1 Introduction

Even with advanced corrosion resistant materials available, carbon steel has been widely employed as a construction materials for pipe work in oil and gas production such as down hole tubular, flow lines and transmission pipelines. Carbon steels are used in mass amounts in marine applications, chemical processing, petroleum production and refining, construction and metal-processing equipment [1–6]. Although their corrosion resistance is limited, these materials are preferred to others because of economic considerations and the fact that their corrosion falls into general attack.

API 5L is a general carbon steel which is used for fabrication of flow and transition pipelines in oil and gas industry [7].

Acid solutions are widely used in industries for lots of purposes, such as acid pickling, industrial acid cleaning, acid descaling and oil well acidizing. Among the acid solutions, hydrochloric acid is one of the most widely used agents. Hydrochloric acid is often used as a pickling acid for iron and its alloys to remove undesirable corrosion products. The chemical acid cleaning will cause metal corrosion upon the already cleaned surface after the elimination of the corrosion products. Due to the exposure of carbon steel to corrosive environments, they are susceptible to different types of corrosion mechanisms; therefore, the use of corrosion inhibitors to reduce metal dissolution and wear costs will be predictable [8, 9].

Many scientific studies have been devoted to the subject of the application of corrosion inhibitors, such as organic inhibitors, for controlling (reducing) the corrosion of steel pipelines [10–14]. Organic compounds containing heteroatoms, such as O, N or S, and multiple bonds, which allow an adsorption on the metal surface [15]. Organic molecules of this type adsorb on the metal surface and form a bond between the heteroatom pair and/or the π-electron cloud and the metal, thereby reducing the corrosion in acidic solutions [16].

In the literature, several Schiff bases have been reported as effective corrosion inhibitors for different metals and alloys in acidic media [17–19]. Increasing popularity of Schiff bases in the field of corrosion inhibition science is based on the ease of synthesis from relatively inexpensive starting-materials and their eco-friendly or low toxic properties [20, 21]. The high inhibitory performance of these compounds results from the presence of a –C=N– group in the molecules. The planarity (π) and lone pairs of electrons present on N atoms are the important structural features that determine the adsorption of these molecules on the metal surface [22]. Adsorption of inhibitors on the metal surface involves formation of two types of interaction (physical adsorption and chemical adsorption). Physical adsorption requires the presence of both electrically charged surface of the metal and charged species in the bulk solution. The second one, chemisorption process involves charge sharing or charge transfer from the inhibitor molecules to the metal surface to form a co-ordinate type bond and takes place in the presence of heteroatoms (P, N, S, O, etc.) with lone pairs of electrons and/or aromatic ring in the molecular structure [23, 24].

In the present paper, in order to obtain as effective inhibitor, synthesized Schiff base N,N′-bis(4-hydroxybenzaldehyde)-1,2-cyclohexandiimine (4-HBC) has been studied by techniques like SEM, atomic force microscopy (AFM), polarization curve and electrochemical impedance to investigate corrosion behavior of API 5L carbon steel in 1 M HCl solution. Thermodynamic parameters for both activation and adsorption processes were calculated.

2 Experimental

2.1 Materials

API 5L steel samples were cut from parent pipe with chemical composition reported as C: 0.086, Si: 0.232, P: 0.011, S: 0.002, Cu: 0.014, A1: 0.015, N: 0.076, V: 0.001, Ti: 0.008, Mo: 0.001, Mn: 1.35, Cr: 0.009 %w, Fe:Rest. The specimens of dimension 1 cm × 1 cm (exposed) × 4.3 mm (isolated with polyester resin) were used for polarization and electrochemical impedance methods. They were polished mechanically using different grade emery papers up to 2000, and washed thoroughly with triple distilled water and degreased with acetone before being immersed in the acid solution.

The aggressive solution of 1 M HCl was prepared by dilution of Merck product HCl. The concentration range of inhibitors employed was varied from 1 × 10−5 M to 2 × 10−3  M. All chemicals used in present work were of reagent-grade Merck product and used as received without further purification. The 4-HBC Schiff base (Fig. 1) was prepared according to the described procedure [25]. The resulting white precipitate is filtered off, washed with warm ethanol and diethyl ether. Analysis calculated for C20H22N2O2 (322.41): C, 73.17; H, 6.25; N, 9.64. Found: C, 73.75; H, 6.01; N, 9.70 %. Identification of structure of synthesized Schiff base was performed by IR, 1H NMR, and 13C NMR spectroscopic techniques.

Fig. 1
figure 1

The chemical structure of the Schiff base

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2.2 Methods

The apparatus for electrochemical investigations consists of computer controlled Auto Lab potentiostat/galvanostat (PGSTAT302N) corrosion measurement system at a scan rate of 1 mV s−1. The electrochemical experiments were carried out using a conventional three electrode cell assembly at 25 ± 2 °C. A rectangular platinum foil was used as counter electrode and saturated calomel electrode as the reference electrode. The EIS experiments were conducted in the frequency range of 100 kHz–0.01 Hz at open circuit potential. The AC potential amplitude was 10 mV. Time interval of 20–25 min was given for steady state attainment of open circuit potential. 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 [26, 27].

The API 5L Grade B specimens of size 1 cm × 1 cm × 0.43 cm were abraded with emery paper (up to 2000) to give a homogeneous surface, then washed with distilled water and acetone. The specimens were immersed in 1 M HCl prepared with and without addition of 2 × 10−3 M at 25 ± 2 °C for 6 h, cleaned with distilled water. The surface morphology of the electrode surface was evaluated by scanning electron microscopy (VEGA) and AFM Nan Surf easyscan2.

3 Results and Discussion

3.1 Electrochemical Results

Polarization curves were obtained for carbon steel in 1 M HCl solution with and without inhibitors. The polarization exhibits Tafel behavior. Tafel lines obtained in various concentrations of 4-HBC in 1 M HCl solutions are shown in Fig. 2, at 25 °C, respectively. The corresponding electrochemical parameters, i.e., corrosion potential (E corr vs. SCE), corrosion current density (I corr ), cathodic and anodic Tafel slopes (β a , β c ) and the degree of surface coverage (θ) values were calculated from these curves and are given in Table 1. The degree of surface coverage for different concentrations of inhibitor is calculated using the following equations [28, 29]:

$$ \theta = \frac{{I - \dot{I}}}{I}, $$
(1)

where I and İ are the corrosion current densities without and with corrosion inhibitor, respectively, determined by the intersection of the extrapolated Tafel lines at the corrosion potential for carbon steel in uninhibited and inhibited acid solution. The presence of 4-HBC shifts both anodic and cathodic branches to the lower values of current densities and thus causes a remarkable decrease in the corrosion rate. It can be clearly seen from Fig. 2 that both anodic metal dissolution of iron and cathodic hydrogen and oxygen evolution reactions were inhibited after the addition of Schiff base to the aggressive solution. This result is indicative of the adsorption of inhibitor molecules on the active sites of carbon steel surface [30]. The inhibition of both anodic and cathodic reactions is more pronounced with the increasing inhibitor concentration. However, the influence is more pronounced in the cathodic polarization plots compared to that of the anodic polarization plots. The cathodic current–potential curves (Fig. 2) giving rise to parallel lines indicates that the addition of 4-HBC to the 1 M HCl solution does not modify the reduction mechanism and the reduction at carbon steel surface takes place mainly through a charge transfer mechanism [31–33]. The slopes do not display an order with the inhibitor concentration; this feature indicates that inhibition occurred by a blocking mechanism on the available metal spaces [34–37]. The corrosion potential displayed small change in the range of −459 to −501 mV versus SCE and curves changed slightly towards the negative direction (Fig. 2). These results indicated that the presence of 4-HBC compounds inhibited iron oxidation and in a lower extent hydrogen and oxygen evolution, consequently these compounds can be classified as mixed corrosion inhibitors, as electrode potential displacement is lower than 85 mV in any direction [38]. The polarization resistance (R p ) from Tafel extrapolation method was calculated using the Stern–Geary Equation (Eq. 2) [39].

$$ R_{p} = \frac{{\beta_{a} \beta_{c} }}{{2.303(\beta_{a} + \beta_{c} )}} \times \frac{1}{{I_{corr} }}. $$
(2)

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 [40].

Fig. 2
figure 2

Anodic and cathodic polarization curves for carbon steel in 1 M HCl without and with various concentration of 4-HBC at 25 °C: (1) 0, (2) 1 × 10−5, (3) 3 × 10−5, (4) 5 × 10−5, (5) 1 × 10−4, (6) 3 × 10−4, (7) 5 × 10−4, (8) 1 × 10−3 and (9) 2 × 10−3 M

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Table 1 Potentiodynamic polarization parameters for the corrosion of steel in 1 M HCl solution in absence and in the presence of different concentrations of 4-HBC at 25 °C
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Figure 3 shows the Nyquist diagrams of API 5L Grade B carbon steel in 1 M HCl solutions containing different concentrations of 4-HBC at 25 °C, respectively. All the impedance spectra exhibit single depressed semicircle. The diameter of semicircle increases with the increase of 4-HBC concentration. The semicircular appearance shows that the corrosion of carbon steel is controlled by the charge transfer and the presence of 4-HBC does not change the mechanism of carbon steel dissolution [41, 42]. 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 [42–45]. This behaviour is usually attributed to the inhomogeneity of the metal surface arising from surface roughness or interfacial phenomena [40, 41], which is typical for solid metal electrodes [46]. The equivalent circuit compatible with the Nyquist diagram recorded in the presence of inhibitor is depicted in Fig. 4b. 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 [47].

$$ Z(\omega ) = R_{s} + \frac{1}{{\frac{1}{{R_{ct} }} + i\omega C}}, $$
(3)

where ω is the frequency in rad s−1, 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 [48]. CPE Q dl , R s and R ct can be corresponded to double layer capacitance, \( Q_{dl} = R^{n - 1} C_{dl}^{n} \) solution resistance and charge transfer resistance, respectively. Computer fitting of the spectrum allows evolution of the elements of the circuit analogue. The aim of the fitting procedure is to find those values of the parameters which best describe the data, i.e., the fitting model must be consistent with the experimental data. The experimental and computer fit results of Nyquist plot for steel in 1 M HCl demonstrated in Fig. 4b. 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 M HCl solution. The data indicate that increasing charge transfer resistance is associated with a decrease in the double layer capacitance. It has been reported that the adsorption of organic inhibitor on the metal surface is characterized by a decrease in C dl [48]. The decreased values of C dl may be due to the replacement of water molecules at the electrode interface by organic inhibitor of lower dielectric constant through adsorption, which suggests that Schiff base inhibitor acts by adsorption at the metal–solution interface. The increase in values of R ct and the decrease in values of C dl with increasing the concentration also indicate that Schiff base acts as primary interface inhibitor and that the charge transfer controls the corrosion of steel under the open circuit conditions [39].

Fig. 3
figure 3

Nyquist plots for carbon steel in 1 M HCl without and with various concentration of 4-HBC at 25 °C: (1) 0, (2) 1 × 10−5, (3) 3 × 10−5, (4) 5 × 10−5, (5) 1 × 10−4, (6) 3 × 10−4, (7) 5 × 10−4, (8) 1 × 10−3 and (9) 2 × 10−3 M

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Fig. 4
figure 4

a Equivalent circuits compatible with the experimental impedance data in Fig. 3 for corrosion of steel electrode at different inhibitor concentrations and b experimental and computer fit result of Nyquist plot for carbon steel in 1 M HCl without 4-HBC at 25 °C

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Fig. 5
figure 5

Anodic and cathodic polarization curves for the chemical structure of the steel in 1 M HCl, a blank and b in presence of 2 × 10−3 M of 4-HBC, at different temperatures. (1) 25, (2) 45 and (3) 65 °C

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Table 2 Impedance data for steel in 1 M HCl solution without and with different concentrations of 4-HBC at 25 °C
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3.2 Effect of Temperature

The effect of temperature on the inhibited acid–metal reaction is highly complex, because many changes occur on the metal surface such as rapid etching and desorption of inhibitor. This was accomplished by investigating the temperature dependence of the corrosion current. The change of the corrosion rate with the temperature was studied in 1 M HCl, both in the absence and in the presence of 4-HBC at the temperature 25, 45 and 65 °C and are given in Fig. 5a, b for exploring the activation energy of the corrosion process and the thermodynamic functions of adsorption of 4-HBC. The various electrochemical parameters were calculated from Tafel plots and summarized in Tables 3 and 4. As it can be seen, raising the temperature has no significant effect on the corrosion potentials; but leads to a higher corrosion rate I corr . It is seen that the investigated 4-HBC has inhibiting properties at all the studied temperatures and the values of inhibition efficiency for 4-HBC decrease with temperature increase. The activation parameters for the corrosion process were calculated from Arrhenius-type plot according to the following equation [49]:

$$ \ln \;I_{corr} = \ln \;A - \frac{{E_{a} }}{RT}, $$
(4)

where E a is the apparent activation corrosion energy, R is the universal gas constant, A is the pre-exponential factor and T is the absolute temperature. Arrhenius plots for the corrosion density of carbon steel in the case of 4-HBC are given in Fig. 6. Values of apparent activation energy of corrosion (E a ) for steel in 1 M HCl with the absence and presence of various concentrations of 4-HBC were determined from the slope of ln(I corr ) versus T −1 plots and shown in Table 5. Inspection of the data shows that the activation energy is lower in the absence of inhibitors than in its presence. It has been reported that higher E a in presence of inhibitor for steel in comparison with blank solution is typically showing physisorption [50]. An alternative formulation of Arrhenius equation is [46]:

$$ I_{corr} = \left( \frac{RT}{Nh} \right)\exp \left( {\frac{{\Updelta S_{a} }}{R}} \right)\exp \left( {\frac{{ - \Updelta H_{a} }}{RT}} \right), $$
(5)

where h is Planck’s constant, N is Avogadro’s number, ΔS a is the entropy of activation and ΔH a is the enthalpy of activation. Figure 7 shows a plot of ln(I corr /T) against 1/T. Straight lines are obtained with a slope of (−ΔH a/R) and an intercept of ln(R/Nh) + ΔS a/R from which the values of ΔH a and ΔS a are calculated, are listed in Table 5. In the system, the positive signs of the enthalpies ΔH a reflect the endothermic nature of the steel dissolution process. Practically E a and ΔH a are of the same order. Large and negative values of entropies ΔS a imply that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex [51–53].

Fig. 6
figure 6

Typical Arrhenius plots of ln I corr versus T −1 for carbon steel in 1 M HCl at different concentrations of 4-HBC: (filled squares) 0, (open squares) 1 × 10−5, (filled rectangles) 3 × 10−5, (asterisks) 5 × 10−5, (dashes) 1 × 10−4, (triangles) 3 × 10−4, (open circles) 5 × 10−4, (diamonds) 1 × 10−3 and (filled circles) 2 × 10−3 M

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Table 3 Electrochemical parameters for the corrosion of carbon steel in 1 M HCl solution in the absence and in the presence of different concentrations of 4-HBC at 45 °C
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Table 4 Electrochemical parameters for the corrosion of carbon steel in 1 M HCl solution in the absence and in the presence of different concentrations of 4-HBC at 65 °C
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Fig. 7
figure 7

Typical Arrhenius plots of ln I corr T −1 versus T −1 for carbon steel in 1 M HCl at different concentrations of 4-HBC: (filled squares) 0, (open squares) 1 × 10−5, (filled rectangles) 3 × 10−5, (asterisks) 5 × 10−5, (dashes) 1 × 10−4, (triangles) 3 × 10−4, (open circles) 5 × 10−4, (diamonds) 1 × 10−3 and (filled circles) 2 × 10−3 M

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Table 5 Activation parameters of the dissolution of steel in 1 M HCl solution in the absence and in the presence of 4-HBC
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Fig. 8
figure 8

Langmuir adsorption plot for steel electrode in 1 M HCl at different temperatures: (diamonds) 25, (dashes) 45 and (triangles) 65 °C

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3.3 Adsorption Isotherm and Thermodynamic Parameters

Adsorption isotherms provide information about the interaction of the adsorbed molecules with the metal surface [30]. 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 [54].

$$ {\text{Org}}_{{({\text{sol}})}} + n{\text{H}}_{2} {\text{O}}_{{({\text{ads}})}} \to {\text{Org}}_{{({\text{ads}})}} + n{\text{H}}_{2} {\text{O}}_{{({\text{sol}})}} , $$
(6)

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 that can give important information on the interaction of inhibitor and metal surface. Ten adsorption isotherms (Langmuir, Temkin, Freundlich, Frumkin, Modified, Langmuir, Henry, Viral, Damaskin, Volmer and Flory–Huggins) [55–57] were tested for their fit to the experimental data. The linear regression coefficient values (R 2) were determined from the plotted curves. According to these results, it can be concluded that the best description of the adsorption behavior of 4-HBC can be explained by Langmuir adsorption isotherm which is given by (Eq. 7).

$$ \frac{C}{\theta } = \frac{1}{{K_{ads} }} + C, $$
(7)

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 versus C, depicted in Fig. 8 suggest that the adsorption of 4-HBC on the steel surface obeyed the Langmuir adsorption isotherm in different temperatures. 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 (ΔG ads ) on steel surface can be evaluated with the following equation:

$$ \Updelta G_{ads} = - RT\ln \left( {55.5K_{ads} } \right). $$
(8)

The value 55.5 in the above equation is the concentration of water in solution in mol l−1 [58]. The negative values of ΔG ads suggest that the adsorption of 4-HBC on the carbon steel surface is spontaneous. Generally, the values of ΔG 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 ΔG ads are listed in Table 6. The ΔG ads values are around −30 kJ mol−1, which means that the absorption of 4-HBC on the carbon steel surface belongs to the physisorption and the adsorptive film has an electrostatic character [59]. Enthalpy and entropy of adsorption (ΔH ads and ΔS ads ) can be calculated using the following equation [7]:

$$ \Updelta G_{ads} = \Updelta H_{ads} - T\Updelta S_{ads} , $$
(9)
$$ \ln \;K_{ads} = \frac{{ - \Updelta H_{ads} }}{RT} + \frac{{ - \Updelta S_{ads} }}{R} - \ln (55.5). $$
(10)

Figure 9 represents the plots of ln K ads versus T −1 for adsorption 4-HBC. The lines obtained represent a slope of (−ΔH ads /R) and intercept of [(ΔS ads /R) − ln(55.5)]. Thermodynamic parameters for the adsorption of inhibitors can provide valuable information about the mechanism of corrosion inhibition. An endothermic adsorption process ΔH ads >0 is attributed unequivocally to chemisorption an exothermic adsorption process ΔH ads <0 may involve either physisorption or chemisorption or a mixture of both the processes. The calculated values of ΔH ads and ΔS ads are −14.56 kJ mol−1 and 68.66 J mol−1 K−1, respectively. Based on the results of this study, the calculated ΔG ads and ΔH ads values for 4-HBC show that 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 ΔS ads arises from substitution process, which can be attributed to the increase in the solvent entropy and more positive water desorption entropy. It is also interpreted that with increase of disorders due to the more water molecules they can be desorbed from the metal surface by one inhibitor molecule [59].

Fig. 9
figure 9

Ln(K ads) versus T −1 for adsorption 4-HBC on steel surface

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Table 6 Thermodynamic and equilibrium adsorption parameters for adsorption 4-HBC on carbon steel surface in 1 M HCl solution
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Fig. 10
figure 10

Potentiostatic current–time transients recorded for steel electrode at −0.4 V versus SCE potential step in 1 HCl solution: (1) without and (2) with 2 × 10−3 M 4-HBC

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3.4 Chronoamperometry

In order to gain more insight about the effect of inhibitor on the electrochemical behavior of steel in 1 M HCl solution, potentiostatic current–time transients were recorded. Figure 10 shows the current transients of steel electrode at −0.4 V versus SCE applied anodic potential. As can be seen, in the absence of inhibitor, initially, the current decreases monotonically with time. The decrease in the current density is due to the formation of corrosion products layer on the anode surface. However, in later times the current reaches a steady state value depending on applied potential. In the presence of inhibitor, increasing current was not observed and in later times the current reaches a steady state value and electrode was inhibited from corrosion due to inhibitor adsorption.

Fig. 11
figure 11

Surface of steel electrode by SEM microscope of carbon steel exposed to a 1 M HCl solution and b in the presence of 2 × 10−3 M of 4-HBC

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3.5 Surface Metallography

The metallographic images presented in Fig. 11 show the morphologies of the electrode surface in 1 M HCl with and without inhibitor. The metallography were recorded for carbon steel surface after exposure to 1 M HCl solution for 6 h. Surface of electrode in the absence of inhibitor was observed in Fig. 11a. It was observed that the sample in contact solution gave high corrosion attack on the other hand, in presence of 4-HBC smooth surface was obtained, and the degree of attack decreased (Fig. 11b). SEM images indicate that corrosion was inhibited in presence of 4-HBC (Fig. 11a, b). This is in good agreement with the result obtained from the EIS tests that increasing 4-HBC concentration in solution increases the exponent n of the double layer capacitance.

Fig. 12
figure 12

2D and 3D of AFM images of carbon steel exposed to 1 M HCl solution, a in the absence of 2 × 10−3 M 4-HBC and b in the presence of 2 × 10−3 M 4-HBC (scan size 49.5 × 50 µm)

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AFM images of the steel after 6 h immersion in 1 M HCl solution with and without inhibitor are shown in Fig. 12. In the absence of inhibitor (Fig. 12a) the surface displayed a very irregular topography due to corrosion attack. The average roughness R a of steel in 1 M HCl solution without inhibitor was calculated as 1.58 µm by AFM (Fig. 12b). In the presence of 4-HBC, smoother surface was obtained and the R a value decreased to 136 nm (Fig. 12b) as a consequence of low corrosion damage and the protective formation of an inhibitor layer on steel surface.

4 Conclusion

The 4-HBC Schiff base was synthesized and investigated as corrosion inhibitor for API 5L carbon steel in 1 M HCl solution with different concentrations using a series of techniques. The following points can be emphasized:

  1. (1)

    4-HBC has an excellent inhibition effect for the corrosion of carbon steel in 1 M HCl solution especially in high concentration. Its inhibition efficiency is both concentration and temperature dependent. The high inhibition efficiency of Schiff base was attributed to the formation of a film on the steel surface.

  2. (2)

    Corrosion current density is increased by increasing the temperature, but, the rate of its increase is lower with the presence of Schiff base compound.

  3. (3)

    Polarization measurements demonstrate that 4-HBC behaved as mixed type corrosion inhibitor by inhibiting both anodic metal dissolution and cathodic hydrogen and oxygen evolution reactions.

  4. (4)

    Impedance measurements indicate that with increasing inhibitor concentration, the polarization resistance (R ct ) increased, while the double-layer capacitance (C dl ) decreased.

  5. (5)

    The adsorption of 4-HBC molecules on carbon steel surface has been described by Langmuir adsorption isotherm. The values of ΔG ads and K ads indicate spontaneous interaction with surface and high adsorption ability of studied inhibitor.

  6. (6)

    The high resolution AFM and SEM micrographs showed that the corrosion of carbon steel in 1 M HCl solution was described by corrosion attack and the addition of inhibitor to the aggressive solutions diminished the corrosion of carbon steel.