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Article

Cathodic Electrodeposition of Cerium-Based Conversion Coatings Using Deep Eutectic Solvents Formulations for Corrosion Protection of AA7075 Aluminum Alloys

by
Aurora Petica
1,
Stefania Costovici
1,
Adrian-Cristian Manea
1,2,
Anca Cojocaru
1,2,
Calin Constantin Moise
1,
Sabrina State
1,
Liana Anicai
1,* and
Marius Enachescu
1
1
Center of Surface Science and Nanotechnology, National University of Science and Technology Politehnica Bucharest, Splaiul Independentei 313, 060042 Bucharest, Romania
2
Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, National University of Science and Technology Politehnica Bucharest, 132 Calea Grivitei, 010737 Bucharest, Romania
*
Author to whom correspondence should be addressed.
Metals 2025, 15(1), 20; https://doi.org/10.3390/met15010020
Submission received: 30 November 2024 / Revised: 23 December 2024 / Accepted: 26 December 2024 / Published: 30 December 2024
(This article belongs to the Special Issue Corrosion and Protection of Lightweight Engineering Materials: Mg Alloys, Al Alloys, Ti Alloys and Other Related Metals—2nd Edition)
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Abstract

The paper presents a new approach towards forming Ce-based nanostructures using deep eutectic solvents (DESs) as new green solvents and large-scale media for the chemical and electrochemical synthesis of advanced functional surfaces and nanomaterials. Some experimental results regarding the cathodic electrodeposition of cerium-based conversion coatings onto AA7075 aluminum alloys involving different DES-based formulations are discussed. Electrolytes containing Ce(NO3)3·6H2O dissolved in choline chloride-glycerine and choline chloride-urea (1:2 molar ratio) eutectic mixtures with additions of H2O2 have been proposed and investigated. The influence of the operating parameters, including the applied current density, process duration and temperature on the quality of the formed Ce-containing conversion layers was studied. Adherent and uniform Ce-based conversion layers containing 0.3–5 wt.%. Ce have been obtained onto Al alloy substrates. Higher values of the applied current density and longer process durations led to higher Ce content when a choline chloride-urea eutectic mixture was used. Several accelerated corrosion tests were performed to evaluate the corrosion performance, respectively: (i) continuous immersion in 0.5 M NaCl for 720 h with intermediary visual examinations, recording of (ii) potentiodynamic polarization curves and of (iii) impedance spectra at open circuit potential in 0.5 M NaCl, as well as (iv) salt mist test for 240 h. The influence of an additional post-treatment step consisting in the electrochemical deposition of a hydrophobic Ce-based layer involving ethanolic solutions of stearic acid and cerium nitrate is also considered. Different corrosion performances are discussed, taking into account the used DES-based systems and electrodeposition parameters.

Graphical Abstract

1. Introduction

The detrimental impact of hexavalent chromium compounds on the environment and human health determined the European Union to restrict their use, with a significant impact on a quite large number of technological processes, including those related to Al and its alloys surface treatments [1]. In the aeronautical industry, a large range of chemical and electrochemical procedures based on Cr(VI) compounds are applied on Al alloys that usually provide high-quality performance in terms of adhesion, appearance and corrosion protection [2,3,4,5].
In the past decades, a multitude of studies have been performed and reported to find suitable candidates able to replace chromates. Some of the most relevant and promising technologies include the trivalent chromium process (TCP), rare-earth chemical conversion coatings (RECC), transition metal oxyanion additives (TMOA), Zr/Ti-based conversion coatings (Zr/TiCC), sol-gel coatings, and smart coatings providing stimulus-related inhibitor release [4,5,6,7,8,9,10,11,12,13].
Among the rare earth elements, lanthanide compounds showed a certain efficiency against the corrosive attack of metallic substrates [5,10]. Cerium compounds are generally the most active [10]. Cerium-based conversion coatings can be applied to Al alloys through a large range of procedures, including electrolytic deposition, spray, swabbing and immersion [10,14,15,16,17,18,19].
Electrochemical deposition represents a potentially attractive route to form cerium oxide layers, able to reduce the process duration and also to provide a coating developing on the whole alloy surface, unlike, for example, immersion coating, which can result in coatings mainly over cathodically active sites. Both anodic and cathodic procedures have been applied, involving different metallic substrates, such as nickel, various types of steel, zinc, Mg alloys, or Ti alloys [20,21,22,23,24,25,26]. However, fewer studies have been reported related to the electrodeposition of ceria layers on Al alloys substrates.
Brunelli et al. [16] reported the cathodic electrodeposition of cerium conversion coatings (CeCCs) onto the AA2024 alloy using aqueous solutions of CeCl3 with additions of H2O2. Conversion layers of about 500 nm were obtained by applying low current density values in the range of 5–8 mA cm−2 for short durations of maximum 120 s. The improved corrosion performance in a chloride environment was assigned to the inhibition of both cathodic and anodic reaction rates. The dry-mud morphology of the coating with the presence of cracks was attributed to the drying process after the deposition. Stoffer et al. [27] proposed an electrodeposition process that produces CeCCs based on a complex electrolyte consisting of water, solvent (i.e., propylene glycol, ethylene glycol, glycerol), oxidizing agent (i.e., H2O2), and cerium ions. Coatings of about 0.1–1 μm could be obtained at low current densities of around 3 mA cm−2 for deposition times of 2–3 min. The produced CeCCs showed similar or better anticorrosive characteristics when compared to the chromated specimens during the salt spray test according to ASTM-B117 for 336 h. Zhao et al. [28] reported the cathodic electrodeposition of CeCCs on LY12 aluminum alloy from diluted CeCl3 electrolyte in the presence of H2O2 at 40 °C. The grown deposit composed of cerium hydroxide/hydrated oxide showed good corrosion resistance. Živković et al. [29] electrochemically synthesized CeCCs on AA6060 alloy using an aqueous solution of 0.05 M Ce(NO3)3 with 0.1 NaNO3 as supporting electrolyte at pH 5.2 and room temperature, applying various cathodic potentials in the range −1.1 ÷ −1.6 V vs. SCE for different deposition periods between 5 and 20 min. Nanostructured ceria films were produced, exhibiting a cubic fluorite structure. The best corrosion protection in 0.5 M NaCl solution was provided by the CeCCs prepared at −1.4 V for 20 min, showing the highest polarization resistance. Quite recently, it has been found that the electrodeposition of CeCCs on anodic alumina layers could provide additional corrosion protection [30,31].
Until now, no robust chromate-free substitutes for Al alloys surface treatments have been found. Therefore, novel procedures and electrolyte formulations are attracting the researchers’ interest.
Ionic liquids (ILs) as green solvents have drawn great interest in recent years and were proposed as potentially benign solvents with enormous potential in a large range of applications. As they are highly polar, non-volatile, chemically inert, thermally stable, and miscible with organic solvent and water, they are suitable for a large range of applications, including organic chemical reactions, polymer synthesis, separations, and electrochemical applications. Other functional features of ILs are related to their wide-range electrochemical window and reasonable ionic conductivity. This makes them suitable as room-temperature electrolytes for electrochemical devices/reactions [32].
In recent years, the use of novel ionic liquids based on eutectic mixtures of quaternary ammonium salts (e.g., choline chloride) with hydrogen bond donor species such as amides, carboxylic acids or alcohols has increased. They have been used as novel electrolytes for a wide range of electrochemical processes, including metals and alloys electrodeposition and metal oxides preparation [33,34,35]. These media, also known as ‘deep eutectic solvents (DESs)’ or ‘ionic liquid analogues (ILAs)’, are easy to prepare, are not reactive to air humidity and have additional advantages in cost, environmental impact and synthesis. They are potentially recyclable, biodegradable and have no negative impact on human health [32,36]. Very few works dealt with the application of ILs or DESs for inorganic materials preparation, mainly for electrodeposited metal oxide films. Generally, metal oxides have been obtained by anodic oxidation of metallic electrodes [37]. Lair et al. [38] electrodeposited homogeneous and adherent CeO2 thin films on stainless steel substrate acting as cathode from mixtures of 1-methyl-3-butylimidazolium bis(trifluoromethyl sulfonyl)imide and ethanol. The X-ray diffraction evidenced the formation of cubic phase ceria whose crystallinity could be improved after thermal treatment. Microscopic analysis showed that the films displayed some cracks and were composed of a dense layer covered by a thicker porous layer made of dispersed clusters.
Lisenkov et al. [39] investigated the incorporation into a protective metallic coating of a corrosion inhibitor that can be released during sacrificial dissolution of the galvanic layer, respectively, the electrodeposition of Al-Ce alloy involving electrolytes consisting of saturated solutions of CeCl3 in AlCl3 -1-butyl-3-methylimidazolium chloride mixtures. The deposition of Al-Ce alloy film was performed under potentiostatic conditions at −1.2 V vs. Al wire ref. on Pt and AA2024 alloy substrates. EDS analysis confirmed the presence of about 1.4 at.% of Ce in the deposit. Preliminary corrosion tests in 0.05 M NaCl showed the prepared Al-Ce film provided additional anti-corrosion characteristics to the AA2024 alloy and could be considered a promising system for active corrosion protection of Al alloys. Molodkina et al. [40] investigated the electrochemical behavior of Ce(III) ions in the absence and in the presence of Fe(II) ions involving a fluorine-free [BMP][DCA] IL (1-butyl-1-methyl-pyrrolidinium dicyanamide) electrolyte through cyclic voltammetry on a Pt(111) working electrode. The microscopic data indicated that a more pronounced roughness could be related to the cathodic IL degradation process. The forming H2 bubbles (resulting from the [BMP]+ reduction) and adsorption of degradation products may lead to suppression of the deposit growth in some regions. EDX spectra revealed that after polarization at −1.71 V vs. Ag/AgCl, the deposit consisted of Ce but also included large amounts of O and C. In addition, based on XPS analysis, it has been shown that the formed Ce species are oxidized chemically to Ce(III)/Ce(IV) oxides/hydroxides. This chemical oxidation can occur via a reaction with residual water, whose concentration remains still noticeable. Marín-Sánchez et al. [41] reported the electrodeposition of coatings formed by nanocomposite material with a cerium oxide reinforced metal zinc matrix from choline chloride-urea eutectic mixtures with the addition of Zn and Ce salts. The applied current density influenced the morphology and composition of the deposit. The Ce content was situated in the range 8.30–1.36 at.% for applied current densities between 1.13 and 0.75 A dm−2. In addition, it has been shown that Ce has been incorporated as oxides with a ratio of 50.6% Ce2O3–49.4% CeO2. The Zn-Ce coating presented an active migration of the Ce species in case of defects in this coating. Quite recently, Aldana-González et al. [42] studied the electrodeposition of Ce onto AA6061 alloy using a choline-chloride-urea eutectic mixture with the addition of Ce salt under potentiostatic conditions at 70 °C. Up to 41.33% Ce was deposited on the Al alloy after applying a potential of −1.45 V for 3000 s. A corrosion inhibition efficiency of 92% was determined with respect to the blank in 3.5% NaCl.
Considering all presented above, the paper aims to explore the use of different DES-based formulations for the cathodic electrodeposition of cerium-based conversion coatings onto AA7075 aluminum alloys. The influence of the operating parameters is presented, including the applied current density, process duration and temperature on the quality of the formed Ce-containing conversion layers. The paper will also report on the corrosion behavior in 0.5 M NaCl aqueous solution. The effect of an additional post-treatment step consisting of the electrochemical deposition of a hydrophobic Ce-based layer involving ethanolic solutions of stearic acid and cerium nitrate on the overall corrosion performance is also discussed.

2. Materials and Methods

To perform experiments, choline chloride-urea (denoted ILU) and choline chloride-glycerol (denoted ILG) eutectic mixtures (1:2 molar ratio) have been initially synthesized by gentle stirring under heating at a temperature in the range of 80–100 °C, until a homogeneous, clear and colorless liquid was formed. Then, the other components were introduced, including cerium metallic salt and hydrogen peroxide, as detailed in Table 1.
All chemicals were of analytical grade and used without further purification. Choline chloride (HOC2H4N(CH3)3Cl) (ChCl) (99%), urea (U) (99.5%), glycerol (Gly) (99%), and ethanol (EtOH) were purchased from Aldrich; Ce(NO3)3.6H2O (99.5%) and 30% H2O2 were supplied by Alfa Aesar and Chimexim (Romania), respectively.
Specimens of AA 7075 alloy (chemical composition shown in Table 2) with dimensions of 35 × 70 × 10 mm have been used as metallic substrates.
The aluminum alloy coupons have been initially degreased in acetone for 30–60 s to remove the surface contaminations, followed by a chemical alkaline degreasing step using an aqueous solution of 30–50 g/L Na2CO3·10H2O + 30–50 g/L Na3PO4·12H2O + 5 g/L NaOH for 5 min at 60–80 °C and then by a chemical deoxidizing in HNO3 1:1 (vol.) solution for 30–60 s at room temperature, then rinsed with water and air dried. The specimens have been rinsed in deionized water between each step, too.
The electrochemical deposition step was performed under mild stirring and galvanostatic (current control) conditions using a DC power supply (N5769A Power Supply Agilent Technologies, Santa Clara, CA, USA, 0–15 A, 0–100 V) and a two-electrode cell configuration. Platinized titanium plates were used as anodes. Current densities between 5 and 20 mA cm−2 and electrolyte temperatures between 25 and 45 °C were applied for electrodeposition durations between 5 and 45 min. In a separate sequence of experiments, an additional post-treatment step was performed, consisting of the electrochemical deposition of a hydrophobic Ce-based layer in ethanolic solutions of 0.1 M Ce(NO3)3·6H2O + 0.2 M stearic acid at a constant voltage (Udc) of 20 V for 20 min at room temperature. After preparation, the specimens were rinsed with ethanol and dried using an air blower.
The coating mass was estimated by the gravimetric method according to MIL-DTL-81706B [43], weighing the specimens after the conversion process and after the coating removal by immersion for 60 s in a freshly made HNO3 1:1 (vol.) solution at room temperature.
To obtain more information on the cathodic behavior of the AA 7075 electrode in the prepared electrolytes, cyclic voltammograms were recorded using a three-electrode glass cell with an Al alloy working electrode (0.785 cm2), a platinum auxiliary electrode and a silver wire as a quasi-reference electrode. Both auxiliary and reference electrodes were immersed in the solutions without using a separate compartment. The voltammetric studies were performed using a PARSTAT 4000 potentiostat (Ametek, Berwyn, PA, USA) controlled by VersaStudio software (2.1 version).
The morphology and elemental composition of the electrochemically grown Ce-based conversion coatings were examined using scanning electron microscopy (SEM) associated with energy dispersive X-ray spectroscopy (EDX) (FIB-SEM platform Hitachi Ethos NX 5000, Tokyo, Japan, associated with Aztec Live detector analyzer from Oxford Instruments, Oxford, UK). An electron accelerating voltage of 20 kV and an in-column secondary electron detector, SE (U), were used.
The phase composition and structure were determined involving X-ray diffractometry (XRD) (High Resolution SmartLab X-ray diffractometer Rigaku, Tokyo, Japan, 9 kW, with rotating anode) using CuKα radiation at a speed of 2 s/step (1 step = 0.05°).
The corrosion behavior of the electrodeposited Ce-based conversion layers has been evaluated by accelerated laboratory tests consisting of: (i) continuous immersion in 0.5 M NaCl at 25 °C for 720 h with intermediary visual examinations and recording of corrosion potential; (ii) recording of potentiodynamic polarization curves at a scan rate of 1 mV s−1; and (iii) of electrochemical impedance spectra (EIS) at open circuit potential in 0.5 M NaCl against Ag/AgCl reference electrode and using a Pt counterelectrode. For both electrochemical investigations, the working electrode, WE, was represented by the investigated conversion coating with a geometrical constant surface of 0.196 cm2. EIS spectra were recorded with 10 mV a.c. voltage within the 100 kHz–100 mHz frequency range and ZView 2.4 software from Scribner Association Inc. (Wellington, FL, USA), Derek Johnson, was used to process the experimental data. The same electrochemical equipment as mentioned above has been involved during this experimental sequence. In addition, the salt mist test (in accordance with IEC 60068-2-11:2021, Ka method [44]) for a total conditioning duration of 240 h was performed, with intermediary examinations after 24, 48, 72, 144, 168, and 192 h. Minimum 3 pcs of each variant (specimens of 70 × 35 mm) were subjected to the test.

3. Results and Discussion

3.1. Electrochemical Studies

In order to obtain information on the cathodic behavior of the AA 7075 working electrode in ILU and ILG-based electrolytes, linear polarization curves in the absence and in the presence of H2O2 and of Ce salt have been recorded, as shown in Figure 1 and Figure 2.
For ILU and ILU:Gly: EtOH systems (see Figure 1a, curves 1 and 2), only a reduction process could be evidenced, starting at about −1.5 V and −1.2 V, respectively. The only possible cathodic reaction occurring is the hydrogen evolution according to the reaction:
2 H2O + 2 e− → H2 + 2 OH−
In addition, due to the presence of oxygen (aerated electrolytes), the simultaneous O2 reduction reaction may be considered:
O2 + 2 H2O + 4 e− → 4 OH−
No additional peak appeared, suggesting that the electrolytes are stable in the investigated potential domain. In the presence of H2O2, as illustrated in Figure 1a curve 3, a cathodic shoulder centered at around −1.56 V is observed that could be assigned to the H2O2 reduction generating hydroxyl ions (OH−), according to the reaction [9]:
H2O2 + 2 e− → 2 OH−
The influence of Ce(NO3)3·6H2O in the absence and in the presence of H2O2 is illustrated in Figure 1b. No cathodic process except hydrogen evolution takes place in the absence of H2O2 (see curve 2 from Figure 1b). The cathodic peak around −0.87 V evidenced in curve 3 from Figure 1b reveals the role of H2O2, related to the production of larger amounts of OH− ions that further promote the formation of Ce(OH)3 [20]:
Ce3+ + 3OH− → Ce(OH)3
Then, the oxidation of Ce(OH)3 produces CeO2 [20,45,46]:
Ce(OH)3 → CeO2 + H3O+ + e−
H2O2 acts also as an oxidant, facilitating the transformation of Ce(III) to Ce(IV) in electrolyte through the formation of a complex ion, Ce(OH)22+ (as the only soluble cerium species during the deposition), leading to the precipitation of a hydroxide or oxide film containing mainly Ce(IV) [9,24,46]:
2Ce3+ + H2O2 + 2OH− ↔ 2Ce(OH)22+
Ce(OH)22+ + 2OH− → CeO2∙H2O + H2O
Ce(OH)22+ + 2OH− → CeO2 + 2H2O
The recorded chronoamperogram at the cathodic peak potential using ILU-Gly-EtOH-H2O2 + 0.1 M Ce(NO3)3·6H2O electrolyte is presented in Figure 1c. The current rapidly decreased in the first 800 s and then seemed to reach a plateau. As shown in the inset of Figure 1c, the deposit was light yellow and appeared quite homogeneous.
The electrochemical behavior of the AA 7075 working electrode in ILG-based systems is illustrated in Figure 2. In a similar manner, no additional processes except the hydrogen reduction occur in the absence of H2O2 (curves 1 and 2 from Figure 2a). A less profiled cathodic shoulder in the −1.2 ÷ −1.3 V region is observed, ascribed to the hydroxyl ions (OH−) generation (Equation (3)). In the presence of both H2O2 and Ce(NO3)3·6H2O, a cathodic peak at around −0.92 V is noticed, as shown in curve 3 from Figure 2b. This is associated with the production of OH− ions that further determine the formation of Ce(OH)3 and then CeO2 (Equations (4)–(8)) [9,24,46]. Figure 2c presents the recorded chronoamperogram at the cathodic peak potential using ILG-EtOH + 0.5 M H2O2 + 0.1 M Ce(NO3)3·6H2O. A continuous decrease of the current up to 1200 s occurs, followed by an increase up to a maximum of 0.08 mA cm−2 that could be associated with a progressive nucleation of ceria and then a slight decrease at a value of 0.06 mA cm−2. The formed coating was yellow and relatively homogeneous on the metallic substrate.
Usually, the compounds containing Ce4+ ions exhibit yellowish hues [19]. Therefore, the yellow-based appearance is attributed to the presence of Ce(IV)-based compounds within the layer as oxides/hydroxides.
It is reasonable to infer that the electrodeposition of the Ce-based layers is influenced by the use of DES-based formulations as electrolytes. It results in a threshold time and then the process occurs quite similarly to that of aqueous solutions.

3.2. Characterization of the Electrodeposited Ce-Based Conversion Coatings

The electrodeposition of Ce-based conversion layers onto AA 7075 substrate using the ILU-Ce system has been performed under galvanostatic conditions, for applied current densities in the range 5–25 mA cm−2, at room temperature (25 ± 2 °C) under mild stirring. Uniform, adherent, light yellow coatings have been obtained under the investigated operation conditions. Figure 3 presents an example of the recorded SEM micrographs of the developed Ce-based coatings onto the AA 7075 alloy at different magnifications and the spatial distribution profiles for the main elemental components. Generally, the Ce-based deposits consist of quite irregular, rounded particles of about 10–15 μm, covering the Al alloy substrate entirely. For the investigated applied current density domain and process durations, no particular changes of the morphology were noticed.
Based on EDX measurements, a Ce content was determined in the range of 0.31–2.44 wt.% depending on the operation conditions. Higher Ce content up to about 4.8 wt.% was noticed for higher current density values in the range of 15–20 mA cm−2 and for process durations of 5–10 min. The presence of C has also been observed, suggesting a potential incorporation of the electrolyte in the coating.
The influence of the main operation parameters, respectively of the applied current density and of the process duration on the coating mass of the Ce-based coatings using the ILU-Ce system is illustrated in Figure 4.
As shown in Figure 4a, an increase of the applied current density from 10 mA cm−2 to 20 mA cm−2 facilitates higher values of the coating mass, from about 0.22 mg cm−2 to 0.45 mg cm−2, respectively. In addition, longer process durations also determine the increase of the coating mass.
A quite similar morphology of the Ce-based coatings has been noticed when the involved electrolyte has been the ILG-Ce system, as exemplified in Figure 5. In addition, a few discrete cracks are also present.
According to preliminary experiments, the electrodeposition process using ILG-Ce electrolyte was found to be more favorable at relatively higher operation temperatures, of about 40 °C, leading to adherent and uniform deposits.
However, according to EDX analysis, a lower value of the Ce content in the conversion layer was observed, in the range of 0.43–1.56 wt.%. A decrease of the process duration from 40 min to 20 min at a constant current density of 5 mA cm−2 determined the diminishing of Ce content, from 1.56 to 0.79 wt.%. A slight decrease of the Ce content against the applied current density has also noticed, respectively from ≈1.56 wt.% at 5 mA cm−2 to ≈0.93 wt.% at 20 mA cm−2.
As illustrated in Figure 6a, the increase of the applied current density determines a slight decrease of the coating mass, from 0.6 mg cm−2 at 5 mA cm−2 to 0.45 mg cm−2 at 20 mA cm−2. Longer process durations of more than 30 min determined the formation of deposits with higher values of the coating mass, of about 1.1–1.2 mg cm−2, as shown in Figure 6b.
The thickness of the electrodeposited Ce-based conversion layers was determined from the cross-sectional SEM micrographs. The prepared coatings using ILU-Ce systems exhibited thicknesses between ≈0.36 μm (10 mA cm−2, 5 min) and ≈1.5 μm (20 mA cm−2, 10 min) (see Figure S1—Supplementary Materials). In the case of ILG-Ce systems, thinner films were obtained, having thicknesses in the range 0.45–0.75 μm (see Figure S2—Supplementary Materials).
XRD analysis has been applied to obtain information on the present phases in the electrodeposited Ce-based conversion films. Figure 7 presents a typical X-ray pattern of the obtained layer involving an ILG-Ce electrolyte.
The films seemed to be poorly crystallized, also containing a certain amount of amorphous phase. A broad region between 26–34° is noticed, with a very low-intensity peak at 28.7° corresponding to the CeO2 (111) plane of the cubic fluorite crystal structure [10,24,47]. The crystallite average sizes calculated by the Scherrer equation were between 2 nm and 6 nm. The intense characteristic peaks located at 38.4°, 44.7°, 64.9°, and 78° were assigned to the Al phase of the AA 7075 alloy [48].

3.3. Corrosion Behavior of Ce-Based Conversion Coatings on AA 7075 Obtained from DES-Based Systems

In order to assess the corrosion performance of the electrodeposited Ce-containing conversion coatings involving ILU-Ce and ILG-Ce electrolyte systems, potentiodynamic polarization curves and electrochemical impedance spectra at open circuit potential in a free-aerated 0.5 M NaCl solution at room temperature have been recorded after various immersion periods. The prepared specimens subjected to the corrosion tests had coating mass values of 0.4 ± 0.1 mg cm−2 and 0.6 ± 0.1 mg cm−2 for ILU-Ce and ILG-Ce, respectively. Figure 8 presents typical polarization plots in semilogarithmic coordinates corresponding to ILU-Ce and ILG-Ce-based layers. For comparison, the same experiments have been carried out for untreated AA7075 alloy.
As shown in Figure 8a, the curves recorded at the initial moment of immersion present a slight shift of the corrosion potential towards the electropositive direction with about 20 mV for the ILU-Ce-based layer as compared to the untreated alloy. A shift of the cathodic branch of the polarization curves to lower current densities could be observed in the case of Ce-containing layers, more significant in the case of ILU-Ce, attributed to the cathodic inhibition of cerium species. After 720 h of conditioning, as illustrated in Figure 8b, slightly lower currents compared to the initial moment are noticed in the case of ILU-Ce-based layers as a result of the protective action of Ce species associated with a displacement of the corrosion potential towards more electropositive values.
The additional formation of a protective aluminum mixed oxide/hydroxide passive film could also be possible, as this is a usual phenomenon in the case of immersion in aerated solutions. The corrosion current density, icorr, and corrosion potential, Ecorr, were determined by Tafel extrapolation and their values for the investigated specimens are summarized in Table 3. The presence of CeCCs improved the corrosion performance of the Al alloy. Thus, at the initial moment of immersion, values of corrosion current density of about 22 µA cm−2 have been determined for bare alloy that decreased up to ≈4.1 µA cm−2 and to ≈1.4 µA cm−2 for ILG-Ce and ILU-Ce, respectively. After 720 h of conditioning, lower corrosion current densities were evidenced in the case of Ce-based conversion coatings prepared in the ILU-Ce system. The CeCCs involving ILG-Ce showed a lower performance in terms of icorr that could be associated with potential defects on the layer that allowed the ingress of the aggressive chloride anion. In the case of untreated AA 7075 alloy, a slight increase of the icorr is noticed, as a result of the continuation of the metal dissolution process.
Figure 9 presents the electrochemical impedance spectra of the Ce-based conversion layers involving ILU-Ce and ILG-Ce systems, as well as of the bare alloy, recorded at open circuit potential (OCP) in 0.5 M NaCl solution for various immersion periods. The proposed electrical equivalent circuit to describe their corrosion behavior is shown in Figure 9j. All Nyquist diagrams present a semi-circle arc in the relative high-frequency range. The diameter of the semicircles is usually related to the film resistance and may be correlated to the rate of corrosion: the larger the resistance, the lower the rate of corrosion. For both Ce-based deposits and untreated AA 7075 one-time constant is noticed in the Bode plot from the initial moment up to the end of the immersion period. The values of the phase angle at the high frequency, also associated with a higher impedance modulus, could be an indicator of the barrier properties of the electrodeposited layer. In the case of CeCCs formed in ILU-Ce systems, a decrease of the phase angle and of the impedance modulus is noticed for the first 15 days of exposure, suggesting a potential damage of the layer with possible infiltration of water and chloride ions. After that, an increase in the resistance is noticed for up to 30 days of exposure and could be attributed to the healing effect of Ce species as well as of the formed aluminum mixed oxide/hydroxide passive film. In the case of Ce-based conversion coatings prepared in ILG-Ce systems, a relatively continuous decrease of the impedance modulus is evidenced, suggesting lower protective performance against the aggressive environment. The untreated AA 7075 alloy presents a slight increase of the impedance modulus for the first 15 days of conditioning that could be ascribed to the presence of an aluminum mixed oxide/hydroxide protective film. Then, the resistance continuously decreased due to the dissolution of the formed passive layer.
In order to fit the experimental electrochemical impedance data and obtain more quantitative information on the corrosion performance of the investigated Ce-based conversion layers, an electrical equivalent circuit has been proposed consisting of a simple RC combination comprising a film capacitor (CF) in parallel with a film resistor (RF) related to the formed cerium-containing conversion layer, in series with the solution ohmic resistance (Rsol). During the fitting of the experimental data, a constant phase element (CPE) has been used to replace the capacitor [19,49] in order to model more accurately the non-ideal behavior of capacitance.
Evolution of R(f) against immersion period for CeCCs formed in ILU-Ce and ILG-Ce systems and for the bare alloy is shown in Figure 10. It should be noticed that R(f) values when the ILG-Ce system was involved were lower as compared to those determined for the ILU-Ce-based conversion. These data are in agreement with those resulting from the analysis of the polarization curves.
In order to improve the corrosion performance of the conversion layers onto Al alloy substrates, an additional post-treatment is often recommended [27,50]. Therefore, an additional post-treatment step has been applied onto the electrodeposited Ce-based conversion layers using ILU-Ce systems, as this procedure showed the best corrosion performance. It consisted in the electrochemical deposition of a hydrophobic Ce-based layer using an ethanolic solution of 0.1 M Ce(NO3)3·6H2O + 0.2 M stearic acid at a constant voltage (Udc) of 20 V for 20 min at room temperature. Measurements of the contact angle (see Table S1—Supplementary Materials) evidenced values of about 151°, suggesting hydrophobic characteristics. Figure 11 illustrates the influence of the applied post-treatment on the overall corrosion performance.
Ecorr of the Ce-based conversion coating obtained from the ILU-Ce system subjected to post-treatment had a value of −0.574 V vs. Ag/AgCl at the initial moment of immersion, significantly more electropositive than that of pristine ILU-Ce-based film, of −0.690 V vs. Ag/AgCl, and of the bare alloy, of −0.634 V vs. Ag/AgCl, as illustrated in Figure 11a (see also Table 3). In addition, the corrosion current icorr was 0.029 μA cm−2, meaning almost two orders of magnitude lower than the pure ILU-Ce-based layer. After 720 h of exposure, a slight displacement of the Ecorr towards more electronegative values was noticed, of −0.635 V vs. Ag/AgCl, and an increase of the corrosion current up to about 0.929 μA cm−2.
As presented in Figure 11b, the Ce-based conversion coating subjected to post-treatment provides the best corrosion protection. The Rtotal value for the initial moment of immersion was two orders of magnitude higher than that of the ILU-Ce-based film. It decreased up to 168 h (7 days), then progressively increased up to 528 h (22 days), suggesting the possible active corrosion protection attributed to the presence of cerium species and to the beneficial influence of the hydrophobic characteristics. While the pristine ILU-Ce-based film showed a quite similar trend of evolution during the conditioning period, due to the healing effect of Ce species, the Rtotal had lower values.
Figure 12 illustrates the appearance of the bare Al alloy and of the electrodeposited Ce-based conversion layer using the ILU-Ce system in the absence and in the presence of the applied post-treatment during the salt mist test for 240 h.
It could easily be observed that the untreated alloy showed the presence of a significant amount of corrosion products after 96 h of exposure. Ce-based conversion layers obtained from the ILU-Ce system displayed a partial removal of the passive layer after 96 h and an initiation of few pitting points. The aspect did not suffer other significant modifications up to the end of the test. On the contrary, the Al alloy specimens subjected to the additional post-treatment step did not show any significant surface modifications after 240 h of conditioning.
The approach towards electrochemical forming of Ce-based conversion layers using DES-based formulations showed promising results. In the presence of water and H2O2 in the electrolyte, the synthesis mechanism seems to occur via a similar mechanism to that proposed in aqueous solutions. However, probably due to their higher viscosity, the deposition process requires longer durations. On the other hand, almost crack-free deposits appeared to form. This makes them potentially suitable as long-term corrosion-resistant coatings. Moreover, the application of an additional electrochemical post-treatment step allowing the formation of a hydrophobic Ce-based layer further improves the corrosion performance, due to the synergic effect of cerium species, which may increase the active corrosion protection and of the hydrophobic characteristics.

4. Conclusions

The performed investigations showed the possibility of having new electrolytes suitable to electrochemically deposit cerium-based conversion layers on AA7075 aluminum alloys based on different DES-based formulations. In addition, their impact on the overall corrosion performance has also been assessed. Therefore, electrolytes containing Ce(NO3)3·6H2O dissolved in choline chloride-glycerol and choline chloride-urea eutectic mixtures (1:2 molar ratio), with additions of H2O2 and other components to provide a proper adhesion and growth rate, have been proposed.
Uniform, adherent, light yellow coatings have been obtained under the investigated operation conditions. Generally, the Ce-based deposits consist of quite irregular, rounded particles of about 10–15 μm, covering the Al alloy substrate entirely. For the investigated applied current density domain and process durations, no particular changes of the morphology were observed. Conversion layers with thicknesses of about 1.2 μm and 0.5 μm have been obtained using ILU-Ce and ILG-Ce systems, respectively.
The EDX investigations confirmed the presence of Ce within the deposited coatings in the range of 0.3–5 wt.%. The electrolytes based on a choline chloride-urea eutectic mixture appeared to facilitate a higher Ce content (up to ~4.8%) depending on the applied operation conditions.
The corrosion performance of the coatings was assessed through polarization measurements and EIS for long immersion periods of 720 h.
Ce-based conversion coatings obtained from the ILU-Ce system showed better corrosion protection as compared to those prepared in the ILG-Ce electrolyte, materialized by corrosion current densities of about 0.8 μA cm−2 and total resistances of ≈56 kΩ cm2 after 720 h of continuous immersion in 0.5 M NaCl.
The application of an additional electrochemical post-treatment step allowing the formation of an additional hydrophobic Ce-based layer improved the corrosion characteristics. They showed total resistances of ≈96 kΩ cm2 after 720 h of continuous immersion in the chloride-containing aggressive medium. This behavior suggests a synergic effect between the Ce species that may enhance the active corrosion protection and the hydrophobic characteristics of the layers.
Still further investigations are scheduled for a much more detailed analysis of the characteristics of the obtained Ce-based conversions to gain a better understanding of the layer formation mechanism associated with the obtained morphology as an important factor in influencing the corrosion protection mechanisms.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/met15010020/s1, Figure S1: SEM cross-sectional view of the Ce based deposit involving ILU-Ce system (10 mA cm−2, 15 min, RT); Figure S2: SEM cross-sectional view of the Ce based deposit involving ILG-Ce system (ILG-Ce, 5 mA cm−2, 20 min, 40 °C); Table S1: Contact angle values.

Author Contributions

Conceptualization, L.A.; Methodology, A.P., S.C., A.-C.M. and A.C.; Validation, A.-C.M., A.C., S.S. and L.A.; Formal analysis, A.P., S.C., A.-C.M., A.C., C.C.M., S.S. and L.A.; Investigation, A.P., S.C., A.-C.M., A.C., C.C.M., S.S., L.A. and M.E.; Resources, M.E.; Writing—original draft, L.A.; Writing—review & editing, L.A.; Visualization, L.A.; Funding acquisition, L.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Romanian Ministry of Education, The Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI) under the ECOCCEALPROT project 311PED/2020.

Data Availability Statement

The original contributions presented in this study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Metals 15 00020 g001
Figure 1. Linear polarization curves obtained on AA 7075 of: (a) ILU, ILU-Gly-EtOH, and ILU-Gly-EtOH-H2O2 and (b) ILU-Gly-EtOH-H2O2, ILU-Gly-EtOH + 0.1 M Ce(NO3)3·6H2O and ILU-Gly-EtOH-H2O2 + 0.1 M Ce(NO3)3·6H2O electrolytes at room temperature (scan rate 50 mVs−1); (c) Chronoamperogram obtained at room temperature on AA 7075 for the cathodic deposition of Ce oxide-based layer at Epeak = −0.870 V vs. Ag. Inset: the photographic image of the deposited coating.
Figure 1. Linear polarization curves obtained on AA 7075 of: (a) ILU, ILU-Gly-EtOH, and ILU-Gly-EtOH-H2O2 and (b) ILU-Gly-EtOH-H2O2, ILU-Gly-EtOH + 0.1 M Ce(NO3)3·6H2O and ILU-Gly-EtOH-H2O2 + 0.1 M Ce(NO3)3·6H2O electrolytes at room temperature (scan rate 50 mVs−1); (c) Chronoamperogram obtained at room temperature on AA 7075 for the cathodic deposition of Ce oxide-based layer at Epeak = −0.870 V vs. Ag. Inset: the photographic image of the deposited coating.
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Figure 2. Linear polarization curves obtained on AA 7075 of: (a) ILG, ILG-EtOH, and ILG-EtOH-H2O2, and (b) ILG-EtOH-H2O2, ILG-EtOH + 0.1 M Ce(NO3)3·6H2O, and ILG-EtOH + 0.5 M H2O2 + 0.1 M Ce(NO3)3·6H2O electrolytes at 40 °C (scan rate 50 mVs−1); (c) Chronoamperogram obtained at 40 °C on AA 7075 for the cathodic deposition of Ce oxide-based layer at Epeak = −0.92 V vs. Ag. Inset: the photographic image of the deposited coating.
Figure 2. Linear polarization curves obtained on AA 7075 of: (a) ILG, ILG-EtOH, and ILG-EtOH-H2O2, and (b) ILG-EtOH-H2O2, ILG-EtOH + 0.1 M Ce(NO3)3·6H2O, and ILG-EtOH + 0.5 M H2O2 + 0.1 M Ce(NO3)3·6H2O electrolytes at 40 °C (scan rate 50 mVs−1); (c) Chronoamperogram obtained at 40 °C on AA 7075 for the cathodic deposition of Ce oxide-based layer at Epeak = −0.92 V vs. Ag. Inset: the photographic image of the deposited coating.
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Figure 3. SEM micrographs (SE(U) images) at different magnifications (a,b) and EDX maps (c–f) showing the distribution profiles of the significant elements within the Ce-based deposit involving the ILU-Ce system (10 mA cm−2, 15 min, RT).
Figure 3. SEM micrographs (SE(U) images) at different magnifications (a,b) and EDX maps (c–f) showing the distribution profiles of the significant elements within the Ce-based deposit involving the ILU-Ce system (10 mA cm−2, 15 min, RT).
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Figure 4. The influence of (a) current density and (b) electrochemical process duration on the coating mass of the Ce-based coating using ILU-Ce system.
Figure 4. The influence of (a) current density and (b) electrochemical process duration on the coating mass of the Ce-based coating using ILU-Ce system.
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Figure 5. SEM micrographs (SE(U) images) at different magnifications (a,b) and EDX maps (c–f) showing the distribution profiles of the significant elements within the Ce-based deposit (ILG-Ce, 5 mA cm−2, 30 min, 40 °C).
Figure 5. SEM micrographs (SE(U) images) at different magnifications (a,b) and EDX maps (c–f) showing the distribution profiles of the significant elements within the Ce-based deposit (ILG-Ce, 5 mA cm−2, 30 min, 40 °C).
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Figure 6. The influence of (a) current density and (b) electrochemical process duration on the coating mass of the Ce-based coating using ILG-Ce system.
Figure 6. The influence of (a) current density and (b) electrochemical process duration on the coating mass of the Ce-based coating using ILG-Ce system.
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Figure 7. X-ray diffractogram for cerium oxide-based films involving ILG-Ce system (10 mA cm−2, 20 min, 40 °C). Inset: the Gaussian fit of the Ce(111) peak.
Figure 7. X-ray diffractogram for cerium oxide-based films involving ILG-Ce system (10 mA cm−2, 20 min, 40 °C). Inset: the Gaussian fit of the Ce(111) peak.
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Figure 8. Polarization curves in semilogarithmic coordinates for Ce-based conversion coatings on AA 7075 obtained from DES systems during continuous immersion test in 0.5 M NaCl: (a) initial moment and (b) after 720 h of conditioning, 25 °C, 1 mV s−1).
Figure 8. Polarization curves in semilogarithmic coordinates for Ce-based conversion coatings on AA 7075 obtained from DES systems during continuous immersion test in 0.5 M NaCl: (a) initial moment and (b) after 720 h of conditioning, 25 °C, 1 mV s−1).
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Figure 9. Comparative Nyquist and Bode plots in 0.5 M NaCl at open circuit potential after various continuous immersion periods for Ce-based conversion coatings involving (a–c) ILU-Ce and (d–f) ILG-Ce systems, as well as for AA 7075 (g–i). Solid lines are the fit to the measured points using the proposed equivalent circuit in (j). The significance of circuit components: Rsol-ohmic resistance of solution; C(f) and R(f)—capacitance and ohmic resistance of the formed film. In the case of AA 7075, R(f) is related to the native oxide film (SWE = 0.196 cm2).
Figure 9. Comparative Nyquist and Bode plots in 0.5 M NaCl at open circuit potential after various continuous immersion periods for Ce-based conversion coatings involving (a–c) ILU-Ce and (d–f) ILG-Ce systems, as well as for AA 7075 (g–i). Solid lines are the fit to the measured points using the proposed equivalent circuit in (j). The significance of circuit components: Rsol-ohmic resistance of solution; C(f) and R(f)—capacitance and ohmic resistance of the formed film. In the case of AA 7075, R(f) is related to the native oxide film (SWE = 0.196 cm2).
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Figure 10. Evolution of: R(f) for Ce-based conversion coatings involving ILU-Ce and ILG-Ce, as well as for AA 7075, against immersion duration in 0.5 M NaCl.
Figure 10. Evolution of: R(f) for Ce-based conversion coatings involving ILU-Ce and ILG-Ce, as well as for AA 7075, against immersion duration in 0.5 M NaCl.
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Figure 11. (a) Comparative polarization curves in semilogarithmic coordinates for AA 7075 untreated and Ce-based conversion coating obtained from the ILU-Ce system subjected to post-treatment during continuous immersion test in 0.5 M NaCl (initially and after 720 h of conditioning, 25 °C, 1 mV s−1); (b) Evolution of Rtotal for AA 7075, Ce-based conversion coating using the ILU-Ce system and after application of the post-treatment during continuous immersion test in 0.5 M NaCl.
Figure 11. (a) Comparative polarization curves in semilogarithmic coordinates for AA 7075 untreated and Ce-based conversion coating obtained from the ILU-Ce system subjected to post-treatment during continuous immersion test in 0.5 M NaCl (initially and after 720 h of conditioning, 25 °C, 1 mV s−1); (b) Evolution of Rtotal for AA 7075, Ce-based conversion coating using the ILU-Ce system and after application of the post-treatment during continuous immersion test in 0.5 M NaCl.
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Figure 12. Photographic images of electrodeposited Ce-based conversion coatings without and with the application of the post-treatment subjected to salt mist test for 240 h.
Figure 12. Photographic images of electrodeposited Ce-based conversion coatings without and with the application of the post-treatment subjected to salt mist test for 240 h.
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Table 1. The composition of the DES-based electrolytes used for the cathodic electrodeposition of Ce-based conversion coatings.
Table 1. The composition of the DES-based electrolytes used for the cathodic electrodeposition of Ce-based conversion coatings.
Electrolyte TypeElectrolyte Composition
ILU-CeILU:glycerol:ethanol (6:1:1 vol.)
+
0.1 M Ce(NO3)3.6H2O + 0.5 M H2O2
ILG-CeILG:ethanol (1:1 vol.)
+
0.1 M Ce(NO3)3.6H2O + 0.5 M H2O2
Table 2. Chemical composition of AA 7075.
Table 2. Chemical composition of AA 7075.
Composition (wt.%)
SiFeCuMnMgCrZnTiOther ElementsAl
Max. 0.40Max. 0.501.2–2.0Max.
0.30		2.1–2.90.18–0.285.1–6.1Max. 0.2Max. 0.15rest
Table 3. Values of corrosion parameters determined from polarization curves for bare 7075 and Ce-based conversion coatings obtained from DES systems immersed in 0.5 M NaCl.
Table 3. Values of corrosion parameters determined from polarization curves for bare 7075 and Ce-based conversion coatings obtained from DES systems immersed in 0.5 M NaCl.
Coating SystemInitialAfter Continuous Immersion for 720 h
Ecorr,
V vs. Ag/AgCl		icorr,
μA cm−2		Ecorr,
V vs. Ag/AgCl		icorr,
μA cm−2
AA 7075 untreated−0.63422.2−0.72125.6
ILG-Ce−0.6104.12−0.6067.17
ILU-Ce−0.6901.43−0.6600.77
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Petica, A.; Costovici, S.; Manea, A.-C.; Cojocaru, A.; Moise, C.C.; State, S.; Anicai, L.; Enachescu, M. Cathodic Electrodeposition of Cerium-Based Conversion Coatings Using Deep Eutectic Solvents Formulations for Corrosion Protection of AA7075 Aluminum Alloys. Metals 2025, 15, 20. https://doi.org/10.3390/met15010020

AMA Style

Petica A, Costovici S, Manea A-C, Cojocaru A, Moise CC, State S, Anicai L, Enachescu M. Cathodic Electrodeposition of Cerium-Based Conversion Coatings Using Deep Eutectic Solvents Formulations for Corrosion Protection of AA7075 Aluminum Alloys. Metals. 2025; 15(1):20. https://doi.org/10.3390/met15010020

Chicago/Turabian Style

Petica, Aurora, Stefania Costovici, Adrian-Cristian Manea, Anca Cojocaru, Calin Constantin Moise, Sabrina State, Liana Anicai, and Marius Enachescu. 2025. "Cathodic Electrodeposition of Cerium-Based Conversion Coatings Using Deep Eutectic Solvents Formulations for Corrosion Protection of AA7075 Aluminum Alloys" Metals 15, no. 1: 20. https://doi.org/10.3390/met15010020

APA Style

Petica, A., Costovici, S., Manea, A.-C., Cojocaru, A., Moise, C. C., State, S., Anicai, L., & Enachescu, M. (2025). Cathodic Electrodeposition of Cerium-Based Conversion Coatings Using Deep Eutectic Solvents Formulations for Corrosion Protection of AA7075 Aluminum Alloys. Metals, 15(1), 20. https://doi.org/10.3390/met15010020

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