Inhibitory influence of cationic Gemini surfactant on the dissolution rate of N80 carbon steel in 15% HCl solution

Strong acids are commonly used in petroleum wells to remove scale layers from the surface of N80 C-steel pipe. The corrosive effects of these acids, on the other hand, pose a significant risk to C-steel pipes. For the first time, we discovered the anti-corrosion properties of cationic Gemini surfactant, 1,2-bis(dodecyldimethylammonio) ethane dibromide (DMAEB), for N80 C-steel pipe in acid washing solution (15.0% HCl). The DMAEB, in particular, can reduce the corrosion rate of N80 C-steel by approximately 97%. DMAEB molecules work as a mixed-type corrosion inhibitor, according to electrochemical results. The DMAEB demonstrated a high inhibition effect at high temperatures, as well as high activation energy against the corrosion process. DMAEB's significant performance is primarily due to physical adsorption on the N80 C-steel surface, as confirmed by adsorption isotherms, SEM, EDX, FT-IR, and theoretical studies. Our findings shed new light on the use of Gemini surfactants as corrosion inhibitors in petroleum wells.

www.nature.com/scientificreports/ Methods. The surfaces of the N80 C-steel specimens were prepared before each experiment according to ASTM G1-03 21,22 . ASTM G31-72(2004) standard method was used to conduct the mass loss experiments and evaluating corrosion rate 23,24 .
The polarization experiments were recorded using a three-electrode cell (working electrode = N80 C-steel, reference electrode = SCE, counter electrode = Pt) and potentiostat instrument (EG/G Model 273A). The polarization experiments were conducted in the potential range ± 250 mV vs. OCP and using a scan rate of 1.0 mV s −1 .
All measurements (potentiodynamic polarisation, gravimetry) were carried out three times under identical conditions. The averages of all data points were recorded.
The critical micelle concentration (CMC) of the DMAEB in the pure water was determined by surface tension measurements using Tensiometer (KRÜSS Scientific).
The surface morphology investigations (SEM and EDX) were conducted using ZEISS/EVO Scanning Electron Microscope fitted with EDX analyzer. FT-IR spectra were recorded via FT-IR spectrophotometer (Shimadzu: IRTracer™-100).
Quantum chemical calculations were studied using the VAMP module in Materials Studio-6.0-software from Accelrys Inc.

Results and discussion
anti-corrosion properties of DMAEB. The mass loss method and electrochemical technique (i.e. polarization test) have been used to investigate the anti-corrosion capabilities of DMAEB for N80 C-steel pipe in the acid washing solution (15% HCl). Table 1 shows the corrosion rate (C R ) from mass loss measurements, as well as the inhibition efficiency (E w %), for N80 C-steel pipe in 15% HCl solution with increasing concentrations of DMAEB at 303 K.
The following relationships were used to calculate the C R and E w % 25, 26 : M is the mass loss in N80 C-steel specimen, t is the time of immersion, A is the surface area, C R0 is the corrosion rate for blank solution and C R is the corrosion rate for inhibited solution.
According to Table 1, the acid solution treatment with DMAEB resulted in a decrease in C R values. The change in DMAEB concentrations had a significant impact on the C R values. This means that increasing DMAEB concentrations causes a decrease in C R values.
The addition of 100 mg/l of DMAEB results in the highest inhibition efficiency (E w % = 96.8) (Table 1). There was no significant change in the E w % value above 100 mg/l.   27 , the CMC value of DMAEB was determined by surface tension measurements, as shown in Fig. 2. According to Fig. 2, the CMC value of DMAEB is 111 mg/l. This indicates that DMAEB's maximum performance was achieved near its CMC value. At the C-steel/solution interface, a complete monolayer of DMAEB was formed at CMC value. There are no spaces available for the adsorption of additional surfactant molecules in this case. This refers to the direct relationship between the surfactant's CMC value and corrosion inhibition.
These findings show that the cationic Gemini surfactant DMAEB has a high efficacy (96.8%) in inhibiting corrosion of N80 C-steel pipe in 15.0% HCl solution at a low concentration (100 mg/l).
The electrochemical parameters of N80 C-steel pipe were evaluated using the polarization method in both 15.0% HCl solution and blank solution treated with Gemini surfactant DMAEB. As shown in Fig. 3, the treatment of 15.0% HCl solution with DMAEB resulted in a reduction of both anodic and cathodic lines. The change in the polarization curves was found to be dependent on DMAEB concentration.
When DMAEB was added to a 15.0% HCl solution, the corrosion potential (E corr. ) shifted to the positive direction (Table 2). At 150 mg/l, the maximum shifting in E corr. was less than 85 mV. DMAEB was identified as the mixed-type corrosion inhibitor based on these findings 28,29 . Furthermore, the addition of DMAEB resulted in a significant decrease in corrosion current density (i corr ) ( Table 2). While i corr decreases, the changes in both Tafel line slopes (b a for anode and b c for cathode) remain nearly constant (Table 2). This discovery supports DMAEB's inhibitory mechanism, which involves blocking the anodic and cathodic sites by surfactant molecules 30,31 .
The following relationship can be used to calculate the inhibition efficiency E p % values based on polarization measurements 32, 33 : i corr(0) is the corrosion current density for blank solution). The increase in DMAEB concentration is directly proportional to E p % values. The highest DMAEB concentration, 100 mg/l, resulted in a 97% reduction in corrosion rate ( Table 2). Figure 4 depicts a histogram comparing the inhibition percentages of the two techniques (polarization and mass loss) for the same conditions. It appears that no significant differences in inhibition percentage values are observed regardless of the techniques used. The inhibition percentages obtained by mass loss measurements were found to be slightly lower than those obtained by polarization measurements. This is because the surface of N80 C-steel is exposed to the acidic solution for a longer period of time when using the mass loss method 34 .
The main mechanism of the Gemini surfactant's anti-corrosion properties is based on its adsorption capability on the surface of the N80 C-steel pipe 35,36 . Because the surface of the steel in acidic solution has a positive charge, the adsorption of Clions from the solution on the steel surface is facilitated 37 . The steel surface was changed to a negatively charged surface in this case. This situation promotes electrostatic interactions between positively charged DMAEB molecules and the steel surface, resulting in the formation of the DMAEB surface layer. This surface layer protects the N80 C-steel pipe's surface from the corrosive acid solution, resulting in a low corrosion rate 38 . The short spacer group connects the two head groups in GS. This facilitates the hydrophobic interaction   www.nature.com/scientificreports/ between the DMAEB's two alkyl tails, lowering the CMC value. Furthermore, the short spacer group promotes the formation of more rigid molecules. This prevents DMAEB molecules from desorbing from the metal surface, resulting in increased inhibition efficiency 39 . Additionally, the long two alkyl chains help to cover the surface of the N80 C-steel pipe 40 . The schematic illustration of the inhibition mechanism is shown in Fig. 5. DMAEB has a higher efficiency in a more corrosive solution when compared to previous works for other corrosion inhibitors of the same family in the literature 41-45 (see Table 3).
Temperature dependence and activation energy. The C R values of the N80 C-steel pipe in 15.0% HCl solution containing 100 mg/l of DMAEB were recorded at temperatures ranging from 303 to 333 K to predict the stability of Gemini surfactant DMAEB at high temperatures ( Table 4). The highest temperature (333 K), as shown in Table 4, reduced the E w % of DMAEB to 92.5%. This finding confirms DMAEB's high stability at high temperatures. Two factors contribute to the slight decrease in E w % of DMAEB at high temperatures. The first factor is an increase in the corrosion rate of steel as temperature rises 46 . The second factor is the high-temperature desorption of some DMAEB molecules from the steel surface (i.e. physical adsorption) 47 .
The relationship between corrosion rate C R and activation energy (E a ) is expressed by the Arrhenius formula (see Eq. 4) 48 .
(4)  The E a was calculated using the Arrhenius plot (straight-line gradient) (see Fig. 6).
The calculated E a for the corrosion reaction in 15.0% HCl solution containing 100 mg/l of DMAEB is 36.74 kJ/mol, which is comparable to the calculated E a in 15.0% HCl solution (13.08 kJ/mol). This finding confirms that the presence of DMAEB in a 15.0% HCl solution raises the energy barrier for corrosion, resulting in a low corrosion rate 49-51 . Adsorption isotherm studies. Because the effectiveness of Gemini surfactant DMAEB is dependent on DMAEB molecules' adsorption capability on the steel surface, it is critical to investigate the adsorption isotherm in this section. The Langmuir adsorption isotherm (Eq. 5) is the best fitting isotherm that describes the adsorption process of DMAEB molecules based on mass loss measurements 52 .
θ is the surface coverage = E w %/100, C inh is the DMAEB concentration, K ads is the equilibrium constant). The linear correlation coefficient (R 2 ) is very close to one in the Langmuir isotherm plot (Fig. 7), confirming the validity of this isotherm 53 . The DMAEB's K ads was determined to be 1.72 × 10 4 M −1 .
Gibbs free energy change (∆G°a ds ) was calculated from the following relation 54 : The ∆G°a ds for the DMAEB was identified to be − 34.62 kJ/mol. A negative ∆G°a ds value confirms the spontaneity of DMAEB adsorption on the steel surface 55 . The value of ∆G°a ds (i.e. − 34.62 kJ/mol) refers to DMAEB physisorption on the surface of the N80 C-steel 56 .
(6) �G • ads = −RTln (55.5K ads ).  In the blank solution, the surface morphology of the N80 C-steel revealed a damaged structure and dense surface roughness (Fig. 8a). The N80 C-steel EDX spectrum in the blank solution (Fig. 8b) revealed characterized signals for N80 C-steel composition and corrosion products (i.e. O, Cl, C, Mn, Si, and Fe).
In the presence of 100 mg/l DMAEB, the N80 C-steel has a smooth surface and is free of corrosion products (Fig. 9a). The presence of characterized DMAEB signals is revealed by the EDX spectrum of N80 C-steel in inhibited solution (Fig. 9b) (i.e. N and C). Furthermore, Cl signals have vanished.
The FT-IR spectra of pure DMAEB and the film formed on the surface of N80 C-steel in the inhibited solution (15.0% HCl + 100 mg/l DMAEB) were analyzed and shown in Fig. 10. The characteristic peaks for C-H stretch, CH 2 bending, CH 3 bending, C-C stretch, C-N, and long-chain CH 2 groups can be seen in the FT-IR spectrum of pure DMAEB.
The FT-IR spectra of the film formed on the surface of N80 C-steel in the inhibited solution showed three changes. The first is a shift in some peaks, such as C-H stretch, CH 2 bending, CH 3 bending, and C-C stretch. The second change is the absence of C-N. The presence of new peaks related to Fe-N bonds at 645-450 cm −1 is the third change. All of these studies support the adsorption of DMAEB molecules on N80 C-steel.   Figure 11a,b show that HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) regions are concentrated on ammonium groups. This demonstrates that the ammonium groups in DMAEB molecules are the active components in the adsorption process 31 . The high value of the HOMO energy (E HOMO = 8.939 eV) refers to the DMAEB molecule's ability to link with the steel surface 57 . Also, the low LUMO energy (E LUMO = 1.577 eV) refers to the DMAEB molecule's ability to gain electrons from the filled Fe d-orbital 58 . Furthermore, the low energy gap (ΔE = E LUMO − E HOMO , 7.362 eV) refers to DMAEB molecules' high inhibition performance 59 . It was discovered that the electron density was distributed throughout the entire DMAEB molecule (see Fig. 11c). This means that DMAEB molecules are adsorbing on the Fe surface in flat-lying orientations 60 . DMAEB's high dipole moment (i.e. μ = 6.94 Debye) indicates a strong electrostatic interaction between DMAEB molecules and the C-steel surface 61 .   where I is the ionization potential = − E HOMO and A is the electron affinity = − E LUMO .
The calculated values of χ and η are 5.258 eV and 3.681 eV, respectively. The high χ value for DMAEB molecules indicates a high ability to attract electrons and, as a result, a high adsorption efficiency 62 . Furthermore, the low η value for DMAEB molecules indicates a strong interaction between the metal surface and inhibitor molecules 63 .
It is worth noting that the Gemini surfactant DMAEB has a high efficacy (96.8%) in inhibiting corrosion of N80 C-steel pipe in 15.0% HCl solution at a low concentration (100 mg/l). DMAEB's anti-corrosion properties were investigated using mass loss, polarization, SEM, EDX, and FT-IR tools. DMAEB acts as a mixed-type