Introduction

The acidification of petroleum wells is a common practice employed in oil extraction processes to avoid the formation of iron scales. Iron deposits tend to obstruct the linings, reducing flowing effectiveness, and most importantly, efficiency is lower when the wells are cloaked by iron, possibly preventing efficient oil yielding. The attack by hydrochloric acid is strong and the higher the concentration and temperature, the higher the corrosion rate1,2,3.

Mild steel, an alloy of iron and carbon, has many applications because of its unique properties. It has a high tensile strength, elasticity, thermal conductivity, electrical conductivity, and ductility. For example, mild steel has been widely used in the construction industry as a reinforcing material in bridges, buildings, and roads. However, mild steel degrades when it interacts with the acid environment, which can limit the steel structures’ useful life4,5.

The inhibition of the corrosion of carbon steel by hydrochloric acid has been the objective of many studies and applied research in the past decades6,7. The presence of organic inhibitors has been shown to have a retarding effect on the rate of steel dissolution and so make many industrial applications safer and more efficient. The protection systems commonly employed include chemical inhibitors and use of protective barriers, whilst modification of the corrosive medium is also considered. Although these inhibitors are highly effective in inhibiting corrosion, there is still significant pollution, chemical reagent misuse, and other disadvantages8.

In recent years, the use of surfactants compounds as corrosion inhibitors for steel in acid media has emerged9,10. These compounds are nontoxic and relatively inexpensive compared to commercially clear key inorganic inhibitors such as chromates and nitrites11,12.

Surfactants are molecules containing both hydrophilic and hydrophobic sites, which are generally insoluble in aggressive mediums and remain in contact with the surface of the metal13. When the surfactant molecules are in the right concentration, they can produce a barrier film between the metal and the aggressive medium and prevent aggression so that balanced protection is created14. The molecule can be physically adsorbed on a metal surface, only when the organic part has adequate chemical reactivity and adsorption energy15. Due to its unique structure, the surfactant also has a unique inhibition mechanism; the surfactant corrosion inhibitor is different from conventional organic inhibitors. Research on surfactants as corrosion inhibitors is relatively recent compared to other fields16. Developing low environmental impact corrosion inhibitors is a crucial aspect of corrosion control in industries where metal degradation poses a significant challenge17.

This paper mainly discusses the progress of using nonionic surfactant (i.e. Polyoxyethylene (7) tribenzyl phenyl ether, abbreviated PETPE) as corrosion inhibitor in petroleum filed. This work presents guidance for using nonionic surfactants as corrosion inhibitors offer a novel approach to mitigating corrosion-related issues.

Nonionic surfactants possess a distinct chemical structure that sets them apart from traditional corrosion inhibitors. Unlike ionic surfactants that carry a net positive or negative charge, nonionic surfactants are electrically neutral18,19,20. This neutral charge allows them to interact with metal surfaces differently and exhibit unique corrosion inhibition properties. The novelty of nonionic surfactant PETPE as corrosion inhibitor lies in its unique chemical structure, versatile interfacial properties, enhanced adsorption and surface coverage, compatibility with various environments, low environmental impact, synergistic effects, and ongoing research in the field21,22. These characteristics distinguish nonionic surfactant PETPE as a promising and innovative approach to corrosion prevention and mitigation.

Experimental part

Materials

EPRI LAB created a nonionic surfactant (polyoxyethylene (7) tribenzyl phenyl ether, abbreviated PETPE) (Fig. 1).

Fig. 1
figure 1

Polyoxyethylene (7) tribenzyl phenyl ether, abbreviated PETPE. Where n  = 7.

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Mild steel contains the following chemical constituents (in weight percentages): 0.17 C, 0.034 Mn, 0.19 Si, 0.029 S, 0.03 P, and remainder iron. The mild steel samples have been cleaned with ethyl alcohol as well as distillation water previous to being investigated, and it was then smoothed using scratch sheets ranging in degree from 400 to 150023.  The test solution is 1.0 M HCl (Sigma Co.).

Methods

Potentiodynamic polarization experiments using potentiostat-3000R-Gamry involve measuring the current response (j) of mild steel electrode (surface area of 0.365 cm2) immersed in 1.0 M HCl solution with and without the presence of PETPE. By varying the applied potential over a range (± 0.50 V vs. OCP), the polarization curves with scan rate of 0.10 mV s−1 can be obtained, which provide information about the corrosion rate (jcorr), corrosion potential (Ecorr), Tafel slopes (βa& βc) and the effectiveness of the PETPE in reducing the current density.

Electrochemical impedance spectroscopy (EIS) was performed using a 5 mV peak-to-peak sinusoidal oscillation within the 100 kHz–0.01 Hz frequency range.

The corrosion test cell contains three electrodes cell (i.e. Pt strip, working electrode, and saturated calomel electrode (SCE) reference electrode).

Sheets of mild steel were cut into parts of 1.0 cm x 1.0 cm x 0.05 cm to assess weight loss (WL) studies. The WL test was performed using the ASTM standard methodology G1-03-2017-e1. Each sample was submerged in the test liquid for 12 h. The average mass loss was calculated after the trials were repeated numerous times.

The corrosion rate (CR) detected from WL testing has been determined using the corresponding equation24:

$$C_{R}=\frac{W (mass\,loss)}{S (surface\,area)\times t(time)}$$
(1)

Theoretical studies

The HyperChem 8.010 application, which was installed on a core i7 laptop, was used to execute the theoretical computations. Complete geometric optimization for PETPE is performed and the charge of each atom is determined. To assess the electronic characteristics of PETPE DFT theory is applied using 6-31G base set. Energy of the highest occupied molecular orbital “EHOMO”, and the energy of the lowest unoccupied molecular orbital “ELUMO”) was obtained. Then, various important quantum descriptors such as: energy gap “ΔEg”, the global hardness “η”, softness “σ”, and the energy of back donation “Eb−d” are evaluated using the following Eqs25,26:

$$\triangle Eg= E_{HOMO}- E_{LUMO}$$
(2)
$$\eta= ( E_{LUMO}- E_{HOMO})/2$$
(3)
$$\sigma\ = 1/\eta$$
(4)
$$E_{b-d}=\frac{\eta}{4}$$
(5)

Results and discussion

Electrochemical measurements

The inhibitory capabilities of the PETPE can be assessed by comparing the polarization curves for mild steel in 1.0 M HCl obtained with and without the nonionic surfactant PETPE, as shown in Fig. 2. The difference in current density between the two samples demonstrates PETPE’s capacity to minimize the corrosion rate jcorr while also protecting mild steel in the HCl conditions. The addition of the nonionic surfactant PETPE are able to change the shape and location of the polarization curve produced from potentiodynamic polarization tests. Analyzing both the cathodic and anodic branches of the polarization curve for mild steel in the absence and in the presence of PETPE provides a comprehensive understanding of the corrosion inhibition mechanism. The presence of PETP changes the anodic polarization (metal dissolution) behavior and corrosion kinetics of steel. The cathodic branch of the polarization curve represents the reduction reaction (hydrogen evolution reaction) occurring at the mild steel surface in the presence of the electrolyte (HCl) and the PETP. In the presence of PETP, the cathodic branch exhibit changes in the reduction kinetics and current density values compared to the uninhibited system.

Table 1 shows the polarization parameters measured with and without the nonionic surfactant PETPE. Once the nonionic surfactant PETPE is added, the corrosion potential (Ecorr) has the potential to shift to a more positive value. This change in behavior suggests that mild steel has a better thermodynamic inclination to keep itself passive or protected27. The jcorr(0) value measures the rate of corrosion in the absence of PETPE. The existence of PETPE is capable of decreasing jcorr, signifying a slower corrosion rate. The reduction is attributable to PETPE’s ability to hinder the electrochemical reactions that cause corrosion. When comparing Tafel slopes (βa& βc) with and without PETPE can assess its inhibitory efficacy. PETPE has been shown to alter the kinetics of both anodic and cathodic Tafel slopes (βa& βc), leading to reduced corrosion rates28.

Fig. 2
figure 2

Polarization curves for mild steel in 1.0 M HCl with and without the nonionic surfactant PETPE at 298 K.

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Table 1 Polarization parameters and mild steel corrosion inhibition efficacy in 1.0 M HCl vary with PETPE content at 298 K.
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According to the corrosion rates jcorr without and with the PETPE identified in Table 1, the inhibition efficiency (Pj%) can be computed applying the equation as follows29,30:

$$P_{j}\%=\frac{j_{corr(0)}-j_{corr}}{j_{corr(0)}}\times 100$$
(6)

It is vital to note that the Pj% of PETPE varies with the concentration of PETPE. The considerable inhibition efficiency result (Pj%=91.3) at a relatively small dose of PETPE (100 ppm) demonstrates that PETPE is effective as a corrosion inhibitor, as it represents a significant reduction in corrosion rate.

A typical electrochemical impedance spectroscopy (EIS) method that offers useful data on the corrosion behavior of mild steel in HCl solution and the efficacy of PETPE is the Nyquist plot (Fig. 3). Figure 4 displays a comparable electrical circuit model that depicts the electrochemical reactions taking place at the interface between the mild steel and the electrolyte. The imaginary part (-Z”) of the impedance values is plotted against the real part (Z’) in a graphical form known as the Nyquist plot31. A semicircular arc or an incomplete semicircle is formed by the data points. The charge transfer resistance (Rct), which is correlated with the corrosion rate, is shown by the semicircle’s diameter in the Nyquist plot32. Double-layer capacitance (Cdl) is represented by a capacitive loop that is frequently seen in the high-frequency area of the Nyquist plot. The variations in the semicircle’s diameter, the capacitive loop’s form, and the frequency dispersion provide understanding on how the PETPE inhibits the corrosion of mild steel. The EIS parameters recorded with and without the nonionic surfactant PETPE are displayed in Table 2. The comparatively low goodness of fit (χ2) demonstrates the precision of the fitting processes. The charge transfer resistance (Rct) of mild steel has been raised by PETPE that adsorbs on the surface with power. This happens when the mild steel is coated with a film of PETPE that prevents the electrochemical processes that cause corrosion. Since there is less ease of electron movement at the mild steel-electrolyte interface, a greater Rct reflects a reduced rate of corrosion. The double-layer capacitance (Cdl) of mild steel can be decreased by a PETPE that creates a protective film on its surface. The adsorption of PETPE and resulting change of the mild-electrolyte interface are responsible for this drop in Cdl, which suggests a more compact or restrictive electrical double layer.

The definition of Cdl can be described as follows33:

$$C_{d1}= \frac{\epsilon\epsilon^{o}}{d}S$$
(7)

where the double layer local and air dielectric constants are denoted by ε and εo respectively. The Cdl’s thickness is represented by the d. S is the mild steel’s area in 1.0 M HCl. As a result, the area of mild steel electrode having direct contact with the HCl solution is significantly reduced when PETPE replaces water molecules, which causes the d value to be trending upward. Thus, it can be demonstrated that the Cdl values decrease more noticeably the more densely PETPE adsorbs on mild steel34.

Fig. 3
figure 3

Nyquist curves for mild steel in 1.0 M HCl with and without the nonionic surfactant PETPE at 298 K.

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Fig. 4
figure 4

Equivalent circuit used to fit the EIS data.

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According to the charge transfer resistance without (Rct0) and with (Rct) the PETPE identified in Table 2, the inhibition efficiency (PR%) can be computed applying the equation as follows33:

$$P_{R}\%=\frac{R_{ct}-R_{cto}}{R_{ct}} \times 100$$
(8)

PETPE is an excellent corrosion inhibitor because it significantly reduces the rate of corrosion, as seen by the notable inhibition efficiency result (PR%=95.4) at a relatively low dose of PETPE (100 ppm).

Table 2 EIS parameters and inhibition efficiency of mild steel in 1.0 M HCl as a function of PETPE concentration at 298 K.
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Weight loss measurements

The WL method has been employed to assess the efficacy of nonionic surfactant PETPE. The approach comprises assessing the change in corrosion rate (CR) of mild steel specimens before and after being exposed to a corrosive environment (1.0 M HCl), with and without the addition of PETPE (see Fig. 5). PETPE are able to minimize the corrosion rate (CR) of mild steel in corrosive environments. The efficiency of PETPE is commonly measured via comparison of corrosion rates with and without PETPE (Fig. 5).

Fig. 5
figure 5

Variation of corrosion rate (CR) and efficiency of PETPE for mild steel in 1.0 M HCl, in the presence of PETPE at different concentrations at 298 K.

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The inhibition efficiency of the PETPE (PW %) can be calculated by using the following formula35 :

$$P_{W}\%=\frac{C_{R0}-C_{R}}{C_{C_{R0}}}\times 100$$
(9)

The data in Fig. 5 revealed that PETPE had a maximum protection percentage of 94.2% at 100 ppm. The polarization test results are consistent with the WL method data, demonstrating PETPE’s effective inhibitory activity on mild steel corrosion in 1.0 M HCl.

Effect of temperature and thermodynamic studies

The effect of temperature on the corrosion rate (CR) of mild steel in corrosive environment (1.0 M HCl) and PETPE’s effective inhibitory activity was shown in Fig. 6.

Fig. 6
figure 6

Variation of corrosion rate (CR) and efficiency of PETPE for mild steel in 1.0 M HCl in the presence of 100 ppm of PETPE at different temperatures.

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Increasing the temperature of the HCl solution normally speeds up the corrosion of mild steel. Higher temperatures increase the acid’s reactivity, resulting in more aggressive corrosive conditions36. In particular, increasing temperature reduced the inhibitory efficacy of PETPE. This implies that the adsorption of PETPE on the surface of mild steel is temperature-dependent. At high temperatures, many of the PETPE adsorbed molecules transform into desorbed molecules. PETPE’s inhibition efficiency remains significant at high temperatures (i.e., 90.4), indicating that PETPE can tolerate the temperature range of the specific application while maintaining inhibitory performance.

Thermodynamic studies aims to help us understand the fundamental mechanisms that control the interaction of PETPE with acid. This understanding is useful in the design and optimization of PETPE for corrosion control in acidic situations. Formulas 10 and 1137 can be utilized in order to determine thermodynamic parameters, including activation energy (Ea), standard entropy (ΔS°), and standard enthalpy (ΔH°).

$$C_{R}=Ae^{\frac{=E_{a}}{RT}}$$
(10)
$$C_{R}=\frac{RT}{Nh}exp\left(\frac{\Delta S^{\circ}}{R}\right)exp\left(\frac{\Delta H^{\circ}}{RT}\right)$$
(11)

R is the gas constant; N is equal to 6.2022 1023 mol−1; h is equal to 6.6261 10−34 m2 kg s−1.

The slope of the Arrhenius plot (Fig. 7a) can be used to calculate the activation energy (Ea). The energy barrier that needs to be overcome through for the process of corrosion to start is measured by the activation energy. An increase in activation energy by adding PETPE (Ea = 13.5 kJ mol−1 in the blank solution, Ea = 26.3 kJ mol−1 at 100 ppm of PETPE) indicates that the PETPE can inhibit the corrosion process. The standard enthalpy change (ΔH°) and entropy change (ΔS°) in the presence of PETPE can be analyzed using the transition state plot (Fig. 7b), which yields significant data. The PETPE can interact with the mild steel surface and increase the enthalpy change ΔH° of the corrosion process (ΔH° = 10.9 kJ mol−1 in the blank solution, ΔH° = 23.7 kJ mol−1 at 100 ppm of PETPE). In the presence of the PETPE, ΔS° moves in direction of less negative value (ΔS° = − 192.9 J mol−1 K−1 in the blank solution, ΔS° = − 174.2 J mol−1 K−1 at 100 ppm of PETPE), which implies that the PETPE increases the disorder or randomness of the system.

Fig. 7
figure 7

Arrhenius plot (a) and transition state plot (b) for mild steel in 1.0 M HCl in the presence of 100 ppm of PETPE.

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Adsorption isotherms describe the extent and character of PETPE adsorption on mild steel surface in the presence of acid. The Langmuir isotherm (Eq. 12) is frequently utilized to describe adsorption behavior. This isotherm helps to determine the inhibitor’s adsorption capacity, equilibrium constant Kads, and surface coverage (\(\theta\))38,39.

$$\frac{C_{inh}}{\theta}=\frac{1}{K_{ads}}+C_{inh}$$
(12)

Figure 8 displays a Langmuir isotherm resulting from weight loss measurements. PETPE adsorption on the mild steel surface follows the conditions of the Langmuir model due to the correlation coefficient (R2 = 0.954) is becoming approximately one. Furthermore, the high Kads of 7524 M−1 of PETPE indicates its strong adsorption capabilities.

Gibb’s free energy (ΔG°) is calculated from the following equation40:

$$\Delta G^{\circ}_{ads}=-RT {\rm ln} (55.5K_{ads})$$
(13)

The ΔG° of the adsorption process is − 32.0 kJ mol−1. A negative ΔG° value implies the potential of PETPE adsorption on the mild steel surface and indicates that the adsorption is effectively favorable. It further indicates that physisorption rather than chemisorption is the predominant mode of adsorption41.

Fig. 8
figure 8

Langmuir isotherm resulting from weight loss measurements for mild steel in 1.0 M HCl in the presence of PETPE.

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Theoretical calculations

An inhibitor’s molecular electronic structure can influence its effectiveness of adsorption. Frontier orbital theory states that the development of a transition state results from a contact between the reactants’ frontier orbitals, with the major sites of reactant reaction being the HOMO and LUMO42,43. In order to better understand the inhibitory mechanism, it was crucial to look into the distribution of HOMO and LUMO (see Fig. 9).

Fig. 9
figure 9

(a) the optimized structure, (b) the charge of each atom, (c) HOMO, and (d) LUMO of PETPE.

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The EHOMO and ELUMO represent the ability of electron to give or accept electron from the surface of the metal, respectively39. A high EHOMO molecule has a propensity to transfer electrons to the vacant orbital of the metal. In addition, several studied declare that raising the values of EHOMO influence the transport process via the adsorbed layer, which in turn promotes adsorption and increases the inhibition efficiency44,45. On the other hand, the difference between the two energy levels EHOMO and ELUMO determine the reactivity of PETPE. Table 3 show lower value of ΔEg which indicate an reactivity and inhibition efficiency.

The global hardness (η) and softness (σ) are parameters derived from conceptual density functional theory (DFT) that are often used in the field of molecular chemistry to understand the reactivity and stability of molecules, including corrosion inhibitors. These parameters provide insights into the electronic structure and chemical reactivity of molecules. A higher σ and a lower η confirm that the molecule is more reactive and may have a stronger tendency to interact with metal surfaces, which could be beneficial for corrosion inhibition46,47.

Besides, the lower value of ΔEg facilitates electron donation process. Furthermore, the lower value of Eb−d confirms the easier back-donation of the electron for the S orbital of the metal to the PETPE48. Figure 9b shows the charge per atoms of PETPE is evaluated to determine the most active centers that attack to the metal surface. All the previous data are matched with literature and confirm the strong inhibition behavior of PETPE49,50.

Table 3 Theoretical parameters of PETPE.
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Conclusions

Based on the study conducted on the use of the nonionic surfactant PETPE as a corrosion inhibitor for mild steel in hydrochloric acid (HCl) solution, the following conclusions can be drawn:

  1. 1.

    PETPE exhibits significant corrosion inhibition properties for mild steel in HCl solution. It effectively reduces the corrosion rate and improves the surface protection of the steel.

  2. 2.

    The corrosion inhibition efficiency of PETPE increases with increasing inhibitor concentration. Higher concentrations of PETPE lead to better corrosion protection. (PR%=95.4) at a relatively low dose of PETPE (100 ppm).

  3. 3.

    The adsorption process of PETPE on the mild steel surface follows Langmuir adsorption isotherm, indicating a monolayer adsorption of the inhibitor molecules.

  4. 4.

    The addition of PETPE results in an increase in activation energy (Ea = 26.3 kJ mol−1 at 100 ppm of PETPE, compared to in the blank solution (Ea = 13.5 kJ mol−1), confirming that PETPE can impede the corrosion process.

  5. 5.

    A negative ΔG° (− 32.0 kJ mol−1) value implies the potential of PETPE adsorption on the mild steel surface and indicates that the adsorption is effectively favorable.

  6. 6.

    The adsorption of PETPE and resulting change of the mild-electrolyte interface are responsible for this drop in Cdl, which suggests a more compact or restrictive electrical double layer.

  7. 7.

    Theoretical calculations demonstrate PETPE’s strong inhibitory behavior.