1 Introduction

Corrosion is worth investigating in oilfield applications, because corrosion problems represent a large portion of the total costs for oil and gas producing companies every year worldwide [1]. Even with advanced corrosion-resistant materials available, carbon steel has been widely employed as the construction material for pipe work in the oil and gas production such as down hole tubular flow lines and transmission pipelines. Steel pipelines play an important role in transporting gases and liquids throughout the world [2]. Moreover, appropriate corrosion control can help avoid many potential disasters that can cause serious issues including loss of life, negative social impacts, and water resource and environmental pollution [3, 4].

Among the corrosion control techniques, the development of new corrosion inhibitors have substantially increased in the recent years because it is believed to be one of the most effective and economic methods to protect metal corrosion in acidic media [5, 6]. Organic inhibitors have been successfully used in these applications even without an understanding of the inhibition mechanism [7].The corrosion inhibition by organic compounds is related to their adsorption properties. Adsorption depends on the nature and the state of the metal surface (microstructure and chemical composition), on the type of corrosive environment and on the chemical structure of the inhibitor [8]. Inhibitors incorporate into the corrosion product layer and form a protective barrier between the base metal and the corrosive media. It is suggested that the structure of the inhibitor must be appropriate to interact with the corrosion products and that they can be effective on iron carbonates or sulfides, but not effective on oxides [8].

The present investigation is concerned with the mechanism and efficiency of Methionine as corrosion inhibitor of carbon steel in 1 M HCl solution.

2 Experimental Part

Experimental data are provided by Nyquist plots. Materials tested were carbon steel samples that were cut from parent pipe with chemical composition reported as C:0.2, Si:0.032, P:0.21, Mn:0.35% wt, Fe:rest. The specimens of dimension 1 cm × 1 cm (exposed) × 4.3 mm (isolated with polyester resin) were used for 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 the dilution of Merck Product HCl. The concentration range of the inhibitor (See Fig. 1) employed varied from 1 × 10−4 to 5 × 10−3 M and the temperature change was studied in 1 M HCl, both in the absence and in the presence of MTI in the temperatures 25, 45, and 65 °C. All chemicals used in the present work were of reagent-grade Merck product and used as received without further purification.

Fig. 1
figure 1

Weight loss specimen and weight loss measurement setups

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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 to 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.

In these experiments rectangular coupons were cut into 5 × 3 × 0.5 cm dimensions used for weight loss measurements.

The surface areas of the coupons were mechanically abraded with 220 up to 2000 grades of emery papers degreased with acetone and rinsed by distilled water just before immersion. Standard weight loss tests were carried out under the procedure according to ASTM G1 and G31 [9].

At this stage accurate weight determination was carried out to minimize the uncertainty. The balance should have an accuracy of at least 0.1 mg and weighing each coupon at least three times. Solutions were prepared with 1 M HCl without inhibitors and with different concentration of inhibitors.

The specimens were submerged in the solutions. The volume of the test solution should be large enough to avoid any appreciable change in its corrosivity during the test, either through exhaustion of corrosive constituents or by accumulation of corrosion products that might affect further corrosion. The selected volume in test was 500 ml that satisfies this requirement and the specimens stored for 24 h [9].

After completing this period of time, coupons should be removed from the solution and be cleaned. Chemical procedure involves immersion of the corrosion test specimen in a specific solution that is designed to remove the corrosion products with minimal dissolution of any base metal. The choice of chemical procedure should be in accordance to Table A.1.1 of ASTM G1 [9, 10].

For surface analysis, the specimens of size 0.32 cm2 were abraded with emery paper (up to 1200) to give a homogeneous surface, then washed with distilled water and acetone. The specimens were immersed in 1 M HCl prepared with and without the addition of 5 × 10−3 M at 25 ± 2 °C for 24 h, cleaned with distilled water. The surface morphology of the electrode surface was evaluated by atomic force microscopy (AFM) Nan Surf easyscan2 and metallographic microscope Neophot 32 (Fig. 2).

Fig. 2
figure 2

Nyquist plots for carbon steel in 1 M HCl without and with various concentrations of MTI at 25 °C: (1) 0, (2) 1 × 10−4, (3) 5 × 10−4, (4) 1 × 10−3, (5) 5 × 10−3

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3 Results and Discussion

3.1 Electrochemical Impedance Spectroscopy (EIS)

The corrosion behavior of steel in 1 M HCl solution in the presence of inhibitor was investigated by the EIS method at 25 °C. The Nyquist plots are shown in Figs. 3, 4, 5, and 6. For Nyquist plots (Figs. 1, 2, 3), it is clear that the impedance diagrams in most cases do not show perfect semicircle. This behavior can be attributed to the frequency dispersion [11] as a result of roughness in the homogenates of the electrode surface. The impedance response consisted of characteristic semicircles for solutions examined indicating that the dissolutions of steel process occur under charge transfer control in other words under activation control and the presence of the inhibitor does not change the mechanism of the acid dissolution. These semicircles are of a capacitive type whose diameters increase with increasing inhibitor concentration. The impedance spectra of the different Nyquist plots (Figs. 1, 2, 3) were analyzed by fitting the experimental data to a simple equivalent circuit model (Fig. 7), which includes the solution resistance R s and the double layer capacitance C dl which are placed in parallel to the charge transfer resistance R ct [12]. The experimental and computer fit results of Nyquist plot for steel in 1 M HCl containing 5 × 10−4 M MTI is demonstrated in Fig. 7. The R ct values were calculated from the difference in impedance at low and high frequencies. The value of R ct is a measure of electron transfer across the surface and inversely proportional to the corrosion rate. The double layer capacitance, C dl was calculated at the frequency f max at which the imaginary component of the impedance is maximal by the following equation:

Fig. 3
figure 3

Nyquist plots for carbon steel in 1 M HCl without and with various concentrations of MTI at 45 °C: (1) 0, (2) 5 × 10−4, (3) 1 × 10−3, (4) 5 × 10−3

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Fig. 4
figure 4

Nyquist plots for carbon steel in 1 M HCl without and with various concentrations of MTI at 65 °C: (1) 0, (2) 1 × 10−3, (3) 5 × 10−3

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Fig. 5
figure 5

Nyquist plots for carbon steel in 1 M HCl with 1 × 10−3 M of MTI at: (1) 65, (2) 45, (3) 25 °C

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Fig. 6
figure 6

Nyquist plots for carbon steel in 1 M HCl at: (1) 65, (2) 45, (3) 25 °C

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Fig. 7
figure 7

Equivalent circuits compatible with experimental and computer fit results of Nyquist plot for carbon steel in 1 M HCl with 5 × 10−4 M of MTI at 45 °C

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

where ω is the frequency in rad s−1, ω = 2πf, and f is frequency in Hz.

The values of R ct, C dl, and %IE for steel in 1 M hydrochloric acid containing different concentrations for the used inhibitor are shown in Tables 1, 2, and 3.

Table 1 Data obtained from EIS measurements for C-steel in 2 M HCl in the presence of different concentrations of inhibitor 25 °C
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Table 2 Data obtained from EIS measurements for C-steel in 2 M HCl in the presence of different concentrations of inhibitor at 45 °C
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Table 3 Data obtained from EIS measurements for C-steel in 2 M HCl in the presence of different concentrations of inhibitor at 65 °C
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Percent inhibition efficiencies (IE) were calculated using the following formula.

$$\% {\text{IE}} = \left[ {1 - (R_{\text{ct}} ,b/R_{\text{ct}} ,i)} \right] \times 100,$$
(2)

where R ct ,i and R ct ,b are values of the charge transfer resistance with and without inhibitor, respectively.

The data indicate that increasing charge transfer resistance is associated with a decrease in the double layer capacitance and increase in the percentage inhibition efficiency. The decrease in C dl values could be attributed to the adsorption of the inhibitor molecules at the metal surface. It has been reported that the adsorption of organic inhibitor on the metal surface is characterized by a decrease in C dl [13]. Furthermore 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.

It is seen that the investigated MTI has inhibiting properties at all the studied temperatures and the values of inhibition efficiency for MTI decrease with temperature increases (Figs. 5, 6). Thus, the inhibitor efficiencies were temperature dependent.

3.2 Application of Adsorption Isotherm

Assuming a direct relationship between inhibition efficiency and the degree of surface coverage, θ, for different concentrations of the inhibitor. Data obtained from EIS measurements were tested graphically for fitting various adsorption isotherms including Langmuir, Frunkin, Temkin, and Frundich. By far the best fit was obtained with the Langmuir adsorption isotherm for the different inhibitors under investigation. According to this isotherm, θ is related to the inhibitor concentration C:

$$\frac{C}{\theta } = \frac{1}{{K_{\text{ads}} }} + C$$
(3)

By plotting C/θ versus C straight lines were obtained as shown in Fig. 8. For inhibitor, the slopes of the straight lines deviate slightly from unity indicating the presence of mutual repulsion or attraction between the adsorbed molecules. Similar behavior was obtained for the adsorption of some organic inhibitors on iron surface in acid solutions [14–16]. The intercept of the straight line permits the calculation of the equilibrium constant K ads, which leads to evaluate the standard free energy of adsorption (∆G ads) from the following equation:

$$\Delta G_{\text{ads}} = - {\text{RT}}\; \ln (55.5K_{ads} )$$
(4)
Fig. 8
figure 8

Langmuir adsorption plot for steel electrode in 1 M HCl at 25 °C

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The value 55.5 in the above equation is the concentration of water in solution in mol l−1 [17], where R is the universal gas constant and T is the absolute temperature. In the present study, the value of ∆G ads was slightly more negative than −20 kJ mol−1, indicating that the adsorption mechanism of the inhibitor on steel in 1 M HCl solution was mainly chemisorption. In fact, the inhibitor molecules form a film whose strength on the steel surface is due to hydrogen and n bonding.

The K ads values showed the strength of the adsorption forces between the inhibitor molecules and steel surface. According to the values obtained for K ads, the interaction between the double layer that exists in the phase boundary and the adsorbed molecules appeared to be relatively strong.

3.3 Weight Loss Measurements

Weight loss of steel at various time intervals, in the absence and presence of different concentrations of inhibitor at 25 °C was studied. The initial total surface area of the specimen (making corrections for the areas associated with mounting holes) and the mass lost during the test are determined. The average corrosion rate may be then obtained as follows:

$$\text{Corrosion rate (}{\text{mpy}}\text{)} = \frac{K \times W}{A \times t \times \rho },$$
(5)

where K = a constant, W = Weight loss (g), t = time of exposure (h), A = Expanded surface area (cm2), ρ = Density of carbon steel sample (g cm−3).

Section 8.1.2 of ASTM G1 determines constant K for calculating corrosion rate with different units. For expressing corrosion rate as mils per year (mpy) this constant is equal to 3.45 × 106. Appendix X1 of ASTM G1 determines the value of density for different alloys. This value for carbon steel is equal to 7.84.

The efficiency percentage of inhibitors is taken as:

$$\% {\text{IE}} = (1 - W_{\text{i}} /W_{\text{o}} ) \times 100,$$
(6)

where W o = weight loss without inhibitor, W i = weight loss with inhibitor, respectively.

The values of the corrosion rate (mg cm−2 min) and the percentage inhibition efficiency are presented in Table 4. It is clear that the decreasing corrosion rate is associated to increase in the %IE. Figure 8 shows the variation of the %IE with the concentration of the additives. The curves obtained indicate that the %IE increases with the increasing concentration of the additives.

Table 4 Data obtained from weight loss measurements for steel in 1 M HCl in the presence of different concentrations of the used inhibitor
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3.4 Mechanism of Inhibition and Molecular Structure

In order to predict the adsorption mechanism of inhibition onto steel surface, corrosion mechanism of metal must be known [18]. We assume that the electrochemical reaction of iron corrosion involves transfer of two electrons, and then there are two steps where each one represents transfer of one electron and one of them may be considered the rate determining step. This result is in good agreement with the mechanisms that are suggested for iron dissolution [19] in:

(i) Aqueous solutions:

$${\text{Fe }} + {\text{ H}}_{ 2} {\text{O }} \Rightarrow \, \left[ {\text{FeOH}} \right]_{\text{ads}} + {\text{ H}}^{ + } + {\text{ e}}^{ - 2} ,$$
(7)
$$\left[ {\text{FeOH}} \right]_{\text{ads}} \Rightarrow \, \left[ {\text{FeOH}} \right]^{ + }_{\text{ads}} + {\text{ e}}^{ - }_{\text{ads}}\,\, \left( {{\text{r}}.{\text{d}}.{\text{s}}.} \right),$$
(8)
$$\left[ {\text{FeOH}} \right]^{ + }_{\text{ads}} + {\text{ H}}^{ + } \Rightarrow {\text{ Fe}}^{ 2+ } + {\text{ H}}_{ 2} {\text{O}} .$$
(9)

(ii) Aqueous solutions containing Cl− ions:

$${\text{Fe }} + {\text{ H}}_{ 2} {\text{O }} + {\text{ Cl}}^{ - } \Rightarrow \, \left[ {\text{FeClOH}} \right]^{ - }_{\text{ads}} + {\text{ H}}^{ + } + {\text{ e}}^{ - 2} ,$$
(10)
$$\left[ {\text{FeClOH}} \right]^{ - }_{\text{ads}} \Rightarrow \, \left[ {\text{FeClOH}} \right]_{\text{ads}} + {\text{ e}}^{ - }_{\text{ads}} \,\,\left( {{\text{r}}.{\text{d}}.{\text{s}}.} \right),$$
(11)
$$\left[ {\text{FeClOH}} \right] \, + {\text{ H}}^{ + } \Rightarrow {\text{ Fe}}^{ 2+ } + {\text{ Cl}}^{ - } + {\text{ H}}_{ 2} {\text{O,}}$$
(12)

where [FeOH]ads and [FeClOH]ads are the adsorbed intermediates where each of them is involved in the rate determining step (Eqs. 7 and 10, respectively) of steel dissolution according to mechanism (i) and (ii), respectively. It must be pointed out that the presence of Cl− ions does not exclude dissolution through the [FeOH]ads intermediate in chloride-free acid media, as the two mechanisms can proceed simultaneously [14].

Inhibition of steel in HCl solution by the MTI can be explained on the basis of adsorption. Four types of adsorption may take place involving organic molecules at the metal solution interface (i) electrostatic attraction between charged molecules and the charged metal, (ii) interaction of n-electrons with the metal, (iii) interaction of uncharged electron pairs in the molecule with the metal, and (iv) a combination of the above [20, 21]. It is apparent that the adsorption of these compounds on the Fe surface could occur directly on the basis of donor acceptor between the lone pairs of the heteroatoms and the extensively delocalized n-electrons of the pyrimidine molecule and the vacant d-orbitals of iron surface atoms [17].

In acidic solution, these compounds can exist as protonated species. These protonated species may adsorb on the cathodic sites of steel surface and decrease the evolution of hydrogen. These compounds are able to adsorb on anodic sites through N atoms, azomethine group, heterocyclic, and aromatic rings which are electron donating groups. The adsorption of these compounds on anodic sites may decrease anodic dissolution of steel.

3.5 Surface Photographs

The surface photographs of steel, which were immersed in blank and containing MTI after 24 h, are presented in Fig. 9. It can be seen clearly from Fig. 9, there is a good surface coverage on the steel surface in the presence of inhibitor which provides a good corrosion inhibition efficiency. This is due to the involvement of inhibitor molecules in the interaction with the reaction sites of steel surface, resulting in a decrease in the contact between iron and the aggressive medium and it sequentially exhibited excellent inhibition effect.

Fig. 9
figure 9

Surface photographs of steel taken after 24 h for corrosion of steel electrode at different inhibitor concentrations

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Figure 10 represents the morphologies of steel specimens after immersion for 24 h in 1 M HCl solutions without and with 5 × 10−3 M MTI. In the absence of inhibitor (Fig. 10a) 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 0.9 μm by atomic force microscopy (Fig. 10a). In the presence of MTI, smoother surface was obtained and the R a value decreased to 108 nm (Fig. 10b) as a consequence of low corrosion damage and the protective formation of an inhibitor layer on steel surface.

Fig. 10
figure 10

Surface of steel electrode by atomic force microscopy after 24 h immersion at OCP in 1 M HCl solution: a without and b with 5 × 10−3 M MTI

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4 Conclusions

The corrosion behavior of carbon steel was investigated in 1 M HCl with and without addition of various concentrations of MTI using a series of techniques. MTI 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. Impedance measurements indicate that with increasing inhibitor concentration, the polarization resistance (R ct) increased, while the double layer capacitance (C dl) decreased. The MTI gives the highest inhibition efficiency at 5 × 10−3 M. The adsorption of these compounds on the steel surface was found to obey Langmuir adsorption isotherm. The high AFM 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.