Intelligent Smart Coatings for Enhanced Corrosion Protection in Carbon Steel ^†

Abstract

This study explores a new approach for corrosion protection of carbon steel, focusing on the application of polymeric coatings. Anticorrosive pigments were synthesized by loading 2-Mercaptobenzothiazole into zirconium oxide particles, and then an epoxy coating was applied on a steel substrate to analyze the corrosion inhibition activity. Analytical techniques like FTIR and XRD confirmed the successful loading of corrosion inhibitors onto zirconium dioxide nanoparticles (ZrO₂), revealing changes in chemical bonding and structural patterns. Scanning electron microscopy (SEM) confirmed the spheroidal morphology of ZrO₂ after inhibitor loading, while contact angle measurements showed improved hydrophobicity due to reduced porosity from the nanoparticles. Electrochemical impedance spectroscopy (EIS) showed enhanced corrosion resistance in the modified coatings compared to reference coatings, demonstrating stable impedance values and delayed electrolyte uptake. These findings suggest the potential of the developed coating system in mitigating carbon steel corrosion, offering insights for its application across various industries.

1. Introduction

Carbon steel is renowned for its affordability and strength, but it faces corrosion challenges. The management and prevention of corrosion have become focal points across various industries by employing corrosion-resistant materials, inhibitors, cathodic protection, and protective layers [1]. Traditional methods, such as organic coatings, act as passive barriers, yet their susceptibility to damage from scratches due to lack of active corrosion protection, as well as their limited durability, necessitates frequent maintenance [2]. Similarly, cathodic protection, while effective, relies on continuous power and risks uneven shielding, potentially leaving certain areas vulnerable to corrosion [3]. The deployment of intelligent coatings to shield metal surfaces has attracted the attention of many researchers. These specialized coatings are developed to deliver efficient protection against corrosion [4]. Nonetheless, it is essential to acknowledge that despite their effectiveness, smart coatings are susceptible to direct mixing of inhibitor with matrix, which can lead to an unwanted reaction and cause defects in the coating that ultimately facilitate the proliferation of corrosion [5]. To advance corrosion prevention in carbon steel, innovative strategies must offer multifaceted solutions. These advancements include robust protection mechanisms with self-repair capabilities and real-time corrosion monitoring facilitated by integrated pH-sensitive carrier/inhibitors systems to emphasize sustainability through the utilization of environmentally friendly materials [6,7]. By addressing the deficiencies of existing methods, this advanced technique of loading inhibitors into carriers aims for efficient release to ensure prolonged infrastructure durability and safety in carbon steel applications across diverse industries. Smart coatings can also contribute to reducing maintenance costs and extending the lifespan of critical assets, thus improving operational efficiency and sustainability. In addition, implementing smart coatings can minimize the environmental impact of protective coating technologies, in line with global standards for sustainable development [8]. In this study, the primary objective is to synthesize an anticorrosive pigment comprising ZrO₂/2-MBT. Furthermore, incorporating anticorrosive pigments into the self-healing epoxy is a key focus. This innovative coating comprises a self-healing epoxy layer specifically tailored for carbon steel. These pigments were thoroughly characterized by utilizing scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). Subsequently, the effectiveness of this novel coating in resisting corrosion will be rigorously evaluated through electrochemical impedance spectroscopy (EIS) tests conducted at various time intervals. Moreover, the study will assess a more sustainable approach to corrosion mitigation in carbon steel applications.

2. Experimental

2.1. Chemicals and Materials

Zirconium oxide (ZrO₂) and 2-Marcaptobenzotriazole (2-MBT) were purchased from Sigma Aldrich (St. Louis, MO, USA). The epoxy resin (EPON RESIN 815C) and the curing agent (EPIKURE) were purchased from Hexion Inc., Columbus, OH, USA, and a local source provided plain carbon steel plates that were cut into the size of 30 × 30 × 1 mm³. The chemical composition of these steel plates was as follows: p = 0.04%, S = 0.04%, Mn = 0.30%, Cu = 0.20%, C = 0.21%, and Fe = 99.18%. The steel coupons were cleaned using locally available silicon carbide emery sheets (SiC) with a grit size of 120.

2.2. Development of Anticorrosive Pigments

Zirconium oxide (ZrO₂₋₎ was used as a carrier, and 2-MBT as a corrosion inhibitor. A solution was made by adding an inhibitor (2-MBT) into 30 mL of acetone, and a 1:2 ratio of the inhibitor to the carrier was used for the loading of 2-MBT into zirconium oxide. The mixture was sonicated for 10 to 15 min to thoroughly disperse the solution and allow the inhibitor to settle down in the ZrO₂ particles. The resultant mixture was allowed to dry overnight at 70 °C while being stirred moderately on the hot plate. The resultant product was dried and loaded with zirconium dioxide nanoparticles after the surplus solvent of acetone was removed, which was ground slowly to obtain fine particles to avoid agglomeration. The loaded particles were named as anticorrosive pigment (ZrO₂/2-MBT) and will be used for the development of modified coatings.

2.3. Preparation of Carbon Steel Sample and Formulation of Coatings

The carbon steel was polished using a silicon carbide emery sheet of 120 grit size, degreased with acetone, washed with distilled water, and dried. The polishing was carried out to obtain a rough surface, which provided mechanical interlocking for a better coating adhesion to the steel sample. The coatings were formulated by incorporating 1 wt.% of the anticorrosive particles into 5 mL of epoxy resin. The epoxy resin and anticorrosive mixture were mixed slowly to avoid bubbles. Furthermore, 1 mL of curing agent was added and subjected to 5 min of sonication to ensure homogeneity.

2.4. Application of the Coatings on the Steel Sample

The polished sample was rinsed again with ethanol before applying the coating to ensure there was no contamination, and the coating formulated was poured on the sample. The coating was uniformly applied on the steel substrate using a doctor’s blade. Two types of coatings were prepared: (a) coatings without any additives, i.e., blank coatings named reference coatings; (b) coatings with an additive of anticorrosive pigments (ZrO₂/2-MBT), named modified coatings. After a competitive process, a dried coating thickness of 150 ± 5 µm was attained after a week at room temperature.

2.5. Characterization

X-ray diffraction (XRD) was conducted using a PAN analytical X’pert Pro Cu (K) with a scanning rate of 2°/min and an angle range of 10° ≤ 2θ ≤ 90°. The morphological characteristics of ZrO₂/2-MBT loaded into zirconium oxide and modified coatings were examined using a field emission scanning electron microscope (FE-SEM-Nova Nano-450, FEI, Eindhoven, Netherlands). To investigate alterations in bonding and the interaction of loaded ZrO₂/MBT with zirconium oxide, Fourier transform infrared (FTIR) spectroscopy analysis was carried out. Transmission mode spectra were captured using the FTIR Frontier (PerkinElmer, Frontier, Shelton, CT, USA) in the range of 4000–500 cm⁻¹. A contact angle goniometry machine was used to assess superhydrophobic properties. After establishing the distance, a water droplet was placed onto the coating sample, and the resulting contact angle was measured. The assessment of corrosion resistance involved conducting an electrochemical impedance spectroscopy (EIS) analysis once the coated sample had thoroughly dried. The counter electrode consisted of graphite, the reference electrode utilized Ag/AgCl, and the working electrode was coated steel. An electrolyte solution of sodium chloride with a concentration of 3.5 g per 100 mL of distilled water, corresponding to the seawater percentage, was employed. The Gamry 3000 (potentiostat/galvanostate/ZRA, Gamry, Warminster, PA, USA) facilitated the EIS analysis within the frequency range of 100 kHz to 10 mHz, proceeding from higher to lower limits. The root mean square (RMS) signal for the two-hour EIS measurement was set at 10 mV.

3. Results and Discussion

FTIR analysis was used to examine the structure and chemical bonding of both modified and unmodified ZrO₂ nanoparticles, as shown in Figure 1. The incorporation of inhibitor molecules with their characteristic functional groups is illustrated by distinct peak differences between the ZrO₂ and ZrO₂/2-MBT spectra [9]. The findings offer important insights into the compositional alterations that occur when ZrO₂ is loaded with an inhibitor. The loading of corrosion inhibitor (2-MBT) into ZrO₂ was confirmed by the presence of some new peaks in the FTIR spectra of ZrO₂/2-MBT, as shown in Figure 1, and no other significant peaks are seen for pure ZrO₂. This offers more proof that the encapsulation inside the carrier matrix was successful as the new peaks emerged due to the presence of 2-MBT. Peaks near 1500–1700 cm⁻¹ on ZrO₂ that are linked to surface hydroxyl groups (-OH) have become less intense due to their interaction with 2-MBT molecules, which may prevent water molecules from adhering to those adsorption sites [10]. New peaks added by 2-MBT can be seen between 1000 and 1500 cm⁻¹, and these distinctive 2-MBT peaks confirm the successful incorporation onto the ZrO₂ surface.

Figure 2 illustrates the XRD analysis of ZrO₂ and ZrO₂/2-MBT, which shows the crystalline nature of both samples. The XRD pattern reveals the presence of a new peak in the range of 11–25° of the loaded particles ZrO₂/2-MBT. However, new peaks emerged at 11.5°, 13.5°, 22.5°, 24.1° and 26.5° in the case of ZrO₂/2-MBT, and this is due to the presence of 2-MBT. This new XRD pattern corresponds to the JCPDS No. for MBT: 00-008-0769 [11]. The presence of 2-MBT is responsible for the observed decrease in intensity and widening of the characteristic patterns of ZrO₂, which confirms the successful loading of corrosion inhibitor 2-MBT into ZrO₂.

Figure 3 depicts the morphological structure of both the ZrO₂ and ZrO₂/MBT observed by the SEM. The ZrO₂ particles retained their spheroidal morphology; however, the size of the loaded particles (ZrO₂/2-MBT) was larger than ZrO₂ before loading, which is due to the slight presence of the inhibitor on the ZrO₂ surface. This observation confirms inhibitor loading without appreciable morphological changes, supporting the XRD and FTIR data.

Figure 4 displays the measured contact angle for both the modified and reference coatings to analyze the hydrophobic nature of the coatings. In comparison to the reference coatings (79.3°), the modified coatings exhibit a comparatively higher contact angle (83.7°), indicating an enhanced hydrophobic nature. This increase in hydrophobicity is due to the addition of anticorrosive pigments, which offer better reduced porosity and intensify the hydrophobic properties of modified coatings [12].

Electrochemical impedance spectroscopy (EIS) is the most important and valuable tool for the characterization of corrosion resistance properties of the coating systems [13]. A potentiostatic EIS test was carried out in 3.5 g per 100 mL NaCl solution for both reference and modified coatings. The EIS spectra obtained after the fitting were analyzed and depicted in Figure 5 and Figure 6 to obtain information on corrosion activity and corrosion protection performance of coatings. Bode plots for the reference coating (ZrO₂) depicted in Figure 5a show the impedance value of above 10 M Ω⋅cm² on the first day of immersion, which decreases even further by one order magnitude after 4 days of immersion due to water uptake and possible corrosion activity. The phase angle value for reference coatings dropped to −10 from −80 at the mid-frequency range. This makes the electrolyte (NaCl solution) easier to reach at active sites of the coating–steel interface, which leads to corrosion activity [14]. In contrast, modified coatings (ZrO₂/2-MBT) show a higher impedance spectra value of 1 G Ω⋅cm² at the low-frequency range in Figure 6a, which is due to the presence of ZrO₂/2-MBT. The presence of anticorrosive pigment not only increases the impedance value but also the phase angle value remains close to −80 over a wide frequency range, as shown in Figure 6b. Moreover, the impedance spectra value for modified coatings increased above 100 G Ω⋅cm² after 4 days of immersion due to the presence of corrosion inhibitor (2-MBT), which makes the coating more compact and reduces the active sites of corrosion [15]. This finding implies that the modified coating’s electrolyte uptake was delayed and that, in comparison to the reference coating, the coating barrier effect was significantly enhanced.

4. Conclusions

The development and characterization of an epoxy coating system for carbon steel, incorporating anticorrosive pigments, have shown promising results in enhancing corrosion resistance. Through comprehensive analyses, it was confirmed that the incorporation of corrosion inhibitors onto zirconium dioxide nanoparticles (ZrO₂) successfully modified the chemical bonding and structural patterns, as evidenced by FTIR and XRD analysis. SEM observations affirmed that ZrO₂ morphology remains the same after inhibitor loading, ensuring no harm to the integrity of ZrO₂ as a carrier. Moreover, contact angle measurements revealed increased hydrophobicity in the modified coatings, attributed to reduced porosity from incorporated nanoparticles. Notably, EIS analysis in a 3.5 g per 100 mL NaCl solution exhibited significantly improved corrosion resistance in the modified coatings compared to reference coatings, maintaining stable impedance values around 100 G Ω⋅cm² and delaying electrolyte uptake due to the hydrophobic nature of the coating. These outcomes collectively highlight the potential of the developed coating system to effectively mitigate corrosion in carbon steel and its applicability across various industrial settings.