β-Enaminoesters as Novel Corrosion Inhibitors for Carbon Steel in Acidic Medium

In this work, two β-enaminoesters, EN1 and EN2, were synthesized, characterized and their anticorrosion effects on American Iron and Steel Institute (AISI) 1020 carbon steel in 0.5 mol L HCl were investigated using gravimetric and electrochemical methods. The results revealed that both compounds inhibit corrosion. Good correlations were observed between results obtained for gravimetric and electrochemical methods. EN1 exhibited highest efficiency of 98% after 24 h of immersion in HCl solution. The adsorption of compounds on steel surface followed the modified Langmuir adsorption isotherm. Surface morphology of carbon steel was examined using scanning electron microscopy showing that the metal surface was protected by EN1 as inhibitor of corrosion.


Introduction
Among the various metallic materials that can suffer corrosion, carbon steel is widely used in several industrial segments because it is easier to handle and more costeffective than more noble materials, and approximately 20% of steel produced is intended for the replacement of parts for corroded equipment, parts or installations.The petroleum and petrochemical industries suffer the most attacks of corrosive agents, causing damages in the entire productive chain, from extraction to refining. 1 Acidic solutions are usually used in industries, such as pickling, acid cleaning and the cleaning of oil refinery equipment, even though they are increasing the corrosion of metallic materials and affect the performance of metals subjected to such processes. 2he knowledge and characterization of the different aggressive means responsible for the chemical and electrochemical reactions that provoke the deterioration of the materials are of fundamental importance so that increasingly effective methods to combat the corrosion can be developed. 3One of the methods used to prevent corrosion is the use of corrosion inhibitors.
Such inhibitors are organic or inorganic substances, which, when added to the corrosive medium, prevent or reduce the development of corrosion reactions.These inhibitors are usually adsorbed, making a very thin and persistent film, which leads to a decrease in the corrosion rate, due to the slowing of the anodic or cathodic reactions (or both). 4he use of organic compounds to inhibit metallic corrosion in acidic media is well established.Compounds containing electron-donating atoms, such as oxygen, nitrogen and sulfur, have shown efficiency as carbon steel corrosion inhibitors.The molecular structure, aromaticity and the presence of π electrons or non-ligand electron pairs are important characteristics to determine how these molecules adsorb on the metal surface and, when adsorbed, how they block the active sites and reduce the corrosion rate. 5,6he general term "enaminone" refers to any compound having the N-C=C-C=O conjugate system.Enaminones are β-enaminocarbonyl compounds, derivatives of β-diketones, β-ketoesters and other β-dicarbonyl compounds.The most common representatives of this class are β-enaminoketones and β-enaminoesters. 7,8Enaminoesters possess structures with acceptor-donor electronic effects, and due to the mesomeric effect of the acyl and amino substituents on the double bond, the delocalization of π electrons resulting from this effect leads to a remarkable polarization of these olefinic carbons, giving these compounds a distinct reactive nature. 9chiff bases are similar compounds that have good corrosion inhibition efficiencies because they have the necessary characteristics for interactions with metals.Recently, Prajila and Abraham 10 studied the anticorrosive activity of three Schiff bases against carbon steel, being observed 94% of corrosion inhibition efficiency for (4-(4-hydroxybenzylideneamino)-4H-1,2,4-triazole-3,5-diyl)dimethanol in the concentration of 1.6 × 10 -4 mol L -1 , 95% for (4-(4-methoxybenzylideneamino)-4H-1,2,4-triazole-3,5-iyl)dimethanol (1.5 × 10 -4 mol L -1 ) and 97% for (4-(3,4-dimethoxybenzylideneamino)-4H-1,2,4-triazole-3,5-diyl)dimethanol (1.4 × 10 -4 mol L -1 ), by electrochemical impedance spectroscopy (EIS) method in 0.5 M HCl. 10 Heydari et al. 11 also investigated as corrosion inhibitors, for mild steel, other two Schiff bases: benzenemine, with 97 and 88%, in 1.0 × 10 -3 and 9.7 × 10 -5 mol L -1 of concentrations, respectively, by weight loss method in 1 M HCl. 11hus, due to some structural similarities, enaminoesters are compounds considered to be potential corrosion inhibitors.Although these compounds are known, there are no reports in the literature of the use of these compounds as corrosion inhibitors, so this application is unprecedented.Therefore, in this work we presented the synthesis and the investigation of the inhibitory effects of two β-enaminoesters (EN1 and EN2) on American Iron and Steel Institute (AISI) 1020 carbon steel in 0.5 mol L -1 HCl, employing potentiodynamic polarization (PP), linear polarization resistance (LPR), EIS, mass loss and scanning electron microscopy (SEM).

Experimental
Materials and instruments 1 H and 13 C nuclear magnetic resonance (NMR) spectra were obtained on Bruker NMR Ultrashield 500 MHz spectrometer, with tetramethylsilane as the internal reference and CDCl 3 or DMSO-d 6 as the solvent; the chemical shifts (d) are reported as ppm.Peak multiplicities are expressed as follows: s, singlet; d, doublet; t, triplet; q, quartet.Coupling constants (J values) are given in hertz (Hz).Elemental analyses were performed on a PerkinElmer 2400 CHN at the Laboratory of Environmental Science of the Universidade Estadual do Norte Fluminense (UENF).Reactions were monitored by thin-layer chromatography (TLC) on Merck silica gel 60 F254 aluminum sheets.TLC spots were visualized by inspection of the plates under UV light (254 and 365 nm).The reagents and solvents were purchased from Sigma-Aldrich and Vetec, respectively.All reagents were used without any further purification.The water used in all experiments was MilliQ level.Ethyl acetate was treated with p-toluene sulfonic acid (p-TSA) in the presence of ethanol as a solvent.After 10 min in magnetic stirring, phenethylamine or p-phenethylamine and molecular sieves were added.The mixture was stirred at 30 o C for 20 h. 12,13Then, the molecular sieves were filtered and the ethanol was evaporated.Dichloromethane and saturated aqueous NaHCO 3 solution were added to the mixture until neutrality.The organic phase was dried with Na 2 SO 4 and filtered; then, the solvent was evaporated, and the product was collected.EN1 was obtained with a 74% yield, and EN2 with an 84% yield.The synthetic route is shown in Figure 1.

Electrochemical tests
The assays were tested at 30 o C using the electrochemical cell of three electrodes: carbon steel with 0.80 cm 2 of area, a platinum electrode as the auxiliary electrode and silversilver chloride (Ag/AgCl, 3.0 mol L -1 KCl) as a reference electrode.
Before each experiment, the work electrode was abraded with 400, 600, and 1200-grade emery paper, washed with distilled water, degreased with ethanol and dried.The tests were carried out in aerated media using the Autolab Potentiostat/Galvanostat model PGSTAT 302N and analyzed with NOVA 1.11 software.

EIS
The open circuit potential (OCP) was monitored before each assay for 30 min until a stabilized potential was observed. 14,15After that, impedance measurements were taken at a frequency interval of 10 kHz to 0.1 Hz with 10 mV of amplitude peak-by-peak.The inhibition efficiency (η) was calculated using equation 1: 2,3,16,17 (1) where R ct is the charge transfer resistance in the presence of the inhibitor, and R 0 ct is the charge transfer resistance in the absence of the inhibitor. 18All values were obtained from the Nyquist diagram of each experiment. 19

LPR
LPR tests were performed using a scan rate of 1 mV s -1 in the potential range of ± 10 mV around the open circuit potential (E OCP ). 20The inhibition efficiency was calculated using equation 2: (2) where R p is resistance of polarization in the presence of the inhibitor, and R 0 p is the resistance in the absence of the inhibitor.The values of R p are the values of the angular coefficient obtained by a linear regression of the graph of current density (i) vs. potential (E).

PP
PP tests were performed using a scan rate of 1 mV s -1 in the potential range of ± 200 mV around E OCP .Inhibition efficiency was calculated using equation 3: 2,3,21 (3) where i corr,0 and i corr are the uninhibited and the inhibited corrosion current densities, respectively.

Weight loss experiment (WL)
The gravimetric measurements were taken using samples of AISI 1020 carbon steel of 3.0 × 3.0 × 0.1 cm in a solution of 0.5 mol L -1 HCl in the presence and absence of EN1 in a concentration of 1.0 × 10 -2 mol L -1 for 3, 6, 18 and 24 h. 22The experiments were conducted in triplicate.The samples were abraded with 400, 600, and 1200-grade emery paper, washed with distilled water, degreased with ethanol and dried.
The assays were tested with the samples of carbon steel immersed in a corrosive medium in the presence and absence of an inhibitor at 30 o C and in naturally aerated media for 3, 6, 18 and 24 h. 13,23The inhibition efficiency was calculated using equation 4: (4)   where W 0 and W are the weight loss in the absence and presence of the studied inhibitors, respectively.

SEM
The samples of carbon steel of 3.0 × 3.0 × 0.1 cm were abraded and washed with ethanol, dried and immersed in the presence or absence of the inhibitor and in the presence of 0.5 mol L -1 HCl for 24 h at room temperature.The specimens were removed, washed with water and ethanol, and dried.The measurements were taken using a Hitachi TM 3000 tabletop microscope. 24

Results and Discussion
Synthesis EN1 and EN2 were obtained in good yields of 74 and 80%, respectively, by synthetic routes adapted from the literature. 25,26The products were isolated in oil form.The characterizations were determined by 13 C and 1 H NMR spectroscopy and compared with the reported data. 26

EIS
The corrosion behavior of AISI 1020 carbon steel in 0.5 mol L -1 HCl solution obtained in the presence and absence of the β-enaminoesters was investigated by EIS at 30 o C after 30 min of immersion, when the stabilization of the OCP was observed.The Nyquist plots obtained by EIS measurements in the absence and presence of EN1 and EN2 are shown in Figure 2.
The diameter of the semicircular Nyquist plots, in both cases, increases with an increment in the concentration of the inhibitors, showing that the corrosion and inhibition processes are charge-transfer-controlled.The charge transfer resistance values (R ct ) were obtained by Z' in the x-axis, where the intersection at high frequency is the ohmic resistance of the solution (R s ), and the intersection at low frequency is the R ct + R s .Thus, the R ct values were calculated by the difference between (R ct + R s ) and R s .
The degree of surface coverage (θ) is proportional to the inhibition corrosion efficiency and this may be calculated by equation 5: (5)   where R ct and R 0 ct are the charge transfer resistances in the presence and absence of the inhibitor, respectively, as obtained by EIS.
For the investigation of impedance plots containing a capacitive semi-circle, the [R(RQ)] circuit was used as shown in Figure 3. 23 According to results, the EIS data are analyzed using the [R(RQ)] equivalent circuit (Figure 3) that includes the solution resistance (R s ), polarization resistance (R p ) and constant phase element (CPE), which can be represented in equation 6: 23 (6)   where Y 0 is the magnitude of the CPE, n is the phase shift, j is the imaginary unit and w is the angular frequency.][22][23] The double layer capacitances (C dl ) for a circuit including a CPE were calculated from the following equation 7: [21][22][23]  (7)   where f max is the frequency at which the imaginary component of the impedance is maximal.The parameters obtained by EIS can be observed in Table 1.
The R ct values observed for EN1 were better than those observed for EN2.Consequently, EN1 is more efficient as an inhibitor than EN2, as shown in Table 1.This increase of R ct values with an increment in the concentration of EN1 and EN2 can be attributed to an increase in the surface coverage on the metal surface by molecules of the β-enaminoester.
The decreased values of C dl indicates a decrease in the local dielectric constant, suggesting that the inhibitor molecules act by adsorbing on the metal/solution interface.
The Bode plots of the evaluated compounds are shown in Figure 4.An increase in the Z mod when the concentration of EN1 and EN2 increases indicates better performance. 27dsorption of the EN1 and EN2 on the metal surface lowered the surface heterogeneities, and as a result, the phase angle increased, approaching 90 o . 17,28,29

LPR
The LPR measurements were performed after the  EIS assays in the presence and absence of the inhibitors using AISI 1020 carbon steel against 0.5 mol L -1 HCl.The obtained data were submitted to the linear regression, and the efficiency values were calculated by equation 2, using the R p values (angular coefficient).The results of corrosion inhibition efficiency obtained by the LPR measurements corroborated the results observed in the EIS assays, where EN1 presented a better performance in the studied concentrations.The R p values and inhibition efficiency values are shown in Table 2. 2 Both studied molecules (EN1 and EN2) presented the same basic structure, where the adsorption on the metallic surface occurs through the enamino portion, but the corrosion inhibition efficiencies observed are different for the two structures.This fact can be associated with the methoxy group being in EN2; this group contains an oxygen atom that can form the hydrogen bond with water molecules, thus approaching the water molecules on the metal surface, increasing the corrosive process.

PP
The experiments were conducted after the LPR measurement at room temperature (25 o C) in 0.5 mol L -1 HCl in the presence and absence of inhibitors.By using the PP technique, it was possible to obtain the current density illustrated by Tafel plots. 3,14In Figure 5, it is possible to observe the polarization curves obtained in the presence and absence of EN1 and EN2.
Table 3 presents the electrochemical parameters obtained by Tafel extrapolation and the corrosion inhibition efficiency.
According to the polarization curves (Figure 5) and Table 3, the presence of inhibitors promotes a decrease in the current densities in cathodic and anodic branches and a slight shift of E corr in the anodic direction.This fact indicates that EN1 and EN2 acted as mixed-type corrosion inhibitors that could retard both metal dissolution and the cathodic process. 3,14,16,23,30hus, when the corrosion inhibition efficiencies for EN1, at the concentration of 1.0 × 10 -2 mol L -1 , were compared using the PP, LPR and EIS techniques, the obtained values, 89, 92 and 90%, respectively, indicated a good correlation.

WL Immersion time
The gravimetric assay was performed only for the inhibitor that showed better efficiency, EN1, in 1.0 × 10 -2 mol L -1 .The weight loss of AISI 1020 carbon steel was determined at 30 o C after 3, 6, 18, and 24 h of  immersion in 0.5 mol L -1 HCl in the presence and absence of the inhibitor.The corrosion rates and the inhibition efficiency were calculated from the mass loss data, and the results are shown in Table 4. 31,32 In Table 4, it is possible to observe the increase of the corrosion efficiency inhibition with the increase of the immersion time.This behavior can be attributed to the interaction of the inhibitor molecules with the metal surface across the formation of a stable and passive film.The lone electron pairs of nitrogen and oxygen atoms, and the π electrons of β-enamine conjugated double bond may be involved in this interaction.][35] The corrosion inhibition efficiency observed for the gravimetric method, 89% after 3 h of immersion time, was similar to the obtained values by electrochemical methods (89, 92 and 90% by PP, LPR and EIS, respectively), for EN1 at the same concentration, showing, thus, a good correlation.

Effect of temperature
The working temperature can influence the corrosion inhibition efficiency, and generally the corrosion rate  increases with an increase of the temperature; this fact is due to the accelerated evolution of H 2 and, consequently, higher dissolution of the metal. 36herefore, the effect of temperature was analyzed at 303, 313, 323, and 333 K, only for the inhibitor that showed better efficiency, EN1, at 1.0 × 10 -2 mol L -1 during 3 h of immersion in the presence of 0.5 mol L -1 HCl solution.The inhibition efficiencies (η (%)) at different temperatures can be seen in Table 5.
Stability in the efficiency of inhibition of EN1 was observed in temperatures of 303, 313, and 323 K, characterizing this compound as an efficient and stable corrosion inhibitor for AISI 1020 carbon steel, in 0.5 mol L -1 HCl solution, at different temperatures.One decrease of only 3% of the inhibition efficiency was observed at 333 K.
The energy barrier for the corrosion reaction is an important thermodynamic parameter that should be observed when studying corrosion inhibitors and can be calculated using the temperature variation.This energy value is represented by the apparent activation energy (E a ).In this work, the E a was calculated for AISI 1020 carbon steel corrosion in the presence and absence of EN1 in acid solution, using an Arrhenius plot in accordance with equation 8: 36,37 (8) where lnW corr is the corrosion rate, E a is the apparent activation energy, A is the frequency factor, T is the absolute temperature and R is the molar gas constant.][39] Table 6 shows the correlation coefficient obtained with a linear regression of the Arrhenius plot (lnW corr vs. 1 / T) and the E a obtained according to equation 6, both in the presence and absence of an inhibitor.
As observed in Table 6, the E a is higher in the presence of EN1 when compared with that of the blank solution.[41]

Adsorption isotherm
The efficiency of an organic inhibitor relies on its ability to be adsorbed onto the metal surface.The interaction between the inhibitor and the metal surface can be evaluated by adsorption isotherms. 42n the present work, attempts were made to fit these values as a function of concentration of EN1 and EN2, employing three isotherms, including Langmuir, Temkin and Frumkin, and the correlation coefficients were evaluated to choose the more adjusted model. 38The corresponding mathematical equations are shown below, and Figure 7 shows the application of the three isotherm models to the study of the adsorption of β-enaminoesters on the carbon steel surface.Langmuir: (9)   Temkin:  Frumkin: (11)   where C is the inhibitor concentration, K ads is the equilibrium constant of the adsorption process, θ is the degree of surface coverage, and a is the interaction parameter.Table 7 shows the values of the parameters of the linearized Langmuir, Temkin and Frumkin adsorption isotherms for EN1 and EN2.
Table 7 shows the correlation coefficient values of three adsorption isotherm models and their slopes.Better correlation coefficients for both compounds (EN1 and EN2) were obtained by the Langmuir isotherm.However, the slope observed for the studied compounds differed by approximately 0.1, as expected from the Langmuir adsorption model, suggesting that each molecule of EN1 and EN2 occupies approximately 1.09 and 1.22 adsorption sites of the metal surface, respectively.Thus, a modified Langmuir adsorption isotherm chosen in accordance with the literature, which is given by equation 12. 38-41,43-47 (12) where n is a correction parameter of slope.The linear coefficient allowed the evaluation K ads and the free energy of adsorption (DG 0 ads ) calculated by equation 13.   (13)   where R is the universal gas constant (J K -1 mol -1 ), T is the temperature (K), and 55.55 is the molar concentration (mol L -1 ) of water in the solution.
The K ads and DG 0 ads values and the modified adsorption Langmuir isotherm parameters are shown in Table 8.
The high values of K ads are indicative of a high adsorption percentage of inhibitor molecules on the metal surface, and the negative values of DG 0 ads indicate a higher spontaneous corrosion inhibition process for these compounds. 46The DG 0 ads values vary from -34.18 to -32.55 kJ mol -1 , indicating that EN1 and EN2 adsorbed onto carbon steel surface via mixed mechanisms. 48,49

SEM
The SEM images of inhibited (EN1) and uninhibited (blank) AISI 1020 carbon steel coupons after immersion in 0.5 mol L -1 HCl for 24 h are shown in Figure 8.The SEM image of the blank, in the absence of an inhibitor, is highly damaged; however, in the presence EN1, the surface is smoother.
Further, the image of carbon steel in the absence of an inhibitor (blank) indicated the presence of white flakes, which could be due to oxidation products that form as result of corrosion processes on the metal surface. 15,50When EN1 was utilized, the presence of flakes was minimized, and it is possible to observe the grooves caused by using emery paper after the preparation of the metallic surface, confirming that EN1 acts as an efficient inhibitor of corrosion because the metal surface was preserved.

Inhibition mechanism
Metallic corrosion is a consequence of the formation of an electric double layer on the metal surface caused by water molecules and chloride ions when in HCl medium.The use of the corrosion inhibitor is very important due to the interaction of the inhibitor/surface, which results in the formation of a protective layer that decreases the double layer, thus retarding the corrosive process.The adsorption process can be physical or chemical, in accordance with the inhibitor structure.Inhibitors that contain heteroatoms are absorbed by chemisorption, donating electrons to metal, but compounds that have π bonds and/or its heteroatoms protonated are adsorbed by physisorption. 22,51igure 9 illustrates a corrosive medium in the absence and presence of an inhibitor.It can be observed that in the absence of an inhibitor flawed regions are formed because of the attachment by chloride and hydronium ions.After an ideal concentration of an inhibitor is added, the inhibitor molecules form a protective barrier that prevents the approach of corrosive particles, such as chloride and hydronium ions, thus preserving the metal surface. 22,51

Conclusions
Both the inhibitors showed a good efficiency of anticorrosive action for AISI 1020 carbon steel in 0.5 mol L -1 HCl, and their efficiency increased with concentration.The best inhibitor was EN1 at a concentration of 1.0 × 10 -2 mol L -1 .Good correlations were observed between the results obtained for PP, LPR, EIS and WL.The results obtained from the polarization curves suggested that EN1 and EN2 act as mixed inhibitors that reduce both the anodic and cathodic current densities.The maximum corrosion inhibitor efficiency for EN1 (1.0 × 10 -2 mol L -1 ) was 98%, observed after 24 h of immersion in the presence of 0.5 mol L -1 HCl.The surface coverage obtained from the EIS measurements fits well with the modified Langmuir   adsorption isotherm, with correlation coefficients and slopes near unity.The SEM analysis showed that the metal surface was protected by the use of EN1 as a corrosion inhibitor.

Figure 2 .
Figure 2. Nyquist plots obtained in absence and presence of EN1 and EN2, against AISI 1020 carbon steel in 0.5 mol L -1 HCl.

Figure 3 .
Figure 3. Equivalent circuit used to fit the impedance spectra in the absence and presence of the inhibitors EN1 and EN2.

Figure 4 .
Figure 4. Bode impedance plots for the carbon steel obtained in 0.5 mol L -1 HCl in the presence and absence of EN1 and EN2.

Figure 5 .
Figure 5. Polarization curves of carbon steel in 0.5 mol L -1 HCl in the presence and absence of EN1 and EN2.

Table 2 .
LPR parameters for AISI 1020 carbon steel in 0.5 mol L -1 HCl solution in the absence and presence of EN1 and EN2

Table 4 .
Weight loss data for AISI 1020 carbon steel in 0.5 mol L -1 HCl solution in the absence and presence of EN1 at 30 o C a Weight loss in the absence of inhibitor; b weight loss in the presence of EN1; c inhibition efficiency.EN1: ethyl-(2Z)-3-[(2-phenylethyl)amino] but-2-enoate.

Table 5 .
Weight loss data for AISI 1020 carbon steel in 0.5 mol L -1 HCl solution in the absence and presence of EN1 at 303, 313, 323, 333 K

Table 6 .
Linear regression parameters obtained from Arrhenius plots

Table 7 .
Values of parameters of linearized Langmuir, Temkin and Frumkin adsorption isotherms for EN1 and EN2 on AISI 1020 carbon steel in 0.5 mol L -1 HCl