Journal of Materials Science & Technology  2019 , 35 (10): 2243-2253 https://doi.org/10.1016/j.jmst.2019.05.045

Orginal Article

A green and effective corrosion inhibitor of functionalized carbon dots

Yuwei Yeab*, Dongping Yangb, Hao Chena*

a The Institute of Engineering Research, Jiangxi University of Science and Technology, Ganzhou, 341000, China
b Key Laboratory of Marine Materials and Related Technology, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, 315201, China

Corresponding authors:   *Corresponding authors at: The Institute of Engineering Research, Jiangxi Uni-versity of Science and Technology, Ganzhou, 341000, China.E-mail addresses: y_w_ye@163.com (Y. Ye), chenhao_168@163.com (H. Chen).*Corresponding authors at: The Institute of Engineering Research, Jiangxi Uni-versity of Science and Technology, Ganzhou, 341000, China.E-mail addresses: y_w_ye@163.com (Y. Ye), chenhao_168@163.com (H. Chen).

Received: 2019-03-16

Revised:  2019-04-30

Accepted:  2019-05-22

Online:  2019-10-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

In this work, a green and effective corrosion inhibitor of functionalized carbon dots (FCDs) was synthesized by the conjugation of imidazole and citric acid carbon dots (CA-CDs). The corrosion inhibition behavior of FCDs for Q235 steel in 1 M HCl solution was systematically investigated by electrochemical analysis, corrosion morphology and adsorption isotherm. The electrochemical results implied that the as-prepared FCDs inhibitor could effectively suppress the corrosion of Q235 steel in 1 M HCl solution. At the same time, the inhibition efficiency of steel in 1 M HCl solution was more than 90% when the inhibitor concentration exceeded 100 mg/L. This excellent property was attributed to the coverage of adsorption film on the steel surface, which conformed to the Langmuir adsorption model. In addition, the analysis of adsorption isotherm displayed that the adsorption mechanism was the physicochemical interaction at the steel/solution interface.

Keywords: Q235 steel ; Carbon dots ; Adsorption ; HCl solution

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Yuwei Ye, Dongping Yang, Hao Chen. A green and effective corrosion inhibitor of functionalized carbon dots[J]. Journal of Materials Science & Technology, 2019, 35(10): 2243-2253 https://doi.org/10.1016/j.jmst.2019.05.045

1. Introduction

Metal materials occupy a very important status in various industries of modern society [[1], [2], [3], [4]]. At the same time, the corrosion of metal is also widespread, and the damage is extremely serious [[5], [6], [7], [8]]. The addition of corrosion inhibitor is a common anti-corrosion method, which has the characteristics of simple process, low cost and strong practicality [[9], [10], [11], [12], [13]]. It can significantly reduce the corrosion rate of metal material and extend the service life of equipment. Some traditional high-efficiency corrosion inhibitors, such as chromate, mercury salts, phosphorus-containing compounds, are difficult to biodegrade and exhibit a certain degree of toxicity [14,15]. Therefore, it is necessary to develop some environment-friendly corrosion inhibitors to replace these traditional corrosion inhibitors [16].

Carbon dots (CDs) and imidazole are two new types of green corrosion inhibitors, which have received plentiful attentions. Due to the advantages of rich raw materials, good water solubility, non-pollution, low cost, and simple synthesis, carbon dots have been used for the research of anti-corrosion field [17,18]. For instance, Cui et al. [19] obtained a novel CDs from aminosalicylic acid and discussed its inhibitive effect for steel in hydrochloric acid environment. The result showed that the corrosion rate of steel in 5 mg/L CDs solution was the lowest among all concentrations, presenting the strongest anti-corrosion ability. Afterwards, they continued to prepare two kinds of carbon dots from p-phenylenediamine (p-CDs) and o-phenylenediamine (o-CDs). By analysis, the corrosion inhibition efficiencies of p-CDs and o-CDs reached up to 88% and 94% at the concentration of 200 mg/L, respectively [20].

As a green high-efficiency corrosion inhibitor, imidazole has become a research hotspot because of its rich chemical properties, good water solubility and excellent inhibitory effect [[21], [22], [23]]. For example, Likhanova et al. [24] studied the influence of ionic liquid with imidazole on the inhibition behavior of mild steel in H2SO4 environment. They discovered that the imidazole compound at 100 ppm presented a high corrosion efficiency for steel in H2SO4 solution. Milošev et al. [25] investigated the inhibition effect of imidazole, benzimidazole and its derivatives for copper in 3 wt% NaCl solution, and manifested that the corrosion current density of copper in imidazole solution was greatly decreased compared with the blank solution.

Therefore, a functionalized inhibitor was synthesized using natural citric acid and imidazole. Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectronic spectroscopy (XPS), transmission electron microscopy (TEM) and scanning probe microscope (SPM) were selected to estimate the structure and component of as-prepared FCDs. Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (Tafel), scanning vibrating electrode technique (SVET) and weight loss measurement were chosen to appraise the inhibition effect of as-prepared FCDs. Scanning electron microscopy (SEM) and laser scanning confocal microscope (LSCM) were used to achieve the surface characterization of Q235 steel after corrosion test.

2. Experimental

2.1. Materials

N-carboxysuccinimide (NHS), (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), citric acid and N-(3-aminopropyl)imidazole were provided by Aladdin Industrial Corporation. Q235 steels (1 × 1 cm2) were used for the corrosion test. All specimens were polished using 400$\widetilde{1}$200 grit sandpapers and then cleaned three times with ethanol.

2.2. Preparation of citric acid carbon dots (CA-CDs)

The citric acid carbon dots were prepared through previous study [26]· In brief, a certain amount of citric acid (1 g) was added into 100 mL of round-bottomed flask and then heated at 200 °C in an oven for 30 min. After that, the pH of obtained solution was adjusted to 7.0 by 50 mL of 1 M NaOH solution. Then, the solution was further purified through 12 h dialysis treatment in deionized water and the water was replaced every 4 h. In the end, the obtained CA-CDs was transferred to a vacuum oven at 65 °C for one day.

2.3. Preparation of functionalized carbon dots (FCDs)

The preparation schematic of FCDs is displayed in Fig. 1. 4 mg of CA-CDs, 2 g of (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and 1 g of N-carboxysuccinimide were dissolved into 200 mL of deionized water and then stirred for 10 min, the CA-CDs solution was obtained. Later, 2 g of N-(3-aminopropyl)imidazole was weighed and dissolved in 40 mL of ethanol, and 1 mL of triethylamine was added into the mixture to obtain the imidazole solution. Finally, the imidazole solution was added into the CA-CDs solution and then stirred for 10 h. The reaction product was dialyzed three times and then evaporated in a vacuum oven at 65 °C for 24 h.

Fig. 1.   Preparation schematic of FCDs.

2.4. Preparation of FCDs solution

In order to analyze the inhibition behavior of FCDs in 1 M HCl solution, the concentrations of FCDs inhibitor were selected as 0, 25, 50, 100 and 200 mg/L. As a comparison, the corresponding concentrations of CA-CDs and imidazole solutions were prepared.

2.5. Structure characterization

Structure of FCDs was characterized using NICOLET 6700 FTIR (Thermo, US). Chemical component and chemical bonding of FCDs were analyzed using AXIS XPS (Kratos, UK). Morphology and grain size of FCDs were studied through Dimension 3100 SPM (Vecco, US) and TF20 TEM (FEI, US). Before observation, a spot of FCDs inhibitors were dissolved in deionized water and then dropped on the silicon surface. After drying, the samples were placed in SPM instrument for morphology observation. Finally, the line scanning (height) was obtained through the SPM software.

2.6. Weight loss measurement

Prior to test, the Q235 steel was weighted three times through precision electronic scale. After that, the steel was soaked in 1 M HCl solution with different concentrations of inhibitors. Ultimately, the steel was cleaned by deionized water and the corrosion rate (υcorr) could be acquired [27]:

υcorr=$\frac{ W1-W2}{ A*T }$ (1)

Among them, W1 and W2 values were the weight before and after test (g), respectively. A and T values were the test area (cm2) and immersion time (h), respectively.

2.7. Electrochemical characterization

The evolution of inhibition behavior for steel was examined through electrochemical workstation (CHI-660E) with classic three electrodes system. The reference electrode was saturated calomel electrode (SCE), the counter electrode was platinum plate with area of 2.5 cm2 and the working electrode was Q235 steel with an exposed area of 1 cm2. Open circuit potential (OCP) measurements were implemented for 30 min to achieve a steady state of steel in test solution. Tafel tests were recorded in a range of -0.2-0.8 V with a scan rate of 1 mV s-1. Electrochemical impedance spectroscopy tests were carried out at an alternate current amplitude of 20 mV and a frequency range of 105-10-2 Hz. The local corrosion situation of steel was determined through scanning vibrating electrode technique with the vibration amplitude of 30 μm, the vibration frequency of 80 Hz and the test area of 2000 μm × 2000 μm.

2.8. Surface analysis

All specimens were washed through deionized water and then dried through N2 gas after test. S4800 SEM (Hitachi, Japan) was used to detect the macroscopic corrosion morphology of Q235 steel surface. 3D LSM700 LSCM (Zeiss, Germany) was selected for the analysis of surface roughness of all specimens.

3. Results and discussion

3.1. Structure analysis of FCDs

Fig. 2 displays the FTIR and XPS spectra of as-prepared FCDs. In the case of FTIR spectrum, a series of characteristic peaks at 1184.6, 1400, 1499.9, 1549.9, 1639.8, 1642.8, 2958.1 and 3412 cm-1 were ascribed to C-OH, COO-, N—H, C-N, N—C=O, C=O, C-H and O—H, respectively (Fig. 2(a)) [26]. The C-OH, COO-, C=O and O—H bonds were from CA-CDs, while the N—H and C-N bonds were derived from the imidazole. In addition, the formation of N—C=O bond implied that the amidogen of imidazole successfully reacted with the carboxyl of CA-CDs. For XPS spectra, C, N and O elements were found on as-prepared FCDs (Fig. 2(b)). In order to reveal the existence form of C and N elements, the C 1s and N 1s fine spectra were acquired through Gaussian fitting. Four peaks were fitted in C 1s fine spectrum, which centered at 284.7, 285.9, 286.4 and 287.9 eV, corresponding to C—C, C=N, C-O and N—C=O bonds, respectively [28]. Meanwhile, the N 1s fine spectrum could be split into N—H, N—C and N=O bonds, which centered at 398.8, 399.9 and 400.9 eV, respectively [29]. These results were in agreement with the analysis of FTIR spectrum.

Fig. 2.   FTIR and XPS spectra of FCDs inhibitor: (a) FTIR; (b) XPS-full spectrum; (c) XPS-C 1s; (d) XPS-N 1s.

3.2. Morphology analysis of CA-CDs, imidazole and FCDs

Fig. 3 presents the TEM and SPM images of CA-CDs, imidazole and FCDs. Obviously, a uniform spherical structure with size of 3-5 nm was observed in the TEM image of CA-CDs. For imidazole, some aggregations were detected and the average size was about to 5-8 nm. After reaction, the average size of FCDs was reduced to 4-6 nm, implying that the connection of CA-CDs could improve the distribution of imidazole. The phenomenon could be further verified by SPM image. The shape of CA-CDs and FCDs was graininess and the dispersion was uniform, while some local agglomeration phenomena were observed for imidazole. By statistics, the height profiles of CA-CDs and agminated imidazole were about 1-2 and 15-25 nm, respectively. After reaction, the height profile of FCDs was about 2-4 nm, which was higher than CA-CDs and lower than agminated imidazole.

Fig. 3.   TEM and SPM images of CA-CDs, imidazole and FCDs specimens: (a) TEM-CA-CDs; (b) TEM-imidazole; (c) TEM-FCDs; (d) SPM-CA-CDs; (e) SPM-imidazole; (f) SPM-FCDs; (g) Height-CA-CDs; (h) Height- imidazole; (i) Height-FCDs.

3.3. OCP analysis

Fig. 4 reveals the OCP curves of Q235 steel in pure HCl and inhibitor solutions. With the immersion time gone on, all OCP curves became stable. At the same condition, the OCP stabilization values of steel in imidazole and FCDs solutions were higher than that in pure HCl solution. Meanwhile, these OCP stabilization values increased as the inhibitor concentration increased. Nevertheless, the OCP stabilization values of steel in CA-CDs solution were different. When the CA-CDs concentration was lower than 100 mg/L, the OCP stabilization value of steel in CA-CDs solution was lower than that in pure HCl solution.

Fig. 4.   OCP curves of Q235 steel in various test solutions: (a) CA-CDs; (b) imidazole; (c) FCDs.

3.4. EIS analysis

Fig. 5 displays the Nyquist and Bode data of Q235 steel in the absence and presence of CA-CDs, imidazole and FCDs with different concentrations. For Nyquist data, the diameter of capacitive reactance arc was increased with the addition of CA-CDs, imidazole and FCDs, implying that the corrosion degree of steel in these solutions was weakened [30,31]. However, the relationships between inhibitor concentration and the diameter of capacitive reactance arc were different. With the inhibitor concentration increased, the diameter of capacitive reactance arc was stable in CA-CDs solution, while an ascend trend was observed in imidazole and FCDs solutions. This was due to the fact that the steel surface could be covered by more imidazole and FCDs inhibitors [32]. In the other words, the adsorption film became denser as the imidazole and FCDs concentration increased. At the same time, two capacitive reactance arcs were detected in all Nyquist curves, corresponding to the charge transfer process and relaxation process.

Fig. 5.   EIS images of Q235 steel in various test solutions after 24 h immersion: (a, b, c) CA-CDs; (d, e, f) imidazole; (g, h, i) FCDs.

In the case of Bode data, the |Z|0.01 Hz could be selected to appraise the corrosion rate of electrode [[33], [34], [35]]. Obviously, the |Z|0.01 Hz was about 28 Ω cm2 when the Q235 steel was immersed into pure HCl solution for one day. As the inhibitor concentration increased, the |Z|0.01 Hz kept around at 250 Ω cm2 in CA-CDs solution. However, the |Z|0.01 Hz presented an increase trend with the increased of imidazole and FCDs concentration. The |Z|0.01 Hz reached up to 582 and 1165 Ω cm2 after one day immersion in 200 mg/L of imidazole and FCDs solutions, respectively. In addition, two time constants were observed on all phase angle curves and the height of peak increased as the inhibitor concentration increased, implying a more powerful response from the adsorption of inhibitor in steel/solution interface.

In order to better understand the corrosion process, Fig. 6 manifests the equivalent circuit model of corrosion test and Table 1 summarized the fitting parameters. Among them, Rs, Rf, Rct and CPE represented the solution resistance, the film resistance, the charge transfer resistance and the constant phase angle element, respectively [36,37]. As seen in Table 1, the similar values of Rs were observed in various concentrations of CA-CDs, imidazole and FCDs solutions. The Rf and Rct values were enhanced after the addition of inhibitors, indicating the steel surface was covered by an effective adsorption film. In the meantime, the both values were gradually enhanced with the inhibitor concentration increased except for CA-CDs. By comparison, the FCDs presented the highest enhancement degree among all inhibitors. Moreover, the change trend of CPEdl was opposite to Rct value as the inhibitor concentration increased. This was due to the fact that the corrosion solution on the surface of steel was gradually replaced by inhibitors. Hu et al. [38] pointed out that the relationship between CPEdl and inhibitor concentration could be indirectly evaluated through Helmholtz model:

CPEdl=$\frac{ε°ε}{d}$ (2)

where ε° and ε are the permittivity of atmosphere and film, respectively. d represents the thickness of electric double-layer and S denoted the exposed proportion of electrode. In general, the thickness of electric double-layer displayed an uptrend and the exposed area of electrode showed a downtrend with the inhibitor concentration increased. These changes would lead to the decline of CPEdl value. The inhibition efficiency (η) could be obtained as follows [39]:

η=$\frac{ R_{ct}-R_{ ct,0} }{ R_{ct}}$ ×100% (3)

Fig. 6.   Equivalent circuit model of corrosion process.

Table 1   Impedance parameters of Q235 carbon steel in various test solutions.

Concentration (mg/L)Rs (Ω cm2)Rf (Ω cm2)Rct (Ω cm2)CPEf (μF cm-2)CPEdl (μF cm-2)η (%)
Blank1.634.2661.6146.7377.21-
CA-CDs251.5110.36209.4123.93279.5125.9
501.1514.89194.9620.94260.1231.1
1001.5815.68207.1616.87248.2834.1
2001.2816.24219.2113.74229.5639.1
Imidazole251.4315.35171.4221.41141.8362.4
501.4416.97405.1316.34104.3572.3
1001.1617.74506.4112.4182.9478.1
2001.2919.61566.813.5765.2282.7
FCDs251.1719.64298.7216.5375.1280.1
501.2725.85626.7711.8552.4386.1
1002.4531.57768.329.3437.3390.0
2002.2436.821108.247.3824.7393.4

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Thereinto, Rct and Rct,0 denoted the charge transfer resistances without and with inhibitor solution, respectively. By calculation, the η values of steel in various inhibitor solutions presented a huge difference. In the case of CA-CDs, the η value revealed a slight increase (25.9%-39.1%) when the inhibitor concentration increased from 25 to 200 mg/L, indicating the weak protective function. However, the η values of Q235 steel in imidazole and FCDs solutions were greatly enhanced as the inhibitor concentration increased. When the imidazole and FCDs concentration was 200 mg/L, the η values reached up to 82.7% and 93.4%, respectively. Meanwhile, the corrosion inhibition effect of FCDs was stronger than that of imidazole, implying that the protective effect of imidazole was improved through the modification of CA-CDs.

3.5. Tafel analysis

Fig. 7 shows the Tafel curves of Q235 steel in different inhibitor solutions and Table 2 summarized the Tafel parameters of corrosion potential (Ecorr), corrosion current density (icorr), anodic Tafel slope (ba), cathodic Tafel slope (bc), degree of surface coverage (θ) and corrosion inhibition efficiency (IE). Thereinto, the values of θ and IE were obtained through Eqs. (4) and (5) [40,41]:

θ= $\frac{i_{corr}^{0}- i_{corr}}{{i_{corr}^{0}}}$ (4)

IE=$\frac{i_{corr}^{0}- i_{corr}}{{i_{corr}^{0}}}$×100% (5)

where i0corr and icorr denote the corrosion current densities without and with inhibitor solution, respectively. The higher the value of ba and bc are, the stronger the inhibition effect is. Clearly, the ba and bc values were enhanced after the addition of inhibitors, suggesting that the anodic and cathodic reactions were suppressed. That is to say, the CA-CDs, imidazole and FCDs were the mixed-type inhibitors. Furthermore, the increase range of ba value was higher than that of bc value, illustrating that the anodic reaction was primarily inhibited.

Fig. 7.   Tafel curves of Q235 steel after 24 h immersion in various test solutions: (a) CA-CDs; (b) imidazole; (c) FCDs.

Table 2   Corrosion parameters of Q235 steel in various test solutions.

Concentration (mg/L)Ecorr (V vs. SCE)icorr (μA cm-2)ba (V dec-1)bc (V dec-1)θIE (%)
Blank-0.475256.55.477-8.711--
CA-CDs25-0.479192.65.126-8.1130.24924.9
50-0.482177.011.772-9.6850.31031.0
100-0.473165.79.785-13.1150.35435.4
200-0.470159.011.772-9.6580.38038.0
Imidazole25-0.48493.410.39-11.8750.63663.6
50-0.48570.47.271-9.5050.72872.8
100-0.48954.29.326-11.8320.78978.9
200-0.49643.19.135-10.8350.83283.2
FCDs25-0.47851.36.467-9.6010.80080.0
50-0.48733.16.125-9.4150.87187.1
100-0.50121.411.759-10.0390.91791.7
200-0.50915.315.466-8.4520.94094.0

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For corrosion potential, the values of imidazole and FCDs shifted to the direction of negative potential with the inhibitor concentration increased. Nevertheless, the movement in both directions was detected for CA-CDs inhibitor. In terms of corrosion current density, a mild reduction of icorr value was observed for steel in CA-CDs solution. After the addition of imidazole and FCDs inhibitors, the icorr value decreased by one order of magnitude and presented a downtrend with the increase of inhibitor concentration, implying that the corrosion rate of steel was inhibited. In addition, the θ and IE displayed a similar pattern to icorr value. When the inhibitor concentration was 200 mg/L, the IE values of steel in CA-CD, imidazole and FCDs solutions reached 38.0%, 83.2% and 94.0%, respectively. These phenomena were consistent with the EIS analysis, indicating that the FCDs was a high-efficiency inhibitor for Q235 steel.

3.6. SVET analysis

As an effective method, the SVET is often applied to investigate the localized corrosion of steel in HCl solution and the corrosion result is presented in Fig. 8. Obviously, the highest positive current density was detected for steel in pure HCl solution, declaring that the anode was severely corroded. After the addition of CA-CDs inhibitor, no significant change of anode current density was found, explaining the weak inhibition effect. Nevertheless, the anode current density of test area revealed a sharp decrease when the steel was immersed into imidazole and FCDs solutions. This discovery indicated that the corrosion situation of electrode was weakened to a certain degree. By comparison, the FCDs exhibited the strongest inhibition effect among all as-prepared inhibitors. In the meantime, the anode current density of steel decreased with the immersion time and inhibitor concentration increased, confirming that the inhibitor could adsorb on the surface of steel and then inhibit the occurrence of corrosion.

Fig. 8.   Current density distributions of Q235 steel in various test solutions and immersion time: (a) 0%-12 h; (b) CA-CDs-200 mg/L-12 h; (c) imidazole-200 mg/L-12 h; (d) FCDs-200 mg/L-12 h; (e) FCDs-100 mg/L-12 h; (f) FCDs-200 mg/L-24 h.

3.7. Corrosion morphology analysis

Fig. 9 reveals the corrosion morphology of Q235 steel in different corrosion solutions. The steel surface presented serious corrosion and desquamation after 24 h immersion in HCl and CA-CDs solutions (Fig. 9(a) and (b)). The successive attack of corrosion ion was the main reason for this phenomenon. However, the desquamation was greatly reduced and some cracks were observed on the steel surface after the addition of imidazole inhibitor, indicating that the corrosion was alleviated by imidazole to some extent (Fig. 9(c)). Afterwards, a smooth surface was found for steel in FCDs solution, implying the slightest corrosion among three inhibitors (Fig. 9(d)). At the same time, the steel presented a relatively severe corrosion in 100 mg/L FCDs solution compared to 200 mg/L FCDs solution, this was because of the augment of adsorption film thickness caused by high concentration inhibitor (Fig. 9(e)). Furthermore, the surface of steel remained smooth as the immersion time increased, verifying the protective function of FCDs inhibitor (Fig. 9(f)). Through EDS detection on part of surface of Fig. 9(f), there were some elements (Fe, C, N) on the surface of corroded steel. Thereinto, iron element was from the electrode surface, C and N elements were derived from the as-prepared FCDs inhibitor, which further affirmed the existence of adsorption film on steel surface.

Fig. 9.   Corrosion morphology and element distribution of Q235 steel in various test solutions and immersion time: (a) 0%-24 h; (b) CA-CDs-200 mg/L-24 h; (c) imidazole-200 mg/L-24 h; (d) FCDs-200 mg/L-24 h; (e) FCDs-100 mg/L-24 h; (f) FCDs-200 mg/L-48 h; (g) Fe; (h) C; (i) N.

The 3D morphology of corroded steel was a forceful tool to evaluate the surface feature and the result was shown in Fig. 10. The inhibitor type, immersion time and inhibitor concentration have a great influence on the surface roughness (Ra) of steel after test. Due to the powerful erosion of corrosion solution, the steel surface was extremely uneven after 24 h immersion in pure HCl solution and the Ra reached 1.791 μm (Fig. 10(a)). The Ra only declined by 0.306 μm when the steel was immersed into 200 mg/L CA-CDs solution (Fig. 10(a)). Differently, the imidazole and FCDs inhibitors vastly reduced the Ra of steel after immersion. In detail, the Ra values of steel in 200 mg/L imidazole and FCDs solutions were about 0.535 and 0.302 μm, respectively, which were 70.13% and 83.14% lower than that in pure HCl solution, respectively (Fig. 10(b)-(d)). By comparison, the FCDs inhibitor displayed the strongest inhibition among all inhibitors, which was well in agreement with the analysis of corrosion morphology. Meanwhile, as the inhibitor concentration and immersion time increased, the Ra of steel showed a downtrend (Fig. 10(e) and (f)).

Fig. 10.   3D morphology of Q235 steel in various test solutions and immersion time: (a) 0%-24 h; (b) CA-CDs-200 mg/L-24 h; (c) imidazole-200 mg/L-24 h; (d) FCDs-200 mg/L-24 h; (e) FCDs-100 mg/L-24 h; (f) FCDs-200 mg/L-48 h.

3.8. Corrosion rate analysis

For further know the precise corrosion loss of Q235 steel in different inhibitor solutions, Fig. 11 summarizes the relationships between inhibitor type, immersion time, inhibitor concentration and corrosion rate. After 48 h immersion, the corrosion rate of steel in pure HCl solution was the highest and the decline range was slight after the addition of CA-CDs (Fig. 11(a)). Nevertheless, the corrosion rates of steel in imidazole and FCDs solutions were one order of magnitude lower compared with pure HCl and CA-CDs solutions. At the same time, the corrosion rate of steel presented a decrease trend as the inhibitor concentration increased, and the FCDs solution presented the strongest inhibition ability on steel. Moreover, an increase in corrosion rate of steel was observed in HCl solution as the immersion time gone on, while the imidazole and FCDs inhibitors displayed an opposite trend, revealing the effective corrosion protection for steel (Fig. 11(b)).

Fig. 11.   Corrosion rate of Q235 steel in various test solutions and immersion time: (a) inhibitor concentration-48 h; (b) immersion time-200 mg/L.

3.9. Adsorption isotherm

The adsorption isotherm could be used to estimate the inhibition mechanism, which was obtained by Eq. (6) [42]:

$\frac{θ}{1-θ}$ =KadsC (6)

where θ, C and Kads are the surface coverage, inhibitor concentration and adsorption equilibrium constant, respectively. The relationship between C and C/θ values was displayed in Fig. 12. By calculation, the linear regression coefficients of R2 values were about 0.99996, 0.99993 and 0.99998 for CA-CDs, imidazole and FCDs inhibitors, respectively, presenting a perfect linear correlation. In the other words, the R2 value close to 1 indicated that the adsorption process of inhibitor conformed to the Langmuir adsorption model. Generally speaking, a high Kads indicates a strong adsorption action [43]. Namely, the corrosion protective behavior of inhibitor can be estimated through Kads. The Kads values of CA-CDs, imidazole and FCDs were about 2.53, 89.52 and 184.16 L/g, respectively. By comparison, the order of Kads of three inhibitors was FCDs > imidazole > CA-CDs. The steel in FCDs solution revealed the highest value of Kads, manifesting the strongest adsorption ability of inhibitor on steel/solution interface. Besides, the adsorption free energy (ΔG0ads) was selected for the judgment of adsorption type, which was obtained through Eq. (7)[9]:

ΔG0ads=-RTln(1000Kads) (7)

where R and T are the molar gas constant and absolute temperature (298 K), respectively. If the value of ΔG0ads was greater than -20 kJ/mol, the type was the physical adsorption. If the ΔG0ads value was lower than -40 kJ/mol, the type was the chemical adsorption [44]. As can be seen from the above formula that the ΔG0ads values of CA-CDs, imidazole and FCDs were about -19.41, -28.25 and -30.04 kJ/mol, respectively. Thus, the adsorption type of CA-CDs was physical adsorption in the steel/solution interface, while the imidazole and FCDs were the physicochemical adsorption.

Fig. 12.   Langmuir adsorption isotherms of Q235 steel surface in various test solutions: (a) CA-CDs; (b) imidazole; (c) FCDs.

3.10. Protective mechanism

Fig. 13 displayed the protective mechanisms of Q235 steel in various test solutions. From above analysis of Langmuir adsorption isotherms, the essence of adsorption film on the surface of steel in CA-CDs solution was the physical adsorption behavior (ΔG0ads > -20 kJ/mol). In this case, the corrosion medium could easily destroy this physical interaction (weak interaction) and then erode the steel, resulting in serious corrosion (Fig. 13(a)). In terms of imidazole inhibitor, the adsorption type changed to the physicochemical adsorption (-40 kJ/mol <ΔG0ads < -20 kJ/mol), and the interaction between solution and steel surface was greatly enhanced. The physical adsorption was attributed to the agglomeration effect of aromatic heterocyclic on the surface of steel. The chemical adsorption was ascribed to the heteroatom of N, which could easily fill in the unoccupied 3D orbitals outside the Fe atoms because of the lone pair electrons, and then formed a coordinate bond between heteroatom and steel. However, a small number of steel surface was exposed to corrosion medium due to the local aggregation and limited dispersion of imidazole (Fig. 13(b)). For FCDs inhibitor, the adsorption behavior was also mixed type and the interaction of steel/solution interface was the strongest duo to the highest Kads value. That is to say, the as-prepared FCDs inhibitor could uniformly adsorb on steel surface and then form a strong physicochemical interaction. Afterwards, the direct contact between corrosion medium and steel surface was avoided through the formation of adsorption film (Fig. 13(c)). In summary, the FCDs inhibitor combined the advantages of both CA-CDs and imidazole, showing the best protective effect for Q235 steel.

Fig. 13.   Protective mechanisms of Q235 steel in various test solutions: (a) CA-CDs; (b) imidazole; (c) FCDs.

4. Conclusions

This paper mainly studied the preparation, structure and corrosion behaviors of functionalized citric acid carbon dots in 1 M HCl solution. The main conclusions were listed as follows:

(1) FTIR and XPS spectra of FCDs inhibitor confirmed the successful reaction between the amidogen of imidazole and the carboxyl of CA-CDs. TEM and SPM images showed that the FCDs inhibitor presented a good distribution and the grain size was about 4-6 nm.

(2) Electrochemical data revealed that all inhibitors were the mixed-type and the FCDs inhibitor presented the strongest inhibition ability for Q235 steel in 1 M HCl solution. Besides, the inhibition efficiency presented an uptrend as the inhibitor concentration increased. These phenomena were also confirmed through 2D and 3D morphologies.

(3) The formation of adsorption film was the main reason for the excellent inhibition effect of FCDs, which was completely consistent with the Langmuir adsorption model. In the meantime, the adsorption type of FCDs inhibitor was the physicochemical adsorption at the steel/solution interface, which was mainly based on the values of Kads and ΔG0ads.

Acknowledgements

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


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