Electrochemical and thermodynamic study on the corrosion performance of API X120 steel in 3.5% NaCl solution

The present work studied the effect of temperature on the corrosion behavior of API X120 steel in a saline solution saturated with CO2 in absence and presence of polyethyleneimine (PEI) as an environmentally safe green inhibitor. The effect of PEI on the corrosion behavior of API X120 steel was investigated using destructive and non-destructive electrochemical techniques. The overall results revealed that PEI significantly decreases the corrosion rate of API X120 steel with inhibition efficiency of 94% at a concentration of 100 μmol L−1. The adsorption isotherm, activation energy and the thermodynamic parameters were deduced from the electrochemical results. It is revealed that the adsorption of PEI on API X120 steel surface follows Langmuir adsorption isotherm adopting a Physi-chemisorption mechanism. Finally, the samples were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques to elucidate the effect of aggressiveness of corrosive media on the surface morphology and the corrosion performance of API X120 steel. The surface topography result indicates that the API X120 steel interface in PEI presence is smoother than CO2 with Cl− ions or Cl− ions only. This is attributed to the compact protective film limits the aggressive ions transfer towards the metallic surface and reduces the corrosion rate. Moreover, PEI inhibition mechanism is based on its CO2 capturing ability and the PEI adsorption on the steel surface beside the siderite layer which give the PEI molecules the ability to reduce the scale formation and increase the corrosion protection due to capturing the CO2 from the brine solution.

the protective scale formation enhances with increasing concentration of CO 2 which reduces the corrosion rate. Moreover, the solubility of the iron carbonate layer decreases with increasing the pH [19][20][21] . In addition, the medium under the scale layer can cause under deposit corrosion 22,23 and that is why removal of scale or changing its morphology is recommended for preventing corrosion. The mechanical degreasing and inhibitors are the commonly applied to prevent scale formation and the other deposits like sands and sulfides in the pipelines 24,25 . Corrosion inhibitors are frequently added to the corrosive media in order to reduce the aggressive attack on the materials and to improve their service performance 26,27 . Corrosion inhibitors are usually organic heterocyclic compounds which contains nitrogen (N), Sulphur (S), and oxygen (O) atoms having lone pair of electrons that can adsorb on the metallic surface. The adsorption of electrons blocks the metal active sites or can create a physical barrier to decrease the attachment between the aggressive ions and the metal surface [28][29][30] . Finsgar et al. investigated the effect of different molecular sizes of polyethyleneimine on the corrosion behavior of stainless-steel alloys in 3% NaCl solution. The results indicate that PEI acts as an effective corrosion inhibitor for pitting and uniform corrosion. Additionally, PEI molecule could be adsorbed in the metal surface to form a dense protective layer which acts as a diffusion barrier from different ionic species with preventing the chlorides attack from saline solution 31,32 . Meanwhile researchers reported that the PEI inhibitor with high molar mass (60,000 g mol −1 ) was highly effective in corrosion prevention of 304 stainless steel than the low molar mass (2000 g mol −1 ) 33,34 . Jianguo et al. confirmed that adding 50 μmol L −1 from PEI (50,000 g mol −1 ) gave the highest inhibition efficiency for the low carbon steel immersed in phosphoric acid 35 . Moreover, Sekine et al. reported that PEI molecules were not effective on reducing the corrosion rate for mild steel in cooling water that contains Ca + and Cl − ions 36 . Zhang et al. have utilized a quaternary polyethyleneimine (QPEI) as a cationic polyelectrolyte inhibitor for Q235 carbon steel in 0.5 M H 2 SO 4 . The results indicated that QPEI formed a protective layer on the metal surface which reduced the corrosion rate of iron in acidic environment. The XPS and SEM results showed that the QPEI can form a protective polymer layer on the metal surface by adsorption 37 . Additionally. Gao et al. differentiate between the adsorbed layer and the polymeric layer which formed from QPEI inhibitor. He hypothesized that the two layer cooperate to form a compact barrier layer on A 3 steel surface in acidic media which prevent metal dissolution and retard H + discharging 38 . Generally, polymeric compounds exhibited a noticeable corrosion inhibition merits compared to their monomer counterparts due to increasing number of active sites. However, the molecular size and steric effect could also influence the adsorption mechanism 39 .
The aim of this research work is to study the effect of polyethyleneimine (PEI) as an environmentally safe corrosion inhibitor on the corrosion behavior of API X120 in 3.5 wt% NaCl solution saturated with CO 2 at ambient and elevated temperatures. To the best of our knowledge, this work has not been previously reported in the literature. Moreover, the adsorption isotherm, activation energy and other thermodynamic parameters were calculated from the electrochemical results. Finally, Bare (as received polished) and rusted samples were characterized by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques to elucidate the effect of the aggressive media on the surface morphology and thus the corrosion performance of API X120 steel. The corrosion inhibition mechanism of the PEI corrosion inhibitor on the metal interface would be elucidated based on all the obtained results.

experimental Work
Materials. API X120 Steel was supplied by Shandong Yineng International Trade Company, China. The chemical composition of the studied steel was determined using an optical emission spectrometer (ARL 3460). The analyzed chemical composition is shown in Table 1. The API X120 steel was manufactured in furnace and casted it into sheets then accelerated quenching was applied reaching to the ambient temperature. The microstructure of API X120 steel sample was obtained using Lecia optical microscope, Spain. The as-received sample was etched using a freshly prepared 2% Nital solution which applied on the surface for 20 sec. Figure 1 shows the representative microstructure of the target specimen as it was found to be pearlitic. On the other hand, the mechanical properties for the API X120 steel is going to be studied in other studies.
Polyethyleneimine (PEI) was purchased from BDH (Germany, Project Code A11033). The chemical structure of PEI is shown in Fig. 2. The sodium chloride (NaCl) was purchased from Sigma Aldrich (Germany).

Sample preparation and test solution.
The steel specimens of size 0.5 cm 2 were cut and grind using different size of silicon carbide (SiC) abrasive papers 500, 800, 1000 and 2000 consecutively. Finally, the ground samples were polished using 4000 grit SiC paper. Further cleaning of the polished samples was performed by acetone followed by ethanol. Finally, the samples were washed with distilled water and dried in air. The 3.5% NaCl aqueous solution was prepared by dissolving stoichiometric amount of NaCl in distilled water. The concentration of the PEI in 3.5 wt% NaCl solution was used from 25, 50, 75 and 100 µmol L −1 . The 3.5 wt% NaCl solution was saturated with CO 2 by purging CO 2 into the solution at a constant flow rate. The CO 2 purging was started 1 hour before the commencement of the experiment and remained continued though out the experiment at a constant flow rate.
Gravimetric measurements. The weight loss experiments were performed on a API X120 steel sheet (2.0 × 2.0 × 0.1 cm) immersed in 3.5 wt% NaCl solution saturated with CO 2 in the absence and the presence of

Results and Discussions
Weight loss measurements. Figure 3 represents the corrosion rate and the inhibition efficiency analysis of API X120 steel immersed in 3.5 wt% NaCl solution saturated with CO 2 without and with (25,50,75 and 100 μmol L −1 ) of PEI corrosion inhibitor for different exposure time at room temperature. The depicted figures were derived from the weight loss measurement as the corrosion rate (CR) was calculated according to the equations below 41 .
= . * * * CR W D A t (mm/y) 87 6 (1) where w is mass loss in mg, ρ is the carbon steel density in g/cm 3 , A is the surface area of sample in square and t is time of test in hours The inhibition efficiency (IE%) and the surface coverage θ, of the inhibitor for the corrosion of HSLA steel was computed as follows,  www.nature.com/scientificreports www.nature.com/scientificreports/ where w° and w are the average weight loss values without and with adding the inhibitor, respectively. It could be noticed from Fig. 3A that the corrosion rate decreases as the PEI corrosion inhibitor concentration increases, suggesting that a greater number of PEI molecules are adsorbed over the active sites of the metal surface hence diminishing the direct contact between the API X120 steel and the corrosive environment 31,42 .
Additionally, Fig. 3B shows that the inhibition efficiency percentage increase with raising the concentration of PEI. This is attributed to the development of coordination bond because of overlap of the lone pair electron of the nitrogen atom with the 3d orbital of the iron atom with increasing the inhibition abilities of the PEI molecules on the metal surface 38,43 . Figure 4 describes the potentiodynamic polarization curves of HSAL steel in 3.5 wt% NaCl solution saturated with CO 2 having various concentrations of Polyethyleneimine (PEI) from 20 to 70 °C. The electrochemical kinetic parameters include corrosion free potential (E corr ), corrosion current density (i corr ), the polarization resistance, (R p ) and cathodic/anodic Tafel slopes (ß c and ß a , respectively) were measured by Tafel extrapolation of the current -potential lines to the corresponding corrosion potentials. The measured data is listed in Table 2. Moreover, the corrosion rates were calculated considering the whole surface of API X120 steel is attacked by the aggressive media without any localized corrosion 44 .

Potentiodynamic polarization analysis.
where 0.13 is the measurement of the time conversion factor, i corr is the corrosion current density (A. cm −2 ), A is the atomic weight of iron (55.6 g. mol −1 ), n is the number of transferred electrons per metal atom, D is the density of iron (7.85 g. cm −3 ), The surface coverage values (θ) was obtained from the following equation 45 : where i°c orr and i corr are the corrosion current densities of API X120 steel without and with the presence of the corrosion inhibitor, respectively. Moreover, the corrosion inhibition efficiency (IE%) is calculated using Eq. 5.
It is worth noting that, the potentiodynamic polarization curves of API X120 steel in 3.5 wt% NaCl solution saturated with CO 2 shows increase in cathodic and anodic current densities with increasing temperature. This is attributed to the hydrogen reduction reaction in the cathodic region and the early dissolution of API X120 steel in the anodic part 46 . Meanwhile, in the presence of PEI corrosion inhibitor, with increasing amount of PEI concentration the cathodic and the anodic current densities are shifted to lower values when compared to the blank. Furthermore, the corrosion potential is slightly shifted to the positive direction. It is reported that if the difference between the E corr value of inhibitor and the E corr value of the aggressive media is more than ±85 mV, the inhibitor will be either cathodic type or anodic, otherwise the corrosion inhibitor will be classified as a mixed type inhibitor. The E corr values in Table 2 confirms that the PEI is a mixed type inhibitor and it has a significant effect in reducing the corrosion rates (mpy) compared to the blank. The higher the concentration of the PEI, the lower are the corrosion rates at all studied temperatures. However, it is further noticed that the increase in temperature increases the corrosion rate at the same concentration of the PEI 47,48 . www.nature.com/scientificreports www.nature.com/scientificreports/ Electrochemical impedance spectroscopic analysis. Figure 5 represents the EIS results for API X120 steel specimens in 3.5 wt% NaCl solution saturated with CO 2 containing different concentrations of PEI (0, 25, 50, 75 and 100 µmol L −1 ) conducted at various temperatures. Clearly, the diameters of the semicircles of the Nyquist graphs of the PEI inhibitor at concentrations of 25, 50, 75 and 100 µmol L −1 are larger compared to the blank one.  www.nature.com/scientificreports www.nature.com/scientificreports/ It is also clear the diameters of the semicircles decrease as the temperature increase at the same concentration of the PEI. In addition, the depressed capacitive loops seen at low frequency in all the Nyquist plots refer to a charge transfer mechanism for the corrosion of API X120 steel in 3.5 wt% NaCl solution saturated with CO 2 . The deviation of the capacitive loop from a complete semi-circle might be because of the heterogeneity and microroughness of the working electrode surface 49,50 . Figure 6 shows the electrical equivalent circuit (EC) that is used for the EIS analysis. It is a two-time constant equivalent circuit in parallel type which commonly used to describe a non-uniform corrosion of electrodes in an electrolyte 51,52 . The EC contains uncompensated solution resistance (R s ), a pore resistance (R po ), a charge transfer resistance (R ct ) and two constant phase elements (CPE1, CPE2) that replaces the capacitive element to obtain a more accurate fit. The non-ideal layer capacitors are predictable in the recorded double layer capacitances. This can be attributed to many reasons such as the non-uniformity and the surface roughness of the tested sample, the current distribution of the inhibitor or the corrosion products, the surface coverage and the corrosion rate 53,54 .
The impedance of the CPE is expressed by Eq. 6 55,56 : where Z Q is the CPE impedance value (Ω cm −2 ); Y o is the CPE constant; j = (−1) 1/2 which apparently equal to the imaginary number; ω = 2πf max is the angular frequency in rad/s and f max the maximum frequency for the imagi-  www.nature.com/scientificreports www.nature.com/scientificreports/ nary part; n is the fitting roughness and its values between 0 and 1. When n = 0, the CPE becomes equivalent to a resistor and when n = 1, the CPE becomes equivalent to an ideal capacitor. All EIS parameters obtained from the Nyquist graphs are summarized in Table 3.
The surface coverage (θ) is estimated using the following equation 31,57 : where R ct1 and R ct2 are the charge-transfer resistances in the absence and the presence of the PEI corrosion inhibitor, respectively. Moreover, the corrosion inhibition efficiency (IE%) was calculated using Eq. 3. It can be noticed that the API X120 steel surface is more corrosion resistant with 100 μmol L −1 of PEI than the lower concentrations of PEI in a 3.5% NaCl saturated CO 2 purging solution. R ct increases while the C dl decreases with increasing the PEI concentration or decreasing the temperature 58 . For instance, comparing the blank solution with the one containing 25 μmol L −1 of PEI at 20 °C, the R ct increases from 345 Ω cm 2 to 1082 Ω cm 2 , and C dl decreases from 119.27 µF to 68.75 µF with an inhibition efficiency of 68.4%. Meanwhile, increasing the inhibitor concentration to 100 μmol L −1 , increases the R ct value to 2416 Ω cm 2 , and the C dl decreases to 49.51 µF with an inhibition efficiency of 85.6%. This observation would be explained by the Helmholtz equation as expressed below 59 .
where δ ads is the thickness of the PEI corrosion inhibitor adsorbed layer, ε o is the air permittivity, ε is the local dielectric constant and A is the area of API X120 steel electrode. This equation shows 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 PEI corrosion inhibitor as its concentration increases in solution. As the protective layer increase, the charge transfer become more sluggish as shown from the R ct and IE% values. Increasing the temperature will promote the desorption rate of the PEI molecules from the API X120 steel surface and raise the dissolution rate of the Fe ions which lead to decrease in the inhibition efficiency. Generally, increasing the PEI concentration shift the "n" values to the less positive direction. This make the constant phase element becoming farther from the ideal capacitor 60,61 . It is worthy to mention that the EIS measurements are in agreement with the potentiodynamic polarization data.
Inhibitor adsorption and thermodynamic analysis. The reaction of metal active sites with the corrosion inhibitor molecules occurs via substitutional replacement of the electrolyte molecules at metal/solution interface [62][63][64] . The adsorption isotherm models determine the type of reaction i.e. whether it is spontaneously or non-spontaneously and whether the interaction is physical or chemical. Figure 7 shows the relation between θ C inh and C inh at different temperatures according to Langmuir adsorption isotherm equation 65 .
T, °C C inh µmol L −1 R s , R po , Y po × 10 −6 s n Ω −1 cm −2 n 1 R ct , Y ct × 10 −6 s n Ω −1 cm −2 n 2 θ IE, % where K ads is the equilibrium constant of the adsorption-desorption process, and C inh, is the inhibitor concentration. Straight lines are obtained with a slope close to 1, with a correlation coefficient (R 2 > 0.99). The values of K ads are calculated from the intercepts of the plotted straight lines with y-axis. The standard free energy of adsorption reaction, ΔG°a ds , in kJ mol −1 , is calculated using K ads values from the following equation 66 where R is the universal gas constant in J mol −1 K −1 and T is the absolute temperature. The values of K ads and ΔG°a ds , are given in Table 4. The higher K ads value indicates the strong adsorption ability of the PEI inhibitor. In addition, the calculated ΔG°a ds values of the used PEI corrosion inhibitor is close to −40 kJ mol −1 at 20 °C and decreases as the temperature increases. Thus, it can be assumed that the PEI molecules adsorbed on the metal surface and demonstrate acceptable desorption properties in the polarization graphs. Finally, the adsorption mechanism of the PEI molecules in the API X120 steel surface in CO 2 saturated solution is followed a mixed physi-chemisorption mechanism 67,68 .

Thermodynamic activation parameters and inhibition mechanism.
To estimate the activation energy (E a ) for the corrosion of API X120 steel in 3.5 wt% NaCl solution saturated with CO 2 in diverse concentrations of PEI corrosion inhibitor at temperature range of 20 °C to 70 °C, the logarithm of the corrosion rate (CR) which can be expressed by log (CR) was plotted against 1/T according to Arrhenius equation 69 : where, A is the Arrhenius constant that depends on the metal type and electrolyte 70 . The E a values calculated from the slopes of the plotted straight lines in Fig. 8 that have high regression coefficient close to unity are listed in Table 5. The addition of PEI corrosion inhibitor increases the activation energy value which indicate a strong physical adsorption of PEI compound on API X120 surface. However, the adsorption of PEI molecules on API X120 steel as shown ΔG ads o occurs through a simultaneous physi/chemisorption process. The activation energy solely would not elucidate the type of the adsorption because there is a rivalry between the PEI corrosion inhibitor and the OH − group from water molecules for adsorbing on Figure 7. Langmuir adsorption plots for API X120 steel in carbonated 3.5 wt% NaCl with saturated CO 2 solution under elevated temperature.  Table 4. Thermodynamic Parameters derived from the Langmuir plots under elevated temperature.
the metal surface and for removing the OH − group away from the metal surface and thus extra activation energy would be required 71,72 . According to the transition state equation, the apparent enthalpy of activation, ΔH a , and entropy of activation, ΔS a , for API X120 corrosion in 3.5 wt% NaCl solution saturated with CO 2 can be calculated from the corrosion rate (corrosion current densities) at different temperatures in the presence and the absence of various concentrations of the PEI corrosion inhibitor 73,74 .
"h" is the Planck's constant, N A , is the Avogadro's number, and R, is the universal gas constant. Plotting log (CR/T) against 1/T yield straight lines relation as shown in Fig. 9. The values of ΔH * and ΔS * are calculated from the slopes of the straight lines and their intercepts with the y-axis, respectively and are tabulated in Table 5. The endothermic nature of the API X120 dissolution reaction is inferred from the positive sign of ΔH * . Increasing the ΔH * values as the concentration of inhibitor increase means that the dissolution of API X120 becomes more sluggish in presence of the PEI inhibitor 75 . The negative values of ΔS * confirms that the activated complex is the rate determining step where association rather than dissociation take place as the reaction goes from the reactants to the activated complex step 76 . Figure 10(A) shows the surface analysis of API-X120 steel after its immersion in 3.5 wt% NaCl for 6 h at room temperature, where a severely corroded surface is observed with mean roughness value (R a ) of 460 nm. Meanwhile, the peaks and the valleys heights reduced to 211 nm after adding 100 μmol L −1 of PEI as seen in Fig. 10(B). On the other hand, Fig. 10(C,D) represent the 3D surface topography images of API X120 steel samples immersed in 3.5 wt% NaCl solution saturated with CO 2 in the absence and the presence of 100 μmol L −1 of PEI, respectively. It can be noticed that there is less damage in comparison with the samples without CO 2 purging. The R a values also reduced from 230 to 131 nm as seen in Fig. 10(C,D) respectively. This is attributed to the affinity of the PEI molecules to be adsorbed on the metal surface through heteroatoms and formed a protective layer which reduces the corrosion rate and the surface roughness 77 . This is attributed to the compact protective film limits the aggressive ions transfer towards the metallic surface and reduces the corrosion rate.

AFM investigation.
Scanning electron microscopic analysis. The surface morphology of API X120 steel when exposed to various aggressive media after 6 hrs of immersion at 20 °C is presented in Fig. 11. Steel surface is extremely corroded and roughened due to a highly aggressive 3.5 wt% NaCl solution with a lot of clear pits as seen in Fig. 11(A). Figure 11(B) depicts the specimen's surface in the presence of the 100 µmol L −1 of PEI in 3.5 wt% NaCl solution. It can be noticed that only a little number of pits can be observed. Figure 11(C) shows a dense corrosion product   www.nature.com/scientificreports www.nature.com/scientificreports/ scale formed in CO 2 saturated brine solution. Meanwhile, in Fig. 11(D) the scale corrosion product influenced further diminished due to the presence of 100 µmol L −1 of PEI to the brine solution, which decreases the steel surface roughness. This indicates that although the scale (corrosion product-siderite) seems to provide some corrosion protection to the metal surface, the PEI molecules have the ability to prohibit the scale formation and increase the corrosion protection due to capturing the CO 2 from the brine solution as shown in Fig. 12. The cross-sectional examination shows that the thickness of the formed layer is significantly diminished in favor of adding 100 μmol L −1 of the PEI corrosion inhibitor as it drops from 7 μm to 900 nm in the absence and the presence of 100 μmol L −1 of the PEI, respectively.

Corrosion inhibition mechanism.
PEI is a branched water-soluble macromolecule polymer and there are a large number of amine groups on it. Each nitrogen atom in amine group contains a lone pair of electrons which provides a strong electron donating affinity to PEI molecule. It is clear that PEI has corrosion inhibition properties  www.nature.com/scientificreports www.nature.com/scientificreports/ towards API X120 steel in neutral media. This is due to the electrostatic interaction between the positive ions on the metal surface and the partially negative charge PEI molecules. Consequently, PEI molecule adsorbs on the steel surface physically and 100 μmol L −1 of PEI gives 88% corrosion inhibition efficiency. On the other hand, the PEI molecule has a CO 2 sorption affinity which influence the CO 2 molecule to interact with the PEI adsorbed  www.nature.com/scientificreports www.nature.com/scientificreports/ molecule instead of the metal surface [78][79][80][81][82][83] . As a result, a dense protective layer formed over the metal surface in the presence of the PEI corrosion inhibitor and it is more compact than the corresponding layer with the CO 2 as seen in Fig. 12. Therefore, the complex PEI film on the steel surface act as a barrier which hinder the aggressive ions to penetrate to the steel surface as well as prohibiting the Fe dissolution 38,84-87 . conclusions The PEI can efficiently inhibit the corrosion of API X120 steel (API X120) in 3.5 wt% NaCl solution saturated with CO 2 as it builds a protective film from adsorbed molecules on the metal surface. PEI molecules exhibit a strong affinity for metal surface therefore shows good inhibition efficiency. The inhibition efficiency of API X120 steel increases with increasing concentration of PEI in 3.5 wt% NaCl solution saturated with CO 2 . Potentiodynamic polarization measurements for APIX 120 steel immersed in CO 2 saturated saline solution confirm that PEI functions as a mixed type corrosion inhibitor as the cathodic and the anodic reaction rate has been reduced. The PEI molecules are exothermically mixed physi/chemisorbed on the API X120 steel surface, as confirmed from the calculated standard Gibbs free energy change (ΔG ) ads o . SEM and AFM images depict that the surface roughness of the exposed steel to CO 2 saturated saline solution is significantly higher than that in the corresponding PEI solution.

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