Experimental and Theoretical Studies of the Corrosion Inhibition Performance of a Quaternary Phosphonium-Based Ionic Liquid for Mild Steel in HCl Medium

Abstract

The corrosion inhibition performance of a quaternary phosphonium-based ionic liquid, i.e., hexadecyltriphenylphosphonium bromide (HPP), on mild steel in 1 M HCl solution was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods. The surface characterization of mild steel was examined by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The results revealed that the inhibition efficiency increases with its increasing concentration, and it can reach up to 99.1% at the concentration of 0.07 mM HPP. PDP data showed that the absorption of HPP conformed to Langmuir adsorption, which served as a mixed-type inhibitor, involving chemisorption and physisorption. SEM analysis confirmed the formation of barrier film on the metal surface, inhibiting the acid attack. Moreover, density functional theory (DFT) calculations and molecular dynamics (MD) simulations were conducted to elucidate the adsorption mechanism of inhibitor molecules on the mild steel surface. A match between the experimental and theoretical findings was evidenced.

1. Introduction

Mild steel is widely applied in the construction, food processing, gas storage tanks, vehicles, bridge fields on account of its excellent mechanical strength, ductility, easy production, and low cost [1,2]. However, the structure and functionality of mild steel will be altered by corrosion. In extreme circumstances, they will cause the material’s surface to crack. This kind of cracking can occasionally result in significant financial losses and casualties [3]. Therefore, it is urgent to prevent mild steel from corrosion. In recent years, various anti-corrosion strategies emerged, mainly including adding corrosion inhibitors, coating isolation, alloying, metal surface modification [4,5]. Compared with other strategies, the application and addition of inhibitor has become the simplest and economical approach [6,7,8]. Inhibitors are chemicals that adhere to the surface of metal to produce a barrier that prevents corrosion on materials [9,10,11]. Without affecting the original performance of metal materials, adding a little amount of corrosion inhibitor can dramatically lower the rate of corrosion of materials in the environmental medium.

Seeking a corrosion inhibitor that can be widely used in industry is urgent, especially in high-risk fields such as the metal pickling and acidization treatment. Recently, ionic liquids, particularly the four varieties depicted in Figure 1, have become increasingly popular due to their distinctive properties, which include low toxicity, high polarity, low cost, non-flammability, low vapor pressure, excellent solubility, and thermal stability [12,13,14]. The researchers discovered that many kinds of ionic liquids can effectively suppress metal corrosion. For example, Ech-chihbi and co-workers investigated the anti-corrosion properties of three imidazolium-based ionic liquids on mild steel in 1 M HCl solution [15]. According to the research, these synthesized ionic liquids exhibited excellent corrosion inhibiting property >90%. According to Liu et al., three imidazole ionic liquids can reduce the corrosion rate of N80 steel at high temperatures and high acid concentrations [16]. They verified that the longer the alkyl chain of the three investigated corrosion inhibitors, the more effective they were at inhibiting corrosion. Furthermore, Zheng and colleagues reported a new benzimidazole derivative, i.e., 1-butyl-3-methyl-1H-benzimidazolium iodide (BMBIMI), as inhibitor for mild steel in 0.5 M H₂SO₄ solution, and they confirmed that their own iodide ion and BMBIMI cations have a synergistic inhibitory effect, which enhances the overall corrosion inhibition performance [17]. On this basis, we introduced a newly generated terminology, viz., homologous self-synergistic inhibition effect, for ionic liquid-based corrosion inhibitors.

Regrettably, neither the inhibition mechanism nor the conformational relationship between inhibition efficacy and molecular structure are fully understood, which still needs further experimental confirmation. Therefore, as a phosphorus-based ionic liquid, hexadecyltriphenylphosphonium bromide (HPP) was selected as a potential corrosion inhibitor for mild steel in HCl solution in this study. Electrochemical experiments were adopted to assess its inhibition performance. The present work offers a workable strategy for developing novel sustainable corrosion inhibitors and aids in understanding the anticorrosive mechanism of related ionic liquid-based inhibitors.

2. Experimental

2.1. Materials and Reagents

Mild steel sheets (purchased from Shanghai Ronghan Industrial Co., Shanghai, China) were used and the chemical composition is as follows: 0.04% S, 0.04% P, 0.13% Mn, 0.18% Si, 0.17% C, and Fe for balance. HPP (purity, 98%) was supplied by Shanghai Aladdin Industrial Corporation (Shanghai, China). HCl (purity, 37%) was purchased from Sinopharm Chemical Reagent. Concentrated HCl and ultrapure water were used to prepare corrosive medium (1 M HCl). HPP with concentrations of 0.01, 0.03, 0.05, 0.07, and 0.09 mM were selected. All chemicals and solvents were used as received without any further purification.

2.2. Electrochemical Measurements

Electrochemical experiments were implemented using a typical three-electrode system by CHI660E electrochemical workstation with its own software, which was purchased from Shanghai Chenhua Instrument Co., Ltd. It consists of a platinum sheet, a saturated calomel electrode (SCE), and mild steel, which were employed as the counter, reference, and working electrodes, respectively. A block of mild steel was made that was 1 cm × 1 cm × 1 cm, then sealed with epoxy resin (exposed area of 1 cm²), which as the working electrode. Before each experiment, the steel sample was polished and with polishing paper of different roughness (120 #, 240 #, 400 #, 600 #, 800 #, 1000 #), cleaned in the distilled water, then degreased with ethanol and dried at room temperature. Firstly, the working electrode’s open circuit potential (E_OC) was tested. Then, testing of the electrochemical impedance curve continued with a consistent open circuit value using the same working electrode. The impedance test applied a frequency range of 100~0.1 Hz with 10 mV amplitude and used ZsimpWin software to analyze the obtained impedance data. Finally, potentiodynamic polarization measurement was conducted scanning within −250 mV to 250 mV potential range at 0.02 mV/s versus E_OC. In order to reduce experimental error, all tests were conducted three times.

2.3. Surface Analysis

The surface morphology and elemental composition of mild steel immersed in inhibited and uninhibited HCl solution for 6 h at 298 K was tested via SEM-EDS (JEOL-JSM-7800F) operated at 20 kV accelerating voltage. SEM examined the morphological changes, EDS, and elemental mappings analysis furnished the composition of the steel surface after dipping into test solutions with and without HPP. To guarantee the accuracy and reliability, all samples were washed with absolute ethanol and ultrapure water, and then dried in a vacuum oven.

2.4. UV-Visible Analysis of the Solution

The UV-vis curves of mild steel after soaking in blank solution and in solution with optimal concentration of HPP inhibitor for 6 h were studied. After that, the resultant solution was examined using a TU-1901 spectrophotometer with a quartz tube. Specifically, this technique was intended to determine whether the inhibitor and Fe forms a complex.

2.5. Computational Methodology

2.5.1. DFT Calculations

To investigate the electronic structures of HPP inhibitor, quantum chemistry calculations were adopted by DFT with the GGA-BLYP exchange correlation function using the Dmol³ module of Material Studio (MS) software (BIOVIA Company, San Diego, CA, USA) [18]. In addition, it is emphasized that a double numeric quality and polarization (DNP) basis set was used with the COSMO implicit solvent model (dielectric constant of water: 78.54), where the reliability of this level of theory in studying organics has been confirmed [19,20]. Vibration analysis was conducted in the test to make the structure reach the minimum point of potential energy surface after optimization. The convergence thresholds for energy, displacement, and force were 1 × 10⁻⁵ Ha, 5 × 10⁻³ Å, 2 × 10⁻³ Ha/Å, respectively.

2.5.2. Molecular Dynamics Simulation

The Forcite module of the MS software was used to explore the adsorption mechanism of HPP on the surface of mild steel [21]. Fe(110) crystal slab was selected as a representative of the steel surface because of its proven most favorable configuration based on the combined factors such as facet area, surface energy, and coordination numbers [22]. The geometry of the cleaved surface was optimized using COMPASSII (condensed phase optimized molecular potentials for atomistic simulation studies II) force field and subsequently enlarged to a 10 × 10 supercell. The nonbond interaction energy of COMPASSII is calculated by the following formula, which is an ab initio force field [23]:

$$E_{\mathrm{nonbond}}={\displaystyle \sum _{i,j}{\epsilon }_{ij}}\left[2{\left(\frac{r_{ij}^0}{r_{ij}}\right)}^9-3{\left(\frac{r_{ij}^0}{r_{ij}}\right)}^6\right]+{\displaystyle \sum _{i,j}\frac{q_i{q}_j}{r_{ij}}}$$

(1)

where ε_ij is the energy parameter between atoms i and j, $r_{ij}^0$ means the dimension parameter, r_ij represents the distance between particles i and j, and q_i and q_j are the charges of i and j atoms, respectively. The Andersen thermostat was used to control the system temperature of 298 K, and a simulation box with periodic boundary conditions with dimensions of 24.8 × 24.8 × 39.1 ų was employed. The step size was set to 1 fs, and the total simulation time was 1000 ps.

3. Results and Discussion

3.1. Open Circuit Potential and PDP Analysis

The variation in the E_OC values of the mild steel electrode with immersion time is analyzed. As depicted in Figure 2a, in comparison to the E_OC of the sample submerged in the corrosion solution without HPP, Figure 2a shows a consistent declining trend when HPP inhibitor is introduced to the corrosion solution. This event demonstrates that the formation of a stable protective film on the metal surface following the addition of HPP inhibitor.

The PDP curve is one of the most frequently used techniques to explore the corrosion kinetics. The potentiodynamic polarization curves of mild steel electrode in 1 M HCl solution containing different concentrations of HPP inhibitor were shown in Figure 2b. As can be observed, the addition of HPP has no discernible impact on the cathode and anode branch shapes of the polarization curve, indicating that the investigated inhibitor does not alter the reaction mechanism of mild steel in 1 M HCl solution [24]. It is generally recognized that the inhibitor molecules mainly inhibit the corrosion process by preventing mild steel from coming into direct contact with the corrosive liquid. Compared with the blank solution, the Tafel curves after adding HPP inhibitor moved to the direction of low current density. The polarization parameters including corrosion current density (i_corr), cathode Tafel slope (β_c), anode Tafel slope (β_a), and corrosion potential (E_corr) were collected via the Tafel extrapolation method [25]. The inhibition efficiency (η_PDP) is measured by the following equation [26]:

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

(2)

where i_corr,₀ and i_corr stand for the corrosion current densities without and with the HPP inhibitor, respectively.

As obtained in

Table 1. Tafel polarization results of mild steel in 1 M HCl solution with different HPP concentrations.

C (mM)	Ecorr (V/SCE)	icorr (μA/cm2)	βc (mV/dec−1)	βa (mV/dec−1)	ηPDP (%)
Blank	−0.426	669.2	−115	82	/
0.01	−0.427	77.39	−105	104	88.4
0.03	−0.464	38.63	−136	134	94.2
0.05	−0.472	21.41	−161	165	96.8
0.07	−0.489	15.79	−176	155	97.6
0.09	−0.477	26.37	−129	177	96.1

, the corrosion potential of the blank condition was −0.426 V, which gradually changed to −0.489 V with the addition of HPP, and the potential change was significantly less than 85 mV. The slopes of anodic and cathodic Tafel lines (β_a and β_c) were lightly varied with the increasing of HPP concentration. This indicates that HPP is a mixed-type inhibitor, suppressing both anodic dissolution of iron and cathodic evaluation of hydrogen gas [27,28]. As can be seen, the inhibition rate at 0.09 mM was lower than 0.07 mM, indicating that 0.07 mM was the optimal inhibition concentration of HPP. The molecular concentration will achieve saturation at concentrations greater than 0.07 mM, generating competitive adsorption that will accelerate the corrosion of mild steel. We can see that when HPP concentration was 0.07 mM, the corrosion current density decreased to 15.79 μA/cm², and the η_PDP value was up to 97.6%.

3.2. EIS Measurement

The impedance technique has been validated as an effective way for evaluating the performance of inhibitors. Herein, in order to investigate the corrosion of mild steel in 1 M HCl in the absence and presence of varying amounts of HPP, EIS measurements were conducted. The Nyquist and Bode plots were obtained and are presented in Figure 3. As depicted in Figure 3a, all the Nyquist curves exhibited similar shape behavior in the inhibited and uninhibited system. Nyquist plots all maintained a consistent shape for all concentrations and show as a single oblate capacitive loop, indicating that the corrosion inhibiting effect in 1 M HCl is controlled by the charge transfer mechanism [29]. The unique depressed capacitive loop results from the influence of frequency dispersive impacts, which can be caused by the electrode surface’s roughness and inhomogeneity [30,31]. Bode diagrams in Figure 3b,c show a one-time constant in all HPP concentrations, indicating that charge transfer is the only relaxation process. Based on the Bode diagram for phase angle vs. logf the phase angle curves were significantly widened and significantly increased with increasing inhibitor concentration at 0.01−0.07 mM.

EIS data was further analyzed by fitting the equivalent circuit model shown in Figure 4. In the equivalent circuit diagram, R_s, R_f, R_ct, CPE_f, and CPE_dl represent solution resistance, corrosion product resistance, charge transfer resistance, constant phase angle element, and constant phase angle element of double-layer capacitance, respectively. The impedance function of CPE is described by the following equation [32,33]:

$$Z_{\mathrm{CPE}}=Y_0^{-1}{\left(j\omega \right)}^{-n}$$

(3)

where Y₀ denotes the proportional factor, ω means the angular frequency, j stands for the imaginary unit, n represents a measure of non-uniform current distribution due to surface heterogeneity and its value lies between 0 and 1. When n = 0, CPE behaves as resistor; an ideal capacitor when n = 1 [34]. The inhibition efficiency (η_EIS) and double-layer capacitance (C_dl) were obtained from the following formula [35,36]:

$${\eta }_{\mathrm{EIS}}\%=\frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}^0}\times 100$$

(4)

$$C_{\mathrm{dl}}=Y_0{\left(R_{\mathrm{p}}^{1-\mathrm{n}}\right)}^{\frac{1}{\mathrm{n}}}$$

(5)

where $R_{\mathrm{ct}}^0$ and R_ct are the charge transfer resistance without HPP inhibitor and with HPP inhibitor, respectively. f_max refers to the maximum frequency at which the imaginary part of the impedance has a maximum value. The fitting degree between experimental data and the simulated results obtained by the proposed equivalent circuit was evaluated by the chi-square (χ²) parameter, which is defined as [37]:

$${\chi }^2={\displaystyle \sum _{i=1}^n\left[\frac{{\left(Z_i^{\prime }\left({\omega }_i,\stackrel{\rightharpoonup }{p}\right)-a_i\right)}^2}{a_i^2+b_i^2}+\frac{{\left(Z_i^{″}\left({\omega }_i,\stackrel{\rightharpoonup }{p}\right)-b_i\right)}^2}{a_i^2+b_i^2}\right]}$$

(6)

wherein, ω_i, a_i, b_i are experimental data points, $\stackrel{\rightharpoonup }{p}$ is a factor connected with the proposed model, $Z_i^{\prime }$ and $Z_i^{″}$ are expected data points. The results incorporated in

Table 2. Corrosion data of mild steel in HCl solution with and without HPP inhibitor.

C (mM)	Rs (Ω cm2)	CPEf		Cf (μF cm−2)	Rf (Ω cm2)	CPEdl		Cdl (μF cm−2)	Rct (Ω cm2)	χ2 (10−3)	θ	ηEIS (%)
		Y0 (10−5 S Sn cm−2)	n1			Y0 (10−5 S Sn cm−2)	n2
Blank	1.003	5.177	1.000	51.7	3.92	415.60	0.686	1241.0	17.1	8.22	/	/
0.01	0.832	4.336	1.000	43.3	13.90	24.74	0.644	45.0	185.2	2.17	0.907	90.7
0.03	1.171	42.20	1.000	422.0	12.07	6.39	0.689	14.8	612.6	2.88	0.972	97.2
0.05	1.106	64.20	0.999	639.2	19.50	4.82	0.664	12.4	1412.0	5.43	0.988	98.8
0.07	0.877	50.59	1.000	505.9	25.56	5.44	0.633	15.0	1986.0	8.10	0.991	99.1
0.09	0.919	78.92	1.000	789.2	17.24	8.19	0.582	16.6	1351.0	9.61	0.987	98.7

show that the values χ² are of the order of 10⁻³, indicating the accuracy of the equivalent circuit model.

As shown in Table 2, the R_ct is 17.1 Ω cm² for the blank condition. However, after the addition of HPP inhibitor, the values of R_ct increased gradually. When the inhibitor concentration reached 0.07 mM, the value of R_ct was changed to 1986.0 Ω cm², and the corresponding inhibition efficiency was as high as 99.1%. However, we also recognize that the value of C_dl decreased compared with blank solution. This phenomenon can be explained by the Helmholtz model [38,39]:

$$C_{\mathrm{dl}}=\frac{\epsilon \times {\epsilon }^0}{d}S$$

(7)

where ε and ε⁰ are the local dielectric constant and the permittivity of vacuum, respectively; S refers to the surface area of working electrode; and d represents the thickness of the protective layer. The inhibitive adsorption was occurred at the steel/solution interface, which hindered the direct contact between metal and corrosive solution, thus reducing the exposed surface area of electrode and/or increasing the thickness of double electrode layer, and finally reducing the value of C_dl [40].

3.3. Adsorption Isotherm

The crucial adsorption information of HPP inhibitor on mild steel surface was discussed by adsorption isotherms. The isotherm was determined using the correlation coefficient (R²) that best matches the acquired investigation data. Different adsorption models, including the Freundlish, Tempkin, and Langmuir isotherms were assessed, and the best fit was found to be the Langmuir isotherm [41]:

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

(8)

where C represents the concentration of HPP, K_ads delegates the equilibrium adsorption constant, θ means the degree of surface coverage, and the θ values were obtained from Table 2.

As shown in Figure 5, a straight line of C versus C/θ is found with the R² value close to 1, indicating that the fitting result is reliable. Moreover, the Gibbs free energy ($\Delta G_{\mathrm{ads}}^0$) can be described by the following equation [42]:

$$\Delta G_{\mathrm{ads}}^0=-RT\ln (55.5K_{\mathrm{ads}})$$

(9)

where 55.5 is the concentration of water molecules in the solution, T is the temperature, and R stands for the universal gas constant. The calculated value of $\Delta G_{\mathrm{ads}}^0$ is −25.692 kJ/mol, which is between −40 and −20 kJ/mol, indicating that the adsorption of inhibitors at the steel/solution interface is a spontaneous process, which belongs to the mixed chemisorption and physisorption process [43].

3.4. SEM-EDS Analysis

We observed the surface morphology of mild steel by SEM after immersing it in the test solution for 6 h. As given in Figure 6a, the polished steel surface was smooth and clean. The surface of mild steel in blank condition was very rough and severely corroded, forming corrosion products such as chloride and oxide, which accumulate on the metal substrate (Figure 6b). As a result of adding HPP inhibitor to the corrosive solution, the surface of mild steel was smoother than it had been before (Figure 6c). This phenomenon was further verified by EDS mapping and point analysis (Figure 6d–i). Br and P elements were observed in the inhibited solution, while the composition of O element was significantly lower than in those without corrosion inhibitor. According to this observation, we can confirm the HPP molecules have been adsorbed onto the surface of the steel.

3.5. UV-Visible Spectroscopy Study

As shown in Figure 7a, the peaks of 204 nm and 250 nm were observed when the mild steel was soaked in blank HCl solution for 6 h, which might be caused by the reaction of Fe with HCl while producing anionic chloride [44]. Additionally, the inhibitor-containing absorption band exhibits the peaks of 231 nm and 274 nm, demonstrating the π-π* electron transition with charge transfer characteristics has taken place (Figure 7b). The change of absorption band positions indicated that HPP and Fe²⁺ may form a complex in acidic solution [45].

3.6. Computational Perspectives

To evaluate the relationship between HPP molecular structure and inhibitory performance, a quantum chemical computation was performed. Figure 8 depicts the optimized molecular structure of HPP, its highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) densities, as well as the molecular electrostatic potential (ESP) map. A closer look at the distributions shows that most of the HOMOs and LUMOs are located around the triphenyl group. HOMO and LUMO densities relate to portions of a molecule that might possibly contribute to electron-donating and electron-accepting abilities, respectively [46]. The triphenyl moiety is richer in electrons thanks to its phenyl rings. When these rings interact with steel surfaces, their reactive sites act as adsorption centers.

In recent years, MD simulations have become a standard tool alongside experiments to understand the anti-corrosion mechanism at an atomic scale, which can bring valuable information to interpret experimental data. Therefore, it is sometimes referred to as “computational microscopy” because of the ability to see like through a microscope the interactions between molecules and metal substrate. Herein, we performed MD simulations with varying numbers of HPP molecules on Fe(110) substrate in aqueous environment. As can be seen from Figure 9, the polar triphenyl groups and bromide ions are tightly attached to the metal surface, while the hydrophobic alkyl chains are tipsily placed in aqueous solutions. Adsorption energy (E_ads) was determined using the following equation in order to measure the strength of the inhibitor adsorbed onto the metal surface [47]:

$$E_{\mathrm{ads}}=E_{\mathrm{total}}-(E_{\mathrm{surf}+\mathrm{solu}}+E_{\mathrm{inh}+\mathrm{solu}})+E_{\mathrm{solu}}$$

(10)

wherein, E_total means the total potential energy of the system, which include metal crystal, the adsorbed inhibitor molecule as well as medium solution; E_surf+solu represents the total energy of metal surface and solution without the inhibitor; E_inh+solu stands for the total energy of the inhibitor and solution; and E_solu represents the potential energy of the solvent molecules. It is commonly accepted that a larger adsorption strength between an inhibitor molecule and metal surface is indicated by a more negative value of E_ads [48]. When the number of HPP molecules is 1, 2, 3, and 4, the corresponding adsorption energies are −345.2, −655.5, −994.1, and −1347.3 kcal/mol, respectively. These negative values indicate that the adsorptive system is stable and spontaneous adsorption can occur, which confirms the experimental finding that the corrosion inhibition efficiency increases with increasing inhibitor concentration within a certain range.

3.7. Analysis of Anticorrosive Mechanism

A clear explanation of HPP inhibitor’s anti-corrosion mechanism in 1 M HCl solution is provided in this section. As shown in Figure 10, below is a description of the anodic dissolution process of steel [49]:

$$\mathrm{Fe}+{\mathrm{Cl}}^{-}\to {\left(\mathrm{FeCl}\right)}_{\mathrm{ads}}+e^{-}$$

(11)

$${\left(\mathrm{FeCl}\right)}_{\mathrm{ads}}\to \left(\mathrm{Fe}{\mathrm{Cl}}^{+}\right)+e^{-}$$

(12)

$$\mathrm{Fe}{\mathrm{Cl}}^{+}\to {\mathrm{Fe}}^2{}^{+}+{\mathrm{Cl}}^{-}$$

(13)

The electrons flowed to the cathode where hydrogen evolution reaction occurred [50]:

$$2{\mathrm{H}}^{+}+2e^{-}\to {\mathrm{H}}_2\uparrow$$

(14)

Earlier in this paper, we speculated that adding HPP to HCl solution would decrease corrosion rates, since HPP formed a barrier film on the steel surface through the adsorption of functional groups. Langmuir adsorption and MD simulations results showed that the hydrophilic part was spontaneously moved to the steel surface during the adsorption process. However, most of the hydrophobic tail was placed in the solution, resulting in the physisorption of the molecules. On the other hand, as a surfactant, HPP possesses an excess of bromine ions, which can accumulate on the steel surface to influence the inhibition process, resulting in a negatively charged metal surface. Afterwards, the positively charged HPP cation was electrostatically attracted to the negatively charged steel surface. Moreover, iron atoms on the steel surface would generate 3d empty orbitals, while electrons on HPP could be transferred to Fe orbitals for chemisorption. Therefore, physisorption and chemisorption occurred simultaneously in the adsorption process due to the self-synergistic inhibition effect, and a self-assembled barrier film was formed to mitigate the corrosion of mild steel.

4. Conclusions

Overall, HPP exhibited an excellent corrosion inhibition performance on mild steel in 1 M HCl solution due to the adsorption effect. The R_ct value and inhibition efficiency increased with the increase of corrosion inhibitor concentration at 298 K, and the inhibition efficiency was up to 99.1% in 0.07 mM HPP. PDP analysis showed that with the increase in HPP, the initial potential became more negative, β_a and β_c were shifted, which proved that the reaction was a complex of physical (electrostatic interaction) and chemical adsorption. SEM-EDX and UV-visible spectroscopy analysis revealed the formation of self-assembly HPP layer on the metal surface. Furthermore, we deduced that the electron donor and acceptor characteristics play a key role in the inhibition process, and the corrosion inhibition performance is further enhanced by the self-synergistic inhibition effect of the ionic liquid itself.