Inhibition Performance and Mechanism of Poly(Citric Acid–Glutamic Acid) on Carbon Steel Corrosion in Simulated Seawater

Abstract

In this investigation, the efficacy of PCA-GLU, a polymer obtained by copolymerizing citric acid and glutamic acid, as a corrosion inhibitor for carbon steel was investigated in a 3.5wt% NaCl solution. Electrochemical impedance spectroscopy (EIS) techniques and potentiodynamic polarization (PDP) measurements were used to evaluate the corrosion inhibition. The findings demonstrate that PCA-GLU has a 96.73% corrosion inhibition efficiency. Additionally, when the inhibitor concentration rises, the corrosion inhibition efficiency rises as well, reaching an ideal concentration of 400 mg/L. Furthermore, PCA-GLU can create an adsorption layer on the surface of Q235. This paper verifies the adsorption mechanism of PCA-GLU through molecular dynamics simulations of the system and quantum chemical calculations of corrosion inhibitors in solution. Ultimately, our research findings validate that PCA-GLU is an efficient corrosion inhibitor in safeguarding carbon steel against corrosion in marine environments.

1. Introduction

Offshore oil extraction uses oilfield water injection operation extensively because it is a cost-effective and efficient technology [1,2]. However, during the actual operation of offshore oilfield water injection, a large amount of seawater needs to be injected, resulting in severe corrosion of the pipelines [3]. This will cause huge economic losses. Carbon steel (CS) is an alloy of iron, carbon, and a small number of other impurities. It has excellent mechanical properties [4], high hardness, and a low price [5], making it the main metal comprising oilfield water injection pipes [6]. Therefore, reducing the corrosion of carbon steel in seawater systems has become a research focus [7].

The corrosion process can be alleviated by applying anti-corrosion coatings, adding corrosion inhibitors to the system, or surface treating the metal. Adding corrosion inhibitors is considered the most effective and direct method because it is more convenient, faster, and cheaper [8,9,10]. Inorganic corrosion inhibitors have a significant environmental impact and are expensive, so their application is limited. Organic corrosion inhibitors can form an adsorption film by interacting with metal surfaces through their heteroatoms. The formed adsorption film is located between the corrosive medium and the metal, reducing the interaction between them and thereby decreasing the corrosion [11,12]. However, many organic molecules have high toxicity and relatively difficult degradability, which requires the development of new corrosion inhibitors [13].

Compared with organic small-molecule corrosion inhibitors, organic polymer corrosion inhibitors are easier to form on metal surfaces and can achieve good corrosion inhibition effects at low concentrations [12]. Therefore, polymer corrosion inhibitors have become a research focus, especially synthetic polymer corrosion inhibitors [14]. Citric acid can adsorb on metal surfaces and has a certain anti-corrosion effect. In Chen et al., the modified polyamide was obtained by polymerizing adipic acid and citric acid with polyamide. This corrosion inhibitor has a significant anti-corrosion effect in a neutral medium and can reduce the corrosion of Q235 carbon steel [15]. The corrosion inhibition rate of this citric acid-modified polymer can reach 86.68% in neutral media containing chloride ions. Meanwhile, recent studies have shown that water-soluble chitosan salts also have excellent corrosion inhibition properties, with a corrosion inhibition rate of up to 96.68% [16]. We synthesized polymers such as poly(citric acid), poly(citric acid-triethanolamine), and poly(citric acid-aspartic acid) and found that poly(citric acid) itself had a poor anti-corrosion effect, but its modifiers all had a good anti-corrosion effect on metals. Citric acid modification is shown to be an effective way to develop efficient green corrosion inhibitors [17,18]. Based on previous research and the relevant literature, it has been found that the addition of amide bonds to citric acid polymers can significantly improve their corrosion inhibition performance. Glutamic acid itself has a poor corrosion inhibition effect [19], but it contains abundant carboxyl and amino groups, and the price of glutamic acid is very low. Therefore, we chose to polymerize citric acid and glutamic acid to form a new type of green polymer corrosion inhibitor, which will be more helpful for us to explore its corrosion inhibition mechanism.

In this study, PCA-GLU was successfully synthesized. The corrosion inhibition effect and possible corrosion inhibition mechanism of polymer on Q235 in a 3.5wt% system were studied through electrochemical experiments. Based on DFT (Density Functional Theory) calculations, further research was conducted on its corrosion inhibition mechanism, providing direction for the development of polymer corrosion inhibitors and future studies on corrosion inhibition mechanisms.

2. Experimental Section

2.1. Reagents and Instruments

Q235 steel was chosen as the testing steel. The 3.5wt% NaCl solution was used as a corrosive medium. Citric acid and glutamate were purchased from Jiangsu Yonghua Fine Chemicals Co., Ltd., Jiangsu, China. Concentrated sulfuric acid and sodium hydroxide were obtained from Shanghai Experimental Reagent Co., Ltd., Shanghai, China, NaCl, Q235steel, N, N-dimethylformamide, sodium chloride, acetone, and ethanol were all come from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

The main experimental instruments used in the preparation, characterization, and electrochemical experiments include an infrared detector (the model is FTIR-8400S, purchased from Shimadzu Corporation in Kyoto, Japan), a magnetic electric heating stirrer (the model is ZNCL-TS, purchased from Yuhua Instrument Co., Ltd. in Gongyi City, Gongyi, China), an ultrapure water machine (the model is Ultra pure UVF, purchased from Shanghai Hetai Instrument Co., Ltd., Shanghai, China), and an electrochemical workstation (the model is CHI660E, purchased from Shanghai Chenhua Co., Ltd., Shanghai, China).

2.2. Synthesis of PCA-GLU Polymer

The preparation method of PCA-GLU refers to in the literature we published previously [20]. The specific preparation process is shown in Figure 1 (which Δ represents heating).

2.3. Electrochemical Tests

The carbon steel is encapsulated with silicone rubber so that only one side of the carbon steel is exposed to the electrolyte, and the exposed area was 10 × 10 mm. Electrochemical tests were then carried out on it. Before testing, the electrode was first sanded step by step with 600,2000 grit sandpaper and then polished with metallographic sandpaper until the surface was smooth; finally, after cleaning and drying, the required carbon was obtained.

A three-electrode system was used in the electrochemical testing. The three-electrode system included a carbon steel electrode, saturated calomel electrode (Shanghai INESA Scientific Instrument Co., Ltd., Shanghai China), and 232 platinum electrode (Shanghai INESA Scientific Instrument Co., Ltd., Shanghai China). Impedance testing was conducted under open-circuit potential and a 5mV AC excitation signal in the frequency range of 0.1 kHz to 100 kHz. The polarization curve scanning range was an open-circuit voltage of E ± 350 mV, and the scanning speed was 1 mV/s.

2.4. Theoretical Calculation

The molecular structure of the corrosion inhibitors has a major influence on corrosion performance. Therefore, the corrosion inhibitors need to be analyzed from the perspective of the molecular structure. The experiments used Gaussian 09 and Material Studio 7.0 software to calculate the corrosion process of the PCA-GLU. In Gaussian 09, a combination of the DFT method and the Becke-type three-parameter (B3LYP) with a 6–31G (d, p) basis set was used to optimize the structure and perform calculations.

The simulation experiment of the interaction energy between the CS and polymer was conducted in a simulation box with periodic boundaries and dimensions of 24.82 × 24.82 × 27.06 Å. This box contained an iron sheet and a self-collecting molecular solution layer containing water molecules (500), chloride ions (7), sodium ions (7), and corrosion molecules (the total mass fraction of sodium ions and chloride ions was 3.5 wt%, and the solution density was 1 g/cm³). In this experiment, the (110) crystal plane was used for iron and extended to a suitable supercell with five layers of iron crystals. The COMPASS force field controls the simulation process.

3. Results and Discussion

3.1. Synthesis of the PCA-GLU

The polymer was tested using FT-IR (Fourier Transform Infrared). As shown in Figure 2, the peaks at 1050 cm⁻¹ and 1634 cm⁻¹ in Figure 2a represent the bending vibration of -N-H and the stretching vibration of -C=O, and the peak at 1108 cm⁻¹ in Figure 2b is the stretching vibration peak of -C-O in the hydroxyl group, while 1754 cm⁻¹, 1724 cm⁻¹, and 1686 cm⁻¹ correspond to the stretching vibration peak of -C=O in the carboxyl group. PCA-GLU was synthesized by the polymerization of glutamic acid and citric acid using the method described in reference [16] and characterized by infrared spectroscopy to obtain Figure 2c. The absorption band at 3364 cm⁻¹ is associated with the -C-N in the secondary amine group and the stretching vibration peak of -N-H and -C=O on the amide is at 1581 cm⁻¹ and 1694 cm⁻¹, while the peak at 1388 cm⁻¹ represents the stretching vibration absorption peak of C-O-C.

In summary, the PCA-GLU has been synthesized successfully.

3.2. PDP Measurements

Figure 3 shows the polarization curve of the CS in a 3.5 wt% NaCl solution. It can be seen that both the cathode and anode branches move toward the more negative and corrected areas around the blank curve, indicating that PCA-GLU simultaneously affects the anode and cathode poles during the corrosion process [21]. However, the cathodic corrosion potential significantly decreases, indicating that PCA-GLU is a mixed-type corrosion inhibitor but is more inclined toward cathodic corrosion inhibitors.

The corrosion current density and corrosion rate of metals in solution with different concentrations of PCA-GLU were calculated, as shown in

Table 1. Corrosion current density and corresponding inhibition efficiencies for Q235 in 3.5 wt% NaCl solution of PCA-GLU.

	Black	100 (mg/L)	200 (mg/L)	300 (mg/L)	400 (mg/L)	500 (mg/L)
−Ecorr (mV)	575	533	534	579	828	899
Icorr (A)	3.872 × 10−5	1.793 × 10−5	1.171 × 10−5	1.643 × 10−6	1.413 × 10−6	2.848 × 10−6
H (%)	-	55.09	69.76	95.76	96.35	92.64

. The corrosion rate is calculated as follows:

$$\eta =\frac{I_0-I}{I_0}\times 100\mathrm{\%}$$

(1)

where I₀ and I represent the values of the corrosion current density after no addition and the addition of the erosion inhibitor.

According to Table 1, it can be found that after the addition of PCA-GLU, the corrosive current density shows a significant tendency to decrease, and the rate of corrosion inhibition rises significantly [22]. At the same time, the current platform is found on the polarization curve caused by polymer adsorption on the carbon steel surface, which results in the formation of a corrosion membrane. Based on the similarity of the Tafel diagram, the change in polymer concentration only alters the corrosion potential, indicating that the inhibitory mechanism of PCA-GLU on carbon steel does not change [23]. Table 1 shows that the corrosion inhibition effect increases with the increase in the PC-GLU concentration, reaching its maximum value at 400 mg/L, which indicates that PCA-GLU slows down corrosion through adsorption. When the concentration of PCA-GLU was 500 mg/L, we found that its effect decreases instead of remaining unchanged. This situation may be caused by the formation of "semimicelles". When the concentration of the polymer exceeds its critical concentration, the adsorbed molecules are easily desorbed and return to the solution [24,25,26].

3.3. EIS Measurements

The EIS measurement results of CS immersed in corrosive media by PCA-GLU are plotted in the Nyquist plot, as shown in Figure 4. In the Nyquist diagram, the diameter of the semicircular impedance curves increases, which means that the corrosion efficiency decreases [27]. It was found that after adding PCA-GLU, the impedance diameter increases significantly, showing that the polymer has a good corrosion inhibitory effect on carbon steel. The best-fitting equivalent circuit model analyzes the EIS spectrum of the PCA-GLU in the image to simulate the electrolytic process of the electrode surface.

The circuit shown in Figure 5 is composed of charge transfer resistance (Rct), film resistance and capacitance (Rf and Qf), solution resistance (Rs), and a constant phase element (CPE). These impedance parameters are listed in

Table 2. The inhibition efficiency and EIS parameters of Q235 in 3.5wt% NaCl solution containing different concentrations of PCA-GLU.

Sample	Rs (Ω·cm2)	CPEdl (S·secn/cm2)	Rf (Ω·cm2)	Rct (Ω·cm2)	η (%)
blank	6.659	21.5 × 10−4	37.55	198	-
100 mg/L	6.492	16.3 × 10−4	22.64	353	51.00
200 mg/L	6.791	8.75 × 10−4	28.52	405	65.84
300 mg/L	17.51	2.47 × 10−4	38.88	1408	92.31
400 mg/L	16.1	2.51 × 10−4	147.1	2161	96.73
500 mg/L	64.92	9.34 × 10−5	61.19	1913	90.12

. Based on the value of the corrosion charge transfer resistance in the table, the corrosion efficiency can be calculated using the following formula:

$$\eta ={\displaystyle \frac{Rct-R{ct}^0}{Rct}}\times 100\mathrm{\%}$$

(2)

where $R{ct}^0$ and $Rct$ represent the total charge transfer resistance of the blank solution and the PCA-GLU solution.

The electrochemical impedance curve is divided into two parts: one is the charge transfer process in the high-frequency region, and the other is the diffusion process in the low-frequency region. As the concentration of PCA-GLU increases (except 500 mg/L), the diameter of the impedance spectrum increases, indicating a higher Rct and a gradual decrease in the corrosion rate. From the fitting parameters of the EIS in Table 2, as the concentration rises, the Rct on the surface of carbon steel decreases, and the corrosion inhibition rate rises. When the concentration of the corrosion inhibitory dose is 400 mg/L, the corrosion rate reaches its peak value, and after this concentration, a threshold phenomenon occurs.

According to the data in Table 2, the change in charge transfer resistance (Rct) is much larger than the change in metal surface film resistance (Rf). Therefore, the calculation of the corrosion inhibition efficiency mainly takes into account the change in the charge transfer resistance value. The significant increase in Rf indicates that the adsorption of the PCA-GLU on the electrode surface forms an efficient shielded effect. The values of Cdl decline with an inhibitor concentration increase. From Table 2, it can be observed that as the concentration of PCA-GLU in the system rises, the CPE value reduces. The CPE value of the system can be described according to Equation (3).

$$CPE={\displaystyle \frac{{\epsilon }^0·\epsilon }{d}}·S$$

(3)

In the formula, d is the thickness of the double layer, S is the surface area of the carbon steel electrode exposed to the corrosive solution, and ε⁰ and ε are the dielectric constant and local dielectric constant of air, respectively. Therefore, we found that at higher concentrations of inhibitors, the surface area of the electrode reveals that the solutions are degraded, effectively inhibiting the corrosion of Q235. In addition, the water on the electrode surface will gradually be replaced by PCA-GLU molecules, reducing the local dielectric constant and thickening the electrical bilayer. These are all reasons for the decrease in CPE_dl values. The electrochemical impedance spectroscopy results confirm that PCA-GLU has a good corrosion inhibition effect on carbon Q235 in a simulated seawater system.

3.4. Quantum Chemical Calculation

The HOMO and LUMO orbitals in the PCA-GLU can help us study the performance of the corrosion inhibitor theoretically. From Figure 6, the HOMO and LUMO distributions of polycitric acid (PCA) and PCA-GLU are located at both ends of the molecules, respectively. This distribution is conducive to the parallel adsorption of polymer molecules on the surface of Q235. The performance of the PCA-GLU can also be predicted by the following calculation parameters: the Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO), Energy Gap (ΔE), Mulliken Electronegativity (χ), Global Hardness (η), parenting index (ω), Chemical Potential (μ), global softness (σ), and electron transfer fraction (ΔN). The above parameter calculation results are obtained by the following formulas:

$$\mu ={\displaystyle \frac{\left(E_{LUMO}+E_{HOMO}\right)}{2}}$$

(4)

$$\chi =-\mu$$

(5)

$$\eta ={\displaystyle \frac{E_{LUMO}-E_{HOMO}}{2}}$$

(6)

$$\sigma ={\displaystyle \frac{1}{\eta }}$$

(7)

$$\omega ={\displaystyle \frac{{\chi }^2}{2\eta }}$$

(8)

$$\begin{array}{c}\Delta E=E_{LUMO}-E_{HOMO}\end{array}$$

(9)

$$\Delta N={\displaystyle \frac{\left(\chi Fe-\chi inh\right)}{2\left(\eta Fe+\eta inh\right)}}$$

(10)

Through the literature, we found that the value of χ_Fe is 7.0 eV/mol, while the value of η_Fe is generally described as 0 eV/mol [28].

Table 3. Quantum chemical parameters for four corrosion inhibitors.

Inhibitors Additive	ELUMO (eV)	EHOMO (eV)	μ	χ	η	σ	ω	ΔN	ΔE
PCA-GLU	−0.798	−6.832	−3.815	3.815	3.017	0.331	2.412	0.528	6.034
PCA	−0.628	−7.138	−3.883	3.883	3.255	0.310	2.316	0.476	6.510
GLU	−0.070	−6.801	−3.436	3.436	3.366	0.297	1.769	0.529	6.731
CA	−0.194	−7.524	−3.956	3.956	3.665	0.273	2.135	0.415	7.330

lists the quantum chemical parameters of four corrosives. E_HOMO reports the capacity of inhibitors to provide electrons to empty orbitals. The greater the E_HOMO value, the stronger the ability of corrosion inhibitors to supply electrons. E_LUMO reports the capacity of inhibitors to receive electrons, and the smaller the E_LUMO value, the stronger the capacity to accept electrons [29]. ΔE can mirror the reaction property of the corrosion inhibitor. The smaller value of ΔE generally means that the material has the possibility of becoming an excellent corrosion inhibitor [30]. From Table 3, we can see that the ΔE order of the four corrosive agents is as follows: CA > GLU > PCA > PCA-GLU. However, it was found that the EHOMO value and ELUMO value of GLU are the highest values among the four compounds, proving that glutamic acid itself can donate electrons easily but cannot easily accept electrons. The rules for χ are the same as for the E_HOMO value of the compound, and the rules for ω are the same as for the E_LUMO value of the compound.

The hardness (η) and softness (σ) go hand in hand with ΔE. The smaller η, the greater the corrosion inhibition performance. The η value of the four corrosive agents is CA > GLU > PCA > PCA-GLU. The order agrees with ΔE.

ΔN represents the probability of moving electrons from inhibitors to the surface. When ΔN = 3.6, the inhibitor forms coordination bonds and transfers electrons to the metal surface. When ΔN < 3.6, the inhibitory effect rises with increasing ΔN [31]. The ΔN value of the four inhibitors is less than 3.6 and the magnitude is as follows: GLU > PCA-GLU > PCA > CA.

Based on the results of the above parameters, it can be inferred that PCA-GLU has a better anti-corrosion effect compared to PCA. This may be because the addition of glutamic acid improves the ability of electrons from the inhibitor to transfer to the iron surface, increasing the corrosion inhibitor to bind more firmly to the iron surface, thereby enhancing its corrosion inhibitory effect.

3.5. Interfacial Adsorption

3.5.1. Interaction Energy Calculation

Figure 7 and Figure 8 (in order to facilitate the observation of the interaction between the orrosion inhibitor and Fe (110), the water in the system was removed from Figure 7 and Figure 8) shows the adsorption position of the PCA-GLU molecules and carbon steel surfaces according to the molecular dynamic simulation. It is obvious that, according to the molecular dynamics simulation, the PCA-GLU molecules are more firmly adsorbed on the Q235 surface, which proves the adsorption mechanism of the molecules on the Q235 surface. The interaction value of the polymer and the Q235 surface can reflect the adsorption performance between both, and the interaction can be calculated using the formula as follows:

$$E_{interaction}=E_{total}-\left(E_{surface}+E_{polymer}\right)$$

(11)

In the formula, E_interaction, E_total, E_surface, and E_polymer, respectively, represent the interaction energy between carbon steel and the corrosion inhibitor, the total energy of the corrosion and the Q235 surface, and the energy and the energy of the Q235 surface.

The smaller the negative value of the interaction, the greater the interaction force between the two, indicating better corrosion performance of the material. The final calculation of the interaction value between PCA-GLU and the CS surface is −224.23 kcal/mol, and the interaction value between GLU and the carbon Q235 is −83.15 kcal/mol. At the same time, the interaction between the PCA and CA and the CS surfaces is calculated, as shown in

Table 4. The value of the interaction energy between the corrosion inhibitor and Fe(110) surface acquired by molecular dynamics simulation in a 3.5 wt% NaCl aqueous solution system.

Inhibitors	PCA-GLU	PCA	GLU	CA
Interaction (kcal/mol)	−224.23	−182.01	−83.15	−77.94

. The interaction value between the PCA and the carbon steel surfaces is −182.01 kcal/mol, while the interaction value between CA and the carbon steel surfaces is −77.94 kcal/mol.

From Figure 7 and Figure 8, it can be seen that polymer molecules are more likely to be adsorbed on the surface of Q235 than small molecules, which means that polymer molecules have better corrosion inhibitory effects on carbon steel than small molecules. Based on the interaction values of the four corrosion inhibitors, it can be found that the citric acid polymer modified with glutamic acid is more easily adsorbed on carbon steel than PCA. The results of the molecular dynamics simulation are the same as those of the quantum chemical calculation. The addition of glutamic acid can enhance the adsorption capacity of poly(citric acid) on the surface of Q235, thereby improving its corrosion performance.

3.5.2. Radial Distribution Function (RDF)

The RDF can analyze the correlation between the given particle and the surface in the system, that is, the adsorption type between the polymer and the iron surface are studied in this experiment to help understand the adsorption type of polymer on the Q235 surface. In general, the initial RDF peak is between 1 Å and 3.5 Å, indicating that the polymer and carbon steel surfaces are chemically adsorbed. The peaks appearing after 3.5 Å may be caused by physical adsorption [32]. Figure 9 shows the radial distribution function between the N and O atoms and the Fe(110) surface in PCA and PCA-GLU. The Fe-O bond lengths in PCA and the Fe-N and Fe-O bond lengths in PCA-GLU were found to be between 1 Å and 3.5 Å, indicating that N and O atoms showed chemical adsorption on the two corrosion inhibitors’ surfaces made of carbon steel.

4. Conclusions

This study synthesized PCA-GLU and for the first time investigated its corrosion inhibition effect on Q235 in a 3.5wt% NaCl system.

(1) The electrochemical test results indicate that PCA-GLU can slow down Q235 corrosion and is considered an effective corrosion inhibitor. As the concentration rises, the corrosion inhibitory effect of the PCA-GLU rises. When the concentration of PCA-GLU is 400 mg/L, the optimal corrosion inhibition efficiency of 96.73% can be achieved.

(2) Through quantum chemical calculations, it was found that the addition of glutamic acid can significantly increase the probability of electron transfer from corrosion inhibitors to carbon steel, effectively improving the corrosiveness of Q235. The results of the molecular dynamics calculation indicate that the interaction between PCA-GLU and carbon steel is stronger than the other three corrosion inhibitors. The N and O atoms on PCA-GLU undergo chemical and physical adsorption on the surface of Q235.