# Mechanisms of novel Cr(III)/Zr(IV) impregnation and sodium silicates sealing of anodized aluminium alloy for enhancing corrosion performance

Céline Mira, Jean-Charles Dupin, Joffrey Tardelli, Laetitia Thomas, Jérôme

Frayret, Virginie Bague, David Sinopoli

► To cite this version:

Céline Mira, Jean-Charles Dupin, Joffrey Tardelli, Laetitia Thomas, Jérôme Frayret, et al.. Mechanisms of novel Cr(III)/Zr(IV) impregnation and sodium silicates sealing of anodized aluminium alloy for enhancing corrosion performance. Surface and Coatings Technology, 2025, 504, pp.131999. 10.1016/j.surfcoat.2025.131999. hal-05022658

HAL Id: hal-05022658

https://univ-pau.hal.science/hal-05022658v1

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Full Length Article

# Mechanisms of novel Cr(III)/Zr(IV) impregnation and sodium silicates sealing of anodized aluminium alloy for enhancing corrosion performance

<figure><img src="image_2.png" /></figure>

Celine Mira<sup>a,b</sup>, Jean-Charles Dupin<sup>a,*</sup>, Joffrey Tardelli<sup>b</sup>, Laetitia Thomas<sup>b</sup>, Jerome Frayret<sup>c</sup>, Virginie Bague<sup>d</sup>, David Sinopoli<sup>e</sup>

ARTICLE INFO

Keywords:

Anodized aluminium alloy

Sealing

Impregnation

Salt spray test

SEM

XPS

ABSTRACT

In the recent development of REACH compliant protective coatings, sulfuric acid anodizing and simple hydrothermal sealing have replaced the previous process based on hexavalent chromium. To optimize this route, a Cr (III)/Zr(IV) impregnation step has been optimized and applied on the sulfuric anodizing layer and prior to the hydrothermal sealing process. This REACH compliant filler improves the effectiveness of the corrosion protection, but this is not yet suitable for industrial certification of the Al2618A T851 alloy. The sealing bath formulation was then improved by adding sodium silicates to increase the level of the protection. Different sequences of coupled $Ar^+$ etching/ XPS analyses highlighted the positive effect of the addition of sodium silicates in the sealing bath, in terms of $ZrO_2$ precipitation enhancement. Indeed, without the use of Si-based additives in the hydrothermal sealing, only soluble non-protective oxyfluorozirconia species are formed reducing the anticorrosive behavior of the coating. In addition, sodium silicates play an undeniable role in the leaching of fluorine from the impregnation layer reagents, promoting the precipitation of more protective $ZrO_2$ entities and then improving corrosion resistance.

## 1. Introduction

In aeronautics, fuel economy and high performance requirements could now be achieved by using suitable materials with a low weight to mechanical strength ratio. Aluminium alloys are good candidates for such considerations and are widely used in the design of many aircraft parts [1,2]. The aluminium alloys of the 2xxx series are particularly interesting in terms of the mechanical properties provided by the addition of copper [3,4]. However, the presence of copper reduces the corrosion protection, and therefore a solution consists in a shielding treatment of the alloy surface to prevent any damage [5]. Among the electrochemical treatments, anodizing and conversion processes have often been carried out over the years with hexavalent chromium baths being favoured. In this case, after an initial degreasing of the alloy surface and a sulfo-chromic ($H_2SO_4 + CrO_3$) pickling, a chromic acid ($CrO_3$) anodizing (CAA) followed by a sealing with potassium dichromate ($K_2Cr_2O_7$) were performed. In fact, chromium (VI) was an excellent

corrosion inhibitor with self-healing properties [6].

Since 2007, the hexavalent chromium has been included in the Annex XIV of the REACH Regulation N° 907/2006 [7] as a carcinogenic substance. Since then, new conversion processes have been developed and optimized using less hazardous chemicals (e.g. TCP for Trivalent Chromium Process) [8]. More recently, an original REACH compliant three steps Cr(VI) free process based on the anodizing has been evaluated. The pickling has been replaced by a new Cr(VI)-free solution [9,10] and the CAA has been replaced by sulfuric acid anodizing (SAA) or sulfo-tartric acid anodizing (TSAA), depending on the application [11,12]. Naturally, the changes in the anodizing electrolyte induced changes in some parameters such as temperature, voltage and anodizing time and the corrosion protection then depends on an appropriate initial alchemy [13]. The resulting porosity is even different from CAA and could have an impact on the sealing performance. All these experimental developments would then imply drastic differences in the composition and efficiency of the protective layers. In the end, the potassium

<footnote><sup>a</sup> Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, Pau, France</footnote>

<footnote><sup>b</sup> IRT-M2P, Duppigheim, France</footnote>

<footnote><sup>c</sup> Inesurf, Grabels, France</footnote>

<footnote><sup>d</sup> Safran Aircraft Engines, Evry, France</footnote>

<footnote><sup>e</sup> Airbus Helicopters, Marignane, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: dupin@univ-pau.fr (J.-C. Dupin).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. A schematic presentation of the tested configurations on the aluminium alloy 2618A substrate and the abbreviation associated.</figcaption></figure>

<table><caption>**Table 1**
Binding energy of chemical references by XPS surfaces analysis.</caption><thead><tr><th>Chemical compounds</th><th>Core peak</th><th>Binding Energy (eV) (Full Width at Half Maximum (eV))</th></tr></thead><tbody><tr><td>ZrO<sub>2</sub></td><td>Zr 3d<sub>3/</sub></td><td>182.4 (1.3) -184.7 (1.3)</td></tr><tr><td>ZrF<sub>4</sub></td><td>2-1/2</td><td>186.3 (1.8) - 188.6 (1.8)</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>Cr 2p<sub>3/2</sub></td><td>577.3 (1.5)</td></tr><tr><td>Cr(OH)<sub>3</sub></td><td></td><td>577.4 (2.6)</td></tr><tr><td>SiO<sub>2</sub></td><td>Si 2p</td><td>103.4 (1.6)</td></tr><tr><td>Na<sub>2</sub>SiO<sub>3</sub></td><td></td><td>102.9 (1.8)</td></tr><tr><td>Na<sub>2</sub>SiO<sub>3</sub></td><td>Na 1 s</td><td>1071.7 (1.8)</td></tr></tbody></table>

<table><caption>**Table 2**
Concentrations of reactive chemicals in solutions 1 and 2 for ICP/AES analysis.</caption><thead><tr><th></th><th>Solution 1 (Na<sub>3</sub>AlF<sub>6</sub> + SiO<sub>2</sub> (s))</th><th>Solution 2 (H<sub>2</sub>ZrF<sub>6</sub> + SiO<sub>2</sub> (s))</th></tr></thead><tbody><tr><td>Bath pH</td><td>~ 4</td><td>~ 1</td></tr><tr><td>Composition</td><td>Na<sub>3</sub>AlF<sub>6</sub></td><td>H<sub>2</sub>ZrF<sub>6</sub></td></tr><tr><td>Concentration (g/L)</td><td>20</td><td>20</td></tr></tbody></table>

dichromate sealing was dropped in favour of a simple hydrothermal sealing.

Among the different types of aluminium alloys, the specific 2618A alloy is difficult to protect against corrosion due to its specific microstructure. In fact, the presence of significant amounts of Fe and Ni generates numerous intermetallic particles as mainly Al<sub>9</sub>FeNi; hence, mechanical resistance is increased at the expense of corrosion protection [14–17]. Indeed, these particles could induce a flawed anodized-sealed coating. In order to improve the corrosion protection and the obstruction of anodized layer pores in a REACH compliant way, some new technologies are now proposed with for instance PEO process incorporating glass powders [18] or simply new sealing solutions containing sodium silicates, an environmentally friendly and inexpensive substance

[15,19–21].

The aim of the work undertaken is to develop a sustainable alternative to previous chromium VI salts solutions used for the protection of anodized aluminium layers. The implementation of a mixed Cr(+III)/Zr (+IV) impregnation process is proving to be a promising route, as the associated salts are initially considered to be far less toxic than chromium VI salts, while also being effective in the emergence of corrosion [22,23]. The current development of these alternative post-treatments has been optimized by attempting to study the effects of the chemical entities Zr(IV) and Cr(III) on the anodized layers and understanding their reciprocal interactions. To make the process more robust, a chemical sealing step with a silicate-based solution was added and compared to conventional hydrothermal sealing. At each stage of the coating construction process (anodization-impregnation-sealing), the distribution of the elements was monitored by SEM-EDS analysis and the chemical state of the constituent species was determined by XPS. All the results gathered finally attested to certain specific interferences by different chemical mechanisms, and were crucial in explaining the very first trends in the behavior of anodized Al2618A alloy under corrosive conditions (neutral salt spray tests).

## 2. Experimental

### 2.1. Surface treatment

Aluminium alloy 2618A T851 sheet specimen were used. The dimensions of the sample were 9.5 cm × 10 cm × 0.5 cm. Its nominal composition (in % by weight) was: Al, 2.5 % Cu, 1.5 % Mg, 1.1 % Fe, 1.1 % Ni, 0.20 % Si, 0.11 % Zn, 0.02 % Mn, 0.06 % Ti, 0.06 % Ti + Zr.

All treatments were carried out in a semi-industrial scale, in a 240 L bath on the surface treatment line of IRT-M2P (Duppigheim, France). Each step of the process was followed by a rinse in demineralized water for 2 min. The samples were chemically degreased in 10 % vol. alkali-based bath for 10 min at 45 ± 5 °C. This was followed by a sulpho-nitro-ferric pickling in a mixture of 42 % vol. acid-based solution

<figure><img src="image_4.png" /><figcaption>Fig. 2. SEM micrographs of a fractography of anodic layer 2618A T851.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Typical SEM micrograph of anodic layer 2618A T851 a) On top surface b) In a crater performed on the top surface with Ar⁺ beam using Glow Discharge Optical Emission Spectrometry (GD-OES).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Cross-sectional SEM micrograph and EDS mapping (colored by element) of treated sample SAA-Cr/Zr sodium silicates sample.</figcaption></figure>

(sulfuric and nitric acid) and 10 vol. ferric sulphate-based solution for 5 min at 50 °C. The samples were anodized in sulfuric acid at 18 °C with a concentration between 180 and 220 g/L for 20 min at voltage of 15–20 V. For the impregnation step, the anodized samples were immersed for 10 min at 40 °C in one single solution with an adjusted pH between 3.7 and 4.2, containing 80–100 mL/L $Cr_2(SO_4)_3$ (part A, associated pH 1.6 ± 0.2) in addition of 70–80 mL/L $K_2ZrF_6$ (part B, associated pH 6.3 ± 0.2). Finally, the anodized impregnated samples were sealed for 20 min at 98 °C in a hot water bath containing a concentration of 20–100 g/L of sodium silicates at alkaline pH (between 11 and 12). For comparison, the classic hot water sealing, without Si-based additives, was also carried out on some materials for 20 min and 98 °C.

In order to understand the specific role of the zirconium and the chromium elements in the impregnation process, some anodized samples were impregnated with “single component” impregnation solutions prepared in the laboratory: a chromium solution prepared in the laboratory (Part A) and a zirconium solution prepared in the laboratory (Part B). Initially, the laboratory impregnations were carried out under the same conditions of temperature, time and pH as the commercial mixed impregnation (10 min at 40 °C and pH between 3.7 and 4.2). The addition of 10 vol. nitric acid (Sigma Aldrich, CAS No. 7697-37-2) to lower the pH for the zirconium impregnation and 10 vol. ammonia (Fisher Scientific, CAS No. 1336-21-6) to raise the pH for the chromium impregnation were used. A second experiment was then carried out

under the same conditions of duration and temperature, but the pH was not adjusted to 3.7–4.2. These samples were sealed with sodium silicates. Hot water sealing without additives was not studied for these laboratory configurations. Fig. 1 shows the different configurations tested in this study. The effect of the addition of sodium silicates in the sealing process is studied on the anodized/fully impregnated system and the role of chromium and zirconium during impregnation is analyzed to anticipate a precipitation mechanism.

## 2.2. Preparation of specimen for SEM cross-section observations and fractography

The samples were cut with a mechanical saw to dimensions of 3 cm × 0.5 cm, rinsed with ethanol and dried with compressed air. A coating was then prepared with 9 g of hardener and 20 g of resin. The following mechanical polishing techniques were used P240, P600 and P1200. Finishing was carried out with 9 µm, 3 µm and 1 µm suspension solutions. Finally, a superfinish of standard colloidal silica suspension (OP-S) completed the polishing process adapted to the SEM cross section observations. Some fractographic observations were made at ambient temperature after a manual failure had been made in the specimen. This intentional fractography was only carried out to observe the pore, entanglement or columnar structure in the SEM. It is not a failure formed during a fatigue test or caused by stress corrosion.
<figure><img src="image_7.png" /><figcaption>Fig. 5. XPS elemental depth profile of elements for samples SAA-Cr/Zr sodium silicates (solid line) and SAA-Cr/Zr hot water (dotted line).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. XPS high-resolution Si 2p core level spectra of sample SAA-Cr/Zr sodium silicates with Ar⁺ etching (a) 120 s (b) 180 s (c) 780 s (d) 1020 s and (e) 1620 s. NB: the percentage value directly corresponds to the relative amount of SiO₂⁻ over the total amount of Si based chemical environments.</figcaption></figure>

## 2.3. Preparation of specimen for XPS analyses

The samples were cut with a mechanical saw to reach 1 cm × 1 cm dimensions, rinsed with ethanol and dried with pressurized purified air.

## 2.4. Chemical and morphological characterization

The morphological characterization was carried out using a ZEISS SIGMA VP 300 field emission scanning electron microscope (FE-SEM) equipped with a BRUCKER X Flash 6160 analyzer for energy dispersive X-ray spectroscopy (EDS) analyses. The EDS analyses were performed at 15.0 kV with a magnification of 2.00 K and a working distance of 8.5 mm. Intentional fractography of the anodized layer edge was observed at 5.00 kV with a magnification of 6.41 K and a working distance of 4.5 mm. The surface was also observed at 2.00 kV with a magnification of

50.00 K and a working distance of 3.5 mm. Chemical analyses were performed using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. The X-ray source corresponds to an Al Kα (1486.6 eV) micro-focused monochromator and the analysis area is 400 µm (X-ray spot size). The pass energy is 20 eV. Charge compensation is performed with a dual beam source: ultra-low energy co-axial electron and Ar⁺ ions. The Ar⁺ beam was used for the depth profile by etching the surface. The etching parameters are: 100 cycles, 60 s, low current, 1000 eV. For the in-depth study, the spectra calibration was based on the Zr 3d₃/₂ orbital binding energy (BE) at 182.4 eV with respect to the ZrO₂ phase present in the protective multilayer coating, except for the laboratory-produced chromium protective coating, which was calibrated on the silicon dioxide Si2p (BE = 103.4 eV) of the sealing layer. Mathematical convolutions of the spectra were performed using the Casa XPS software version 2.3.24. A standard BE database of materials, compiled from XPS
<figure><img src="image_9.png" /><figcaption>Fig. 7. XPS high-resolution Cr 2p$_{3/2}$ core level spectra of SAA-Cr/Zr sodium silicates with Ar$^+$ etching (a) 180 s (b) 780 s and (c) 1620 s; of SAA-Cr/Zr hot water with Ar$^+$ etching (d) 180 s (e) 780 s and (f) 1620 s.</figcaption></figure>

surface analyses carried out in the laboratory to understand the expected chemical mechanisms studied in this work, is given in Table 1. All of the materials listed below could effectively be part of the overall anodized-impregnated-sealed system:

- Zirconium dioxide, ZrO₂ (powder, CAS n° 1314-23-4, Sigma Aldrich®)

- Zirconium(IV) fluoride, ZrF₄ (powder, CAS n° 7783-64-4 Sigma Aldrich®)

- Oxide of chromium, Cr₂O₃ (10688 Chromium(III) oxide, Puratronic®, 99.97 % (metals basis), powder, CAS n° 1308-38-9, Alfa Aesar®)

- Chromium(III) hydroxide, Cr(OH)₃, 95 % (powder, CAS n° 1308-14-1 Sigma Aldrich\_Combi\_Blocks®)

- Silicon(IV) oxide, SiO₂, 99.5 % (metals basis) (powder, CAS n° 14808-60-7 Alfa Aesar®)

- Sodium silicates, Na₂SiO₃ (Liquid, SurTec® 345 Hardwall)

For XPS preparation, the powders were finely ground in a mortar in a controlled atmosphere glove box before being placed on copper tape, which was attached to the sample holder to ensure conductivity during analysis.

For the analysis of standard liquid sodium silicates, an aliphatic organic compound, hexatriacontane C₃₆H₇₄, was finely ground by hand in a mortar and then deposited on a copper adhesive (conductive) attached to the sample holder. A drop of solution was then applied directly to the powder. After natural evaporation in a fume cupboard, a solid agglomerate was obtained.

## 2.5. Assay of sealing bath and prepared mixture by inductively coupled plasma-atomic emission spectrometry (ICP/AES)

To complement the XPS study of the coated Al alloy, some ICP/AES analyses of sealing baths were carried out after immersion of the anodized impregnated material to determine the amount of leached fluorine. A Jobin Yvon Panorama sequential atomic emission spectrometer equipped with a cross-flow nebulizer, a Scott-type spray chamber, a peristaltic pump and a quartz plasma torch was used. An ORION 9409 fluoride specific electrode, an ORION 9002 double junction reference electrode and an ORION 720A ion meter enabled to measure the free fluoride entities. The standard addition method was used for the calibration of the ICP/AES.

### 2.5.1. Determination of the fluorine concentration in the sealing bath: hot water vs. sodium silicates

The aim of this experiment is to understand the leaching action of fluorine as a function of the sealing nature. Two anodized and Cr/Zr impregnated samples were prepared and sealed in hot water and sodium silicate baths respectively described in Section 2.1.

### 2.5.2. Evaluation of solubility between silicon and fluorine

In order to understand the precipitation mechanisms during the sealing after impregnation, two solutions were prepared in order to study the exchange between the silicon and the fluorine.

The insoluble, reactive solid, silica SiO₂, was mixed with the soluble species AlF₆⁻ (solution 1) and ZrF₆²⁻ (solution 2), respectively. The concentrations of the two mixtures are given in Table 2. In order to reach the equilibrium, both solutions were kept at room temperature for a
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** XPS high-resolution Zr 3d$_{5/2-3/2}$ core level spectra of SAA-Cr/Zr sodium silicates with Ar$^+$ etching (a) 180 s (b) 780 s and (c) 1620 s; of SAA-Cr/Zr hot water with Ar$^+$ etching (d) 180 s (e) 780 s and (f) 1620 s. NB: the percentage value directly corresponds to the relative amount of ZrO$_x$F$_y$ over the total amount of Zr based chemical environments.</figcaption></figure>

week before dissolved species were filtrered on a 0.45 µm cellulose acetate.

## 2.6. Evaluation of corrosion protection

According to the ISO 9227:2017 standard, some neutral salt spray tests (NSST) were finally performed on coated samples (3 samples for each specific configuration studied) then exposed for 500 h in a neutral pH sodium chloride fog (5 ± 1 %) at 35 ± 2 °C. Some visual observations of the appearance of corrosion pits were carried out after 24 h, 148 h, 309 h, and 476 h of NSST exposure. The number of pits was counted for a surface area of a m$^2$, using a simple visual methodology. Three panels per specimen were simultaneously exposed to the salt spray test. The NSST tests carried out are only designed to compare the different coatings at technology readiness level 3.

## 2.7. Crosscut adhesion tests

In order to assess the suitability of impregnated-sealed SAA-Cr/Zr samples, a good adhesion ability onto the Al2618 substrate is mandatory. The evaluation of the coatings adhesion on the aluminium alloy was performed with an Elcometer 1542 Cross Hatch Adhesion Tester Basic Kit. The results were analyzed and interpreted according to the ASTM D 3359-B standard which offers guidance on the interpretation of crosscut tests findings.

# 3. Results and discussion

## 3.1. The sulfuric anodic layer

Fig. 2 illustrates the morphology of the anodic layer with a pore entanglement structure, characteristic of the 2xxx series of aluminium alloy [16,17]. A micrometric intermetallic particle is clearly visible in a section of the anodic layer, causing structural heterogeneity and mechanical weakening (Fig. 2). Unlike the 2024 alloy which contains intermetallic particles based on high standard electrode potential (e.g., S-Al$_2$CuMg) that dissolve during the anodizing, a large amount of iron-rich intermetallic phases Al$_9$NiFe of the 2618A alloy are not completely dissolved and are present inside the oxide layer. After etching with an Ar$^+$ beam using Glow Discharge Optical Emission Spectrometry (GD-OES), the anodization layer surface becomes very rough (Fig. 3a) but it allows us to consider that the pore diameter, evaluated on the SEM micrograph with the software ImageJ, varies in the range of 10 to 20 nm (Fig. 3b). This porous configuration with many heterogeneities is not a sufficient protective barrier for the 2618A T851 substrate.

## 3.2. Study of chemical interaction between the sodium silicates sealing on anodized impregnated samples, SAA-Cr/Zr-Sodium silicates.

### 3.2.1. Chemical profile of anodic layer

SEM-EDS analysis of a cross section of SAA-Cr/Zr-Sodium silicates sample (Fig. 4) shows that silicon (in blue) is located on the top surface, above the impregnation layer formed by zirconium (in red) and
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** XPS high-resolution Al 2p core level spectra of SAA-Cr/Zr sodium silicates with Ar⁺ etching (a) 180 s (b) 780 s and (c) 1620 s; of SAA-Cr/Zr hot water with Ar⁺ etching (d) 180 s (e) 780 s and (f) 1620 s. NB: the percentage value directly corresponds to the relative amount of AlOₓFᵧ over the total amount of Al based chemical environments.</figcaption></figure>

chromium (in green). Zirconium and chromium precipitated in the upper part of the anodic layer in apparently high concentrations in hollow defect zones of the anodic layer.

The XPS depth profile of SAA-Cr/Zr-sodium silicates (in straight line, Fig. 5) confirms the presence of an additional physical layer of silicon oxide on the top of the impregnation layer after the addition of sodium silicates in the hydrothermal sealing bath. Such a new phase does not exist in the classic hot water sealing (dotted line for SAA-Cr/Zr hot water sample in Fig. 5), which consists in the sole pore closure of the anodized layer with (oxi)hydroxide bridges as predicted in some previous works [24–26]. The high-resolution in-depth Si 2p core spectra of the SAA-Cr/Zr-sodium silicates system (Fig. 6) attest to the main precipitation into insoluble SiO₂ oxide phase, a known efficient passive anticorrosion barrier [27]. In addition to the silica component, a less intense component (10 % of the Si content) appears on the low BE side, indicating the presence of a SiO₂⁻ chemical environment type of silicon. This amount

rises up to 30 % in depth (e.g. after 1600 s of Ar⁺ etching).

Irrespective of the sealing (hot water or sodium silicates), chromium and zirconium co-exist at the same depth location of the material surface as previously reported in Fig. 5. The maximum amounts of metals are found on the top surface of the anodized layer (when the Al signal of the anodized layer starts to rise). The high-resolution Cr 2p₃/₂ core level spectra in the Fig. 7 do not allow to clearly evidence some variations in the chemical environment of Cr(III) type (oxide and/or hydroxide of chromium) to be clearly identified with the type of the sealing process or even to be related to the chromium location in depth. In addition, the Ar⁺ etching induced an artificial Cr(III) type reduction with a gradually increasing component at 574.5 eV (Cr°). The Zr 3d core level spectra (Fig. 8) consist into two 3d₅/₂–₃/₂ doublets associated with ZrO₂ (182.4 eV – 184.8 eV) as found for the reference material and ZrOₓFᵧ phases (183.3 eV – 185.6 eV) in variable relative proportions depending on the seal type [28]. Under alkaline and acidic conditions, ZrO₂ usually acts as
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** XPS elemental depth profile of fluorine (in at.%F) in SAA-Cr/Zr (no sealing), SAA-Cr/Zr hot water and SAA-Cr/Zr sodium silicates materials.</figcaption></figure>

<table><caption>**Table 3**

Concentration of fluorine (mg/L) in sealing silicates and hot water baths after SAA-Cr/Zr panel immersion.</caption><thead><tr><th></th><th>Sodium silicates sealing bath</th><th>Hot water sealing bath</th></tr></thead><tbody><tr><td>Concentration of fluorine (mg/L)</td><td>178.0 ± 8.0</td><td>37.1 ± 0.9</td></tr></tbody></table>

an efficient barrier [18] and almost exclusively ZrO₂ is present in the in SAA-Cr/Zr-sodium silicate material (c.a. 95 % of the total zirconium amount, Fig. 8a-c) which is very promising for the expected corrosion resistance.

The situation is different for hot water sealing (SAA-Cr/Zr-hot water), as ZrO₂ is mostly present very close to the surface (after a 180 s Ar⁺ etching, Fig. 8d), whereas 75 % of the zirconium is present in the form of ZrOₓFᵧ deeper in the material (e.g. after 1600 s etching, Fig. 8f). For hot water sealing, the Al 2p core level spectra also reveal the significant presence of aluminium oxyfluorides AlOₓFᵧ in-depth (Figs. 9e-f) whereas aluminium appears to be present in many forms (oxi-hydroxides) after sodium silicate sealing (Figs. 9b-c) [29].

In the close-up survey of the XPS depth profile (Fig. 10) the relative percentage of fluorine increases to about 13 at.% after hot water sealing (1000 s Ar⁺ etching). Under these conditions, fluorine would then react with zirconium to form ZrOₓFᵧ phases, which are mainly present deep in the impregnation layer as seen elsewhere (Fig. 8e-f). In contrast to usual zirconium oxide or hydroxide phases, and as described in Section 3.4, fluoro zirconates can still dissociate as long as they are in an aluminium hydroxide environment, even if their dissociation constant is very low, equal to 10⁻²⁸.³ [30,31].

Considering these initial results, the amount of fluorine is relatively high when the SAA-Cr/Zr system is not sealed (c.a. 22 at.%, Fig. 10), but it is almost undetectable (~ 2 at.%) when operating with silicate sealing. Fluorine is then released out of the impregnation/sealing layer in the presence of silicates after the treatment. The ICP/AES analyses confirm this trend as the concentration of fluorine in the silicate sealing bath after the immersion of the SAA-Cr/Zr panel is almost 5 times higher than the concentration found in the hot water sealing bath, as shown in the Table 3. Fluorine has then a much greater capability to migrate to the silicate bath.

### 3.2.2. Comparison of neutral salt spray tests (NSST) and adhesion tests on SAA-Cr/Zr sodium silicates vs. hot water sealing

For the different hot water sealed tested panels of 9.5 cm × 10 cm surface, NSST shows a fairly stable number of corrosion pits, 10 pits ± 1 pit (i.e. 1050±100 pits / m²), whatever the exposure time but not compatible enough with a proper corrosion resistance state (Fig. 11 and Fig. 12). On the other hand, SAA-Cr/Zr system sealed with silicates remains virtually undamaged for the first 250 h of salt spray exposure. After this period, the surface condition evolves slightly with the appearance of about 2 to 4 corrosion pits (i.e. 200 to 425 pits / m²). These results provide a first level of information on the surface protection. For example, based on these surface observations the sodium silicate sealed system would provide better corrosion protection rather than hot water on the 2618A T851 aluminium alloy. To go deeper in the comparison of corrosion performance, the depth of the pits could also be evaluated in longer term exposures as prescribed by R.E. Melchers [32].

Results of the adhesion test show comparative Grade 0 trend for both SAA-Cr/Zr hot water and SAA-Cr/Zr silicates materials (Table 4) as edges of cuts are completely smooth and no flakes of the coating are detached at the intersections of the cuts. These observations would meet the requirements for an efficient coating and attest the different chemical on Al2618 alloy did not compromise the possible of a future painting application.

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Number of corrosion pits under NSST exposure (in hours): SAA-Cr/Zr hot water (in blue) and SAA-Cr/Zr sodium silicates (in yellow) (3 samples tested per sealant).</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 12. Specimen SAA- Cr/Zr sealed a) sodium silicates sealing b) hot water sealing after 476 h of alkaline salt spray exposure.</figcaption></figure>

**Table 4**

A comparative table with results of crosscut tests of specimens studied in experiments.

<table><thead><tr><th>Crosscut view</th><th>2mm</th><th>5mm</th></tr></thead><tbody><tr><td>ASTM D 3359-BAdhesion Grade Specimen</td><td>0<br>SAA-Cr/Zr-hot water</td><td>0<br>SAA-Cr/Zr-sodium silicates</td></tr></tbody></table>

### 3.3. Study of the role of zirconium and chromium during impregnation: partially impregnated SAA-Cr-sodium silicate systems prepared in the laboratory vs. partially impregnated SAA-Zr-sodium silicate systems prepared in the laboratory.

#### 3.3.1. The role of chromium at two different pH: 1.6 vs. 4

When the impregnation solution is used at the original pH of 1.6, the XPS chromium signal of the laboratory produced SAA-Cr material is not detected regardless of the seal (Fig. 13). Once the pH is adjusted to 4 with 10% ammonia (to set the same conditions as the global commercial Cr/Zr impregnation process), the amount of chromium in the material surface depth is very low, less than 0.5 at.%, which is not significant to consider a real pH effect. In view of these observations, chromium would not precipitate and could not play a role if it were used alone in the impregnation layer. Whatever the pH, the relative silicon concentration, throughout the depth analyzed (Fig. 13), is about 10 at.% even after 4000 s of Ar⁺ etching, a content much more higher than for SAA-Cr/Zr-sodium silicates system (Si content 1 at.% at maximum Fig. 5). This trend is similar to a simple treatment configuration (SAA sodium silicates, anodization/sealing). The main silicon chemical environment for SAA-Cr-sodium silicates is SiO₂ but the SiO₃⁻ relative percentage increases strongly with depth (up to 40% of the total Si content already after 180 s of Ar⁺ etching and at very acid pH 1.6) (Fig. 14). A better precipitation of SiO₂ could then be pH sensitive.

Salt spray tests on SAA-Cr-sodium silicates showed a generalized corrosion after 168 h of exposure demonstrating a severe lack of protection with the only chromium impregnation alone, despite the silicate

sealing combination (Fig. 15).

#### 3.3.2. The role of zirconium at two different pH: pH = 4 vs. pH = 6

The XPS depth profile of the SAA-Zr-sodium silicates system shows the zirconium precipitation in the upper part of the impregnation layer in both tested conditions (pH 4 or 6). However, the depth distribution of zirconium seems to be influenced by the type of seal (Fig. 16). At pH 6, the zirconium behaves as in the full SAA-Cr/Zr-sodium silicate impregnation, with a maximum content below the sealing layer, followed by a continuous decrease (after 1000 s of Ar⁺ etching). At pH 4, the zirconium content increases progressively, even reaching the anodic layer after 2000 s of Ar⁺ etching. Despite these differences, the Zr 3d core spectra of both systems (pH 4 and pH 6), clearly show a predominant ZrO₂ phase (and a minor ZrOₓFᵧ phase) in the impregnation layer (Fig. 17). The evolution of the Si 2p spectra (Fig. 18) suggests that the amount of SiO₃⁻ is almost twice less on the surface than deeper in the impregnation layer, and the phenomenon is enhanced at pH 4. Insoluble SiO₂ species remain in the majority at all pH conditions. The corrosion resistance performances evaluated by salt spray test showed numerous corrosion pits at 168 h for SAA-Zr-sodium silicates systems, despite the precipitation of normally protective species such as ZrO₂ and SiO₂ (Fig. 19). The corrosion protection could then be essentially due to the synergy between all the species introduced by the impregnation and sealing processes: chromium, zirconium and silicon.

### 3.4. Mechanisms of impregnation and sealing precipitation

In the light of all the XPS results, several chemical mechanisms emerge during the complex construction of the protective system (the so-called anodized/impregnated/sealed multilayers).

#### a) Commercial Cr(III)/Zr(IV) impregnation

After the complete Cr(III)/Zr(IV) impregnation, the XPS analyses showed that zirconium was mainly present in the form of ZrO₂ following to an initial chemical reaction (1) corresponding to the formation of AlF₆⁻ and Zr(OH)₄.

$$ZrF_6^{2-}(aq) + Al(OH)_3(s) + OH^{-} \rightleftharpoons AlF_6^{3-}(aq) + Zr(OH)_4(s) \quad K = 10^{12.1} \quad (1)$$

The reaction constant K has been calculated with the dissociation and solubility constants as followed [26]:
<figure><img src="image_15.png" /><figcaption>Fig. 13. XPS elemental depth profile of samples SAA-Cr (pH 4) sSodium silicates (triangular symbol) and SAA-Cr (pH 1.6) sSodium silicates (solid line).</figcaption></figure>

$$K = \frac{[AlF_6^{2-}][Zr(OH)_4]}{[ZrF_6^{2-}][Al(OH)_3][OH^-]} = \frac{Kd(ZrF_6^{2-})Ks(Al(OH)_3)}{Kd(AlF_6^{3-})Ks(Zr(OH)_4)}$$

with

$$Kd(ZrF_6^{2-}) = \frac{[Zr^{4+}][F^-]^6}{[ZrF_6^{2-}]} = 10^{-28.3}$$

$$Kd(AlF_6^{3-}) = \frac{[Al^{3+}][F^-]^6}{[AlF_6^{3-}]} = 10^{-19.8}$$

$$Ks(Zr(OH)_4) = [Zr^{4+}][OH^-]^4 = 10^{-54.1}$$

$$Ks(Al(OH)_3) = [Al^{3+}][OH^-]^3 = 10^{-33.5}$$

The high value of the reaction (1) constant ($K = 10^{12.1}$) would indicate that the reaction should theoretically be complete. This reaction is responsible for the exchange of fluorine from $ZrF_6^{2-}$ with the aluminium to form $AlF_6^{3-}$. Even if $K_d(ZrF_6^{2-})$ value is low, stable hexa-fluoro zirconate species can release some fluorine atoms in a $Al(OH)_3$ surface surrounding. Indeed, the comparison of the solubility constants of aluminium hydroxide ($K_s = 10^{-33.5}$) and zirconium hydroxide ($K_s = 10^{-54.1}$) shows that $Zr(OH)_4$ is more stable than $Al(OH)_3$. Before the association with fluorine atoms, aluminium hydroxide eliminates some $OH^-$ groups systematically captured by zirconium (+IV) ions to form $Zr(OH)_4$ and then $ZrO_2$ after dehydration. The possible release of fluorine atoms by zirconate is then due to high affinity between the hydroxides and zirconium ions. A part of hydroxides could also be kept by the chromium cations according to the reaction (2).

$$Cr^{3+} + Al(OH)_3 \rightarrow Cr(OH)_3 (s) + Al^{3+} K = 10^{-2.5} \quad (2)$$

In reality, however, the XPS results indicate a small percentage of zirconium (less than 2 %) that can be attributed to $ZrO_xF_y$ species considering the $Zr$ 3d doublet on the low energy side (183.3 eV – 185.6 eV). The formation of $ZrO_xF_y$ would result from another reaction that could be involved in the mechanism of precipitation mechanism, but this chemical reaction (3) seems to be very limited.

$$ZrF_y^{4-y} (aq) + xOH^- \rightleftharpoons ZrO_xF_y^{4-y} (s) \quad (3)$$

The XPS analyses also showed that the most of chromium would be present in the form of a multiple oxi-hydroxides. The chromium hydroxides are stable phase in the impregnation layer as their solubility constant is quite low ($K_s(Cr(OH)_3) = 10^{-31}$). However, presence of chromium III could allow a further exchange of fluorine atoms according to the reaction (4):

$$ZrF_6^{2-} (aq) + Cr(OH)_3 (s) + OH^- \rightleftharpoons CrF_3 (aq) + Zr(OH)_4 (s) + 3 F^- K = 10^{-4.5} \quad (4)$$

Furthermore, the dissociation constant of $CrF_3$ ($K_d(CrF_3) = 10^{-10.2}$) is higher than that of $AlF_6^{3-}$ ($K_d(AlF_6^{3-}) = 10^{-19.8}$) [30], suggesting that fluorine is more attractive to aluminium than to chromium. The $CrF_3$ phase could then release fluorine atoms and $Cr^{3+}$ ions would afterwards surround with hydroxide groups coming from the anodized layer (reaction (4)) and generating a higher acidification of the surface, useful for the sealing of sodium silicates.

b) Partial Cr (III) impregnation

In order to understand the role of chromium in the protective effect of the coating, a partial impregnation was carried out. The reaction (2)
<figure><img src="image_16.png" /><figcaption>Fig. 14. XPS high-resolution Si 2p core level spectra of SAA-Cr (pH 4) sodium silicates with Ar⁺ etching (a) 180 s (b) 780 s (c) 1020 s and (d) 1620 s; of SAA-Cr (pH 1.6) sodium silicates with Ar⁺ etching (e) 180 s (f) 780 s (g) 1020 s and (h) 1620 s.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 15. Specimen SAA-Cr (pH 4) sodium silicates after 168 h of alkaline salt spray exposure.</figcaption></figure>

of the Cr(III) type impregnation with the aluminium hydroxide is too
limited to take place whatever the pH conditions (pH 1.6 or 4) [33]. At
the same time, reaction (1) does not take place because of the absence of
zirconium and also because the solubility constant of the Al(OH)₃ (K₃Al
(OH)₃) = 10⁻³³.₅) is lower than that of Cr(OH)₃ (K₅Cr(OH)₃) = 10⁻³¹).
This means that the aluminium hydroxide is more stable than chromium
hydroxide, which is unable to release hydroxide functions in the me-
dium and then to precipitate.

c) Partial Zr (IV) impregnation

In the case of partial zirconium impregnation, reaction (1) is not total
because ZrOₓFᵧ is formed in similar proportions whatever the pH.
However, reaction (3) is more likely because the percentage of fluo-
rozirconia species is twice as high as in the SAA-Cr/Zr-sodium silicate
system (less than 5 %).
<figure><img src="image_18.png" /><figcaption>Fig. 16. XPS elemental depth profile of samples SAA-Zr (pH 4) sodium silicates (triangular symbol) and SAA-Zr (pH 6) sodium silicates (solid line).</figcaption></figure>

d) Sealing

The sealing was carried out on the anodized impregnated layer containing AlOₓFᵧ, ZrOₓFᵧ and ZrO₂ phases in variable amounts. The sodium silicate solution used for the sealing is alkaline (pH 11.5) and precipitation of silicon in the form of silicon hydroxide depends on the pH change on the anodic surface (pH below 10.8). The decrease in pH is due to the consumption of hydroxide ions during the impregnation process (reaction (1)). The precipitation of silica gel Si(OH)₄ (s) and its dehydrated form SiO₂, is described in detail in reaction (5). The presence of SiO₃²⁻ corresponds to silicates initially present in the sealing bath which did not react.

$$ \mathrm{SiO_3^{2-}(aq)} + 3 \mathrm{H_2O} \rightleftharpoons \mathrm{Si(OH)_4(s)} + 2 \mathrm{OH^{-}} \quad (5) $$

Moreover, a fluorine exchange between AlFₓ species and silicon oxides could take place as shown in the complementary experiment carried out with the direct reaction between Na₃AlF₆ solution and some

particles of SiO₂. Indeed, a significant concentration of silicon was measured by ICP/AES in the solution 1 after equilibrium (Table 5) confirming the transfer of fluorine atoms from AlF₆³⁻ to SiO₂ and the partial solubilization of silicon [34]. The same experiment carried out between a H₂ZrF₆ solution and some SiO₂ particles showed no transfer of silicon in the solution 2 (Table 5). Reaction (6) illustrates the mechanism of the increase in the consumption of hydroxide ions. As a result, the silicon could only react with the fluorine coming from aluminium and not with the zirconium based species.

$$ \mathrm{AlF_6^{3-}(s)} + \mathrm{SiO_3^{2-}(aq)} + 3 \mathrm{H_2O} \rightleftharpoons \mathrm{Al(OH)_3(s)} + \mathrm{SiF_6^{2-}(s)} + 3 \mathrm{OH^{-}} \quad (6) $$

4. Conclusion

According to the results of this study, the impregnation and sealing of Al2618 alloy anodized layer with consecutive solutions containing Cr (+III)/Zr(+IV) and silicates salts constitutes a promising alternative to usual solutions containing Cr(+VI).

The multiscale approach using SEM-EDS and the combination of ionic etching / XPS analyses made it possible to highlight the mechanisms of precipitation of Cr(III)/Zr(IV) impregnation and sodium silicates sealing species. They promoted the formation of ZrO₂ and Cr₂O₃/ Cr(OH)₃ as an impregnation layer. In the proposed system, the dominant sealant species are silica SiO₂, which provide an additional protective barrier layer, in contrast to a simple hot water sealing, which did not create an additional interphase. Understanding this chemical organisation offers new prospects for improving the protection of Al2618 alloys in the future.

The maximum corrosion protection of SAA-Cr/Zr-sodium silicates system mainly depends on the presence of oxide phases, the ability to withdraw fluorine out of the protective layer and the synergy between the two impregnating elements, zirconium and chromium.

It has also been shown that the absence of a physical impregnation layer allows silicon to precipitate deeper into the anodised layer, but the protective effect is not efficient in this case. The formation of ZrO₂ in the impregnation layer is the main key of the alloy protection even if the presence of ZrOₓFᵧ phase could reduce the overall performance.

<figure><img src="image_19.png" /><figcaption>Fig. 17. XPS high-resolution Zr 3d$_{5/2-3/2}$ core level spectra of SAA-Zr (pH 4) sodium silicates with Ar⁺ etching (a) 180 s (b) 780 s (c) 1020 s and (d) 1620 s; of SAA-Zr (pH 6) sodium silicates with Ar⁺ etching (e) 180 s (f) 780 s (g) 1020 s and (h) 1620 s.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. 18.** XPS high-resolution Si 2p core level spectra of SAA-Zr (pH 4) sodium silicates with Ar⁺ etching (a) 180 s (b) 780 s (c) 1020 s and (d) 1620 s; of SAA-Zr (pH 6) sodium silicates with Ar⁺ etching (e) 180 s (f) 780 s (g) 1020 s and (h) 1620 s.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Fig. 19.** Specimen SAA-Zr (pH 4) sodium silicates after 168 h of alkaline salt spray exposure.</figcaption></figure>

**Table 5**

Concentration (g/L) of silicon, aluminium and zirconium respectively present in solution 1 and solution 2 after a week of reaction.

<table><thead><tr><th></th><th colspan="4">Solution 1 (Na<sub>3</sub>AlF<sub>6</sub> + SiO<sub>2</sub> (s))</th><th colspan="4">Solution 2 (H<sub>2</sub>ZrF<sub>6</sub> + SiO<sub>2</sub> (s))</th></tr></thead><tbody><tr><td>Bath pH</td><td colspan="4">~ 4</td><td colspan="4">~ 1</td></tr><tr><td>Bath Composition after equilibrium</td><td>Si</td><td>Δ</td><td>Al</td><td>Δ</td><td>Si</td><td>Δ</td><td>Zr</td><td>Δ</td></tr><tr><td>Concentration (g/L)</td><td>52.47</td><td>0.51</td><td>3.7</td><td>0.1</td><td>No detection</td><td>-</td><td>9.1</td><td>0.4</td></tr></tbody></table>

However, only a synergy between the Cr(III) and Zr(IV) chemical species of the impregnation and the addition sodium silicate sealing appears as the best configuration to improve the corrosion protection of

the aluminium alloy 2618A T851.

**CRediT authorship contribution statement**

**Celine Mira:** Writing – review & editing, Writing – original draft, Visualization, Conceptualization. **Jean-Charles Dupin:** Writing – review & editing, Resources, Project administration, Methodology, Investigation, Funding acquisition. **Joffrey Tardelli:** Writing – review & editing, Resources, Project administration, Methodology, Investigation, Conceptualization. **Laetitia Thomas:** Resources, Project administration. **Jerome Frayret:** Writing – review & editing, Supervision, Conceptualization. **Virginie Bague:** Supervision, Conceptualization. **David Sinopoli:** Supervision, Conceptualization.
Declaration of competing interest

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

Acknowledgements

The authors thank the ANR funding and the consortium of the COCA
consortium: Safran Aircraft Engines, Safran Tech, Airbus Helicopters,
IRT-M2P, INEOSURF and IPREM (UMR5254 - UPPA/CNRS). The au-
thors are grateful to the members of IRT-M2P and INEOSURF for the
sample preparation, the SEM-EDS analyses, the fractography SEM- ob-
servations and the neutral salt spray tests, in Duppigheim, and for the
ICP-AES analyses, in Grabels.

Data availability

No data was used for the research described in the article.

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