AEO7 Surfactant as an Eco-Friendly Corrosion Inhibitor for Carbon Steel in HCl solution

The impact of AEO7 surfactant on the corrosion inhibition of carbon steel (C-steel) in 0.5 M HCl solution at temperatures between 20 °C and 50 °C was elucidated using weight loss and different electrochemical techniques. The kinetics and thermodynamic parameters of the corrosion and inhibition processes were reported. The corrosion inhibition efficiency (IE%) improved as the concentration of AEO7 increased. In addition, a synergistic effect was observed when a concentration of 1 × 10−3 mol L−1 or higher of potassium iodide (KI) was added to 40 µmol L−1 of the AEO7 inhibitor where the corrosion IE% increased from 87.4% to 99.2%. Also, it was found that the adsorption of AEO7 surfactant on C-steel surface followed the Freundlich isotherm. Furthermore, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements indicated that AEO7 was physically adsorbed on the steel surface. The surface topography was examined using an optical profilometer, an atomic force microscope (AFM), and a scanning electron-microscope (SEM) coupled with an energy dispersion X-ray (EDX) unit. Quantum chemical calculations based on the density functional theory were performed to understand the relationship between the corrosion IE% and the molecular structure of the AEO7 molecule.

explained on the basis of adsorption of ethoxylated fatty alcohols molecules on the metal surface through their ethoxy groups 25 . Additionally, a novel eco-friendly Gemini surfactant is synthesized by Mobin et al., namely, Ethan-1,2-diyl bis(N,N-dimethyl-N-hexadecyl ammonium acetoxy) dichloride (16-E2-16). This mixed-type corrosion inhibitor considerably increases the corrosion inhibition efficiency (IE%) of mild steel to 98% in 1 M HCl at 60 °C 26 . Furthermore, three novel nonionic dithiol surfactants are synthesized based on 2-mercaptoacetic acid and polyethylene glycol with different molecular weights to obtain the di-mercaptoethanoate polyethylene glycol: SH600, SH1000 and SH1500. It is proved that the corrosion IE% increases with the molecular weight 27 . The combination of cetyl trimethyl ammonium bromide (CTAB) and indigo carmine significantly enhances the corrosion IE% of indigo carmine/CTAB for C-steel to 98.5% at 5 × 10 −5 M in 0.5 M HCl 28 . In addition, a new green inhibitor extracted from Morus alba pendula leaves (MAPLE) is used. The corrosion IE% increases from 93% at a concentration of 0.4 g L −1 to 96% after adding1 mM of KI 29 . Moreover, ionic liquids e.g. tetra-n-butyl ammonium methioninate is used as a corrosion inhibitor owing to their electrostatic interaction with the iron surface (following Freundlich isotherm) with a maximum efficiency of 93.8% at 1.59 × 10 −3 M of the inhibitor in 1 M HCl 30 . The aim of this work is to investigate AEO7 nonionic surfactant as an eco-friendly corrosion inhibitor for C-steel at ambient and elevated temperatures in 0.5 M HCl, which to the best of our knowledge, has not been previously reported literature. In addition, the adsorption isotherm and the thermodynamic parameters for the adsorption of the AEO7 inhibitor molecules on the C-steel surface are calculated. Furthermore, the synergistic effect of various concentrations of KI on the corrosion IE% of AEO7 for C-steel are studied. Moreover, quantum chemical parameters based on the density functional theory method are investigated. For example, the energies of the lowest unoccupied molecular orbitals (E LUMO ) and the highest occupied molecular orbitals (E HOMO ) as well as the energy gap (ΔE) are calculated. Moreover, the electron affinity (A), the ionization potential (I), the electronegativity (X), the global softness (δ), the dipole moment (μ) and the global hardness (γ) are figured out to provide a theoretical explanation for the corrosion inhibitive behavior of AEO7 and to understand the relation between the corrosion IE% and its molecular structure.
Weight loss measurements. The C-steel sheets with dimensions 2.5 × 2.0 × 0.2 cm 3 were used for weight loss measurements. The weight loss measurements were performed in a 200 mL glass beaker containing 100 mL of 0.5 M HCl with and without adding different concentrations of AEO7 at room temperature. In order to investigate the effect of time on the rate of corrosion in the presence of the inhibitor, the performance was tested in a range from 30 min to 3 h. Afterwards, the specimens were removed and treated according to the method described in the ASTM G1-90 standard 31 . The corrosion rate (mpy) was computed based on Equation (1).
Corrosion rate (mpy) 534 (1) where W is mass loss in mg, ρ is the C-steel density in g cm −3 , A is the surface area of the sample in cm 2 and t is the time of the test in h. The corrosion IE% and the surface coverage θ of the inhibitor used for the corrosion of C-steel were calculated as follows 15,32 , where W 0 and W are the average weight loss values without and with adding the inhibitor, respectively.
Electrochemical measurements. Electrochemical measurements were performed in a three-electrode double-jacketed cell. The C-steel working electrodes were prepared according to the aforementioned procedures described in Section 2.1. It had an exposed area of 0.5 cm 2 . A graphite sheet with the same exposed area was used as a counter electrode and a saturated calomel electrode (SCE) acted as a reference electrode. The reference electrode was coupled with a Luggin capillary to minimize the IR potential drop. A Julabo F12 thermostat (GmbH, Seelach, Germany) was utilized to control the temperature of the solution as all electrochemical tests Surface morphology. The surface of C-steel was examined using different characterization techniques. A Leica optical profilometer was utilized to explore the surface topography and surface roughness at a microscale. At the same time, an Asylum Research MFP-3D, atomic force microscope, AFM (Santa Barbara, CA, USA) was used to measure the surface roughness and surface topography in nanoscale and compared to the results that were obtained from the optical profilometer. The surface condition was checked using an FEI NOVA NANOSEM 450 high field emission scanning electron microscope, HFESEM, (Hillsboro, OR, USA) to document the corrosion of C-steel.

Computational.
A quantum chemical study was carried out using the density functional theory (DFT) method as implemented in the GAUSSIAN09 suit of programs 33 . Geometric optimization was performed using the B3LYP functional combined with the cc-pvdz basis set. The B3LYP functional (with Becke's three-parameter functional (B3) and a mixture of HF with DFT exchange terms linked to the gradient-corrected correlation functional of Lee, Yang, and Parr (LYP) 34,35 ) is known to produce a good estimate of molecular properties related to reactivity. The consistent-correlated polarized valence double zeta basis set (cc-pvdz) basis set 36,37 became a state of the art for correlated calculations. The DFT, among other functions, seems promising for revealing changes in the electronic structure responsible for the inhibitory action of compounds on metal surfaces. The geometry of AEO7 was fully optimized using a gradient minimization technique. The optimum structure was characterized as having a zero-gradient norm. By diagonalizing the matrix of the second derivatives, positive harmonic vibrational frequencies were observed. The Gaussian 09 software was utilized to calculate the quantum chemical parameters necessary to explain the atomic and molecular interactions involving the compounds. GaussView 5.0 software was further applied in visualizing the electron density graphical isosurfaces and the quantum chemical parameters, which were calculated in the domain of Koopmans' theory 38 such as the energy of the highest occupied molecular orbital (E HOMO ) and the energy of the lowest unoccupied molecular orbital (E LUMO ). Moreover, the ionization potential (I) and the electron affinity (A) associated with the energies of HOMO and LUMO, respectively, where I = −E HOMO , A = −E LUMO and the energy gap Δ = − E E E ( ) HOMO LUMO , as well as the chemical potential (μ), the absolute hardness (χ), and the absolute softness were calculated.
The global hardness (γ) reflects the resistance to a charge transfer while the global softness (δ) describes the ability to receive electrons and were estimated by: Homo Lumo HOMO LUMO Electronegativity (X) is the power of an atom to attract electrons towards itself and was calculated using Koopmans' theory Homo Lumo

Results and Discussion
Weight loss measurements. Effect of inhibitor concentration on corrosion of C-steel. Figure 2 shows the weight loss transients for C-steel upon immersion in 0.5 M HCl in the presence and absence of 20, 30, 35, and 40 μmol L −1 of AEO7 at 25 °C. It reveals that the higher the concentration of AEO7 is, the lower the weight loss we measure i.e. the higher the corrosion IE% we calculate. The result can be attributed to the ability of AEO7 to be adsorbed on the surface of C-steel because of the existence of seven oxygen atoms arranged in a kind of chain sequence in AEO7 which helps the 3d orbitals of an iron atom to interact with the lone pairs of electrons from the oxygen atoms [39][40][41] . This, consequently, increases the corrosion IE%. In addition, the high molecular weight of AEO7, because of the presence of a long chain of hydrophobic ethylene groups, increases the corrosion IE%. Therefore, in the presence of AEO7, the corrosion rate is an indicator for the number of free corrosion sites remaining after effectively blocking the other sites by the adsorbed inhibitor molecule 25,42 .
Effect of immersion time on corrosion of C-steel. Assuming that corrosion takes place only at the free sites while neglecting the corrosion rates of the covered sites, the corrosion IE% can be calculated by Equation 2. Table 1 shows the results for corrosion IE% of different concentrations of AEO7 on C-steel after various immersion times at 25 °C. The increase in the corrosion rate with time is attributed to the entanglement of the polymer chains with each other with time which lowers the overall entropy surface energy meanwhile exposes more active sites for corrosion 43 EIS measurements. Figure 3 shows the EIS Nyquist plots for C-steel in 0.5 M HCl solution in the absence and presence of 20, 30, 35 and 40 µmol L −1 of the AEO7 corrosion inhibitor. Clearly, the diameters of the semicircles in the presence of AEO7 inhibitor are larger compared to the blank one. It is also lucid that the diameters of the semicircles decrease as the temperature increases at the same concentration of the AEO7 inhibitor. In addition, the depressed capacitive loops at low frequency in all the Nyquist plots refer to a charge transfer-controlled mechanism for the corrosion of C-steel in the 0.5 M HCl solution. The deviation of the capacitive loop from a complete semi-circle can be referred to the heterogeneity and microroughness of the working electrode surface 45,46 . Figure S1 (Supporting Information) depicts the Bode plots and phase angle curves for the same EIS data represented by the Nyquist plots shown in Fig. 3. In contrast to the effect of increasing the temperature, raising the AEO7 concentration increases the phase angles and shifts their peaks to lower frequencies [47][48][49][50] . It can be seen that the phase angles for the inhibited cases are much higher than the uninhibited ones. The increased values of the phase angle for the inhibited specimens indicated that the surface becomes smoother owing to the inhibitor adherence to the metallic surface and the formation of a protective film of inhibitors over the metallic surface. The broadening and the shift to lower frequencies of the single maximum in the Bode plots further supports the protective film formation by inhibitor molecules 51,52 . Figure 4 shows a one-time constant equivalent electrical circuit (EC) that is used for fitting the measured EIS data of a uniformly corroding electrodes in an electrolyte 53,54 . The EC is composed of an uncompensated solution resistance (R s ), a charge transfer resistance (R ct ) and a constant phase element (CPE) that replaces the capacitive element to obtain a more accurate fit. CPE is used for a non-ideal double-layer capacitor. The non-ideal behavior of a double layer can be attributed to many reasons; e.g, surface roughness and a non-uniform (i) surface coverage, (ii) corrosion rate and/or (iii) current distribution.
The impedance of the CPE is expressed by Equation 6 55 : , ω is the angular frequency in rad s −1 and the value of n ranges between 0 and 1. When n = 1, the CPE becomes equivalent to an ideal capacitor, and when n = 0, the CPE becomes equivalent to a resistor. The double layer capacitance (C dl ) can be calculated using the following equation 56 : ct n The surface coverage (θ) is estimated using the following equation:  where R ct1 and R ct2 are the charge-transfer resistances in the presence and absence of the inhibitor, respectively. Moreover, the corrosion IE% is calculated using Equation 2 55,56 . All EIS parameters obtained from the Nyquist and Bode graphs are summarized in Table 2.
It can be noticed that the higher the concentration of the AEO7 inhibitor is, the larger the charge transfer resistance we measure which is typically opposite to the behavior of the C dl which decreases with increasing the AEO7 concentration or decreasing the temperature 57,58 . For instance, comparing the blank solution with the one containing 20 μmol L −1 of AEO7 at 20 °C, the R ct increases from 43 Ω cm 2 to 128 Ω cm 2 , and C dl decreases from 119.75 µF to 82.57 µF (IE% is 66.4%). Meanwhile, increasing the inhibitor concentration to 40 μmol L −1 increases the R ct value to 341.6 Ω cm 2 , and the C dl decreases to 59.51 µF with an IE% of 87.6%. This observation can be explained using the Helmholtz equation 55 .
ads o dl where δ ads is the thickness of the adsorbed layer of the AEO7 corrosion inhibitor, ε o is the vacuum permittivity, ε is the local relative dielectric constant and A is the area of C-steel electrode. This equation shows that C dl is inversely proportional to δ ads i.e. the decrease of the C dl value is attributed to the growth of the adsorbed film of AEO7 corrosion inhibitor as its concentration increases in solution. As the protective film grows, the charge transfer becomes more sluggish as indicated by the high R ct and corrosion IE% values. Increasing the temperature  will decrease the corrosion IE% because the desorption rate of the inhibitor molecules from the C-steel surface increases which leads to an increase in the dissolution rate of the steel 59 . In general, the higher the inhibitor concentration is, the less positive the n values are. This indicates that the behavior of the constant phase element is becoming farther from the ideal capacitor as the AEO7 concentration increases 60,61 . The electrochemical kinetic parameters such as the corrosion free potential (E corr ), corrosion current density (i corr ), the polarization resistance (R p ) and the cathodic and anodic Tafel slopes (ß c and ß a , respectively) which are obtained by the Tafel extrapolation method are listed in Table 3. The corrosion rates are calculated using Equation 10 assuming that the whole surface of the C-steel is attacked by the aggressive media, and no localized corrosion is detected 62 3.15 * 10 7 is the number of seconds in one year and 10 is a factor to change cm to mm. M is the atomic weight of iron (g mol −1 ) while n is the number of transferred electrons per metal atom, F is Faraday's constant (96497 C mol −1 ), A is the exposed area of the electrode, d is the density of iron, and i corr is the corrosion current density in A cm −2 .

Potentiodynamic polarization measurements.
The polarization resistance (R p ), can be calculated using the Stern-Geary equation 55 . The values of surface coverage (θ) are calculated using the following equation: corr corr corr where ′ i corr and i corr are the corrosion current densities in the absence and presence of the AEO7 corrosion inhibitor. In addition, the corrosion IE% is calculated using Equation 2 55 .
It is worth noting that the presence of AEO7 corrosion inhibitor decreases the anodic and cathodic current densities and the corrosion potentials are shifted towards the nobler direction compared to the blank. This is a typical behavior for the mixed-type corrosion inhibitors 64 . It is reported that an inhibitor to be considered as cathodic or anodic one requires a shift of ±85 mV in the E corr between the uninhibited and inhibited systems, otherwise it is classified as a mixed type one 44,65 . The E corr values, in Table 3, confirm that AEO7 is of the mixed type category. The higher the concentration of the AEO7 is, the lower the corrosion rates are at any temperature. This can be explained by the increase in the local electron density at the steel surface by which the affinity of C-steel towards the adsorption of negatively charged Cl − ions decreases and consequently the corrosion rate is lowered. On the other hand, raising the temperature increases the corrosion rate at the same concentration of the AEO7 due to the increase in the desorption rate of the corrosion inhibitor from the C-steel surface 66 . It is worth mentioning that the polarization data shown in Fig. 5 confirm the EIS ones in Figs 3 and S1.   To determine the synergistic effect of KI, the synergism parameter S 1 is calculated using Aramaki and Hackerman's Equation 70 : where θ 1+2 = (θ 1 + θ 2 ) − (θ 1 × θ 2 ), θ′ 1+2 represents the measured surface coverage for a certain concentration of KI in combination with a 40 μmol L −1 of AEO7, θ 1 is the surface coverage of KI at a certain concentration and θ 2 is the surface coverage of 40 μmol L −1 AEO7. The synergistic effect of KI with other inhibitors is common. However, in this research the interaction between KI and AEO7 is changing according to the concentration of iodide ions as seen in Table 5. Three conditions are reported with altering the KI concentration leading to different S 1 values. If S 1 values are more than unity, then the synergistic effect of KI is proved. However, if S 1 values approach unity, it means that there is no interaction between potassium iodide and the AEO7 molecule. If S 1 values are less than unity, an antagonistic behavior is expected which is a characteristics of a competitive adsorption process 69,71,72 . The synergism is originated from the double role that iodide ions play. The protective iodide ions compete with the chloride corrosive ones for the adsorption on the surface of the metal and at the same time they increase the adsorbability of AEO7 molecules on the C-steel surfaces 43,44 . Table 5 shows the values of S 1 that are calculated after the addition of different concentrations of KI to a 0.5 M HCl solution with 40 μmol L −1 of AEO7. It is evident   that the values are greater than unity for all KI concentrations that are higher than 1 × 10 −3 M which proves the synergism between KI and AEO7.
Inhibitor Adsorption and thermodynamic analysis. The equilibrium constants for the adsorption of AEO7 on the C-steel surface in a 0.5 M HCl solution at different temperatures can be derived from the proper adsorption isotherm. Freundlich adsorption isotherm shows the best fitting for the surface coverage of AEO7 on C-steel with its concentration in solution 73 . This isotherm can be summarized by Equations 14 and 15, as given below: n ads θ = + . K n C log log 2 303 log (15) ads where C is the concentration of the corrosion inhibitor, K ads is the equilibrium constant for the adsorption process and θ is the surface coverage (obtained from the potentiodynamic polarization analysis shown in Table 3) while n is a function in the strength of adsorption. Equation 15 is used to create Fig. 7 at various temperatures. R 2 values are close to unity confirming the properness of using the Freundlich isotherm 30,74 . From the intercepts of the graphs, K ads values are computed. K ads constants are also used to calculate the values of the standard Gibbs free energy change of adsorption ΔG°a ds according to the following Equation 75,76 ads ads o where R is the universal gas constant in J mol −1 K −1 , T is the temperature in K and 55.5 is the number of moles of water per liter, according to Bouklah et al. 77 . Calculated values of R 2 , K ads and ΔG°a ds are listed in Table 6. In addition, the enthalpy change of adsorption, ln K ads , was plotted versus 1/T, which resulted in a straight line, as shown in Fig. 8, following the Van't Hoff equation.
There are different types of adsorption that represent the interaction between the metal surface and the corrosion inhibitor molecule e.g. physical, chemical or chemi/physisorption. From the results in Table 6  K ads values are often used to describe the strength of the bond between adsorbent and adsorbate 63 . It can be seen in Table 6 that increasing the temperature decreases the K ads as the bonds between C-steel and AEO7 molecules are weakened.
ΔG ads o is connected to ΔH ads o (standard enthalpy) and ΔS ads o (standard entropy) of adsorption according to Equation 17: ads ads a ds Δ°H ads is obtained from the slope of the Van't Hoff plot and Δ  S ads is obtained from Equation 17 and listed in Table 6.
The Δ°H ads values can provide a vital information pertaining to the adsorption mechanism of an inhibitor. In the present study, Δ°H ads value is negative implying exothermic adsorption. Similar results are reported by Esmaelili et al. 78 . The negative value of Δ°H ads value is confirmed by the decrease in the corrosion IE% with increasing the temperature. On the other hand, the calculated values of Δ°S ads are positive which is attributed to the increase in the solvent energy in addition to the increase in entropy due to H 2 O desorption 79 where CR stands for the corrosion rate at a temperature (T) which is expressed by the corrosion current density (i), E a represents the apparent activation energy and A is the Arrhenius constant that depends on the metal type and electrolyte. R is the universal gas constant (8.314 J mol −1 K −1 ). Figure 9 shows the relation between log i and 1/T for C-steel in 0.5 M HCl solution with and without AEO7. From the slopes, Ea can be calculated for each concentration.
The activation enthalpy (∆H*) and activation entropy (∆S*), for the corrosion of C-steel in a 0.5 M HCl with and without AEO7 are calculated from the transition-state equation, as shown by Shuklar et al. 83 .  Table 6. Thermodynamic parameters derived and calculated based on Freundlich adsorption isotherms and polarization measurements. where CR is the rate of corrosion which is expressed by the current density, R is the universal gas constant in J mol −1 . K −1 , T is the temperature in K and h is the Planck's constant. To obtain ∆H * and ∆S * , log i/T versus 1/T are plotted at different concentrations of AEO7 corrosion inhibitor as displayed in Fig. 10.
ΔH* values are computed from the slopes of the plots while ΔS* values are calculated from the intercept of the plots in Fig. 10. The computed values of Ea, ΔH* and ΔS* are presented in Table 7. Clearly, it is seen that ΔH* values in the presence of AEO7 corrosion inhibitor are smaller than those for AEO7-free HCl solutions. In addition, raising the concentration of AEO7 increases Ea and ΔS* values. This trend supports the physisorption mechanism which is proposed for the adsorption of AEO7 molecules on the C-steel surface in a 0.5 M HCl solution.
A similar interpretation can be made using the thermodynamic equation below 30,84 : The values of ΔH* are smaller compared to those of Ea. The ΔS* values are large and negative. The ΔS* value becomes less negative with increasing AEO7 concentration indicating a decline in the level of perturbation of the AEO7 molecule to move from the reactant state to the activated complex one 82 .    Microscope Analysis. Figures 11 and S2 compare the surface morphology of (A) a polished C-steel coupon using the SEM (Fig. 11) and optical profilometry ( Figure S2) with another coupon after immersion for 24 h in 0.5 M HCl in the (B) absence and (C) presence of 40 µmol L −1 of AEO7 alone and (D) in presence of 1 × 10 −3 M of KI along with 40 µmol L −1 of AEO7. The polished coupons did not exhibit any noticeable defects, except polishing scratches (Fig. 11A) with a surface roughness (R a = 0.02 μm). After immersion in 0.5 M HCl without any inhibitor, heavy corrosion attack takes place (Fig. 11B) and R a increases to 3.2 μm. In Figs 11C and 13C, where the AEO7 corrosion inhibitor exists, the corrosion is decreased significantly compared with that in its absence (Figs 11B and S2B) and Ra decreases to 0.12 μm. In presence of 1 × 10 −3 M KI in addition to 40 µmol L −1 of AEO7, a synergistic effect take place as the surface look clean and R a decreases significantly to 0.06 μm.
AFM analysis. The three-dimensional AFM technique is one of the ideal approaches for exploring the surface topography at the nano and microlevels. It is also a powerful method for elucidating the efficiency of the corrosion inhibitor 84 . The optical profilometry provides qualitative images at the microscale level. However, the quantitative details for the surface roughness of C-steel can be determined by AFM where the mean roughness factor (R a ) can be measured. The R a values show that the roughness of C-steel immersed in 0.5 M HCl increases from 20 nm for the unexposed specimen to 320 nm for the coupon corroded in an uninhibited-HCl solution.
However, the addition of 40 µmol L −1 of AEO7 reduces the R a to 120 nm due to the formed adsorbed protective layer. Moreover, upon adding 1 × 10 −3 M KI to the AEO7, the R a is reduced to 65 nm. These results are clear in the AFM micrographs shown in Fig. 12. Both SEM and AFM measurements confirm the high corrosion IE% of AEO7 especially in the presence of KI.
Quantum Chemical Calculations. Figure 13 represents the optimized structure of the inhibitor as well as the corresponding frontier molecular orbitals namely HOMO and LUMO. The molecule consists of a polar part composed of alternative eight oxygen atoms and ethylene groups linked to a zigzag non-polar hydrocarbon. The population of the electron density is focused on the oxygen atoms suggesting that heteroatoms behave as  active sites with the highest ability to interact with the metal surface. The presence of these electronegative atoms increases the polarity of the molecule as well as their interaction with the charged sites on the metal surface enhancing the adsorption probability and then preventing interaction of the surface with the environment 85,86 . Figure 14 depicts the Mulliken charge distributions of AEO7. The highest electron densities are located at oxygen atoms (≈−0.34 au) indicating that oxygen atoms are the active centers, which have the strongest ability to interact of the metal surface. This finding is in line with the HOMO populations. This interaction with the metal surface with several numbers of active centers provides a good protective layer on the metal surface.
The quantum chemical parameters are calculated and listed in Table 8. The large energy gap decreases the ability of the inhibitor to form coordinate bonds with the d-orbital of the metal by donating and accepting electrons. Moreover, large global hardness and small global softness values reveal the inhibitor resistance to charge   transfer and decreased the ability to receive electrons. However, the polarity of the molecule as inferred from the value of the dipole moment suggests a good electrostatic interaction with the metal surface and in turn enhances the inhibition ability. Therefore, these results are in good agreement with the experimental data i.e. the inhibitor favors the physical adsorption on the metal surface 87 .

Conclusion
AEO7 is an effective corrosion inhibitor for C-steel in 0.5 M HCl solutions. The presence of KI synergistically enhanced the corrosion IE% and the adsorption of AEO7 at the C-steel surface at ambient temperature. Weight loss results reveal a high corrosion IE% especially at room temperatures. EIS and polarization measurements confirm the adsorption of AEO7 corrosion inhibitor molecules on the C-steel surface and the formation of a protective layer that suppresses the C-steel oxidation and the hydrogen ion reduction reactions at the anodic and cathodic sites, respectively. AEO7 molecules are exothermically physisorbed on the C-steel surface, and the adsorption follows the Freundlich isotherm. SEM, optical profilometry and AFM measurements reveal a smooth surface of the C-steel coupon when immersed in an AEO7-inhibited HCl solution compared to the cases when an uninhibited HCl solution is used where severe corrosion takes place, which increases the surface roughness significantly.

Data Availability Statement
The raw data required to reproduce these findings can be shared at any time based on direct requests to the authors.