Corrosion Behavior of Metallic Materials in Acidic-Functionalized Ionic

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Corrosion Behavior of Metallic Materials in Acidic-Functionalized Ionic Liquids Ying Ma, Feng Han, Zhen Li,* and Chungu Xia State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: This paper describes the influence of temperature, water content, and anionic type of acidic-functionalized ionic liquids (ILs), 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([BsMIM][HSO4]) and 1-(4-sulfobutyl)-3-methylimidazolium toluenesulfonate ([BsMIM][OTs]), on the corrosion behavior of Fe, Ni, and 304 stainless steel (304SS). Electrochemical methods including electrochemical impedance spectroscopy (EIS) and Tafel plots were used to investigate it. Also, scanning electron microscopy (SEM) was used to characterize the nature of the corrosion morphology. The obtained electrochemical results indicated that increasing temperature accelerates the corrosion, while decreasing IL concentration retards the corrosion. The corrosion process is controlled by charge transfer. Moreover, the bisulfate anion (HSO4−) has an effect on the corrosion rate more significantly than the p-toluenesulfonate anion (OTs−) does. The SEM spectrum showed that the corrosion situation of Fe is more serious than Ni and 304SS performed in IL-based solutions, especially in [BsMIM][HSO4]. Also, the protective layer formed on the 304SS surface is more uniform. On the basis of these consistent finds, the corrosion mechanism is assumed. KEYWORDS: Corrosion, Ionic liquid, Potentiodynamic polarization, Electrochemical impedance spectroscopy, Scanning electron microscopy

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viscosity of the fluid that makes the diffusion of electrochemical species strenuous.9−24 Perissi et al.13 investigated the corrosion behavior of copper, nickel, brass, Inconel 600, and AISI 1018 steel exposed to [bmim]NTf2 at high temperatures. It was found that Inconel 600 exhibits the lowest corrosion current density of 0.112 μA· cm−2 at 25 °C. Lebedeva et al.15 studied the influence of water traces in [bmim]Cl and [bmim]NTf2 for the corrosion of copper and nickel. It was found that the corrosion current density of Cu (2.12 μA·cm−2) and Ni (0.41 μA·cm−2) in dry [bmim]NTf2 is higher than that in water-containing ionic liquid. Barham et al.17 discussed the concentration of [bmim][DCA] and temperature in an aqueous carbonated solution of monoethanolamine for carbon steel corrosion. It was found that in 1.0 M [bmim][DCA] at 40 °C, the passivation potential of carbon steel ranges from −0.55 to

INTRODUCTION Ionic liquids (ILs) are salts composed of distinct cations and anions that are capable of facile tuning. They have some attractive features, such as low melting points and viscosities, excellent chemical stabilities, wide potential windows, good electrical conductivities, and ion transport properties.1,2 Particularly, task-specific ILs have displayed superior performance. They are playing more and more important roles in many areas of electrochemical processes. Indeed, quite a number of task-specific ILs behave not only as solvents suitable for electrochemical devices and methods but also as unique and robust electrolytes.3−8 On the other hand, metal corrosion is common daily in industry. Since the main construction materials for IL-operated instruments are considered metallic materials, the corrosion problem has attracted particular attention.9 Electrochemical, spectroscopic, and gravimetric methods as well as morphological analysis techniques have been used to study the corrosion behavior of metals and alloys in various ILs.9−12 Accordingly, the corrosivity of IL media strongly depends on the chemical nature of the cationic and anionic types,9 temperature,13,14 even the presence of water.15,16 The corrosion behavior seems not only due to the formation of an IL protective layer on the metal surface but also due to the high © XXXX American Chemical Society

Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: September 5, 2015 Revised: January 14, 2016

A

DOI: 10.1021/acssuschemeng.5b00974 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering −0.35 V (vs SCE), and the corrosion rate can decrease to 10 μA·cm−2. In this paper, we concentrated on corrosion properties of two acidic-functionalized ILs, 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([BsMIM][HSO4]) and 1-(4-sulfobutyl)-3methylimidazolium toluenesulfonate ([BsMIM][OTs]), diluted in different contents of water in contact with Fe, Ni, and 304SS at temperatures ranging from 30 to 70 °C. The chemical structure of the two selected ILs is shown in Chart 1. Chart 1. Chemical structure of two acidic-functionalized ILs

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EXPERIMENTAL SECTION

The ILs (purity >99%) employed in this study were purchased from Research Group of Youquan Deng in Lanzhou Institute of Chemical Physics. The corrosion tests were performed in a three-electrode glass cell with a temperature control unit. A saturated calomel electrode (SCE) and Pt wire were used as the reference electrode (RE) and counter electrode (CE), respectively. Three metallic material electrodes with an exposure surface area of 3.14 × 10−2 cm2 were used as the working electrodes (WE). The electrolytes were obtained by dissolving 1 g of IL into 100, 300, and 500 mL of bidistilled water, abbreviated as 1 g IL/100 mL water, 1 g IL/300 mL water, 1 g IL/500 mL water, respectively. Before testing, the WE was abraded with metallographical emery papers of grade 1200, 1500, and 2000 mesh, polished with alumina powder of particle sizes from 1 to 0.05 μm, cleaned with bidistilled water and ethanol, and finally dried under air flow to obtain a reproducible surface as a mirror. The volume of tested electrolytes in the electrolytic cell was 60 mL each time. The electrolytes were purged to remove dissolved oxygen under a nitrogen atmosphere. After reaching the open-circuit potential (Eocp) in 120 min, the EIS technique was performed three times over a frequency range of 10 mHz to 100 kHz with a signal amplitude perturbation of 5 mV. Potentiodynamic polarization studies were performed later with a scan rate of 0.1667 mV·s−1 in the potential range from 600 mV below Eocp to 600 mV above Eocp. All potentials were recorded with respect to the SCE. The nature of corrosion morphology was investigated by scanning electron microscopy (SEM, JSM-5600 LVD, Kevex Corporation, U.S.A.) in a low vacuum condition after immersing three metallic materials (50 mm × 25 mm × 1 mm) into 1 g IL/100 mL water at 40 °C for 48 h. The acceleration voltage was 20 kV.

Figure 1. Tafel plots of Fe in 1 g [BsMIM][OTs]/300 mL water (a), Ni in 1 g [BsMIM][HSO4]/500 mL water (b), and 304SS in 1 g IL [BsMIM][HSO4]/100 mL water (c) at 30−70 °C.

RESULTS AND DISCUSSION Influence of Temperature on Corrosion. To study the comparative effect of temperature on the corrosion of metallic materials in diluted IL electrolytes, experiments were conducted on the basis of the single-factor method. Figures 1a−c show a portion of the Tafel plots recorded on Fe, Ni, and 304SS in ILbased electrolyte at 30, 50, and 70 °C, respectively. Values of corrosion potential (Ecorr), cathodic (βc) and anodic (βa) Tafel slopes, polarization resistance (Rp), and corrosion current density (jcorr) are calculated from the corresponding Tafel plots (Tables S1−S3, Supporting Information). As shown, the active dissolution region for Fe ranges from about −0.65 to −0.4 V at 30−50 °C in 1 g [BsMIM][OTs]/ 300 mL water, and the corrosion current increases near the corrosion potential. There no clear passivation tendency

appears, but a very small one appears at the corrosion current of 0.01 μA at 70 °C. For Ni, the Tafel plots can be divided into three parts including an active zone, passive zone, and transpassivation zone (Figure 1b). In the case of 1 g [BsMIM][HSO4 ]/500 mL water, the region of active dissolution extends from about −0.4 to 0.1 V at 30 °C and shifts toward negative with a rise in temperature. The passive zone ranges from about 0.15 to 0.25 V, and the passivation current reaches at 0.32 μA. It was quite obvious to see that the passivation current density increased with an increase in temperature. This may be attributed to the passive layer formed on the surface of Ni damaged with temperature and caused the corrosion rate to increase.14−17 At about 400 mV above the corrosion potential, the passive layer is destroyed, and the anodic current increases again. The fluctuations may be due to

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DOI: 10.1021/acssuschemeng.5b00974 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering formation of a surface layer with some protective properties.9 For 304SS, it corrodes in the active−passive states in ILs. The active dissolution region moves in a more positive direction, and the passivation state remains stable over a 400 mV wide range with increased voltage. All the cathodic Tafel curves give rise to parallel lines, indicating that change in temperature does not modify the mechanism of corrosion. The calculated parameters (Tables S1−S3, Supporting Information) reveal that the polarization resistance of the solution decreases with a rise in temperature. The corrosion potential of the tested materials in IL-based solutions moves to negative, and the corrosion current density increases. This is mainly because the increase in temperature accelerates the anodic reaction. In addition, βc and βa usually represent the complexity of the electrochemical reaction. The higher values of βc than βa listed in Tables S1−S3 in the Supporting Information verify that metal dissolution occurs easily. As demonstrated, Fe shows the highest corrosion current density of 1.69 μA·cm−2 in 1 g [BsMIM][HSO4]/100 mL water at 70 °C, while 304SS shows the lowest jcorr of 4.87 × 10−4 μA· cm−2 in 1 g [BsMIM][OTs]/500 mL water at 30 °C (Tables S1−S3, Supporting Information). EIS is a powerful, nondestructive, and informative technique used for rapid characterization and study of corrosion behavior.9 In this paper, it was used to determine more information about the corrosion mechanism and to improve the results extracted from the Tafel plots. The resulting Nyquist plots and Bode plots of Ni in 1 g IL/300 mL water are shown in Figures 2−4. The electrical equivalent circuit obtained by

Figure 4. Bode impedance magnitude plots of Ni in 1 g IL/300 mL water at 30−70 °C.

Zsimp Win software is presented in Figure 5, where Rs is the solution resistance, CPE is a constant phase element related to

Figure 5. Equivalent circuit compatible with the experimental impedance data.

the double layer capacitance, and Rpore denotes pore resistance correlated to the charge transfer resistance. L refers to inductance, and RL represents the inductive resistance. The fitted EIS parameters are listed in Tables S4−S6 in the Supporting Information. Figure 2 clearly shows that the Nyquist plots of Ni present a capacitive loop at high frequency and an inductive loop at the fourth quadrant, which is associated with the adsorption of the intermediate on the Ni surface. The shape of the capacitive loop suggested that the corrosion mechanism is controlled by the charge transfer process. The semicircle of the capacitive loop decreased with increasing temperature owing to the desorption of ILs from the surface of the Ni electrode, resulting in a decreases in Rpore (Table S5, Supporting Information). However, for 304SS, its Nyquist plots are not perfect semicircles, which may due to the frequency dispersion of the interfacial impedance. This phenomenon is a typical impedance feature of solid metal electrodes in the corrosion process resulting from the surface roughness, chemical heterogeneity of the surface, and adsorption−desorption process of the medium on the metal surface.15−22 Figure 3 depicts a one phase peak at the center frequency range of Ni in IL-based electrolytes at 30−70 °C related to the electrical double layer. A small peak at low frequency is related to adsorption. Interestingly, the peak width of 304SS is larger than those of Fe and Ni. This may be attributed to its good protective property of the formed passive film on the surface,25 which can be verified by the wider passivation potential range shown in Figure 1c and the larger phase shift (n) of CPE calculated based on the equivalent circuit (Table S6, Supporting Information). As n represented the degree of surface inhomogeneity, the larger value of n, the more homogeneity of the metal surface. Direct analysis can be

Figure 2. Nyquist plots of Ni in 1 g IL/300 mL water at 30−70 °C.

Figure 3. Phase angle plots of Ni in 1 g IL/300 mL water at 30−70 °C.

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DOI: 10.1021/acssuschemeng.5b00974 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering observed from the SEM images. The increased proportional factor, Y0, of CPE (Tables S4−S6, Supporting Information) can be correlated with the decreased dielectric constant and/or increased thickness of the double layer.17 The decreased value of n can be attributed to the desorption of ILs on the active sites and the deteriorated surface heterogeneity at high temperatures. Moreover, the phase angle and impedance, |Z|, of the metal electrode decreased, indicating that high temperatures accelerate the metal dissolution speed. These are in good agreement with the information obtained from the Tafel plots. Influence of Concentration on Corrosion. It has been reported by pioneers that an acidic medium makes a great impact on the corrosion behavior of metallic materials.26−28 We investigated the effect of concentration of electrolytes on the corrosion behavior of Fe, Ni, and 304SS. For 304SS, the range of active dissolution moves in the positive direction and the values of corrosion current density decrease (Table S3, Supporting Information), suggesting that dilution of ILs retards corrosion. An interpretation of this behavior may be the lower concentration of ILs, the less active sites adsorbed on the metal surface. In addition, it is clear from the Table S3 of the Supporting Information, that the solution polarization resistance increases due to the decreased conductivity of the electrolyte with the decrease of IL concentration. For Fe, with the dilution of [BsMIM][OTs], its corrosion potential remains almost constant and the corrosion current depressed at 50 °C as shown in Figure 6a. For Ni, the corrosion potential shifts toward the positive direction and causes a decrease in the corrosion rate as shown in Figure 6b. Moreover, Tafel plots of tested metal electrodes have a similar shape, indicating that there is no change in the corrosion mechanism on the effect of ILs concentration. Figure 7 shows Nyquist plots of 304SS in various concentrations of IL-based electrolytes at 30 °C. It exhibits with an imperfect capacitive loop, and the diameter of the semicircles increases with a decrease in IL concentration, suggesting that the resistance to charge transfer increased. In the Bode plots, phase angle (Figure 8) and absolute impedance (Figure 9) increased due to less active sites available for charge transfer. From parameters calculated (Table S6, Supporting Information), the values of Rpore and n increased with decreasing IL concentration, while Y0 decreased. This indicated that the decreasing concentration of ILs retards the dissolution process of metal electrodes. In addition, the values of Y0 decreased due to the decreased surface oxide layer thickness and the influence of the kinetics based on the changed oxide layer.17 Influence of Anionic Structure on Corrosion. Not only does the concentration of the acidic medium affect the corrosion behavior of Fe, Ni, and 304SS in IL-based electrolytes, but also the anionic structure influences the corrosion process. Figure 10 shows that the region of active dissolution for Fe, Ni, and 304SS is similar in both [BsMIM][HSO4] and [BsMIM][OTs] electrolytes. The values of jcorr in [BsMIM][OTs] are lower than in [BsMIM][HSO4]. This indicated a lower corrosion rate of the tested materials in [BsMIM][OTs]. Presumably, this consequence may due to the lower acidity and larger volume of OTs−, adsorbed on the metal surface and blocking active sites against approaching oxidizing species. The shapes of the Tafel curves for Fe, Ni, and 304SS remain the

Figure 6. Tafel plots of Fe (a), Ni (b), and 304SS (c) in various concentrations of [BsMIM][OTs] electrolytes at 50 °C.

Figure 7. Nyquist plots of 304SS in various concentration of electrolytes at 30 °C.

same in these two IL-based solutions, suggesting that the same corrosion mechanism was controlled by charge transfer. D

DOI: 10.1021/acssuschemeng.5b00974 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

water and ethanol, dried, and then SEM images were taken. These photos are displayed in Figure 11a−f.

Figure 8. Phase angle plots of 304SS in various concentration of electrolytes at 30 °C.

Figure 11. SEM micrographs of metals immersed in IL-based solutions for 48 h at 40 °C: (a) Fe in 1 g [BsMIM][HSO4]/100 mL water, (b) Fe in 1 g [BsMIM][OTs]/100 mL water, (c) Ni in 1 g [BsMIM][HSO4]/100 mL water, (d) Ni in 1 g [BsMIM][OTs]/100 mL water, (e) 304SS in 1 g [BsMIM][HSO4]/100 mL water, and (f) 304SS in 1 g [BsMIM][OTs]/ 100 mL water.

Figure 9. Bode impedance magnitude plots of 304SS in various concentration of electrolytes at 30 °C.

For Fe, its surface was rough, covered with nonuniform corrosion products as shown in Figure 11a and b. Only a minor difference in the morphology can be observed in the two ILbased solutions. For Ni, a dense film of corrosion products is deposited on the surface in the [BsMIM][HSO4] solution, while the damage is obviously reduced in the [BsMIM][OTs] solution. It is slightly corroded when contrasted with Fe. For 304SS, there is a cleaner surface in the [BsMIM][OTs] solution with several cracks, even the grinding finishing line can be seen. Also a uniform and dense film of the corrosion product can be seen on the 304SS surface in the [BsMIM][OTs] solution. This reveals that the corrosion extent of the metals is more significant in the [BsMIM][HSO4] solution. The corrosion product adsorbed on the 304SS surface is more uniform and protective to stop the contact between the metal and solution. Corrosion Mechanism. Regarding the corrosion mechanism of metals in acidic solutions, a lot of work has been done. In general, spontaneous corrosion of metals undergoes two processes: anodic reactions for metal dissolution and cathodic reactions for hydrogen evolution or oxygen reduction.9 On the basis of the above results and discussion, the corrosion mechanism in this work can be described as follows.19,25−28

Figure 10. Tafel plots of Fe, Ni, 304SS in 1 g [BsMIM][HSO4]/300 mL water at 30 °C.

Also, from the Nyquist plots, the capacitive loops of the metallic material in the [BsMIM][OTs] solutions are larger than those in the [BsMIM][HSO4] solutions, which is an indication of faster interfacial electron transfer. The Bode plots show that the absolute impedance values of the electrode are much lower in [BsMIM][HSO4] solutions. Moreover, Rpore decreased, while Y0 of CPE increased in the [BsMIM][HSO4] solutions (Tables S4−S6, Supporting Information). This phenomenon may be attributed to the increased resistance and charge recombination of [BsMIM][HSO4] in electrolytes. Therefore, it is not surprising to see that the corrosion rate order obeys [BsMIM][HSO4] > [BsMIM][OTs]. SEM Analysis. SEM analysis was performed to study the corrosion morphology of the metal surface. After immersion tests, Fe, Ni, and 304SS specimens were washed with bidistilled

Anodic reactions:

E

M + HSO4 −(OTs−) → MHSO4 −(MOTs−)

(1)

MHSO4 −(MOTs−) → MHSO4 (MOTs) + e

(2)

DOI: 10.1021/acssuschemeng.5b00974 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering MHSO4 (MOTs) → MHSO4 +(MOTs+) + e MHSO4 +(MOTs+) → M+ + HSO4 +(OTs−)

(3) (4)

(5)

+

(6)

MH + e → MH +

MH + H + e → M + H 2

HSO4−

(7) −

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CONCLUSION The corrosion properties of Fe, Ni, and 304SS in [BsMIM][HSO4] and [BsMIM][OTs] diluted solutions have been studied by electrochemical methods and characterized by SEM. The following results can be drawn: (1) The investigated ILs, [BsMIM][HSO4] and [BsMIM][OTs], exhibit different extents of corrosive characteristics to Fe, Ni, and 304SS. (2) The corrosion rate increases with increasing temperature and decreases with dilution in two IL-based solutions, especially in [BsMIM][OTs]. (3) The corrosion behavior is controlled by charge transfer. The results obtained indicate that materials selection for technical equipment used in IL-based chemical processes is a very important issue and requires further investigations. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00974. Values of corrosion potential (Ecorr), cathodic (βc) and anodic (βa) Tafel slopes, linear polarization resistance (Rp), corrosion current density (jcorr), which is calculated from the corresponding Tafel plot, are listed in Tables S1−S3. Also, values of solution resistance (Rs), charge transfer resistance (Rpore), inductance (L), inductive resistance (RL), proportional factor (Y0), and phase shift (n) are shown in Tables S4−S6. (PDF)

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ACKNOWLEDGMENTS

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REFERENCES

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It is assumed that and OTs anions in [BsMIM][HSO4] and [BsMIM][OTs] can be adsorbed onto the positively charged metal surface by Coulombic attraction. Then under the effect of electrostatic attraction, the molecule of IL adsorbed between the positively charged molecules and the negatively charged metal surface forms monomolecular layers on the metal surface, which can protect metal surface from attack by HSO4− or OTs− ions.

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The authors thank the National Natural Science Foundation of China (Project Nos. 21133011, 21473225, and 21303231) and National Basic Research Program of China (973 Program, No. 2011CB201404) for financial support.

Cathodic reactions: M + H+ → MH+

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-0931-4968056. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of Ying Ma, Feng Han, Zhen Li, and Chungu Xia. Ying Ma, Feng Han, and Chungu Xia contributed equally. Zhen Li made the largest contribution. All authors have given approval for the final version of the manuscript. Notes

The authors declare no competing financial interest. F

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DOI: 10.1021/acssuschemeng.5b00974 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX