J. Cent. South Univ. (2018) 25: 1914-1919

DOI: https://doi.org/10.1007/s11771-018-3881-x

Synergistic corrosion inhibition behavior of rare-earth cerium ions and serine on carbon steel in 3% NaCl solutions

LIU Xia(刘瑕)¹, YUAN Yi-zhi(袁艺智)¹, WU Zhu-ying(吴柱莹)¹,TIAN Gao-deng(田高登)¹, ZHENG Yu-gui(郑玉贵)²

1. Department of Applied Chemistry, Shenyang University of Chemical Technology,Shenyang 110142, China;

2. CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research,Chinese Academy of Sciences, Shenyang 110016, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: The synergistic inhibition effect of CeCl₃ (Ce) and serine (Ser) on the corrosion of carbon steel in a 3% NaCl solution was investigated by electrochemical methods and surface analysis. The results showed that both CeCl₃ and Ser, when used alone, had limited inhibition effect toward carbon steel corrosion in the 3% NaCl solution. In contrast, the combination of CeCl₃ with Ser produced a strong synergistic effect on the corrosion inhibition behavior of carbon steel, improving the inhibition efficiency significantly. The polarization curves showed that the mixture of CeCl₃ and Ser acts as a cationic-type inhibitor. Scanning electron microscopy and Fourier transform infrared spectroscopy showed that the synergistic inhibition effect was due to complex formation between the cerium ions and amino acid molecules.

Key words: synergistic inhibition effect; rare earth; complex; amino acid

Cite this article as: LIU Xia, YUAN Yi-zhi, WU Zhu-ying, TIAN Gao-deng, ZHENG Yu-gui. Synergistic corrosion inhibition behavior of rare-earth cerium ions and serine on carbon steel in 3% NaCl solutions [J]. Journal of Central South University, 2018, 25(8): 1914–1919. DOI: https://doi.org/10.1007/s11771-018-3881-x.

1 Introduction

Metal corrosion is common in various national industries and can cause enormous economic loss, waste of resources, and major catastrophic incidents. Among various anti-corrosion technologies, corrosion inhibitors have become increasingly popular because of their significant effect, even when used in small amounts [1–3]. As green environmental-corrosion inhibitors, rare earth inhibitors have attracted significant interest. Many researchers have reported the corrosion inhibition properties of rare-earth salts [4–6], but most of these studies have been limited to aluminum, copper, and magnesium alloys, while studies on carbon steel corrosion are less common [7–10]. Moreover, in the case of these rare-earth salts, repulsion between the positively charged rare-earth ions and the metal surface occurs, which is detrimental for the corrosion inhibition efficiency of rare-earth inhibitors.

Synergistic effects are often used to increase the inhibition efficiency of corrosion inhibitors and reduce their dosage [11–17]. However, studies on the synergistic effects of rare-earth corrosion inhibitors are rare. To improve the efficiency of rare-earth inhibitors, the main objective of this study is to evaluate the synergistic effect between rare-earth salts and amino acids.

In this study, a cerium salt was chosen as the rare-earth inhibitor because of its abundance and low cost. The synergistic inhibition effect of CeCl₃ and serine toward the corrosion of carbon steel in 3% NaCl solution was investigated using electrochemical methods and surface analysis. Morphological studies of the carbon steel electrode surface were performed using scanning electron microscopy (SEM). The films formed on the metal surface in the presence of the inhibitors were analyzed by infrared (IR) spectroscopy. The general mechanism underlying the synergistic inhibition effect of the rare-earth cerium salt and serine on the corrosion of carbon steel in 3% NaCl solution was also determined.

2 Experimental

A standard three-electrode system, with a P110 working electrode, saturated calomel reference electrode, and graphite counter electrode, was used for all electrochemical measurements. The chemical composition of the P110 carbon steel is listed in Table 1. The exposed surface of the electrode(1 cm²) was polished with 800-grit silicon carbide abrasive paper, rinsed with ethanol, and degreased with acetone. The molecular structure of serine (Ser, analytical grade reagent) is shown in Figure 1. All experiments were performed in 3% NaCl solutions with inhibitor concentrations of 100×10^–6.

Table 1 Chemical composition of tested P110 carbon steel (wt%)

Figure 1 Chemical structure of serine

Electrochemical measurements were made using a PARSTA 2273 electrochemical measurement system and a JSM-6360LV scanning electron microscope was used for observation of the corrosion surface morphology. A Nicolet 6700 IR spectrometer was used to analyze the surface films.

Potentiodynamic polarization curves were obtained at a scan rate of 0.166 mV/s. EIS measurements were carried out at an open circuit potential with an AC voltage of 5 mV in the frequency range of 10 mHz to 10 kHz.

3 Results and discussion

3.1 Potentiodynamic polarization measurements

Potentiodynamic polarization curves for the P110 carbon steel in 3% NaCl solution in the absence and presence of inhibitors, at 20 °C, are shown in Figure 2. The electrochemical parameters such as corrosion potential (φ_corr), corrosion current density (J_corr), anodic Tafel slope (β_a), and cathodic Tafel slope (β_c) were calculated using the Tafel fitting and are listed in Table 2. The corrosion inhibition efficiency was calculated using the following equation:

η=[(J⁰_corr–J_corr)/J⁰_corr]×100%                 (1)

where J⁰_corr and J_corr are the corrosion current densities in the absence and presence of inhibitors, respectively.

Figure 2 Potentiodynamic polarization curve for P110 carbon steel in 3% NaCl solution in absence and presence of corrosion inhibitors

It is well known that the anodic corrosion reaction of iron involves the dissolution of iron (Eq. (2)) [15, 18]:

Fe→ Fe²⁺ +2e^–                                           (2)

The cathodic corrosion reaction in acidic medium occurs as follows:

Table 2 Polarization parameters for P110 mild steel in 3% NaCl solution in absence and presence of corrosion inhibitors

4H⁺+O₂+4e^-→2H₂O                      (3)

From Figure 2, it can be seen that the corrosion inhibitors mainly affected the cathodic reaction. However, the cathodic reactions were not significantly inhibited by the presence of either CeCl₃ or Ser alone. In the 3% NaCl solution containing both Ce and Ser, corrosion was remarkably inhibited. This indicates a synergistic effect between Ce and Ser toward inhibiting corrosion of the mild steel in a 3% NaCl solution [19]. The combination of Ce and Ser produced more significant effects on the cathodic reactions and shifted φ_corr in the cathodic direction compared to that of the blank solution. The results above indicate that the corrosion inhibitors are predominantly cathodic inhibitors that retard the transfer of O₂ to cathodic sites.

3.2 Electrochemical impedance spectroscopy measurements

The impedance spectra with and without corrosion inhibitors are presented in Figure 3 as Nyquist plots. The fitted values of the impedance parameters are listed in Table 3, and the inhibition efficiency (η%) was calculated using the following equation:

η%=[(R_ct,i–R_ct,b)/R_ct,i]×100%                 (4)

where R_ct,i is the charge transfer resistance with inhibitors and R_ct,b is the charge transfer resistance without inhibitors.

Figure 3 Nyquist plots for P110 steel in 3% NaCl with and without corrosion inhibitors

Table 3 EIS parameters for P110 mild steel in 3% NaCl solutions in absence and presence of inhibitors

From Figure 3, it can be seen that the diameter of the capacitive loop slightly increases with the addition of either CeCl₃ or Ser alone, which showed that neither CeCl₃ nor Ser is an effective inhibitor of the corrosion of carbon steel in 3% NaCl solution. In contrast, the combination of Ce and Ser significantly increased the diameter of the capacitive loop, indicating a synergistic effect between Ce and Ser leading to an enhanced inhibition efficiency. The inhibition efficiencies obtained from the potentiodynamic polarization and

electrochemical impedance spectroscopy measurements are in good agreement.

3.3 Infrared reflection spectrum

FTIR spectra are powerful tools used to determine the type of bonding present in inhibitors adsorbed on metal surfaces [17]. To analyze the interfacial species formed on the metal surface in the presence of inhibitors, FTIR reflection spectra were obtained for the P110 samples after immersion in 3% NaCl solutions containing Ce–Ser for 24 h. The spectra were prepared according to published procedures [18, 19]. For comparison, IR spectra of Ser and Ce–Ser were measured in the region of 400 to 4000 cm^–1 and are shown in Figure 4. From Figure 4, it can be seen that the bands at approximately 1469 and 1411 cm^{–1 in Ser(Figure 4(a)) shift to 1494 and 1422 cm–1}, respectively (Figure 4(b)). These results provide evidence for the interaction of Ce with Ser, which likely results in the formation of a Ce–Ser complex [20, 21]. The FTIR spectrum of the film formed on the metal surface (Figure 4(c)) is similar to that of the Ce–Ser complex. Therefore, it can be concluded that the protective film on the carbon surface is primarily composed of the complex. The formation and deposition of a protective Ce–Ser complex on the carbon surface inhibited the cathodic partial reaction and prevented the steel surface from being corroded.

Figure 4 IR spectra of Ser (a), Ce–Ser (b), and FTIR spectrum of adsorbed layer with Ce–Ser complex (c)

3.4 Scanning electron microscopy

The surface morphologies of the P110 carbon steel specimens in 3% NaCl solution with and without inhibitors, after 24 h of immersion, were examined using SEM. The SEM images are shown in Figure 5. In the absence of inhibitors, a very rough surface was observed due to the rapid corrosion of the carbon steel. In the presence of either CeCl₃ or Ser alone, the roughness of the surface was reduced, indicating an inhibitory effect of Ce and Ser on the surface of the metal. In the presence of the inhibitor blend, Ce–Ser, the carbon steel surface was smoother compared to those imaged in the presence of CeCl₃ or Ser alone.

This behavior is evidence for the proposed synergistic effects between Ce and Ser leading to a denser film on the surface of the metal. The synergism between the rare-earth Ce(III) and the amino acid lies in the formation of the Ce–Ser complex. It has been reported that metal complexes have better corrosion inhibition efficiency than that of organic ligands alone [22–24]. The complexes have greater inhibition efficiency due to a larger size and more adsorption sites on the metal surface because of the coordination between the metal ions and the ligands.

To further explore the interaction between the inhibitors and the metal surface, energy dispersion spectroscopy (EDS) of the surfaces of the samples immersed in 3% NaCl solution with inhibitors was performed and the EDS spectra are shown in Figure 6. The results presented in Figure 6 indicate the presence of Ce and other elements like O, N, and Fe. Hence, it can be concluded that deposits of Ce–Ser precipitated on the metal surface. As noted by other authors [15], this precipitation suggests the formation of a Ce–Ser complex film.

4 Conclusions

1) Ce and Ser can inhibit the corrosion of P110 carbon steel in 3% NaCl solutions, but their inhibition efficiency is less than 50%.

2) A strong synergistic effect was observed between Ce and Ser due to interactions between these corrosion inhibitors. The mixture of Ce and Ser mainly acts as a cathodic-type inhibitor of carbon steel corrosion in 3% NaCl solutions.

3) A Ce–Ser complex film was formed on the steel surface, significantly retarding the cathodic process of metal corrosion and effectively protecting iron against corrosion.

Figure 5 SEM images of P110 steel in 3% NaCl solutions in absence and presence of inhibitors

Figure 6 EDS spectrum of P110 carbon steel in 3% NaCl solution containing Ce–Ser

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(Edited by YANG Hua)

中文导读

稀土铈离子和丝氨酸对碳钢的缓蚀协同作用研究

摘要：本文通过电化学和表面分析方法研究了稀土铈离子和丝氨酸在3%氯化钠溶液中对碳钢的缓蚀协同作用。研究结果表明, 单独的铈离子或丝氨酸在3%氯化钠中对碳钢的腐蚀抑制作用均有限，相比之下，铈离子与丝氨酸复配之后，稀土缓蚀剂的缓蚀性能显著增强，即稀土铈离子与丝基酸之间存在缓蚀协同效应。扫描电子显微镜和傅里叶红外反射光谱表明，稀土铈离子和丝基酸在金属表面形成了稀土离子–氨基酸的配合物薄膜，阻碍了阴极腐蚀过程，从而有效地抑制了金属腐蚀。

关键词：协同效应；稀土元素；配合物；氨基酸

Foundation item: Project(2017210101002066) supported by the Crosswise Project of Liaoning province, China

Received date: 2017-03-15; Accepted date: 2017-09-24

Corresponding author: LIU Xia, PhD, Associate Professor; Tel: +86–24–89383296; E-mail:laura-lx@163.com; ZHENG Yu-gui, PhD, Professor; Tel: +86–24–23928381; E-mail: ygzheng@imr.ac.cn; ORCID: 0000-0001-6798-628x