Na-CDs as an eco-friendly and efficient corrosion inhibitor for Q235 in 1 M HCl

2.1 Materials

Sodium citrate and copper chloride were of analytical grade, purchased from Macklin. Q235 carbon steel samples (0.80 wt% Mn, 0.50 wt% C, 0.37 wt% Si, 0.25 wt% Cu, 0.25 wt% Cr, 0.25 wt% Ni, 0.045 wt% S, 0.04 wt% P and balance Fe) were acquired from Shandong Shengxin Science and Technology Co., Ltd. The specifications of the Q235 samples used for electrochemical tests were 1 cm³, the electrodes were encapsulated using an epoxy resin and exposed to a surface of 1 cm² due to electrochemical testing. The Q235 samples used for weight loss experiments were initially 3.55 cm × 2.8 cm × 0.4 cm in size, with a diameter of 0.4 cm. All Q235 carbon steels are polished with 200 to 2,000 pieces of silicon carbide paper before testing, and samples are cleaned using ultrasonic waves using industrial ethanol. Concentrated hydrochloric acid solution purchased from Tianjin Damao Chemical Reagent Factory. Deionized water was produced in-house in the laboratory.

2.2 Preparation of Sodium-Doped carbon dots (Na-CDs)

Na-CDs were prepared using a hydrothermal method, as shown in Fig. 1, resembled Ren et al. (X. Ren et al., 2015) in method. Initially, 0.1 M sodium citrate and 0.1 M copper chloride were dissolved in 100 mL of deionized water. The resulting solution was transferred to a 200 mL polytetrafluoroethylene (PTFE)-lined autoclave. The autoclave was maintained at 200 °C for 5 h. After the solution cooled naturally to room temperature, a 0.22 µm aqueous system filter membrane was used to filter out insoluble large particles from the resultant brown solution. The filtered solution was then placed into a 1000 kDa dialysis bag and dialyzed in deionized water for 24 h, with the water changed every 6 h. The dialyzed solution was lyophilized to obtain a yellow–brown solid product, which is the Na-CDs.

Close

Fig. 1

Schematic diagram of Na-CDs preparation process.

Export to PPT

2.3 Characterization of Na-CDs

The structural morphology, size distribution, and crystalline characteristics of the synthesized Na-CDs were investigated using XRD on a Panalytical Empyrean system (Netherlands) and TEM on a FEI Talos F200X G2 instrument (USA). Raman spectroscopy, employing a Horiba LabRAM HR Evolution (Japan) with a laser excitation wavelength of 532 nm, was utilized to assess the graphitization level of Na-CDs across a spectral range of 800–2200 cm⁻¹. FTIR analyses were performed on a Thermo Fisher Nicolet iS50 (USA) within the range of 600–4000 cm⁻¹, to elucidate the functional groups present in Na-CDs. The electronic structure and elemental composition were further elucidated by XPS on a Thermo Scientific K-Alpha system (USA). The colloidal stability of Na-CDs in a 1 M HCl solution was characterized by measuring their zeta potential using a zeta potential analyzer (Brookhaven, USA).

2.4 Weight loss measurements

The dimensions of the Q235 carbon steel samples for the weight loss experiments are detailed in Table S1. Samples were immersed in 1 M HCl solution both in the absence and presence of varying concentrations of Na-CDs for a duration of 24 h, after soaking, the Q235 sample was slowly washed with a small flow of deionized water. The mass of the samples was recorded with a precision of 0.1 mg before and after immersion, and the measurements were repeated thrice to obtain an average value. The experiments were conducted in a 100 mL double-walled beaker to maintain a constant temperature of 289 K throughout the process.

2.5 Electrochemical measurements

Electrochemical assays were designed as follows: Na-CDs were dissolved in 1 M HCl solution to prepare inhibited solutions with concentrations of 50, 100, 150, and 200 mg/L, respectively. ACS2350M electrochemical workstation (CORRTEST, China) was utilized to perform the measurements using a three-electrode system. This system consisted of a saturated calomel electrode (reference electrode), a 1.0 cm² platinum sheet (counter electrode), and a Q235 carbon steel working electrode with a 1.0 cm² area exposed, the latter being encapsulated in epoxy resin (Jin et al., 2022; J. Wang et al., 2021). Prior to electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) tests, a stable open circuit potential was established. Before electrochemical tests, a stable open circuit potential (E_OCP) was obtained. The amplitude of sinusoidal potential was 10 mV, and the measurement frequency of electrochemical impedance spectroscopy (EIS) was 10⁵Hz ∼ 10^-2Hz, and the resulting data were processed using ZSimpWin software. The polarization curves were carried out with E_OCP ± 250 mV, and the scanning rate was 0.5 mV/s, and the results were analyzed with CView2 software (Cen et al., 2019b). All electrochemical experiments were performed in triplicate to ensure the reliability of the data.

2.6 Surface characterization

Post weight loss testing, the Q235 carbon steel samples were retrieved, rinsed with deionized water, and subsequently dried using a stream of cold, dry air. Surface morphology of the carbon steel, was examined using a Scanning Electron Microscope (TESCAN MIRA LMS, Czech Republic). Energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the elemental composition of the metal surfaces under various conditions.

3.1 Structural characterization of Na-CDs

TEM and HRTEM results are shown in Fig. 2a and 2b, respectively. A size distribution of 100 randomly selected Na-CDs, as shown in the inset of Fig. 2a, indicates that the sizes are primarily concentrated between 2 and 5 nm, with an average size of 3.62 nm. As shown in Fig. 2b, the lattice stripes of Na-CDs are very clear, the illustration shown is a high resolution image of the selected carbon points, showing the (1 0 2) and (1 0 0) crystal faces of Na-CDs with a spacing of 0.25 and 0.21 nm, respectively (Cen et al., 2019a; Dang et al., 2018; L. Wang et al., 2014). Raman spectrum test was performed at 532 nm laser wavelength, as shown in Fig. 2d. The two peaks at 1355.48 and 1591.71 cm⁻¹ are the D and G bands of Na-CDs, respectively. These bands are associated with the vibrations of sp³ and sp² hybridized carbon within the hexagonal lattice of graphite. The intensity ratio of the D to G bands (ID/IG) is 1.02, reflecting the degree of graphitization of Na-CDs (Q. Yang et al., 2018).

Close

Fig. 2

(a) TEM images and particle size distribution of Na-CDs, (b)HRTEM images, (c) and (d) are XRD and RAMAN data images of Na-CDs, respectively.

Export to PPT

The infrared spectroscopy data are presented in Fig. 3. The peaks at 3423, 2962, 1571, 1394 and 1072 cm⁻¹ can be attributed to the vibrations of O–H, C–H, C = C, C–H and C-O-C, respectively (W. Li et al., 2019; Rashid et al., 2012).

Close

Fig. 3

FTIR spectra.

Export to PPT

The elemental composition and corresponding valence states of Na-CDs were obtained by XPS. The full spectrum of XPS measurement is shown in Fig. 4a, and it can be seen that only C, O, and Na elements are contained. The three elements were then analyzed at high resolution. The high-resolution map of C1s is shown in Fig. 4b, where 284.8, 286.5 and 288.3 eV are the characteristic peaks of C–C, O-C = O and C-O-C, respectively (Zhu et al., 2022). Fig. 4c presents the high-resolution O1s spectrum, where peaks at 531.2 eV for C-O, 533.1 eV for C = O, and a feature at 535.5 eV resulting from the sodium Auger peak (Na KLL) were identified through deconvolution (S. Ren et al., 2022). The characteristic peak at 1071.1 eV (Fig. 4d) is attributed to Na 1 s (X. Liu et al., 2022). The results of TEM, XRD, FTIR and XPS show that the prepared Na-CDs have relatively uniform particle size distribution and are rich in functional groups. Because of its rich functional groups and the larger atomic diameter of sodium atoms, the crystal face spacing of Na-CDs is larger than that of graphene. However, no matter what kind of characterization method, the successful preparation of sodium ion doped carbon dots has been proved.

Close

Fig. 4

(a) XPS full spectrum of Na-CDs, and (b)-(d) high-resolution XPS images of C1s, O1s and Na1s, respectively.

Export to PPT

3.2 Weight loss measurements

Weight loss measurement is one of the common methods to evaluate the performance of corrosion inhibitors. By controlling the temperature and time of corrosion reaction and measuring the mass loss of metal samples in corrosive media containing different concentrations of corrosion inhibitors, the performance of inhibitors was analyzed. Corrosion rate (V) and corrosion inhibition efficiency (IE_w%) can be calculated by formulas (Eq. (1)) and (Eq. (2)) (Cen et al., 2019b):

In formula (1) and (2), W₀ and W are the weight of Q235 carbon steel before and after weight loss test, respectively. A and t are the surface area and soaking time of the sample, respectively. V₀ and V are the corrosion rates of samples in 1 M HCl solution without and with different concentrations of Na-CDs, respectively. Fig. 5 shows the corrosion inhibition rate of Q235 carbon steel in 1 M HCl solution without and with different concentrations of Na-CDs and the corrosion rate of Q235 carbon steel. The average values of mass loss (ΔW), corrosion rate (V) and corrosion inhibition efficiency (IE_W) of carbon steel at different Na-CDs concentrations are shown in Table 1. All the data of weight loss test are shown in Table S1.

Close

Fig. 5

Corrosion inhibition rate (red) and corrosion rate (blue) of Q235 Carbon steel in 1 M HCl solution containing different concentrations of Na-CDs at 289 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Export to PPT

Compared with the corrosion rate of Q235 carbon steel in 1 M HCl solution without Na-CDs, the addition of Na-CDs significantly reduces the corrosion rate. With the increase of the concentration of Na-CDs, the corrosion rate first increased and then decreased. When the concentration of Na-CDs was 150 mg/L (as show Table S1), the corrosion inhibition efficiency was the highest, reaching 96.07 %, and the corrosion rate was reduced from 0.1635 g·cm⁻²·h⁻¹ to 0.0064 g·cm⁻²·h⁻¹, 25.55 times decrease. It is worth noting that even at 50 mg/L, the inhibition efficiency of Na-CDs is about 90 %. However, unlike common CDs corrosion inhibitors, the corrosion inhibition rate increases with the increase of concentration, and when the concentration exceeds 150 mg/L, the inhibition efficiency decreases when reaching 200 mg/L. This may be due to increased interactions between Na-CDs at higher concentrations, or it may be due to increased desorption rates of Na-CDs from metal surfaces at higher concentrations. But overall, the results of the weight loss measurements clearly show that Na-CDs provides excellent protection against carbon steel in 1 M HCl solutions.

3.3 Potentiodynamic polarization (PDP) analysis

Fig. 6(a) displayed the potentiodynamic polarization data for Q235 carbon steel in 1 M HCl solutions without and with various concentrations of Na-CDs at 298 K. It was clearly observed that the addition of Na-CDs to 1 M HCl resulted in a noticeable negative shift in the corrosion potential compared to the blank solution, with the maximum shift being 56.95 mV, which is less than 85 mV. This indicated that Na-CDs acted as a mixed-type corrosion inhibitor (Qiang et al., 2017; Zhao et al., 2022). The cathodic branches of all PDP curves showed a parallel trend, suggesting that the mechanism of the cathodic reaction remained unchanged upon the addition of Na-CDs, while the significant reduction in corrosion current density demonstrated that Na-CDs effectively suppressed the cathodic reaction (Xu et al., 2023). Moreover, the addition of Na-CDs resulted in a distinct plateau region around −0.33 V on the anodic branches, this is due to the adsorption of Na-CDs in the anode region, forming an adsorption film, thus inhibiting the anode reaction, there is an obvious anode passivation zone. When the polarization potential exceeded this value, the current density increased sharply, a phenomenon typically referred to as the desorption potential (E_des) (Desimone et al., 2011), where the Na-CDs rapidly desorbed from the metal surface, causing the anodic curve to coincide or nearly coincide with that of the blank condition (Cen et al., 2019a).

Close

Fig. 6

PDP data result (a), EIS data result (b-d), and fitted circuit diagram (e).

Export to PPT

The corrosion potential (E_corr), corrosion current density (i_corr), and the Tafel slopes for the anode and cathode (b_a and b_c) were obtained by fitting the data in the Tafel region near the corrosion potential using CView2 software, as shown in Table 2. The inhibition efficiency IE_PDP was calculated using Equation (3) (R. Cui et al., 2011).

In the formula, where $i_{\mathit{corr}}^0$ and $i_{\mathit{corr}}$ are the corrosion rates without and with inhibitor, respectively.

It is noteworthy that, unlike many carbon dot inhibitors, the inhibition efficiency of Na-CDs does not increase monotonically with the concentration, and reaches the maximum when the concentration is 150 mg/L. At this concentration, the plateau region was widest and the E_des highest, indicating that at this point, the protective film formed by Na-CDs on the metal surface had the strongest adhesion (X. Zhang et al., 2001). When the concentration of Na-CDs exceeds 150 mg/L, the corrosion current will increase sharply, and the corrosion inhibition efficiency will decrease when the concentration reaches 200 mg/L. This phenomenon might be attributed to an enhanced aggregation effect of Na-CDs at higher concentrations, resulting in reduced film formation on the metal surface, resulting in an increase in the area of the metal exposed to the corrosive solution.

3.4 Electrochemical impedance spectroscopy analysis

Fig. 6(b-d) shows the Nyquist and Bode diagrams obtained by electrochemical impedance testing of Q235 carbon steel at 298 K temperature in 1 M HCl solution without or with different amounts of Na-CDs added. In the Nyquist plot (Fig. 6b), in the absence of Na-CDs, a large number of corrosion products are formed on the metal surface, and a tailing phenomenon occurs in the capacitor loop. After the addition of Na-CDs, the radius of the capacitor ring significantly increased, indicating that the exposed active site area was reduced, and Na-CDs formed a protective layer on the metal surface, reducing the charge transfer rate, thereby inhibiting the corrosion of Q235 carbon steel in 1 M HCl solution (Luo et al., 2021b; Ouakki et al., 2020). The |Z| value at low frequencies also reflected the inhibitive performance of Na-CDs, with the |Z| value at 0.01 Hz increasing from 11.01 Ω·cm² in the blank to 175.09 Ω·cm² at a concentration of 150 mg/L, a 15.9-fold increase. As shown in Fig. 6c, the log10 (|Z|) increased from 1.04 to 2.25 Ω·cm². The phase angle data, as depicted in Fig. 6d, After the addition of Na-CDs, both show a wider Angle, which looks like the overlap of two angular peaks, indicating that the adsorption of Na-CDs on the Q235 carbon steel surface might be due to two reasons: 1) the double-layer capacitance (C_dl); and 2) the adsorption and desorption of Na-CDs at the metal/solution interface (Xu et al., 2023).

To better understand the EIS data, we used the equivalent circuit model as shown in Fig. 6f to fit the data, where I and II correspond to the fitted circuit after blank solution and Na-CDs addition, respectively. In the circuit, solution resistance, film resistance and charge transfer resistance are denoted by Rs, Rf and Rct respectively. At the same time, considering the non-homogeneity of the electrode, the ideal film capacitor (C_f) and double layer capacitor (C_dl) are replaced by CPE_f and CPE_dl. The impedance function of CPE can be calculated by equation (4) (Tan et al., 2020):

The impedance function of CPE was calculated using Equation (4), where Y₀ is the magnitude of the constant phase element, j is the imaginary unit, ω is the angular frequency, and n is the dispersion coefficient. When n equals −1, 0, or 1, the CPE represents an inductor, resistor, or capacitor, respectively. When n is between −1 and 1, it represents non-ideal capacitive or inductive behavior, and n can also be interpreted as an index of surface heterogeneity (Dong et al., 2023). The formulas for C_f and C_dl are shown in Equation (5).

The inhibition efficiency IE_EIS of Na-CDs on Q235 carbon steel in 1 M HCl was calculated using Equations (6) and (7) (Y. Zhang et al., 2021). The resulting data, as shown in Table 3.

As indicated in Table 3, the EIS data were consistent with the weight loss tests and potentiodynamic polarization data, all of which demonstrated the highest efficiency at a concentration of 150 mg/L. In Table 3, the values of n₁ and n₂ were not integers in the absence of Na-CDs, suggesting that the electrochemical reactions on the metal surface under blank conditions were of a non-ideal dielectric nature. After the addition of the inhibitor, n₁ was consistently 1, while n₂ remained non-integer, indicating that the protective film formed by the inhibitor exhibited ideal capacitive behavior with the 1 M HCl solution, but the interface between the film and the metal was still characterized by non-ideal dielectric behavior. This could also be interpreted as the film formed by Na-CDs on the metal surface being heterogeneous. The resistances through the solution/film interface (R_f), the film/metal interface (R_ct), and their sum (R_p) revealed that both R_ct and R_p were at their maximum at a concentration of 150 mg/L. This could be understood to mean that at this concentration, the adsorption of Na-CDs on the metal surface was the most compact and the interaction with the metal was the strongest, resulting in the most dense and complete protective film. However, when the concentration exceeded this level, reaching 200 mg/L, the resistance provided by the film dropped sharply, indicating that the adsorption on the metal surface weakened and the film became less dense and complete, a conclusion consistent with that drawn from potentiodynamic polarization studies.

3.5 Adsorption isotherm and thermodynamic analyses

The adsorption process of inhibitors is of significant importance for studying the mechanism of corrosion inhibition. Different inhibitor systems are suitable for different adsorption isotherms. Goodness of fit R² can be used to judge whether the data fit the model adequately, and its value range is [0,1]. The closer the value is to 1, the better the fit. In the field of corrosion inhibitors, the most commonly used models are Langmuir and Freundlich isotherms. However, when the main body of the inhibitor is nanoparticles, these two adsorption isotherms are not suitable for direct application. This is because the aggregation effect and heterogeneity inherent to nanoparticles can have a certain impact on the adsorption process. Equations (8), (9), and (10) are the Langmuir, Freundlich, and Redlich-Peterson equations, respectively (Porkodi & Vasanth Kumar, 2007).

q_m: Maximum adsorption capacity, unit: mg/L.

K_Langmuir: Langmuir adsorption equilibrium constant, unit: L/mg.

C_e: Equilibrium concentration, unit: mg/L.

K_Freundlich: Adsorption equilibrium constant of Freundlich.

n: indicates the adsorption strength of the adsorbent.

A, B, α are parameters of the Redlich-Peterson model.

The Redlich-Peterson (R-P) adsorption isotherm is a three-parameter model that can, under certain conditions, be reduced to the Langmuir or Freundlich models (Wu et al., 2010). It incorporates characteristics of both the Langmuir and Freundlich models, allowing the R-P equation to describe the adsorption process on heterogeneous layers more accurately than the Langmuir and Freundlich isotherms (Wu et al., 2010). Considering the tendency of nanoparticles to aggregate and the microscopically uneven state of the metal surface, the R-P model was chosen to fit the data. According to Cen et al. (Cen et al., 2020) and Wu et al. (Wu et al., 2010), the linear exponential expression has higher fitting accuracy than the linear logarithmic expression, converting Equation (10) to Equation (11).

In the adsorption process of nanoparticles as corrosion inhibitors, the value of α can represent the heterogeneity of the adsorption process. The closer the α value is to 1, the more it resembles monolayer adsorption. Ignoring the heterogeneity of the metal surface and considering it as an ideal solid surface, the effect of nanoparticle aggregation on the effective concentration for corrosion inhibition can be judged by the α value; the closer α is to 1, the less effect it has on the effective inhibition concentration (Cen et al., 2020; Dong et al., 2023).

The adsorption quantity q_e can typically be inferred by the surface coverage rate θ. The surface coverage rate θ is expressed as follows (Eq. (12)):

The IE_EIS in Equation (10) can be obtained through Equation (5). By calculating and fitting the data using Excel and Origin, the α value (α = 1.15) was obtained. Substituting the α value into (Ce)^α, the (Ce)^α value was obtained. A linear regression graph of Ce/qe versus (Ce)^α was plotted (Fig. 7a) with an R² value of 0.9987, slightly higher than that of the Langmuir fitting (Fig. 7b) with an R² of 0.9974. This result indicates that the R-P adsorption isotherm is more suitable than Langmuir isotherm to describe the adsorption process of carbon dot inhibitors on metal surface. As for the Freundlich isotherm, the results were clearly not applicable (Figure S1). Furthermore, according to the definition of the R-P equation and the fitting results, the adsorption of Na-CDs on the metal surface was very close to the ideal state described by the Langmuir adsorption. If the minimum heterogeneity of the metal surface and the weak interactions between the Na-CDs adsorbed on the metal surface, the adsorption–desorption equilibrium constant K_ads can be calculated to be 264.94 L/g according to Equation (8). The K_ads value depends on the adsorption capacity of the sample; the larger the K_ads value, the stronger the adsorption capacity. In addition, the standard Gibbs free energy $\mathrm{\Delta }{G}_{\mathit{ads}}^0$ is an important physical quantity for studying the adsorption of inhibitors. When $\mathrm{\Delta }{G}_{\mathit{ads}}^0$ is greater than -20 kJ/mol, it indicates a physisorption process, and when it is less than -40 kJ/mol, it indicates a chemisorption process. This can be calculated through Equation (13) (Zeng et al., 2022).

Close

Fig. 7

Isotherm adsorption curves of (a) R-P, (b) Langmuir.

Export to PPT

In this formula, R is 8.314 J/(mol·K), and T is the absolute temperature, which in this study is 289 K. By calculation, the value of $\mathrm{\Delta }{G}_{\mathit{ads}}^0$ was found to be -30 kJ/mol. The value of Na-CDs is between -20 and -40 kJ/mol, which indicates that Na-CDs is adsorbed to the metal surface through a combined action of physical and chemical adsorption.

3.6 Corrosion morphology analyses

To better understand the corrosion inhibition mechanism of Na-CDs, SEM and EDS were used to analyze the corrosion morphology of Q235 carbon steel in its original state, as well as after soaking for 24 h in 1 M HCl solution without Na-CDs and with 150 mg/L Na-CDs (Fig. 8). It can be observed that the original metal surface (Fig. 8a) only exhibits scratches from polishing and is relatively smooth. In the 1 M HCl solution without Na-CDs, a large amount of corrosion products appeared (Fig. 8b), and the surface was significantly rougher than in its original state. After adding 150 mg/L Na-CDs, the corrosion products on the metal surface were greatly reduced, and the surface products appeared more like a thin film (Fig. 8c), which was obviously smoother than the metal surface soaked in the Na-CDs-free solution for 24 h. To further investigate the surface state of the metal and the film-forming nature of Na-CDs on the metal surface, EDS was performed on the region of Fig. 8 a1-c1 to analyze the elemental composition of the metal surface under different states.

Close

Fig. 8

Q235 carbon steel morphologies in different states.

Export to PPT

The results obtained from EDS are shown in Fig. 9a-c, Figure S2, and Table S2. From Fig. 9a, it shows that the original Q235 steel mainly consists of Fe and C. After soaking for 24 h in 1 M HCl solution (Fig. 9b), the content of Fe atoms on the metal surface dropped sharply from 84.89 at% to 49.54 at%, the O content increased from 0.99 at% to 34.71 at%, and Cl atoms appeared on the surface (4.63 at%), indicating severe corrosion of the metal. In contrast to the metal in 1 M HCl solution, after adding Na-CDs (Fig. 9c), the Fe atom content on the metal surface increased, from 49.54 at% to 69.99 at%, while the O content decreased sharply (from 34.71 at% to 7.69 at%). This is because Na-CDs form a dense protective film on the Q235 carbon steel surface through both physical and chemical adsorption, inhibiting the dissolution of HCl on the steel (Zhu et al., 2021).

Close

Fig. 9

EDS data of Q235 carbon steel in different states.

Export to PPT

There are two noteworthy points. First, by comparing the metal surface states under different Na-CDs concentrations, it can be found that although the corrosion inhibition rates of different concentrations of Na-CDs is different, what is common is that they can all form good films on the metal surface (Figure S3), providing excellent protection to Q235 carbon steel, which corresponds to the results from the earlier weight loss tests and electrochemical tests. Another point to note is that by comparing the state of the metal at a concentration of 150 mg/L with and without slow rinsing with deionized water (Figure S4), it can be found that the morphology of the products on the metal surface is different, but after slow rinsing with deionized water, there will no longer be large particle agglomerations on the metal surface, and the contents of Cl and O decrease sharply. This indicates that Na from Na-CDs combines with Cl ions to form NaCl on the metal surface, which is rapidly lost with the rinsing water, consistent with the earlier speculation.

3.7 Inhibition mechanism for Na-CDs

As shown by FRIT and XPS, Na-CDs are composed of Na, C, O, and are rich in hydroxyl, carboxyl, and carbonyl groups. The good agreement between the weight loss tests and electrochemical test results suggests that Na-CDs are most effective at 150 mg/L in 1 M HCl solution for Q235 carbon steel. The Zeta potential results show that Na-CDs become positively charged (24.8 mV) in 1 M HCl solution, while the iron surface is negatively charged in hydrochloric acid. Therefore, Na-CDs will adsorb onto the metal surface through electrostatic attraction. Additionally, through adsorption analysis and analysis of different states of the metal surface, it is known that the film formation of Na-CDs on the metal surface is formed by the combined action of physical and chemical adsorption, which is consistent with the electrochemical test results. At 150 mg/L, Na-CDs have the strongest adsorption capacity on the metal surface, and the film formed is the most complete and dense. However, when the concentration exceeds 150 mg/L, the interaction between nanoparticles increases, leading to desorption of Na-CDs from the metal surface, thereby reducing the density and integrity of the protective film. To more intuitively explain this phenomenon, this study proposes the corrosion inhibitor mechanism shown in Fig. 10. As illustrated in Fig. 8, when Na-CDs are added to the solution, they first adsorb onto the metal surface to form a film, with Cl ions playing a bridging role. When the concentration of the inhibitor is further increased, the interaction between Na-CDs strengthens, leading to desorption from the metal surface and reducing the density and integrity of the film.

Close

Fig. 10

Corrosion inhibition mechanism diagram.

Export to PPT