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Article

Chitosan and Its Derivatives as a Barrier Anti-Corrosive Coating of 304 Stainless Steel against Corrosion in 3.5% Sodium Chloride Solution

by
Ana Alejandra Aguilar-Ruiz
1,
Reyna Guadalupe Sánchez-Duarte
1,*,
Víctor Manuel Orozco-Carmona
2,
Germán Eduardo Devora-Isiordia
1,
Yedidia Villegas-Peralta
1,* and
Jesús Álvarez-Sánchez
1
1
Departamento de Ciencias del Agua y Medio Ambiente, Instituto Tecnológico de Sonora, 5 de Febrero 818 Sur, Ciu-dad Obregón C.P. 85000, Sonora, Mexico
2
Centro de Investigación en Materiales Avanzados S.C., Ave. Miguel de Cervantes 120, Complejo Industrial, Chihuahua C.P. 31136, Chihuahua, Mexico
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(10), 1244; https://doi.org/10.3390/coatings14101244 (registering DOI)
Submission received: 25 July 2024 / Revised: 28 August 2024 / Accepted: 20 September 2024 / Published: 29 September 2024
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Abstract

:
This study investigates the corrosion resistance of chitosan and its crosslinked form coatings applied on stainless steel as substrate using various analytical techniques. Fourier transform infrared spectroscopy (FTIR-ATR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) were employed for surface characterization. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) techniques were used to analyze the electrochemical behavior. Four coatings were evaluated along with naked stainless steel (ss): chitosan (Chi), chitosan crosslinked with ammonium paratungstate (Chi/PTA), chitosan crosslinked with polyethylene glycol (Chi/PEG), and chitosan crosslinked with polyvinylpyrrolidone (Chi/PVP). Electrochemical measurement parameters analysis assessed the coating corrosion resistance, such as impedance modulus (|Z|) and corrosion potential (Ecorr). Results indicate varying degrees of corrosion resistance among the coatings. Chi/PTA exhibited notable characteristics in the electrochemical tests, showing promising polarization resistance (Rp) and impedance behavior trends. Conversely, Chi/PEG showed differing electrochemical responses, suggesting higher susceptibility to corrosion under the study conditions. These findings contribute to understanding the electrochemical performance of chitosan-based coatings on stainless steel, highlighting their potential in corrosion protection applications.

Graphical Abstract

1. Introduction

The shortage of drinking water has significant impacts worldwide. However, these impacts are even more common and severe in arid areas. Therefore, approximately 40% of the world’s population faces a water crisis, leading to the use of unsafe water sources, contributing to the increase in disease development and up to 30% of deaths worldwide [1,2]. As population growth creates this imbalance between the natural supply of water and its growing demand, the need for efficient and low-cost solutions that do not compromise the environment is urgent [3]. Since most of the water on Earth is salty and not suitable for direct human consumption, seawater desalination emerges as a viable solution to address this problem, for which various technologies have been implemented and have significantly advanced over time [4,5]. Among the most commonly used technologies to carry out the desalination process is the multi-effect evaporation desalination process; however, this procedure presents significant challenges, one of the most relevant being the corrosion of the metal structures used in the construction of the plants due to constant and prolonged exposure to high concentrations of seawater salt, leading to corrosive processes which result in material losses, decreased process quality, and efficiency in water processing, as well as high repair costs. Stainless steel is a widely used material in the construction of structures intended for desalination due to its high resistance to wear in aggressive conditions compared to other metals [6,7,8]. However, its superior wear resistance does not exempt it from the corrosive processes that metals face. Corrosion in stainless steel is usually localized by pitting and occurs in incredibly aggressive environments, such as marine environments [9,10].
Corrosion or oxidation is the destructive attack or transformation of metallic compounds due to chemical or electrochemical reactions. Different technologies have been developed to prevent this phenomenon, including protecting the exposed surface through chromatin, phosphating techniques, or using polymer coatings, such as polypropylene or paints [11]. However, the use of these techniques often involves the use of toxic compounds that significantly impact health and the environment [12,13,14,15]. To prevent corrosion, technologies are being researched and developed to effectively protect metals against this attack, which are also environmentally friendly and economical. Barrier protection is one of the most effective and simple alternatives [16,17]. This technique forms coatings based on components such as resins, oxides, salts, or polymers on the metal surface that act as a physical barrier against aggressive ions in corrosive media [18,19,20]. The barrier protection method can be defined as the degree to which a metal susceptible to corrosion can be mechanically isolated from the environment; this is a crucial strategy to prevent corrosion in stainless steel [21,22]. Organic coatings, specifically those based on chitosan, have shown good characteristics that can be used as barrier protectors to prevent corrosion. Chitosan, recognized for its biocompatibility, excellent film-forming ability, non-toxic behavior, and chelating activity, offers a promising alternative to synthetic polymers [23,24,25,26]. Thanks to the functional groups of chitosan (hydroxyl and amine groups), it can adhere to negatively charged surfaces and spontaneously adsorb on metal surfaces and form complexes with metal ions and gels with polyanions, which makes these chitosan-based barriers attractive for applications where active protection against corrosion is required, in addition to regard for the environment since chitosan is a renewable resource. On the other hand, the main disadvantage of barrier protection is the delamination process; however, the exact mechanism of delamination is not fully understood, so its study has captured researchers’ interest in corrosion and its prevention [18,27,28,29]. Moreover, the study of the metal–polymer interface has been recognized as a crucial aspect to ensure the success of barrier techniques since the predominant reactions are electrochemical [21,30,31]. The exchange of electrons between anodic and cathodic reactions produces an electronic current flow through the metal interface, known as corrosion potential [32]. Electrochemical techniques such as linear polarization, polarization resistance, and potentiodynamic polarization have been successfully used to investigate the fundamental aspects of corrosive processes, the effects of alloying elements, and reaction kinetics. However, researchers point out that it is crucial to recognize the limitations of direct current techniques, such as the influence of ohmic resistance. For this reason, electrochemical impedance spectroscopy (EIS) has gained ground as a more appropriate alternative for studying corrosion [11,33]. The use of electrochemical impedance techniques is essential to determine the effectiveness of coatings as they provide detailed information on the resistance of the protective film against the infiltration of corrosive agents, which is crucial to evaluate its corrosion prevention capacity. Consequently, the use of chitosan-based coatings to combat corrosion in metal structures of solar desalination plants emerges as a promising alternative whose effectiveness can be enhanced by incorporating crosslinking agents and corrosion inhibitors, thus contributing to the safety, efficiency, and sustainability of installations constructed from metal structures. Therefore, the present work aims to study and evaluate the anticorrosive performance of chitosan-based coatings, as well as their crosslinking with ammonium paratungstate (PTA), polyethylene glycol (PEG), and polyvinylpyrrolidone (PVP) applied through the sol–gel technique for their future application in the structure of metal surfaces of solar desalination plants. The thickness of the coatings was measured, and the effectiveness of the coatings was evaluated through electrochemical impedance spectroscopy (EIS) studies, potentiodynamic polarization studies (PDP), and scanning electron microscopy (SEM-EDS). Results obtained for the different types of chitosan-based coatings and their modifications were analyzed to determine the performance of the coatings in terms of corrosion inhibition efficiency. In contrast to previous research where modifications such as glutaraldehyde have been used to enhance anticorrosive protection, this study highlights that combining chitosan with PTA offers additional benefits. It demonstrates increased charge transfer impedance, indicating a better barrier against corrosion in aggressive environments.

2. Materials and Methods

2.1. Reagents

This study used 5 cm × 5 cm plates of 304 stainless steel. Chitosan was obtained through alkaline deacetylation of chitin sourced from shrimp shell waste following the methodology by Sánchez-Duarte et al. [34]. The materials, 99% acetic acid and 99.5% acetone, were purchased from the FAGA laboratory. Polyethylene glycol (PEG) and 37.4% hydrochloric acid (HCl) were bought from Merck. Glutaraldehyde (Glu) at 25% and polyvinylpyrrolidone (PVP) were purchased from Sigma-Aldrich. The coatings were prepared following the methodology presented by Aguilar-Ruiz et al. [35]. The inhibitory effect against corrosion of the coatings [36] was evaluated using electrochemical tests, specifically electrochemical impedance spectroscopy (EIS), as the primary technique under experimental conditions using a three-electrode electrochemical cell: working electrode (WE), a silver chloride electrode as a reference electrode (RE), and a platinum electrode as a counter electrode (CE).

2.2. Pre-Treatment of Stainless Steel Plates

All plates to be used were pre-treated, starting with sanding using 200, 400, 1000, 1200, and 2000 grit sandpaper. Subsequently, they underwent one to two rounds of ultrasonic cleaning for 10 min each in acetone solution to remove any residual metallic particles left from sanding. Next, the cleaned plates in the ultrasonic bath were rinsed with distilled water and allowed to dry at room temperature. The clean plates were then pre-treated by chemical etching and immersed for 30 s in a 0.1 M hydrochloric acid solution, followed by thorough rinsing with distilled water. The treated plates were dried in an oven at 60 °C for 24 h and stored in a desiccator until further use.

2.3. Barrier Inhibitor Coating Synthesis

For the preparation of pure chitosan coatings, the steps described by Aguilar-Ruiz et al. [36] were followed. A solution of chitosan dissolved in 2% acetic acid was prepared. Pure chitosan (Chi) coatings were prepared from the chitosan solution. Crosslinked chitosan coatings were prepared from the same chitosan-based solution with polyethylene glycol (PEG) and glutaraldehyde (Glu), respectively, to obtain Chi/PEG crosslinked coatings. Similarly, polyvinylpyrrolidone (PVP) and glutaraldehyde (Glu) were added to the pure chitosan-based solution to obtain Chi/PVP coatings. The preparation of Chi/PTA coatings involved impregnating pure chitosan (Chi) coatings in a PTA solution using the dip-coating technique at a constant speed. The pre-treated plates were immersed in the chitosan and its variants solutions to apply the coatings using the sol–gel method. Immediately after immersion, the plates were removed at a constant speed of 5 cm/min and dried at room temperature for 24 h. All coated plates were prepared in triplicate, dried, and stored until further use.

2.3.1. Morphological Characterization

To determine the morphological characteristics of chitosan-based plate coatings, metallographic analysis was performed using an inverted microscope (LEICA-DM-IRM) (Leica Microsystems, Germany), and samples were mounted in resin and polished to obtain smooth cross-sections. The prepared sections allowed for precise measurement of coating thickness. Additionally, the surface morphology and elemental composition of the coatings were examined using a Hitachi SU3500 scanning electron microscope (SEM)(CIMAV, Chihuahua, México ). The samples were coated with a thin conductive layer to enhance imaging quality. Cross-sectional and surface images were taken at a magnification of 2500×, providing detailed views of the samples’ microstructure. The analysis was conducted under an accelerating voltage of 10–15 kV, with a focus on observing coating thicknesses in the range of 1–3 microns. Elemental mapping was performed to visualize the distribution of key elements across the coating surface.

2.3.2. FTIR Characterization

Changes in molecular vibrations were investigated in the coatings through Fourier transform infrared spectroscopy (FTIR) (Thermo Scientific Spectrum model Nicolet iS5, Waltham, MA, USA) in attenuated total reflectance mode (ATR) using a Thermo Scientific Nicolet iS5 FTIR spectrometer (Waltham, MA, USA). The spectra were recorded under controlled conditions using KBr, covering a wave number range of 4000–500 cm−1 with a spectral resolution of 100 cm−1.

2.4. Electrochemical Analysis

Electrochemical analyses were conducted to determine the corrosion resistance behavior of the chitosan coatings. These analyses provided a detailed understanding of the electrochemical double-layer dynamics and interface, which is essential for evaluating the coatings’ corrosion resistance properties.

2.4.1. Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) was used to study the electrochemical double layer and interface, providing a detailed understanding of the electrical circuit of the system based on the ASTM standards D8370-22 methodology [37]. EIS tests were performed using SI 1287 SOLARTRON equipment) (Solartron Instruments, Famborrough, Hampshire, UK). The experiments were carried out by applying a direct current (DC) potential of 1 V and an alternating current (AC) amplitude of 10 mV open circuit potential (Vs Open Circuit). A frequency scan was conducted from 100,000 Hz to 0.1 Hz in logarithmic steps of 10 per decade, using a three-electrode electrochemical cell: the coated stainless steel as the working electrode (WE), a platinum electrode as the counter electrode (CE), and a silver chloride reference electrode (Ag/AgCl). All Nyquist graphs were fitted to an equivalent circuit (EC) consisting of a constant phase element CPE in parallel to polarization resistance (Rp) in series with a solution resistance (Rs). Tests were conducted following ASTM standards G106-89 (Reapproved 2023) [38]. The electrochemical parameters obtained are analyzed using Nyquist plots [8,9].
Nyquist plots were used to visualize the system’s response to applied potential frequencies. The corrosion inhibition analysis of the coatings was carried out through electrochemical tests using a three-electrode cell and 3.5% NaCl as the electrolytic medium. The metallic stainless steel plates under study were the working electrode (WE). At the same time, platinum was used as the counter electrode (CE) and silver chloride (Ag/AgClsat) as the reference electrode (RE). The instrument used was an AUTOLAB 780i potentiostat-galvanostat) (Metrohm Autolab B.V., The Netherlands). Cview, Zview, Csimp, and Origin software were used for data acquisition and graph plotting.

2.4.2. Potentiodynamic Polarization (PDP)

Potentiodynamic polarization tests were conducted using the cell setup described in the EIS section under consistent environmental conditions to assess the corrosion resistance of the coatings applied to stainless steel according to the ASTM standards G5-17 (2021). A potential scan ranging from −0.6 V to +1.0 V versus the open circuit potential (OCP) was used at a scan rate of 0.8 mV/s. The working electrode surface area was 0.79 cm2. Polarization data were analyzed to determine the electrochemical information in accordance with ASTM standards G102-23. All tests were conducted under controlled conditions, maintaining oxygen levels in the solution below 5 ppm through nitrogen aeration to minimize the influence of dissolved oxygen on corrosion. Tafel analysis was performed based on the polarization curves obtained to evaluate the corrosion properties of the samples. Tafel slopes were used to estimate the overpotential coefficients for both oxidation and reduction processes, providing a detailed insight into the electrochemical behavior of the coatings. The instrument used was a SI 1287 SOLARTRON potentiostat-galvanostat. Cview, Zview, Csimp, and Origin software were used for data acquisition and graph plotting.

3. Results and Discussion

3.1. Chitosan Coating Characterization

3.1.1. Physicochemical Characterization

The results obtained from chitosan characterization were as follows: the average molecular weight of chitosan was 587.11 kDa, the degree of deacetylation (%) was >90, humidity content (%) was 6.46, and ash content (%) was 0.93. The characterization results are consistent with the literature reports [39,40,41].

3.1.2. Morphological Characterization (SEM-EDS)

The characterization of chitosan-based coatings and chitosan crosslinked with PEG, PVP, PEG, and PTA on stainless steel plates was performed using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). SEM analysis, conducted at an acceleration voltage of 10 kV and 100× magnification, allowed for observing the morphology and distribution of coatings on stainless steel surfaces. The images obtained revealed important details regarding the uniformity and adhesion of the coatings. EDS spectroscopy provided information on the elemental composition of the coatings, identifying the main elements present in the samples. The data confirmed the presence of elements such as carbon, nitrogen, and oxygen, corresponding to chitosan and its crosslinked components and additional elements indicative of the other polymers used. This information is crucial for evaluating the effectiveness and quality of the applied coatings.
Figure 1 shows the SEM image of the chitosan coating along with its elemental analysis. A relatively smooth surface with some imperfections can be observed. The coating uniformity is good, suggesting good adhesion. The EDS spectrum reveals the presence of carbon (C), nitrogen (N), and oxygen (O) as the main components, consistent with the chemical composition of chitosan. Peaks of substrate elements such as iron (Fe), chromium (Cr), and nickel (Ni) are also observed, originating from the underlying stainless steel.
The results of the morphological analysis of Chi/PTA are presented in Figure 2. The obtained results indicate that the coating crosslinked with PTA shows superior uniformity compared to pure chitosan, without significant defects. The EDS spectrum of this coating shows an elemental composition like the result of pure chitosan, but with the addition of tungsten (W), corresponding to the PTA structure (NH4)10(H2W12O42)·4H2O. Thus, the presence of W confirms that the crosslinking process was effective. The results suggest that both the adhesion and uniformity of the Chi/PTA coating were positively impacted by the crosslinking with PTA, thereby improving the properties of the chitosan coating.
The results of the analysis of the chitosan coating crosslinked with PVP (Chi/PVP), presented in Figure 3, show some irregularities and surface defects compared to the other coatings, indicating lower uniformity. The EDS spectrum reveals a high presence of carbon (C) and oxygen (O), with a lower proportion of nitrogen (N), corresponding to the PVP structure. The elemental distribution might suggest that the crosslinking was partial, which could explain the less uniform morphology observed in the SEM image.
The chitosan surface crosslinked with the PEG (Chi/PEG) coating exhibits more imperfections and less uniformity. The EDS spectrum of this coating indicates a high presence of carbon (C) and oxygen (O), with additional peaks of ethylene (C2H4) characteristic of PEG. The lower uniformity observed in the SEM analysis could correlate with the elemental distribution, suggesting partial crosslinking between chitosan and PEG, negatively affecting the coating quality, as depicted in Figure 4.
In general, SEM images of chitosan coatings and their modifications indicate that the pure chitosan coating exhibited moderate porosity with micropores that could potentially diminish its effectiveness as a corrosion barrier. Conversely, the Chi/PTA coating displayed a more homogeneous surface with reduced porosity, suggesting improved cohesion and corrosion resistance. The Chi/PEG coating showed increased porosity, likely due to PEG’s effect, which may affect its protective capabilities. Meanwhile, the Chi/PVP coating demonstrated low porosity and a uniform surface like Chi/PTA, indicating high efficacy as a corrosion barrier. However, to obtain more specific results and a detailed understanding of the performance of these coatings, further targeted studies are necessary.

3.1.3. Metallographic Analysis of Coating Thickness

A metallographic analysis was carried out using a Leica DM-IRM microscope to measure the thickness of the coatings. Before analysis, samples underwent embedding, polishing, and cross-sectional cutting to expose the layers. High-resolution images were captured with a pixel resolution of 2048 × 1536 × 24 bpp, corresponding to a calibrated size of 241.72 × 181.29 µm. A calibration factor 0.118 was applied, translating to one pixel, equating to 0.12 microns. This enabled precise measurement of coating thicknesses using image analysis software, as shown in Figure 5. Average results are presented in Table 1.
The metallographic analysis revealed varying thicknesses for the different coatings applied to stainless steel samples. The Chi/PEG coating exhibited an average thickness of 2.424 ± 0.309 microns. In contrast, the Chi/PTA coating showed a slightly thinner layer, averaging 2.109 ± 0.191 microns. The Chi/PVP coating was still thinner, measuring around 1.877 microns on average, but with a slightly higher uncertainty (±0.292 microns) than Chi/PTA. The thickest coating in this study was pure chitosan, with an average thickness of 3.437 ± 0.768 microns. Standard deviation could indicate variability in coating application.
Thicker coatings are generally anticipated to offer superior physical barrier properties against corrosion on metallic substrates. However, this expectation is not always met, as observed in the study. For instance, despite being the thickest, the Chi coating exhibited noticeable surface irregularities (Figure 1), which could compromise its protective effectiveness. Similar irregularities were noted in the Chi/PEG and Chi/PVP coatings. In contrast, the Chi/PTA coating, despite being thinner, presented a more uniform surface (Figure 2). Some studies suggest that a homogeneous coating may enhance corrosion protection by improving barrier integrity [42,43]. These findings are promising in comparison to other chitosan-based coatings and highlight their potential efficacy in corrosion protection applications.
The results obtained are consistent with reports from similar studies on coating thickness, which ranges from 1.6 to 20 microns [44]. Hua et al. [45] emphasized that the corrosion protection of barrier-type materials is significantly influenced by film thickness. Specifically, very thin films offer inadequate protection, while intermediate thicknesses provide optimal corrosion resistance. Conversely, excessive thickness can introduce defects that undermine effectiveness. Additionally, improving the smoothness of the coating is crucial, as a well-applied, even film enhances protective performance. Thus, achieving optimal corrosion resistance necessitates both an appropriate thickness and a smooth, uniform coating.
Regardless of its greater thickness, the Chi coating exhibited significant surface irregularities, which potentially compromised its ability to effectively block Cl− ions. Similar irregularities were observed in the Chi/PEG and Chi/PVP coatings [15]. In contrast, the Chi/PTA coating, although thinner, demonstrated a more uniform surface, which may enhance its protective performance by providing a more consistent barrier against aggressive species. Research suggests that a smooth and homogeneous coating can be more effective in preventing the ingress of Cl− ions and reducing corrosion. These findings emphasize the critical role of both coating thickness and surface morphology in determining corrosion resistance to effectively block the corrosive ion pathway [46].

3.1.4. FTIR Structural Analysis

The molecular structure of the coatings was investigated through Fourier transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR), the results of which are presented in Figure 6. The spectrum reveals different regions, marked from “(a)” to “(d)”, where structural modifications due to crosslinkers are highlighted. It is noted that the fingerprint of the organic compounds in FTIR appears from the 1500 cm−1 band [47]. In the region “(a)” of 1660–1590 cm−1, peaks corresponding to the vibrations of amide I and amine groups (C=O, N-H) are present, evident in all coatings. The amide I peak, specifically around 1660 cm−1, corresponds to the stretching of the C=O bond, while the peak around 1590 cm−1 is attributed to amide II, related to the bending of the N-H bond [48,49]. The intensity of these peaks varies depending on the crosslinker used, suggesting differentiated interactions of these groups with PVP, PEG, and PTA. In region “(b)” (1450 –1200 cm−1), CH2 bending bands show additional interactions with the PVP and PEG crosslinkers, though less prominently, indicating a lower degree of interaction of these groups with chitosan. On the other hand, the range of 1300–1250 cm−1 reveals vibrations of C-N and C-O bonds, with a notable peak around 1250 cm−1, confirming the integration of PVP and PEG into the chitosan matrix [30]. In region “(c)” (1150–1000 cm−1), peaks associated with glycosidic bridges (C-O-C) and C-O bonds show the involvement of PVP, PEG, and PTA in the crosslinking of the coating. The most substantial peaks are observed around 1100 cm−1, reflecting high activity in glycosidic structures. Finally, in region “(d)” (900–850 cm−1), peaks around 890 cm−1, associated with P-O and W-O-W bond vibrations confirm the incorporation of PTA into the chitosan structure [50,51,52].
These results propose that the crosslinkers induce specific modifications in the chitosan structure, as reflected in the variations observed in the different regions of the FTIR spectrum. The differences in the areas of amide I and II bands indicate that the interaction of chitosan with crosslinkers directly affects the vibrations of key functional groups, generating differentiated crosslinking depending on the type of modifier used.

3.2. Coatings Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) tests were performed using the SI 1287 SOLARTRON equipment to evaluate the anticorrosive behavior of the coatings applied to stainless steel plates. The electrochemical parameters obtained are presented in Table 2 and analyzed using Nyquist plots [9,10].
Figure 7 shows the Nyquist plot for naked stainless steel (Naked SS), chitosan (Chi) and chitosan crosslinked with PTA (Chi/PTA), PEG (Chi/PEG), PVP (Chi/PVP). SS Naked exhibits a clear absence of a semicircle, possibly indicating a Warburg-type resistive behavior. This may suggest high ion diffusion in the solution and therefore direct interaction between the metal and the electrolyte. Results could indicate a slow corrosion process through a porous corrosion product layer or at the metal–electrolyte interface [27].
Results show a solution resistance (Rs) of 165.76 Ω cm2 and a high polarization resistance (Rp) of 15,960 Ω cm2, indicating moderated corrosion resistance expected for the SS.
The absence of clearly defined semicircles in the Nyquist plots for chitosan (Chi) and crosslinked chitosan coatings on stainless steel could be attributed to different factors. Specifically, the Chi/PVP coatings show relatively low polarization resistance (Rp). The lower values may indicate reduced corrosion resistance and suggest that the charge transfer process occurs rapidly, hindering the formation of a clear semicircle in the Nyquist plot. In systems with low charge transfer resistance, the semicircle may not be well defined. Additionally, the dispersed capacitive behavior of these coatings, as indicated by the values of the constant phase elements (CPE-P), may contribute to the distortion or absence of expected semicircles. Such phenomena are typically observed in coatings that present limited capacity to resistance corrosion.
The Nyquist plot for the Chi/PVP exhibits a partially defined semicircle, indicating a relaxation process associated with the phase constant and parallel resistance. The solution resistance (Rs) for Chi/PVP is 227.6 Ω cm2, while its polarization resistance (Rp) is considerably lower (13,520 Ω cm2). This indicates a reduced ability to resist corrosion.
Chi/PEG behavior presented a smaller partial semicircle, reflecting comparatively lower resistance than the other systems, with a solution resistance (Rs) of 213.8 Ω cm2 and a polarization resistance (Rp) of 35,560 Ω cm2. The Chi/PEG coating does not exhibit protective capacity, as indicated by the noticeable capacitive behavior reflected.
According to the Nyquist plot presented for the Chi system in Figure 7, and considering the trend observed for the coating, the impedance values are lower compared to the other coatings, thus suggesting a lower charge transfer resistance and greater ease of charge carrier movement. With a solution resistance (Rs) of 222.1 Ω cm2 and a polarization resistance (Rp) of 219,560 Ω cm2, Chi exhibits high conductivity and low corrosion resistance, consistent with the absence of a semicircle shape for this system.
The Nyquist Chi/PTA system shows a considerably larger partially formed semicircle, indicating high charge transfer resistance and a more pronounced relaxation effect. The solution resistance (Rs) of 225.6 Ω cm2 and a polarization resistance (Rp) of 247,840 Ω cm2 indicate that Chi/PTA could offer protection in aggressive environments. The Chi/PTA coating shows improvement in corrosion resistance, which could benefit the system’s long-term stability by providing better adhesion and cohesion within the coating, as observed in the SEM studies.
The chitosan coatings applied to the SS samples enhanced corrosion resistance capability compared to uncoated stainless steel. The Chi/PTA coating stands out as the most promising coating, offering superior polarization resistance and a more favorable CPE-P parameter than its counterparts. The SEM analysis and chitosan coating thickness study revealed the presence of surface defects and a non-homogeneous distribution of the material in the chitosan coatings, which explains the lower Rp compared to the uncoated substrate despite having the most significant thickness.
Results obtained in this study advise that chitosan coatings could improve corrosion resistance when their structure is modified with crosslinking agents. In contrast to previous research where modifications such as glutaraldehyde were used to enhance anticorrosive protection [53], this study highlights that combining chitosan with PTA may offer additional benefits. The increased charge transfer impedance could indicate a more effective barrier against coating corrosion in certain aggressive environments, such as NaCl solution. This enhancement could be attributed to the coating’s ability to form a dense and homogeneous matrix on the metallic plate surface, reducing the diffusion of corrosive agents and extending the coating’s lifespan. These results underscore the importance of considering specific coating modifications to optimize corrosion resistance and electrochemical performance in practical applications.

3.3. Coatings Potentiodynamic Polarization Tests (PDP)

To assess the corrosion resistance of the coatings applied to stainless steel, potentiodynamic polarization tests were conducted using the cell setup described in the EIS section under identical environmental conditions. A potential scan was applied from −0.6 V to +1.0 V vs. open circuit potential (OCP), with a scan rate of 0.8 mV/s. The working electrode surface area was 0.79 cm2. Polarization data were used to determine electrochemical parameters. All tests were conducted under controlled conditions, with oxygen levels in the solution maintained below 5 ppm by nitrogen purging to minimize the influence of dissolved oxygen on corrosion. Tafel analysis was performed using the polarization curves obtained to evaluate the corrosion properties of the samples. Tafel slopes were used to estimate overpotential coefficients for both oxidation and reduction processes, providing a detailed insight into the electrochemical behavior of the coatings.
The polarization curves obtained for the different coatings and stainless steel (SS) are shown below in Figure 8. Electrochemical parameters derived from these curves, including corrosion current (Icorr), corrosion potential (Ecorr), anodic (βa) and cathodic (βc) slopes, and corrosion rate (in mm per year) were obtained from the analysis of the results obtained from the test in the Cview (3.5) software and then plotted in Origin software 2024b version. Results are presented in Table 3.
In Figure 8, the Tafel plot for the uncoated steel shows a curve with a relatively constant slope across the evaluated current density range. This suggests uniform reaction kinetics, possibly controlled by charge transfer. The extremely electronegative corrosion potential and steeper slope indicate a high corrosion rate, which is expected for steel without any protective coating.
Results show that the Tafel curve for Chi/PTA presents a notably lower slope in the low current density region. This suggests improved corrosion protection at low current densities. In the high current density region, there is an inflection. Still, the slope is less steep than Chi/PVP and chitosan cases, indicating that Chi/PTA may maintain better protective effectiveness under more severe conditions. The presence of PTA could provide additional adsorption sites or better chemical stability, thereby enhancing corrosion resistance.
The Tafel plot for chitosan displays a behavior with a precise inflection in the Tafel curve. However, the low current density region has a lower slope than its homologs, suggesting that chitosan might offer better corrosion protection at low current densities. The increased slope indicates a decrease in coating effectiveness at high current densities. This could be attributed to the polymeric structure of chitosan, which provides physical and chemical barriers against corrosion, though it could be potentially less stable compared to other coatings.
These results suggest that Chi/PEG is less effective in corrosion protection, showing a high susceptibility to corrosion. The Tafel plot for Chi/PEG demonstrates behavior with two distinct regions, like other coatings. The slope in the low current density region is low, indicating good corrosion protection. However, the slope increases significantly in the high current density region, suggesting a decrease in coating effectiveness under severe conditions. This could be attributed to the hydrophilic nature of PEG, which might facilitate the penetration of water and corrosive agents into the coating.
Chi/PVP shows intermediate corrosion resistance, with moderately low current and corrosion rate values. Its behavior can be observed in Figure 8. The Tafel plot for Chi/PVP displays two distinct regions: one with low current density showing a moderate slope and another at high current density where the slope increases significantly. This behavior suggests that the Chi/PVP coating offers some protection against corrosion at low current densities, but its effectiveness diminishes at high current densities. The inflection in the curve could indicate a limit in the coating’s protective capacity, possibly due to saturation of adsorption sites or degradation of the layer under severe conditions.
At high current densities, the effectiveness of the coatings decreases, with Chi/PTA exhibiting the best relative resistance. This suggests that Chi/PTA may be more suitable for applications under severe conditions where high current densities are expected. This is consistent with findings reported by Arwati et al. [54], who conducted detailed potentiodynamic polarization analyses investigating the inhibitory capacity of chitosan. Their results suggested that chitosan coatings affect both anodic and cathodic activities, reporting high efficiency as a corrosion inhibitor, differing from reports by Zhang et al. [55], who attributed the performance of chitosan and its coatings to the physical barrier conferred by the coatings, rather than an effect on electrochemical activity. Therefore, the results indicate that the Chi/PTA coating offers the best corrosion resistance, with low corrosion current and rate. On the other hand, Chi/PEG exhibits the poorest corrosion resistance with the highest corrosion current and rate. Chi and Chi/PVP show intermediate corrosion resistance, while stainless steel exhibits the highest susceptibility to corrosion. It could be inferred that chitosan and its crosslinkages enhance resistance and consequently anticorrosion performance by not only impeding electrolyte diffusion to the metal surface but also by preventing electrochemical reactions [56].
The differences in corrosion resistance among the coatings can be attributed to variations in the anodic and cathodic slopes and corrosion potentials. This could demonstrate that protection against corrosion phenomena extends beyond only a physical barrier, and electrochemical processes could be prevented. The electrochemical analyses, complemented by morphological and structural characterization, point out that Chi/PTA coating improves corrosion resistance, thus demonstrating the stability and adhesion of the coating, also suggesting its effectiveness as a barrier in moderately aggressive corrosive environments such as NaCl solutions. However, further optimization of the coating process could enhance the protective barrier benefit. Future optimizations could significantly boost the coatings’ performance, reinforcing their potential for industrial applications in aggressive environments by improving coating uniformity and eliminating surface defects, potentially increasing the protective barrier’s effectiveness.

3.4. Resistance Corrosion Efficiencies

Determining protective anticorrosive efficiency in percentages is critical for assessing the effectiveness of protective coatings in this study. This analysis allows for a direct comparison of how each coating reduces the corrosion rate relative to untreated stainless steel, providing a quantitative measure of their ability to protect the metal substrate. Results are presented in Figure 9. The inhibition efficiencies were determined from Equation (1),
P r o t e c t i v e   A n t i c o r r o s i v e   E f f i c i e n c y   % = C R 0 − C R C R 0 × 100 ,
where CR represents the corrosion rates in mmPY of each coating and CR0 represents the corrosion rate in mmPY of the naked stainless steel.
Results reflect that chitosan crosslinked coatings improve corrosion potential and reduce corrosion rates by approximately 38.98% to 65.67%, underscoring their effectiveness in corrosion protection applications. These findings are consistent with those reported by other researchers in the field in Table 4 [50,51,57,58]. Notably, the efficiencies obtained in this work are higher than those reported for chitosan (CS) and carboxymethylcellulose (CMC) on steel, which showed efficiencies under 45% [59]. Nevertheless, efficiencies obtained for this work are lower compared to some advanced coatings reported in the literature such as chitosan/PTA applied on zinc substrates (95%) [50], chitosan–ZnO used on steel plates (85%) [60] and chitosan-graphene oxide applied on steel substrates (98%) [61,62]. Results still represent a substantial improvement over an untreated metal surface. The temporary nature of the coating is considered an advantage in specific contexts, allowing the maintenance of optimal protective performance over time, potentially addressing issues of degradation or reduced efficacy.
The enhanced performance of the Chi/PTA compared to the other coatings could be attributed to a more stable structure achieved through ionic crosslinking neutralizing the positively charged amine groups of chitosan, facilitating hydrogen bond formation, reducing permeability, and decreasing chitosan swelling. The SEM images validate these findings, revealing a uniform and flawless Chi/PTA coating surface.

4. Conclusions

Chitosan-based coatings and their crosslinked forms were evaluated for corrosion resistance studies on stainless steel using surface characterization techniques and electrochemical tests. The metallographic analysis revealed varying thicknesses for the different coatings applied to stainless steel samples.
Scanning electron microscopy (SEM) provided insights into the coatings’ morphology and elemental composition, revealing uniform coverage and chemical stability.
Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) techniques were employed to assess the coating’s electrochemical behavior. Chitosan crosslinked with ammonium paratungstate (Chi/PTA) showed promising results with higher polarization resistance (Rp) and impedance modulus (|Z|), indicating enhanced corrosion protection.
From these results, it can be suggested that chitosan-based coatings, in particular Chi/PTA, could be a promising coating method that works as a corrosion barrier on stainless steel surfaces. Future research could focus on optimizing coating formulations and exploring their performance in more aggressive environments to further validate their industrial applications.

Author Contributions

All authors contributed to the study’s conception and design. Material preparation, data collection, and analysis were performed by A.A.A.-R., R.G.S.-D. and V.M.O.-C.; Y.V.-P.; G.E.D.-I. and J.Á.-S. carried out the coating characterization studies. The first draft of the manuscript was written by A.A.A.-R. and edited by R.G.S.-D. and Y.V.-P. after all authors commented on previous versions of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

We acknowledge projects ITSON PROFAPI-2024-012, ITSON PROFAPI-2024-015, and project CONAHCYT, Ciencia de Frontera CF-2023-G-1395.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM image at 500× magnification of stainless steel plate coated with pure chitosan (a) and EDS spectrum of pure chitosan coating (b).
Figure 1. SEM image at 500× magnification of stainless steel plate coated with pure chitosan (a) and EDS spectrum of pure chitosan coating (b).
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Figure 2. SEM image at 500× magnification of stainless steel plate coated with Chi/PTA (a) and EDS spectrum of Chi/PTA coating (b).
Figure 2. SEM image at 500× magnification of stainless steel plate coated with Chi/PTA (a) and EDS spectrum of Chi/PTA coating (b).
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Figure 3. SEM image at 500× magnification of stainless steel plate coated with Chi/PVP (a) and EDS spectrum of the Chi/PVP coating (b).
Figure 3. SEM image at 500× magnification of stainless steel plate coated with Chi/PVP (a) and EDS spectrum of the Chi/PVP coating (b).
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Figure 4. SEM image at 500× magnification of stainless steel plate coated with Chi/PEG (a) and EDS spectrum of the Chi/PEG coating (b).
Figure 4. SEM image at 500× magnification of stainless steel plate coated with Chi/PEG (a) and EDS spectrum of the Chi/PEG coating (b).
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Figure 5. High-resolution metallographic image of coating thickness measurement using Leica DM-IRM microscope for (a) Chi/PEG, (b) Chi/PTA, (c) Chi/PVP, and (d) chitosan.
Figure 5. High-resolution metallographic image of coating thickness measurement using Leica DM-IRM microscope for (a) Chi/PEG, (b) Chi/PTA, (c) Chi/PVP, and (d) chitosan.
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Figure 6. FTIR-ATR of Chi, Chi/PTA, Chi/PVP, Chi/PEG coating spectra are shown in different regions marked from (a) to (d). (a) 1660–1590 cm−1; (b) 1450–1200 cm−1; (c) 1150–1000 cm−1; (d) 900–850 cm−1.
Figure 6. FTIR-ATR of Chi, Chi/PTA, Chi/PVP, Chi/PEG coating spectra are shown in different regions marked from (a) to (d). (a) 1660–1590 cm−1; (b) 1450–1200 cm−1; (c) 1150–1000 cm−1; (d) 900–850 cm−1.
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Figure 7. Nyquist plot for Chi/PEG, SS Naked, Chi/PTA, Chi/PVP, and Chi coatings with their fitted data to an equivalent circuit.
Figure 7. Nyquist plot for Chi/PEG, SS Naked, Chi/PTA, Chi/PVP, and Chi coatings with their fitted data to an equivalent circuit.
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Figure 8. Tafel polarization curve for Chi/PVP, Chi/PTA, Chi/PEG and Chi coatings and for Naked SS sample.
Figure 8. Tafel polarization curve for Chi/PVP, Chi/PTA, Chi/PEG and Chi coatings and for Naked SS sample.
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Figure 9. Comparison of protective anti-corrosion efficiency of Chi, Chi/PTA, Chi/PEG, and Chi/PVP coatings on stainless steel in 3.5% NaCl solution.
Figure 9. Comparison of protective anti-corrosion efficiency of Chi, Chi/PTA, Chi/PEG, and Chi/PVP coatings on stainless steel in 3.5% NaCl solution.
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Table 1. Coating thicknesses measured by metallographic analysis.
Table 1. Coating thicknesses measured by metallographic analysis.
CoatingThickness (µm)
Chi/PEG2.424 ± 0.309
Chi/PTA2.109 ± 0.191
Chi/PVP1.877 ± 0.292
Chi3.437 ± 0.768
The results represent the average of n = 5 measurements with their respective standard deviations.
Table 2. Electrochemical parameters of crosslinked chitosan coatings and Naked SS.
Table 2. Electrochemical parameters of crosslinked chitosan coatings and Naked SS.
SampleRs (Ω cm2)CPE-T (µF/cm2)CPE-P (µF/cm2)Rp (Ω cm2)
Naked SS165.764.2422 × 10−50.894815,960
Chi222.16.15 × 10−50.8083219,560
Chi/PTA225.67.21 × 10−50.8465247,840
Chi/PEG213.80.000106550.806135,560
Chi/PVP227.69.07 × 10−50.826413,520
Table 3. Electrochemical parameters and corrosion rate of chitosan coatings evaluated using polarization curves.
Table 3. Electrochemical parameters and corrosion rate of chitosan coatings evaluated using polarization curves.
SamplesIcorr (A/cm2)−Ecorr (V)βa (mV/D)−βc (mV/D)Corrosion Rate (mmPY)
SS4.14 × 10−6−0.3231.33 × 107308.520.042
Chi2.16 × 10−6−0.237536.14342.750.022
Chi/PTA1.22 × 10−6−0.2651.77 × 107210.340.014
Chi/PEG2.52 × 10−6−0.4303.26 × 107118.720.025
Chi/PVP2.90 × 10−6−0.205146.1159.750.023
Table 4. Efficiency of chitosan and other coatings for corrosion protection.
Table 4. Efficiency of chitosan and other coatings for corrosion protection.
Coating TypeEfficiency (%)Reference
Chitosan/PTA on Zinc95%[50]
Chitosan (CS), Carboxymethylcellulose (CMC) on Steel45%[59]
Graphene Oxide–Chitosan–Silver on copper nickel alloy98%[62]
Chitosan–TiO2 on cupper92%[63]
Chitosan–ZnO Steel75%[60]
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MDPI and ACS Style

Aguilar-Ruiz, A.A.; Sánchez-Duarte, R.G.; Orozco-Carmona, V.M.; Devora-Isiordia, G.E.; Villegas-Peralta, Y.; Álvarez-Sánchez, J. Chitosan and Its Derivatives as a Barrier Anti-Corrosive Coating of 304 Stainless Steel against Corrosion in 3.5% Sodium Chloride Solution. Coatings 2024, 14, 1244. https://doi.org/10.3390/coatings14101244

AMA Style

Aguilar-Ruiz AA, Sánchez-Duarte RG, Orozco-Carmona VM, Devora-Isiordia GE, Villegas-Peralta Y, Álvarez-Sánchez J. Chitosan and Its Derivatives as a Barrier Anti-Corrosive Coating of 304 Stainless Steel against Corrosion in 3.5% Sodium Chloride Solution. Coatings. 2024; 14(10):1244. https://doi.org/10.3390/coatings14101244

Chicago/Turabian Style

Aguilar-Ruiz, Ana Alejandra, Reyna Guadalupe Sánchez-Duarte, Víctor Manuel Orozco-Carmona, Germán Eduardo Devora-Isiordia, Yedidia Villegas-Peralta, and Jesús Álvarez-Sánchez. 2024. "Chitosan and Its Derivatives as a Barrier Anti-Corrosive Coating of 304 Stainless Steel against Corrosion in 3.5% Sodium Chloride Solution" Coatings 14, no. 10: 1244. https://doi.org/10.3390/coatings14101244

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