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# Early-stage surface oxidation of the equiatomic CoCrFeMnNi high entropy alloy studied in situ by XPS

Luntao Wang, Sandrine Zanna, Dimitri Mercier, Vincent Maurice, Philippe Marcus

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Luntao Wang, Sandrine Zanna, Dimitri Mercier, Vincent Maurice, Philippe Marcus. Early-stage surface oxidation of the equiatomic CoCrFeMnNi high entropy alloy studied in situ by XPS. Corrosion Science, 2023, 220, pp.111310. 10.1016/j.corsci.2023.111310. hal-04119608

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Submitted on 6 Jun 2023

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# Early-stage surface oxidation of the equiatomic CoCrFeMnNi high entropy alloy studied in situ by XPS

Luntao Wang, Sandrine Zanna, Dimitri Mercier<fnref n="1" />, Vincent Maurice<fnref n="2" />, Philippe Marcus<fnref n="3" />

PSL University, CNRS- Chimie ParisTech, Institut de Recherche de Chimie Paris, Physical Chemistry of Surfaces
Group, 11 rue Pierre et Marie Curie, 75005 Paris, France

**Abstract:**

The mechanisms of initial oxide growth on the equiatomic CoCrFeMnNi high entropy alloy were studied in situ by X-ray photoelectron spectroscopy on clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides were formed at 100 °C, and Mn(II) at 200 °C. Data quantification revealed a transition mechanism from selective initial oxidation to 3D kinetically-controlled growth, leading to stratification of the oxide film. Formation of the Co-rich outer layer above the Cr-rich inner layer first formed implies predominant cationic outward oxide growth supported by the overall stoichiometry of the film.

**Keywords:**

High entropy alloy; Oxidation mechanisms; Surface analysis; in situ XPS

<footnote n="1">¹ Corresponding author: dimitri.mercier@chimieparsitech.psl.eu</footnote>

<footnote n="2">² Corresponding author: vincent.maurice@chimieparsitech.psl.eu</footnote>

<footnote n="3">³ Corresponding author: philippe.marcus@chimieparsitech.psl.eu</footnote>
Introduction

High entropy alloys (HEAs), a subset of multi-principal element alloys (MPEAs) [1], are
single-phase alloys of high mixing entropy composed of five or more elements in equiatomic
percentage [2, 3]. Among them, the CoCrFeMnNi alloy, introduced by Cantor et al. in 2004
[4], has attracted much interest as it combines desirable mechanical properties such as yield
strength, tensile strength and ductility [5-8]. The corrosion properties of this HEA have been
extensively discussed [9-16], and it has been shown that the anti-corrosion performance is
lower than that of 304 stainless steels. Surface analysis, including X-ray photoelectron
spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and
transmission electron microscopy (TEM), have been applied to further understand the
corrosion protection provided by the oxide films formed on CoCrFeMnNi surfaces [9, 10, 13].
Both the native oxide film formed in air and the passive film formed in sulfuric acid aqueous
solution have a bilayer structure mixing Fe, Co and Cr in the exchange outer layer and Cr and
Mn in the barrier inner layer. The stratification of the surface oxide film is similar to that of the
protective films formed on Cr-containing Fe-based and Ni-based alloys [17, 18]. However, the
MnCr₂O₄ oxide formed on CoCrFeMnNi surfaces is detrimental to the corrosion resistance,
especially to the resistance to localized corrosion by pitting in chloride-containing environment
[14].

To date, several studies of long-term oxidation (a few to thousands of hours) and surface
evolution at high temperatures of MPEAs combining Cr, Mn, Fe, Co and Ni have been
published [19-26]. On CoCrFeNi surfaces oxidized in air at 1000 °C, the oxide scale develops
a complex structure with an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄
and NiCr₂O₄, and an inner layer of Cr₂O₃ [27]. On CoCrFeMn surfaces oxidized in air at 700 °C
and 900 °C, the oxide scales are enriched in MnFe₂O₄, MnFeO₂ and Cr₂O₃ [28]. On
CoCrFeMnNi surfaces of varied compositions oxidized in air at 650 °C and 750 °C for 1100
hours, a high Mn content in the alloy was found to reduce the oxidation resistance by
facilitating the formation of a less protective spinel-type oxide [29].
Single-phase *bcc* HEAs with high melting point transition metal elements such as Mo, Nb, and V maintain enhanced mechanical properties at high temperature (>800°C) [30]. This is not the case for single-phase *fcc* HEAs that require hardening by adding secondary (often *bcc*) phases [31, 32]. However, these single-phase *fcc* HEAs, including the CoCrFeMnNi HEA, can be used in applications where the operating conditions are at ambient temperature or at temperatures slightly above room temperature (up to 100-200°C). Thus, to improve the properties of these alloys, it is important to understand the evolution of the native/passive films and mechanisms describing the oxide growth under such conditions. This work focuses on this point, starting with a clean metallic surface.

Reports on the surface oxidation of CoCrFeMnNi alloys at relatively low temperatures are only a few. Recently, early-stage oxidation (for several minutes) of Ni₃₈Cr₂₂Fe₂₀Mn₁₀Co₁₀ surfaces was investigated at 120 and 300 °C [33]. The oxide films formed at these two temperatures were reported to consist of a Cr-rich inner layer and a Fe, Mn, Co-rich outer layer. In another study, the thermal stability from room temperature to 200 °C under ultra-high vacuum (UHV) of the passive oxide film pre-formed in sulfuric acid aqueous solution on the equiatomic CoCrFeMnNi HEA has been investigated *in situ* by ToF-SIMS [34]. The bilayer structure of the passive film, with Fe, Co-rich outer and Cr, Mn-rich inner layers, was found to be thermally unstable above 150°C with reduction of the Fe and Co oxides and formation of Mn oxide in the outer layer. At 200 °C, the thermally-treated passive film was mainly composed of outer Mn oxide and inner Cr oxide, with only small amounts of Fe and Co oxides left in the film.

In the present work, the mechanisms of initial, early-stage oxidation of the equiatomic CoCrFeMnNi high entropy alloy were studied at 100 and 200 °C. XPS analysis was applied *in situ* starting from a surface prepared oxide-free in UHV conditions and exposed stepwise to gaseous oxygen, in a controlled manner at low pressure (10⁻⁷ mbar). The choice to work under ultra-high vacuum with a low O₂ pressure allows a fine analysis of the oxide growth without competition with secondary reactions due to atmospheric environment. Previous works performed on stainless steel [35-37], Ni-based alloy [38] and *fcc* HEA [34] surfaces have shown that pre-formed surface oxide films are not significantly modified at these temperatures.
The growing oxide species formed in this relatively low temperature range are comparable to those formed in native or electrochemical passive films formed at ambient conditions, which is not the case at higher temperature where new specific species can be formed.

Considering the superposition of Auger peaks in the binding energy regions of the Mn 2p, Fe 2p, Co 2p and Ni 2p core levels as demonstrated in previous work [13], an unconventional XPS analytical methodology was applied in which the high-resolution spectra of the Cr 3p, Mn 3p, Fe 3p, Co 3p and Ni 3p core levels were used to quantify the growth of the oxide film and the evolution of its composition as well as that of the modified underlying alloy region. The chemical states of the oxide species and the stoichiometry of the oxide film are also discussed.

The results bring new insight on how equiatomic CoCrFeMnNi surfaces evolve in the very first stages of oxidation at relatively low temperature.

## 2. Experimental

### 2.1. Sample preparation

A fcc single-phase HEA sample of 20.3Cr-20Mn-19.8Fe-20Co-19.9Ni (at.%) composition was used. Detailed manufacturing and heat treatment process of the alloy have been described previously [13]. Before introduction into the multi-chamber XPS spectrometer, the sample surface was mechanically polished down to 0.25 µm with diamond paste and then successively rinsed with acetone, ethanol and deionized water in ultrasonic baths for 10 minutes each.

Experiments and analysis were performed in a Thermo Electron Escalab 250 spectrometer (base pressure <10<sup>-9</sup> mbar) including an attached preparation chamber. The preparation chamber is equipped with an Ar<sup>+</sup> cold cathode ion gun for sputtering and a resistive heating filament for thermal treatment. The temperature of the sample is controlled by a thermocouple connected to the sample surface. Cycles of Ar<sup>+</sup> ion sputtering (P<sub>Ar</sub> = 5 × 10<sup>-6</sup> mbar, 20 µA, 1 keV, 10 minutes) and annealing (500 °C, 2 hours) were performed on the polished surface in order to remove the surface contamination and pre-formed oxides. The prepared surface was directly transferred under UHV to the analysis chamber for XPS analysis. The surface was
considered clean when having no detectable oxygen.

For surface oxidation, gaseous O₂ (99.999% pure) was introduced via a leak valve at the partial pressure of 10⁻⁷ mbar. Oxidation was performed stepwise by cumulating exposure to oxygen gas with the sample temperature pre-set at 100 or 200 °C. Cumulated exposure times were 1, 5, 15, 45, 105 and 165 minutes at 100 °C, and 1, 15, 30, 60, 105 and 165 minutes at 200 °C. After each oxidation step, survey and high-resolution XPS spectra were recorded at room temperature.

## 2.2 XPS analysis

XPS analysis was performed using a monochromatic Al Kα X-ray source (hv = 1486.6 eV). The analyser pass energy was set to 50 eV and 20 eV for recording the survey and high resolution spectra with a step size of 1 eV and 0.1 eV, respectively. Photoelectrons were collected at 90° emission angle.

Data processing (curve fitting) was performed using the *Thermo Avantage* software (Version 5.956), subtracting a modified Shirley type background for the spectra of the alloying elements. For curve fitting, asymmetric and symmetric Gaussian-Lorentzian mixed line shapes were used for the metallic and oxide components, respectively. A pure Co sample was oxidized *in situ* in order to obtain the reference line shape of the Co 3p component. For the other alloying elements, the exact fitting parameters were taken from previous work performed using the same spectrometer in the same analytical conditions [13]. The plasmon peaks were not considered in the XPS peak decomposition, since measurements on pure metallic substrates showed that their intensities are lower than 2% of the main 3p peak intensity when observed.

Like in previous work [13], a single-layer model of XPS intensity attenuation was used to quantify oxide film thickness and surface composition. The model assumes the formation of a uniformly thick oxide layer, completely covering a substrate surface of modified composition with respect to the bulk alloy. The model yields the following set of 11 equations with 11 unknowns ($k, d_{oxide}, D_{Cr}^{Met}, D_{Mn}^{Met}, D_{Fe}^{Met}, D_{Co}^{Met}, D_{Ni}^{Met}, D_{Cr}^{Ox}, D_{Mn}^{Ox}, D_{Fe}^{Ox}, D_{Co}^{Ox}$):
$$I_X^{Met} = k \sigma_X \lambda_X^{Met} D_X^{Met} T_X \exp(-\frac{d_{oxide}}{\lambda_X^{Ox}}), \text{ with } X=\text{Cr, Mn, Fe, Co and Ni} \quad (1)$$

$$I_Z^{Ox} = k \sigma_Z \lambda_Z^{Ox} D_Z^{Ox} T_Z \left[ 1 - \exp\left(-\frac{d_{oxide}}{\lambda_Z^{Ox}}\right) \right], \text{ with } Z=\text{Cr, Mn, Fe and Co} \quad (2)$$

$$D_{Cr}^{Met} + D_{Mn}^{Met} + D_{Fe}^{Met} + D_{Co}^{Met} + D_{Ni}^{Met} = D^{Met} \quad (3)$$

$$D_{Cr}^{Ox} + D_{Mn}^{Ox} + D_{Fe}^{Ox} + D_{Co}^{Ox} = D^{Ox} \quad (4)$$

where $I_X^Y$ is the intensity of the photoelectrons (cps eV) emitted by elemental core level X from the matrix Y (Met or Ox), k a constant characteristic of the spectrometer, $\sigma_X$ the photoionization cross-section of the considered core level X, $\lambda_X^Y$ the inelastic mean free path of photoelectrons emitted by the considered core level X in the matrix Y (in nm), $T_X$ the transmission function of the analyser for the considered core level X, $d_{oxide}$ the thickness of the oxide layer (nm), and $D_X^Y$ the density of the element X in the matrix Y (mol.cm⁻³). $D_X^Y$ is calculated from $M_Y$, the molar mass of the matrix Y (g.mol⁻¹), and $x_X^Y$, the atomic percentage of the element X in the matrix Y. The values of the photoionization cross-sections ($\sigma_X$) at 1486.6 eV are taken from Scofield and the inelastic mean free paths ($\lambda_X^Y$) calculated by the TPP2 formula. All values are compiled in the previous study [13]. The systematic error for calculating the oxide thickness has been estimated at ±0.2 nm [39].

# 3. Results and discussion

## 3.1 Clean CoCrFeMnNi HEA surface

The XPS survey and Cr-Mn-Fe-Co-Ni 3p, O 1s and C 1s core level spectra obtained after preparation of the clean surface are presented in Fig. 1. The survey spectrum does not show any specific elements apart from the expected alloying elements. In the Cr-Mn-Fe-Co-Ni 3p core level region, only metallic components are identified: Cr 3p at 41.7 eV, Mn 3p at 46.9 eV, Fe 3p at 52.8 eV, Co 3p at 59.0 eV, and a Ni 3p$_{3/2}$-p$_{1/2}$ spin-orbit doublet at 66.3-68.1 eV. No carbon nor oxygen signal was detected in C 1s and O 1s core level spectra, confirming that oxides and carbonaceous contaminants on the sample surface prepared by sputtering and annealing are below the XPS detection limit (< 1 at.%). The surface composition in alloying elements was
determined as 18Cr-13Mn-20Fe-24Co-25Ni (at.%). The depletion in Mn and enrichment in Co and Ni as compared to the nominal bulk composition of 20.3Cr-20Mn-19.8Fe-20Co-19.9Ni (at.%) suggests that the less reactive Co and Ni elements are enriched by the preparation procedure of an oxide-free surface, rather than by surface segregation at the annealing temperature of 500 °C. Indeed, Mn has the lowest surface energy compared to the other metal elements [40], and is expected to segregate if equilibrium is reached between surface and bulk compositions.

## 3.2 Surface oxidation

The evolutions of the XPS Cr-Mn-Fe-Co-Ni 3p core level region upon cumulative exposure time to oxygen at 100 and 200 °C are shown in Fig. 2. No carbon contamination was detected in the C 1s spectra during the oxidation process at these temperatures.

From 1 up to 15 minutes of oxidation at 100 °C (Fig. 2(a)), new components at 43.2, 47.8, 55.5 and 59.9 eV, characteristic of oxidized chromium, manganese, iron and cobalt species, noted $Cr_{ox}$, $Mn_{ox}$, $Fe_{ox}$, and $Co_{ox}$, respectively, appear and increase in relative intensity whereas no oxidized Ni is observed. The metallic components at 41.7 eV ($Cr_{met}$), 46.9 eV ($Mn_{met}$), 52.8 eV ($Fe_{met}$), 59.0 ($Co_{met}$), and 66.3-68.1 eV ($Ni_{met}$) decrease in relative intensity as expected for a growing surface oxide film increasingly attenuating the intensity of the photoelectrons emitted by the underlying metallic substrate. These observations show that the oxidation of the $CoCrFeMnNi$ surface is a fast process even in low oxygen pressure conditions. Beyond 15 minutes of exposure, only very small modifications are observed suggesting that oxide growth has reached saturation in the tested conditions.

At 200 °C (Fig. 2(b)), the surface evolution is similar to that observed at 100 °C. The $Cr_{ox}$, $Mn_{ox}$, $Fe_{ox}$, and $Co_{ox}$ species appear at the same binding energy (BE) values and increase in relative intensity faster than at 100 °C, while the $Cr_{met}$, $Mn_{met}$, $Fe_{met}$, $Co_{met}$ and $Ni_{met}$ decrease. This confirms the formation and growth of a surface oxide film increasingly attenuating the photoelectrons emitted by the alloy substrate. Like at 100°C, no oxidized Ni is formed at 200 °C. The $Mn_{ox}$, $Fe_{ox}$, and $Co_{ox}$ components increase further in relative intensity until 60 minutes
exposure time, indicating that these species are more abundant at the surface compared to the
film formed at 100 °C. This temperature effect is not observed for the Cr<sub>ox</sub> component oxide
which intensity decreases with ongoing oxidation. This may be caused by the attenuation of
the signal emitted by the Cr<sub>ox</sub> species preferentially formed in the very first initial stage of
oxidation and located in the inner part of the oxide layer at the interface with the substrate.
Beyond 60 minutes of exposure time, the variations are quite small indicating near-saturation
of the growth of the oxide.

3.3 Oxide film growth

The intensities of the oxide (Cr<sub>ox</sub>, Mn<sub>ox</sub>, Fe<sub>ox</sub>, and Co<sub>ox</sub>) and metallic (Cr<sub>met</sub>, Mn<sub>met</sub>, Fe<sub>met</sub>, Co<sub>met</sub> and Ni<sub>met</sub>) components obtained by curve fitting the Cr-Mn-Fe-Co-Ni 3p core level spectra were used to extract the equivalent thickness of the oxide layer as well as the compositions in alloying elements of the oxide layer and modified alloy underneath. This provides insightful information to discuss the underlying mechanisms that govern the oxide growth process. The growth kinetics of the oxide films formed at 100 and 200 °C is first discussed (Fig.3).

At 100 °C, the film thickness increases initially quite fast before levelling off and nearly
reaching a plateau beyond 15 minutes of oxidation. This growth kinetics is consistent with
observations from a series of studies performed on stainless steel surfaces (Fe-Cr-Ni alloy) at
relatively low oxidation temperature using XPS combined with scanning tunnelling
microscopy (STM) [41-43]. In these studies, an initial mechanism (mostly 2D) of nucleation
and subsequent coalescence of oxide islands was clearly revealed by STM local analysis. On
the present HEA surface, a similar mechanism of fast nucleation and mostly 2D growth of
oxide islands until saturation of the surface followed by 3D growth of the oxide film most
likely takes place. The equivalent thickness of 1.2 nm reached after 1 minute of oxidation
suggests the formation of a 3D film already at this stage since the equivalent thickness of a
monolayer of oxide is typically of about 0.25 nm [41, 44]. Hence, the very first stages of oxide
film growth by nucleation and 2D growth until complete coverage of the surface occur within
the first minute of oxygen exposure at the low pressure of 10<sup>-7</sup> mbar. Exposures at even lower
oxygen partial pressure and shorter time would be required to time-resolve early nucleation
and 2D growth of the oxide film [41-43]. At 15 minutes of oxidation, the equivalent thickness is 1.8 nm and it only increases to 2 nm after 165 minutes, clearly showing that further 3D growth is limited by atomic transport.

At 200 °C, the oxide film nucleates and grows even faster during the first minute of oxidation reaching 1.7 nm in equivalent thickness, suggesting that initial oxidation is promoted by the increase of surface mobility. Then, 3D growth becomes increasingly slower reaching 4.5 nm in equivalent thickness after 60 minutes of oxidation, beyond which 3D growth levels off reaching only 4.8 nm in equivalent thickness after 165 minutes. Thus, 3D oxide growth and its saturation level increase at higher oxidation temperature. This is consistent with a higher temperature promoting atomic transport within the oxide film and between surface and sub-surface of the sample, thus resulting in the formation of thicker oxide films at equivalent exposure time.

The growth of the oxide layer under UHV can also be limited by the evaporation of chromium oxide, as shown previously for the oxidation of pure chromium [45]. The oxide growth kinetics can then be fitted by Eq.(1) [36, 37, 45]:

$$t = \frac{k_p}{k_v^2} \left[ -\frac{k_v}{k_p}x - \ln \left( 1 - \frac{k_v}{k_p}x \right) \right] \quad [1]$$

where $k_p$ is the parabolic growth constant, $k_v$ the volatilization constant, $x$ the oxide film thickness, and $t$ the time. The fitted curves are also shown in Fig.3. They were obtained with parabolic and volatilization constants of 0.0035 nm²/s and 0.0017 nm/s at 100 °C, and 0.0102 nm²/s and 0.0021 nm/s at 200 °C, respectively. The values obtained at 200 °C are of the same order as those, 0.0019 nm²/s and 0.001 nm/s, respectively, obtained on a Ni-based alloy oxidized at 250 °C under 10⁻⁶ mbar oxygen pressure [46]. However, we note that at lower oxidation temperature and oxygen pressure, the equiatomic CoCrFeMnNi HEA shows relative higher parabolic and volatilization values compared to the Ni-based alloy, thus showing lower oxidation resistance than the Ni-based alloy. Considering the self-limiting oxide thicknesses not exceeding a few nanometers and the relatively low oxidation temperatures of the experiments, it cannot be excluded that the observed oxide growth kinetics may be explained also by the Cabrera and Mott (C-M) theory [47, 48] in which the oxide growth is limited by
the electron tunnelling effect.

3.4 Composition of oxide film and modified alloy underneath

The variations of the composition of the oxide film in alloying elements and of the underlying
modified alloy with increasing exposure time to oxygen at 100 and 200 °C are shown in Fig.4.
No oxidized nickel is detected along the entire oxidation process at these temperatures.

In the oxide film formed at 100 °C (Fig.4(a)), the composition in cationic species is 36Cr<sub>ox</sub>-28Mn<sub>ox</sub>-20Fe<sub>ox</sub>-16Co<sub>ox</sub> (at.%) after 1 minute of oxygen exposure, indicating preferential oxidation of Cr and to a lesser extent of Mn in the very first oxidation stages. With ongoing oxidation, only slight variations of the concentration in oxidized Fe (~2 at.% increase) and oxidized Mn (~2 at.% decrease) are observed. In contrast, the variation of the concentration in oxidized Cr and Co is more pronounced (~6 at.%) and rapid with a decrease to 30 at.% Cr<sub>ox</sub> and increase to 22 at.% Co<sub>ox</sub> after 15 minutes of oxidation. Afterwards, both the Cr<sub>ox</sub> and Co<sub>ox</sub> concentrations follow the same but slower trend decreasing to 28 at.% and increasing to 24 at.%, respectively, after 45 minutes of oxidation. After 165 minutes, the composition is stabilized as 28Cr<sub>ox</sub>-27Mn<sub>ox</sub>-22Fe<sub>ox</sub>-23Co<sub>ox</sub> (at.%). The surface oxide formed at growth saturation at 100 °C remains thus slightly enriched in Cr and Mn oxides. The enrichment in Cr<sub>ox</sub> and Mn<sub>ox</sub> results from preferential oxidation occurring mostly in the initial oxidation stages. The enrichment in Cr and Mn oxides observed after 1 minute of oxidation is in accordance with the values of the Gibbs free energy of formation of relevant oxide compounds compiled in Table 1 [49]. Regarding the minor formation of Co oxide while no Ni oxide forms, the Gibbs free energy for CoO formation is slightly more negative than that for NiO, showing the higher affinity of Co towards oxygen. Observations of the oxidation behaviour of the NiCo alloy at low oxygen partial pressure (10<sup>-7</sup> mbar) and low temperature also showed that Co oxide is mainly formed in the oxide film [50].

In the modified alloy region underneath the oxide film formed at 100 °C (Fig.4(b)), the
elemental composition varies from 18Cr-13Mn-20Fe-24Co-25Ni (at.%) before oxygen
exposure to 13Cr-9Mn-21Fe-28Co-29Ni (at.%) after 1 minute of oxygen exposure,
consistently with the preferential formation of Cr<sub>ox</sub> and Mn<sub>ox</sub> species in the very first oxidation stages. With ongoing oxygen exposure, the elemental composition slightly evolves to 12Cr-9Mn-19Fe-28Co-32Ni (at.%) after 15 minutes of oxidation and reaches 12Cr-6Mn-19Fe-29Co-34Ni (at.%) after 165 minutes oxidation. The continuous increase of the atomic ratio in metallic Ni is consistent with the persisting absence of oxidation of this element. Co, despite being oxidized after the very first oxidation stages (Fig.3(a)), remains enriched in the modified alloy, suggesting a preferential supply by atomic transport from the sub-surface region.

At 200 °C, the evolution of the composition in the oxide film takes place within the first 60 minutes of oxidation (Fig.4(c)), beyond which the percentages in alloying elements are stable according to the XPS observations. After 1 minute of oxidation, the composition in cationic species is 29Cr<sub>ox</sub>-28Mn<sub>ox</sub>-27Fe<sub>ox</sub>-16Co<sub>ox</sub> (at.%). Preferential formation of Cr<sub>ox</sub> and Mn<sub>ox</sub> species is confirmed in the very first oxidation stages like observed at 100 °C. However, Fe<sub>ox</sub> is concomitantly formed at 200 °C. After 15 minutes of oxidation, Co<sub>ox</sub> species steeply increases to 23 at.% while Cr<sub>ox</sub> decreases to 24 at.%, Mn<sub>ox</sub> is stable at 29 at.% and Fe<sub>ox</sub> decreases to 24 at.%. Beyond this point, Mn<sub>ox</sub> and Fe<sub>ox</sub> keep stable at 29 at.% and 24 at.%, respectively, during further oxide growth. The main modification is the balance between Co<sub>ox</sub> and Cr<sub>ox</sub>. At 60 minutes of oxidation, the oxide film becomes enriched in Co<sub>ox</sub> (32 at.%), and depleted in Cr<sub>ox</sub> (15 at.%) and this inverted balance is maintained beyond 60 minutes of oxygen exposure. The oxidation behaviour in this later stage of oxide growth follows the same trend as observed at 100 °C with increasing Co<sub>ox</sub> and decreasing Cr<sub>ox</sub> formations, the difference being that higher temperature (200 °C) promotes Co<sub>ox</sub> formation which results in the larger Co<sub>ox</sub> enrichment of the oxide film. These results suggest a stratification of the grown oxide layer with the part formed in the very first oxidation stages enriched in Cr oxide, and the part enriched in Co oxide formed in the later oxidation stages. Whether Cr oxide enriches the inner or outer part of the oxide film depends on the predominant mechanism of 3D growth, by outward cationic or inward anionic transport, respectively. This is further discussed below.

Underneath the oxide film formed at 200°C (Fig.4(d)), the composition of the alloy is 12Cr-8Mn-18Fe-30Co-32Ni (at.%) after 1 minute of oxidation to be compared with 18Cr-13Mn-
20Fe-24Co-25Ni (at.%) for the clean, oxide-free surface. The 6 at.% decrease in metallic Cr and 5 at.% decrease in metallic Mn are in accordance with the preferential initial formation of Cr and Mn oxides. The slight preferential formation of Fe oxide (27 at.%) is also reflected by the depletion in metallic Fe, however less marked (2 at.%) than for metallic Cr (6 at.%) and Mn (5 at.%). This might be caused by faster supply of Fe from the sub-surface than of Cr and Mn. The enrichment in metallic Co and Ni is because these more noble elements are less or non-oxidized in this initial stage. Upon further oxidation, the modified alloy composition evolves to 11Cr-6Mn-16Fe-25Co-42Ni and 12Cr-7Mn-17Fe-10Co-54Ni (at.%) after 30 and 45 minutes of oxidation, respectively. The percentage in metallic Cr, Fe and Mn can be considered stable within ± 1 at.% accuracy. The main modification is the preferential oxidation of cobalt leading to its depletion compensated by the enrichment of nickel. This trend is confirmed after 60 minutes of oxidation beyond which there is no further significant changes of the modified composition of the alloy.

The oxide film formed at saturation level after 165 minutes of oxidation time at 100 and 200 °C have a composition of 28Cr<sub>ox</sub>-27Mn<sub>ox</sub>-22Fe<sub>ox</sub>-23Co<sub>ox</sub> (at.%) and 14Cr<sub>ox</sub>-29Mn<sub>ox</sub>-24Fe<sub>ox</sub>-33Co<sub>ox</sub> (at.%), respectively. The higher temperature of 200 °C promotes the enrichment in cobalt oxide compensated for the most part by the depletion in chromium oxide. In the modified alloy underneath the oxide film, the composition is 12Cr-6Mn-19Fe-29Co-34Ni and 12Cr-7Mn-17Fe-10Co-54Ni (at.%) at 100 and 200 °C, respectively, confirming that increasing the temperature promotes the preferential oxidation of cobalt in the later stages of oxidation and that its depletion from the modified alloy region is compensated by the enrichment of Ni that is not oxidized in the tested conditions.

The oxidation behaviour of a non-equiatomic, single phase fcc Ni<sub>38</sub>Cr<sub>22</sub>Fe<sub>20</sub>Mn<sub>10</sub>Co<sub>10</sub> (at.%) multi principal element alloy (MPEA) after 5 minutes exposure in 10 mbar O<sub>2</sub> at 120 and 300 °C was studied by atom probe tomography (APT) [33]. The oxide films formed on this MPEA surface at 120 and 300 °C were composed of 22Cr<sub>ox</sub>-10Mn<sub>ox</sub>-20Fe<sub>ox</sub>-9Co<sub>ox</sub>-39Ni<sub>ox</sub> (at.%) and 25Cr<sub>ox</sub>-11Mn<sub>ox</sub>-19Fe<sub>ox</sub>-7Co<sub>ox</sub>-37Ni<sub>ox</sub> (at.%), respectively. It is unclear why these oxide films have compositions nearly identical to the MPEA alloy bulk composition and do not show
enrichment and depletion effects reflecting the competitive oxidation process revealed in the present work. A possible explanation lies in the surface preparation and oxidation conditions, with the thermal oxide films formed on initially native oxide-covered surfaces on the MPEA alloy whereas, in the present work on the HEA alloy, thermal oxidation was performed on initially oxide-free surface

Compared to the native and passive oxide films formed on the same HEA at room temperature [13], which compositions are 47Cr<sub>ox</sub>-15Mn<sub>ox</sub>-30Fe<sub>ox</sub>-8Co<sub>ox</sub> (at.%) and 65Cr<sub>ox</sub>-14Mn<sub>ox</sub>-16Fe<sub>ox</sub>-5Co<sub>ox</sub> (at.%), respectively, the oxide film formed at 100 and 200 °C in low pressure gaseous oxygen show lower Cr<sub>ox</sub> and higher Mn<sub>ox</sub> and Co<sub>ox</sub> contents. This confirms that higher oxidation temperature promotes the formation of Mn and Co oxides.

3.5 Chemical states in oxide film

Besides oxide film thickness and composition, identifying the chemical states of each oxide
species is helpful to better understand the oxidation behaviour of the studied CoCrFeMnNi
HEA. XPS 2p core level spectra of the alloying elements are insightful and commonly used for
this purpose. However, in the present case of an alloy consisting of five transition elements,
their analysis is blurred by the interference of Auger lines which makes curve fitting and the
quantification uncertain.

The XPS O 1s, Cr 2p, Fe 2p, Co 2p, Mn 2p, and Ni 2p core level spectra were recorded at saturation of the oxide films grown at 100 °C and 200 °C (Fig.5). At both temperatures, the O 1s spectra show only one peak component at a BE value of 530 eV and assigned to O<sup>2-</sup> anions of the oxide film matrix [17, 51, 52], demonstrating pure oxide formation after exposure to gaseous oxygen. Thanks to the absence of interference of Auger lines in Cr 2p spectra, the Cr 2p<sub>3/2</sub> peak at 576 eV BE, corresponding to Cr(III) cations in an oxide matrix [18, 43, 53], could be easily identified for the films grown at 100 and 200°C. It is interesting to note that Cr(III) in Cr<sub>2</sub>O<sub>3</sub> shows an asymmetrical shape due to a discrete multiplet structure as observed in Fig 5(b). In the Fe 2p spectra, there is the interference of Co L<sub>2</sub>M<sub>23</sub>M<sub>45</sub> and Ni L<sub>3</sub>M<sub>23</sub>M<sub>45</sub> Auger transitions both peaking in the 712-713 eV BE range [54-58]. However, the satellite
features observed at 732 eV BE indicates that the chemical state of Fe oxide species formed at 100 °C and 200 °C is Fe(III) [59]. In the Co 2p spectra, there are the Fe L₃M₄₅M₄₅ and Ni L₃M₂₃M₂₃ Auger transitions interfering at 783 and 777 eV BE, respectively [57, 58, 60]. The 3/2-1/2 satellites of CoO oxide, located at 786-803 eV BE, are consistent with the formation of Co(II) oxide species at both temperatures [61].

As for the identification of the Mn oxidation states, two different states of this element should be considered for oxidation at 200 °C. Indeed, two components at 48.1 and 50.4 eV BE were used to fit the Mn 3p spectra while, for oxidation at 100 °C, only one peak at 48.1 eV BE was used (Fig. 2). The peak at 50.4 eV BE was first observed after 15 minutes oxidation at 200 °C. Then, with prolonged exposure to oxygen, both peak components increased in intensity, meaning continuous formation of Mn oxide of mixed valence states. In the Mn 2p core level spectra shown in Fig.5(e), the Ni Auger transition L₂M₂₃M₄₅ interferes at 641 eV BE [58]. In the Mn 2p₁/₂ region, the peak observed at both temperatures is located at 653 eV BE, indicating that the chemical state is Mn(II) or Mn(III) [62-64]. Considering that Mn(II)O has a satellite feature at ~647 eV BE, not present for Mn(III)₂O₃ [65], it is suggested that, for oxidation at 100 °C and for 1 minute at 200 °C, Mn(III) oxide is mainly formed since the Mn(II) satellite feature at ~647 eV BE is not observed. Then, for longer oxidation at 200 °C, the Mn(II) satellite feature is observed, and the oxide film contains both Mn(II) and Mn(III) oxidation states. Consistently, the peak components at 48.1 and 50.4 eV BE in the Mn 3p core level region are assigned to the Mn(II,III) and Mn(II) oxides, respectively [66].

In the Ni 2p core level spectra (Fig.5(f)), there are Mn and Co Auger transitions interfering at 851.7 and 867.7 eV BE. Only metallic Ni 2p₃/₂-1/₂ satellites are observed at 859 and 875.8 eV BE, which confirms that no oxidized Ni is formed at the two oxidation temperatures.

The chemical states of the alloying elements present in the oxide films being identified, the Gibbs free energy of formation of the corresponding Cr(III), Fe(III), Co(II), Mn(II,III) and Mn(III) oxides can be discussed comparatively to help further understand the oxidation behaviour of the CoCrFeMnNi HEA at 100 °C and 200 °C. As shown in Table 1, Cr₂O₃, Mn₂O₃ and Fe₂O₃ have lower Gibbs free energies of formation compared to CoO at the two studied
temperatures (100 °C and 200 °C), and are preferentially formed in that order. For longer
oxidation at 100 °C, although Cr and Mn have higher oxygen affinity than Fe and Co, the
decrease of the Cr(III) and Mn(III) atomic ratios and the increase of the Fe(III) and Co(II) is
linked to the depletion in metallic Cr and Mn in the modified alloy layer underneath the oxide
film, preventing sustained preferential formation of Cr(III) and Mn(III) oxides. At this
oxidation stage, the oxidation mechanism becomes a kinetically controlled process, with the
oxide formation not only determined by competitive oxidation of the Cr, Fe, Mn, Co and Ni
elements, but also by their atomic mobility, as also observed for the thermal oxidation
behaviour of 304 stainless steels [43]. At 200 °C, the gradual enrichment of Co(II) oxide is
observed at the expense of Cr(III) oxide and also of Fe(III) oxide. This Co enrichment cannot
be explained only by faster atomic supply from the sub-surface since the Co atomic ratio in the
modified alloy layer is even lower than that of metallic Cr after exposure to oxygen for 165
minutes. One possible explanation for this Co oxide enrichment at 200 °C is that a Cr(III)-rich
oxide inner layer is formed in the very first stages and then slows down the transport of Cr(III),
Mn(III) and Fe(III) cations more efficiently than that of Co(II) cations. Thus, mostly Co(II)
cations would diffuse through this Cr(III)-rich inner layer leading to Co(II)O formation on the
outer part of the growing oxide film. This result is in accordance with Co content decrease in
the underlying layer. This explanation implies that the oxide film would mainly grow by
cationic outward transport.

3.6 Oxide film stoichiometry

The evolution of the oxygen content and overall stoichiometry of the oxide films with
increasing oxidation time is presented in Fig.6. At 100 °C, oxygen content and overall
stoichiometric ratio remain unchanged during oxide growth at values 65 at.% and 1.86,
respectively. At 200 °C, the oxygen content slightly decreases from 67 at.% after 1 minute of
oxidation to 63 at % after 165 minutes. The overall stoichiometric ratio has an initial value of
~2 and then decreases to 1.7. The overall stoichiometry measured at 100°C and initially at
200°C suggests an average valence of about +2 of the mixed oxide film. This is consistent with
the formation of Co(II) and Mn(II) oxide species but not of Cr(III), Mn(III) and Fe(III) oxide
species. A possible explanation is the presence of cation vacancies ($V_{Cr}'''$, $V_{Mn}'''$, $V_{Fe}'''$) in the
growing oxide. With ongoing oxidation at 200°C, the decrease of the overall stoichiometric
ratio to 1.7 is consistent with the increasing formation of Co(II) oxide species and with the
presence of cation $V_{Co}''$ vacancies. The presence of cation vacancies at both temperatures also
implies that the oxide film would mainly grow by cationic outward transport, like suggested
by the evolution of the composition of the oxide film formed at 200°C.

Conclusions

XPS high resolution core level analysis was applied *in situ* in well-controlled environment to investigate the initial, early-stage oxidation mechanisms of equiatomic CoCrFeMnNi HEA surfaces exposed to low-pressure gaseous oxygen at 100 °C and 200 °C.

The results show that the clean, oxide-free surface prepared under UHV by Ar ions sputtering
and annealing at 500°C is enriched in Co and Ni elements at the expense of Mn and Cr. Upon
surface oxidation at 100 °C, Cr(III), Mn(III), Fe(III) and Co(II) oxide species are formed while
at 200 °C manganese also oxidizes to Mn(II). No nickel oxidation was detected at these
temperatures.

Quantification of the oxide film growth and surface composition revealed a very fast first stage
of nucleation, 2D growth and early 3D growth of the oxide film, initially enriched in Cr and
Mn oxides owing to preferential oxidation of these elements. Accordingly, the modified alloy
underneath the oxide film is initially depleted in Cr and Mn. With increasing oxidation time,
3D growth progressively slows down before levelling off at near saturation level. At 200°C,
3D growth reaches near saturation slower and the oxide film grows thicker than at 100°C owing
to atomic transport promoted in the oxide film and from the alloy sub-surface. The oxide films
initially enriched in Cr and Mn become depleted due to the preferential formation of Co oxide
in these later stages. This transition from a selective oxidation process to a kinetically-
controlled growth process is consistent with the formation of a stratified structure of the oxide
film, like observed on the same alloy after annealing pre-formed passive oxide films. The Co-
rich oxide species form in the outer part above the Cr-rich oxide initially formed in the inner
part, consistently with a predominant cationic outward oxide growth mechanism supported by
the overall stoichiometry of the oxide film.

CRediT authorship contribution statement

Luntao Wang: Investigation, Formal analysis, Writing – original draft.

Sandrine Zanna: Investigation, Validation, Writing – review & editing.

Dimitri Mercier: Validation, Writing – review & editing, Supervision.

Vincent Maurice: Validation, Writing – review & editing, Supervision, Funding acquisition

Philippe Marcus: Conceptualization, Writing – review & editing, Supervision, Funding acquisition, Project management.

Declaration of Competing Interest

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.

Acknowledgements

This project has received funding from the European Research Council (ERC) under the
European Union's Horizon 2020 research and innovation program (ERC Advanced grant no.
741123).

Data Availability

All data presented herein will be provided upon reasonable request.
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<table><caption>Table 1 Gibbs free energies for oxides formation at two different temperatures [49].</caption><thead><tr><th>Oxide</th><th>400K/127°C (kJ/mol O<sub>2</sub>)</th><th>500K/227°C (kJ/mol O<sub>2</sub>)</th></tr></thead><tbody><tr><td>MnO</td><td>-710.656</td><td>-695.984</td></tr><tr><td>Mn<sub>2</sub>O<sub>3</sub></td><td>-569.719</td><td>-552.468</td></tr><tr><td>Mn<sub>3</sub>O<sub>4</sub></td><td>-641.292</td><td>-623.801</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>-687.099</td><td>-669.510</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>-476.338</td><td>-458.480</td></tr><tr><td>CoO</td><td>-412.624</td><td>-397.742</td></tr><tr><td>NiO</td><td>-403.996</td><td>-385.684</td></tr></tbody></table>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Fig.1 XPS spectra of the clean CoCrFeMnNi HEA surface prepared by Ar ion sputtering and annealing: (a) survey spectrum, (b) Cr-Mn-Fe-Co-Ni 3p, (c) O 1s and (d) C 1s core level regions.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig.2 Time evolution of the XPS Cr-Mn-Fe-Co-Ni 3p core level region for the CoCrFeMnNi surface exposed to low pressure oxygen (10⁻⁷ mbar) at (a) 100 °C and (b) 200 °C.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 3 Time evolution of the equivalent thickness of the oxide films growing on the CoCrFeMnNi surface exposed to low pressure oxygen (10⁻⁷ mbar) at 100 °C and 200 °C.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig.4 Time evolution of the surface composition of the CoCrFeMnNi alloy exposed to low pressure oxygen (10⁻⁷ mbar) at (a,b) 100 °C and (c,d) 200 °C: (a,c) cationic relative concentration in the oxide film and (b,d) metallic relative concentration in the modified alloy underneath the oxide film.</figcaption></figure>
<figure><img src="image_10.png" /></figure>

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

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Fig.5 XPS core level spectra for oxide films formed on the CoCrFeMnNi surface exposed to low pressure oxygen (10⁻⁷ mbar) at 100 °C and 200 °C for indicated times: (a) O 1s, (b) Cr 2p, (c) Fe 2p, (d) Co 2p, (e) Mn 2p and (f) Ni 2p regions.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig.6 Time evolution of the oxygen atomic percentage (dots) and oxygen-to-cation stoichiometric ratio (lines) of the oxide films growing on the CoCrFeMnNi surface exposed to low pressure oxygen (10⁻⁷ mbar) at 100 °C and 200 °C.</figcaption></figure>