Comparative study of N-[(4-methoxyphenyl) (morpholin-4-yl)methyl]acetamide (MMPA) and N-[morpholin-4-yl(phenyl)methyl]acetamide (MPA) as corrosion inhibitors for mild steel in sulfuric acid solution

2.1 Materials

Mild steel strips with the composition carbon = 0.07%; sulfur = 0%; phosphorus = 0.008%; silicon = 0%; manganese = 0.34% and iron = remainder and size of 4 × 1 × 0.025 cm were used for weight loss and effect of temperature studies. Mild steel cylindrical rods of the same composition embedded in polytetrafluoroethylene (PTFE) with an exposed area of 1 cm² were used for potentiodynamic polarization and impedance measurements. The electrode was polished using a sequence of emery papers of different grades and then degreased with acetone. N-[(4-methoxyphenyl)(morpholin-4-yl)methyl]acetamide (MMPA) and N-[morpholin-4-yl(phenyl)methyl]acetamide were synthesized (Raman et al., 2004), and purified by recrystallization from ethanol to analytical purity grade. Its purity was confirmed by elemental analysis and characterized by FT-IR and NMR spectroscopy. The name and molecular structure of studied Mannich bases are given in Fig. 1.

Close

Figure 1

Name and molecular structure of studied Mannich bases.

Export to PPT

The acid solution (1.0 M H₂SO₄) was prepared by the dilution of an analytical grade H₂SO₄ with double distilled water. All tests were conducted at different temperatures in magnetically stirred solutions.

2.2 Weight loss measurements

Weight loss measurements were done according to the method described previously (Jamal Abdul Nasser, 2005; Jamal Abdul Nasser and Anwar Sathiq, 2011). Weight loss measurements were performed at 303 ± 1 K for 2 h by immersing the mild steel coupons into acid solution (100 mL) without and with various amounts of inhibitor. After the elapsed time, the specimens were taken out, washed, dried, and weighed accurately. All the tests were performed in triplicate and the average values were reported. All the concentrations of an inhibitor for weight loss and electrochemical study were taken in M.

The surface coverage (θ) and inhibition efficiency (IE%) were determined by using the following equation:

2.3 Effect of temperature

The loss in weights was calculated at different temperatures from 303 to 333 K. Each experiment was duplicated to get good reproducibility. Weight loss measurements were performed in 1.0 M H₂SO₄ with and without the addition of the inhibitor at their best inhibiting concentrations. Percentage inhibitions of the inhibitor at various temperatures were calculated.

2.4 Potentiodynamic polarization measurements

Both cathodic and anodic polarization curves were recorded (mVs⁻¹) using the corrosion measurement system BAS (Model: 100 A) computerized electrochemical analyzer and PL-10 digital Plotter. A platinum foil and Hg| Hg₂SO₄| 1.0 M H₂SO₄ electrode was used as auxiliary and reference electrodes respectively. The Tafel polarization curves were obtained by changing the electrode potential automatically from ± 0.200 V at open circuit potential with a scan rate 1.0 mV s⁻¹.

2.5 Electrochemical impedance measurements

Electrochemical measurements were carried out in the conventional three-electrode, cylindrical Pyrex glass cell with a capacity of 1000 mL. A saturated calomel electrode (SCE) and a platinum disc electrode were used, respectively, as reference and auxiliary electrodes. The temperature was controlled at 303 K by a thermostat. The working electrode was abraded with silicon carbide paper (grade P1200), degreased with AR grade ethanol and acetone, and rinsed with double distilled water before use. All potentials are reported versus SCE. EIS measurements were carried out in a frequency range of 100,000–0.01 Hz with an amplitude of 10 mV using AC signals at open circuit potential.

2.6 Standard free energy of adsorption

The standard adsorption free energy changes in 0.01 M concentration (best inhibition) of MMPA and MPA at different temperatures (313 K to 333 K) were calculated (Quraishi et al., 2010) using the equation ΔG⁰ = −RT ln (K 55.5) kJ/mole, Where ΔG⁰ = adsorption free energy, R = Gas Constant T = Temperature and K = Adsorptive equilibrium constant. The value of 55.5 is the concentration of water in solution expressed in mol L⁻¹. The value of K was calculated from K = θ/C (1−θ). Where θ = Surface coverage (Inhibition efficiency/100) and C = inhibitor concentration.

2.7 SEM analysis

The specimens used for surface morphological examination were immersed in acid containing various concentrations of inhibitor and blank for 2 h. Then they were removed, rinsed quickly with rectified spirit, and dried. The analysis was performed on HITACHI-model S-3000 H SEM.

2.8 FT-IR spectroscopy

The percentage transmission is recorded against wave number. The mild steel specimens were immersed in various test solutions for a period of 2 h. After 2 h, the specimens were taken out and dried. The surface film was scratched carefully and its FT-IR spectra were recorded using Perkin–Elmer make model spectrum RXI instrument.

3.1 Weight loss measurements

The effect of concentration of MMPA and MPA on the corrosion of mild steel in 1.0 M H₂SO₄ is given in Table 1. The corrosion rate decreased considerably with an increase in concentration of both the inhibitors and reached the minimum value in the range of 0.1 M concentrations. It is obvious that inhibition efficiency values for the two tested Mannich bases increases with the increase in inhibitor concentration, and this increase in the inhibition efficiency, at a given inhibitor concentration, enhances in the following order: MPA < MMPA. The greater inhibitive power of MMPA is due to the presence of electron donating p-OCH₃ group that increases the electron density of the phenyl ring. The inhibition action of Mannich bases can be explained by considering the following mechanism: Fe (Inh)_ads, reaction intermediates (Bockris and Drazic, 1962): Fe + Inh ↔ Fe (Inh)_ads ↔ Feⁿ⁺ + ne⁻ + Inh. At first, when there is not enough Fe (Inh)_ads to cover the metal surface, because the inhibitor concentration is low or because the adsorption rate is slow, metal dissolution takes place in sites on the mild steel surface free of Fe (Inh)_ads. With high inhibitor concentration, a compact and coherent inhibitor over the film is formed on the mild steel which reduces chemical attacks on the metal.

3.2 Effect of temperature

The effect of temperature on inhibition reaction is highly complex, because many changes may occur on the metal surface such as rapid etching, rupture, desorption of inhibitor and the decomposition and/or rearrangement of inhibitor (Ramesh Saliyan and Vasudeva Adhikari, 2008). The effect of temperature on the corrosion inhibition with and without inhibitor is shown in Table 2. It can be seen that the weight loss increases with temperature in the absence and presence of inhibitor. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between two processes at a particular temperature. With the increase of temperature, the equilibrium between the adsorption and desorption processes is shifted leading to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant. It explains the lower inhibition efficiency at higher temperatures (Chaudhary et al., 2007: Sanyal and Kumar, 2010).

In order to calculate activation parameters for the corrosion process, Arrhenius Eq. (3) and transition state Eq. (4) were used (Quraishi et al., 2010).

The apparent activation energy (E_a) at the optimum concentration of MMPA and MPA determined by linear regression between log (CR) and 1000/T in 1 M H₂SO₄ medium has been shown in Fig. 2. The results are shown in Table 3.

Close

Figure 2

The Arrhenius plots of log Rate vs 1000/T for the effect of temperature on the performance of MMPA and MPA at the optimum inhibition concentration of 0.01 M.

Export to PPT

Radovici classifies the inhibitors into three groups according to temperature effects:

1. Inhibitors whose inhibition efficiency (I.E.) decreases with temperature increase. The value of the apparent activation energy E_a found is greater than that in the uninhibited solution.

2. Inhibitors whose I.E. does not change with temperature variation. The apparent activation energy does not change with the presence or absence of inhibitors.

3. Inhibitors in whose presence the I.E. increases with the temperature increase while the value of E_a for the corrosion process is smaller than that obtained in the uninhibited solution. This is an indication, according to the literature, for a specific type of adsorption of the inhibitors (Radovici, 1965).

From the present set of Mannich bases in 1 M H₂SO₄, the inhibition efficiency decreases with temperature and the E_a values are greater as compared to blank acid value (Table 3). The increase in the apparent activation energy may be interpreted as physical adsorption that occurs in the first stage (Quraishi et al., 2010; Parameswari et al., 2010).

The relationship between log (CR/T) and 1000/T is shown in Fig. 3. Straight lines are obtained with a slope $(-\mathrm{\Delta }{H}_a^o/2.303R)$ and an intercept of log $(R/\text{Nh}+\mathrm{\Delta }{S}_a^o/2.303R)$ from which the values of $\mathrm{\Delta }{H}_a^o$ and $\mathrm{\Delta }{S}_a^o$ were calculated and is presented in Table 3.

Close

Figure 3

The plots of log (CR/T) vs 1000/T for the effect of temperature on the performance of MMPA and MPA at the optimum inhibition concentration of 0.01 M.

Export to PPT

Inspection of these data revealed that activation parameters for dissolution reaction of mild steel in 1.0 M H₂SO₄ in the presence of Mannich bases are higher (49.24 kJ mol⁻¹ for MMPA and 51.78 for MPA) than those in the absence of inhibitors (5.14 kJ mol⁻¹). The positive signs of $\mathrm{\Delta }{H}_a^o$ reflect the endothermic nature of the mild steel dissolution process suggesting that the dissolution of mild steel is slow (Guan et al., 2004) in the presence of inhibitors.

The thermodynamic values obtained are the algebraic sum of the adsorption of organic molecules and desorption of water molecules (Branzol et al., 2000). Hence, the gain in entropy is attributed to the increase in solvent entropy and to more positive water desorption enthalpy (Ateya et al., 1984). The less negative value (when compared to free acid) of entropy of activation for Mannich bases also suggests that an increase in disordering takes place in going from reactants to the metal/solution interface, which is the driving force for the adsorption of inhibitors onto the mild steel surface.

3.3 Potentiodynamic polarization studies

The values of corrosion potential (E_corr), corrosion current density (I_corr), and anodic (b_a) and cathodic (b_c) Tafel slopes can be evaluated from anodic and cathodic regions of the Tafel plots. The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential to obtain corrosion current densities (I_corr).

The inhibition efficiency was evaluated from the measured I_corr values using the relationship: $$\text{Inhibition efficiency}=i_{\text{corr}}^o-i_{\text{corr}}^o/i_{\text{corr}}^o\times 100$$ Where, $i_{\text{corr}}^o$ and $i_{\text{corr}}^i$ are values of corrosion current density in the absence and in the presence of inhibitor, respectively.

Figs. 4 and 5 represent the potentiodynamic polarization curves of mild steel in 1.0 M H₂SO₄ in the absence and presence of various concentrations of the MMPA and MPA. It can be seen from Figs. 4 and 5 that, in the presence of inhibitors, the curves are shifted to lower current regions, showing the inhibition tendency of the Mannich bases. There was no definite trend observed in the E_corr values in the presence of both the Mannich bases. In the present study, shift in E_corr values is in the range of 0.200–0.250 V suggesting that they all act as mixed type inhibitors (Singh and Quraishi, 2010).

Close

Figure 4

Potentiodynamic polarization curve for mild steel in 1 M H₂SO₄ in the absence and presence of various concentrations of MMPA.

Export to PPT

Close

Figure 5

Potentiodynamic polarization curve for mild steel in 1 M H₂SO₄ in the absence and presence of various concentrations of MPA.

Export to PPT

The values of various electrochemical parameters derived by Tafel polarization of the inhibitors are given in Table 4. Investigation Table 4 revealed that the values of b_a change slightly in the presence of both the inhibitors, whereas a more pronounced change occurs in the values of b_c, indicating that both anodic and cathodic reactions are affected but the effect on the cathodic reactions is more prominent. Thus, both the MMPA and MPA acted as mixed type corrosion inhibitors, but predominantly cathodic inhibitor (Jamal Abdul Nasser and Anwar Sathiq, 2011).

3.4 Electrochemical impedance spectroscopy

The corrosion of mild steel in 1.0 M H₂SO₄ solution in the absence and presence of MMPA and MPA were investigated by EIS at the open circuit potential condition. Nyquist plots for mild steel obtained at the interface in the absence and presence of inhibitors at optimum concentrations are given in Figs. 5 and 6.

Close

Figure 6

Impedance diagram for mild steel in 1 M H₂SO₄ in the absence and presence of various concentration of MMPA.

Export to PPT

The Nyquist diagram obtained with 1.0 M H₂SO₄ shows only one capacitive loop and the diameter of the semicircle increases on increasing the inhibitor concentration suggesting that the formed inhibitive film was strengthened by the addition of inhibitors. All the obtained plots show only one semicircle and they were fitted using one time constant equivalent model (Randle's model) with double layer capacitance (C_dl), corrosion current density (I_corr) and charge transfer resistance (R_t). All the main parameters deduced from the impedance technique are given in Table 5.

Deviation from perfect circular shape, often known as frequency dispersion, was attributed to surface roughness and inhomogeneities of the solid surface (Lenderink et al., 1993). The change of i_corr and R_t values can be related to the gradual replacement of water molecules and /or hydroxyl ions by Mannich bases molecules on the mild steel surface and consequently to a decrease in the number of active sites necessary for the corrosion reaction.

Moreover, the values of double layer capacitance (C_dl) decreased with increasing inhibitor concentrations. The decrease in C_dl is probably due to the decrease in local dielectric constant and/or an increase in the thickness of the protective layer at electrode surface, enhancing, therefore, the corrosion resistance of the studied mild steel.

The inhibition efficiency was evaluated from the measured I_corr values using the relationship. $$\text{Inhibition efficiency}=i_{\text{corr}}^o-i_{\text{corr}}^i/i_{\text{corr}}^o\times 100$$ Where, $i_{\text{corr}}^o$ and $i_{\text{corr}}^i$ are values of corrosion current density in the absence and in the presence of inhibitor, respectively.

The results indicate a good agreement between the values of corrosion inhibition efficiency from weight loss, potentiodynamic polarization and impedance measurements. It is concluded that the corrosion rate depends on the chemical nature of the electrolyte rather than the applied technique (Ostovari et al., 2009). The differences are sometime as high as >2–3% but the order of magnitude is the same for all the methods.

3.5 Adsorption isotherm

The establishment of isotherms that describe the adsorptive behavior of a corrosion inhibitor is an important part of its study, as they can provide important clues to the nature of metal–inhibitor interaction. Several adsorption isotherms were assessed. The simplest, is the Langmuir isotherm, based on the assumption that all adsorption sites are equivalent and that particle binding occurs independently from nearby sites being occupied or not. Interactions between adsorbed species complicate the problem by making the energy of adsorption a function of surface coverage. One of the isotherms that include this possibility is the Temkin isotherm. The Temkin adsorption isotherm was found to provide the best description of the adsorption behavior of the Mannich bases: this has the following surface coverage, θ, and the bulk concentration, C, relationship: f (θ,x) exp(−2a θ) = KC. Where f (θ,x) is the configurational factor, which depends on the physical model and the assumptions underlying the derivation of the isotherm, θ is the degree of surface coverage, x is the size ratio, “a” is the molecular interaction parameter, C is the inhibitor concentration in the electrolyte and K is the equilibrium constant for adsorption. (See Fig. 7).

Close

Figure 7

Impedance diagram for mild steel in 1 M H₂SO₄ in the absence and presence of various concentration of MPA.

Export to PPT

The equation expressing Temkin's adsorption isotherm is: K_ads C = e^f θ Where K_ads is the adsorption equilibrium constant, θ is the degree of surface coverage, C is the molar concentration of inhibitor (mol L⁻¹) and “f” is the molecular interaction constant. The plots of log C against θ displayed a straight line for the inhibitor tested. The linear plot with high correlation coefficient (0.9872 and 0.9500 for MMPA and MPA, respectively) clearly revealed that the surface adsorption process of inhibitors on the mild steel surface obeys the Temkin adsorption (Fig. 8).

Close

Figure 8

Temkin adsorption isotherm plot for the adsorption of MMPA and MPA on mild steel in 1.0 M H₂SO₄ solution.

Export to PPT

3.6 Standard free energy of adsorption

The negative values of $\mathrm{\Delta }{G}_{\text{ads}}^0$ ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the mild steel surface (Keles et al., 2008: Singh and Quraishi, 2010). It is usually accepted that the value of $\mathrm{\Delta }{G}_{\text{ads}}^0$ around −20 kJ mol⁻¹ or lower indicates the electrostatic interaction between charged metal surface and charged organic molecules in the bulk of the solution while those around −40 kJ mol⁻¹ or higher involve charge sharing or charge transfer between the metal surface and organic molecules (Moretti et al., 1996). In the present study, the $\mathrm{\Delta }{G}_{\text{ads}}^o$ values obtained for the Mannich bases on mild steel in 1.0 M H₂SO₄ solution are ranging between −28 and −27 kJ mol⁻¹ for MMPA and −27 to −25 kJ mol⁻¹ for MPA, which are higher than −20 kJ mol⁻¹ and lower than −40 kJ mol⁻¹ (Table 6); this indicates that the adsorption is neither typical physisorption nor typical chemisorption but it is a complex mixed type that is the adsorption of inhibitor molecule on the mild steel surface in the present study involves both physisorption and chemisorption and chemisorption is the predominant mode of adsorption.

3.7 SEM analysis

SEM photographs of the metal sample in the absence and presence of MMPA and MPA are shown in Figs. 9, 9a and 9b, respectively. The inhibited metal surface is smoother than the uninhibited surface indicating a protective layer of adsorbed inhibitor preventing acid attack.

Close

Figure 9

SEM image of mild steel in 1 M H₂SO₄ solution.

Export to PPT

Close

Figure 9a

SEM image of surface of mild steel after immersion for 2 h in 1 M H₂SO₄ in the presence of 0.01 M MMPA.

Export to PPT

Close

Figure 9b

SEM image of surface of mild steel after immersion for 2 h in 1 M H₂SO₄ in the presence of 0.01 M MPA.

Export to PPT

3.8 FT-IR spectroscopy

FT-IR spectra were recorded to understand the interaction of inhibitor molecules with the metal surface (Tamil Selvi et al., 2003). Figs. 10 and 11 show the IR spectra of pure MMPA and MPA and Figs. 10a and 11a represent the spectra of the scraped samples obtained from the metal surfaces after corrosion experiments in 1.0 M H₂SO₄, respectively.

Close

Figure 10

FT-IR spectrum of pure sample of MMPA.

Export to PPT

Close

Figure 11

FT-IR spectrum of pure MPA.

Export to PPT

Close

Figure 10a

FT-IR spectrum of scraped sample of MMPA obtained from the metal surface after corrosion experiment in 1 M H₂SO₄.

Export to PPT

Close

Figure 11a

FT-IR spectrum of scraped sample of MPA obtained from the metal surface after corrosion experiment in 1 M H₂SO₄.

Export to PPT

In pure MMPA, the IR bands observed at 3399.18, 1610.55 and 1109.57 cm⁻¹ have been assigned to –NH, amide –C⚌O and –C–N–C of morpholine group, respectively (Table 7) (Geary, 1971; Raman et al., 2004).

In the scraped sample obtained from the mild steel surface after corrosion experiments in 1.0 M H₂SO₄, the IR bands that appeared at 3394.18, 1620.50 and 1103.66 cm⁻¹ have been assigned to –NH, amide –C⚌O and –C–N–C of morpholine group respectively (Table 7). All the bands displayed substantial shifts with fairly low intensity indicating coordination through the oxygen of amide moiety and nitrogen of morpholine ring (Kerridge, 1988; Raman et al., 2004).

In pure MPA, the IR bands observed at 3429.84, 1713.12 and 1112.18 cm⁻¹ have been assigned to –NH, amide –C⚌O and –C–N–C of morpholine group, respectively (Table 7) (Geary, 1971; Raman et al., 2004).

Similarly, in the scraped sample obtained from the mild steel surface after corrosion experiments in 1.0 M H₂SO₄, the IR bands that appeared at 3405.52, 1706.89 and 1113.30 cm⁻¹ have been assigned to –NH, amide –C⚌O and –C–N–C of the morpholine group respectively (Table 7). All the bands displayed substantial shifts with fairly low intensity indicating coordination through the oxygen of amide moiety and nitrogen of morpholine ring (Kerridge, 1988; Raman et al., 2004).

3.9 Mechanism of corrosion inhibition

From the results obtained from different electrochemical and weight loss measurements, it was concluded that both the Mannich bases MMPA and MPA inhibit the corrosion of mild steel in 1.0 M H₂SO₄ by adsorption at mild steel/solution interface.

It is a general assumption that the adsorption of organic inhibitors at the metal surface interface is the first step in the mechanism of the inhibitor action. Organic molecules may be adsorbed on the metal surface in four types, namely

1. Electrostatic interaction between the charged molecules and the charged metal,

2. Interaction of unshared electron pairs in the molecule with the metal,

3. Interaction of p-electrons with the metal and/or

4. A combination of types (a–c) (Schweinsberg et al., 1988; Shorky et al., 1998; Singh and Quraishi, 2010).

In general, both the Mannich bases, MMPA and MPA contain one amide linkage, one aromatic ring and a residue of morpholine ring.

The inhibition of active dissolution of the metal is due to the adsorption of the inhibitor molecules on the metal surface forming a protective film. The inhibitor molecules can be adsorbed onto the metal surface through electron transfer from the adsorbed species to the vacant electron orbital of low energy in the metal to form a co-ordinate type link.

The inhibition efficiency depends on many factors (Noor, 2005; Bentiss et al., 2009) including the number of adsorption centers, mode of interactions with metal surface, molecular size and structure.

It is well known that iron has co-ordinate affinity toward nitrogen, sulfur and oxygen-bearing ligands (Donald, 1990; Snyder et al., 1989; Elayyachy et al., 2006; Lece et al., 2008). Hence, adsorption on iron can be attributed to co-ordination through amide linkage, hetero atom (N and O) and π-electrons of aromatic ring.

If one considers the structures of investigated compounds (Fig. 1) several potential sources of inhibitor–metal interaction can be identified. In both the Mannich bases, there are the unshared electron pairs on N and O, capable of forming a co-ordination σ-bond with iron (Shriver et al., 1994). Further, the double bonds in the molecule allow back donation of metal d-electrons to the π∗-orbital and this type of interaction cannot occur with amines.

The better inhibition performance of MMPA than MPA is due to the presence of p-OCH₃ group in its structure, because of electro donating mesomeric effect, p-OCH₃ group increases the electron density of aromatic ring and makes the π-electrons more available to interact with mild steel surface. Thus, MMPA is more effectively adsorbed. In the above facts, the investigated Mannich bases follow type (d) mechanism. Thus, the inhibition efficiency of the studied compounds follows the order MPA < MMPA. The results of weight loss, EIS and potentiodynamic polarization follow the same trend.