Single and Double Alkyl Chain Quaternary Ammonium Salts as Environment-Friendly Corrosion Inhibitors for a Q235 Steel in 0.5 mol/L H₂SO₄ Solution

Abstract

In this work, the corrosion inhibition effects of octadecyl trimethyl ammonium chloride (OTAC) and dioctadecyl dimethyl ammonium chloride (DDAC) on Q235 steel in a 0.5 mol/L H₂SO₄ solution are studied. The results of the electrochemical experiment, contact angle measurement, and scanning electron microscopy indicate that the two ionic liquids belong to mixed-type corrosion inhibitors with good anti-corrosion performance. Additionally, OTAC has a better anti-corrosion ability than DDAC, implying that the steric hindrance effect of the double alkyl chain is not conducive to the adsorption of DDAC on the electrode surface.

1. Introduction

Carbon steel has been widely employed in various fields, such as the chemical industry, marine engineering, building materials, and pipeline transport, due to its unique advantages, including its high mechanical property, low cost, and easy processing [1,2,3]. Unfortunately, during the process of use in a corrosive environment, oxidized corrosion products are prone to form on the surface of steel equipment [4]. As a result, diluted sulfuric acid is usually used as the pickling solution to remove corrosion products [5]. However, a pickling solution also corrodes the surface of carbon steel, owing to its corrosive nature [6]. Therefore, it is necessary to protect carbon steel against corrosion in sulfuric acid solution. One of the most effective strategies is to add corrosion inhibitors to reduce the corrosion rate of carbon steel in a sulfuric acid solution [7,8,9,10]. Corrosion inhibitors include inorganic and organic compounds, but some metal salts or organic compounds containing sulfur and phosphorus cause a series of environmental problems [11]. Accordingly, it is urgent and necessary to develop environment-friendly corrosion inhibitors.

In recent years, ionic liquids have attracted more attention in academic and industrial fields, owing to their excellent thermal stability, high ionic conductivity, negligible vapor pressure, non-volatility, and lower toxicity [12,13]. Compared with organic corrosion inhibitors, ionic liquids are biodegradable and environment-friendly. Many ionic liquids, such as imidazole, ammonium, pyridine, and phosphonium, have been proven to possess excellent anti-corrosion performance [14,15]. According to the literature, ionic liquids could adsorb on metal surfaces to form a protective layer [16,17]. Some reports have investigated the influence of the single alkyl chain length of ionic liquids on the corrosion inhibition performance [18,19]. For example, Zhang’s group demonstrated that the inhibition efficiency of imidazolium-type ionic liquids increased with the increase in alkyl chain length attached to an imidazolium ring [20]. However, a few reports have studied the steric hindrance effect of single and double alkyl chains on the corrosion inhibitory performance of ionic liquids.

In this work, the anti-corrosion abilities of OTAC and DDAC for Q235 steel in a 0.5 mol/L H₂SO₄ solution are investigated using electrochemical experiments and surface characterizations. The focus of this work is to systematically explore the steric hindrance effect of single and double alkyl chains on corrosion inhibitory performance. The molecular structures of the two ionic liquids are shown in Figure 1. The results show that OTAC and DDAC could slow down the corrosion rate of Q235 steel in a 0.5 mol/L solution. Additionally, OTAC has better corrosion inhibition performance than DDAC.

2. Experimental

2.1. Materials

Octadecyl trimethyl ammonium chloride and dioctadecyl dimethyl ammonium chloride were purchased from Energy Chemical (Shanghai, China). Sulfuric acid (≥98.3%) and anhydrous ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The 0.5 mol/L H₂SO₄ solution was formulated by diluting concentrated sulfuric acid. The studied metal was Q235 steel, and its chemical composition (wt.%) was Fe, 98.397; C, 0.27; Si, 0.31; Ni, 0.021; P, 0.014; Ti, 0.042; S, 0.052; Cr, 0.074; Cu, 0.82, respectively.

2.2. Preparation of Self-Assembly Monolayers (SAMs)

Ethanol was used as the solvent to prepare different concentrations of OTAC and DDAC solutions. The Q235 steel specimens were meticulously polished with 400–2000 grit sandpapers to obtain a tidy surface and then cleaned with ultrapure water and ethanol to obtain a clean surface. Finally, they were dried under a stream of cold air to acquire prepared samples. After that, the samples were immersed in 10 mM inhibitor solution for different self-assembly times (0.5, 1, 2, and 3 h) at 298 K. In addition, the samples were immersed in a different concentration (5, 8, 10, and 12 mM) of inhibitor solutions for 2 h.

2.3. Electrochemical Measurements

The electrochemical experiments were conducted on a CHI660E electrochemical workstation with a three-electrode system. The Q235 steel with a bare area of 1.00 square centimeters was used as the working electrode, the saturated calomel electrode with an area of 4 square centimeters was used as the reference electrode, and the platinum plate was used as the counter electrode. First, the Q235 steel electrodes were immersed in 0.5 mol/L H₂SO₄ solution for 1200 s to reach a stable open circuit potential (E_OCP). Then, electrochemical impedance spectroscopy (EIS) was measured at E_OCP with a ±10 mV amplitude signal in the frequency range from 100 kHz to 0.01 Hz. Finally, the polarization curves were measured from −700 mV to +200 mV at a scan rate of 1 mV/s [21,22,23]. The inhibition efficiency (η) was calculated from polarization curves and EIS according to the following equations [24]:

$${\eta }_{\mathrm{P}}=\frac{i_{\mathrm{corr}}-i_{\mathrm{corr},0}}{i_{\mathrm{corr},0}}\times 100$$

(1)

$${\eta }_{\mathrm{E}}=\frac{R_{\mathrm{ct}}-R_{\mathrm{ct},0}}{R_{\mathrm{ct}}}\times 100$$

(2)

Herein, i_corr,₀ and i_corr are the corrosion current densities of the Q235 steel electrodes without and with the inhibitors, respectively. R_ct,0 and R_ct are the charge transfer resistances of Q235 steel without and with the inhibitors, respectively.

2.4. Surface Characterizations

The Q235 steel specimens were immersed in 0.5 mol/L H₂SO₄ solution for 4 h, then washed with ethyl alcohol and dried under cold air. The wetting ability of the specimens’ surface was investigated by contact angle measurement (Biolin PD-200, Beijing, China). The surface morphology of the specimens was studied using scanning electron microscopy (Hitachi SU8010, Hong Kong, China).

3. Results and Discussion

3.1. Optimum Conditions for the Formation of the SAMs

3.1.1. Effect of Self-Assembly Time

The polarization curves of the Q235 steel samples measured in a 0.5 mol/L H₂SO₄ solution after immersion in 10 mM inhibitor solutions for different self-assembly times are exhibited in Figure 2. As compared with the blank, the cathodic and anodic current densities of the modified Q235 steel electrodes obviously decreased, owing to the studied inhibitors absorbed on the electrode surfaces to form SAMs. With the increase in self-assembly time, the current density of the specimen surfaces covered with the inhibitors presented a decreasing trend. In addition, the current density reached the minimal value when the self-assembly time was 2 h. However, the current density increased when the self-assembly time was 3 h, which was due to the adsorption of the inhibitors reaching a critical value and rearrangement on the metal. Thus, corrosive sulfate ions could easily attack the substrate surface through the interspace [25]. Furthermore, it was obvious that the polarization curves of the cathodic branch presented a parallel trend, implying that the two inhibitors did not change the cathodic reaction mechanism [26].

It is widely known that Bockris and Drazic exhibit the dissolution mechanism for steel in a H₂SO₄ medium as follows [27]:

Fe + H₂O ↔ FeOH_ads + H⁺ + e⁻

FeOH_ads → FeOH⁺ + e⁻

FeOH⁺ + H⁺ ↔ Fe²⁺ + H₂O

The expression of the process of cathodic hydrogen reduction reaction is as follows:

Fe + H⁺ ↔ (FeH⁺)_ads

(FeH⁺)_ads + e⁻ → (FeH)_ads

(FeH)_ads + H⁺ + e⁻ → Fe + H₂O

The adsorption mechanism of the two ionic liquids on the Q235 steel surface can be described as follows:

Fe + lnh ↔ (Felnh)_ads

Accordingly, it can be concluded from the above reaction formulas that the two inhibitors’ adsorption on the surface of Q235 steel can effectually slow down the dissolution of anodic Fe²⁺. On the other hand, the area exposed to the corrosive medium is reduced owing to the two inhibitors absorbed, thereby reducing the active sites of hydrogen ion reduction.

The polarization parameters, including corrosion potential (E_corr), corrosion current density (i_corr), cathodic slope (β_c), and anodic slope (β_a), were obtained through Tafel linear extrapolation, as shown in

Table 1. Polarization parameters of Q235 steel specimens measured in 0.5 mol/L H₂SO₄ solution.

Samples	t (h)	icorr (μA/cm2)	Ecorr (V/SCE)	−βc (mV/dec)	βa (mV/dec)	ηp (%)	SD a
Blank	-	1804 ± 18	0.446	150	120	-	-
OTAC	0.5	226 ± 16	0.455	119	73	87.47	0.0064
	1	165 ± 20	0.460	116	75	90.85	0.0088
	2	96 ± 2	0.471	118	60	94.67	0.0006
	3	113 ± 27	0.468	117	63	93.73	0.0090
DDAC	0.5	501 ± 21	0.451	122	101	72.22	0.0094
	1	236 ± 23	0.456	136	83	86.91	0.0100
	2	171 ± 1	0.467	132	74	90.52	0.0004
	3	188 ± 26	0.462	134	79	89.57	0.0110

^a standard deviation of 3 independent measurements.

. Furthermore, the corrosion inhibition efficiency (η_p) of the Q235 steel specimens covered with the inhibitors was calculated by Equation (1) [28,29]. Compared to the blank, the corrosion potential values of the Q235 steels covered with OTAC and DDAC deviated to the negative direction, indicating that the influence of the two inhibitors on the cathodic reaction was more significant than that of the anodic reaction [6]. In addition, the moving values of corrosion potential were less than 85 mV, indicating that the two inhibitors were regarded as mixed-type inhibitors [30]. Furthermore, it was obvious that OTAC (90.18%, 2 h) presented better corrosion inhibition efficiency than DDAC (85.95%, 2 h) under the same experimental conditions because the steric hindrance effect of the double alkyl chain was not conducive to the adsorption of DDAC on the surface of the work electrodes [31].

The anti-corrosion performance of the two molecules was further investigated using electrochemical impedance spectroscopy. Figure 3 presents the Nyquist and bode plots of the modified specimens measured in a 0.5 mol/L H₂SO₄ solution for different self-assembly times. As shown in Figure 3a,c, the semicircle was composed of an inductive reactance arc and a capacitive reactance arc. With the extension of the self-assembly time, the radius of the capacitive loops significantly increased, indicating an increase in the number of inhibitor molecules adsorbed on the surface of the Q235 steel electrode. On the other hand, the semicircles presented a flat shape, which was attributed to the roughness and inhomogeneity of the electrode surfaces [32]. As compared with the blank, the semi-circular shape of the modified specimens remained unchanged, implying that the adsorption of the two inhibitors on the surface of Q235 steel did not change the electrode reaction mechanism [5]. In addition, OTAC had better inhibitory performance than DDAC when the self-assembly time was the same. The results are in agreement with the polarization curves.

It can be seen from Figure 3b,d, with the extension of the self-assembly time, that the values of the impedance modulus and phase angle of the steel covered with two inhibitors showed an increasing trend. Compared to the blank, the impedance modulus of the modified specimens increased up to one order of magnitude in the low-frequency region. Furthermore, it was obvious that a clear peak appeared in the phase angle, indicating the existence of a time constant. This time constant was due to the relaxation effect caused by the adsorption and desorption of the two molecules onto the surface of the work electrodes [33].

In order to determine the impedance parameters using simulations correlated with the experimental data obtained, a complex non-linear least squares (CNLS) simulation was utilized [34]. And the suitable equivalent circuit diagram was chosen to match the Nyquist plot in Figure 4. The equivalent circuit is made of solution resistance (R_s), charge transfer resistance (R_ct), constant phase angle element (CPE_dl), and inductive elements (L and R_L). The expression of the impedance function of the CPE was as follows [35]:

$${\mathrm{Z}}_{\mathrm{CPE}}=\frac{1}{Y_0{(j\omega )}^n},$$

(3)

where Y₀ represents the value of CPE, j is the imaginary unit, ω is the angular frequency, and n is the deviation index. The value of n ranges from −1 to 1.

The expression of the double layer capacitance is as follows [36]:

$$C_{\mathrm{dl}}=Y_0{(\omega )}^{n-1}=Y_0{(2\pi f{Z}_{\mathrm{im}\text{-}\mathrm{Max}})}^{n-1}.$$

(4)

Here, the fZ_im-Max value is the frequency corresponding to the maximum imaginary part of the impedance.

The relevant fitting parameters of the impedance spectrum data are shown in

Table 2. Fitting parameters of impedance spectrum data of the Q235 steel specimens measured in 0.5 mol/L H₂SO₄ solution.

Samples	t (h)	Rs (Ω cm2)	Y0 × 10−6 (Ω cm2)	n	Cdl (μF cm−2)	Rct (Ω cm2)	L (Ω cm2)	RL (Ω cm2)	ηE (%)	SD a
Blank	-	3.09	118.60 ± 6.96	0.91	108.46 ± 0.41	14.85 ± 0.97	183	268 ± 23	-	-
OTAC	0.5	2.37	65.09 ± 6.78	0.90	47.38 ± 9.56	98.89 ± 1.19	1743	442 ± 8	87.84	0.0150
	1	2.76	68.29 ± 5.32	0.91	48.22 ± 4.33	129.50 ± 0.91	5379	755 ± 22	90.44	0.0008
	2	2.52	53.61 ± 4.88	0.91	35.12 ± 1.17	205.01 ± 8.56	1494	1333 ± 50	92.76	0.0031
	3	3.00	60.38 ± 5.98	0.91	41.38 ± 5.16	175.50 ± 5.88	1297	1424 ± 63	92.40	0.0029
DDAC	0.5	2.90	91.36 ± 0.72	0.80	55.07 ± 7.05	51.69 ± 1.33	2310	413 ± 34	71.40	0.0070
	1	2.85	85.14 ± 2.24	0.86	57.28 ± 8.05	100.40 ± 3.33	3416	1154 ± 22	85.30	0.0045
	2	2.88	69.40 ± 9.25	0.82	40.65 ± 4.90	138.31 ± 1.84	2113	2510 ± 24	89.26	0.0014
	3	2.77	81.61 ± 7.23	0.82	48.13 ± 6.94	119.80 ± 3.51	1425	1425 ± 17	87.60	0.0036

^a standard deviation of 3 independent measurements.

. And the corrosion inhibition efficiency (η_E) of the Q235 steel specimens covered with the inhibitors was calculated by Equation (2). With the extension of the self-assembly time, the values of R_ct presented an increasing trend, which was due to the corrosion reaction being inhibited. On the other hand, the values of C_dl presented a decreasing trend, owing to an increase in the thickness of the electrical double layer and a decrease in the local dielectric [37]. Furthermore, it was obvious that OTAC (90.44%, 1 h) presented better corrosion inhibition efficiency than DDAC (85.3%, 1 h) under the same experimental conditions, which was consistent with the results of the polarization curves.

3.1.2. Effect of the Concentrations of the Inhibitor Solutions

The polarization curves of the Q235 steel specimens measured in a 0.5 mol/L H₂SO₄ solution after immersion in different concentration solutions of the two inhibitors for 2 h are exhibited in Figure 5. The Tafel extrapolation method was used to determine the polarization parameters, including the corrosion potential (E_corr), corrosion current density (i_corr), cathodic slope (β_c), and anodic slope (β_a), as shown in

Table 3. Polarization parameters of the Q235 steel specimens measured in 0.5 mol/L H₂SO₄ solution.

Samples	C (mM)	icorr (μA/cm2)	Ecorr (V/SCE)	−βc (mV/dec)	βa (mV/dec)	ηp (%)	SD a
Blank	-	1804 ± 18	0.446	150	120	-	-
OTAC	5	198 ± 1	0.457	119	79	89.02	0.0006
	8	141 ± 5	0.463	116	67	92.18	0.0016
	10	96 ± 2	0.471	115	60	94.67	0.0006
	12	127 ± 8	0.466	118	72	92.96	0.0032
DDAC	5	303 ± 13	0.452	136	84	83.20	0.0057
	8	215 ± 16	0.456	134	78	88.08	0.0056
	10	171 ± 1	0.467	130	73	90.52	0.0004
	12	197 ± 2	0.461	131	74	89.07	0.0007

^a standard deviation of 3 independent measurements.

. The corrosion inhibition efficiency (η_p) of the Q235 steel specimens covered with the inhibitors was calculated using Equation (1).

Compared to the blank, the corrosion current density of the modified Q235 steel electrodes significantly decreased in Figure 5. It was demonstrated that the two ionic liquids could effectively inhibit metal corrosion. It was obvious that the corrosion potential values of the Q235 steel covered with OTAC and DDAC shifted in a negative direction. With the increase in the concentration of the solutions of the two inhibitors, the current density of the specimen surfaces covered with the inhibitors presented a decreasing trend. It was shown that the polarization curves of the cathodic branch presented a parallel trend; see Figure 5. Moreover, the current density reached the minimal value when the concentration of the studied molecules was 10 mM. It was obvious that OTAC (88.21%, 5 mM) presented a better corrosion inhibition efficiency than DDAC (81.96%, 5 mM) under the same experimental conditions, as shown in Table 3.

The Nyquist and bode plots of the Q235 steel specimens measured in a 0.5 mol/L H₂SO₄ solution after immersion in the different concentration solutions of the two inhibitors are presented in Figure 6. Compared to the blank, the diameter of the semicircle of the modified Q235 steel electrodes obviously increases in Figure 6a,c. Moreover, when the concentration solution of the two inhibitors was 10 mM, the corrosion inhibition efficiency reached a maximum. Furthermore, with the increased concentration in the two inhibitor solutions, the values of the impedance modulus and phase angle show an increasing trend in Figure 6b,d.

A suitable equivalent circuit diagram was used to fit the impedance data in Figure 4. The characteristic electrochemical parameters related to the impedance plot, including the charge transfer resistance (R_ct), the double layer capacitance (C_dl), the solution resistance (R_s), the modulus with capacitance (Y₀), and the inductive elements (L and R_L), are shown in

Table 4. Fitting parameters of impedance spectrum data of Q235 specimens measured in 0.5 mol/L H₂SO₄ solution.

Samples	C (mM)	Rs (Ω cm2)	Y0 × 10−6 (Ω cm2)	n	Cdl (μF cm−2)	Rct (Ω cm2)	L (Ω cm2)	RL (Ω cm2)	ηE (%)	SD a
Blank	-	3.01	118.60 ± 6.97	0.91	108.46 ± 0.4	14.85 ± 0.97	183	268 ± 23	-	-
OTAC	5	3.28	91.48 ± 8.7	0.92	67.08 ± 1.61	134.60 ± 1.71	598	605 ± 34	88.97	0.0014
	8	2.83	80.61 ± 6.21	0.81	43.25 ± 4.21	162.90 ± 1.67	874	1178 ± 23	90.88	0.0009
	10	2.52	53.61 ± 2.98	0.91	35.44 ± 2.77	205.00 ± 5.82	1494	1333 ± 46	92.76	0.0020
	12	3.19	64.82 ± 7.61	0.90	44.22 ± 6.68	180.00 ± 3.00	701	1213 ± 30	91.75	0.0013
DDAC	5	2.85	88.68 ± 17.82	0.84	57.57 ± 2.95	88.19 ± 1.75	1054	591 ± 18	83.16	0.0032
	8	2.75	86.92 ± 6.04	0.86	57.44 ± 0.43	108.20 ± 4.10	1410	1212 ± 404	86.27	0.0056
	10	2.88	69.40 ± 14.16	0.82	40.65 ± 5.54	138.30 ± 3.02	2113	2510 ± 186	89.26	0.0022
	12	2.65	79.95 ± 2.60	0.82	46.85 ± 4.86	121.80 ± 1.93	1831	1429 ± 47	87.81	0.0020

^a standard deviation of 3 independent measurements.

. The corrosion inhibition efficiency (η_E) of the Q235 steel specimens covered with the inhibitors was calculated by Equation (2). With the increase in the concentration of the solutions of the two inhibitors, the corrosion inhibitory efficiency of the two inhibitors improved, indicating the formation of a protective film on the surface of the Q235 steel electrode. Additionally, it was obvious that OTAC (88.97%, 5 mM) presented better corrosion inhibition efficiency than DDAC (83.16%, 5 mM) under the same experimental condition.

3.2. Contact Angle Measurements

A contact angle measurement is applied to study the wetting ability of the surfaces of the specimens. Figure 7 shows the pictures of sessile water drops on the surfaces of polished steel and modified steel. According to the literature, a water contact angle value of <90° is associated with a hydrophilic surface, while a value of >90° is associated with a hydrophobic surface [38].

The polished steel was affirmed to have a hydrophilic surface with a water contact angle of 32.8°, while the values for the steel surfaces covered with inhibitors were 69.1° and 80.2°, respectively. The obvious increase in the contact angle is due to the formation of a protective film by ionic liquids adsorbed onto the sample surface. Additionally, the contact angle value of steel surfaces covered with OTAC was higher than that of DDAC, implying that OTAC adsorbed on Q235 steel surfaces to form a denser protective film.

3.3. SEM Analysis

The surface morphology of the sample was observed by SEM. The images of the Q235 steel specimens before and after immersion in the 0.5 mol/L H₂SO₄ solution for 4 h are shown in Figure 8. The polished specimen presented a tidy surface with neat scratches because the specimen surface was worn by different sandpapers, as shown in Figure 8a. However, the polished specimen immersed in the 0.5 mol/L H₂SO₄ solution for 4 h showed a rough and rugged surface, which was due to the attack of corrosive ions in Figure 8b. The specimen’s surface became smooth and tidy compared with Figure 8b, which indicated that the two ionic liquids adsorbed onto the surface of the Q235 steel specimens to form a protective film in Figure 8c,d. Additionally, the specimen modified with OTAC presented a smoother surface than that of DDAC, implying that OTAC adsorbed on the Q235 steel surface to form a denser protective film. These results are consistent with electrochemical experiments.

3.4. Adsorption Isotherm

In order to further investigate the adsorption information of the two inhibitors onto the metal surface, adsorption isotherms were deduced through surface coverage as a function of the concentrations of the two ionic liquids. A series of adsorption isotherm models were considered to fit the results of Tafel curve experiments. As a result, the Langmuir adsorption isotherm (Figure 9) was found to be the most suitable mode due to the linear regression coefficients (R²) closing to 1 [39]. This adsorption isotherm was calculated by the following formula:

$$\frac{\theta }{1-\theta }=K_{\mathrm{ads}}\times C$$

(5)

where K_ads is the equilibrium constant of the adsorption, θ is the surface coverage, and C represents the concentration of the inhibitor.

The energy of adsorption ($\mathrm{\Delta }{G}_{\mathrm{ads}}^0$) is calculated as follows:

$$\mathrm{\Delta }{G}_{\mathrm{ads}}^0=-RT\ln (1\times 10^6{K}_{\mathrm{ads}}),$$

(6)

where R is the universal gas constant and T is the absolute temperature.

The relevant thermodynamic parameters, such as the equilibrium constant of the adsorption (K_ads) and the energy of the adsorption ($\mathrm{\Delta }{G}_{\mathrm{ads}}^0$) are listed in

Table 5. Thermodynamic parameters for the adsorption of the two inhibitors.

Inhibitor	T (K)	Kads (L·mol)	$\mathbf{\Delta }{\mathit{G}}_{\mathbf{ads}}^{\mathbf{0}}$ (kJ/mol)
OTAC	298	14.84	23.80
DDAC	298	4.64	20.92

. Obviously, the value of K_ads for OTAC was higher than that for DDAC, indicating that OTAC presented better anti-corrosion performance than DDAC. Moreover, the adsorption of the two molecules on the Q235 steel surface was considered a spontaneous process because the values of ∆G⁰ were negative. The calculated values of ∆G⁰ were between −40 kJ/mol and −20 kJ/mol, implying that the adsorption of the studied molecules on the Q235 steel surface was physical and chemical adsorption [40].

4. Conclusions

In summary, these systematic studies provide a direction for the spatial structure of corrosion inhibitors. The findings of electrochemical measurements and surface characterizations are as follows:

(1)

OTAC and DDAC, as environmental-friendly inhibitors, have good corrosion inhibition effects on Q235 steel in a 0.5 mol/L H₂SO₄ solution. The two ionic liquids are considered to be mixed-type inhibitors whose cathodic reactions are significantly suppressed.

(2)

Surface characterization results are consistent with electrochemical measurement results. The two ionic liquids adsorb on a metal surface to form self-assembly monolayers, which can prevent corrosive media from contacting Q235 steel.

(3)

OTAC has better anti-corrosion performance than DDAC because the steric hindrance effect of the double alkyl chain is not conducive to the adsorption of DDAC on the surface of steel electrodes.