Facile synthesis of N-doped carbon dots (N-CDs) for effective corrosion inhibition of mild steel in 1 M HCl solution

N-CDs, as a novel and eco-friendly inhibitor, was synthesized easily by hydrothermal carbonization technique aiming to inhibit mild steel (MS) corrosion in 1 mol/L HCl. XRD, TEM, SEM, FTIR, UV-vis spectrophotometer and photoluminescence (PL) were utilized to characterize N-CDs. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques along with the complementary surface studies were combined to investigate the corrosion inhibition capability of N-CDs for MS. N-CDs were found nanometer-sized (≈ 4 nm) with quasi-spherical morphology and high crystallinity. Inhibition efficiency, directly proportional to concentration but inversely with temperature, was measured as high as 96.73% (PDP) and 95.21% (EIS) at 200 mg/L. Inhibition mechanism refered mainly to adsorption process that good obeyed Langmuir adsorption isotherm. The surface studies, quantitatively verified by EDX, showed a smoother surface of MS in presence of the N-CDs. Furthermore, the UV-visible spectroscopy effectively revealed the complexations between iron and metal surfaces.

Corrosion can lead to structural damage, reduced efficiency, and even failure of equipment, which can be costly to repair or replace [9]. Recently, corrosion has become a major issue in locations where mild steel is predominantly used. Steel's impressive strength has made it a preferred material for a wide range of applications, including industries, construction supplies, infrastructure, equipment, ships, trains, vehicles, machines, and appliances. However, when exposed to acid-based cleaning processes, mild steel is highly susceptible to severe corrosion [10]. As a result, the development of effective corrosion inhibitors is a critical area of research. Traditional corrosion inhibitors, such as heavy metals and organic compounds, have several drawbacks, including toxicity and environmental concerns [11,12].
In the context of corrosion inhibition, carbon dots have shown great potential due to their ability to form a protective film on the surface of metals. This film acts as a barrier, preventing corrosive agents from encountering the metal surface and slowing down the corrosion process [13][14][15][16][17]. Additionally, carbon dots can also act as electron donors or acceptors, which can further enhance their corrosion inhibition properties [18].
C-Dots offer several advantages over traditional corrosion inhibitors. They are non-toxic, biocompatible, and environmentally friendly, making them an attractive alternative for corrosion protection [19,20]. Additionally, C-Dots can be easily synthesized using inexpensive and readily available precursors, making them a cost-effective solution for corrosion prevention [21,22].
Multiple studies have shown that C-Dots can effectively inhibit corrosion in different metals, such as steel, copper, and aluminum [23][24][25][26]. For instance, synthesized C-Dots based complex with sulfosalicylic acid modification were used as a corrosion inhibitor for carbon steel in HCl solution, significantly reducing its corrosion rate [27]. Similarly, biomass derived C-Dots were used as a corrosion inhibitor for Q235 steel in 1 M HCl solution, inhibiting corrosion by self-aggregation and adsorption on metal surface [28]. Other studies also demonstrated the effectiveness of C-Dots as corrosion inhibitors for carbon steel in different acidic solutions, attributing the reduction in corrosion rate to the formation of a protective film on the metal surface [29][30][31][32][33]. Overall, C-Dots derived from different sources and with various properties have been reported to be promising corrosion inhibitors for different metals and environments. Ongoing research in this area holds the potential to improve the efficiency and durability of metal-based products, thereby reducing maintenance costs and extending the lifespan of equipment. In our current study, we utilized the hydrothermal carbonization method to synthesize corrosion inhibitor N-CDs from citric acid monohydrate and L-arginine, with the aim of developing an effective corrosion inhibitor. Subsequently, we investigated the inhibitory properties of the synthesized N-CDs on mild steel corrosion in HCl solution using various techniques including PDP, EIS, and surface studies. Furthermore, we evaluated the potential application of N-CDs in corrosion science by estimating their inhibition efficiency through the used electrochemical techniques.

Chemicals and reagents
L-Arginine 99.0% and 8-hydroxyquinoline (extra pure) 99% from Loba Chemie company, citric acid monohydrate from El Nasr Pharmaceutical Chemicals company, (3-aminopropyl) triethoxysilane ≥ 98.0% from Sigma-Aldrich (http:// www. sigma aldri ch. com), and cerium (III) chloride.7H 2 O 98.5% from Chem-Lab NV were all purchased to use in both the preparation and application processes. Distilled H 2 O was used throughout the preparations and measurements. All reagents are of high purity and were used without further purification.

Synthesis of nitrogen doped carbon dots
N-doped carbon dots were synthesized according to Ye et al. with some modifications [34]. The general schematic illustration of N-CDs synthesis is shown in Scheme 1. Typically, an equal molar ratio of 80 mmol of citric acid monohydrate as a carbon source and 80 mmol of L-arginine as both carbon and nitrogen sources were first dissolved in 50 mL of distilled water with aid of an ultrasonicator. The mixture was then transferred into a Teflon-lined stainless-steel autoclave. After the solution was heated at 200 °C for 3 h and cooled down to room temperature naturally, a deep green solution was obtained.

Characterization of N-CDs
Size and morphology of N-CDs were detected by two techniques, first is TEM using JEOL JEM-2100 transmission electron microscope (Tokyo, Japan), at an acceleration voltage of 200 kV for ultrahigh resolution and imaging of nanoscaled samples, and second is SEM using high-performance JSM-6400 scanning electron microscope (JEOL Ltd., Japan) instruments. FTIR spectra using a German FTIR spectrophotometer (TENSOR 27, Bruker) (http:// www. bruker. com/) was used to record the function groups present on N-CDs surface. The structural analysis, crystallinity, and purity of the synthesized N-CDs was confirmed by XRD using A GNR-X-Ray Diffractometer (Model; APD 2000 PRO, USA) with CuK α radiation (λ = 1.50598 Å) at 35 mA and 35 kV. The scan was performed over the 2θ range (10°-80°) at 22.1 °C in 0.03° step/scan with an integration time of 1 s/step. The Bragg law (2d sinθ = nλ) was used to calculate the interlayer spacing [35], where d is the interplanar spacing in nm, θ denotes the Braggʼs/scattering angle in degrees, n is the diffraction order and for first-order n = 1, and λ is the X-ray wavelength (CuK α radiation = 0.154064 nm). A Fluorolog (Horiba Jobin-Yvon) instrument with a slit width of 5 nm for both excitation and emission was used to perform photoluminescence properties of N-CDs. A high-performance T80 Double Beam UV-Vis spectrophotometer from PG Instruments Ltd with a wavelength range of 190-1100 nm was used to investigate the absorption spectra of highly dilute aqueous solution of N-CDs in a quartz cuvette.

Electrode and working solutions
The mild steel reinforcing rod utilized in all corrosion inhibition experiments was commercially purchased (Fig. S1, Electronic Supporting Material, ESM) with the chemical composition given in Table 1, (as analyzed by FOUNDRY-MASTER optical emission spectrometer (OES), Model/ Hitachi High-Tech, America). For the surface preparation, the exposed surface was polished up to 1500 grade using a series of successive finer grades of SiC sandpapers of 160, 400, 800, 1000, 1200, and 1500 grift, respectively. As a result, a fairly flat surface was obtained that provides a mirror finish. The electrodes were then treated with ethanol to remove traces of contaminants, washed with distilled water, dried, and used without further storage. Analytical grade 1 M HCl prepared by diluting 32% HCl using distilled water was used in all experimental corrosive electrolytes. 0.2 g of each inhibitor was dissolved into 1000 mL of 1 M HCl solution to form 200 mg/L of corrosion inhibitor solution (stock test solution) and other concentrations of 100, 50, and 25 mg/L were prepared through appropriate dilution. Meanwhile, the total volume of the test solution used in the electrochemical and surface analysis studies was 100 mL.

Electrochemical measurement system and electrochemical cell
Metrohm Autolab PGSTAT128N 12 V/800 mA Potentiostat/ Galvanostat electrochemical cell Analyzer (Autolab N series, Serial no: AUT86580, Manufacture: Metrohm Autolab B.V., Utrecht, The Netherlands) is employed for potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) measurements. It is powered by a computer with Nova 1.11 software, that enables data acquisition and processing. A classical three-electrode Pyrex glass cell with a capacity of 100 mL 1 M HCl solution with 0 to 200 mg/L of the inhibitors was used in the present study for electrochemical measurements at various temperatures 298-328 K. Ag/ AgCl and mild steel cylindrical electrode with an exposed surface area of 0.385 cm 2 was employed as a reference and working electrodes, respectively. The counter electrode was made of platinum (Pt). All experiments were conducted at constant temperatures (within ± 1 C) using a water bath to regulate the temperature. Before starting the measurements and for 30 min, the working electrode was immersed in the test solution to reach a steady potential "OCP test". At E ocp , electrochemical impedance spectroscopy measurement was realized by applying an alternating perturbation within the frequency spectrum 10 4 -10 −2 Hz using an amplitude signal of ± 5 mV. Potentiodynamic polarization measurements were executed immediately after EIS by displacing the working To confirm data reliability and reproducibility, the measurements for each experimental condition were repeated more than one time, and the acceptable reproducibilities were documented. Notably, the current results are displayed as current density (A/cm 2 ) to take into account the electrode surface area.

Surface analysis
The morphological aspects of corroded surfaces were captured by FE-SEM (QUANTA FEG 250 instrument, Netherland) at 1000 × and 30.000 × magnifications, that is equipped by EDAX (Genesis) Energy Dispersive X-ray Analyzer, England. The previously mentioned UV-Vis spectrophotometer was also used to perform absorption spectra on the inhibited solution with 200 mg/L N-CDs at 298 K, before and after immersion of the mild steel for 24 h. presence of many surface defects bound with the surface of N-CDs [36]. The XRD pattern (Fig. 1c) showed only the characteristic peak of carbon dots around 2θ = 20-30°, confirming the synthesis of pure N-CDs. The characteristic peak was observed at ~ 2θ = 25.55°, corresponding to the 002 crystallographic planes of graphite. The interplanar spacing within N-CDs is 0.348 nm, very close to graphite's 0.34 nm, indicating the predominance of highly conjugated sp 2 domains in the cores of N-CDs nanoparticles [37,38]. N-CDs exhibited a good degree of graphitization and crystallization (58.91%) because of the similar d-spacing to pure graphitic carbon along with the sharp peak observed.

Chemical composition and structural features of N-CDs
The FTIR results, as shown in Fig. 1d, indicate the evolution of graphene/graphite-like structures due to the stretching vibrations of C-H aliphatic groups at 2965 cm -1 [39][40][41]. The edges are formed with chemical structures such as -COOH, -CONR, -OH, -NH 2 at 1400 cm -1 , 1659 cm -1 [42], 3413 cm -1 [43,44], and 3193 cm -1 [45], respectively, confirming the success of the amidation process. Moreover, the typical absorption of cumulated double bonds (N = C = O and C = C = N) at 2060 cm -1 is characteristic of N-CDs, distinguishing them from their precursor analogs [46]. Thus, it can be concluded that N atoms were successfully doped into the N-CDs structures. Additionally, nitrogen atoms could be present on the surface of the nanoparticles as amine and amide groups and in the core as aromatic structures.

Spectroscopic analysis
The UV-Vis absorption spectrum of N-CDs nanoparticles in aqueous solution is shown in Fig. S2a (ESM). The N-CDs typically exhibit strong absorption bands in the UV region, ranging from 230 to 350 nm [47]. The high-energy UV absorption peak at 235 nm was assigned to the π → π* transitions of aromatic C = C bonds for the carbon core. In contrast, the low-energy UV absorption peak at 335 nm was attributed to n → π* transitions by non-bonding orbitals present in the N-CDs structure, such as O − H, N − H, C = N/C = O, and/or C − N bonds or other connected surface functional groups. The tail observed in the visible region indicates the absence of other types of nanocarbon formation during partial carbonization [48]. This finding confirms the purity of the carbon dots, which is in accordance with previous analyses results.
Figures S2b, c (ESM) show the PL spectra plots of the N-CDs. The strongest PL emission was observed in the blue region of the visible spectrum, with a λ em of approximately 410 nm. The carbon dots also exhibited excitation-dependent photoluminescence, suggesting that the composition, structure, and luminescent centers of each type of carbon nanodot were similar [49]. While the mechanism of PL in C-Dots is not well-understood, the investigated C-Dots' PL mechanism was mainly attributed to the appearance of sp 2 aromatic domains and surface defects resulting from dehydration, oxidation, and amidation [50]. The nanoscale of such complex carbonaceous systems with a large degree of dispersion of structural units, conjugated C = C domains, and various functional groups on the surface clearly illustrated the hydrophilic and polar nature of the N-CDs. This nature leads to fluorescence and contributes to chemical and optical stability. Figure S3 (ESM) illustrates the variation of OCP of mild steel with exposure time to 1 M HCl, both in the absence and presence of varying concentrations of synthesized N-CDs at 298 K. The unstable OCP oscillation of MS in free HCl indicates a greater susceptibility to corrosion. However, in N-CDs solutions, the trend of OCP is similar, with the value initially increasing sharply to more negative values and then reaching a plateau. This indicates that the corrosion reaction slows down over time and reaches a quasi-equilibrium rate within the presented time interval [51]. This may be due to the rapid migration rate of N-CDs, resulting in the formation of more stable, insoluble, and protective nanosize films.

Open-circuit potential measurements (OCP)
The OCP in N-CDs solution oscillated towards the negative sites. The adsorption of N-CDs nanoparticles on the MS surface may inhibit H + adsorption and discharge at the cathode region, resulting in an increase in the overpotential of the hydrogen evolution process. This shift in the OCP to more negative values indicates that the N-CDs inhibitor inhibits the cathodic reaction better than the anodic dissolution [52]. Moreover, an increase in the concentration of the inhibitor could shift the OCP more negatively.

Electrochemical impedance spectroscopy (EIS) measurements
The corrosion behavior of MS electrodes in a 1 M HCl solution in the absence and presence of synthesized N-CDs at varying concentrations was studied using EIS. The measurements were presented in the Nyquist and Bode plots. Nyquist plots for the N-CDs inhibitor (Fig. 2a) consisted of a single semicircle at high frequencies, followed by straight lines at low frequencies, reflecting charge transfer and diffusion processes. All Nyquist plots showed an increase in the diameter of the capacitive loops with increasing N-CDs concentration in the electrolyte, indicating a significant increase in the corrosion resistance of MS. A feature of all Nyquist plots was the presence of charge transfer on heterogeneous electrodes due to the imperfect semicircles. Surface heterogeneity may be related to the MS composition, corrosion products, or heterogeneity of the inhibitor film adsorbed on the surface [53]. The Bode modulus plots show that the impedance modulus magnitude at the low-frequency region (|Z| f = 0.01 Hz) is closely linked to a material's anti-corrosion performance [54]. Figure 2b shows that the values of |Z| f = 0.01 Hz increased significantly with increasing N-CDs concentration, indicating an upward trend of ~ 1-2 orders at different inhibitor concentrations. The value of |Z| f = 0.01 Hz in the blank solution was 41.59 Ω.cm 2 , but it reached 1008.28 Ω. cm 2 at 200 mg/L of C-Dots, confirming a decrease in the corrosion rate of the MS electrodes. A unique time constant in the middle frequency range of the Bode-phase spectra was observed, which was related to the charge transfer on the metal/electrolyte interface. The phase shifts of the sinusoidal current to the potential were less than 90º, indicating non-ideal capacitors. For uninhibited samples, the size of the loop was narrow, while it became broader and the broadening increased with increasing inhibitor concentration. The results suggest that all inhibitors acted as interfacial inhibitors, exhibiting increasingly capacitive behavior with increasing concentration [55,56].
Structural impedance modeling was performed for quantitative evaluation and interpretation of impedance spectra. The spectra recorded for MS in 1 M HCl or containing different concentrations of N-CDs were modeled by the classic Randle's cell, [R(Q[RW])], with an additional Warburg element (Fig. 2c). This was evidence of the presence of diffusion processes [44]. In the equivalent circuits, constant phase element (Q dl ) was used instead of the ideal capacitor (C dl ) to approximate the values of the respective capacitances of the double electric layer and film layer. The impedance function of the CPE, as well as that of the pure capacitors, is a frequency-dependent parameter and is given as follows [53,56,57]: where, (Y o ) in Ω −1 s n /cm 2 denotes CPE magnitude, (j) is the imaginary number (j = √ −1 ), and ω is the angular frequency. "n" is the surface heterogeneity parameter, and is used as a valuable criterion of the nature of the metal surface, in particular, that of CPE, i.e., CPE represents either an inductor, resistor, Warburg impedance, or pure capacitor if n = − 1, 0, 0.5 or 1, respectively. The double-layer capacitance (C dl ) [53] and relaxation time constant (τ) of a surface state, were computed by the following equations: Tables 2 presents the values of the corrosion process parameters obtained by the fitting of the spectra to the equivalent circuit, and η EIS % was calculated using Eq. 3.4 [54].
where R p and R p o are the polarization resistance in the inhibited and uninhibited solutions, respectively.
According to the numerical values obtained from impedance modeling, the addition of N-CDs increased the values of R ct . These values were further enhanced by increasing the concentration of N-CDs, indicating the formation of a protective film at the MS/electrolyte interface. The CPE dl values were attributed to the capacitive behavior of the electrochemical double layers and showed an increasing trend with an increase in N-CDs concentration. Furthermore, the capacitance of the electrochemical double layer can be approximated by the capacitance of a two-plate condenser as follows [53]: where ɛ o is the permittivity of the atmosphere, ɛ is the dielectric constant of the medium, A is the surface of the electrode, and d is the thickness of the protective layer. The decreasing trend of CPE dl with increasing concentration in the solution could then be considered "expected" due to a decrease in the dielectric constants of the electrochemical double layer resulting from the replacement of more H 2 O molecules with N-CDs, and an increase in the thickness of the protective layer (d) [53,58,59] For all experimental impedance data that was fitted, the heterogeneity parameter values ranged from 0.75 to 1. This indicates a non-ideal capacitive response of the CPE. The appearance of the Warburg impedance element was attributed to the electrochemical reactions at the MS/ electrolyte interface under diffusion control. In the presence of inhibitors, the value of τ was slightly higher for N-CDs (τ = 0.0228 s at 200 mg/L) compared to the blank (τ = 0.0061 s), indicating the spontaneous adsorption process of N-CDs [56].
The data shows that as the concentration of N-CDs increases, the corrosion inhibition efficiencies (η EIS %) also increase, reaching maximum values of 95.92% at 200 mg/L of N-CDs. This indicates that N-CDs nanoparticles provide high surface corrosion protection. Furthermore, the calculated trend, in combination with the trend in CPE values, suggests that the corrosion protection efficiency is dependent on the surface concentration (coverage) of N-CDs nanoparticles that form the protective layer [42,44,54,59].

Potentiodynamic polarization measurements
To determine the action mechanism of N-CDs, specifically whether it is anodic, cathodic, or mixed, and to verify the data obtained from EIS measurements, PDP measurements were performed for MS in a 1 M HCl solution in the absence and presence of different concentrations of N-CDs. The results of Fig. 3 show that as the concentration of N-CDs increased, both the cathodic and anodic currents decreased, indicating protection of the MS surface by the N-CDs film. Additionally, the corrosion potential shifted to a more negative direction with an increase in N-CDs concentration, which is consistent with the results in Fig. S3. This suggests that N-CDs inhibit the cathodic corrosion reaction slightly more than the anodic reaction, although the function of N-CDs is mixed. Furthermore, increasing the concentration of N-CDs resulted in a shift in both the anodic and cathodic Tafel lines to lower current densities. The quasi-parallel cathodic and anodic potentiodynamic polarization branches imply that the adsorption of the investigated N-CDs affected the kinetics of corrosion reactions, rather than a change in the corrosion mechanism itself [60]. This result confirms the adsorption of N-CDs nanoparticles on the mild steel surface, and the extent of adsorption is related to the N-CDs concentration, indicating film formation [61]. Table 3 shows the extrapolated polarization parameters including corrosion current density, i corr , corrosion potential, E corr , slopes of the anodic and cathodic Tafel curves, β a and − β c , polarization resistances, R p , corrosion rates, CR. Tafel method was used to determine the corrosion inhibition efficiency according to Eq. 3.6 [62]: where i • corr and i corr denote the corrosion current densities of the uninhibited and inhibited steel, respectively.
The continued decrease in i corr values and corrosion rate of MS was observed with an increase in the concentration of N-CDs. The corrosion rate of MS was found to be very rapid and scored 20.6 mm/year in 1 M HCl medium but dropped sharply to 0.67 mm/year at 200 mg/L of N-CDs, as investigated. In comparison with the i corr value of 1761.2 mA/ cm 2 in the uninhibited medium, a smaller value of 57.56 was observed.
The N-CDs were found to be adsorbed on the MS surface, leading to intensive surface coverage and enhanced IE%. The corrosion inhibition efficiency first increased rapidly and then gradually approached and yielded a maximum protection efficiency of 96.73% at 200 mg L − 1 of N-CDs. In Table 4, we compared our synthesized N-CDs inhibitor with recently published inhibitors for their corrosion inhibition  73 The present work efficiency on mild steel in an acidic medium. The comparison revealed that N-CDs possess distinct advantages over the others, such as being easily synthesized, eco-friendly, and highly efficient. Both the anodic and cathodic slopes appeared to be similar at all N-CDs concentrations, indicating that the addition of N-CDs to the electrolyte did not influence the mechanisms of MS corrosion reactions. At an inhibitor concentration of 200 mg L − 1 of N-CDs, E corr was displaced to values not exceeding ± 85 mV, thus confirming the mixed character of N-CDs when they interacted with the MS surface. In addition, the carbon dots inhibited the corrosion reaction mainly based on the surface blocking mechanism [69]. The effectiveness of N-CDs adsorption on the MS surface in acidic solutions could be attributed to the presence of -NH 2 , -COOH, N-C = O, OCH 3 , and other groups in the N-CDs structure, which caused the blockage of active sites.
The accordance between the data from the three techniques is obvious, which validates the accuracy of the data and the approaches used to analyze it. Hence, the synthesized N-CDs can be used as effective inhibitor of MS corrosion in applications that use acidic electrolytes.

Application of adsorption isotherm
The remarkable inhibitive action of using carbon dots as inhibitors could be attributed to their adsorption behavior on the MS surface. Thus, the adsorption isotherms were an important complement that could determine the inhibition mechanism of N-CDs on the metal surface. Figure 4a and b show the representation of both the Langmuir and kinetic thermodynamic adsorption isotherms that were used to fit the potentiodynamic experimental results of corrosion of MS in 1 M HCl in the presence of N-CDs nanoparticles [70].
The adsorption parameters are represented in Table 5. A good agreement for the adsorption of N-CDs on the surface of the mild steel was observed according to either the Langmuir or kinetic-thermodynamic adsorption isotherms. However, the comparison of R 2 values revealed that Langmuir gave the best fit (R 2 ≅ 1). Moreover, the Langmuir adsorption isotherm suggested one-layer adsorption of the inhibitor on the mild steel and the interactions between adsorbed particles were negligible [71]. Concerning the kinetic thermodynamic model, the inverse of the slopes (1/y) of the obtained straight lines was not much larger than unity, confirming that each active site could adsorb only a molecule of N-CDs.
Where K ads is the equilibrium constant of the adsorption process and K ads = Kʹ (1/y) , C inh denotes the inhibitor concentration, y is the number of inhibitor molecules occupying one active site of the metal surface. The K ads values derived from the intercepts of Langmuir plots were exploited in calculating the standard free energy of adsorption (ΔG  where, R (8.314 J/mol.K) stands for the universal gas constant, T (K) represents the absolute temperature, 1000 value (g/L) represents the mass concentration of water in solution as the unit of K ads was reported in terms of L/g, and ΔG • ads (J/mol) is the free energy of adsorption. K ads and the thermodynamic adsorption parameter ( ΔG

Temperature effect and thermodynamic activation parameters of the corrosion process
The effect of temperature on MS corrosion inhibition was investigated in the absence and presence of 200 mg/L of N-CDs in 1 M HCl at different temperatures ranging from 298 ± 1 to 328 ± 1 K with an interval of 10 K, using PDP. The results are shown in Fig. S4, and the extrapolated data is recorded in Table S1. The results showed that regardless of the absence (Fig. S4a) or presence of N-CDs inhibitor (Fig. S4b) in the electrolyte, the corrosion reaction kinetics increased with an increase in temperature, due to an increased corrosion rate. It was expected that N-CDs would desorb at higher temperatures, resulting in lower inhibition efficiency. However, by comparing the polarization resistance (R p ) values at a fixed temperature, it was evident that the synthesized N-CDs acted as good corrosion inhibitor in the entire temperature range and can be considered a robust MS corrosion inhibitor in 1 M HCl. The 2D-variations expressed with increasing temperature are documented in Fig. 5. As compared to the blank, the increase in R p values for N-CDs is much less than that measured for the same concentration at room temperature (Table S1). However, the N-CDs inhibitor still showed remarkable efficiency in preventing corrosion at high temperatures, indicating that C-Dots provide more efficient protection for MS from 298 to 328 K [59,72]. The desorption or degradation of the inhibitor layer at elevated temperatures appears to be the main contributing factor behind this observation, causing increased exposure of the metal surface to the acidic corrosive solution. A decrease in η% with an increase in temperature for the synthesized C-Dots suggested some physical adsorption, and the weak Van der Waals forces responsible for such type of interaction tended to disappear at elevated temperatures [73]. However, the high-density chemical adsorption process was predominant.
The dependence of the corrosion current density (i corr ) on temperature be considered an Arrhenius-type process, and its rate is given by [55]: where the symbol A symbolizes the Arrhenius frequency factor that varies with the metal type and electrolyte, E a is defined as the activation energy of the corrosion process. Figure 6a shows the plot of Ln i corr as a function of the inverse of the absolute temperature (1000/T) for MS electrodes in the absence and presence of 200 mg/L of N-CDs inhibitors.
Arrhenius plots yielded a straight line of slope (-E a /R), which allowed us to calculate the values of the activation energies E a (included in Table 6). The temperature effect was also verified by determining the changes in activation enthalpy ∆H* and activation entropy ∆S* using transition state equation as follows [74]: where, N and h are t he Avogadro's number (6.023 × 10 23 mol −1 ) and Planck's constant (6.626 × 10 −34 m 2 .kg/s). The transition state plots of Ln i corr /T versus 1000/T are given in Fig. 6b. ∆H* and ∆S* are calculated respectively from the slopes ( −ΔH * R ) and intercepts  Table 6.
The activation energy of the inhibited mild steel is higher than the uninhibited one. The E a value of much lower than ± 80 kJ/mol was due to chemical and physical adsorption. A higher E a value indicates an increase in the double layer thickness, which improves the activation energy of the corrosion process. This is achieved by increasing the energy barrier for corrosion activity [57].
The value of ∆H* was positive and increased interestingly in the presence of N-CDs. It indicates that the adsorption of N-CDs on the MS surface was endothermic and thus slowed down the dissolution of iron. The E a value was greater than those of enthalpy (ΔH*) analogs, confirming that the corrosion process involved a gaseous reaction, that is, the hydrogen evolution reaction [55].
The difference value of E a -ΔH* is approximately equal to the average RT value (2.477 kJ/mol) in the studied solutions. This indicates that both corrosion and inhibition processes were unimolecular interactions. The value of ΔS* was large and negative, indicating a decrease in disorderliness as the reaction proceeded from reactants to activated complex. This indicates that most of the MS surface becomes inhibited with N-CDs, reducing the occurrence of metal dissolution (disorder) [72]. In summary, the previous data shows that N-CDs molecules exhibit good performance at higher temperatures.

Effect of immersion time
To investigate the inhibition behavior of N-CDs over a time scale of 0.5 to 48 h, we conducted EIS measurements on steel electrodes immersed in 1 M HCl, both in the absence and presence of 200 mg/L of the studied N-CDs. As shown in Fig. 7a, b, the diameter of the semicircle and the impedance modulus decreased over time for MS/1 M HCl, indicating a faster corrosion rate at MS surface in this solution. In contrast, in the presence of N-CDs, we observed an increase in the charge transfer resistance (R ct ) at the metal/solution interface, leading to an increase in inhibitive efficiency, as seen in Fig. 7c, d and Table S2.
The value of |Z| f = 0.01 Hz increased from 1008.28 to 2026.02 Ω.cm 2 for MS electrodes immersed in 1 M HCl with 200 mg/L N-CDs after 48 h, and maximum phase angles increased with immersion time. These results suggest a slower corroding system or better protection provided by the N-CDs during long-term immersion. We fitted the EIS data for steel immersed in solutions containing N-CDs with the same circuit shown in Fig. 2c. In contrast, the electrical circuit shown in Fig. 7e, with a one-time constant and an arrangement of [R(QR)], was used to fit the EIS data of MS in blank solutions. Table S2 represents the variation of the electrochemical parameters of MS in 1 M HCl, in the absence and presence of N-CDs over time. The values of R ct and C dl depend on the packing of the monolayer of the inhibiting compound. The more densely packed the monolayer of the inhibitor, the  higher the diameter of the semi-circle, resulting in higher R ct and lower C dl values. In the case of MS/1 M HCl alone, the values of C dl increased with time, whereas the values of R ct decreased. This confirmed serious corrosion and more conformation of corrosion products. However, the increase in the values of R ct for the inhibited solutions was attributed to the higher surface coverage and formation of a more compact protective film of N-CDs at the MS/electrolyte interface.
The potentiodynamic polarization technique was also used to test the inhibition of the corrosion of MS in 1 M HCl by the N-CDs inhibitor at 200 mg/L. The obtained values are recorded in Table S3 and graphically represented in Fig. 8a, b. Figure 8a shows the worsening of the corrosion rate of MS/1 M HCl over time, while the opposite variation was observed in the presence of the N-CDs inhibitor. In Fig. 8b, an improvement in the corrosion inhibition efficiency was observed with increasing immersion

Synergetic inhibitive effect of N-CDs and other inhibitors
The synergetic effect is the effect whereby the combined action of two (or more) parameters exceeds the individual action of each one of them. For this reason, electrochemical measurements were performed after exposing MS samples to solutions of 1 M HCl containing 200 mg/L of either cerium chloride (III), 8-hydroxyquinoline, N-CDs, or mixtures thereof for 0.5 h. The results of the polarization measurements are shown in Fig. 9, where the highest current density values were observed for the mixture of N-CDs and Ce (III). The current densities decreased for the mixture of N-CDs and 8-HQ, reaching the minimum current density for the mixture of Ce (III), N-CDs, and 8-HQ. These behaviors are well demonstrated in Fig. 10a, b, where the impedance spectra were recorded in the form of Nyquist and Bode plots. The impedance figures show that the largest diameter of the capacitive semicircle and the value of the impedance modulus (|Z| f = 0.01 Hz) belonged to the spectra of the mixture between N-CDs + Ce (III) + 8-HQ, which indicates the highest corrosion resistance. For a quantitative evaluation of the synergetic effect, the polarization resistance (R p ) values acquired from the measured polarization curves in a narrow potential range (OCP ± 0.030 V) were used. The obtained experimental data are represented in Table S4. From the table, it is evident that the corrosion rate of MS in the presence of N-CDs in combination with Ce (III) is further reduced in comparison to N-CDs alone. It is observed that the mixtures of (N-CDs + 8-HQ) or (CD N-CDs + Ce (III) + 8-HQ) increased the IE more than N-CDs alone, indicating a synergistic effect. However, the synergistic effect of the inhibitor at the mentioned concentration ratios did not produce appreciable degrees of inhibition. Rather, the use of N-CDs alone would be more beneficial.

Scanning electron microscopy and energy dispersive X-ray analysis
Figures S5 and 11 summarize SEM micrographs at low and high magnifications of the surface of MS samples after 6 h exposure to a corrosive medium, without and with 200 mg/L of N-CDs inhibitors. Additional quantitative analyses were performed by EDX analysis, exactly on the full area belonging to micrographs (a) in the SEM analysis, to confirm the suggestions made by SEM observations. As can be seen from Fig. S5 (micrographs a and b), pitting corrosion proceeded in the absence of the inhibitor. The MS exposed to the corrosive medium suffered severe corrosion. Pores were created due to pitting growth and the presence of an oxide layer. Its EDX spectrum (Fig. S5c) shows the notorious appearance of oxygen and chloride, and low iron intensity, which was indicative of serious corrosion of steel. The low magnification SEM micrograph (Fig. 11a) shows that MS exposed to N-CDs inhibitor maintained an almost smooth and intact surface. In addition, no pitting spots or deposits were observed. The high magnification micrograph (b) clearly shows crystals that are nearly spherical and welldistributed over the entire surface of the sample. Unlike the previous case, the surface contained lower quantities of O and Cl − and higher quantities of Fe and C. A new peak for N appeared in the EDX-diagram (Fig. 11c), a characteristic of N-doped C-Dots adsorption on MS surface.

Conclusion
N-doped C-Dots were successfully synthesized using a hydrothermal method that employed L-arginine and citric acid monohydrate. These N-CDs have an average particle size of approximately 4.4 nm and exhibit a graphene/graphite-like structure with edges containing several functional groups, such as C = O, -OH, -NH 2 , and/or -NCO, C = N, C − N, and -COOH. These groups make the N-CDs hydrophilic, fluorescent, and stable. PDP, EIS, and Surface morphological analysis confirmed that N-CDs act as an effective corrosion inhibitor for MS in a 1 M HCl solution through an adsorption mechanism, forming a protective hydrophobic film on the MS surface. The corrosion inhibition by N-CDs and its effectiveness in 1 M HCl solution may be attributed to the several adsorption sites available in the C-Dots molecules, such as -NH 2 , -COOH, -OH, NCO, and phenyl ring. These groups facilitate adsorption and consequently block the active sites created on the MS surface, resulting in the formation of a surface adsorbed monolayer of N-CDs. This layer offers a protective hydrophobic film to the transport. The inhibition efficiency of N-CDs increases with increasing concentration. Moreover, the addition of 8-HQ or a mixture of Ce(III) and 8-HQ increases the inhibition efficiency of N-CDs, while the addition of Ce(III) alone decreases it. Overall, the synthesized N-CDs offer a novel, eco-friendly, and effective long-term corrosion inhibitor for MS in a 1 M HCl solution.
Author contributions F. Nasr did the experimental work and wrote the first paper draftE. A. Matter supervised the experimental work, share writing the paper, explaination of data and revising the manuscriptG. A. El-Naggar supervised the experimental work, share writing the paper, explaination of data and revising the manuscriptGaber Hashem Gaber Ahmed supervised the experimental work, share writing the paper, explaination of data and revising the manuscript