# Enhanced corrosion resistance of 2024 aluminum alloys with Cr2O3 thin layers by Atomic Layer Deposition

D. Mercier, R. Zazpe, X. Wang, M. Michaux, J. Rodriguez-Pereira, S. Zanna, A. Seyeux, J.M. Macak, P. Marcus

► To cite this version:

D. Mercier, R. Zazpe, X. Wang, M. Michaux, J. Rodriguez-Pereira, et al., Enhanced corrosion resistance of 2024 aluminum alloys with Cr2O3 thin layers by Atomic Layer Deposition. Corrosion Science, 2025, 256, pp.113167. 10.1016/j.corsci.2025.113167. hal-05356626

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# Enhanced corrosion resistance of 2024 aluminum alloys with Cr₂O₃ thin layers by Atomic Layer Deposition

D. Mercier<sup>a,1,*</sup>, R. Zazpe<sup>b,c,1</sup>, X. Wang<sup>a</sup>, M. Michaux<sup>a</sup>, J. Rodriguez-Pereira<sup>b,c</sup>, S. Zanna<sup>a</sup>, A. Syeux<sup>a</sup>, J.M. Macak<sup>b,c,**</sup>, P. Marcus<sup>a,*</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Atomic Layer Deposition

Cr₂O₃ thin film

2024 Al alloy

Time-of-flight secondary ions mass

spectroscopy

Corrosion resistance

## ABSTRACT

This research explores the use of chromium oxide (Cr₂O₃) thin layers grown by Atomic Layer Deposition (ALD) as protective coating to enhance the corrosion resistance of 2024 aluminum alloys. In order to obtain sufficiently dense and uniform Cr₂O₃ layers, the ALD process was tailored in terms of alloy surface pretreatment before the main Cr₂O₃ ALD process. The corrosion resistance of both Cr₂O₃ coated and non-coated aluminum alloys was evaluated in a corrosive 0.1 M KOH environment using *in situ* optical microscopy and *ex situ* surface analysis techniques, including X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and a neutral environment containing chlorides. Findings revealed that the Cr₂O₃-coated samples exhibited significantly reduced reactivity, highlighting the excellent corrosion protection provided by the Cr₂O₃ thin films. Although surface analysis revealed the presence of submicron defects within the Cr₂O₃ layer, which could act as corrosion initiation sites, the occurrence of these defects was mitigated with increasing Cr₂O₃ layer thickness. Additionally, after the corrosion test, an enrichment of copper and aluminum oxides at the layer surface was observed, suggesting preferential attack at intermetallic phases in corrosive environment.

## 1. Introduction

Aluminum and its alloys have become indispensable in a wide range of industrial sectors owing to their unique combination of properties, which confer exceptional versatility. As of 2024, global primary aluminum production has reached approximately 48 million tons, establishing it as the most extensively utilized non-ferrous metal. The transportation and construction industries together account for nearly 50 % of this consumption. The incorporation of alloying elements enables the development of materials with an enhanced strength-to-weight ratio, a key parameter for structural and lightweight applications. Pure aluminum exhibits outstanding corrosion resistance, primarily due to the spontaneous formation of a nanometer-scale passive alumina (Al₂O₃) film [1]. This passive layer affords effective protection across a broad range of aggressive environments and a relatively wide pH window.

However, in contrast to Fe- or Ni-based alloys—where alloy design

strategies are employed to simultaneously optimize mechanical properties and corrosion resistance—modifications in aluminum alloys predominantly target improvements in their mechanical performance, often at the expense of the corrosion resistance. In particular it is now well established for different series of aluminum alloys -particularly the 2XXX and 7XXX series - that improvement in mechanical properties induced by the presence of intermetallic particles (IMPs, widely discussed in the literature [2], is accompanied by a significant reduction in the corrosion resistance [3]. This decrease is primarily attributed to differences in electrochemical reactivity between the matrix and the intermetallics. The corrosion mechanisms of such alloys have been extensively investigated in the literature, and it is now clearly recognized that intermetallic particles play a critical role in the initiation of localized corrosion, and the chemical composition of the intermetallic phases has a major influence on the corrosion behavior of the alloys [4, 5]. Nanogalvanic coupling occurs both in the heterogeneous corroded

<footnote><sup>a</sup> PSL University, CNRS-Chimie ParisTech, Institut de Recherche de Chimie Paris, Research Group of Physical Chemistry of Surfaces, Paris, France</footnote>

<footnote><sup>b</sup> Center of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, Nam. Cs. Legii 565, Pardubice 530 02, Czech Republic</footnote>

<footnote><sup>c</sup> Central European Institute of Technology, Brno University of Technology, Purkynova 123, Brno 61200, Czech Republic</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>** Corresponding author at: Center of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, Nam. Cs. Legii 565, Pardubice 530 02, Czech Republic.</footnote>

<footnote>E-mail addresses: dimitri.mercier@chimieparistech.psl.eu (D. Mercier), jan.macak@upce.cz (J.M. Macak), philippe.marcus@chimieparistech.psl.eu (P. Marcus).</footnote>

<footnote n="1">¹ These authors contributed equally.</footnote>
IMPs and between these particles and the surrounding matrix. These effects are particularly pronounced for S-phase particles compared to θ-phase particles [6].

More recently, studies on model alloys have revealed differences in the chemical nature of the oxide films covering the IMPs, as compared to those formed on the matrix [7]. These findings offer new insights into the localized corrosion processes driven by microstructural heterogeneities.

The corrosion protection of aluminum alloys is typically ensured through the formation of barrier layers that limit interfacial interactions with the environment. In the aeronautic industry, chromate-based treatments containing hexavalent chromium have been widely employed due to their excellent anticorrosion properties [8,9]. These are attributed to the formation of a protective barrier layer that effectively hinders chloride ion penetration. However, owing to the toxicity of hexavalent chromium and the associated REACH regulations, such treatments are currently being phased out.

Trivalent chromium process (TCP) coatings have emerged as a promising alternative to replace conventional hexavalent chromate-based treatments. Numerous studies have demonstrated the effectiveness of TCP layers in providing corrosion protection [10,11]. Nevertheless, the performance of these coatings is strongly influenced by the underlying microstructure, particularly the nature of intermetallic particles (IMPs) [12] and the surface preparation protocols employed prior to their deposition [13].

More recently, new methods such as Chemical vapor deposition (CVD), and more specifically Atomic Layer Deposition (ALD), have been developed to deposit thin protective layers. Unlike PVD methods, which often result in films with inherent heterogeneities, limiting their effectiveness for corrosion protection [14], CVD processes can yield highly uniform thin films. ALD is a derivate CVD technique that enables the deposition of dense, conformal, and uniform (ultra)thin films with low defect densities, even on complex and large-scale substrates [15,16]. ALD processes are well-established, allowing for the deposition of various metals [17], oxides [18–20], sulfides [21], fluorides [22,23], and nitrides [24] onto most metallic substrates. Among a wide number of applications, previous works have reported ALD for the deposition of nanoscale protective coatings, including Al₂O₃, TiO₂, Ta₂O₅, HfO₂ or mixed oxides, which demonstrated the enhanced corrosion resistance on several metals and alloys, including stainless steel [25–27], Ti [28], Cu [29–31], and Mg alloys [32] and also prevention of photo-corrosion of hematite [33]. Regarding aluminum and its alloys, a number of studies report an improvement in corrosion resistance for TiO₂ or Al₂O₃ coatings, for low-alloy aluminum (AA1060) [34], alloys used in the food industry (AA3104) [35] or alloys used in aeronautics (AA2024) [36,37].

Unlike Al₂O₃ layers, thin Cr₂O₃ layers by ALD have been studied to a very limited extent. The Cr₂O₃ ALD has been reported using different Cr precursors, such as tris(2,2,6,6-tetramethyl heptane 3,5-dione) chromium (III) - Cr(thd)₃ [38], chromium oxychloride - CrO₂Cl₂, [39–42] and chromium acetylacetonate - Cr(acac)₃ [43].

This work aims to deposit thin Cr₂O₃ layers by ALD with a controlled stoichiometry and thickness to enhance the corrosion protection of aeronautical aluminum alloy (2024 Al alloy). Chromium oxide (Cr₂O₃) is a material with outstanding properties as protective coating, due to its excellent resistance to corrosion, wear, and oxidation.

This research focused on optimizing Cr₂O₃ ALD processes, conducting an in-depth chemical characterization of the deposited layers using X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and evaluating the reactivity of these coatings in aggressive alkaline and chloride environments.

## 2. Methods

### 2.1. Sample preparation

A 2024 Al alloy with the following composition was used in this

work: Cu (3.8–4.9 wt%), Mg (1.2–1.8 wt%), Mn (0.30.9 wt%), Fe (0.5 wt%), Si (0.5 wt%), Zn (0.25 wt%), Ti (0.15 wt%), Cr (0.1 wt%), and Al (balance). Samples were cut into disks of 15 mm in diameter.

Before the ALD deposition, samples were mechanically polished to a mirror finish with SiC paper of 1200, 2400 and 4000 grades, followed by diamond polishing with 6, 3, 1 and 0.25 µm suspensions. Afterwards, the samples were cleaned by sonication in acetone, ethanol and deionized water (Merk Millipore, resistivity of 18.2 MΩ.cm) and finally dried in a flow of compressed air.

### 2.2. Atomic Layer Deposition (ALD) process

The 2024 Al alloy samples and reference silicon wafers were cleaned before the Cr₂O₃ ALD processes by placing them subsequently in acetone, isopropanol and deionized water baths for 5 min under sonication. Samples were dried in N₂ flow. The deposition of Cr₂O₃ was carried out using 2024 Al alloy and silicon wafers as a substrate in a thermal ALD system (TFS-200, Beneq) at chamber pressure 2 mbar applying a deposition temperature of 300 °C. The surface of the Al alloy samples was pre-treated before the deposition of Cr₂O₃ by 5 pulses (500 ms) of ultrapure water (18 MΩ cm⁻¹) to increase the number of hydroxyl active sites on the substrate surface, followed by 5 ALD cycles of Al₂O₃ using trimethyl aluminum (TMA) as Al precursor and water. One Al₂O₃ ALD cycle was composed of the sequence: TMA (200 ms) – N₂ purge (5 s) – water (500 ms) – N₂ purge (5 s). Then the deposition of Cr₂O₃ was performed using chromium acetylacetonate, Cr(acac)₃, (Strem, 97.5 %) as Cr precursor, and ozone as the oxygen source. Chromium acetylacetonate was heated up to 170 °C to get sufficient vapor pressure. One Cr₂O₃ ALD cycle was defined by the sequence: Cr (acac)₃ (3 s) - N₂ purge (15 s) - Cr(acac)₃ (3 s) - N₂ purge (15 s) - O₃ (3 s) - N₂ purge (10 s). The number of ALD cycles applied was 1000 and 2250. The calculated growth rate was ≈ 0.028 nm/cycle. High purity N₂ (99.999 %) was used as carrier gas at a flow rate of 500 standard cubic centimeter per minute.

### 2.3. Scanning electron microscope (SEM) characterization

The structure and morphology of the deposited Cr₂O₃ were characterized by a field emission SEM (FE-SEM JEOL JSM 7500F) using a backscattered electron (BSE) detector applying at a voltage of 5 kV, a current of 20 µA and a low angle backscattered electron (LABE) detector (), with no conductive coating.

### 2.4. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) characterization

Chemical in-depth profiling was performed with an IONTOF ToF-SIMS 5 spectrometer (with a pressure of 10⁻⁸ mbar or lower). The depth profiles were recorded in the dual-beam mode. A pulsed 25 keV Bi⁺ primary ion source delivering 1.2 pA of target current over a 100 × 100 µm² area was used for analysis. Analyses were interlaced with sputtering using a 1 keV Cs⁺ sputter beam delivering 50 nA of target current over a 300 × 300 µm² area. Negative ions were recorded for their higher sensitivity to fragments coming from oxide matrices. Data acquisition and processing were done with the IONSPEC 6.5 software.

### 2.5. X-ray photoelectron spectroscopy (XPS) characterization

XPS analyses were performed with a Thermo Electron ESCALAB 250Xi spectrometer (pressure of 10⁻⁹ mbar) using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). High-resolution spectra of the Cr 2p, Al 2s, Cu 2p, O 1s and C 1s core levels were recorded with a pass energy of 20 eV and a step size of 0.1 eV. Data acquisition and processing were done with the Avantage software.
## 2.6. Corrosion resistance experiments

The corrosion resistance of ALD $Cr_2O_3$-coated 2024 Al alloy was evaluated via immersion in a 0.1 M KOH solution for 30 min. This test solution was chosen because aluminum oxide is not stable in it, so this is a very corrosive environment for the alloy. Real-time, in situ monitoring of the corrosion process was conducted using a Keyence VHX-5000 digital microscope at 100x magnification

## 2.7. Electrochemical measurements

Electrochemical measurements were performed using a Multi-PalmSens4 potentiostat (PalmSens, The Netherlands) in a conventional three-electrode cell with an alloy sample as a working electrode, a Pt wire as a counter electrode and a saturated calomel reference electrode (SCE). Measurements were carried out at room temperature in $K_2SO_4$/ NaCl with concentration of 0.01 M / 0.1 M, respectively (Analab Nor- mapur, analytical reagent, VWR). Before each electrochemical polariza- zation, the open circuit potential was recorded for 30 min, after which the potential was stable. Potentiodynamic polarizations were performed by the linear scan voltammetry at a scanning rate of 1 mV/s.

# 3. Results and discussion

## 3.1. Optimization of the ALD process

The uniformity and morphology of the ALD $Cr_2O_3$ thin layers deposited on 2024 Al alloy samples were characterized by SEM using BSE detector. The as-obtained images of samples coated without pre-treatment with 1000 ALD cycles revealed that the uniformity of the $Cr_2O_3$ thin layers was not optimal (see Fig. 1a and b).

In order to enhance uniformity, the surface of the 2024 Al alloy substrates was treated with 5 ALD $Al_2O_3$ cycles (in addition to the 5 pulses of water). The BSE SEM images obtained from the $Al_2O_3$ pre-

treated $Cr_2O_3$ thin layers (see Fig. 1c-d) revealed a significant improvement in their uniformity as compared to their non-pretreated counterparts (Fig. 1a-b), confirming the positive impact of the $Al_2O_3$ pre-treatment on the density of the $Cr_2O_3$ thin layers. Accordingly, all following $Cr_2O_3$ ALD processes described in this work were carried out with the surface pre-treatment of 5 pulses of water and 5 pulses of $Al_2O_3$ (see Section 2.2). This $Al_2O_3$ surface pre-treatment provides to the surface of the substrate of a higher number of active site (principally hydroxyl groups), which allows a more uniform/denser $Cr_2O_3$ thin layer to be obtained.

## 3.2. Characterization of the ALD layer

Following the tailored ALD process described above, $Cr_2O_3$ thin layers were deposited on 2024 Al alloy substrates and silicon wafers applying 1000 and 2250 $Cr_2O_3$ ALD cycles. The resulting $Cr_2O_3$ thin layers on Al alloy substrates were characterized by SEM and showed a rough granular surface (Fig. 2a and b). SEM images of the polished surfaces before ALD coating are presented in the SI file (Fig. S1).

Cross-sectional SEM images of the $Cr_2O_3$ thin layers deposited on Si after 1000 and 2250 ALD cycles (Fig. 3) confirmed the deposition of continuous thin layers with a thickness of 28 and 63 nm, respectively.

To observe possible defects within the coating, AFM imaging was performed on a thin $Cr_2O_3$ layer deposited after 2250 ALD cycles. Fig. 4 shows a representative image of the overall surface. The surface displays nanometric grains. Although it appears homogeneous, various types of defects can be observed. A polishing scratch is visible. The coating follows this scratch closely, confirming that it is sufficiently conformal to mitigate the influence of surface roughness. Two other types of defects were also identified. The first, indicated by a green arrow, corresponds to a circular heterogeneity with a diameter of approximately 400 nm and a depth of 40 nm. Given the coating thickness (~60 nm), this defect is not through the entire layer. The second type, marked by a blue arrow, consists of heterogeneities of similar diameter (~500 nm), but with a

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a-b) BSE SEM images (at different magnification) of $Cr_2O_3$ thin layers deposited on 2024 Al alloy samples after 1000 ALD cycles without $Al_2O_3$ pre-treatment. (c-d) BSE SEM images of $Cr_2O_3$ thin layers deposited on 2024 Al alloy samples after 1000 ALD cycles with $Al_2O_3$ pre-treatment.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. SEM top view images (taken at different magnification) of Cr₂O₃ thin films deposited after (a) 1000 ALD cycles and (b) 2250 ALD cycles on 2024 Al alloy substrate.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Cross-sectional SEM images of Cr₂O₃ thin layers deposited after (a) 1000 and (b) 2250 Cr₂O₃ ALD cycles.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. AFM image (4 × 4 µm²) of a Cr₂O₃ thin layer deposited after 2250 ALD cycles.</figcaption></figure>

depth of around 60 nm, meaning this defect is fully through the layer.

These observations confirm that while the ALD coating effectively conforms to and covers pre-existing surface heterogeneities, the process also generates nanometric defects, some of which may be through the layer.

Cr₂O₃ layers deposited after 1000 and 2250 ALD cycles were further characterized using ToF-SIMS and XPS. Fig. 5 presents a representative ToF-SIMS in-depth profile acquired in negative polarity from the Cr₂O₃ layer deposited after 2250 ALD cycles. Selected secondary ions, indicative of both oxide and metallic components of the Al 2024 alloy, are shown. CrO₂ (m/z 83.9300) is associated with Cr₂O₃, while AlO₂ (m/z 58.9727) and CuO (m/z 78.9246) correspond to Al₂O₃ and Cu₂O/CuO,

<figure><img src="image_8.png" /><figcaption>Fig. 5. ToF-SIMS negative ions in-depth profile for an ALD Cr₂O₃-coated 2024 Al alloy (2250 ALD cycles).</figcaption></figure>

respectively. MgO₂ (m/z 55.9757) was selected as the marker of Mg oxide. Al₂ (m/z 53.9637) and Cu (m/z 62.9314) represent metallic Al and Cu, respectively, from the bulk alloy.

All in-depth profiles exhibited similar features, revealing the deposited Cr₂O₃ outer layer. Beneath this layer, an intermediate layer mainly composed of Al₂O₃ was observed. A small amount of magnesium is also detected in this layer. Within this intermediate layer, a weak shoulder in the CuO⁻ signal near the Cr₂O₃ / Al₂O₃ interface suggests a minor enrichment of copper oxide in this region. The subsequent sharp increase in the Cu⁻ and Al₂⁻ signals mark the transition to the underlying bulk alloy. The Cu⁻ profile exhibits a maximum intensity at the Al₂O₃/Al interface, potentially indicating a slight Cu enrichment at this interface.

The profile obtained for a freshly polished 2024 alloy is provided in the Supporting Information file for comparison (Fig. SI2). A broadening of the interface is observed after the ALD process (45 s and 85 s before and after ALD deposition, respectively). The evolution of the AlO₂⁻ and
MgO$_2^-$ signals, which show similar trends, suggests that the oxide growth is due to the temperature increase rather than an increase in the thickness of the Al$_2$O$_3$ pre-treatment layer.

XPS analysis of the Cr$_2$O$_3$ layer deposited after 2250 ALD cycles is illustrated in Fig. 6. Chromium oxide is the dominant surface species. Notably, Al and Cu signals (Al 2s and Cu 2p) are absent, indicating a negligible presence of these elements at the surface. The detection limits under conditions, where the signal-to-noise ratio is optimal — that is, by increasing the number of scans — can approach 0.05 at% [44]. This observation indicates the presence of a homogeneous, and continuous chromium oxide layer with a thickness >10 nm. Further analysis of the Cr 2p spectrum, along with comparison to a reference spectrum of thermally grown Cr$_2$O$_3$, confirmed the chemical state of the chromium oxide as Cr$^{3+}$ in Cr$_2$O$_3$.

### 3.3. Reactivity of the Cr$_2$O$_3$ layer obtained by ALD

To assess the corrosion resistance of ALD-formed layers, a novel method was developed as outlined in Fig. 7. Based on prior chemical ToF-SIMS in-depth profiling, specific regions of interest were prepared. Two craters were formed by an ion etching in the ToF-SIMS chamber, targeting the bulk substrate and the mid-point of the Cr$_2$O$_3$ layer. The same parameters as those used to analyze the substrates were applied to create the craters. The sample was then immersed in a 0.1 M KOH solution. At this pH (13), thermodynamic considerations (E-pH diagrams for Al and Cr [46]) indicate that Cr$_2$O$_3$ remains stable in the aggressive solution, while Al undergoes rapid corrosion (~1 mg.cm$^{-2}$h$^{-1}$ [46]). This approach, combined with surface chemical analyses (XPS and ToF-SIMS), allowed to assess, thanks to the high reactivity of Al substrates in alkaline media, the stability of the deposited layers as well as the presence and impact of possible defects in the deposited coating. This methodology is then complemented by more conventional analysis in a chloride-containing environment, assisted by electrochemical techniques

The corrosion behavior at the different locations of the Cr$_2$O$_3$ layers was investigated in situ using optical microscopy and ex situ using XPS and ToF-SIMS. Fig. 8 shows optical micrographs of the sample at various immersion times (1, 10, 20, and 30 min) in the alkaline solution of 0.1 M KOH.

Different thicknesses of Cr$_2$O$_3$ layers were tested. The results shown in Fig. 7 correspond to 63 nm-thick Cr$_2$O$_3$ layer. The left crater exposes the bulk substrate, while the right crater reaches the mid-layer (31 nm of remaining oxide).

The different regions exhibited distinct corrosion behaviors. The 2024 Al alloy (crater on the left) underwent a strong corrosion immediately upon immersion in 0.1 M KOH, characterized by the substantial visual alteration and massive hydrogen gas evolution. Profilometry revealed pronounced attack, with a marked increase in crater depth of 3.8 µm corresponding to a corrosion rate of 2 mg.cm$^{-2}$h$^{-1}$, confirming

the alloy instability in this alkaline environment.

In contrast, the Cr$_2$O$_3$-coated substrate, both outside the crater and in the crater with 50 % of the layer removed, demonstrated significantly reduced reactivity. Microscopic examination revealed minimal morphological changes, although localized darkening was observed after 20 min in the intermediate crater (indicated by red arrows in Fig. 7). Hydrogen gas evolution was markedly lower and more localized [47,48]. After 30 min only a few corroding spots appear on the original coated surface. These observations indicate the protective role of the Cr$_2$O$_3$ layer, with however a few localized defects that may serve as corrosion initiation sites. Profilometric measurements further supported the superior corrosion resistance of the thinned Cr$_2$O$_3$ layer, as no significant increase in the crater depth was detected.

### 3.4. Chemical characterization after KOH immersion

ToF-SIMS analyses were carried out in the craters, as well as on the surface covered by the original Cr$_2$O$_3$ layer. Given the small size of the craters, XPS analyses (beam size 900 µm) were performed only outside the craters.

ToF-SIMS in-depth profiles after KOH immersion are presented in Fig. 9 for the 63 nm thick (un-thinned) layer (Fig. 8a) and the remaining 31 nm thick layer in the crater after etching (Fig. 8b). The Cr$_2$O$_3$/Al$_2$O$_3$ interface, defined as the point where the CrO$_2$ ion signal is at 80 % of its maximum, exhibited similar sputtering times for the substrate before and after the immersion (590 and 575 s, respectively). This means that the Cr$_2$O$_3$ layer thickness is unchanged. Some studies [49] have reported the dissolution of Cr$_2$O$_3$ at higher KOH concentrations (>1 mol/L) and longer immersion times, but no change in the layer thickness was observed here. For the thinned layer, the sputtering time to the interface was 292 s, similar to the 290 s measured before KOH exposure. This result confirms the previous conclusion. Similar measurements on thinner substrates (1000 ALD cycles) led to the same conclusions (see Figure S3 in the Supplementary Information file).

A detailed analysis of the profiles for AlO$_2$, CuO$^-$ and MgO$_2$ revealed some changes after exposure to KOH. The rapid drop of the signals in the early stages of profiling suggested the presence of Al, Mg and Cu-oxides at the Cr$_2$O$_3$ surface. Their intensities, significantly higher than in the reference, confirm their incorporation into the layer.

Despite these small changes, the Al$_2$O$_3$ layer at the Cr$_2$O$_3$/metal interface was apparently unaffected by a possible electrolyte ingress.

On the same sample (1000 cycles), 3D chemical imaging was performed using ToF-SIMS in order to correlate possible defects with the alloy microstructure. The 2D chemical maps extracted at various depths during the profiling process are shown in Fig. 10. To ensure a representative analysis of the microstructure, the 3D profiling was conducted over an area of 100 × 100 µm$^2$. To limit the number of figures, only the images corresponding to CrO$_2^-$, AlO$_2^-$, Cu$^-$ and Al$^+$ ions are presented.

To enhance the signal visibility, each 2D image corresponds to a

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** XPS spectra of Cr 2p, Al 2s and Cu 2p core level regions for an ALD Cr$_2$O$_3$ coated 2024 Al alloy (2250 ALD cycles). The spectrum in red is from a lab made Cr$_2$O$_3$ reference obtained by thermal oxidation of a bulk chromium metal sample [45].</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Outline of the experimental protocol for assessing the behavior of the Cr₂O₃ ALD layers on 2024 Al alloy in a corrosive environment (0.1 M KOH).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. In situ optical micrographs obtained for an ALD Cr₂O₃-coated 2024 Al alloy (2250 ALD cycles) during immersion in an alkaline solution (0.1 M KOH) for 1, 10, 20 and 30 min. Before exposure to KOH, the left crater goes down to the 2024 Al alloy and the right crater depth is 50 % of the Cr₂O₃ deposited layer (craters: 300 x 300 µm²).</figcaption></figure>

4 nm thick slice. Within the first 28 nm—corresponding to the Cr₂O₃ surface layer—the maps reveal a surface primarily composed of Cr₂O₃. The CrO₂⁻ ion distribution appears quite homogeneous, however localized defects can be observed (indicated by white arrows). This distribution remains largely unchanged throughout the Cr₂O₃ layer, which may indicate the presence of through-thickness voids. The green arrow shows a defect, which disappeared during the chemical profiling.

The AlO₂⁻ and Cu⁻ ion maps reveal the presence of these species within the Cr₂O₃ layer. Their distributions are not uniform, but are instead concentrated in the regions corresponding to the observed defects. This suggests that these species may originate from corrosion products trapped within the voids during the chemical attack. Additionally, through-thickness defects that do not contain any corrosion products are also observed (highlighted by white circles). At the bottom of these craters, Cr₂O₃ is observed (slices 22–26, white circles), indicating that these pores are not through-thickness. This observation is consistent with AFM data and confirms that only through-layer defects appear to initiate the corrosion.

Beyond a depth of 20 nm, the underlying Al₂O₃ layer i reached. This layer displays micrometer-scale circular heterogeneities. When compared with the chemical maps of Cu⁻ and CrO₂⁻, these heterogeneities are located at the same positions as the previously identified defects in the Cr₂O₃ layer and are characterized by Cu enrichment.

Once this Al₂O₃ layer is etched away, the metallic substrate is exposed. The Cu⁻ ion maps clearly reveal spherical particles, which can

<figure><img src="image_12.png" /><figcaption>Fig. 9. ToF-SIMS in-depth profiles for Cr₂O₃-coated 2024 Al alloy (2250 ALD cycles) (a) as-deposited layer and (b) half-thinned layer, after exposure to 0.1 M KOH for 30 min.</figcaption></figure>

be attributed to intermetallic phases. The images also show that the size of these features is significantly smaller (300–400 nm) than that of the secondary phases (5–10 µm).

Fig. 11 displays the Cr 2p, Al 2s, and Cu 2p spectra obtained after 30 min immersion in 0.1 M KOH. The unaltered shape of the Cr 2p spectrum, identical to the pre-immersion state (Fig. 6), confirms the excellent resistance of the Cr₂O₃ layer. While ToF-SIMS detected Al, XPS analysis exhibits only very small signal (<0.05 at%). However, XPS did confirm the presence of trace amounts of Cu (~0.05 at%), in agreement with the ToF-SIMS findings. Similar analyses conducted for thinner oxide layers (1000 ALD cycles, 28 nm, Fig. 9) showed higher levels of XPS-detectable Cu, suggesting that the ALD layer thickness is an important factor for the corrosion resistance.

Regarding Mg, although some studies [50] have reported surface enrichment during alkaline etching due to the precipitation of Mg(OH)₂, the XPS and ToF-SIMS analyses conducted here did not reveal the presence of Mg(OH)₂ on the substrate surface. This suggests that the dissolved magnesium concentration remains too low to reach the precipitation threshold.

To investigate the influence of the Cr₂O₃ layer on the diffusion of chemical species, ToF-SIMS depth profiles of Cu⁻ ions were recorded for two thicknesses (28 and 63 nm) before and after immersion in 0.1 M
<figure><img src="image_13.png" /><figcaption>Fig. 10. Extraction of 2D chemical depth profiles obtained from a 3D depth profile by ToF-SIMS for Cr₂O₃-coated 2024 Al alloy (1000 ALD cycles) after exposure to 0.1 M KOH for 30 min. The sum is the overlapping of CrO₂ (red), AlO₂ (green) and Cu⁻ (blue) fragments.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 11. XPS spectra of Cr 2p, Al 2s and Cu 2p core levels for ALD Cr₂O₃-coated 2024 Al alloy (a: 1000 ALD cycles and b: 2250 ALD cycles) after immersion in 0.1 M KOH for 30 min.</figcaption></figure>

KOH. For comparative analysis, all profiles were normalized to the maximum intensity of the Al₂ fragment. Fig. 10 shows the results, including profiles obtained within the craters previously produced by the ion etching, down to the middle of the Cr₂O₃ layer. Initial in-depth profiling consistently revealed the presence of Cu on the Cr₂O₃ surface after KOH immersion, regardless of the layer thickness. This observation suggests the corrosion of the underlying metal substrate and the subsequent Cu migration and deposition onto the oxide surface. Furthermore, for a given Cr₂O₃ thickness, craters (extending to mid-layer) exhibited a significant increase in surface Cu⁺ signal intensity (Fig. 12). The comparison of profiles for 28 nm and 63 nm layers demonstrated a significant reduction in Cu⁺ ion intensity for the thicker film, indicative of enhanced corrosion protection by the thicker Cr₂O₃ layer. Increasing the number of ALD cycles is anticipated to lower the density and size of local defects appearing during the deposition process, thereby improving the overall protective properties of the Cr₂O₃ layer. Electrochemical measurements also revealed the presence of local defects (see below) (fig. 13)

### 3.5. Electrochemical tests

The electrochemical reactivity of 2024 Al alloy without and with an ALD Cr₂O₃ coating (2250 cycles) was investigated using a dynamic polarization in a K₂SO₄ (0.01 M) / NaCl (0.1 M) electrolyte.

Fig. 13 presents the results, demonstrating a significant reduction in the surface reactivity for the coated alloy. The current density at the corrosion potential was determined using the Tafel extrapolation to be 6 µA cm⁻² for the 2024 Al alloy and 0.1 µA cm⁻² for the Cr₂O₃-coated alloy. The decreased cathodic and anodic current densities also indicate

<figure><img src="image_15.png" /><figcaption>Fig. 12. ToF-SIMS Cu⁺ in-depth profiles for an ALD Cr₂O₃-coated 2024 Al alloy (a) 1000 ALD cycles and (b) 2250 cycles (in black: reference sample before immersion, in red: as-deposited layer, and in blue: at the mid layer) after immersion in 0.1 M KOH for 30 min.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 13. (a) Potentiodynamic polarization curves for the 2024 Al alloy without and with an ALD Cr₂O₃ coating in K₂SO₄ / NaCl electrolyte (0.01 M/0.1 M) at a scan rate of 1 mV·s⁻¹ and (b): ToF-SIMS Cl⁻ in-depth profile obtained after polarization.</figcaption></figure>

a strong reduction in the surface reactivity of the Cr₂O₃-coated alloy. While a distinct passivation plateau was not observed, a gradual increase in the anodic current density was noticed. This finding could indicate the presence of defects within the Cr₂O₃ layer, facilitating chloride ion penetration to the oxide/metal interface [51–53]. The ToF-SIMS Cl⁻ in-depth profile obtained after polarization (Fig. 13b) confirms the presence of Cl within the film. The enrichment is more pronounced in the outer region of the Cr₂O₃ coating and appears to correlate with the presence of through-thickness defects. This localized Cl accumulation may explain the breakdown of the underlying Al₂O₃ layer, leading to the initiation of the corrosion; Since AFM observations revealed a globular surface morphology, the high Cl content observed in the early stages of profiling can be attributed to the roughness induced by the globules.

### 3.6. Discussion

In this study, a tailored ALD process was successfully developed to produce dense and conformal Cr₂O₃ thin layers on 2024 Al alloy, with precise control over composition and thickness. The deposited layers exhibited excellent structural integrity and chemical stability, as confirmed by XPS and ToF-SIMS analyses.

A major finding is the excellent chemical stability of the deposited Cr₂O₃ film in alkaline solution (0.1 M KOH, pH 13). In contrast to conventional Al₂O₃-based passive films, which are known to dissolve or destabilize at high pH [46], the 28 and 63 nm Cr₂O₃ layer (1000 and 2250 cycles respectively) showed no detectable degradation in thickness or composition after 30 min of immersion. These results demonstrate a distinct advantage of using Cr₂O₃ over the more commonly studied ALD coatings like Al₂O₃ or TiO₂, particularly for alkaline corrosion environments.

However, localized corrosion phenomena were observed, likely
initiated at submicron defects within the Cr₂O₃ film. AFM and ToF-SIMS 3D profiling identified through-thickness defects that appear to allow electrolyte ingress. These defects coincide spatially with Cu-rich regions within the substrate. The size of these defects (400–500 nm) is much smaller than the IMPs size, but would still favor the preferential corrosion. These observations suggest that the submicron defects observed in the ALD Cr₂O₃ layer are located at sites of intermetallic particles, possibly due to local variations in the surface chemistry or topography affecting the ALD nucleation and growth mechanisms. Such behavior is consistent with previous reports highlighting the catalytic role of Cu-containing IMPs in the corrosion of 2xxx series aluminum alloys [5,6]. The detection of Cu⁻ and AlO₂⁻ fragments at the surface of the Cr₂O₃ layer after immersion further supports this corrosion pathway.

The effect of the Cr₂O₃ layer thickness was particularly evident in the alkaline medium. Comparative ToF-SIMS and XPS analyses revealed that the thicker coatings (63 nm) exhibited significantly lower surface enrichment in Cu after immersion in 0.1 M KOH, compared to thinner layers (28 nm). This suggests a more effective barrier against electrolyte penetration and subsequent corrosion of the underlying intermetallic-rich substrate. The improved performance is attributed to a reduced density of through-thickness defects in thicker films, consistent with the self-limiting and layer-by-layer nature of the ALD process, which progressively seals microstructural imperfections with increasing cycle number. These results align with established ALD literature that correlates increased cycle count with lower defect density and enhanced barrier performance.

Electrochemical polarization measurements confirmed the protective effect of the ALD Cr₂O₃ coating in chloride-containing environments. The coated 2024 Al alloy exhibited a notable decrease in the corrosion current density—by nearly two orders of magnitude—compared to the uncoated alloy. This indicates a strong inhibition of both anodic and cathodic reactions. However, the absence of a well-defined passivation plateau and the slight increase in the current at higher potentials suggest the presence of localized defects within the oxide layer. These defects likely act as preferential pathways for chloride ingress, leading to incipient localized corrosion beneath the film. Despite these imperfections, the Cr₂O₃ layer clearly enhanced the corrosion, resistance of the alloy in aggressive environment.

## 4. Conclusion

This work presents a successful strategy to deposit conformal and chemically stable Cr₂O₃ thin films on 2024 Al alloy by a tailored ALD process. The introduction of an initial Al₂O₃ pre-treatment significantly enhanced the nucleation and uniformity of the Cr₂O₃ layer. Structural and compositional analyses (SEM, XPS, ToF-SIMS) confirmed the formation of stoichiometric, continuous Cr₂O₃ films with thicknesses ranging from 28 to 63 nm.

Corrosion resistance assessments in alkaline (0.1 M KOH) and chloride-containing environments demonstrated that the Cr₂O₃ coatings provided effective barrier protection. The layers remained chemically unaltered after their immersion, confirming their intrinsic stability. However, localized corrosion events, evidenced by hydrogen evolution, Cu enrichment, and morphological change, were consistently associated with discrete submicronic defects within the Cr₂O₃ film. Advanced 3D ToF-SIMS profiling revealed that these defects were through-thickness and spatially correlated with underlying intermetallic phases, and facilitate electrolyte access to the alloy-coating interface.

Increasing the film thickness reduced the density of such defects and significantly improved the corrosion performance, as confirmed by both surface analysis and electrochemical tests. The coated samples exhibited a two-order-of-magnitude decrease in the corrosion current compared to uncoated alloys.

Overall, these results highlight the promise of Cr₂O₃ ALD coatings as a robust and environmentally friendly alternative to chromate-based treatments. The control of sub-micrometer defect formation during

deposition remains a key factor to maximize protective performance in demanding environments.

### CRediT authorship contribution statement

D. Mercier: Investigation, Methodology, Writing - Original Draft, Writing - Review & Editing. R. Zazpe: Investigation, Methodology, Supervision, Writing - Original Draft, Writing - Review & Editing. X. Wang: Resources. M. Michaux: Resources. J. Rodriguez-Pereira: Investigation, Methodology. S. Zanna: Investigation. A. Seyeux: Investigation. J.M Macak: Investigation, Methodology, Supervision, Writing - Review & Editing, Funding acquisition. P. Marcus: Methodology, Supervision, Writing - Review & Editing, Funding acquisition.

### 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.

### Acknowledgments

The financial support from the Ministry of Education, Youth and Sports of the Czech Republic for the CEMNAT large research infrastructure (LM2023037) is gratefully acknowledged for ALD, SEM, XPS operation. Région Ile-de-France is acknowledged for partial funding of the ToF-SIMS equipment.

### Supporting information

SEM top-view images and ToF-SIMS depth profiles of Cr₂O₃ thin films deposited by ALD on 2024 Al alloy substrate.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.corsci.2025.113167.

### Data availability

Data will be made available on request.

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