The Effect of CeO₂ Doping on the Prevention of the Corrosion of Montmorillonite on Mild Steel in Hydrochloric Acid Solution

Abstract

This study examined the ability of a coating made from nano-CeO₂-doped montmorillonite (NCM) nanoclay to inhibit corrosion on carbon steel when immersed in a 1 M HCl solution. The coating was produced by combining CeO₂ nanoparticles with montmorillonite nanoclay, and its characteristics were analyzed using SEM and XRD techniques. The corrosion inhibition effects were assessed through weight loss and potentiodynamic polarization (PDP) methods. The findings indicated that the NCM nanoclay serves as an effective inhibitor, exhibiting a mixed-type behavior that impedes both the anodic and cathodic reactions on the steel surface in an acidic environment. The investigation demonstrated that the NCM coating achieved remarkable inhibition efficiencies of 95% (using the weight loss method) and 99% (using the PDP method) in the acidic solution. SEM was utilized to capture images of the surface at various phases of the corrosion inhibition process for mild steel. XRD was employed to analyze the structural properties of the coating’s nanoparticles. This modified and eco-friendly NCM nanoclay has enhanced the corrosion resistance of mild steel in acidic environments.

1. Introduction

Mild steel is an alloy made primarily of carbon and iron, containing up to 0.35% carbon. It also includes trace amounts of other elements like manganese, copper, zinc, and chromium. Due to its low cost and ease of manipulation, mild steel is widely used in various industrial and household applications [1,2,3,4,5,6,7,8]. Surface corrosion and deterioration pose significant and costly challenges for mild steel used in corrosive environments. The worldwide expense related to corrosion is estimated to exceed USD 3 trillion, which accounts for roughly 3% of the global GDP of USD 105.4 trillion [6]. These statistics highlight the urgent need for innovative and effective methods to combat metal corrosion.

For the purpose of cleaning industrial machinery, hydrochloric acid (HCl) is frequently utilized as one of the primary solutions [9]. Corrosion-resistant materials, or corrosion inhibitors, are commonly applied to safeguard metal surfaces from deterioration [1,2,3,4,5,6,7,8,9]. Nevertheless, these substances can be detrimental and costly due to their chemical properties. Organic corrosion inhibitors are prevalent in steel protection; however, they tend to have issues with stability and toxicity [10]. Natural and eco-friendly corrosion inhibitors are employed in steel protection [11]. Extensive research is currently being conducted to identify the most effective corrosion inhibitor for a variety of industrial uses, particularly in acidic environments [12,13,14,15,16,17].

Investigators have explored the corrosion prevention capabilities of various coatings applied to steel surfaces. For instance, sodium lignosulfonate in hydrochloric acid has been analyzed using three distinct monitoring methods: weight loss assessments, electrical conductivity measurements, and potentiodynamic polarization studies [18,19,20]. Numerous researchers have examined the corrosion resistance of steel and other metals in different environments, including hydrochloric acid [21,22,23,24,25,26,27,28,29]. The inhibition mechanism is influenced by several factors, such as the type of metal, the nature and concentration of the medium, the characteristics of the inhibitor, and the temperature [30]. Natural clays are cost-effective materials that possess excellent mechanical properties and nearly negligible toxicity levels. Montmorillonite is a type of silicate mineral characterized by a typical TOT layered structure, featuring an interlaminar space within the TOT layers that is highly effective for adsorption and various other properties. A review has been conducted on the recent developments in the organic modification process of montmorillonite composites, examining how the adsorption on various modifiers on montmorillonite takes place, the structural characteristics, and its gel performance along with the mechanisms of influence [31].

Studies have shown that the natural montmorillonite clay can effectively protect against corrosion when used as a coating, and the incorporation of clay nanolayers enhances the corrosion resistance of the coating [31,32]. Farahi et al. presented a novel method for enhancing a coating formulation by including sodium montmorillonite to bolster the corrosion resistance properties of carbon steel exposed to HCl [33]. AlShamaileh et al. investigated the anticorrosive properties of a coating that incorporates modified montmorillonite nanoclay as an inorganic green inhibitor for mild steel in a 1.0 M HCl solution. Their findings demonstrated that montmorillonite nanoclay serves as an effective mixed-type inhibitor for steel in acidic solutions. Additionally, montmorillonite nanoclay has been shown to be an eco-friendly substance that enhances the corrosion resistance of mild steel in acidic environments [34]. Messinese et al. conducted a thorough study on how surface finishing affects the localized corrosion resistance of stainless steel in chloride-rich conditions [35]. Tambovskiy et al. outlined a technique for safeguarding low-carbon steel utilizing potentiodynamic polarization curves in a chloride solution [36]. Howyan et al. researched the efficiency of protecting carbon steel through a clay-based coating and achieved a protective efficiency of 81.4% based on electrochemical assessments [37].

Cerium oxide nanoparticles (CeO₂ NPs) have garnered significant interest due to their remarkable catalytic properties. The cerium atom can rapidly and significantly adjust its electronic structure to adapt to its surrounding environment [38]. Various physical and chemical approaches for the synthesis of CeO₂ NPs have been documented in prior research [39]. There is a lack of research in which the catalytic properties of rare earth metals, such as cerium compounds, are utilized in corrosion inhibition processes. This fact makes it challenging to find better inhibitors for metal surfaces. Therefore, we proposed a CeO₂-doped montmorillonite and hoped that it will provide a novel solution to this issue. Furthermore, doping with CeO₂ could benefit the adsorption process and hence improve the inhibition process.

A coating doped with CeO₂ has nanoparticles with cerium in the +4 oxidation state. In a corrosion process, the charged nanoparticles are absorbed on the cathode in the electric field and resist the corrosion. The nanoparticle inhibitors have excellent corrosion-inhibiting properties due to their large surface-to-volume ratio which increases the blocking surface active sites. Also, the thermal stability and hardness are improved.

In this research, CeO₂ NPs were incorporated into modified natural clay montmorillonite (MMT) to create a coating aimed at protecting the surface of mild steel in an acidic medium (1 M HCl). The anticorrosion effectiveness is assessed through weight loss and electrochemical methods, specifically potentiodynamic polarization. Scanning electron microscopy (SEM) is employed to analyze the surfaces that were tested. X-ray diffraction (XRD) is utilized to investigate the structural properties of the CeO₂–montmorillonite nanoclay.

2. Materials and Methods

2.1. Materials

The modified montmorillonite (MMT) utilized in this research and the nitrate hexahydrate were sourced from Aldrich, Steinheim, Germany (surface-modified montmorillonite nanoclay containing 35–45 wt% dimethyl dialkyl (C14-C18) amine, SKU: 682624-500G, with a particle size of ≤20 µm). All other reagents (NaOH and HCl) were acquired from local suppliers, ensuring the highest available purity (analytical grade). Mild steel sheets, each 2 mm thick, were obtained locally from Jordan Steel Group in Amman, Jordan, and were cut into squares measuring 2 cm × 2 cm for weight loss assessments. The composition of the mild steel by weight is 0.15% C and 99.7% Fe, along with small amounts of Ni, Cr, Si, and Mn. Weighing was performed using a digital micro-balance (Model SEJ205, company, Taipei, Taiwan). For the potentiodynamic polarization experiments, the area of the steel surface exposed (working electrode) was set to approximately 2.6 cm². The electrodes were polished using fine-grade emery paper (800), degreased with acetone, rinsed with distilled water, and then dried prior to each experiment. Distilled water was utilized to prepare all 1 M HCl solutions.

2.2. NCM Coating Preparation

CeO₂ nanoparticles were synthesized through the homogeneous precipitation of a cerium nitrate hexahydrate solution combined with dilute aqueous ammonia, resulting in a yellow solution. The obtained product was subsequently washed with ethanol and distilled water. It was then dried at 90 °C for 24 h before being calcined for 6 h at 650 °C in an air atmosphere to yield the desired CeO₂ nanoparticles. In order to create the nano-CeO₂-doped montmorillonite (NCM) inhibitor, 5.0 g of MMT and 0.25 g of CeO₂ nanoparticles were mixed in ethanol for 30 min at room temperature to achieve a uniform mixture. The resulting pale yellow mixture was washed with distilled water and then dried in a vacuum oven at 80 °C for 24 h. The prepared NCM solution was used to coat a cleaned steel substrate by immersing it for 24 h, followed by thorough washing with distilled water and drying at 80 °C for approximately 2 h.

2.3. Weight Loss Measurements

For the measurements related to weight loss, steel samples were ground, cleaned, and dried using a nitrogen stream, followed by precise weighing on a 5-digit analytical balance. Each sample was then completely submerged in a 50 mL beaker filled with a 1 M HCl solution. The beakers were maintained in a water bath at room temperature. After one day and one week of immersion, the steel samples were taken out, rinsed with distilled water, dried, and weighed again. A minimum of three steel samples was utilized to obtain an average weight loss value. This procedure was also conducted with the samples coated in NCM.

2.4. Electrochemical Measurements

Electrochemical experiments were conducted using a VoltaLab PGZ 100 potentiostat (Radiometer S.A.S, Neuilly-Plaisance, France) within a three-electrode glass cell. Before the measurements, the surface of the auxiliary electrode (carbon) was meticulously polished using alumina slurry, thoroughly rinsed multiple times with distilled water, and subsequently sonicated for approximately 10 min. All potential values are referenced against the saturated calomel electrode (SCE), and all measurements took place at ambient temperature. The working electrode comprised both bare and coated steel samples. The samples were immersed in the testing solution for several minutes until a stable open circuit potential (OCP) was established. The polarization curves were recorded from a cathodic potential of −100 mV to an anodic potential of +100 mV, relative to the open circuit potential (OCP) at a scan rate of 10 mV/s. The resulting Tafel plots were analyzed through extrapolation to determine the corrosion potential (Ecorr) and the corrosion current densities (Icorr). The experiments were repeated to ensure the data’s reproducibility.

2.5. XRD Measurements

XRD analysis was conducted on MMT, CeO₂, and CeO₂-doped MMT samples using a Malvern Panalytical Aeris instrument (Cu kα1, 0.15406 nm, 0.01 step angle, Malvern Panalytical, Almelo, The Netherlands). The analysis of Ce-MMT involved comparing the peak positions and intensities with spectra from the XRD diffraction data library. The diffraction patterns were recorded in the 2θ range of 4–60°.

2.6. SEM Investigation

SEM examinations were carried out to examine the various stages of corrosion inhibition in mild steel samples, both with and without coatings. The surface morphology of the steel samples was captured prior to and following treatment with NCM nanoclay and MMT alone. The SEM of the Inspect F50-FEI Company, based in Eindhoven, the Netherlands, was utilized for microstructural analysis. The prepared specimens were thoroughly washed with distilled water and alcohol, dried, and then mounted onto aluminum stubs using a double-adhesive carbon sticker before the SEM analysis. An as-received mild steel specimen, approximately 1 cm² in area, was ground using emery paper of various grades from 200 to 1200, ensuring thorough washing between each grinding step. Appropriate diamond pastes were used for polishing the specimen prior to the SEM measurements. The same procedure was followed for the coated steel surfaces.

3. Results and Discussion

3.1. Weight Loss Study

Table 1. Inhibition efficiency percentage of MMT and Ce-MMT from weight loss experiments for mild steel in 1 M HCl. The time of immersion was 1 day or 1 week. The surface area was approximately 9.0 cm².

Parameter/Sample	Clean Steel	MMT-Coated Steel	Ce-MMT-Coated Steel
Initial Mass (g)	6.1054	6.0441	6.0832
Mass after immersion for 1 day (g)	6.0519	6.0392	6.0806
Mass change after 1 day g	0.0535	0.0049	0.0026
Corrosion rate (mm/Y)	0.00259036	0.000237248	0.000125887
Mass after immersion for 1 week (g)	5.8027	5.9644	6.0583
Mass change after 1 week g	0.3027	0.0797	0.0249
Corrosion rate (mm/Y)	0.01465613	0.003858916	0.001205609
1 day inhibition efficiency %	-	90.84	95.14
1 week inhibition efficiency %	-	73.67	91.77

displays the outcomes of the weight loss experiments conducted on all samples tested. The inhibition efficiency percentage of MMT and Ce-MMT for mild steel in 1 M HCl from the weight loss experiments is summarized. An immersion period of either 1 day or 1 week was selected for the tests. The surface area of the specimens was approximately 9.0 cm², which is regarded as a suitable exposure area. The results clearly indicate that the NCM coating considerably reduces mild steel corrosion by about 95% after 1 day of exposure, outperforming MMT alone, which achieved around 90%. To evaluate the durability of the coatings, an experiment was conducted over the course of one week. The results for inhibition efficiency exhibited a consistent trend with a high value of approximately 92%. The MMT coating demonstrated a significant reduction in inhibition efficiency after one week of exposure, dropping to around 74%. This phenomenon is linked to the effect of cerium doping concerning the adsorption process. The inhibition efficiency was determined using the formula IE = (m1 − m2/m1) × 100%, where m1 and m2 represent the mass loss without and with the inhibitor, respectively. The corrosion rate (CR) was calculated using the equation CR = 87.6 m/dAt in mm/y, where m is the mass loss in milligrams, d is the density (7.87 g/cm³), A is the surface area of the specimen in cm², and t is the elapsed time in hours. The weight loss results can be indicative of the long-term stability of the coating, which is important in many applications. Here, we demonstrated, as a proof of concept, that doping improves the efficiency for a whole week. We assume that an extrapolation is valid but needs confirmation in another dedicated project.

The potentiodynamic polarization curves for mild steel, both uncoated and coated with various inhibitors, after being immersed for 1 day in a 1 M HCl solution are illustrated in Figure 1. It is evident that the application of the coatings impedes both cathodic and anodic reactions across all samples. The polarization curves were obtained for the untreated steel sample (clean Fe) during acid immersion (black curve in Figure 1), for the MMT-coated steel (red curve), and for the Ce-MMT-coated steel (blue curve). Key electrochemical parameters, including corrosion current density (Icorr), corrosion potential (Ecorr), and Tafel constants βa and βc, along with the inhibition efficiency percentage, were derived from the Tafel plots (

Table 2. Electrochemical corrosion parameters of steel in 1 M HCl before and after immersion for 1 day for coated and uncoated steel samples.

	Mild Steel	MMt-Fe	Ce-MMT-Fe
Atomic mass (g/mol)	55.8	55.8	55.8
Density (g/cm3)	7.8	7.8	7.8
E(I = 0) (mV)	−625.0	−765.9	−785.8
Corrosion Current (Icorr)	94.2154 µA/cm2	5.4056 µA/cm2	14.9637 nA/cm2
Rp	61.62 ohm.cm2	856.41 ohm.cm2	286.42 kohm.cm2
Beta anodic (mV)	29.5	26.8	23.3
Beta cathodic (mV)	−36.0	−31.2	−30.6
Coefficient	0.9949	0.9910	0.9909
Corrosion Rate	1.102 mm/Y	63.22 µm/Y	175.0 nm/Y
Corrosion Inhibition Efficiency %	-	94.26%	99.98%

). The inhibition efficiency percentage recorded for the MMT-coated Fe sample was 94.26%, while the Ce-MMT-coated Fe sample demonstrated an outstanding value of 99.98%.

The data indicate that both coatings reduce corrosion by diminishing current density while leading to lowered Ecorr values. The Tafel constants βa and βc show minor variations with different coatings, suggesting that both coatings function as mixed-type inhibitors. The inhibition efficiency percentage is calculated using the following equation [40]:

IE% = (I^ocorr − Icorr)/I^ocorr × 100

where IE% is the inhibition efficiency, (I^ocorr is the corrosion current of mild steel and Icorr is the corrosion current of the coated mild steel.

The results indicate that the incorporation of cerium significantly enhances corrosion inhibition efficiency, effectively blocking the exposed steel surface in the 1 M HCl solution.

3.2. XRD Analysis

Figure 2 displays the distinctive XRD patterns for MMT (top-black), CeO₂ (bottom-red), and the combined Ce-MMT powder (middle-blue) within the 2θ range of 4–60°. A comparison of these patterns indicates that the incorporation of CeO₂ has enhanced the crystallinity of the MMT powder. The diffraction peak observed at 26.6° 2θ shows an increase in d-spacing from 3.2 Å in MMT to 13.0 Å in the Ce-MMT (peak at 6.81° 2θ). This expansion serves as evidence of changes within the layers and suggests a mixed crystalline–amorphous structure across the majority of the angular range.

3.3. SEM Study

SEM imaging investigates the corrosion inhibition stages in mild steel samples with and without coatings both before and after exposure to corrosive HCl solution. Figure 3 displays images of untreated and coated steel surfaces prior to and following immersion in 1 M HCl. The immersion time was set to one week as the corrosion was not evident after just one day, making longer exposure more suitable for comparison. Figure 3A shows a micrograph of the clean steel surface where the grain boundaries are clearly visible, representing a typical clean iron surface. Following a week of immersion in 1 M HCl, the surface exhibited clear signs of corrosion (Figure 3B). The as-prepared MMT-coated steel surface and the Ce-MMT-coated steel surface are depicted in Figure 3C and Figure 3E, respectively. Both coated surfaces were immersed in 1 M HCl for one week, and the resultant SEM images are presented in Figure 3D,F. It is important to note that at least three samples underwent the same experiment, and the samples that were not immersed in HCl were dried and kept in a nitrogen atmosphere prior to SEM analysis. The coated steel samples demonstrated minimal corrosion compared to the untreated steel sample. In the case of the Ce-MMT-coated steel, the level of surface protection is quite pronounced (Figure 3F).

4. Conclusions

This study found that a coating composed of nano-CeO₂-doped montmorillonite (NCM) nanoclay on mild steel exhibited an impressive inhibition efficiency of 99% in acidic HCl solution as determined through electrochemical methods and of 95% via traditional weight loss approaches. Compared to the MMT-coated iron, the inhibition efficiency of the Ce-MMT-coated iron increased significantly due to the inclusion of CeO₂ nanoparticles. The synthesized coating was characterized using XRD, and the surfaces were examined with SEM both before and after exposure to the corrosive HCl medium. In general, CeO₂ acts on the steel surface by blocking its reaction to the chloride ions, and this provides corrosion inhibition.