion depth profiles for the MPEA-15Cr10Mo surfaces directly passivated in 0.05 M H₂SO₄ + 0.2 M NaCl at 0 VₗMSE for 1 h without (a) and with (b) pre-passivation in Cl⁻-free 0.05 M H₂SO₄, and in 0.05 M H₂SO₄ + 4.7 M NaCl at 0 VₗMSE for 1 h without (c) and with (d) pre-passivation in Cl⁻-free 0.05 M H₂SO₄. The red arrows point the shoulder feature of FeO₂⁻, NiO₂H⁻ and MoO₂⁻ ions profiles after their maximum intensity. For a better visibility all 37Cl⁻, CrO⁻, MoO₂⁻ and MoO₃⁻ signals have been adjusted and the correction is given in brackets in the legend.</figcaption></figure>

the pre-passivated surfaces, suggesting a significantly higher Cl⁻ content in the oxide film directly passivated in 4.7 M Cl⁻ electrolyte. Noteworthy in this case is the low intensity shoulder of the Cl⁻ profile observed at the entry in the Cr-rich inner oxide region, suggesting penetration in the inner barrier layer of the passive film however without triggering passivity breakdown according to the electrochemical data. Without pre-passivation, the Cl⁻ penetration is more extensive for MPEA-25Cr5Mo, which can be linked to the amount of Mo in the oxide layer previously determined⁵⁸ indicating a role of the Mo oxide on the Cl⁻ penetration (20 and 31% Mo in the outer oxide layer for MPEA-25Cr5Mo and MPEA-15Cr10Mo, respectively)

Similar unchanged oxide film stratification was observed for the MPEA-25Cr5Mo passivated in 0.05 M H₂SO₄ containing 0.2 or 4.7 M Cl⁻ (not shown) and for the MPEA-35Cr surfaces passivated in 0.05 M H₂SO₄ containing 0.2 M Cl⁻, with or without Cl⁻-free pre-passivation (Fig. 4). The oxide film stratification remains the same as the one observed for the surfaces passivated in the absence of Cl⁻,⁵⁸indicating that the presence of up to 4.7 M Cl⁻ does not modify the bilayer structure of the oxide film.

Like for MPEA-15Cr10Mo, the maximum Cl⁻ intensity remains located in the outmost part of the passive oxide films on the MPEA-35Cr substrate passivated in 0.2 M Cl⁻-containing solution (Fig. 4). No deeper Cl⁻ penetration is observed. Thus, the three studied MPEAs only exhibit shallow incorporation of Cl⁻ into the external
<figure><img src="image_13.png" /><figcaption>**Figure 3.** ToF-SIMS $^{37}$Cl⁻ profiles for the MPEA-15Cr10Mo and MPEA-25Cr5Mo surfaces passivated in 0.05 M H₂SO₄ + 4.7 M NaCl at 0 V for 1 h with or without Cl⁻-free pre-passivation. The margin of error for the maximum and interface Cl⁻ signal intensities is smaller than 5%.</figcaption></figure>

part of the outer oxide layer. Deeper penetration of Cl⁻ in trace amounts and blocked at the entry in the Cr-rich inner oxide region is only observed in the most severe tested conditions, i.e. for direct passivation in 4.7 M Cl⁻-containing H₂SO₄ solutions.

Based on these ToF-SIMS results, the bilayer model of the passive film previously established⁵⁸ for these MPEA surfaces passivated in the absence of Cl⁻ was then used to quantify the average composition and thickness of the passive oxide films formed in the Cl⁻-containing sulfuric acid, as well as the composition of the modified alloy underneath. The inner layer is considered to contain only Cr oxide, neglecting the minor content in Fe, Ni and Mo (hydr) oxides and Cr hydroxide indicated by ToF-SIMS, and the outer layer is considered to be a mixture of Mo oxides, Fe and Co (hydr)oxides and Ni and Cr hydroxides, in agreement with the literature.¹¹,¹⁹,²²,²⁴,⁶²⁻⁶⁵ The surface roughness and concentration gradients are not taken into account, i.e., the diffuse interfaces between layers are simplified to sharp interfaces. The amount of Cl⁻ ions was calculated by attributing the XPS intensity of Cl⁻ to the outer layer since ToF-SIMS profiles show that the penetration of Cl⁻ ions is limited to the external part of the oxide film. The values of the photoionization cross-section were taken from Scofield⁶⁶ and the values of the inelastic mean free paths of the emitted photoelectrons were calculated according to Tanuma et al.⁶⁷ The alloy density is 8.3 g.cm⁻³ and an average density of 4.5 g.cm⁻³ was used for the oxide film. The calculated equivalent thickness (±0.1 nm) and cationic composition (±2–3 at.%) of the oxide films, as well as the elemental composition (±2–3 at.%) of the modified alloy underneath the passive films are presented in Figs. 5, 6 and 7 for the passivated MPEA-15Cr10Mo, MPEA-25Cr5Mo and MPEA-35Cr, respectively. For comparison, the values calculated for the surfaces passivated in the Cl⁻-free electrolyte, extracted from previous work,⁵⁸ are also incorporated herein.

Figure 5a shows no evident modifications of the oxide layer thicknesses for the MPEA-15Cr10Mo surfaces passivated in Cl⁻-containing solution (0.2 M or 4.7 M) compared to those passivated in Cl-free solution. The thickness of the mixed outer layer remains around 1.7–1.8 nm, whereas that of the thin inner Cr-rich layer is only 0.3–0.4 nm. Similarly, considering the 2–3 at. % uncertainty of the calculations estimated by adjusting the XPS background and fitting, no significant variations on the cationic composition of the outer layer are observed (Fig. 5b). The contents

of oxidized Cr and Mo in the oxide film remain rather stable. Regarding the composition of the modified alloy underneath the oxide film (Fig. 5c, it is also unmodified compared to the surface passivated in the absence of Cl⁻. Nevertheless, compared to the bulk composition, Ni enrichment and Cr depletion are observed due to preferential Cr oxidation, which is commonly observed on Fe- and Ni-based Cr-containing alloys.¹⁰,²¹,²²,²⁸ Comparing the surface passivated in the presence of Cl⁻ without and with Cl⁻-free pre-passivation, no marked differences are observed for the thicknesses and composition of the oxide films, nor the composition of the modified alloy, indicating that the Cl⁻-free pre-passivation treatment before exposure to Cl⁻-containing (up to 4.7 M) electrolytes does not modify the thickness and chemical composition of the passive oxide film. The other passivated quinary MPEA-25Cr5Mo surfaces studied in this work also show the same passive film composition and thickness and underlying alloy composition, independently of the tested passivation conditions (Fig. 6).

In contrast, the quaternary Mo-free MPEA-35Cr (Fig. 7) shows significant Cl⁻-induced modifications of thickness of outer and inner oxide layer thicknesses and composition of the outer oxide layer.

After passivation in sulfuric acid containing 0.2 M Cl⁻, without or with Cl⁻-free pre-passivation, the total oxide film thickness appears unchanged, however with a significantly thinner outer layer and thicker Cr-rich inner layer compared to the passive oxide film formed in the absence of Cl⁻. Regarding the composition of the oxide film, the Cr content in the outer layer is significantly reduced by Cl⁻ addition in the electrolyte, and balanced by increases of the Ni and Fe contents. Interestingly, although less Cr hydroxide is formed in the outer layer, the total content of Cr ions in the global film (about 82 at. %) remains similar due to the thickening of the inner layer reflecting the increased enrichment in Cr oxide. The diminution in Cr content and increase in Fe content in the outer layer of the passive oxide film induced by the presence of Cl⁻ in the electrolyte have been previously reported for 316L SS. Nevertheless, although unchanged total Cr content was observed with pre-passivation in Cl-free solution, a lower total Cr content was observed without pre-passivation.¹⁵ The composition of the MPEA-35Cr alloy underneath the passive film is also not markedly affected by the presence of Cl⁻ in the electrolyte during passivation compared to passivation in the absence of Cl⁻. Similarly to the two Mo-containing quinary MPEAs presented above, Cl⁻-free pre-
<figure><img src="image_14.png" /><figcaption>**Figure 4.** Normalized ToF-SIMS negative ion depth profiles for the MPEA-35Cr surface passivated in 0.05 M H₂SO₄ + 0.2 M NaCl at 0 V for 1 h without (a) and with (b) pre-passivation in Cl⁻-free 0.05 M H₂SO₄.</figcaption></figure>

passivation before exposure to the Cl⁻-containing solution did not result in significant modifications on the composition and thickness of the oxide film (Fig. 7).

Table II presents the amount of Cl⁻ measured in the passive films formed on the three MPEAs in the studied passivation conditions. The absolute amount of Cl⁻ in the outer oxide layer is expressed as surface density (at.cm⁻²), obtained by weighting the calculated density of Cl⁻ (in at.cm⁻³) by the thickness of the outer oxide layer. In the case of direct passivation in the 0.2 M Cl⁻-containing electrolyte, the MPEA-25Cr5Mo has the highest amount of Cl⁻ among the three passivated alloys. However, with Cl⁻-free pre-passivation, the amount of Cl⁻ for MPEA-25Cr5Mo is markedly reduced. In contrast, the change induced by Cl⁻-free pre-passivation was much less marked in the case of the passivated MPEA-15Cr10Mo and MPEA-35Cr surfaces. In contrast, in the case of

direct passivation in the 4.7 M Cl⁻-containing solution, the content of Cl⁻ in the passive oxide film is significantly higher for both Mo-bearing MPEAs compared to direct passivation in the 0.2 M Cl⁻-containing solution. However, when a Cl⁻-free pre-passivation was applied, the Cl⁻ content is markedly reduced, which is consistent with the reduced maximum intensity of the Cl⁻ signal observed on the ToF-SIMS depth profiles.

These results reveal that the Cl⁻ concentration in the electrolyte influences the amount of Cl⁻ incorporated in the passive oxide film, and that the pre-passivation treatment in Cl⁻-free solution significantly enhances the resistance against Cl⁻ incorporation. The efficiency of the pre-passivation enhancement is dependent on the alloy composition and Cl⁻ concentration in the environment. No simple correlation could be established to relate the electrolyte Cl⁻ concentration or efficiency of pre-passivation to the content of one
<figure><img src="image_15.png" /><figcaption>**Figure 5.** Thickness of oxide layers (a), cationic composition of the outer oxide layer (b), and composition of the modified alloy (c) obtained for the passivated MPEA-15Cr10Mo surfaces.</figcaption></figure>

single alloying element, suggesting that the resistance against Cl⁻-penetration/adsorption of the passive oxide film is not provided by only one alloying element.

## Discussion

**Role of pre-passivation.**—The data presented above for the three studied MPEAs show that no significant modifications of the cationic compositions of the outer layers of the passive oxide films and elemental compositions of the underlying metallic substrates are observed for surfaces passivated in the presence of Cl⁻ without or with Cl⁻-free pre-passivation. Besides, the XPS O 1s spectra show that the OH⁻ and O²⁻ ratios are also unmodified for these two passivation procedures tested on the three MPEAs (spectra not shown). Thus, it can be stated from our measurements that on these MPEA alloys, the pre-passivation does not result in a strong modification/optimization of the composition or thickness of the passive oxide films.

Nevertheless, in most cases, Cl⁻ adsorption/penetration in the outer oxide layer is strongly decreased when pre-passivation is applied. Following previous works on stainless steels, the explanations can be based on the nanostructure of the passive films. It has been shown that passive oxide films formed on stainless steels¹²,²⁰,³¹,³² are composed of nanoscale crystalline grains or amorphous phases depending on passivation conditions. The inter-granular boundaries between grains or between grain and amorphous phases may be more susceptible to cation dissolution and Cl⁻ adsorption and penetration.²⁰,³⁸ Increasing passivation time and/or potential is known to promote the crystallization of the oxide film, resulting in the extension and coalescence of the grains.¹²,³¹ Thus, pre-passivated oxide films should contain fewer defective sites such as intergranular boundaries than native oxide films exposed at OCP, leading to fewer defective sites susceptible to Cl⁻ attack. Consequently, when subsequently exposed to the Cl⁻-containing solution, less Cl⁻ would adsorb onto or penetrate into the pre-passivated oxide films. Thus, the lower Cl⁻ content obtained with
<figure><img src="image_16.png" /><figcaption>**Figure 6.** Thickness of oxide layers (a), cationic composition of the outer oxide layer (b), and composition of the modified alloy (c) obtained for the passivated MPEA-25Cr5Mo surfaces.</figcaption></figure>

Cl⁻-free pre-passivation may reflect an enhancing effect of pre-passivation on combatting Cl⁻-induced passivity breakdown.

**Role of Mo.**—Regarding Mo, it is a well-known alloying element enhancing the resistance to localized corrosion.43-45 The exact role of Mo is still under debate in the literature and can result from a combination of several mechanisms. In a previous work, it has been shown that Mo addition in MPEAs (MPEA-25Cr5Mo and MPEA-15Cr10Mo) provides the alloy much higher resistance to Cl⁻-induced passivity breakdown and initiation of localized corrosion compared to MPEA not containing Mo (MPEA-35Cr).57 In order to discuss the possible mechanisms by which Mo enhances the corrosion resistance, observations done on the present MPEA alloys are summarized in Table III and discussed in order to clarify the role of Mo addition in stabilizing passivity and increasing resistance to localized corrosion.

Previous work has shown that under the conditions investigated in the present work, localized corrosion is not initiated yet on the two

Mo-containing MPEAs independently of the Cl⁻ concentration, while it is on the Mo-free alloy MPEA-35Cr at 4.7 M Cl⁻.57 For the 0.2 M Cl⁻ concentration, the ToF-SIMS depth profiles reveal that the Cl⁻ ions do not penetrate deeper in the passive oxide film on the Mo-free alloy MPEA-35Cr than on the other two Mo-containing MPEAs (Figs. 2 and 4). Moreover, as shown in Table II, the amount of Cl⁻ adsorbed onto or penetrating into the oxide film is higher for the Mo-bearing MPEAs than the Mo-free MPEA-35Cr. Thus, in the acidic solution containing 0.2 M Cl⁻, the effect of Mo in hindering Cl⁻ adsorption/penetration into the passive oxide film is not obvious. For the higher Cl⁻ concentration (4.7 M), the two Mo-bearing MPEAs still show no global deeper Cl⁻ penetration, except for traces blocked at the entry in the inner barrier layer. It cannot be concluded whether this superficial location of Cl⁻ is because of the hindrance effect of Mo oxides or due to the mechanism by which Cl⁻ ions act on the passivating oxides (penetration, or adsorption favoring oxide dissolution). It seems that, whatever the mechanism by which Cl⁻ ions act in the present tested conditions, the outer
<figure><img src="image_17.png" /><figcaption>**Figure 7.** Thickness of oxide layers (a), cationic composition of the outer oxide layer (b), and composition of the modified alloy (c) obtained for the passivated MPEA-35Cr surfaces.</figcaption></figure>

<table><caption>**Table II.** The amount of Cl⁻ ions in the outer oxide layer of the three studied MPEAs.</caption><thead><tr><th rowspan="3">Alloy</th><th colspan="4">Surface density of Cl<sup>-</sup> ions (10<sup>14</sup> at.cm<sup>-2</sup>)</th></tr><tr><th colspan="2">0.2 M Cl<sup>-</sup></th><th colspan="2">4.7 M Cl<sup>-</sup></th></tr><tr><th>Passivation procedure 1</th><th>Passivation procedure 2</th><th>Passivation procedure 1</th><th>Passivation procedure 2</th></tr></thead><tbody><tr><td>MPEA-15Cr10Mo</td><td>1.0 (±0.1)</td><td>0.9 (±0.1)</td><td>6.0 (±0.3)</td><td>1.1 (±0.1)</td></tr><tr><td>MPEA-25Cr5Mo</td><td>1.6 (±0.1)</td><td>0.3 (±0.1)</td><td>5.5 (±0.4)</td><td>1.7 (±0.1)</td></tr><tr><td>MPEA-35Cr</td><td>0.7 (±0.1)</td><td>0.4 (±0.1)</td><td>—</td><td>—</td></tr></tbody></table>

layer, containing Mo(VI) and other (hydr)oxides, can already provide sufficient protection against the detrimental impacts of Cl⁻. However, the role of Mo in reinforcing the Cr-depleted weak sites of the inner barrier layer discussed in previous work for the two quinary MPEAs is still illustrated here in Figs. 2c and 3 by the entry of Cl⁻ ions limited to the outer part of the inner layer, which is more important on MPEA-25Cr5Mo containing less Mo than MPEA-15Cr10Mo, in conditions (4.7 M Cl⁻) where no breakdown of passivity occurs.

In contrast, the passive film formed on the Mo-free MPEA-35Cr already presents passive film modifications (thinning of the outer layer and thickening of the inner layer) at 0.2 M Cl⁻ which are not observed on the two Mo-bearing alloys even at 4.7 M Cl⁻ (Fig. 5 to Fig. 7), evidencing that the passive oxide films of the two Mo-bearing MPEAs present higher stability when being immersed in Cl⁻-containing electrolyte than the MPEA-35Cr. Thus, in the

present case of the Mo-bearing MPEAs, molybdenum may enhance the corrosion resistance mainly by presenting a stabilization effect against Cl⁻-induced modification of the passive film, e.g., possibly acting as a dissolution moderator.⁶⁸ In the work of Wang et al.,⁶⁹ it was shown that the Mo oxide in the outer oxide layer of SS 316 L can prevent the outward diffusion of Cr, which may coincide with the possibility of being a dissolution moderator. To further verify the suppression effect of Mo on Cr dissolution, measurement of the quantity of dissolved Cr in the electrolyte for Mo-bearing and Mo-free alloys could be useful. Regarding the possibility of Mo being a self-repairing promoter,⁴³,⁷⁰ it cannot be confirmed based on the present data that show no significant enrichment in Mo of the metal underneath the passive oxide films whatever the tested passivated conditions.
<table><caption>Table III. Possible effects of Mo, corresponding expected outcomes, and experimental observations from present work.</caption><thead><tr><th rowspan="2">Possible effects of Mo</th><th rowspan="2">Expected outcomes for passivation in Cl<sup>-</sup>-containing electrolyte</th><th colspan="3">Experimental observations after passivation in Cl<sup>-</sup>-containing electrolyte</th><th rowspan="2">Conclusion</th></tr><tr><th>MPEA-15Cr10Mo</th><th>MPEA-25Cr5Mo</th><th>MPEA-35Cr</th></tr></thead><tbody><tr><td rowspan="2">Impediment to deep Cl<sup>-</sup> incorporation</td><td>Deeper Cl<sup>-</sup> penetration in passive film on Mo-free than on Mo-containing alloys</td><td>No deep penetration at 0.2 M Cl<sup>-</sup></td><td>No deep penetration at 0.2 M Cl<sup>-</sup></td><td>No deep penetration at 0.2 M Cl<sup>-</sup></td><td>Consistent</td></tr><tr><td>Cl<sup>-</sup> content in passive film decreases with increasing Mo content</td><td>Traces reaching inner layer at 4.7 M Cl<sup>-</sup> but no passivity breakdown<br>Higher Mo content (~13 at. % Mo (IV) and ~17 at. % Mo(VI)) while Cl<sup>-</sup> content in outer oxide layer not always lower.</td><td>Traces reaching inner layer at 4.7 M Cl<sup>-</sup> but no passivity breakdown<br>Lower Mo content (~10 at. % Mo (IV) and ~6 at. % Mo(VI)) while Cl<sup>-</sup> content in outer oxide layer not always higher.</td><td>Not tested at 4.7 M Cl<sup>-</sup> since passivity breakdown<br>No Mo oxides and low Cl<sup>-</sup> content in outer oxide layer, but not tested at 4.7 M Cl<sup>-</sup> since passivity breakdown</td><td>Unconclusive since influenced by multiple factors.</td></tr><tr><td>Promotion of self-repair</td><td>Increased content of Mo metal due to Mo precipitation at local sites exposed by breakdown</td><td>No evident enrichment in Mo metal underneath passive film</td><td>No evident enrichment in Mo metal underneath passive film</td><td>—</td><td>Unconclusive</td></tr><tr><td>Suppression of Cr dissolution from outer oxide layer</td><td>Cr content in passive film decreases for Mo-free but not for Mo-containing alloys</td><td>No observed compositional and structural modifications of passive oxide film</td><td>No observed compositional and structural modifications of passive oxide film</td><td>Thinner outer oxide layer with less Cr, but thicker inner layer</td><td>Consistent</td></tr></tbody></table>
Conclusions

Surface characterization combining XPS and ToF-SIMS analysis was carried out on MPEA-15Cr10Mo, MPEA-25Cr5Mo and MPEA-35Cr surfaces passivated in 0.05 M H₂SO₄ aqueous solution containing 0.2 or 4.7 M NaCl to investigate the Cl⁻-induced alterations of the passive oxide films, as well as the effect of Cl⁻-free pre-passivation and the role of Mo in combating Cl⁻ attack.

The results show that the passive films formed on the two Mo-bearing MPEAs have a bilayer structure with an inner layer mostly consisting of Cr oxide and an outer layer containing Cr hydroxide, Ni hydroxide, Mo oxides, and Fe (hydr)oxide, and are not significantly altered and exhibit high stability even when formed in 4.7 M Cl⁻ solution. However, the passive oxide film formed on the Mo-free MPEA exhibited already significant thickness and compositional changes at 0.2 M Cl⁻. The inner Cr oxide layer is thicker, and the outer layer thinner with decreased Cr content and increased Ni and Fe contents. Yet, the global Cr(III) content remains unchanged owing to the loss in the outer layer balanced by gain in the inner layer.

In all cases investigated in this work, the incorporation of Cl⁻ is limited to only the external part of the outer oxide layer, showing no deeper penetration except for traces reaching the inner layer in the most severe tested conditions (direct passivation in 4.7 M concentrated Cl⁻-containing solution). The amount of Cl⁻ adsorbed onto or incorporated into the outer layer of the passive films is much higher with 4.7 M Cl⁻ than with 0.2 M Cl⁻ in the electrolyte. Pre-passivation performed in the absence of Cl⁻ prior to exposure to the Cl⁻-containing electrolyte helps reducing the amount of adsorbed/incorporated chlorides, which is assigned to a consolidation of the nanostructure rather than to compositional enhancement of the passive films. For the Mo-bearing MPEAs studied in this work, the beneficial effect of Mo enrichment in the passive films on localized corrosion resistance is concluded to provide the oxide layers a high resistance against Cl⁻-induced compositional changes. The reinforcing effect of Mo on the Cr-depleted weak sites of the barrier inner layer is reflected by the absence of Cl⁻ penetration until the interface with the substrate in the most severe tested conditions where traces access the barrier inner layer. It complements the effects of the Mo enrichment in the outer layer that already counteracts the destabilizing effect of the chloride ions.

Acknowledgments

This work was supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (ERC Advanced Grant agreement No 741123).

ORCID

Dimitri Mercier https://orcid.org/0000-0002-9637-1484

Antoine Seyeux https://orcid.org/0000-0002-3062-8961

Vincent Maurice https://orcid.org/0000-0001-5222-9972

Philippe Marcus https://orcid.org/0000-0002-9140-0047

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