API-5L-X65 samples were cut from parent pipe with chemical composition reported as C:0.12, Mn:1.3, Si: 0.22, Cr: 0.25, Mo: 0.18, Cu: 0.12, Ni: 0.1, P: 0.01, S: 0.003, Al: 0.02, Nb: 0.05, Sn: 0.01, V: 0.05, Ti: 0.005, and Fe: Rest. The specimens of dimension 1 cm × 1 cm (exposed) × 4.3 mm (isolated with polyester resin) were used for electrochemical methods. They were polished mechanically using different grade emery papers up to 2000, washed thoroughly with doubly distilled water, and degreased with acetone before being immersed in the acidic solution.

The DHBP Schiff base inhibitor (Fig. 1) was provided in high yield (97 %) by the condensation of 2,4-Dihydroxybenzaldehyde (2 mmol) with 1,3-diaminopropane (1 mmol) in a stirred ethanolic solution and heated to reflux for 4 h according to the described procedure [13]. The resulting precipitate was filtered off, washed with warm ethanol and diethyl ether. Identification of structure of synthesized Schiff base was performed by ¹HNMR spectroscopy and elemental analysis. The concentration range of inhibitor employed was varied from 1 × 10⁻⁴ to 2 × 10⁻³ M.

The apparatus for electrochemical investigations consists of computer controlled Auto Lab potentiostat/galvanostat (PGSTAT302N) electrochemical measurement system. Potentiodynamic polarization was performed at a scan rate of 1 mV s⁻¹. The electrochemical experiments were carried out using a conventional three electrode cell assembly at 20 ± 2 ℃. A rectangular platinum foil was used as counter electrode and saturated calomel electrode as the reference electrode. Open circuit potentials (OCP) were obtained by potentiostat/galvanostat. The EIS experiments were conducted in the frequency range of 100 kHz to 0.01 Hz at open circuit potential with AC potential amplitude 10 mV. Time interval of 20-25 min was given for steady state attainment of open circuit potential. The electrochemical impedance spectra were also recorded at different anodic and cathodic biased potentials other than open circuit potential and the double layer capacitance values were plotted against the applied DC potentials to determine PZC. The PZC has lowest capacitance value in variation of double layer capacitance versus applied anodic and cathodic potentials. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of home written least square software based on Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [14,15].

For surface analysis, the specimens were immersed in 1 M HCl prepared with and without addition of 1×10⁻³ M inhibitor at 20±2 ℃ for 6 h, cleaned with distilled water. The surface morphology of steel was evaluated by scanning electron microscopy (VEGA\TESCAN).

Fig. 2 shows the Nyquist diagrams of API-5L-X65 in 1 M HCl solutions containing different concentrations of DHBP. All the impedance spectra exhibit single depressed semicircle due to the charge transfer resistance and double laye capacitance. The diameter of semicircle increases with the increase of DHBP concentrations. The addition of DHBP causes a remarkable decrease in the corrosion rate. These results are indicative of the adsorption of inhibitor molecules on the active sites of steel surface [16]. The inhibition of steel corrosion increases with increasing inhibitor concentrations. Moreover, the inhibition efficiencies show no considerable increase with concentrations of inhibitor higher than 2×10⁻³ M and almost are constant. The semicircular appearance shows that the corrosion of steel is controlled by the charge transfer and the presence of DHBP does not change the mechanism of steel dissolution [17,18]. In addition, these Nyquist diagrams are not perfect semicircles. The deviation of semicircles from perfect circular shape is often referred to the frequency dispersion of interfacial impedance [18-20]. This behaviour is usually attributed to the inhomogeneity of the metal surface arising from surface roughness or interfacial phenomena [20,21], which is typical for solid metal electrodes [22]. The equivalent circuit compatible with the Nyquist diagram recorded in the presence of inhibitor is depicted in Fig. 3. 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, both in series with another resistance R_s [23]:

where 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. 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 [24]. 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 1 illustrates the equivalent circuit parameters for the impedance spectra of corrosion of steel in 1 M HCl Solution. The degree of surface coverage and the inhibition efficiency (IE% = θ ×100) for different concentrations of inhibitor is calculated using the following equation [25]:

The charge transfer resistance increases with increasing inhibitor concentrations due to the higher inhibition efficiencies. The data indicate that increasing charge transfer resistance associates with a decrease in the double layer capacitance. The decrease in Q_dl values cause by adsorption of inhibitor indicates that the exposed surface 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 the inhibitor acts by adsorption at the metal-solution interface and the formation of a protective layer on the electrode surface [18]. The thickness of this protective layer increases with the increase in inhibitor concentration, since more inhibitor will electrostatically adsorb on the electrode surface. This trend is in accordance with Helmholtz model, given by the following equation [10,18]:

where e is the thickness of the protective layer, ε is the dielectric constant of the medium, ε₀ is the vacuum permittivity and S is the effective surface area of the electrode.

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

The adsorption of organic compounds on the metal surface generally depends on the surface charge of the metal and the charge or dipole moment of the inhibitor molecule and other ions that are specifically adsorbed on to the metal surface [26]. The surface charge of the metal is determined from the open circuit potential with respect to the PZC [27]. The dependency of the double layer capacitance on the applied dc potential is shown in the Fig. 4. The values of E_ocp and PZC for steel in the inhibited and uninhibited solutions are given in the Table 2. The surface charge of steel is found at the open circuit potential using the equation E_r = E_ocp − E_pzc, where E_r is antropov’s rational corrosion potential [28]. The surface charge of steel at the OCP is found to be positive in the inhibited and uninhibited HCl solutions.

In the presence of the inhibitor, the metal surface is positively charged with respect to PZC [27-29], hence chloride ions will first get adsorbed in the metal surface after that the protonated inhibitor molecules will be adsorbed and the metal ion dissolution will be effectively protected [30]. In the uninhibited HCl solution, the mechanism of anodic dissolution involves the reversible adsorption of the anion Cl⁻ on steel surface, release of electrons from the anions adsorbed onto the metal surface and desorption of adsorbed species with Fe⁺² ions after picking up electrons from the steel surface [27,29]. The inhibitor molecules existing in the protonated form through the heteroatoms N or O in HCl solution are in equilibrium with the corresponding molecular (unprotonated) form. Thus, it is very difficult for the positively charged inhibitor to approach the positively charged metal surface because of electrostatic repulsion. The protonated molecules can get adsorbed on the steel surface via Cl⁻ ions that form interconnecting bridges between the positively charged metal surface and the protonated organic inhibitors [31,32].

Fe + Cl⁻↔ (FeCl⁻)_ads+Inh(FeCl⁻Inh⁺)_ads

In addition to this type of physical adsorption, the adsorption of inhibitor molecules may also take place through donor acceptor interactions between π electrons of N = C, benzene ring and the lone pair of electrons on N, O and S atoms present in the unprotonated form of inhibitor molecules and the vacant ‘d’ orbitals of surface iron atoms [23].

Adsorption isotherms provide information about the interaction of the adsorbed molecules with the metal surface [16]. The efficiency of schiff base molecules as a successful corrosion inhibitor mainly depends on their adsorption ability on the metal surface. The adsorption process consists of the replacement of water molecules at a corroding interface according to following process [33]:

where Org_(sol) and Org_(ads) are the organic molecules in the solution and adsorbed on the metal surface, respectively, and n is the number of water molecules replaced by the organic molecules. It is essential to know the mode of adsorption and the adsorption isotherm. Different adsorption isotherms (Langmuir, Temkin, Freundlich, Frumkin, Modified Langmuir, Henry, Viral, Damaskin, Volmer and Flory-Huggins) [34,35] were tested for their fit to the experimental data. The linear regression coefficient values (R²) determine from the plotted curves. According to these results, it can be concluded that the best description of the adsorption behavior of DHBP can be explained by Langmuir adsorption isotherm which is given by:

where θ is the surface coverage degree, C is the concentration of inhibitor and K_ads is the adsorptive equilibrium constant. The linear relationships of C θ ⁻¹ versus C, depicted in Fig. 5 suggest that the adsorption of DHBP on the steel surface obey the Langmuir adsorption isotherm. Langmuir’s isotherm assumes that the adsorption of organic molecule on the adsorbent is monolayer and the adsorbed molecules occupy only one site and there are no interactions with other adsorbed species. The standard free energy of adsorption of inhibitor (∆G_ads) on steel surface can be evaluated with the following equation:

The value 55.5 in the above equation is the concentration of water in solution in mol.l⁻¹ [10]. The negative values of ∆G_ads suggest that the adsorption of DHBP on the steel surface is spontaneous. Generally, the values of ∆G_ads around or less than −30 kJ mol⁻¹ are associated with the electrostatic interaction between charged molecules and the charged metal surface (physisorption); while those around or higher than −40 kJ mol⁻¹ 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 1.1×10⁻⁴ l mol⁻¹ and −33.03 kJ mol⁻¹, respectively. The ∆G_ads values are around −30 kJ mol⁻¹, which means that the absorption of DHBP on the steel surface belongs to the physisorption and the adsorptive film has an electrostatic character [11,16].

Potentiodynamic polarization was obtained for steel electrode in 1 M HCl solution with and without inhibitor. Polarization curves in various concentrations of DHBP in 1 M HCl solutions are presented in Fig. 6, at 20 ℃. The polarization exhibits Tafel behavior. The corresponding electrochemical parameters, i.e., corrosion potential (E_corr versus SCE), corrosion current density (I_corr), cathodic and anodic Tafel slopes (β_a,β_c), polarization resistance (R_p) and the inhibition efficiency were obtained by extrapolation of the Tafel lines and are shown in Table 3. The addition of DHBP shifts both anodic and cathodic branches to the lower values of corrosion current densities and thus causes a remarkable decrease in the corrosion rate. It can be clearly seen from Fig. 6 that, both anodic metal dissolution of iron and cathodic hydrogen ions reduction reactions are inhibited after the addition of schiff base to the aggressive solution. These results are indicative of the adsorption of inhibitor molecules on steel surface. The inhibition of both anodic and cathodic reactions increases with the increasing inhibitor concentrations. The cathodic current-potential curves give rise to parallel lines indicates that the addition of DHBP to the 1 M HCl solution does not modify the reduction mechanism. The corrosion potential shows a small change in the range of −495 to −505 mV vs. SCE. These results indicated that the presence of DHBP compound inhibits iron oxidation and the hydrogen evolution reaction. Consequently these compounds can be classified as the mixed corrosion inhibitors, as electrode potential displacement is lower than 85 mV in any direction [36]. The polarization resistance (R_p) from Tafel extrapolation method is calculated using the Stern-Geary equation [37]:

By increasing the inhibitor 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 which is in agreement with impedance spectroscopy results.

In order to evaluate the conditions of the steel surfaces in contact with hydrochloric acid solution, surface analysis was carried out. Surface was observed after 6 h of immersion in 1 M HCl in the absence and presence of DHBP. Scanning electron microscopy (SEM) images of the surface in the absence and presence of inhibitor are presented in Fig. 7. The SEM images reveal the presence of corrosion attack and some pits on the surface in the absence of inhibitor while such damages diminish in the presence of inhibitor. As can be seen from these figures, it is obvious that the surface looks more uniform in the presence of inhibitor.