Synthesis, characterization and corrosion inhibition efficiency of N-(4-(Morpholinomethyl Carbamoyl Phenyl) Furan-2-Carboxamide for brass in HCl medium

2.1 Synthesis of CFC

N-(4-Carbamoylphenyl)Furan-2-Carboxamide (CFC) was prepared by a procedure similar to the method reported in the literature (Jumade et al., 2009). 4 -Amino benzamide (3.00 g, 0.0220 mol) and 2-furoyl chloride (3.428 g, 0.0264 mol) were dissolved in mixture of MDC (70 ml) and THF (25 ml). Triethylamine (7.77 g) was added, and the mixture was stirred in the presence of nitrogen atmosphere for 24 h. The reaction mixture was washed with water, filtered and dried over high vacuum pump. The CFC was characterized by spectral techniques such as FT-IR, NMR, and LC-MS. Fig. 1 represents the synthesis of CFC.

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Figure 1

Synthesis of CFC.

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2.2 Characterization of CFC

Yield 93%, white solid, m.p.186–190 °C., IR (KBr, νmax cm⁻¹; 3387, 3179 (NH, St, Amide), 1658 (C⚌O), 1617 (NH, Bend, Amide), 1400 (CN, Amide), 1474, 1527 (C⚌C), 1179 (C—O, furan), 841 (CH, Ar, oop). ¹H NMR (400 MHz, DMSO-d₆) d(ppm): 6.71(1H, Furan), 7.36 (1H, furan), 7.80–7.84 (4H), 7.85–7.87 (2H, Amide), 7.95 (1H, furan), 10.36 (1H, Amide). ¹³C NMR (400 MHz, DMSO-d₆) d(ppm): δ 112.1, 115.0, 119.2, 128.1, 129.1, 141.1, 145.0, 147.2, 156.2 (C⚌O), 167.3 (C⚌O). MS (EI): m/z (%) = 231. Figs. 2a, 2b, 2c, 2d represent the FT-IR, ¹³C, ¹H and LC-MS of CFC respectively.

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Figure 2a

FT-IR spectrum of CFC.

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Figure 2b

¹³C spectrum of CFC.

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Figure 2c

¹H spectrum of CFC.

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Figure 2d

LC-MS spectrum of CFC.

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2.3 Synthesis of MFC

The mixture of CFC (0.0130 mol, 3 g), morpholine (0.0130 mol, 1.1363 g), and formaldehyde (0.01956 mol, 0.587 g) was dissolved in ethanol. The reaction mixture was refluxed for 48 h at 80 °C. The white solid obtained was filtered, washed with cold ethanol and followed by petroleum ether. The resulted mass is dried and recrystallized from ethanol (Nagham et al., 2013; Karthik et al., 2011). The MFC was characterized by spectral techniques such as FT-IR, NMR, and LC-MS. Fig. 3 represents the synthesis of MFC.

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Figure 3

Synthesis of MFC.

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2.4 Characterization of MFC

Yield 90%, white Solid, m.p.176–180 °C., IR (KBr, νmax cm⁻¹; 3305 (NH), 3177 (CH, Ar), 2958, 2843 (CH, Aliph), 1666 (C⚌O), 1646 (NH, bend), 1548, 1472 (C⚌C), 1304 (C—N, amide), 1183 (C—O, furan) 1115 (C—O, morpholine), 1023 (C—N, Amine), 852 (CH, Ar, oop). ¹H NMR (400 MHz, DMSO-d₆) d(ppm): 2.50 (4H), 3.55 (4H), 4.15 (2H), 6.72 (1H, furan), 7.39 (1H, furan), 7.83–7.89 (4H), 7.96 (1H, furan), 8.72(1H, Amide), 10.40 (1H, Amide). ¹³C NMR (400 MHz, DMSO-d₆) d(ppm): □ 50.62, 61.23, 66.5, 112.7, 115.6, 119.9, 128.7, 129.6, 141.7, 146.4, 147.7, 156.8 (C⚌O), 166.9 (C⚌O). MS (EI): m/z (%) = 329.87. Figs. 3a, 3b, 3c, 3d represent the FT-IR, ¹³C, ¹H and LC-MS of MFC respectively.

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Figure 3a

FT-IR spectrum of MFC.

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Figure 3b

¹³C spectrum of MFC.

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Figure 3c

¹H spectrum of MFC.

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Figure 3d

LC-MS spectrum of MFC.

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2.5 Medium

The solubility of MFC was found to be 0.5 g in 100 ml of 1 M HCl. The standard solution of 1 M hydrochloric acid was prepared using double distilled water. The experiments were carried out in the presence and absence of MFC at various concentration ranges from 0.3 mM to 2.1 mM, and temperature ranges from 30 °C to 60 °C.

2.6 Material

Brass with the composition of Cu (60.66%), Zn (36.58%), Sn (1.02%) and Fe (1.74%) was used and exposed geometrical area was 7.6 cm². The specimens were abraded using emery papers of different grades and cleaned with double distilled water, acetone and finally dried (Yohai et al., 2011).

2.7 Weight loss method

The Weight loss method was carried out according to the method described previously (Jamal Abdul Nasser et al., 2011, 2012). The Weight loss method was performed from 30 °C to 60 °C for 2 h by immersing the brass coupons in 1 M hydrochloric acid solution in the presence and absence of inhibitor. After the immersion time, the specimens were taken out, washed with double distilled water dried and weighed accurately (Laamari et al., 2011, 2012). The entire test was performed in triplicate, and the average values were reported. The following equation determined the inhibition efficiency (I.E.) and surface coverage (θ).

2.8 Electrochemical measurement

The electrochemical experiments were performed in a conventional three-electrode system in which saturated calomel electrode (SCE) was used as a reference electrode, platinum sheet as a counter electrode and brass coupons of 1.0 cm² area exposed as working electrode (Zhang et al., 2008). Experiments were carried out in the presence and absence of inhibitor. In electrochemical measurements, a stabilization period of one hour was allowed, which is enough to attain stable E_corr value.

2.9 Potentiodynamic polarization

Polarization studies were carried out in Electrochemical Workstation Model 600 D/E Series. Both cathodic and anodic polarization curves were recorded. Tafel polarization curves were obtained by changing the electrode potential value from ±0.200 V at open circuit potential with a scan rate of 1.0 mV S⁻¹. Corrosion parameters such as corrosion potential (E_corr), corrosion current (i_corr) and Tafel slopes both cathodic b_c and anodic b_a were calculated. The inhibition efficiency (IE) and corrosion rate (CR) were calculated by using the formulae

3.1 Scanning Electron Microscope (SEM)

The brass specimen was polished with various grades of emery sheet, rinsed with distilled water, degreased with acetone, dried and then immersed in 1 M hydrochloric acid in the presence and absence of MFC for 2 h. The surface morphology of brass was recorded using a scanning electron microscope.

4.1 Weight loss method

The effect of concentration of MFC on the corrosion of brass in 1 M HCl was shown in Table 1. It has been observed that the inhibition efficiency (IE) of MFC increases with an increase in the concentration of inhibitor and temperature (Sharma et al., 2011). The increase in inhibition efficiency was due to the blocking effect on the surface of the metal by adsorption, film formation mechanism and also due to the presence of protonated nitrogen and the oxygen atom of MFC. This makes it absorbs quickly on the surface of brass forming an insoluble, stable film. The greater recital of MFC was due to the presence of amide moiety and one morpholine ring.

4.2 Tafel polarization measurements

The potentiodynamic polarization curves of brass in 1 M HCl in the absence and presence of various concentrations of MFC at 60 °C is shown in Fig. 4 The values of corrosion potential (E_corr), corrosion current density (i_corr), anodic (b_a) and cathodic (b_c) Tafel slopes were evaluated by Tafel extrapolation method. The value of corrosion rate of brass decreases as the concentration of MFC increases with respect to temperature. The presence of inhibitor decreases the corrosion rate and i_corr prominently with an increase in the concentration of inhibitor associated with a shift of corrosion potential (E_corr) to more positive (Khaled et al., 2012, 2014). Further, the inhibition efficiency of MFC increases with an increase in concentration and temperature. This is due to physisorption of an inhibitor molecule adsorbed at low temperature, which is changed to chemisorptions at a higher temperature. The maximum inhibition efficiency of MFC was found to be 79.43% in 2.1 mM at 60 °C.

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Figure 4

Potentiodynamic polarization curves of MFC for brass in 1 M HCl at 60 °C.

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From Table 2 it was observed that the addition of MFC shows a positive shift in E_corr value. It was reported that if the corrosion potential shift exceeds with ±85 mV with respect to the corrosion potential of the uninhibited solution, the inhibitor acts as either anodic or cathodic type. In the present study E_corr vary within ±50 mV, which indicates that MFC acts as a mixed type inhibitor and it inhibits both cathodic reaction (hydrogen evolution) and anodic reaction (metal dissolution) (Singh, 2012). It was observed that no specific variations obtained for cathodic Tafel slope and anodic Tafel slope which indicates that MFC was first adsorbed on the surface of the metal by blocking the reaction sites of the metal surface without altering the cathodic and anodic reaction mechanism. Both cathodic and anodic curves show lower current density in the presence of inhibitor than that of HCl solution. The result obtained in this method was a good agreement with conventional weight loss method.

4.3 Electrochemical impedance spectroscopy

The impedance behavior of brass in 1 M HCl was studied in the presence of MFC, and the results were compared with Tafel polarization experiments. A Nyquist plot was recorded for the inhibition of brass with various concentrations of MFC at 60 °C in 1 M hydrochloric acid as is shown in Fig. 5. The Nyquist plots obtained with MFC show only one capacitive loop with high frequency due to charge transfer resistance and time constant of the electrical double layer and the diameter of semicircle increases with an increase in the concentration of inhibitor, which indicates that the presence of inhibitor strengthens the inhibitive film (Benabdellah et al., 2011a,b). All the Nyquist plots show a semicircle and they were fixed using one time constant equivalent model (Randle’s model) with charge transfer resistance (Rct), double layer capacitance (Cdl) and corrosion current (i_corr).

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Figure 5

AC impedance curves of MFC for brass in 1 M HCl at 60 °C.

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A simple Randle’s equivalent circuit was shown in Fig. 6a and b in the absence and presence of MFC to fit the Nyquist plot in 1 M hydrochloric acid solution. The circuit contains a solution resistance (Rs), charge transfer resistance (Rct), one constant phase element (CPE) and Warburg impedance (W) for brass in 1 M hydrochloric acid. In the presence of MFC a constant phase element CPE1 was assigned to explain the heterogeneity of the system (Quartarone et al., 2008; Sudeshna et al., 2009).

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Figure 6

a and b Equivalent circuit for brass in 1 M HCl in the absence and presence of MFC at 60 °C.

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It could be seen from the Table 3 that the Cdl value decreases as the concentration of inhibitor increases. The decrease in Cdl value is due to increase in an electrical double layer on the surface of the metal solution. It implies that the inhibitor undergoes adsorption on the surface of the metal with dissolution. The value of Rct (charge transfer resistance) increases with an increase in the concentration of inhibitor, which indicates that the charge transfer process was mainly controlled by corrosion.

4.4 Cyclic voltammetric studies

The cyclic voltammogram for brass with and without inhibitor was shown in Fig. 7. It can be seen that bare brass shows two oxidation peaks at the forward scan at 0.198 V (SCE) and 0.223 V (SCE). The first peak is attributed to the formation of CuCl₂⁻_(aq) and the second peak is the further oxidation of Cu⁺ to either to Cu²⁺ or due to the formation of the insoluble Cu₂O. In the reverse sweep, there is only one large reduction peak occurring at −0.401 V (SCE) which is due to the reduction of Cu²⁺ formed during the oxidation process.

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Figure 7

CV: for brass in 1 M HCl in the absence and presence MFC at 60 °C.

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The cyclic voltammogram as shown in Fig. 7 also shows the effect of the addition of the various concentrations of the inhibitor, and it is interesting to note that two main changes have occurred with the addition of the inhibitor. First one exhibits only one peak for brass in both forward as well as reverse sweep at around −0.12 V(SCE) for the forward scan and +0.214 V(SCE) for the reverse sweep. The reduction in the Volt is attributed to adsorption of the inhibitor on the brass surface. The Second change is the reduction of the oxidation and reduction peak, which diminishes drastically with the addition of the inhibitor. This observation indicates that the inhibitor added to the solution is adsorbed on the brass surface effectively and reduces the oxidation of the copper in the brass.

Inhibition of corrosion and initial characterization of inhibitor under investigation was done by carrying out cyclic voltammograms of brass with various concentrations of the inhibitor at voltage range of −1.2 V to 1 V and scan rate was 0.05 V (Fig. 7). The range was fixed to take into account the oxidation and reduction potential of Zn and copper ions in various oxidation states as reported by researchers (El-Sayed Sherif et al., 2008; Du et al., 2012).

4.5 Mechanism of corrosion

The mechanism of the corrosion of copper in brass in aqueous solution and the main reaction that can take place in the acidic medium is given below (Chen et al., 2012; Sudheer and Quarishi, 2013; Akabueze and Itodo., 2012)

CuCl₂⁻ may be further oxidized in the acid medium to copper (II)

4.6 Dezincification factor

Dezincification factor is studied by analyzing the amount of copper and zinc ion present in the solution from a weight loss method using atomic absorption spectroscopy (Elico-India). Dezincification factor is defined by the following equation (Bag et al., 1996) where the concentration of zinc and copper is in ppm.

The result of the dezincification in 1 M HCl and the optimum concentration of the inhibitor (700 ppm) are presented in Table 4. It is evident from the results that both the copper and zinc are leached into the solution. The ratio of copper to zinc in the solution was found to be much smaller than in the alloy.

This indicates that diffusion controls the dissolution of the alloy and the related growth of the surface film. Copper is less leached in the solution than zinc, because E^o_(cu2+/Cu) for copper is positive with a value of +0.34 V against the value for zinc whose E^o_(Zn2+/Zn) is −0.76 and also diffusion depends on the size of the ion, and zinc (II) ion having an atomic radius of 0.074 nm diffuses faster than the copper (II) ion which has atomic radius of 0.096 nm. With the addition of the inhibitor, the results indicate both copper leaching and zinc leaching are minimized, but the percent inhibition of the zinc is found to be much higher than the copper. The dezincification factor with the inhibitor is 30.07 compared to the factor with 1 M HCl, which is 59.65. The addition of inhibitor has reduced the dezincification factor of 29.58. The results indicate that the inhibitor at optimum concentration effectively inhibits the dezincification of brass in 1 M HCl.

4.7 Adsorption isotherm

Various adsorption isotherms such as Langmuir, Freundlich, Temkin, Hill de Boer, Flory–Huggins, Frumkin, Dhar-Flory Huggins, Parsons and Bockris‐Swinkels isotherms were used, which are most frequently used for determination of the adsorption studies follows the general formula

4.8 Langmuir adsorption isotherm

The adsorption of MFC on the brass surface is modeled with Langmuir isotherm equation

Fig. 8 shows the plot of C/θ vs. C for various temperatures and the values of R² and slope are listed in the Table 5 used to describe the adsorption process that is based on three assumptions. 1. There is a monolayer adsorption, and it cannot proceed beyond monolayer. 2. All the adsorption sites are equal denoting the probability of adsorption is same everywhere. 3. There is no interaction between the adsorbing molecule and the adsorption process is independent of the neighboring occupied sites (Abdel-Rehim et al., 2011; Kharchouf et al., 2011). The slope of the straight line obtained from the isotherm for all the temperatures is nearly 1.12, greater than 1 which suggests that 1 molecule of MFC occupies more than 1 site approximately 1.12 adsorption site on the brass surface.

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Figure 8

Plot of C/θ vs. C for various temperature with MFC.

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The Surface coverage θ gives valuable information about the corrosion inhibition mechanism of the inhibitory molecules on the brass alloy. Tafel polarization method is employed in this work to find the surface coverage θ at different inhibitor concentrations. Surface coverage (θ) can be calculated from the following equation. Where Vo is, the corrosion rate without inhibitor and V is the corrosion rate with inhibitor. The value of θ is used to fit the different isotherm and the isotherm best described by the adsorption process at each temperature is determined.

By using Eq. (13) ΔG^∘_ads is calculated at 60 °C and it was found to be −33.14 kJ/mol. The High value of K_ads and the negative sign of ΔG^∘_ads indicate that MFC is strongly adsorbed on the surface of the brass. The chemisorption of MFC on the brass surface can also be inferred from the value of ΔG^∘_ads. Normally a value of ΔG^∘_ads around −20 kJ/mol indicates that physisorption due to the Vander walls attraction between the alloy and the inhibitor is the main process, but when the value of ΔG^∘_ads becomes −30 kJ/mol or more negative then chemisorptions would dominate.

From Table 5 it is inferred that with an increase in temperature the value of ΔG^∘_ads becomes more and more negative which indicates that chemisorptions dominate at high temperature. Coordinate bonding between the metal surface and the MFC molecule leads to the higher adsorption of the inhibitor at a higher temperature.

4.9 Effect of temperature

The effect of temperature on the corrosion rate of brass in 1 M hydrochloric acid and the inhibition efficiency of MFC was studied at different temperature ranges from 30 °C to 60 °C using potentiodynamic polarization studies. The temperature has a diverse effect on the corrosion of the Brass in an acid medium. With the increase in temperature corrosion increases in metal without the inhibitor. With inhibitor, the corrosion may increase or decreases depending upon the inhibitor used. Corrosion rate increases with an increase in the temperature in both blank and also with inhibitor. The temperature effect on the corrosion is multifaceted, with many changes occurring on the surface that includes rapid etching of the metal, chemisorption of the inhibitor, and there may be decomposition of inhibitor, rearrangement of inhibitor or rapid desorption of inhibitor on the metal surface (Saliyan and Adhikari, 2009). Corrosion rate normally increases with an increase in the temperature, when there are chemisorptions taking place between the inhibitor and in the brass the inhibition efficiency increases with an increase in temperature.

There is an equilibrium that exists between the inhibitor and the brass at a particular temperature when the temperature is changed the equilibrium shifts, and there is a new equilibrium established with different K value. At lower temperature physisorption dominates when the temperature increase there is an increase in the chemisorptions of MFC on the brass surface.

Arrhenius equation and transition state equation are used for the calculation of the activation energy, enthalpy of adsorption and entropy of adsorption. The Arrhenius and transition state plots are shown in Fig. 9.

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Figure 9

The Arrhenius plot of log I_corr vs. 1/T 10⁻³ for the effect of temperature on the performance of MFC on brass in 1 M HCl.

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The plot of log (I_corr) Vs 1/(T × 10⁻³ K) for blank and various concentrations of inhibitors are shown in Fig. 10. A straight line was obtained. E_a is calculated from the slope of the line, and the results are tabulated in Table 6. It is seen that E_a decreases with an increase in temperature. The MFC obeys Radovici classification on the Brass. Radovici classified the inhibitors into three groups according to the temperature effects.

1. The value of the activation energy is greater in the inhibitor solution than in the blank; then the inhibition efficiency (I.E.) of the inhibitor decreases with an increase in temperature.

2. The value of activation energy is equal to both the inhibitor solution and blank; then the inhibition efficiency (I.E.) does not change with the temperature.

3. The value of activation energy is less in the inhibitor solution than in the blank; then the inhibition efficiency (I.E.) of the inhibitor increases with an increase in temperature. MFC showed activation energy lesser in the inhibited solution than the activation energy of the uninhibited solution and hence with an increase in temperature the inhibition efficiency increases.

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Figure 10

Transition state plot for brass corrosion with and without MFC in 1 M HCl at 60 °C.

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MFC forms a chemical bond with the metal surface at high temperature. Chemisorptions of the MFC is explained by the formation of coordinate bond between the hetero atom (Nitrogen, oxygen atom) present in the inhibitor with d orbital of the metal especially copper thereby reducing the corrosion of the Brass in HCl (Hamdy Hassan et al., 2007). With the increase in temperature, there is a considerable increase in the adsorption of the MFC on the brass surface which decreases the activation energy. With the increase in a temperature increase in the chemisorptions leads to the considerable higher amount of adsorbed molecule on the surface, so the equilibrium shifts to more of adsorption and less of desorption. As a result, the lesser surface of brass is exposed to HCl and hence there is a decrease in the corrosion of the alloy in the solution.

Table 6 shows that at the inhibitor concentration of 700 ppm the E_a value is the lowest which is also the optimum condition of inhibitor concentration. The decreasing values of E_a with an increase in temperature clearly show that there is a chemical adsorption on the brass surface and MFC molecule. Arrhenius frequency factor also reduces with increase in temperature that can be referred to the decrease in the rate of corrosion of brass.

The plot of log (I_corr/T) Vs 1000/T was shown in Fig. 10. A straight line is obtained and from the slope of the line ΔH^∘ and ΔS^∘ are calculated from the slope and the intercept and the values are given in Table 6. ΔH^∘ and ΔS^∘ values at optimum condition of inhibitor in 1 M HCl on the brass surface (21.39 kJ/mol and −178.47 J/(mol K)) are less than the values in the absence of inhibitor (29.68 kJ/mol and −140.73 J/(mol K)). A negative value of the entropy suggests that there is higher adsorption of the inhibitor on the surface of the brass compared to the water molecule replacement on the brass surface. The positive value of ΔH^∘ also suggests that adsorption is endothermic nature.

5.1 Quantum chemical calculations

In this present work, Quantum chemical calculations were performed to explain the relationship between the molecular structure and the inhibition action of the inhibitor. According to Fukui’s frontier molecular orbital theory the ability of inhibitor is related to the frontier molecular orbital- highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and dipole moment (I). Based on this theory the formation of the transition state is due to the interaction between the frontier orbitals HOMO and LUMO of the reactants. E_HOMO is related to the electron donating ability of the molecule. If the value of E_HOMO is high, then the molecule has a tendency to donate electrons to appropriate acceptor molecules with low energy empty molecular orbitals thereby increasing the efficiency of the inhibitor. If the value of E_LUMO is low, then the molecule has the tendency to accept electrons. Moreover, the energy gap between E_HOMO and E_LUMO of the molecule is used to develop the theoretical models that are capable of explaining the structure and conformation in a molecular system. It has been observed that smaller the value of energy gap, higher the inhibition efficiency due to the adsorption and polarization of the molecule on the metal surface (Obaid et al., 2013).

The computed quantum parameters such as E_HOMO, E_LUMO, ΔE, Mulliken charge and dipole moments are shown in Table 7. It can be seen from the table that MFC has higher E_HOMO value (−0.06277) which indicates that it has the tendency to donate electrons to an appropriate acceptor molecule, whereas the value of E_LUMO is less (−0.02824) indicates that it accepts electrons. Subsequently the value of energy band gap is low (0.03453) which indicates that the stability of MFC on the surface of metal improves the corrosion resistance of brass in 1 M HCl. The dipole moment (I) of MFC is 4.9349 which is greater than water (1.88 Debye), reveals that there is a strong dipole–dipole interaction between MFC and the metal surface (Li et al., 2014).

Fig. 12a, (b) and (c) represent the optimized molecular structure, HOMO and LUMO of MFC. Frontier orbital theory is also useful in predicting the adsorption centers of inhibitor molecules responsible for the interaction on the surface of metal atoms. In Fig. 12b the distribution of orbital is even in (LUMO) which indicates that the amide moiety and the morpholine ring present in MFC are responsible for the inhibition. This is confirmed by the Mulliken charge analysis. The Mulliken charge analysis is used to calculate the adsorption center of inhibitor, and it was observed that, if the heteroatom has a more negative charge then adsorption is more on the surface of the metal. Mulliken charges for MFC were shown in Table 8. From the table it was inferred that the Mulliken charge of hetero atom (N) is more negative, indicates that the adsorption is due to the electron donation from an electronegative atom (N) to the metal surface.

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Figure 12a

Optimized molecular structure of MFC.

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Figure 12

(b) and (c) Frontier molecular orbital density distribution of MFC.

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