Introduction

Weak organic acids, such as acetic acid, formic acid, and propionic acid, are commonly present in oil field brines [1]. The total quantity of organic acids in water produced in oil and gas systems is in the range of 500–3,000 ppm, of which acetic acid contributes 50–90 % of the total [2]. Organic acids adsorb onto steel surfaces via the electrons of the carbonyl oxygen. Acetic acid will be used as a representative of small molecule organic acids which are known to contribute to the corrosion rate [3–5]. Many studies have reported [6–8] that carbon steel pipeline failure is directly related to the content of organic acids and results in great economic loss, accidents, and environmental pollution. So, it is important to investigate the comparative corrosion and corrosion inhibition of carbon steel in the presence organic acids. The use of surfactants as corrosion inhibitors is increasing day by day [9, 10]. Surfactants are environmentally acceptable and are very economical and easily available.

Inhibition of metal corrosion by surfactants is a result of adsorption of surfactant molecules or ions at the metal surface to form a protective layer. The extent of adsorption depends on the nature of the metal, the metal surface condition, the mode of adsorption, the chemical structure of the surfactants, and the type of liquid medium [11, 12]. The present study aimed to investigate the efficiency of sodium lauryl ether sulfate (SLES) as a corrosion inhibitor for L80 carbon steel in acetic acid. An attempt was also made to clarify the effects of adding ethanol on the inhibition efficiency of the studied surfactant. Potentiodynamic polarization curve measurements were performed and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) examinations of the electrode surface were carried out.

Experimental

The corrosive electrolyte was prepared by dissolving the appropriate amount of acetic acid (Merck) in doubly distilled water. SLES (Cognis) and ethanol (Merck) were used without further purification. All electrolyte solutions were freshly prepared from analytical grade chemical and doubly distilled water.

All electrochemical measurements were performed using a conventional three-electrode cell, consisting of a saturated calomel electrode (SCE), L80 carbon steel, and platinum foil as reference, working, and counter electrodes, respectively. The working electrodes for electrochemical tests were made of L80 pipeline carbon steel with a chemical composition in wt% of C 0.39 %, Si 0.25 %, Mn 1.73 %, P 0.024 %, S 0.007 %, Mo 0.15 %, Cr 0.001 %, Ni 0.07 %, Al 0.03 %, Cu 0.06 %, and Fe 97.288 %. The unexposed faces and edges of the working electrode were coated with epoxy resin with a resulting exposed area of 0.642 cm2. Electrodes were abraded with up to 1,000 grit silicon carbide paper, rinsed with deionized water, and degreased in acetone. Experiments were carried out at a constant temperature of 25 ± 1.0 °C using a water thermostat. The potentiodynamic current–potential curves were recorded using a potentioscan type potentiostat/galvanostat (EG&G model 273). The experiments were performed by sweeping the potential between −2.0 and +2.0 V (SCE) from open circuit potential at a scan rate of 1.0 mV s−1.

The morphological structure of the L80 carbon steel surface of some samples was determined after the desired electrochemical tests by SEM. The composition and structure of the film formed on the electrode surface were examined by XRD (model Philips p.w./no. 1730) and EDX (Traktor TN-2000 energy-dispersive spectrometer) analysis.

Results and Discussion

Corrosion Behavior of L80 Carbon Steel in Acetic Acid Solutions

The potentiodynamic anodic polarization curves for L80 carbon steel in acetic acid solutions of various concentrations (from 2 to 10 %) are shown in Fig. 1.

Fig. 1
figure 1

Potentiodynamic anodic polarization curves for L80 carbon steel in various concentrations of acetic acid solutions at 25 °C

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Region I in Fig. 1 represents L80 carbon steel corrosion. At region II the passivation process begins. The passivation may be due to the formation of a light coat of a protective material, such as metal oxide, to create a shell against corrosion. Region III represents the passivation region. The passive layer destruction begins in region V (the transpassive region) where oxygen evolution starts. It is clear from Fig. 1 that the anodic part of the polarization curves for L80 carbon steel in acetic acid exhibits an active–passive behavior. The active region displays one anodic peak which is characterized by two values, the anodic peak potential (E ap) and its current density (j ap).

The dissolution of L80 carbon steel in acetic acid solutions can be represented by the following steps:

$$ {\text{Fe + CH}}_{ 3} {\text{COO}}^{ - } \to \left[ {{\text{Fe}}\left( {{\text{CH}}_{ 3} {\text{COO}}} \right)} \right]_{\text{ads}} {\text{ + e}}^{ - } $$
(1)
$$ \left[ {{\text{Fe}}\left( {{\text{CH}}_{ 3} {\text{COO}}} \right)} \right]_{\text{ads}} \to \left[ {{\text{Fe}}\left( {{\text{CH}}_{ 3} {\text{COO}}} \right)} \right]^{ + } {\text{ + e}}^{ - } $$
(2)
$$ \left[ {{\text{Fe}}\left( {{\text{CH}}_{ 3} {\text{COO}}} \right)} \right]^{ + } { + }\,{\text{H}}^{ + } \Leftrightarrow {\text{Fe}}^{{ 2 { + }}} { + }\,{\text{CH}}_{ 3} {\text{COOH}} $$
(3)

Inspection of the data in Fig. 2 reveals that an increase in the acetic acid concentration increases the current density of the anodic peak j ap and shifts the anodic peak potential E ap to more negative values.

Fig. 2
figure 2

Variation of j ap and E ap with acetic acid concentration

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As a result of the formation of iron oxide on the metal surface, the dissolution current density decreases rapidly to a small value (j pass) indicating the onset of passivation [13]. The latter extends to about approximately 1.5 V at which the j pass starts to increase rapidly. The rise of the current density could be attributed to evolution of oxygen [14]. The relation between passive current density j pass versus acid concentration is depicted in Fig. 3. The increase in passive current density with acid concentration was found to be primarily due to the increase in the concentration of Fe2+ ions.

Fig. 3
figure 3

Variation of j pass with acetic acid concentration

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Effect of Sodium Lauryl Ether Sulfate (SLES)

The influence of the addition of various concentrations of SLES on the anodic potentiodynamic polarization curves for L80 carbon steel was investigated in 10 % acetic acid solution at 25 °C (Fig. 4). The addition of SLES decreases the current density of the anodic peak j ap and passivation current j pass and shifts the anodic peak potential E ap to more positive potential. These data suggest that this surfactant inhibits the dissolution and corrosion of L80 carbon steel in acid media.

Fig. 4
figure 4

Potentiodynamic anodic polarization curves for L80 carbon steel in 10 % acetic acid solution with and without SLES at 25 °C

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Figure 5 represents the relationship between the concentration of SLES and j ap and E ap. It is clearly seen that the corrosion of L80 carbon steel decreases when increasing the surfactant concentration. On other hand, E ap moves to more positive values with increasing the surfactant concentration, indicating that the SLES acts predominately as an anodic inhibitor [15].

Fig. 5
figure 5

Variation of j ap and E ap with SLES concentration

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The values of the inhibition efficiency (η %) were calculated from polarization data as reported in our previous work [16].

$$ \eta \;\% = \left[ {{{\left( {j_{\text{ap}}^{0} - \;j_{\text{ap}} } \right)} \mathord{\left/ {\vphantom {{\left( {j_{\text{ap}}^{0} - \;j_{\text{ap}} } \right)} {\left( {j_{\text{ap}}^{0} } \right)}}} \right. \kern-0pt} {\left( {j_{\text{ap}}^{0} } \right)}}} \right]\; \times \;100 $$
(4)

where \( j_{\text{ap}}^{0} \) and j ap are the current density of the anodic peak in the absence and presence of surfactant, respectively.

The variation of the inhibition efficiency η % as a function of the logarithmic concentration of SLES is shown in Fig. 6. The plot shows that the inhibition effect of SLES increases with its concentration increase. The highest inhibition efficiency of SLES was found at 400 mg L−1 (η = 82 %), and further increase in SLES concentration did not cause any significant change in the value of the inhibition efficiency.

Fig. 6
figure 6

Plot of the inhibition efficiency η % versus the logarithm of SLES concentration

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This surfactant seems to function as an inhibitor by being adsorbed on the metal surface. The adsorption process involves the displacement of water and acid from the metal surface [17]. In this case, the adsorbed surfactant can block some active sites for L80 carbon steel dissolution and consequently retards the anodic dissolution of L80 carbon steel. The effectiveness of SLES as a corrosion inhibitor depends on its structure. It is apparent from the molecular structure of SLES that the surfactant is able to adsorb on the metal surface through the lone pairs of electrons of S and O atoms; besides, the long hydrocarbon chain means a more bulky molecule, which screens the surface from attack [18].

Adsorption isotherms can give us useful information on the interaction between the inhibitor and metal surface. In order to obtain adsorption isotherms, the surface coverage values (θ) for different concentrations of SLES in acetic acid solution were obtained from Eq. 5 [19, 20] and tested graphically for fitting a suitable adsorption isotherm.

$$ \theta = {{\eta \;\% } \mathord{\left/ {\vphantom {{\eta \;\% } {100}}} \right. \kern-0pt} {100}} $$
(5)

Attempts were made to fit the experimental data of the corrosion of L80 carbon steel in acetic acid in the presence of varying concentration of SLES to the Freundlich isotherm model, which is described by Eq. 6 [21]:

$$ \theta = k_{\text{F}} C^{1/n} \,\left( {\text{nonlinear form}} \right) $$
(6)
$$ \log \theta = \log k_{F} + 1/n\log C\,\left( {\text{linear form}} \right) $$
(7)

where C is the concentration of inhibitor, k F is the Freundlich isotherm constant, and 1/n is the adsorption intensity.

The value of standard free energy of adsorption (\( \Delta G_{\text{ads}}^{0} \)) is related to k F by the following relation [22]:

$$ \Delta G_{\text{ads}}^{0} = - RT\ln \left( {55.5\,k_{\text{F}} } \right) $$
(8)

where R is the ideal gas constant and T is the absolute temperature.

Figure 7 represents the fitting of polarization data to the Freundlich isotherm at 25 °C. The linear regression coefficient (R 2) is close to unity (R 2 = 0.9949), suggesting that the adsorption of SLES obeys the Freundlich isotherm model. Because the 1/n value is above zero and below one (1/n = 0.9), this indicates that the adsorption process is easy [23]. The Freundlich isotherm equilibrium constant k F value calculated is 271.39 L mg−1. From Eq. 8, \( \Delta G_{\text{ads}}^{0} \) can be calculated as −23.79 kJ mol−1. The value of \( \Delta G_{\text{ads}}^{0} \) has a negative sign indicating that the adsorption reaction proceeds spontaneously and is accompanied by a highly efficient adsorption of the surfactant [24].

Fig. 7
figure 7

Fitting of the corrosion data of L80 carbon steel in the presence of SLES to the Freundlich adsorption isotherm

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The morphology of the L80 carbon steel surface in acetic acid solution in the absence and presence SLES specimens was analyzed by SEM. Figure 8a shows a SEM image of polished L80 carbon steel. The entire surface is smooth and homogeneous. Figure 8b shows the corroded sample after exposure for 24 h; the surface image shows that the surface was covered with a high density of corrosion products. Figure 8c shows the specimen of L80 carbon steel immersed in acetic acid containing 400 mg/L of SLES. Less corrosion attack was found for the sample exposed to acetic acid containing SLES.

Fig. 8
figure 8

SEM of a polished L80 carbon steel, b after immersion in 10 % acetic acid solution for 24 h, c after immersion for 24 h in 10 % acetic acid solution containing 400 mg/L SLES

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Figure 9 shows the EDX analysis of the islands that are shown in Fig. 8b. The EDX spectra show the characteristics peaks of some elements constituting the L80 carbon steel samples (Fe, Mn, and C). In addition, iron oxide is present, as indicated by the Fe and O signals.

Fig. 9
figure 9

EDX spectra of the islands that are shown in Fig. 8b

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An XRD study was performed to investigate the adsorption of SLES on the electrode surface. Figure 10a shows the XRD pattern taken on the corroded surface of L80 carbon steel in acetic acid solution. The formed oxides are mainly of FeO and Fe2O3. Upon addition of 400 mg/L of SLES in acetic acid solution as shown in Fig. 10b, the intensity of the Fe2O3 peak decreases and new peaks appear for C, Na, O, and S, which could be attributed to the presence of SLES molecules on the electrode surface.

Fig. 10
figure 10

XRD pattern for corroded surface of L80 carbon steel a in acetic acid solution, b acetic acid solution containing 400 mg/L SLES

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Effect of Alcohols on Inhibition Efficiency of SLES

In this section, we explore the effect of ethanol on the inhibition efficiency of SLES for L80 carbon steel corrosion in acetic acid solution. Figure 11 depicts the effect of adding increasing concentration of ethanol (from 0.1 to 1.0 mol/L) on the anodic potentiodynamic polarization responses for L80 carbon steel in 10 % acetic acid solution containing 100 mg/L of SLES. It is obvious from Fig. 11 that the addition of ethanol drastically influences the polarization curves.

Fig. 11
figure 11

Potentiodynamic anodic polarization curves for L80 carbon steel in 10 % acetic acid solution containing 100 mg/L SLES in the absence and presence of different concentrations of ethanol at 25 °C

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Figure 12 shows the variation of the current density of the anodic peak j ap with the amount of alcohol added. It is obvious that the values of j ap increased with increasing alcohol content in the solution and reached a maximum value at about 0.4 mol/L for ethanol and then decreased. This behavior is due to two opposing processes [25, 26]:

Fig. 12
figure 12

Variation of j ap with alcohol concentration

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  1. 1.

    The presence of ethanol in the ad-micelle will enlarge the space among the polar groups of SLES molecules and increase the contact of L80 carbon steel surface with acid solution. Consequently, the inhibition efficiency of SLES can be decreased and the corrosion rate can be increased by the addition of alcohol.

  2. 2.

    The concentration of free H+ in the solution decreases with the addition of ethanol because alcohol in acid solution can combine with H+ to form the C2H5OH2 + complex. This leads to the decrease in the corrosion of L80 carbon steel.

At relatively low concentrations of ethanol, the effect of alcohol on the concentration of free H+ is negligible, but the decrease of the coverage of SLES on the L80 carbon steel surface with addition of alcohol is the predominant factor. When the concentrations of alcohols are higher than those required to achieve j ap max, the effect of alcohols on the concentration of free H+ ions becomes the predominant factor and consequently the values of j ap max achieved start to decrease. It is clear that the presence of higher concentrations of ethanol exhibits a synergistic effect with SLES.

In order to judge whether synergism is taking place, one has to calculate the synergism parameter as initially proposed by Murakawa et al. [27] to describe the combined inhibition behavior of SLES and alcohol. Generally, for the interaction of SLES and alcohol, this synergism parameter is defined as follows:

$$ S_{\text{I}} = {{\left( {1\; - \;\eta_{\text{SLES}} \;\% \; - \;\eta_{\text{alcohol}} \;\% + \eta_{{ ( {\text{SLES + alcohol)}}}} \;\% } \right)} \mathord{\left/ {\vphantom {{\left( {1\; - \;\eta_{\text{SLES}} \;\% \; - \;\eta_{\text{alcohol}} \;\% + \eta_{{ ( {\text{SLES + alcohol)}}}} \;\% } \right)} {\left( {1\, - \,\eta_{{({\text{SLES + alcohol)}}}} \;\% } \right)}}} \right. \kern-0pt} {\left( {1\, - \,\eta_{{({\text{SLES + alcohol)}}}} \;\% } \right)}} $$
(9)

where η SLES and η alcohol % are the inhibition efficiencies observed with surfactant and alcohol acting alone, respectively, and η (SLES+alcohol) % is the experimentally observed inhibition efficiency for the mixture surfactant and alcohol. The calculated S I values are presented in Table 1. The S I values nearly equal to unity suggest that the enhanced inhibition efficiencies caused by the addition of these alcohol to surfactant are due mainly to the synergistic effect [28].

Table 1 Synergism parameter (S I) for different concentrations of SLES in the presence of 1.0 mol L−1 of alcohol
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Conclusions

  1. 1.

    The anodic polarization curves for the corrosion of L80 carbon steel in acetic acid solution exhibit a active–passive transition.

  2. 2.

    Results obtained from the experimental data show that SLES acts as an inhibitor of the corrosion of L80 carbon steel in acetic acid.

  3. 3.

    Inhibition efficiency increases with increase in the concentration of the studied inhibitor.

  4. 4.

    Maximum inhibition efficiency (82 %) was obtained at a concentration of 400 mg L−1.

  5. 5.

    The adsorption of SLES on L80 carbon steel surface from acetic acid obeys the Freundlich model isotherm.

  6. 6.

    The adsorption of SLES on L80 carbon steel surface was a spontaneous process.

  7. 7.

    The addition of low concentrations of ethanol reduces the inhibition action of SLES. Beyond certain concentrations of ethanol, a synergistic effect of alcohol and SLES is observed.