N,O-Chitosan Containing 1,3,4- Thiadiazole/CMC/ Nanoparticles and Study Corrosion Inhibition of Mild Steel

INTRODUCTION 
Corrosion represents a critical challenge within the industrial sector, resulting in profound economic consequences. These losses primarily stem from the degradation of material structural integrity, which necessitates extensive restoration efforts and frequently causes maintenance expenditures to exceed initial budgetary projections [1]. To mitigate these effects, techniques such as cathodic protection and the use of inhibitors are frequently implemented. Since eliminating corrosion entirely is often impossible, the focus shifts toward the more practical goal of controlling and reducing its rate [2-4]. One of the most cost-effective and accessible methods for corrosion mitigation involves the addition of inhibitors to the substrate’s environment. Inhibitors are chemical compounds that, when introduced in low concentrations to the electrolyte, significantly reduce the corrosion rate by interfering with the electrochemical kinetics of the system [5]. Corrosion is characterized as the deterioration of metallic elements and their alloys through electrochemical interactions with thier environment. This process occurs due to the inherent tendency of metals to revert to their thermodynamically stable mineral forms [6,7]. While unavoidable, corrosion can be managed through various preventive measures, including: cathodic protection, anodic protection, coating, alloying and using inhibitors, etc8. Out of these methods, inhibitors are highly effective as they reduce the aggressiveness of the corrosive aqueous environments by creating a protective molecular film on the metal surface [8]. Recent studies highlight chitosan as an effective inhibitor solution in acidic environments, particularly for protecting low-carbon steel. The efficiency of chitosan typically improves as its concentration increases, up to a specific optimal threshold [9-11]. Chitosan (CS), is a naturally occurring polycationic linear polysaccharide that is derived from the chitin, is a copolymer made up of N-acetylglucosamine and glucosamine [12-14]. Chitosan (CS) is a naturally occurring, positively charged linear polysaccharide produced from chitin. It functions as a copolymer composed of N-acetyl-D-glucosamine and D-glucosamine units. Following cellulose, chitin is the most prevalent natural polymer on Earth, acting as a fundamental structural component in the outer shells of crustaceans (including shrimp and crabs), as well as in insects and the cell walls of fungi [15-18].
Chitosan (CS) is extensively acknowledged for its biocompatibility, biodegradability, and non-toxic profile, making it a highly secure material. Its biological profile encompasses antimalignant, antitumor, and antimicrobial capabilities, and it is regularly utilized as a blood thinner within pharmaceutical applications [19-22]. The existence of hydroxyl (-OH) and amino (-NH2) functional groups within the structure of CS [23] allows for its easy conversion into a diverse array of functionalized derivatives [24-28]. 1,3,4-Thiadiazole derivatives play significant roles as scaffolds for corrosion inhibition owing to the chemical nature of groups such as C=N, C-S, amine, and N-N linkages. Furthermore, the thiadiazole ring possesses π -electrons, which allow it to bond easily to the surface of mild steel (MS), thereby reducing the corrosion rate [29]. Consequently, chitosan has been modified with 1,3,4-thiadiazole compounds to create derivatives with enhanced solubility, biocompatibility, biological activity, and hydrophilicity [30-32]. Carboxymethyl cellulose (CMC) is a water-soluble, negatively charged derivative of cellulose. Cellulose itself is a long-chain polymer consisting of anhydro-glucose units [33]. CMC exhibits excellent biocompatibility, leading to its widespread use in cleaning products, cosmeceuticals, pharmaceuticals, textiles, paper, adhesives, ceramics, and the food industry [33]. CMC exhibits excellent biocompatibility, making it widely used in cleaning products, cosmeceuticals, medicines, paper, textiles, adhesives, ceramics, food and pharmaceutical industries [34]. The presence of two types of reactive sites (carboxyl and hydroxyl) within its structure, combined with its high water solubility, enhances its chemical reactivity and facilitates numerous further modifications [35-38]. CMC is a linear polymer that exists as a colorless, fragrance-free, non-toxic, water-soluble powder and a highly transparent gel. Soluble in mild alkaline solutions, exhibiting superior dispersion and binding properties [39]. CMC has environmentally beneficial characteristics, including safety, biodegradability, biocompatibility, non-toxicity, and environmental sensitivity. The availability of CMC would facilitate its entry into the pharmaceutical, clinical, and food industries [40,41]. CMC chains can interact with oppositely charged polyelectrolytes, such as chitosan, to form three-dimensional hydrogel networks. It has been effectively utilized as an anticorrosive agent for low-carbon steel in aqueous media [42]. For example, Rajendran et al. examined the corrosion patterns of carbon steel in the presence of a carboxymethyl cellulose (CMC) and 1-hydroxyethane-1,1-diphosphonic acid (HEDP) system; their findings underscore that the application of polymers and their derivatives as sustainable ‘green’ corrosion inhibitors for metals and alloys remains a significant area of research [43].

MATERIALS AND METHODS 
Materials
Powder of Copper oxide Nano (CuONPs, size 40nm), Silver Nano(AgNPs, size 20nm) and Zinc oxide (ZnONPs, size 10-30nm) by US, Research Nanomaterils, Inc. All Chemicals were provided by BDH, SCR and CDH.    

Instrumentation
(FT-IR) spectra were recorded using a Shimadzu FTIR-8400S spectrometer over a spectral range of 400–4000 cm-¹. The ¹HNMR spectra were obtained using an Ultra Shield 400 MHz spectrometer from Bruker, University of Tehran, Iran. TMS was employed as an internal standard with DMSO serving as the solvent. Field Emission scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) were conducted at the University of Tehran, Iran.

Methods of Synthesis
Synthesis of 2,5-dimercapto-1,3,4-Thiadiazole [1] [44]
A mixture of 99% NH2NH2.H2O (5 mL, 0.10 mol) and CS2 (1.2 mL, 0.02 mol) in dry pyridine (50 mL) was refluxed for 5 hours. Following the reaction, the excess solvent was removed under reduced pressure via distillation. The resulting solid was precipitated by the addition of water (25 mL) and concentrated HCL (5 mL). The crude product was collected by filtration and purified via recrystallization from ethanol. 

Synthesis of Compound[2] [45]
An aliquot of 2,5-dimercapto-1,3,4-thiadiazole [1] (0.01mol) was combined with anhydrous sodium carbonate (0.04mol) in (15ml) of distilled water. Following this, chloroacetic acid (0.01mol) was added to the mixture. The resulting solution was heated under reflux for 6 hours. Upon completion, the mixture was acidified to pH 2 using concentrated hydrochloric acid to precipitate the product. The crude solid was isolated via filtration, washed thoroughly with distilled water, and purified by recrystallization from absolute ethanol to yield compound [2]. 

Synthesis of Compound [3] [46]
A solution of compound [3] (0.01mol) and thionyl chloride (0.02 mol) in 15 ml of anhydrous benzene was heated at reflux for 8 hours. Following the completion of the reaction, the solvent and residual thionyl chloride were removed under reduced pressure to yield the crude product.

Synthesis of O- Chitosan derivatives [4] [47]
Chitosan (0.5 g) was suspended in 25 mL of 2M H2SO4, followed by the addition of 0.01 mol of compound [2]. The mixture was heated under reflux for eight hours and then allowed to cool to room temperature. The solution was neutralized to pH 7 using NaHCO3. The resulting product was precipitated in acetone, collected via filtration, and washed thoroughly with additional acetone to remove any residual acid. Finally, the product was dried in an oven at 60°C for 24 hours.

Synthesis of O,N Chitosan derivatives [5] [48]
Initially, 0.5 g of chitosan was immersed in a 50 mL mixture of chloroform and pyridine (1:1 ratio) for duration of 10 hours. Subsequently, compound [3] was added to the mixture while maintained in an ice-water bath. The resulting solution was subjected to continuous stirring at 100°C for 14 hours.
Following this period, the mixture was allowed to cool and was then poured into 25 mL of methanol. After being further chilled to 4°C, The resulting solution underwent filtration, after which the collected precipitate was washed extensively with methanol and subsequently dried at 50°C. 

Synthesis of Polymers Blend [6,7] [49]
Polymer blends were prepared using the solvent casting method. The modified chitosan (CS) solution was prepared by dissolving the polymer in 2% (v/v) aqueous acetic acid under constant stirring at ambient temperature. Simultaneously, carboxymethyl cellulose (CMC) was dissolved in distilled water to achieve a 5 wt% concentration. The two solutions were then combined and homogenized using stirrering for 60 minutes. The modified CS/CMC blends were synthesized at a weight ratio of 5:5.

Synthesis of Modified CS/ CMC/Nanocomposites [8-13] [50]
A 100 mg sample of the dried, modified CS/CMC blend [6, 7] was immersed in 50 mL of a CuO, Ag, or ZnO solution (250 mg/L). The mixture was agitated using a hotplate stirrer for 3 hours to facilitate the bonding of the copper, silver, or zinc nanometals within the blend matrix [26].

RESULTS AND DISCUSSION
The synthesis of new nanocomposites beginning with 2,5-dimercapto-1,3,4-Thiadiazole [1] is demonstrated in Fig. 1. Compound [1] have been created by reacting hydrazine hydrate (99%) with 2mol of carbon disulfide in dry pyridine. The compound’s FT-IR [1] revealed appearance bands at (2547) and (1639) cm-1 respectively, owing to SH group and (C=N). 
Compound [2] was synthesized under basic conditions via the reaction of compound [1] with chloroacetic acid in distilled water. The FTIR spectrum of [2] confirmed the formation of the product, displaying a characteristic broad absorption band between 3400–2400 cm-¹ (O-H stretch) and a sharp band at 1684 cm-¹ corresponding to the carboxylic carbonyl group (C=O).
The 1H-NMR spectrums further supported the structure, exhibiting a broad singlet signal wth chemical shift at δ 12.07 ppm integrating for two protons, assigned to the carboxylic acid groups. Additionally, a signal appeared at δ 4.35 ppm attributed to two protons of SH group. Compound [2] reacted with thionyl chloride in dry benzene to product compound [3]. The FTIR spectrum of compound [3] confirmed the absence of the characteristic carboxylic acid bands at (1684) cm-1 (C=O) and (3400-2500) cm-1(O-H). Instead, a new absorption band appeared at 1737cm-1, corresponding to the carbonyl group of the acyl chloride. O-chitosan derivatives [4] produced through the reaction between compound [2] and chitosan in distilled water in acidic media. The presence of O-H and N-H groups in polymer [4] was confirmed by a strong, broad signal at 3294 cm-¹ in its FT-IR spectrum. This broadening confirms the presence of extensive intra- and intermolecular hydrogen bonding within the chitosan framework. Additionally, a new absorption band appeared at 1714 cm⁻¹, corresponding to the C=O stretching vibration of the ester group.
O,N-chitosan derivatives [5] were synthesized via the reaction of [3] with chitosan, utilizing a mixture of pyridine and trichloromethane as the solvent system.The FT-IR spectrum of polymer [5] exhibited a broad absorption band at 3365 cm-1, attributed to the N-H and O-H stretching vibrations involved in intra- and intermolecular hydrogen bonding within the chitosan framework. The successful formation of the derivative was further confirmed by the emergence of new characteristic bands at 1740 cm-1and 1678 cm-1, corresponding to the carbonyl groups (C=O) of the ester and amide functionalities, respectively.
In the 1H NMR spectrum of polymer [5], a distinct singlet signal appeared at δ 9.66 ppm, which is assigned to the protons of the NHC=O group. Additionally, a singlet signal at δ 4.87 ppm was observed, corresponding to the hydroxyl OH protons of the chitosan, 3.04 - 3.75 (s,H-1, H-3, H-4, H-5, and H-6 the non-anomeric proton of chitosan), singlet signal at δ (5.31-5.94) ppm for eight proton of SCH2 groups. By casting method, blend polymers [6,7] are made from modified CS[4,5] with CMC, as revealed by FT-IR data. The band broadening in the 2400-3600 cm-1 region indicates strong intermolecular hydrogen bonding between the amino groups of CS and the hydroxyl groups of CMC. Additionally, a peak at 1632 cm-1 corresponds to the C=N bond, while a peak at (1710) cm-1 is attributed to the carbonyl of the ester. By stirring alone, the synthesis of nanocomposites [8-13] based on CuO, Ag, or ZnO nanoparticles (NPs) combined with polymer blends [6,7] exhibits characteristic peaks at (3189) cm-¹. These peaks reveal O-H stretching from intramolecular and intermolecular hydrogen bonds. Additionally, shifting is observed in the asymmetric and symmetric C-H stretching vibrations of alkyl groups at (2889,2960) cm-¹, while peaks between 400 and 800 cm-¹ indicate metal-oxide bonding (Cu, Ag, Zn), which confirms the successful formation of the nanoparticles. 

FESEM Studies
FESEM micrographs have been utilized to study changes in surface morphology for prepared blend polymer[7] and nanocomposites [12],[13] as shown in Figs. 4-7, respectively. In FESEM images, blend polymer the pores’ average pore size is compared to the nanocomposites pore size, the size of the pores in the blend polymer was (0.08-0.12) um, while the size of the pores in the nanocomposites was (52-80), (32-63) nm. After the nanoparticles-to-blend polymer interaction, the FESEM images showed that the surface of the produced blends had undergone considerable changes, as we observed a homogeneous distribution of polymers on the surface of the polymer blend. It was observed that the AgNPs and ZnONPs have homogeneous distributions on the surface of the matrix50.

Transmission Electron Microscopy (TEM) 
The AgNPs are well dispersed and have a semi-spherical morphology, according to the TEM image of the modified chitosan/CMC suspension drop-cast with AgNPs. Although the particles are oriented differently and are closed, there are no signs of agglomeration and had sizes in the range of (5-50) nm, Fig. 7. The particles appeared spherical, with a thin layer of silver around modified CS/CMC. As a result, the produced AgNps in modified chitosan/CMC are more stable. The modified chitosan /CMC appeared to be coated with a layer of silver particles validating the generation of CS/CMC/AgNPs [50].

Corrosion inhibition [51]
The chemical composition of carbon steel samples is as follows: C, 0.22; Fe, 99.20; Cu, 0.19; Si, 0.28; Mn, 0.03; Ca, 0.02; and S, 0.06. Possibly the most widely used method for assessing inhibition is the gravimetric approach, which measures weight loss. The weight-loss method has demonstrated such simplicity and reliability that it is now the established baseline for measurement in several scheduled corrosion monitoring programs. Weight-loss measurements were conducted using 250 ml beakers containing 100 ml of the test solution at room temperature. The iron coupons were weighed and then fully submerged in the beakers, suspended from a rod by a hook. After being immersed for seven hours, the coupons were rinsed with distilled water, dried, and reweighed. The difference in weight before and after immersion in the various testing solutions was then used to calculate the total weight loss, expressed in grams.
Table 1 shows Inhibition effectiveness of polymers and nanocomposites.

I.E. represents the inhibitor solution’s inhibition efficiency.
Wi is the weight loss in the inhibitor solution, and Wu is the weight loss in the control solution.
Modified CS[5], modified CS/CMC[7], modified CS/CMC/CuONPs[11], modified CS/CMC/AgNPs[12], modified CS/CMC/ZnONPs[13] demonstrated a high degree of corrosion inhibition against carbon steel corrosion in an acidic medium. This can be attributed to the presence of p electrons in aromatic systems, multiple bonds, and the electronegative atom (N) in inhibitor molecule structures. This group has an inductive effect that makes the aromatic ring more active and boosts electron density. These modifications can make it much easier for the inhibitor to absorb, which boosts both protection and absorption. The study indicates that compounds that adhere to the metal surface work as adsorption inhibitors and stop corrosion. It is important to note that nanoparticles, along with Chitosan (CS) and Carboxymethyl Cellulose (CMC) compounds, can significantly enhance corrosion protection by forming a thin, passive layer on a material’s surface. This defensive membrane precludes the contact of degradative chemicals with the metal base. Specifically, this method works by either inhibiting the redox (reduction-oxidation) processes within the corrosion system or by neutralizing the effects of dissolved oxygen. Furthermore, the integration of Copper (Cu), Silver (Ag), and Zinc (Zn) nanoparticles helps stabilize the formation of these protective layers, effectively shielding metal surfaces against environmental degradation [52].

Mechanism of Corrosion Inhibition
Chemical adsorption of inhibitor molecules is facilitated by the electron-accepting nature of the unoccupied d-orbitals in iron (Fe) atoms. Specifically, coordinate bonds are formed through the overlap of the vacant 3d-orbitals of Fe with the p-orbital lone pair of the inhibitor.
The presence of additional hydroxyl substitution groups on the aromatic ring further enhances this process. The inductive effect of these groups increases the electron density and activates the aromatic ring, leading to more efficient adsorption and improved corrosion protection. This indicates that these substances function as effective adsorptive inhibitors by adhering strongly to the metal surface. Furthermore, the substantial molecular size and high molecular weight of these compounds and nanoparticles contribute to their superior inhibition efficiency [53].

Corrosion / EIS measurement [54]
The Corrosion/EIS measurement setup consists of a host computer, a thermostat, and a magnetic stirrer, interfaced with a Vertex. One Potentiostat/Galvanostat/EIS (Ivium Technologies, Netherlands). The electrochemical cell is a double-walled Pyrex glass cell (100 mL capacity) featuring internal and external bowls for temperature control. Measurements were performed using a standardized three-electrode cell: 

Working Electrode (WE): Carbon steel, used to determine the electrochemical potential.

Counter (Auxiliary) Electrode (CE): A platinum electrode with a surface area of 1 cm².
Reference Electrode (RE): A Saturated Calomel Electrode (SCE, Hg/Hg2Cl2in sat. KCl) as reference electrode.
Before measurements, the working electrode was immersed in the test solution for 15 minutes to achieve a steady-state Open Circuit Potential (Eocp). All experiments were conducted at 298, 308, and 318 K using a cooling-heating circulating water bath to maintain precise thermal control.

Polarization Curves [55,56]
Corrosion characteristics were evaluated based on the experimental data. The corrosion potential (Ecorr) and corrosion current density (Icorr) were derived by extrapolating the anodic and cathodic Tafel slopes in a 0.1M HCl medium, both with and without the addition of inhibitor molecules. Additionally, the Tafel constants for the anodic (ba) and cathodic (bc) regions were determined. Table 2 presents the results for the Tafel slopes (mV/dec), corrosion potential Ecorr (mV), corrosion current density ICD (A/cm²), and protection efficiency PE:

Where (icorr) o is the corrosion current density in the absence of inhibitors, and (icorr) is the corrosion current density in the presence of inhibitors.

CONCLUSION
The goal of this study is to create new nanocomposites through a sequence of reactions. This process involves synthesizing new compounds, grafting them onto chitosan, blending the modified polymers with carboxymethyl cellulose (CMC), and finally coating the resulting polymer matrix with various nanoparticles. The structural and morphological properties of these compounds and polymers were characterized using FT-IR, 1H-NMR, FESEM, and TEM. Using the weight loss method and electrochemical techniques (Tafel curves), the corrosion inhibition of modified polymers, blended polymers, and nanocomposites in 0.1 M hydrochloric acid on mild steel was investigated. The nanocomposites showed superior inhibition efficiency compared to the modified CS, blended polymer (modified CS/CMC). The presence of nanoparticles in the nanocomposites is responsible for their higher inhibition efficiency when compared to the blended and modified polymers, yielding the following efficiency sequence: [modified CS/CMC/AgNPs [12] > modified CS/CMC/ZnONPs [13] > modified CS/CMC/CuONPs[11] > modified CS/CMC [7].The reason for the improved inhibition efficiency with increasing inhibitor concentration. The presence of multiple electron-donating groups (such as O, S, N, -COOH, and -OH) facilitates the formation of strong coordination bonds with the iron surface and nanoparticles. This increases the surface coverage of the chitosan (CS) particles on the metal across various concentrations and temperatures. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.