Acidic-Functionalized Ionic Liquid as Corrosion Inhibitor for 304

Research Article pubs.acs.org/journal/ascecg

Acidic-Functionalized Ionic Liquid as Corrosion Inhibitor for 304 Stainless Steel in Aqueous Sulfuric Acid 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 ABSTRACT: The corrosion behavior of three acidic media, 98% sulfuric acid, 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([BsMIM][HSO4]) and 1-(4-sulfobutyl)-3-methylimidazolium tetrafluoroborate ([BsMIM][BF4]), was studied by electrochemical impedance spectroscopy (EIS) and Tafel plots. It indicated that the corrosion mechanism is controlled by charge transfer. And the two acidic-functionalized ionic liquids (AFILs) exhibited lower corrosion rate. On the basis of the electrochemical corrosion results, the inhibition ability of AFILs in 1.0 M sulfuric acid toward 304 stainless steel was investigated at various concentrations and temperatures using gravimetric measurement, combined with the characterization of 304SS surface morphology by scanning electron microscopy (SEM). The results showed that the two AFILs act good as inhibitors, especially at low temperatures. The inhibition efficiency increases with the inhibitor concentration, and decreases sharply above 60 °C. The thermodynamic parameters suggested that the adsorption of AFILs on the 304SS surface occurred by physical and chemical adsorption. And the corrosion inhibition mechanism is proposed. KEYWORDS: Electrochemical impedance spectroscopy, Tafel plot, Gravimetric measurement, Scanning electron microscope, Mechanism

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INTRODUCTION

adsorbed on the metal surface via a coordinate covalent bond or the electrostatic interaction.5 ILs are salts composed of cations and anions. Their physicochemical properties have been investigated by pioneers being utilized in many fields, without exception of the corrosion and corrosion inhibitive characters. It reported that the corrosion behavior of ILs depends on their nature of chemical structures, pH, temperature, specific interaction, etc. And, the inhibition abilities were generally considered as the formation of a protective film on the metal surface.6,15,16 Likhanova et al.17 investigated the corrosion inhibition of mild steel by imidazolium and pyridinium ILs in 1 M aqueous solution of sulfuric acid. The two ILs displayed corrosion protection efficiency within 82−88% even at 100 ppm at room temperature. Also, it suggested that corrosion inhibition occurred by a chemical adsorption process. Zhang et al.4 reported that [BMIM][HSO4] exhibited a corrosion inhibition efficiency of 78.6% at 1 mM at 303 K against the corrosion of mild steel in hydrochloric acid, which performed superior than [BMIM]Cl. The inhibition efficiency was more pronounced with inhibitor concentration, and the adsorption of the inhibitors on the metal surface obeys Langmiur adsorption isotherm.

Corrosion is common daily in industry connected with operating and equipment maintenance, which induces recurrent partial and even total process shutdown, leading to severe economic losses and environmental pollution.1 Thereby, passive metals, such as carbon steels and stainless steels, have been widely used as structural materials.2 However, even then, in the acidic medium, the only thing we can do still is to control and minimize the damage it caused. Pioneers have tried a lot to study the metal corrosion and corrosion inhibition. Results showed that adding organic inhibitor in acidic medium is one of the most practical methods to protect metals against corrosion.3−14 Li et al.11 employed tetradecylpyridinium bromide (TDPB) as a corrosion inhibitor on the corrosion of aluminum in 1 M HCl by weight loss and electrochemical methods. In the presence of 1 mM TDPB, the maximum inhibition efficiency at 20 °C is 95.6%; at 40 °C, 87.2%. Ehasni et al.12,13 synthesized 3,3′-(1,4-phenylene)bis(2-imino-2,3dihydrobenzo[d]oxazole-5,3-iyl)bis(4-thylbenezenesulfonate) (1,4-Ph(OX)2Ts2) and 1-(4-nitrophenyl)-5-amino-1H-tetrazole, confirmed their inhibitive abilities in sulfuric acid by means of electrochemical and quantum chemical. In the presence of 1 mM 1,4-Ph(OX)2Ts2 toward 1005 aluminum alloy against 0.5 M sulfuric acid solution, the corrosion inhibition efficiency reaches 65.06% at 298 K and 68.32% at 318 K. It was found that those could be called inhibitors usually © XXXX American Chemical Society

Received: June 30, 2016 Revised: August 13, 2016

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DOI: 10.1021/acssuschemeng.6b01492 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

All potentials were recorded with respect to the Hg/Hg2SO4 at 30 °C, and converted with respect to SCE. Gravimetric Analysis. Gravimetric experiments were performed as per the ASTM method described previously.20 Thin disks of 304SS (5 cm × 2.5 cm × 0.1 cm) were grinded with SiC paper down to 1200 grit in order to achieve a reproducible surface finishing. The samples were well cleaned, weighed, and then hung in a 500 mL beaker, immersed in 300 mL corrosive solution at different temperatures range from 30 to 70 °C for 12 h. The concentration of inhibitor varies between 10−4 to 10−2 M. After the immersion tests, the 304SS disks were thoroughly washed, dried under air and reweighed. The corrosion rate (CRW0, CRW represents the corrosion rate in the absence and presence of inhibitor, respectively), the surface coverage (θ) and the inhibition efficiency (η) were calculated by eqs 1−3, where W′ is the average weight loss (mg), S is the surface area of 304SS (cm2), and t is the immersion time (s).

Acidic-functionalized ILs (AFILs), which can provide acidic functional groups linked with their cations or anions, display favorable physicochemical properties,1,18 such as environmental friendly, negligible vapor pressure, nonflammability, high thermal and chemical stability, wide electrochemical window, and adjustable solvent power for organic and inorganic substances, etc. As a kind of catalysts, AFIL is more effective than a conventional catalyst, sulfuric acid, in chemical reactions due to its strong acidity and excellent physicochemical properties. Notably, the corrosion of metallic materials by acidic catalysts is an important characteristic that determines their practical applications, also the economy and safety of processes. It is well-known that sulfuric acid is a strong corrosive acid in spite of good catalytic performance. How about the AFILs? In the previous work, the corrosion behaviors of two AFILs, [BsMIM][HSO4] and [BsMIM][OTs], toward three metallic materials were studied. It pointed out that the corrosion rate is slowly, and the corrosion mechanism is controlled by charge transfer.6 Then, how about their corrosion inhibitory effect? Maybe AFILs themselves possess corrosion inhibition properties on 304SS because of their structures. However, there is quite a few studies about AFILs corrosion inhibitory effect in acidic media. The aim of the present work was to study the corrosion inhibitory effect of two AFILs, [BsMIM][HSO4 ] and [BsMIM][BF4], against the corrosion of 304SS in aqueous sulfuric acid by gravimetric method and SEM characterization.

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C RW = θ=

η=

W′ St

(1)

0 C RW − C RW 0 C RW 0 C RW − C RW 0 C RW

(2) × 100%

(3)

Surface Analysis. The surface morphology and composition changes of the 304SS disks in the absence and presence of inhibitors were characterized by a JSM-5600 LV scanning electron microscopy (SEM). The abraded 304SS specimen were immersed in 1 M H2SO4, 1 M H2SO4 with addition of 5 mM [BsMIM][HSO4], and 1 M H2SO4 with addition of 5 mM [BsMIM][BF4] for 24 h at 30 °C first. Then, after cleaned with bidistilled water and ethanol, the dried surfaces were examined at a magnification of 2000×. The acceleration voltage was 20 kV.

EXPERIMENTAL SECTION

Materials. [BsMIM][HSO4] and [BsMIM][BF4] used in this work were synthesized and purified in our laboratory. The synthetic method was similar to those described previously.17−19 Water content below 3% was tested by Karl Fischer titration (HB43-S, Mettler Toledo). The molecular structures of the two AFILs are depicted in Chart 1.

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RESULTS AND DISCUSSION EIS Study. EIS is a rapid characterization method to obtain information about corrosion mechanism and dynamic parameters. In this paper, the technique of EIS was used to investigate the relationship of impedance varies with frequencies. These corresponding results are depicted in Figures 1, 2, and 3. In the Nyquist plots as shown in Figure 1, three loops can be seen clearly, which represented the relation between real part and imaginary part of impedance recorded on 304SS in 98% H2SO4, [BsMIM][HSO4], and [BsMIM][BF4] at 30 °C, respectively. The formation of these imperfect semicircles is a typical electrochemical behavior happened on solid electrode

Chart 1. Molecular Structures of [BsMIM][HSO4] and [BsMIM][BF4]

The corrosion solution, 1 M H2SO4, was prepared by dilution of analytical grade 98% H2SO4 of predetermined normality with double distilled water. The 304SS materials were purchased from AiDa HengSheng Science Technology Corporation consisted of 0.17% Cr, 0.18% Ni, 0.06% Si, 0.06% Al, 0.32% C, 0.15% O, and balance Fe. Electrochemical Tests. The corrosion tests were carried out in a typical three-electrode glass cell with temperature control unit. Hg/ Hg2SO4 electrode and Pt wire were used as the reference electrode (RE) and counter electrode (CE), respectively. 304SS with the exposure surface area of 3.14 × 10−2 cm2 was used as the working electrode (WE). Before testing, the WE was abraded with 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 electrolyte was 20 mL, purged with nitrogen to remove dissolved oxygen each time. After the open-circuit potential (Eocp) was reached in 120 min, EIS technique was carried out three times over a frequency range of 10 mHz to 100 kHZ with a signal amplitude perturbation of 5 mV. Potentiodynamic polarization study was performed later with a scan rate of 0.05 mV·s−1 in the potential range from ±300 mV around Eocp.

Figure 1. Nyquist plots of 304SS in 98% H2SO4, [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C. B


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resistance (Rpore), constant phase element related to the capacitance of the formed film on 304SS surface (CPEf), and the double layer capacitance at the interface between 304SS and solution in the film pores (CPEdl) are shown in Table 1. The charge transfer resistance (Rct) equals to Rf + Rpore. As seen from Table 1, [BsMIM][HSO4] shows the best electroconductivity for its lowest solution resistance. The proportional factor of CPEf tends to decrease, and the CPEdl tends to increase in ILs, due to the decreased dielectric constant and/or the increased double electric layer thickness. In addition, both Rf and Rpore increased in ILs, suggested that the charge transfer resistance increases. Moreover, the values of phase shift nf in ILs equal to 1, depicted that the CPEf can be regarded as a pure resistor. Generally speaking, the corrosion rate follows the order: [BsMIM] [BF4] < [BsMIM] [HSO4] < 98% H2SO4. Tafel Plots. Tafel curves recorded on 304SS in three tested acidic media were shown in Figure 5. In [BsMIM][HSO4], the dissolution potential ranges from −0.3 to −0.25 V, and at about 90 mV above corrosion potential, the anodic current slightly changed with potential. This indicated a passive state. Perhaps [BsMIM][HSO4] blocked the active site on the surface of 304SS and acted as a barrier for oxidizer species.17 In [BsMIM][BF4], near the corrosion potential, the corrosion current increases abruptly by orders of magnitude. The dissolution zone starts from −0.33 V. Furthermore, the cathodic current is the lowest, suggested the tardy hydrogen evolution rate. In sulfuric acid, the dissolution zone ranges from about −0.22 to −0.15 V. The passive potential extends from −0.15 to 0.1 V, and the corrosion current reaches at 10−6 A. The anodic current increases with orders of magnitude when contrasted with ILs. This verified the high dissolution speed of 304SS. The paralleled cathodic branches indicated the similar corrosion mechanism for 304SS in three tested media. Noticeably, the corrosion potential of 304SS in ILs shifts to more negative than that in sulfuric acid solution. However, it is reasonable, as the corrosion rate is mainly measured by corrosion current density, while corrosion potential is the means by which the anodic and cathodic reactions are kept in balance. The corrosion parameters calculated from Tafel plots are listed in Table 2. Ecorr denotes corrosion potential, Rp refers to polarization resistance, jcorr is corrosion current density, βc refers to cathodic Tafel slope, and βa refers to anodic Tafel slope. Corrosion rate CRP was obtained by eq 4, where MFe is the molecular weight of Fe, n is the number of electrons transferred in the corrosion reaction, and ρ is the density of 304SS (7.96 g·cm−3).

Figure 2. Phase angle plots of 304SS in 98% H2SO4, [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C.

Figure 3. Bode impedance magnitude plots of 304SS in 98% H2SO4, [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C.

caused by the frequency dispersion of interfacial impedance.17−19 Furthermore, in [BsMIM][BF4], the diameter of semicircle reaches the largest. This revealed the strongest charge transfer resistance and the slowest corrosion rate for 304SS. Figure 2 shows the phase angle plots of 304SS in three acidic electrolytes. Obviously, there are two phase peaks at the frequency range indicated two time constants in ILs. Interestingly, the phase peak of sulfuric acid is much wider for the overlap of time constants. Noticeably, the value of phase angle in two ILs performs no great differences at high frequency, as well as the absolute value of Z in the Bode impedance magnitude plots have shown in Figure 3. The equivalent circuit R(Q(R(QR))) obtained by Zsimp Win software is shown in Figure 4. The parameters of interests such as electrolyte resistance (Rs), film resistance (Rf), pore

⎛ mm ⎞ j MFe C RP⎜ ⎟ = 3.268 × 10−3 corr nρ ⎝ y ⎠

(4)

It is clear that the corrosion rate of 304SS is significantly reduced as a result of the reduction in jcorr. All the values of βa are larger than those of βc in the tested media, which indicated that the corrosion process is controlled by the anodic reaction. Interestingly, the value of βc shows negligible change in AFILs when compared with the sulfuric acid. Here, the value of βc is a speed indicating of the cathodic reactions. As the hydrogen evolution mainly occurs in cathode for 304SS in the three tested corrosive solutions, the βc depicts similar values. However, the changes of βa is outstanding due to the acidity differences among them following by the order: [BsMIM][BF4]

Figure 4. Equivalent circuit used to fit the EIS data earned for 304SS in acidic solutions. C


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Table 1. Summary of the Parameters Extracted from the EIS Method for 304SS in 98% H2SO4, [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C sample

98% H2SO4

Rs (Ω·cm2) CPEf − Y0 (S·sn·cm2) CPEdl − Y0 (S·sn·cm2) Rf (Ω·cm2) Rpore (Ω·cm2) CPEf − nf CPEdl − ndl

1.000 × 2.684 × 7.212 × 4.469 1.337 × 0.5996 0.9433

[BsMIM][HSO4]

10−7 10−5 10−6

6.079 × 6.560 × 2.125 × 3198 3.414 × 1 0.7816

105

Table 2. Summary of the Parameters Extracted from the Polarization Curve of 304SS in 98% H2SO4, [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C (E vs SCE) sample

98% H2SO4

[BsMIM][HSO4]

[BsMIM][BF4]

−0.2223 16.567 39.793 15.519 0.1784 104373

−0.3193 15.370 37.746 1.1994 0.01379 1279482

−0.3313 14.263 17.740 0.7515 0.00864 1456832

105

[BsMIM][BF4] 8.351 6.604 2.574 3117 1.714 1 0.69

× 10−7 × 10−9 × 10−5 × 106

However, it is sure that the corrosion process of 304SS in sulfuric acid solution is retarded by introducing IL. The two AFILs can be used as corrosion inhibitors. In the absence of AFILs, the corrosion rate reaches as high as 9 mg·cm−2·h−1 at 70 °C. On the contrary, in the addition of 10−4 M ILs, the corrosion rate depressed by a factor of about 1.5. Also, the inhibition efficiency increases with increasing concentration of IL. The function of ILs performed may attributed to that the atoms of the imidazolium ring and the −CN− group can form a big π bond. Then, the electron of ILs can enter the unoccupied orbitals of Fe. Meanwhile, the π* orbital can accept the electron of d orbital in Fe to form more adsorption center.19 Influence of Temperature on Corrosion Inhibition. Not only concentration of inhibitors influences the corrosion behavior of 304SS in 1 M H2SO4 solution but also temperature has an effect on the inhibition process. In this paper, various temperatures were studied. It can be observed obviously that the acidic solutions changed from colorless to dark green accompanied by evolution of hydrogen gas at high temperatures during the immersion tests. This declares that the interaction between metal and acidic medium is sensitive to temperature. The corresponding results are listed in Table 3. It is directly to see from Table 3 the corrosion rate increases with the rise of temperature in the absence and presence of inhibitors. This behavior can be interpreted on the basis that high temperature accelerated the dissolution process of 304SS. Interestingly, rising temperature from 30 to 50 °C does not substantially increase the weight loss of 304SS. The inhibition efficiency almost remains constant below 50 °C, and sharply increases above 60 °C. Generally speaking, the inhibition efficiency of [BsMIM][HSO4] and [BsMIM][BF4] shows no great differences. Adsorption Isotherm. Organic molecules can depress metallic corrosion by adsorption at the metal/solution interface. The adsorption isotherm gives important information about the interaction mechanism between the metal surface and the inhibitor. Frumkin, Temkin and Langmuir isotherms are favored, especially Langmuir isotherm model. It supposes that the solid surface contains a fixed number of adsorption sites and each site holds one adsorbed species.19 Figure 6 shows the three adsorption isotherms of 304SS in 1 M sulfuric acid solution containing different concentrations of [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C. It was found that our experimental data fit well with Langmuir adsorption isotherm, suggested the adsorption process of ILs on 304SS surface in 1 M H2SO4 solution obeys the Langmuir adsorption rules. Thereby, a monolayer of adsorbed molecules is formed. Thermodynamic Calculations. The thermodynamic parameters play an important role in understanding the inhibition mechanism of inhibitors. The adsorption -

Figure 5. Tafel plots of 304SS in acidic media at 30 °C.

Ecorr (V) βc (mV/dec) βa (mV/dec) jcorr (μA·cm−2) CRP (mm/y) Rp (Ω·cm2)

10−8 10−9 10−5

< [BsMIM][HSO4] < H2SO4.21,22 In addition, the Tafel slopes in [BsMIM][BF4] depicted the smallest, as well as jcorr, verified the results obtained from EIS that [BsMIM][BF4] occurs the slowest corrosion rate. Immersion Tests. As mentioned above, the corrosion rates of 304SS in the two AFILs are much slower than that in 98% sulfuric acid solution. Additionally, on the basis of the previous reports, the organic molecular with −CN− group and electro negative nitrogen performs excellent as a corrosion inhibitor in acidic solutions.3−5 Can [BsMIM][HSO4] and [BsMIM][BF4] act as inhibitor against corrosion of 304SS in 1 M aqueous sulfuric acid? The influence of concentration and temperature will be discussed later by means of immersion tests. Influence of Concentration on Corrosion Inhibition. The corrosion parameters of 304SS in 1 M sulfuric acid solution in the absence and presence of [BsMIM][HSO4] and [BsMIM][BF4] with different concentrations are summarized in Table 3. It can be observed that the corrosion rate decreases with the rise of ILs concentration. This may attributed to the increased surface coverage of [BsMIM][HSO4] and [BsMIM][BF4] at 304SS/solution interface, which prevented the attack of sulfuric acid solution to 304SS surface, slowed down the dissolution of 304SS. The concentration effect is not obviously below 60 °C. D


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Table 3. Corrosion Parameters Obtained from Weight Loss Measurements of 304SS after 12 h Immersion in 1 M H2SO4 Solution with and without Addition of Various Concentrations of [BsMIM][HSO4] and [BsMIM][BF4] at Various Temperatures T (°C) 30

blank 1.00 × 5.00 × 1.00 × 5.00 × 1.00 × blank 1.00 × 5.00 × 1.00 × 5.00 × 1.00 × blank 1.00 × 5.00 × 1.00 × 5.00 × 1.00 × blank 1.00 × 5.00 × 1.00 × 5.00 × 1.00 × blank 1.00 × 5.00 × 1.00 × 5.00 × 1.00 ×

40

50

60

70

a

C (M) 10−04 10−04 10−03 10−03 10−02 10−04 10−04 10−03 10−03 10−02 10−04 10−04 10−03 10−03 10−02 10−04 10−04 10−03 10−03 10−02 10−04 10−04 10−03 10−03 10−02

CRWa (mg/cm2·h)

ηa (%)

CRWb (mg/cm2·h)

ηb (%)

0.7869 0.03096 0.02275 0.02212 0.02117 0.01200 2.2188 0.03222 0.02975 0.02465 0.02166 0.02075 3.6228 0.05656 0.03349 0.03318 0.02397 0.02240 7.2973 3.0108 2.9416 2.2806 2.1151 1.7704 9.1841 6.1565 4.7457 4.0611 3.1746 3.0145

0 96.07 97.11 97.19 97.31 98.48 0 98.55 98.66 98.89 99.02 99.06 0 98.44 99.08 99.25 99.34 99.18 0 58.74 59.69 68.75 71.02 75.74 0 32.97 48.33 55.78 65.43 67.18

0.7869 0.03318 0.02338 0.02296 0.02243 0.01343 2.2188 0.03569 0.03207 0.02559 0.02369 0.02169 3.6228 0.04249 0.03717 0.03453 0.02480 0.02289 7.2973 4.3456 3.3551 2.7551 2.1792 0.2722 9.1841 6.2303 5.5798 5.5287 4.3457 0.4459

0 95.78 97.03 97.08 97.15 98.29 0 98.39 98.55 98.85 98.95 99.02 0 98.83 98.97 99.05 99.32 99.37 0 40.45 54.02 62.24 70.14 96.27 0 32.16 39.24 39.80 52.68 95.14

[BSMIM][HSO4]. b[BSMIM][BF4].

However, in the presence of AFILs, the corrosion damage is reduced, even the grinding line can be observed. Panels c and 7 show the inhomogeneous 304SS surface with several crackers. Also, there are no great differences for 304SS in the two ILs at 30 °C. This is in line with the inhibition efficiency conclusion as shown in Table 3. These observations proved that [BsMIM][HSO4] and [BsMIM][BF4] can perform as corrosion inhibitors for 304SS in 1 M H2SO4 solution. Corrosion Inhibition Mechanism. On the basis of previous reports23,24 and our experimental data, the inhibition mechanism of two AFILs in 1 M sulfuric acid was proposed, and the results are shown in Figure 8. Anodic reactions:

desorption equilibrium constant (Kads) and standard free energy (−ΔGads) for 304SS in 1 M sulfuric acid solution in the presence of 5 × 10−3 M inhibitors at various temperatures are calculated by eqs 5 and 6. The summarized data are shown in Table 4. θ C(1 − θ )

(5)

ΔGads = −RT ln(55.5K ads)

(6)

K ads =

The negative values of ΔGads indicated that adsorption process of ILs on the metal surface occurred spontaneously. Normally, values of ΔGads around −20 KJ·mol−1 or less negative are characteristic of physical adsorption. Whereas, ΔGads values between −80 and −400 KJ·mol−1 signal that chemical adsorption takes place.23 It can be seen from Table 4 that the values of ΔGads for the two ILs are ranged between −20 and −40 KJ·mol−1, which probably suggests that both chemisorption and physisorption exist in the interaction of the ILs and the 304SS surface.23,24 Morphological Analysis. The morphology of 304SS surface before and after immersion tests is shown in Figure 7. Panel a depicts the freshly grind 304SS disk without immersion in the corrosive solution, the grinding line can be seen clearly. In contrast, in the solution of 1 M sulfuric acid, the surface of 304SS is strictly corroded, full of crackers as shown in panel b.

Fe + nH 2O → Fe(H 2O)n Fe(H 2O)n + SO4 2 − → [Fe(H 2O)n SO4 2 −] [Fe(H 2O)n SO4 2 −] + [BsMIM]+ → [Fe(H 2O)n SO4 2 −BSMIM+] → [Fe(H 2O)n SO4 −][BsMIM]+ + e → [Fe(H 2O)n SO4 BsMIM]−

Cathodic reactions: Fe + (BsMIM)+ → Fe(BsMIM)+ Fe(BsMIM)+ + e → Fe(BsMIM) E


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Figure 6. Temkin (a), Frumkin (b), and Langmuir adsorption (c) isotherm plots for 304SS in 1 M H2SO4 solution containing various concentrations of [BsMIM][HSO4] and [BsMIM][BF4] at 30 °C.

Table 4. Values of Kads and −ΔGads for 304SS in 1 M H2SO4 solution with 5 × 10 −3 M Inhibitors at Different Temperatures −ΔGads (KJ·mol−1)

Kads T (°C)

[BsMIM] [HSO4]

[BsMIM] [BF4]

[BsMIM] [HSO4]

[BsMIM] [BF4]

30 40 50 60 70

7234.9 20208.2 30103.0 21521.2 378.5

6817.5 18847.6 29211.8 469.8 222.7

32.50 36.25 38.48 38.74 28.38

32.35 36.07 38.40 28.15 26.87

It is assumed that SO42− in the sulfuric acid combined with the positively charged metal surface by coulomb force first. Then, the inhibitor molecule adsorbed between the positively charged molecular and negatively charged metal surface through electrostatic interactions. These adsorbed molecules interacted with [Fe(H2O)nSO42−] to form a monomolecule layer. This is the physical adsorption process. On the other hand, the cations of ILs accept electron from metal surface to reach electroneutrality. The anions of ILs, can be regarded as donating groups, increased the electron density of N atom in the −C N− group. And chemisorption occurs by a retro-donation process. Thus, the protective layer is deposited on the surface to against corrosion.

Figure 7. SEM photos of 304SS surface at 30 °C: (a) without immersion in corrosive solution; (b) after 24 h immersion in 1 M H2SO4 without inhibitor; (c) after 24 h immersion in 1 M H2SO4 containing 5 mM of [BsMIM][HSO4]; and (d) after 24 h immersion in 1 M H2SO4 containing 5 mM [BsMIM][BF4].

studied by gravimetric method and characterized by SEM. The following results can be drawn: (1) 98% H2SO4, [BsMIM][HSO4], and [BsMIM][BF4] performed different extent corrosive properties for 304SS. And the corrosion rate increased following the order of [BsMIM][BF4] < [BsMIM][HSO4] < 98% H2SO4. The corrosion mechanism of 304SS in these

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CONCLUSION The corrosion properties of 304SS in 98% H2SO4, pure [BsMIM][HSO4], and [BsMIM][BF4] have been studied by electrochemical methods. And the corrosion inhibitive behaviors of two AFILs in aqueous sulfuric acid have been F


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Figure 8. Corrosion inhibition mechanism of experimental ILs on 304SS in the aqueous sulfuric acid.

acidic media is mainly controlled by charge transfer process. (2) [BsMIM][HSO4] and [BsMIM][BF4] shown good inhibition properties for the corrosion of 304SS in 1 M H2SO4 solution, especially at low temperatures. And the inhibition efficiency was influenced by inhibitor concentration and temperature. The inhibiting efficiency shows no great differences between [BsMIM][BF4] and [BsMIM][HSO4]. (3) The adsorption of the inhibitor molecules on the 304SS surface from 1 M H2SO4 solution is influenced by both physical and chemical adsorption.

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AUTHOR INFORMATION

Corresponding Author

*Zhen Li. Tel.: +86-0931-4968056. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support of the National Natural Science Foundation of China (Project No. 21133011, No. 21473225, and No. 21303231).

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REFERENCES

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Acidic-Functionalized Ionic Liquid as Corrosion Inhibitor for 304

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