J. Cent. South Univ. (2016) 23: 249-257

DOI: 10.1007/s11771-016-3068-2

Adsorption isotherm and inhibition effect of a synthesized di-(m-Formylphenol)-1,2-cyclohexandiimine on corrosion of steel X52 in HCl solution

A. karimi¹, I. Danaee¹, H. Eskandari¹, M. RashvanAvei²

1. Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran;

2. Department of Chemistry, K. N. Toosi University of Technology, Tehran, Iran

 Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The potential of di-(m-Formylphenol)-1,2-cyclohexandiimine as an environmentally friendly corrosion inhibitor for steel was investigated in 1 mol/L HCl using potentiodynamic polarization, electrochemical impedance spectroscopy and chronoamperometry measurements. All electrochemical measurements suggest that this compound is an excellent corrosion inhibitor for mild steel and the inhibition efficiency increases with the increase in inhibitor concentration. The effect of temperature on the corrosion behavior of mild steel with the addition of the Schiff base was studied in the temperature range from 25 °C to 65 °C. It is found that the adsorption of this inhibitor follows the Langmuir adsorption isotherms. The value of activation energy and the thermodynamic parameters such as ΔH_ads, ΔS_ads, K_ads and ΔG_ads were calculated by the corrosion currents at different temperatures using the adsorption isotherm. The morphology of mild steel surface in the absence and presence of inhibitor was examined by scanning electron microscopy (SEM) images.

Key words: corrosion; inhibitor; Schiff base; adsorption; Langmuir adsorption isotherm

1 Introduction

Corrosion is a fundamental process playing an important role in economics and safety, particularly for metals and alloys. Steel has found wide application in a broad spectrum of industries and machinery [1–2]. Steel X52 is used for pipeline transportation systems in the petroleum and natural gas industries. Internal corrosion in wells and pipelines is influenced by different parameters such as acidic content due to CO₂ and H₂S. Moreover, in most industrial processes, acidic solutions are commonly used for the pickling, industrial acid cleaning, acid descaling, oil well acidifying, etc. Unfortunately, iron and its alloys could corrode during these acidic applications particularly with the use of hydrochloric acid and sulphuric acid, which results in terrible waste of both resources and money [3–4].

Corrosion prevention systems favor the use of corrosion inhibitors with low or zero environmental impacts. Inhibitors are chemicals that react with a metallic surface or the environment. Inhibitors decrease corrosion processes by increasing the anodic or cathodic polarization behavior, decreasing the movement or diffusion of ions to the metallic surface and increasing the electrical resistance of the metallic surface [5–6].

The inhibiting actions of organic compounds are usually attributed to their interactions with the metal surface via their adsorption. These compounds in general are adsorbed on the metal surface, blocking the active corrosion sites. Three types of adsorption may take place by organic molecules at metal/solution interface: (1) electrostatic attraction between the charged molecules and the charged metal, (2) interaction of unshared electron pairs or π electron in the molecule with the metal, and (3) combination of (1) and (2) [7–8].

The choice of the inhibitor is based on two considerations, first is economic consideration, and second should contain the electron cloud on the aromatic ring or the electronegative atoms such as N, O in the relatively long chain compounds. Generally, the organic compounds containing hetero atoms like O, N, S, and P are found to work as very effective corrosion inhibitors. The efficiency of these compounds depends upon electron density present around the hetero atoms, the number of adsorption active centers in the molecule and their charge density, molecular size, mode of adsorption, and formation of metallic complexes [9–11]. These features obviously can be combined within the same molecule such as Schiff bases. In the past few years, the inhibition of various metals such as iron, copper, zinc, aluminum, and carbon steel corrosion in acid solutions by Schiff bases with environmental considerations has attracted more attention [12–13]. Schiff bases can conveniently be synthesized from relatively cheap starting materials. It is also reported that Schiff bases show more inhibition efficiency than corresponding amines [14].

The aim of the present work is to investigate the effect of synthesized Schiff base corrosion inhibitor di-(m-Formylphenol)-1,2-cyclohexandiimine on the corrosion of mild steel in 1 mol/L HCl, evaluated by Tafel polarization and electrochemical impedance spectroscopy (EIS) data. The effects of concentration and temperature on the inhibition efficiencies of the selected Schiff base were studied. Thermodynamic parameters such as enthalpy, entropy and Gibbs free energy were calculated from experimental data of the inhibition process at different temperature.

2 Materials and methods

2.1 Preparation of di-(m-Formylphenol)-1,2- cyclohexandiimine (m-FP-DACH)

All chemicals used in the present work were of reagent-grade Merck product and used as received without further purification. The m-FP-DACH Schiff base (Fig. 1) was prepared in high yield (97%) by the condensation of m-Formylphenol (0.244 g, 2 mmol) with 1,2- Cyclohexanediamine (0.114 g, 1 mmol) in a stirred ethanolic solution and heated to reflux for 5 h according to the described procedure [6]. The resulting precipitate was filtered off, washed with warm ethanol and diethyl ether. Identification of structure of synthesized Schiff base was performed by IR and ¹HNMR spectroscopy and elemental analysis. The resulting Schiff bases are shown in Fig. 1.

Fig. 1 General structure of investigated Schiff bases

2.2 Sample preparation

Working electrodes were prepared from steel X52 specimens of the chemical composition: C 0.3, Mn 1.350, P 0.03, S 0.03 (mass fraction, %), and the balance Fe. Samples were cut from a flat plate by the wire cut method. The dimension of each sample was 1 cm×1cm. Working electrodes were prepared by embedding the sample in epoxy resin, exposing a geometrical surface area of 1 cm² to the aggressive electrolyte. The exposed areas of the electrodes were mechanically abraded with 400, 800, 1200 and 2000 grades of emery paper, rinsed by distilled water and degreased with acetone before each electrochemical experiment.

The measurements were performed in 1 mol/L HCl in the absence and presence of Schiff bases in the concentration range from 5×10^–5 to 2×10^–3 mol/L. Distilled water was used to prepare the test solutions.

2.3 Electrochemical measurements

Electrochemical measurements were carried out in a conventional three-electrode system. The reference electrode was a saturated calomel electrode (SCE) and the auxiliary electrode was a platinum sheet. All experiments were repeated three times. To reach the steady state condition, before each experiment, the working electrode was immersed in the test cell for    20 min. The electrochemical measurements were carried out using computer controlled ZAHNER potentiostat/ galvanostat. Electrochemical impedance spectroscopy (EIS) was recorded in frequency range of 100 kHz to  10 mHz with 0.01 V peak-to-peak amplitude using AC signals at open circuit potential. Polarization curves were scanned at a scan rate of 1 mV/s from –700 mV to   –100 mV. All tests were done at constant temperature by controlling the cell temperature using a water bath. Polarization curve was used to calculate corrosion current density by the Tafel extrapolation method. The polarization resistance was calculated from the slope of the potential versus logarithm of current plots. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of home written least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [15–16].

The morphology of steel surface after 12 h exposure to 1 mol/L HCl solution in the absence and presence of 2×10^–3 mol/L m-FP-DACH was observed by scanning electron microscope (SEM) model VEGA\TESCAN.

3 Results and discussion

3.1 Tafel polarization measurements

Figure 2 shows the anodic and cathodic polarization plots of steel in 1 mol/L HCl in the absence and presence of different concentrations of inhibitor at 25 °C. Table 1 gives the electrochemical corrosion parameters such as corrosion potential (E_corr vs SCE), corrosion current density (J_corr), cathodic and anodic Tafel slopes (β_a, β_c), the degree of surface coverage (θ) and inhibition efficiency (IE, E_i) obtained by extrapolation of the Tafel lines. The degree of surface coverage and inhibition efficiency are calculated using the following equations [17]:

                              (1)

E_i=θ                                        (2)

where  and J_corr are the corrosion current densities determined by the intersection of the extrapolated Tafel lines in uninhibited and inhibited acid solution, respectively. Both the anodic and cathodic reactions of mild steel corrosion are inhibited in the presence of the inhibitor. This result suggests that the addition of the m-FP-DACH decreases the anodic dissolution and also retards the hydrogen evolution reaction. It can be seen that the IE increases by increasing inhibitor concentration which indicates that more inhibitor molecule are absorbed on the metal surface and provide wider surface coverage. Thus, these compounds are acting as an adsorption inhibitor.

No definite trend is observed in the shift of E_corr values in the presence of different concentrations of the m-FP-DACH, suggesting that this compound behaves as mixed-type inhibitor [18]. Polarization resistance (R_p) values are determined using Stern–Geary equation which is given below [19]:

                     (3)

Fig. 2 Anodic and cathodic polarization curves of steel electrode in 1 mol/L HCl at 25 °C without and with different concentrations of inhibitor

By increasing the m-FP-DACH concentration, the polarization resistance increases in the presence of compound, indicating adsorption of the inhibitor on the metal surface to block the active sites efficiently and inhibit corrosion [20].

3.2 Electrochemical impedance spectroscopy

Figure 2 shows the Nyquist plots recorded for the corrosion of steel in 1 mol/L HCl solution without and with different concentrations of inhibitor obtained at E_corr. The plots show a depressed capacitive loop which arises from the time constant of the electrical double layer and charge transfer resistance. The impedance of the inhibited steel increases with increasing the m-FP-DACH concentrations and consequently the inhibition efficiency increases. The equivalent circuit compatible with the Nyquist diagrams recorded in the presence of inhibitor is depicted in Fig. 4. The simplest approach requires the theoretical transfer function Z(ω) to be represented by a parallel combination of a resistance R_ct and a capacitance C_dl, both in series with another resistance R_s [21]:

                       (4)

where ω is the frequency in rad/s, ω=2πf and f is frequency in Hz. To obtain a satisfactory impedance simulation of steel, it is necessary to replace the capacitor (C) with a constant phase element (CPE) Q in the equivalent circuit [22]. The most widely accepted explanation for the presence of CPE behavior and depressed semicircles on solid electrodes is microscopic roughness, causing an inhomogeneous distribution in the solution resistance as well as in the double layer capacitance [22]. Constant phase element Q_dl, R_s and R_ct can be corresponded to double layer capacitance,  solution resistance and charge transfer resistance, respectively. To corroborate the equivalent circuit, the experimental data are fitted to equivalent circuit and the circuit elements are obtained. Table 2 illustrates the equivalent circuit parameters for the impedance spectra of corrosion of steel in 1 mol/L HClsolution. The results demonstrate that the presence of inhibitor enhances the value of R_ct obtained in the pure medium while the values of Q_dl decrease. The decrease in Q_dl is caused by adsorption of inhibitor, indicating that the exposed area decreases. On the other hand, a decrease in Q_dl, which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that Schiff base inhibitor acts by adsorption at the metal/solution interface [23].

Table 1 Potentiodynamic polarization parameters for corrosion of steel in 1 mol/L HCl solution in absence and presence of different concentrations of inhibitor at 25 °C

Fig. 3 Nyquist plots for steel in 1 mol/L HCl at 25 °C without and with various concentrations of inhibitor

Fig. 4 Equivalent circuits compatible with experimental impedance data for corrosion of steel electrode in different inhibitor concentrations

Table 2 Impedance spectroscopy data for steel corrosion in 1 mol/L HCl solution with and without different concentrations of inhibitor at 25 °C

As the Q_dl exponent (n) is a measure of the surface heterogeneity, values of n indicates that the steel surface become more and more homogeneous as the concentration of inhibitor increases as a result of its adsorption on the steel surface and corrosion inhibition [23]. The increase in values of R_ct and the decrease in values of Q_dl with increasing concentration also indicate that schiff base inhibitor acts as primary interface inhibitor and the charge transfer controls the corrosion of steel under the open circuit conditions.

3.3 Effect of temperature

The adsorption phenomenon has been successfully explained by thermodynamic parameters [24-25]. The change of the corrosion rate with the temperature was studied in 1 mol/L HCl. For this purpose, polarization curves were performed at different temperatures from  25 °C to 65 °C in the absence and presence of different concentrations of m-FP-DACH (Figs. 5 and 6). For    45 °C and 65 °C, the electrochemical parameters were extracted and summarized in Tables 3 and 4. It is obvious that the values of J_corr increase and the efficiency value decreases by increasing the temperature. Figures 5 and 6 show that raising the temperature has no significant effect on the corrosion potentials but leads to increasing corrosion rate. According to the Arrhenius equation, the apparent activation energy (E_a) of metal corrosion in both media (blank and inhibited) can be calculated from the following equation [26]:

                           (5)

where E_a represents the apparent activation energy, R is the gas constant, A is the pre-exponential factor, and T is the absolute temperature. Figure 7 shows the logarithm of J_corr against the reciprocal of temperature T^{–1 in the absence and presence of m-FP-DACH. The activation energy E}_a is calculated from the slope of the plots (-E_a/R). The calculated value of E_a in the absence of inhibitor is 57.47 kJ/mol, while in the presence of 2× 10^–3 mol/L of inhibitor, it is 63.91 kJ/mol. It has been reported that higher E_a in the presence of the inhibitor for mild steel in comparison with blank solution typically shows physisorption [27]. Enthalpy and entropy ofactivation (ΔH_a and ΔS_a) are calculated from the transition state theory as [27]:

               (6)

where h is the Plank constant and N is the Avogadro’s number. A plot of ln(J_corr T^–1) versus 1/T in straight lines is shown in Fig. 9 for mild steel dissolution in 1 mol/L HCl in the absence and presence of differentconcentrations of m-FP-DACH. Straight lines are obtained with a slope of –△H_a/R and an intercept of ln(R/Nh)+△S_a/R. The values of E_a, A, △H_a and △S_a are calculated and given in Table 5. The positive values of △H_a mean that the dissolution reaction is an endothermic process. Practically, E_a and △H_a are of the same order. Also, the entropy △S_a increases more positively in the presence of the inhibitor than the non-inhibited one, which reflects that the activated complex in the rate determining step represents dissociation rather than an association step, meaning that an increase in disordering takes place on going from reactants to the activated complex [6].

Fig. 5 Anodic and cathodic polarization curves of steel electrode in 1 mol/L HCl without inhibitor at different temperatures

Fig. 6 Anodic and cathodic polarization curves of steel electrode in presence of 2×10^–3 mol/L of inhibitor at different temperatures

Table 3 Potentiodynamic polarization parameters for corrosion of steel in 1 mol/L HCl solution in absence and presence of different concentrations of inhibitor at 45 °C

Table 4 Potentiodynamic polarization parameters for corrosion of steel in 1 mol/L HCl solution in absence and presence of different concentrations of inhibitor at 65 °C

Fig. 7 Typical Arrhenius plots of lnJ_corr vs 1/T for steel in 1 mol/L HCl at different concentrations of inhibitor

Fig. 8 Variation of lnJ_corr/T vs 1/T for steel in 1 mol/L HCl at different concentrations of inhibitor

3.4 Adsorption isotherm

Adsorption isotherms provide information about the interaction of the adsorbed molecules with the metal surface. The adsorption of organic compounds can be expressed by two main types of interactions: physical adsorption and chemical adsorption. There are some factors that influence the adsorption processes including the nature and charge of metal, the chemical of inhibitor, and the type of electrolyte [28].

The adsorption of an organic adsorbate at metal/solution interface can be presented as a substitution adsorption process between the organic molecules in aqueous solution, Org_(sol), and the water molecules on metallic surface, H₂O_(ads):

Org_(sol)+xH₂O_(ads)Org_(ads)+xH₂O_(sol)                     (7)

where Org_(sol) and Org_(ads) are the organic species dissolved in the aqueous solution and adsorbed onto the metallic surface, respectively, H₂O_(ads) is the water molecule adsorbed on the metallic surface, H₂O_(sol) is the water molecule in solution, and x is the size ratio and represents the number of molecules of water replaced by the inhibitor molecule.

Different adsorption isotherms, Langmuir, Temkin, Freundlich, Frumkin, Modified, Langmuir, Henry, Viral, Damaskin, Volmer, and Flory-Huggins, [29–30] were tested for their fit to the experimental data. The linear regression coefficient values (R²) were determined from the plotted curves. According to these results, it is found that the experimental data obtained from polarization readings could be fitted by Langmuir’s adsorption isotherm. According to this isotherm, the surface coverage is related to inhibitor concentration by [30–31]:

                                (8)

                                (9)

where K_ads is the equilibrium constant of the inhibitor adsorption process, C is the inhibitor concentration and θ is the surface coverage calculated by Eq. (1).

A fitted straight line is obtained for the plot of C/θ versus C with slopes close to 1 as seen in Fig. 9. The strong correlation (R²>0.99) suggests that the adsorption

of m-FP-DACH on the mild steel surface obeys Langmuir’s adsorption isotherm. This isotherm assumes that the adsorbed molecules occupy only one site and there are no interactions with other adsorbed species [30]. The K_ads values can be calculated from the intercept lines on the C/θ-axis. This is related to the standard free energy of adsorption (ΔG_ads) with the following equation [32]:

                     (10)

where R is the gas constant and T is the absolute temperature. The constant value of 55.5 is the concentration of water in solution in molar. The enthalpy and entropy of adsorption (ΔH_ads and ΔS_ads) can be calculated using the following equations [23]:

                       (11)

             (12)

Table 5 Activation parameters of dissolution of steel in 1 mol/L HCl solution in absence and presence of inhibitor

Fig. 9 Langmuir adsorption isotherm C/θ vs C of inhibitor in 1 mol/L solution

The negative values of △G_ads suggest that the adsorption of m-FP-DACH on the steel surface is spontaneous. Generally, the values of -△G_ads around or less than 20 kJ/mol are associated with the electrostatic interaction between charged molecules and the charged metal surface (physisorption); while those around or higher than 40 kJ/mol mean charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of metal bond (chemisorption). The values of K_ads and △G_ads are listed in Table 6. The ΔG_ads values are around –28 kJ/mol, which means that the absorption of inhibitor on the steel surface belongs to both physisorption and chemisorption, and the adsorptive film has an electrostatic character [20]. Figure 10 represents the plots of lnK_ads versus 1/T for adsorption m-FP-DACH. The obtained lines can provide valuable information about the mechanism of corrosion inhibition. An endothermic adsorption process △H_ads>0 is attributed unequivocally to chemisorption, and an exothermicadsorption process △H_ads<0 may involve either physisorption or chemisorption or a mixture of both processes [33]. The calculated values of △H_ads and △S_ads are –1.46 kJ/mol and 84.59 J/(mol·K), respectively. The calculated △G_ads and ΔH_ads values for inhibitor show that the adsorption mechanism is not completely physical or chemical, and a combination of physisorption and chemisorption exists between the inhibitor and metal surface. The positive sign of △S_ads arises from the substitution process, which can be attributed to the increase in the solvent entropy and more positive water desorption entropy. It is also interpreted with an increase of disorders due to the more water molecules which can be desorbed from the metal surface by one inhibitor molecule [21].

Table 6 Thermodynamic and equilibrium adsorption parameters for adsorption of inhibitor on steel surface in 1 mol/L HCl solution

Fig. 10 ln K_ads vs 1/T for inhibitor adsorption on steel surface

3.5 Chronoamperometry

In order to gain more insight about the effect of m-FP-DACH on the electrochemical behavior of steel in 1 mol/L HCl solution, potentiostatic current–time transients were recorded. Figure 11 shows the current transients of steel electrode at –0.3 V (vs SCE) applied anodic potential. Initially the current decreases monotonically with time, and the decrease in the current density is due to the formation of corrosion products layer on the electrode surface. However, in later time, the current reaches to a steady state value due to the steel dissolution depending on applied potential (Fig. 11). In the presence of inhibitor, the dissolution current decreases and electrode inhibits from corrosion due to inhibitor adsorption.

Fig. 11 Current transients of steel electrode at -0.3 V vs SCE

3.6 Scanning electron microscopy

In order to evaluate the conditions of the steel surfaces in contact with hydrochloric acid solution, surface analysis was carried out. Surface was observed before and after 12 h of immersion in 1 mol/L HCl in the absence and presence of m-FP-DACH at 25 °C. Scanning electron microscopy (SEM) studies of the surface in the absentce and presence of inhibitor are presented in Fig. 12. It is revealed that the presence ofcorrosion attack and some pits on the surface in the absence of inhibitor while such damages are diminished in the presence of inhibitor.

Fig. 12 Scanning electron microscopy images of steel exposed to 1 mol/L HCl solution:

4 Conclusions

1) The synthetic Schiff base m-FP-DACH acts as an inhibitor for mild steel corrosion in 1 mol/L HCl solution, especially in high concentration.

2) Inhibition efficiency of this compound increases with increasing their concentrations due to the formation of a film on the steel. The inhibitor decreases both anodic and cathodic currents, which shows the mixed mode of action of the inhibitor molecules.

3) The inhibition of mild steel in 1 mol/L HCl solution at different temperatures is found to obey the Langmuir adsorption isotherm.

4) The negative values of △G indicate the spontaneous adsorption of the inhibitor on the surface of mild steel.

5) Surface studies show that the surface of sample in solution with inhibitor molecules looks more flat and more uniform with lower roughness than that in the uninhibited solution.

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(Edited by FANG Jing-hua)

Received date: 2015-03-10; Accepted date: 2015-06-09

Corresponding author: I. Danaee; Tel: +98–6153329937; E-mail: danaee@put.ac.ir