Corrosion Protection of SAE 1020 Steel Using CeO₂ Coatings Prepared Via Ionic Liquid Method

Abstract

Corrosion of SAE 1020 carbon steel, extensively used in the petrochemical industry, remains a persistent challenge. Replacing toxic chromate-based coatings with sustainable alternatives is equally critical. This study introduces an innovative, environmentally friendly surface modification method using the thermal decomposition of cerium chloride heptahydrate (CeCl₃·7H₂O) dissolved in the ionic liquid 1-methylimidazolium hydrogen sulfate, a non-toxic solvent not previously used for the deposition of anticorrosion protective coatings. Coatings were calcined at temperatures from 400 to 600 °C. Electrochemical analyses revealed that the coating treated at 550 °C exhibited the highest corrosion resistance, even after prolonged immersion in corrosive media. This outcome was attributed to the formation of a dense, homogeneous, and adherent CeO₂ layer. The method proved effective in forming protective CeO₂ films, confirming the potential of ionic liquids in producing high-performance anticorrosion coatings. Thus, the developed approach represents a promising and sustainable alternative for corrosion protection in demanding industrial environments, particularly within the oil and gas sector.

Keywords:
Carbon steel SAE 1020; Corrosion; CeO₂; Thermal decomposition; Ionic liquid

1. Introduction

Corrosion remains a significant challenge across various industrial sectors, particularly in the petrochemical industry. Despite advances in materials science, equipment used in oil production stages, such as extraction, transportation, and refining, continues to suffer from corrosive attacks due to the presence of aggressive environments, many of which are unique to the oil industry. While renewable energy sources are gaining traction, the demand for oil and gas exploration and production remains substantial, as the petrochemical industry is a critical driver of the global economy, including Brazil¹-³. Given the socioeconomic importance of the oil industry, developing strategies to combat corrosion in the face of these aggressive media is essential.

Produced water, a particularly corrosive agent in the petrochemical industry, is characterized by its high salinity and the presence of salts such as chlorides, sulfates, and calcium carbonates, among other chemicals. These components significantly contribute to corrosion and the formation of inorganic deposits, particularly in extraction, transportation, and refining facilities. SAE 1020 carbon steel, frequently used in effluent treatment plant filters, is especially vulnerable to these corrosive effects. As a result, produced water is considered one of the most aggressive agents in the oil industry, accelerating corrosion processes and leading to increased equipment failure rates. These failures not only disrupt operational production schedules but also pose safety risks and result in substantial maintenance costs⁴-⁶.

Moreover, the unwanted release of produced water, which contains various toxic elements, into the environment can generate severe ecological impacts, depending on factors such as the concentration and toxicity of the effluent⁷,⁸. The primary environmental hazards associated with produced water include high salinity, suspended solids, dissolved and dispersed organic compounds, and the presence of other pollutants⁵. In light of these challenges, developing new methods to enhance the corrosion resistance of SAE 1020 carbon steel in the oil industry context is crucial.

Due to its low cost, SAE 1020 carbon steel is one of the most used metal alloys in several industries. However, its low corrosion resistance requires frequent replacement of corroded parts⁹. The oil industry, a significant consumer of carbon steel for tanks and pipelines, is particularly vulnerable to corrosion, which can damage the entire production chain from extraction to refining³,⁹,¹⁰. To address this issue, the development of effective protective coatings is essential.

Due to its low cost, SAE 1020 carbon steel is one of the most used metal alloys in several industries. However, its low corrosion resistance requires frequent replacement of corroded parts. The petroleum industry, a large consumer of carbon steel for tanks and pipelines, is particularly vulnerable to corrosion, which can damage the entire production chain, from extraction to refining³,⁹,¹⁰. To solve this problem, the development of effective protective coatings is essential.

Recent studies have highlighted the potential of ionic liquids as corrosion inhibitors for carbon steel in harsh environments, such as HCl solutions, which simulate severe industrial conditions¹¹,¹². Additionally, rare-earth-based salts, such as cerium, have emerged as promising alternatives to chromates, which are known to be polluting, toxic, and carcinogenic. Cerium-based coatings, produced using the sol-gel method, have shown promise due to their low cost and minimal environmental impact¹³-¹⁶. However, there is limited research on the use of thermal decomposition to deposit rare-earth-based chlorides/salts using ionic liquids as solvents for corrosion protection.

The use of ionic liquids in the deposition of conductive metal oxides has been investigated for applications in water and wastewater treatment due to their electrocatalytic properties and physical stability¹⁷-²⁰. However, applying this method to deposit protective oxides for corrosion resistance is an innovative approach that has not yet been reported. We hypothesize that this pioneering application of oxide deposition using ionic liquids as solvents for the precursor metals could yield highly effective coatings for protecting SAE 1020 carbon steel against corrosion.

Therefore, the objective of this study was to propose a novel surface coating method involving the thermal decomposition of cerium chloride heptahydrate (CeCl₃·7H₂O) as a precursor solution, dissolved in the ionic liquid 1-methylimidazolium hydrogen sulfate, to enhance the corrosion resistance of SAE 1020 carbon steel a material widely used in the oil industry. This innovative method, which creates cerium oxide layers at different calcination temperatures, can significantly improve carbon steel's corrosion resistance compared to traditional chromate-based coatings, offering an environmentally friendly and effective alternative for the oil industry. To evaluate the effectiveness of the CeO₂ coatings developed at various calcination temperatures, we employed atomic force microscopy (AFM) and scanning electron microscopy (SEM) for characterization. Additionally, open circuit potential (OCP) monitoring, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) tests were conducted in a 3.5% NaCl solution at room temperature (25 °C) to assess the corrosion resistance of the substrate and the CeO₂ layers.

2. Materials and Methods

2.1. Preparation of test specimens

SAE 1020 carbon steel samples were fabricated into discs with a diameter of 2 cm and a thickness of 3 mm. The steel electrodes were first polished using 100 and 220-grit sandpaper to eliminate burrs, cracks, and surface scratches. Subsequently, the samples were thoroughly cleaned in a solution comprising acetone, isopropyl alcohol, and distilled water to remove any oil and grease residues. The chemical composition of the SAE 1020 carbon steel used as a substrate for the coatings is detailed in Table 1.

Table 1
Chemical composition of SAE 1020 carbon steel.

Element	Percentage (%)
C	0.20
Mn	0.22
Si	0.04
P	0.01
S	0.01
Al	0.007
Fe	99.513

2.2. Deposition by thermal decomposition of chloride using ionic liquid as solvent

The precursor solution for cerium oxide deposition was prepared by dissolving cerium chloride heptahydrate (CeCl₃·7H₂O) at a concentration of 20 g L⁻¹ in 1.0 ml of the ionic liquid 1-methylimidazolium hydrogen sulfate¹⁷. The mixture was heated to 90 °C to facilitate complete dissolution. Deposition of the cerium-based solution onto the steel substrates was achieved using the dip-coating method. Following deposition, the samples were subjected to calcination at various temperatures (400, 450, 500, 550, and 600 °C) for 1 hour, with a heating rate of 5 °C min⁻¹. This step was carried out to investigate the effect of calcination temperature on the corrosion resistance of the resulting CeO₂ layers.

2.3. Morphological characterization

The morphology of the CeO₂ layers, deposited using the ionic liquid method and calcined at different temperatures, was examined using AFM. AFM analysis was conducted to assess the uniformity and smoothness of the films, ensuring they were free from cracks. Images were captured using a Nanosurf EasyScan 2 system (Nanosurf AG, Switzerland) with uncoated silicon AFM probes featuring a force constant of 48 N/m and a resonance frequency of 190 kHz. Additionally, the surface morphology of the CeO₂ layers was further analyzed using a JEOL/JSM-6510LV scanning electron microscope.

2.4. Electrochemical tests

Electrochemical tests were conducted using Autolab PGSTAT 302N potentiostat/galvanostat connected to a microcomputer running GPES version 4.9 (General Purpose Electrochemical System) software for data acquisition and analysis. The tests employed a conventional three-electrode electrochemical cell, with a saturated calomel electrode as the reference, a platinum counter electrode, and the SAE 1020 carbon steel (exposed area of 3.14 cm²) as the working electrode.

The open circuit potential (OCP) of the SAE 1020 carbon steel, both uncoated and CeO₂-coated, was monitored for 2 hours in a 3.5% NaCl solution at room temperature (25 °C). After the OCP had stabilized, corrosion resistance was evaluated through potentiodynamic polarization tests conducted at a scan rate of 1 mV s⁻¹ and EIS, with a frequency range of 10 kHz to 1 mHz and a sinusoidal voltage amplitude of 10 mV. Additionally, to assess the long-term corrosion behavior of the substrate and the electrode coated with CeO₂ calcined at 550 °C, EIS measurements were performed at various immersion times (0, 1, 2, 6, 9, 10, 14, 16, 21, 22, and 30 days).

3. Results and Discussion

3.1. Morphological characterization

Figure 1 presents SEM micrographs of the carbon steel surfaces coated and calcined at different temperatures. The coatings treated at 450 and 500 °C exhibit non-uniform and heterogeneous morphologies, with visible discontinuities and localized porosity, which may compromise their protective performance. In contrast, the coatings calcined at 550 and 600 °C display more compact and homogeneous layers. Notably, the sample calcined at 550 °C presents the most uniform morphology, suggesting superior barrier properties and improved corrosion resistance.

Figure 1
SEM images taken for SAE 1020 carbon steel surfaces coated with CeO₂ layers synthesized from cerium chloride heptahydrate dissolved in 1-methylimidazolium hydrogen sulfate. Coatings were calcined at different temperatures using a controlled heating rate of 5 °C min⁻¹.

AFM analysis (Figure 2) further corroborates these observations by revealing distinct topographical features. The surface calcined at 450 °C exhibits marked irregularities, including cavities and exposed substrate regions, indicating poor film formation. At 500 °C, the coating appears denser, yet surface roughness remains significant, with persistent microcracks that could serve as pathways for electrolyte ingress. At 550 °C, a smoother, continuous, and defect-free surface is observed, indicating the formation of an effective physical barrier against corrosive agents. However, at 600 °C, localized valleys and microcracks re-emerge, likely due to thermal stress or over-sintering, potentially compromising the coating’s integrity. These results underscore the critical role of calcination temperature in determining the microstructure and protective efficiency of the coating. Optimal surface uniformity and minimal defect density were achieved at 550 °C, which aligns with the electrochemical findings and supports its selection as the optimal processing temperature for corrosion protection.

Figure 2
Three-dimensional AFM images taken for SAE 1020 carbon steel surfaces coated with CeO₂ layers synthesized from cerium chloride heptahydrate dissolved in 1-methylimidazolium hydrogen sulfate. The coatings were calcined at various temperatures using a heating rate of 5 °C min⁻¹.

3.2. Protection mechanism of cerium oxide coatings

Cerium-based conversion coatings are well known for their self-healing and passivating properties, particularly when incorporated into oxide layers on metal substrates. The anti-corrosion performance of cerium oxide coatings is primarily associated with the formation of insoluble Ce-containing compounds that serve as physical and chemical barriers at localized corrosion sites.

When the steel substrate undergoes mechanical damage, such as cracks or scratches, localized anodic and cathodic reactions are initiated at the coating–metal interface. The anodic dissolution of iron (Equation 1) results in the release of Fe²⁺ ions, while the cathodic reduction of water or dissolved oxygen (Equation 2) generates OH⁻ ions, leading to an increase in local pH. These conditions promote the formation of iron hydroxides and oxides (Equations 3-5), which accumulate at the damaged sites as corrosion products.

Simultaneously, Ce³⁺ ions present in the coating are highly sensitive to changes in pH. Under alkaline conditions, these ions migrate toward cathodic regions and precipitate as cerium hydroxide or oxide species (Equations 6-10). This leads to the in situ formation of an insoluble Ce-based film that passivates the exposed steel surface by blocking further electrochemical reactions²¹,²². The overall protective mechanism relies on the dual role of cerium species: (i) physical obstruction of ionic transport and (ii) chemical passivation through redox activity between Ce³⁺ and Ce⁴⁺ species. These processes contribute to the inhibition of further corrosion propagation and prolong the service life of the coated substrate.

1. Anodic reaction:

   $$Fe\to F{e}^{2+}+2{\text{e}}^{-}$$(1)

2. Cathodic reaction:

   $$2{\text{H}}_2\text{O}+2{\text{e}}^{-}\to 2O{H}^{-}+{\text{H}}_2$$(2)

3. Formation of corrosion products:

   $$F{e}^{2+}+2O{H}^{-}\to Fe{(OH)}_2$$(3)
   $$2Fe{(OH)}_2+{\text{H}}_2\text{O}+1/2{\text{O}}_2\to 2Fe{(OH)}_3$$(4)
   $$2Fe{(OH)}_3\to F{e}_2{\text{O}}_3.{\text{H}}_2\text{O}+2{\text{H}}_2\text{O}$$(5)

4. Formation of cerium-based protective layers:

   $$C{e}^{3+}+3{(OH)}^{-}\to Ce{(OH)}_3$$(6)
   $$2Ce{(OH)}_3\leftrightarrows C{e}_2{\text{O}}_3+3{\text{H}}_2\text{O}$$(7)
   $$4C{e}^{3+}+{\text{O}}_2+4{(OH)}^{-}+2{\text{H}}_2\text{O}\to 4Ce{(OH)}_2{}^{2+}$$(8)
   $$Ce{(OH)}_2{}^{2+}+2{(OH)}^{-}\to Ce{(OH)}_4$$(9)
   $$Ce{(OH)}_4\leftrightarrows 2{\text{H}}_2\text{O}+C{e}_2\text{O}$$(10)

The accumulation of these Ce-based species within the coating’s microstructure enhances its barrier properties by reducing ionic permeability and promoting passivation, thereby mitigating further degradation.

3.3. Open circuit potential and potentiodynamic polarization curves

Figure 3 illustrates the OCP monitoring over 2 hours for all tested conditions. Among the samples, the coating calcined at 550 °C exhibited the highest corrosion potential, suggesting the formation of a stable and more corrosion-resistant oxide layer. The sample calcined at 500 °C showed an initial decrease in corrosion potential, likely due to the initial corrosive processes or partial dissolution of the oxide layer. However, after approximately 1500 seconds, the potential stabilized, indicating the repassivation of the oxide layer. A similar behavior was observed for the coating calcined at 450 °C. Interestingly, the sample calcined at 600 °C displayed a stable potential without significant oscillation, reaching values comparable to those of the 450 °C sample.

Figure 3
Open-circuit potential measurements of uncoated SAE 1020 carbon steel and samples coated with cerium oxide layers calcined at different temperatures. Tests were conducted in a 3.5% NaCl solution at room temperature (25 °C).

The OCP data also reveal that within the first 1000 seconds, the uncoated substrate exhibited higher potential values than the sample calcined at 400 °C, indicating that lower calcination temperatures may not be effective in forming protective layers. However, after this period, the potential for the uncoated substrate decreased, highlighting the low corrosion resistance of this material. This observation is consistent with a report in the literature, where SAE 1020 carbon steel demonstrated low resistivity in an aqueous medium of 0.5 mol L⁻¹ H₂SO₄ and 2 ppm HF at 80 °C, as it showed lower corrosion potential compared to a Ni-Mo-Cr-P coated substrate. According to the authors, this behavior is attributed to the formation of a stable passive film on the coated metal surface, enhancing its corrosion resistance²³.

Berbel et al.²⁴ also verified the effectiveness of cerium-based coatings on SAE 1010 carbon steel, reporting higher corrosion potentials through OCP curves for the coated electrode, which conferred greater corrosion resistance to the material. Notably, the cerium-based electrode in their study exhibited potentials ranging between −0.25 V and −0.40 V. In contrast, the present study observed more positive potentials, ranging from −0.1 V to 0.2 V, for the electrodes coated and calcined at 450, 500, 550, and 600 °C. This suggests that the use of an ionic liquid as a solvent may play a significant role in enhancing the formation of protective layers, contributing to the improved corrosion resistance observed in this work.

The potentiodynamic polarization curves presented in Figure 4 indicate that, aside from the uncoated substrate, the samples exhibit predominantly active corrosion behavior, lacking the formation of a stable passive layer. This is in agreement with the findings reported by Ananthkumar et al.²⁵, who observed similar active-passive transitions in coated reinforcement steels. In their study, heterostructured carbon-based coatings significantly reduced the corrosion rate by acting as a barrier layer, impeding the diffusion of aggressive ions and water molecules to the underlying steel surface. The improved performance was attributed to the presence of both crystalline and amorphous carbon phases, which synergistically enhanced the coating’s compactness and adhesion.

Figure 4
Potentiodynamic polarization curves of uncoated SAE 1020 carbon steel and samples coated with cerium oxide layers prepared at different calcination temperatures. Measurements were performed in a 3.5% NaCl solution at room temperature (25 °C).

The high corrosion resistance of the coating calcined at 550 °C is demonstrated by the significant shift of polarization curves toward more positive corrosion potentials and reduced current densities. As summarized in Table 2, the cerium oxide-coated electrode treated at this temperature exhibited the highest corrosion potential and the lowest corrosion current among all tested conditions. These findings are consistent with the mechanism described by Chanda et al.²³, who associated the enhanced corrosion resistance of coated SAE 1020 carbon steel with the formation of a stable, adherent, and passivating oxide film on the metal surface. In the present study, this improvement can be attributed to the formation of a cerium-based protective layer. During redox reactions, the local increase in pH promotes the migration of Ce³⁺ ions toward cathodic regions, where they react with hydroxide ions to form insoluble cerium oxide and/or hydroxide species. These compounds serve as an effective barrier, unlike iron oxides or hydroxides, which are permeable to aggressive species such as chloride ions.

Table 2
Corrosion potentials (E_corr) and corrosion current density (I_corr (A cm^-2)) of uncoated SAE 1020 carbon steel and samples coated with CeO₂ layers calcined at different temperatures, obtained from potentiodynamic polarization measurements in 3.5% NaCl solution at 25 °C.

Condition	Ecorr(VSCE)	Icorr(A cm-2)
Bare	-0.503	1.43 × 10-5
400 °C	-0.491	11.60 × 10-5
450 °C	-0.300	2.16 × 10-5
500 °C	-0.125	1.59 × 10-5
550 °C	-0.030	0.61 × 10-5
600 °C	-0.265	2.03 × 10-5

The surface morphological characterization of the cerium oxide layers supports the electrochemical results. 3D AFM measurements (Figure 2) reveal that the coating calcined at 550 °C exhibits a smooth, uniform, and crack-free surface, suggesting the formation of an efficient corrosion protection layer. In contrast, the layers prepared at other temperatures presented heterogeneous surfaces with cracks and fissures, which could act as preferential sites for corrosive attack, thus allowing the solution to reach the underlying surface of the SAE 1020 carbon steel. Furthermore, SEM images (Figure 1) confirm that the layer calcined at 550 °C was more homogeneous, adherent, and compact, likely reducing the permeation of the Cl⁻ ion to the metal surface.

Carvalho et al.¹⁴ previously highlighted the low corrosion resistance of SAE 1020 carbon steel, compared to electrodes coated with cerium oxide using the sol-gel method and calcined at different temperatures (200, 300, 400, and 500 °C), showing a lower corrosion potential of −500 mV. Similarly, Detlinger et al.²⁶ also reported the high corrosion resistance of SAE 1020 carbon steel coated with niobium oxide using the Pechini method and calcined at 450 °C. Their findings demonstrated a higher potential and lower corrosion current density, specifically, − 0.65 ± 0.05 V and 11.6 µA cm⁻², for the coated electrode. As shown in Table 2, this aligns with the results of the present study, where carbon steel coated and calcined at 550 °C exhibited the highest potential (0.030 V) and the lowest corrosion current (0.61 A cm⁻² × 10⁻⁵). This further underscores the effectiveness of using an ionic liquid as a solvent in enhancing corrosion protection.

Moreover, the superior corrosion resistance observed for carbon steel coated and calcined at 550 °C may be attributed to the formation of a more stable and compact oxide layer, possibly consisting of cerium oxides and/or insoluble cerium hydroxides that precipitate in high-pH regions. This effect is further supported by the enhanced precipitation of the Fe₃C (cementite) phase and the increased retention of Fe²⁺ ions within these particles, both of which contribute to localized pH shifts and promote the deposition of protective cerium oxide and/or hydroxides.

This interpretation aligns with recent work by Gupta et al.²⁷, who investigated the corrosion resistance of low-alloy steel subjected to tempering at various temperatures. Their results indicated that tempering at 600 °C yielded improved corrosion resistance, which was attributed to an increase in the Fe₃C phase and enhanced Fe²⁺ retention. The presence of these features facilitated a local pH increase and favored the precipitation of a compact FeCO₃ layer on the steel surface, acting as an effective corrosion barrier.

Therefore, the improved electrochemical performance observed at 550 °C in our study may stem from similar physicochemical processes. The formation of a homogeneous and adherent oxide film, combined with microstructural modifications induced by thermal treatment, likely contributes to the enhanced impedance and lower corrosion current density observed in the EIS and polarization data. This supports the hypothesis that 550 °C represents an optimal calcination temperature for promoting the development of corrosion-resistant coatings on carbon steel substrates.

However, it is important to note that increasing the calcination temperature to 600 °C led to a rise in the corrosion current density. Ghasemi et al.²⁸ reported that elevating the calcination temperature during sol–gel processing reduced the corrosion resistance of 316L stainless steel, a phenomenon attributed to the crystallization of CeO₂. The formation of crystalline CeO₂ can induce structural defects such as microcracks, which facilitate the ingress of corrosive agents and thus accelerate degradation.

Additionally, the reduced corrosion resistance observed at 600 °C for the coated carbon steel may be associated with the increased formation of cementite (Fe₃C) and the potential for galvanic coupling between the cerium-based oxide/hydroxide coating and the cementite phase. As calcination temperature increases, the extent of cementite crystallization tends to rise, potentially introducing internal or residual stresses due to mismatched thermal expansion. These residual stresses can promote microcrack formation, compromising the integrity of the protective layer and acting as initiation sites for localized corrosion. Therefore, optimizing the balance between phase composition, processing temperature, and resulting microstructure is critical to achieving effective corrosion protection in aggressive environments²⁹,³⁰.

3.4. Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) measurements were carried out on both the uncoated substrate and samples with coatings calcined at various temperatures. Consistent with previous electrochemical tests, the coating calcined at 550 °C exhibited the best corrosion protection, as shown in the Nyquist diagram (Figure 5). In a Nyquist plot, the corrosive behavior of a system is linked to charge transfer processes, characterized by the shape of the capacitive arcs. A larger semicircle diameter on the real impedance axis (Z') indicates greater corrosion resistance.

Figure 5
Nyquist plots obtained by electrochemical impedance spectroscopy for uncoated SAE 1020 carbon steel and samples coated with cerium oxide layers prepared at 400, 450, 500, 550, and 600 °C. Measurements were conducted in a 3.5% NaCl solution at room temperature (25 °C). Inset: enlarged view of the high-frequency region to highlight differences in charge transfer resistance.

Table 3 lists the electrolyte resistance (R_e) and polarization resistance (R_p) values, revealing that the carbon steel coated and calcined at 550 °C achieved the highest resistance, thus demonstrating the best corrosion protection performance. This finding is consistent with the SEM and AFM analyses (Figures 1 and 2), where the 550 °C coating showed the formation of a uniform, protective layer on the SAE 1020 carbon steel surface. Additionally, compared to the uncoated metal, the R_p values for the coatings calcined at 450, 500, and 600 °C were all higher, confirming the effectiveness of cerium-based coatings in enhancing polarization resistance. However, the coating calcined at 400 °C exhibited the lowest R_e and R_p values, highlighting the inefficacy of low calcination temperatures in forming protective layers using the ionic liquid method.

Table 3
Electrolyte resistance (Re) and polarization resistance (Rp) values for uncoated SAE 1020 carbon steel and samples coated with CeO₂ layers calcined at different temperatures, obtained from Nyquist plots in 3.5% NaCl solution at 25 °C.

Condition	Re (Ω cm2)	Rp (Ω cm2)
Bare	1.64 x 101	0.39 x 103
400 °C	1.01 x 101	0.28 x 103
450 °C	1.03 x 101	0.80 x 103
500 °C	3.46 x 101	1.79 x 103
550 °C	28.74 x 101	28.80 x 103
600 °C	14.28 x 101	1.33 x 103

Dastgheib et al.³¹, who investigated the corrosion resistance of carbon steel (0.06% C) coated with varying concentrations of cerium nitrate in a 3.5% NaCl medium, observed an electrolyte resistance between 4 and 6 Ω cm² for coated electrodes, as measured by electrochemical impedance. In comparison, the present study (Table 3) shows significantly higher electrolyte resistance, with a value of 28.74 × 10¹ Ω cm² for the electrode coated and calcined at 550 °C. This substantial difference underscores the enhanced effectiveness of the cerium-based coating and the ionic liquid used as a solvent in forming robust protective layers that significantly improve corrosion resistance.

The Bode plot (Figure 6a), which presents impedance modulus as a function of frequency, reveals two distinct regions for the uncoated sample and the coatings calcined at 400, 450, 500, and 600 °C. The high-frequency region (10² to 10⁴ Hz) corresponds to the electrolyte response, while the low- to mid-frequency region reflects the capacitive behavior of the passive film. Conversely, the coating calcined at 550 °C displayed a linear trend across the entire frequency range. This trend aligns with the Nyquist plot findings (Figure 5), where the 550 °C coating demonstrated the highest impedance modulus values, confirming its superior corrosion resistance. The coatings calcined at 500 and 600 °C followed, while those calcined at 400 and 450 °C showed impedance modulus values similar to the uncoated substrate.

Figure 6
Bode diagrams obtained from electrochemical impedance spectroscopy for uncoated SAE 1020 carbon steel and samples coated with cerium oxide layers thermally treated at 400, 450, 500, 550, and 600 °C: (a) Logarithmic impedance modulus as a function of logarithmic frequency, and (b) phase angle (degrees) versus logarithmic frequency. All measurements were conducted in a 3.5% NaCl solution at room temperature (25 °C).

This observation is consistent with reports in the literature on the low corrosion resistance of SAE 1020 carbon steel in a 3.5% NaCl medium, which has the lowest impedance modulus compared to steel coated with hydrophobic films²⁵. For instance, Akhtar et al.³² reported that the impedance modulus of an SAE 1020 carbon steel substrate was approximately 3 Ω.cm², nearly 3 times lower than that of a superhydrophobic coating. This finding supports the results of this study, where the uncoated electrode (Figure 6a) shows an impedance modulus of approximately 2.5 Ω cm², while the coating calcined at 550 °C, with higher corrosion resistance reached an impedance modulus of 4.0 Ω.cm².

Finally, in the phase angle plot (Figure 6b), the cerium-based coating calcined at 550 °C achieved the highest capacitive angle in both the low- and high-frequency regions, demonstrating superior corrosion protection among all the electrodes. The uncoated carbon steel substrate displayed a broad region with a single time constant corresponding to the metal’s corrosion process.

Regarding the corrosion behavior at different immersion times, the Nyquist plot in Figure 7a shows the capacitive arc behavior of uncoated SAE 1020 carbon steel at various immersion times in a 3.5% NaCl solution at room temperature (25 °C). Lower impedance values were observed during the first 24 hours of immersion compared to the initial measurement, indicating accelerated corrosion. After this period, the impedance values stabilized, possibly due to the formation of oxide layers on the electrode surface, which temporarily block aggressive ions. However, the corrosion process resumed by the 6^th day of immersion, as evidenced by a decrease in the capacitive arc. By the 30^th day, the polarization resistance and capacitive arc values increased again (Table 4 and Figure 7a), approximating the initial impedance, thus suggesting a new passivation stage.

Figure 7
(a) Nyquist plot of uncoated SAE 1020 carbon steel after different immersion times; (b) Nyquist plot of samples coated with cerium oxide layers calcined at 550 °C, also evaluated at varying immersion durations. All measurements were conducted in a 3.5% NaCl solution at room temperature (25 °C). Inset: magnified view of the high-frequency region to facilitate comparison of charge transfer resistance over time.

Table 4
Electrolyte resistance (Re) and polarization resistance (Rp) of uncoated SAE 1020 carbon steel after various immersion times in a 3.5% NaCl solution at room temperature (25 °C), determined from EIS measurements.

Immersion Time	Re (Ω cm2)	Rp (Ω cm2)
1ª Impedance	1.64 × 101	3.93 × 102
1 day	0.59 × 101	2.86 × 102
6 days	0.39 × 101	2.02 × 102
21 days	0.78 × 101	2.58 × 102
30 days	0.27 × 101	3.85 × 102

In contrast, the electrode coated with cerium oxide and calcined at 550 °C, prepared using the ionic liquid method (Figure 7b), exhibited different behaviors compared to the uncoated electrode. Initially, during the first 10 days of immersion, there was a noticeable reduction in the capacitive arc, likely due to the dissolution of poorly adhered parts of the coating or a corrosive attack on the metal surface. This decrease in impedance continued at a slower rate over the next 30 days, indicating a significant slowdown in the dissolution or corrosion process.

However, after 14 days of immersion, the capacitive arc magnitude increased and remained approximately constant for the following seven days. Between the 22^nd and 30^th day of immersion, a further increase in impedance values was observed, clearly indicating a process of recovery or stabilization of the coating. According to the literature, cerium in solution tends to remain or oxidize to Ce⁴⁺ in the presence of atmospheric oxygen, condensed moisture, and alkaline conditions. In addition, as seen in Equations 8 and 9, the Ce³⁺ ion can be converted to Ce⁴⁺ by the precipitation of cerium hydroxide in cathodic regions due to the increase in pH in the vicinity of surface defects. Thus, as previously mentioned, the presence of insoluble cerium oxides and/or hydroxides has an inhibiting effect against corrosion¹³.

It should also be noted that Ce⁴⁺ from an inorganic coating can be released into solution and encounter reducing conditions, such as those present on an exposed metal surface due to corrosion, and be reduced to Ce³⁺, with low solubility. As a result, hydrated cerium oxide precipitates as a protective layer that inhibits further corrosion¹³,³³-³⁵. This suggests the formation of an insoluble deposit on the corroded surface after the 22^nd and 30^th day of immersion due to the release of Ce from the layer and the precipitation of protective cerium hydroxides and/or oxides, which corroborates with the self-regeneration mechanism presented by Jian et al.³⁶. According to Buchheit et al.³³, coatings composed of mixed Ce³⁺ and Ce⁴⁺ exhibit greater corrosion resistance, particularly in adverse conditions such as exposure to salt spray. This ability to form stable protective layers is critical to improving the durability of metal surfaces in aggressive environments.

The corrosive behavior of the system can be quantitatively assessed using the R_p values derived from the impedance curves. Table 5 shows that the initial impedance measurement yielded the highest polarization resistance value for the electrode coated with cerium oxide and calcined at 550 °C. However, after 30 days of immersion, the R_p value increased again, likely due to the regeneration of the coating and the stagnation of the corrosive process.

Table 5
Electrolyte resistance (Re) and polarization resistance (Rp) of SAE 1020 carbon steel coated with CeO₂ and calcined at 550 °C after different immersion times in a 3.5% NaCl solution at 25 °C, as determined by EIS data.

Immersion Time	Re (Ω cm2)	Rp (Ω cm2)
1ª Impedance	28.74 × 101	28.78 × 103
14 days	2.50 × 101	1.68 × 103
30 days	0.39 × 101	3.03 × 103

In line with these observations, Thiruvoth and Ananthkumar³⁷ demonstrated the protective effect of cerium oxide nanoparticles in epoxy-based coatings after prolonged exposure to saline environments. Their results showed that cerium oxide contributes to corrosion resistance through the formation of a dense, adherent, and homogenous protective film that effectively blocks chloride ion penetration. Additionally, their study highlighted the reduced solubility of iron in the presence of cerium-modified coatings, reinforcing the idea that cerium oxides play a dual role as physical barriers and chemical inhibitors. These findings are consistent with our results, particularly in the case of samples calcined at 550 °C, where improved film integrity and higher impedance values were observed.

When comparing the impedance values of the coated and uncoated SAE 1020 carbon steel electrodes, Figure 8 reveals that initially, the electrode with the cerium oxide protective layer calcined at 550 °C exhibited signs of corrosion. However, after 10 days in the 3.5% NaCl solution, a passivation process commenced, halting the progression of the corrosive attack, due to the formation of cerium oxide and/or hydroxide, as explained previously. Importantly, at all immersion times studied, the coated SAE 1020 carbon steel demonstrated higher impedance values than the uncoated substrate, indicating superior corrosion resistance for the coated sample.

Figure 8
Impedance modulus values for uncoated SAE 1020 carbon steel and samples coated with cerium oxide layers calcined at 550 °C, evaluated as a function of immersion time in a 3.5% NaCl solution at room temperature (25 °C).

4. Conclusion

This study investigated the corrosion behavior of SAE 1020 carbon steel coated with cerium oxide (CeO₂) layers synthesized via thermal decomposition of cerium chloride heptahydrate in the ionic liquid 1-methylimidazolium hydrogen sulfate, followed by calcination at temperatures ranging from 400 to 600 °C. Electrochemical analyses, including open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy, alongside morphological characterizations, demonstrated that both the calcination temperature and resulting coating morphology play critical roles in enhancing corrosion resistance.

Among the tested conditions, the coating calcined at 550 °C exhibited the most favorable electrochemical performance, with increased corrosion potential, reduced current density, and higher impedance values, both in initial and prolonged immersion in 3.5% NaCl solution at room temperature. These results confirm the superior protective behavior associated with the formation of a dense, adherent, and homogeneous CeO₂ layer. Based on these findings, CeO₂-based coatings synthesized using an ionic liquid and calcined at 550 °C are recommended for applications requiring high corrosion resistance for SAE 1020 carbon steel. The 550 °C calcination temperature optimizes the morphology and enhances the corrosion protection of the material in saline environments.

The use of ionic liquids as non-toxic solvents in the oxide deposition process represents a sustainable and innovative strategy, especially when combined with rare earth salts such as cerium chloride. This approach not only replaces environmentally hazardous chromates but also aligns with green chemistry principles by offering low environmental impact and high-performance corrosion protection. Future work should focus on evaluating the long-term durability of these coatings under real-world conditions, including salt spray exposure and industrial field testing, to fully validate their applicability in corrosion-prone environments.

5. Acknowledgments

The authors thank the Brazilian National Counsel of Technological and Scientific Development - CNPq (grants: 305287/2022-2, and 307866/2022-0), the Coordination for the Improvement of Higher Education Personnel - CAPES (grant: 001), and the Sergipe State Research and Technological Innovation Foundation (FAPITEC) for the scholarships and financial support for this research.

• Data Availability
  The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Edited by

• Associate Editor:
  José Daniel Biasoli de Mello.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Publication Dates

• Publication in this collection
  04 Aug 2025
• Date of issue
  2025

History

• Received
  08 Oct 2024
• Reviewed
  12 Apr 2025
• Accepted
  23 June 2025