Evaluation of the inhibition behavior of carbon dots on carbon steel in HCl and NaCl solutions

1. Introduction

With the development of industry, many media containing Cl^- were easy to cause severe corrosion of steel, which would induce enormous destruction and potential threat to the metal equipment, reducing its service life and safety [[1], [2], [3], [4]]. The emergence of corrosion inhibitor was an effective method to reduce the corrosion of metal in Cl^- contained solutions [[5], [6], [7], [8]]. Some traditional inhibitors (chromate, mercury salts, phosphorus-containing compounds) were difficult to biodegrade and exhibited a certain degree of poisonousness, causing serious environment pollution problems [9]. Thus, the exploration and development of inhibitor with high corrosion inhibition efficiency and low toxicity were urgent [[10], [11], [12], [13]].

As a kind of zero-dimensional nanomaterial, carbon dots (CDs) attracted a lot of attention because of their excellent photochemical property and good biocompatibility [[14], [15], [16], [17]]. At the moment, the application of CDs was mainly focused on bioimaging, biosensors, electroanalysis, and nanocarriers fields [18,19]. Sun et al. [20] prepared a kind of CDs via laser ablation, and found that the as-prepared CDs with size of 5 nm showed a good potential in bioimaging applications. Jiang et al. [21] prepared a new type of N-doped CDs (N-CDs) by a one-step microwave irradiation method without any solvent and catalyst. By measurement, the as-prepared N-CDs revealed a highly sensitive electrochemical response. At the same time, N-CDs was also applied to the corrosion protection field due to its good water solubility, non-toxicity and low cost [[22], [23], [24]]. Cui et al. [25] obtained the N-CDs through antibiotic aminosalicylic acid and then discussed the corrosion inhibition effect of steel in N-CDs solution. The result showed that the addition of N-CDs could significantly reduce the corrosion rate of steel in HCl solution. Ye et al. [26] selected the imidazole and citric acid as raw material to prepare a novel N-CDs. After test, the inhibition efficiency of steel in 1 M HCl solution was more than 90 % when the inhibitor concentration exceeded 100 mg/L. After that, they continued to develop a green corrosion inhibitor of N-CDs, and pointed out that the corrosion current density of steel in N-CDs solution was reduced by one order of magnitude [27].

In this research, a novel N-CDs was prepared via hydrothermal method between methacrylic acid and ethyl(methyl)amine. Compared with the above mentioned traditional inhibitors, the as-prepared N-CDs owned the advantages of environmental friendliness, strong chemical inertness and convenient preparation [28]. Moreover, the N-CDs presented a higher corrosion inhibition efficiency and lower cost (93.93 % and 10-15 kg/\$) than many green inhibitors, such as Rollinia occidentalis extract (85.1 % and 250-280 kg/\$) [29], Ginkgo leaf extract (92.0 % and 200-300 kg/\$) [30], Konjac glucomannan (92.4 % and 180-200 kg/\$) [31], Pyridazinium ionic liquids (84.0 % and 100-200 kg/\$) [32], Glycine max extract (67.1 % and 20-26 kg/\$) [33], Cuscuta reflexa extract (80.7 % and 30-42 kg/\$) [33] and Salvia officinalis plant extract (87.0 % and 20-25 kg/\$) [34].

2. Experimental

2.1. Material preparation

Methacrylic acid, ethyl(methyl)amine, pure hydrochloric acid and sodium chloride were sourced from Aladdin Industrial Corporation. The aggressive solution was prepared by diluting hydrochloric acid and sodium chloride with deionized water. The Q235 steel was purchased via Sheng Xin technology co. LTD. Before use, the steel was polished with 400 #, 800 #, and 1200 # abrasive paper and then washed through deionized water.

2.2. Preparation of N-CDs

Fig. 1 shows the preparation method of N-CDs. 4.30 g of methacrylic acid and 2.96 g of ethyl(methyl)amine were added into 30 mL of deionized water. Afterwards, the mixture was heated to 180 °C by hydrothermal method for 2 h. Subsequently, the solution was centrifuged via centrifugal machine to enhance its purity and then the resultant solution was dialyzed using a dialysis membrane. The dialysis time was 24 h and the deionized water was renewed every 3 h. Finally, the filtrate was filtrated through PVDF membrane, the as-prepared N-CDs was placed to the vacuum oven at 60 °C for 24 h.

2.3. Characterization of N-CDs

Fourier transform infrared spectroscopy (FTIR) and ultraviolet spectrophotometer (UV-Vis) spectrometers were used to measure the structure of N-CDs. X-ray photoelectron spectroscopy (XPS) was selected to obtain the chemical component of N-CDs. Scanning probe microscope (SPM) and transmission electron microscopy (TEM) were chosen to observe the morphology and distribution of N-CDs. Before test, the silicon wafer and copper mesh were immersed in a low concentration of inhibitor solution for 3 min, and then dried 24 h in the vacuum oven.

2.4. Weight loss measurement

Corrosion rate was obtained according to the NACE standard. All specimens were weighed three times before test to obtain the original weight. After that, these specimens were immersed in different test environments for different time, then rinsed by deionized water, dried by N₂ gas and re-weighed to acquire the average corrosion rate (υ_corr), which was obtained through the following equation [35]:

υ_corr=$\frac{ΔA}{ S*t }$ (1)

where ΔA was the weight loss before and after immersion (g), S was the test area (cm²), t was the immersion time (h).

2.5. Electrochemistry characterization

Corrosion tests were evaluated via CHI660E electrochemical workstation at room temperature. The reference electrode was saturated calomel electrode (SCE) and the counter electrode was platinum plate. The electrochemistry test could be performed after the open current potential (OCP) was stable. After 24 h immersion, the Tafel test was measured through scanning the potential in the region of ±250 mV around the OCP and the scanning rate was 0.5 mV/s. EIS was measured at OCP with a 5 mV sinusoidal disturbance signal in the frequency range from 100 kHz to 50 mHz. Furthermore, the scanning vibrating electrode technology (SVET) with vibration amplitude of 30 μm was used to analyze the current density of steel surface in various solutions. Prior to test, the steel was wrapped with AB glue and the exposed area was 1 cm². The test environments were 1 M HCl and 3.5 wt%. NaCl solutions with different concentrations of N-CDs (0, 25, 50, 100, 200 mg/L).

2.6. Corrosion product characterization

Before characterization, the deionized water was used to wash the surface of steel after corrosion. Afterwards, all samples were dried by N₂ gas and then stored in vacuum oven for morphology observation. Scanning electron microscope (SEM) was selected to observe the two-dimensional morphology of steel surface. The three-dimensional morphology of steel surface was obtained via laser scanning confocal microscopy (LSCM). Raman, XPS and EDS were chosen to analyze the corrosion products.

3. Results and discussion

3.1. Structure characterization

Fig. 2 shows the FTIR, UV-vis, and XPS spectra of N-CDs. As seen in FTIR spectrum, a series of absorption peaks were presented in N-CDs, which located at 1373.9, 1461.8, 1637.5, 2831.3, 2994.3 and 3458.9 cm^-1, corresponding to -CH₂, N—C = O, C-H, -CH₃ and O—H vibrations (Fig. 2(a)) [36,37]. Thereinto, the appearance of N—C = O peak suggested that the amino of ethyl(methyl)amine was successfully reacted with the carboxyl of methacrylic acid. It could be seen from UV-vis spectrum that one absorption peak was observed at 380 nm, which might be related to the π-π* electronic transition (Fig. 2(b)) [27]. In order to obtain the composition of as-prepared N-CDs, the fine spectroscopy was analyzed. The C 1s XPS spectrum of N-CDs was divided into four peaks, including C—C, C—N, C—O and O=C-N bonds at 284.6, 285.1, 286.3 and 287.9 eV (Fig. 2(c)) [38]. Nevertheless, the N 1s XPS spectrum could be separated into three peaks: N—H at 398.9 eV, N—C at 399.9 eV, N—C at 401.8 eV, indicating the successful preparation of N-CDs (Fig. 2(d)) [39].

The high resolution TEM image was used to analyze the distribution of as-prepared N-CDs. From the TEM result, it could be seen that a lot of well-dispersed nanoparticles with size of 3.0-4.0 nm were discovered (Fig. 3(a) and (b)), implying a good distribution in deionized water. In the meantime, the morphology and distribution state could also be confirmed by SPM image. As seen in Fig. 3(c) and (d), the N-CDs presented a well-dispersed state and the height was about 2-8 nm, which was in accordance with the analysis of TEM.

3.2. Electrochemical studies

Fig. 4 shows the EIS data of steel in various test solutions. Due to the generation of adsorption film and corrosion reaction, all EIS data contained two capacitive loops (Fig. 4(a) and (d)). As the inhibitor concentration increased, the diameter of capacitive loop increased because the steel surface was covered by more inhibitor molecules, resulting in an increase in the thickness of adsorption film. The lowest frequency impedance value (|Z|_{f=0.01 Hz}) could use to appraise the corrosion resistance of steel in N-CDs solution [[40], [41], [42]]. It could be seen from Fig. 4(b) and (e) that the |Z|_{f=0.01 Hz} value in blank HCl and NaCl solutions was the lowest, suggesting that the steel was severely corroded by corrosive ions. However, the addition of N-CDs significantly enhanced the |Z|_{f=0.01 Hz} value of steel in both solutions. In addition, the highest |Z|_{f=0.01 Hz} value of steel was detected in 200 mg/L of N-CDs solution among all conditions, demonstrating that the N-CDs could endow steel excellent corrosion resistance, especially in HCl solution.

According to the Bode-phase angle plots, two time constants were shown in Fig. 4(c) and (f), implying the response of film and the charge transfer process [43]. In order to deeply understand the corrosion kinetics process, Fig. 4(b) and (e) shows the EIS parameters fitted by the equivalent circuit. Among them, R_s, R_f, CPE_f, R_ct and CPE_dl denoted the solution resistance, film resistance, film capacitance, charge transfer resistance and double-layer capacitance, respectively [[44], [45], [46]]. As seen in Table 1, the R_s value was stable in all test solutions, while some obvious variations were found in other parameters. In the meantime, the R_f and R_ct values were proportional to the concentration of N-CDs, implying that an effective adsorption film was formed on the surface of steel. However, the CPE_f and CPE_dl values were dramatically decreased compared to the blank condition, this was due to that the water molecules were replaced by the as-prepared N-CDs with low dielectric constant, thereby reducing the exposed area of steel in corrosion media.

After 24 h immersion, Fig. 5 shows the Tafel data of all specimens in various test solutions. Obviously, the addition of N-CDs could sharply reduce the corrosion current density (i_corr) of steel in both solutions, manifesting that the corrosion reaction was suppressed through the addition of N-CDs. By calculation, Table 2 summarized the i_corr, corrosion potential (E_corr), anodic Tafel slope (b_a), cathodic Tafel slope (b_c), polarization resistance (R_p), degree of surface coverage (θ) and corrosion inhibition efficiency (IE), which was obtained from Tafel curve. Thereinto, the θ and IE values could be obtained as follow [47,48]:

θ=$\frac{i_{ corr }^{0}-i_{ corr } }{ i_{ corr }^{0}}$ (2)

IE=$\frac{i_{ corr }^{0}-i_{ corr } }{ i_{ corr }^{0}}$×100% (3)

where i^°_corr and i_corr represented the corrosion current densities without and with inhibitor solution, respectively. It could be seen from Table 1 that the i_corr value was reduced by one order of magnitude after the addition of N-CDs. However, the R_p value displayed an opposite trend, implying that the anticorrosion ability of steel was gradually enhanced. Besides, the IE showed an uptrend with the increase of N-CDs concentration and reached the highest values of 93.93 % (HCl) and 88.96 % (NaCl) at 200 mg/L of N-CDs.

Fig. 6 reveals the local corrosion of steel surface in various test solutions, which was obtained through SVET. In the case of blank HCl and NaCl solutions, the anodic current density was almost positive in the test region and the value was the highest compared to other samples, suggesting severe corrosion in the anodic region. As the addition of N-CDs, the anodic current density in the test region manifested a significant reduction in both solutions, implying that the attack of corrosion ion was weaken to a certain degree. At the same time, the concentration of N-CDs had a great influence on the inhibition effect. Through observation, the anodic current density displayed a downtrend with the increase of N-CDs concentration, which confirmed that the adsorption film could availably suppress the damage of steel.

3.3. Weight loss measurement

Fig. 7 presents the average corrosion rates of steel in various conditions. After 24 h immersion, the corrosion rates of steel in pure HCl and NaCl solutions were about 2.05 × 10^-3 and 1.88 × 10^-3 g cm^-2 h^-1, respectively, which were the highest values among all conditions. After the addition of N-CDs, the corrosion rates of steel in both solutions decreased greatly (Fig. 7(a)). Meanwhile, the descend range decreased as the N-CDs concentration increased. When the N-CDs concentration increased to 200 mg/L, the corrosion rates reduced to the lowest values of 2.08 × 10^-4 and 7.43 × 10^-4 g cm^-2 h^-1 in HCl and NaCl solutions, which were reduced by 89.85 % and 60.47 % than in blank solution, respectively. The relationship between corrosion rate and immersion time was shown in Fig. 7(b). Clearly, the corrosion rate of steel in blank solution increased with the increase of immersion time. Differently, the corrosion rate of steel in 100 mg/L of N-CDs solution was reduced with time, further proving the corrosion protection effect.

3.4. Corrosion morphology and product analysis

Fig. 8 shows the 3D morphology and surface roughness of steel after corrosion test. It could be seen that the inhibitor concentration and immersion time greatly affected the corrosion situation of steel. As shown in Fig. 8(a) and (b), the surface of steel was the roughest in blank solution. By measurement, the average roughness (R_a) was about 1.872 (HCl) and 1.063 (NaCl) μm, which were the highest values among all conditions. The surface of steel became smoother and the average R_a value was reduced as the increase of N-CDs concentration. For instance, the average R_a values of steel were about 0.844 and 0.763 μm in HCl and NaCl solutions containing 25 mg/L of N-CDs, which were 54.91 % and 28.22 % lower than the blank solution, respectively (Fig. 8(c) and (d)). After that, the average R_a value continued to decline as the N-CDs concentration increased to 200 mg/L (Fig. 8(e) and (f)). Moreover, the average R_a values of steel in 200 mg/L of CDs solution reduced by 8.21 % (HCl) and 1.70 % (NaCl) when the immersion time increased from 24 to 48 h, respectively (Fig. 8(g) and (h)).

Fig. 9 demonstrates the 2D micromorphology of steel in various conditions. A large number of pores and cracks were observed on the surface of steel in blank solution, implying severe corrosion caused by Cl^- contained solutions (Fig. 9(a) and (b)). Compared with blank solution, the damage of all specimens was significantly suppressed after the addition of as-prepared N-CDs (Fig. 9(c) and (d)). Meanwhile, the corrosion degree was inversely proportional to the concentration of N-CDs (Fig. 9(e) and (f)). Furthermore, the effect of immersion time on the corrosion degree was similar to the concentration of N-CDs (Fig. 9(g) and (h)). Through the morphology observation, it could be seen from the EDS spectra that four kinds of elements (Fe, C, O and Cl) were detected on the surface of steel. By comparison, the sign intensity of Cl and O elements in blank solution was stronger than that in N-CDs solution (Fig. 9(i) and (k)). As the N-CDs concentration increased, the intensity of Cl and O elements presented a downtrend, suggesting that the corrosion degree was alleviated to a certain degree (Fig. 9(j) and (l)).

In order to quantitatively analyze the corrosion products, the Raman and XPS results were shown in Fig. 10. In the case of Raman spectrum, a series of characteristic peaks at 176.2, 212.3, 293.2, 323.4, 399.1, 514.3 and 690-710 cm^-1 were detected, which was related to the Fe₂O₃, γ-FeOOH, α-FeOOH, Fe₃O₄ and β-FeOOH, respectively (Fig. 10(a)) [49]. By contrast, the as-prepared N-CDs in HCl and NaCl solutions could effectually inhibit the formation of corrosion product. Specially, the peaks of Fe₃O₄, α-FeOOH, γ-FeOOH and Fe₂O₃ almost disappeared when the steel was immersed in 200 mg/L of N-CDs solution, showing the weakest corrosion degree. From the XPS results, the content of Fe was the highest among all elements, which was derived from the steel substrate. Except for Fe element, the contents of O and Cl elements were the highest in blank solutions, and presented a downtrend with the increase of N-CDs concentration, demonstrating the decrease of corrosion degree (Fig. 10(b)).

3.5. Adsorption isotherm

According to the previous study, the adsorption isotherm was calculated [50]:

$\frac{θ}{1-θ}= K_{ads}C $ (4)

where θ, C, and K_ads were the surface coverage, inhibitor concentration and adsorption equilibrium constant, respectively. As shown in Fig. 11, the fitting degree (R² value) of steel in HCl and NaCl solutions was about 0.9976 and 0.9995, respectively, indicating that the adsorption process of N-CDs was in accord with Langmuir adsorption model during corrosion test. In the meantime, the adsorption free energy (ΔG⁰_ads) was calculated [51]:(5)ΔGads0=-RTIn(1000Kads)where R and T represented the molar gas constant (8.314 J mol^-1 K^-1) and the absolute temperature (298 K), respectively. Through calculation, the K_ads values of steel in HCl and NaCl solutions were -27.94 and -27.16 kJ/mol, respectively. In general, the K_ads value presented a positive correlation with adsorption strength [52]. Thus, the adsorption strength of N-CDs in HCl environment was stronger than that in NaCl environment, which was well in agreement with the result of EIS data. Moreover, the adsorption type could be distinguished by the value of ΔG°_ads. The type of interface adsorption was physical interaction if the ΔG°_ads was higher than -20 kJ/mol. The type of interface adsorption was chemical interaction if the ΔG°_ads was lower than -40 kJ/mol [53]. Thus, the adsorption mechanism of as-prepared N-CDs on the interface was physicochemical interaction.

4. Conclusion

Novel N-CDs inhibitor was successfully prepared by hydrothermal method, and its microstructure and corrosion production behavior in 1 M HCl and 3.5 wt% NaCl solutions were systematically investigated. The results showed that the as-prepared N-CDs with average size of 3.0-4.0 nm exhibited a good distribution. Through electrochemical measurement, the impedance modulus of steel in HCl and NaCl solutions increased from 13.2 and 185.7 Ω cm² to 1281.4 and 684.6 Ω cm² when the N-CDs concentration increased from 0 to 200 mg/L, respectively. In this case, the inhibition efficiency of steel obtained from Tafel curve reached up to 93.93 % and 88.96 %, respectively. Through the calculation result of Langmuir adsorption isotherm, the K_ads values of steel in HCl and NaCl solutions were about -27.94 and -27.16 kJ/mol, suggesting that the adsorption type of as-prepared N-CDs on the interface was physicochemical interaction.

Acknowledgements

This work was financially supported by Scientific Research Foundation of Jiangxi University of Science and Technology (No. 205200100421), the Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology, the Natural Science Foundation of Jiangxi Province (No. 20181BBE58001), the Natural Science Foundation of Jiangxi Education Department (No. GJJ180431), the Research and Development Project of Ganzhou and the Science and Technology Innovation Talents Program of Ganzhou.

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