Electrochemical evaluation of cephalothin as corrosion inhibitor for API 5L X52 steel immersed in an acid medium

3.2.1 Influence of the inhibitor concentration and electrode rotation speed

Fig. 3 shows the experimental Nyquist, see Fig. 3A, and Bode, see Fig. 3B, plots recorded for the API 5L X52 immersed in 1 M HCl, at stagnant conditions and 25 °C, adding different inhibitor concentrations. The diameter of the corresponding semicircle (which is usually associated with the charge transfer resistance) increased as a function of the inhibitor concentration, indicating that there was a decrease on the API 5L X52 corrosion rate. When the electrode rotation speed was varied from 40 to 1000 rpm, see Fig. 3C, for 50 ppm cephalothin the diameter of the corresponding semicircle slightly decreased as a function of the electrode rotation speed; the same occured with the respective blanks (0 ppm), see Fig. 3D.

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Figure 3

Experimental (A) Nyquist and (B) Bode plots recorded at stagnant conditions and 25 in. the system API 5L X52/1 M HCl varying the cephalothin concentrations as indicated in the figures. (C) Nyquist plots recorded after imposing a cephalothin concentration of 50 ppm and different rotation speeds to the electrode. (D) The blanks (0 ppm) impedance plots evaluated at different rotation speeds.

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Considering the characteristic features depicted by the impedance plots recorded in the absence and in the presence of inhibitors, it was decided to use the equivalent electrical circuits (ECC) shown as insets in Fig. 4, in order to simulate the experimentally generated impedance diagrams.

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Figure 4

Comparison of experimental EIS data (circles), measured for pipeline steel samples immersed in 1 M HCl + 25 ppm of cephalothin at 25 °C and different rotation speeds: (A) 40, (B) 100, (C) 500 and (D) 1000 and the result of fitting (line), the equivalent electrical circuits used to emulate the impedance spectra are shown in each figure.

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Fig. 4 shows some comparisons of experimental impedance plots with the corresponding ones, generated by EECs using the Z_View impedance fitting program. It is clear that the measured impedance plots are in accordance with the equivalent circuit used. The circuit consists of the solution resistance R_s, the charge transfer resistance, R_ct, the resistance of the organic compound present at the interface R_mol and the constant phase element, CPE. For the description of a frequency independent phase shift between an applied AC potential and its current response, a constant phase element (CPE) is used, which is defined in impedance representation as follows:

From the Bode plots shown in Fig. 5 it is possible to note that there are two time constants coupled, as also noted from the Nyquist plot. The impedance (Z) recorded at low frequency values is higher at 500 rpm, see Fig. 5B, that in static conditions, see Fig. 5A; this indicates that a better corrosion protection is achieved when the rotation rate is increased (Yıldız et al., 2014). The corrosion inhibition efficiency, η, of the cephalothin was calculated from the charge-transfer resistance (R_ct) values; see Tables 1, 2, 4 and 5, according to Eq. (3):

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Figure 5

Bode plots recorded in the system API 5L X52/1 M HCl, 25 ppm cephalothin at 25 °C and different rotation speeds: (A) 0 and (B) 500 rpm.

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Table 1 shows the summary of the results of the impedance parameters obtained under static conditions; in this case the R_ct increased with increasing inhibitor’s concentration, which indicates that the inhibition efficiency increases. These results suggest that an increase in the concentration of the inhibitor’s adsorbed species on the steel surface, increased the charge transfer resistance at the metal interface, simply by blocking the active reaction sites available on the metal, that were formerly susceptible to be corroded (Benabdellah et al., 2012). Furthermore, the values of the electrochemical double layer capacitance, C_dl, diminish with respect to the inhibitor concentration increase. The inhibitor’s adsorption at the metal–solution interface creates a gradual substitution of the water molecules, which were originally adsorbed at the metal surface by the inhibitor molecules (Bochris and Swinkels, 1964; Ouici et al., 2013). Therefore, it can be assumed that the overall current is proportional to the amount of sites available for corrosion that are being blocked by the adsorbates. Therefore, the thickness of the protective surface layer at the metal–solution interface improves notably (Solmaz, 2014). Similar results have been reported when thiophene derivatives were used as corrosion inhibitors (Yurt et al., 2004; Issaadi et al., 2011; Daoud et al., 2014; Yadav et al., 2014).

The same behavior became evident after evaluating the inhibitor at 40 rpm, see Table 2, as compared to the previous case, although the inhibition efficiency was smaller by 9%.

Fig. 6 shows a comparison of the η as a function of the inhibitor concentration for different electrode rotation speeds. The results clearly show that in all cases the η increases as a function of the inhibitor concentration, reaching a maximum value when the inhibitor concentration is greater than 25 ppm. However, when increasing the electrode’s rotation rate to 40 rpm, an important η diminution took place, compared to static conditions; notwithstanding when increasing the electrode’s rotation rate, the η increased until attaining a maximum at 500 rpm, although it did not surpassed the η obtained at static conditions.

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Figure 6

Variation of the η of the cephalothin evaluated in the system API 5L X52/1 M HCl, × ppm cephalothin at 25 °C as a function of [cephalothin] at different working electrode’s rotation speed, indicated in the figure.

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3.2.2 Thermodynamic analysis

In order to understand the corrosion inhibition mechanism, it becomes necessary to use different adsorption isotherms, i.e. as those given by the Frumkin, Langmuir, and Temkin models, among others (Espinoza-Vázquez et al., 2014b), from which the degree of metal surface coverage with the inhibitor molecules (θ) is related with the inhibitor concentration, C, in solution. For the study of the cephalothin, the best model that describes the interaction inhibitor–metal surface is that of Langmuir’s isotherm for the different rotation rates, see Fig. 7, expressed as Eq. (4) (Zheng et al., 2014):

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Figure 7

Comparison between the experimental adsorption isotherm (markers) for cephalothin on API 5L X52 steel in 1 M HCl at 25 °C and different rotation speeds, indicated in the figure, with the theoretical model (lines) following the Langmuir model, Eq. (4).

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The Gibbs standard free energy of adsorption of the organic inhibitor was estimated by means of Eq. (5) (Deyab and El-Rehim, 2014; Tansug et al., 2014; Wang et al., 2014a,b):

Table 3, summarizes the estimated thermodynamic parameters, from where it can be noted that for all cases the $\mathrm{\Delta }{G}_{\text{ads}}^{°}$ value is lower than −20 kJ mol⁻¹. According to Yıldız et al. (2014) if $\mathrm{\Delta }{G}_{\text{ads}}^{°}<-20\text{kJ}{\text{mol}}^{-1}$ the undergoing process is physisorption and that those above such relative limit of −40 kJ mol⁻¹ means that a chemisorptive process is in control. Therefore, for all the cases considered in this work it is possible to state that electrostatic interaction or weak bonding (i.e. hydrogen bonds-type) between the inhibitor molecules and the metallic surface is occurring through a physisorption process (Tang et al., 2013).

3.2.3 Effect of the exposure time on the η

It was thought appropriate to determine how efficient would the inhibitor be at a fixed 50 ppm concentration where the η reached a maximum value regardless of the electrode rotation speed, see Fig. 6; thus, a study was carried out as a function of immersion time for a prolonged period of up to 30 days. Fig. 8 shows the comparison made for different immersion times. It can be seen that for 240 h, a maximum of 800 Ω cm² was attained, after which the value decreased to 400 Ω cm².

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Figure 8

Experimental electrochemical impedance data (markers) and fitted (lines) Nyquist plots, for API 5L X52 steel immersed in 1.0 HCl in the presence of 50 ppm cephalothin, under stagnant conditions and 25 °C, during different immersion times indicated in the figure. The equivalent electric circuit used to obtain the impedance parameters is shown in Fig. 4B.

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Table 4 shows the values of the C_dl, R_ct and η as a function of the immersion time at a 50 ppm inhibitor concentration. The R_ct increased until attaining a maximum value at 168 h immersion; thereafter, it diminished to 765 Ω cm² but increased again. Fig. 9 shows the η as a function of the immersion time, and it is possible to observe that within the experimental error, the η almost keeps constant, around 95%, regardless of the immersion time.

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Figure 9

Variation of the cephalothin corrosion η as a function of the immersion time recorded in the system API 5L X52/1 M HCl, 50 ppm cephalothin at stagnant conditions and 25 °C.

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3.2.4 Effect of temperature

From the Nyquist diagram shown in Fig. 10, it can be observed the effect of temperature variation while keeping constant the inhibitor concentration at 50 ppm under static conditions on the impedance response of the system. Table 5, summarizes the electrochemical parameters obtained at a constant concentration of 50 ppm at different temperatures.

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Figure 10

Experimental electrochemical impedance diagrams, Nyquist plots, for the pipeline mild steel immersed in HCl in the presence of 50 ppm cephalothin, under stagnant conditions and different temperatures indicated in the figure.

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Fig. 11 shows the corrosion η variation with respect to the temperature at a constant concentration of 50 ppm and stagnant conditions. It can be observed that from 25 to 40 °C the η diminish up to 66%, after which it becomes practically stable. The temperature at which the best η was obtained was at 25 °C, with a value of 88%.

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Figure 11

Inhibition efficiency as a function of temperature evaluated in the system at 50 ppm of cephalothin in API 5L X52 steel in 1 M HCl.

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3.2.5 SEM analysis

Fig. 12, shows the results obtained through SEM analysis of the surfaces of the sample of the API 5L X52 steel after 48 h immersion at 50 ppm inhibitor concentration in the 1 M HCl medium, see Fig. 12(a) and (b); it can be observed that the surface has been partially attacked, although when compared with the surfaces of steel samples exposed to 1 M HCl without inhibitor, the difference becomes notable, particularly in reference to the morphology obtained, see Fig. 12(c) and (d). Further, surface pits can be easily noted, as indeed similar to those formed in chloride-rich media. In agreement with the observations from the polished material, Fig. 12(d) and (e), it becomes plain that the inhibitor worked correctly since it impeded pits’ formation over the material surface.

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Figure 12

Secondary electron images of the API 5L X52 steel surfaces: (a) and (b) taken after 48 h of immersion in 1 M HCl and 50 ppm of cephalothin, (c) and (d) after 48 h in 1 M HCl and (e) and (f) prior to exposure to the acid medium.

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4.1.1 Inhibitor concentration

From Fig. 5 it is possible to note that, in all cases, the inhibition efficiency increases with concentration until it reaches a maximum value, and most of the inhibitors display this η vs. concentration behavior (Mondal and Taylor, 2014); however, the concentration at which the highest corrosion inhibition is reached depends strongly on the nature of the inhibitor and that is why this sort of plot must be experimentally obtained for each inhibitor. In this respect it is important to mention that the economic viability of a proposed inhibitor depends on the amount that must be used in order to achieve the desired inhibition efficiency effect that regularly must be higher than that 80%. In the present case, the highest η value is reached at 25 ppm that is small enough compared to other cases reported in the literature; for instance, Shukla and Quraishi (2010) evaluated cephalexin and reported an optimum concentration at 400 ppm. In this sense it is relevant to mention that the highest η value is reached when the working electrode surface coverage (θ) is close to 1. From the thermodynamic analysis made, see Section 3.2.2, it becomes plain that the cephalothin molecules adsorb on the steel surfaces following the Langmuir isotherm, which strongly suggest that the principal interaction occurs between the inhibitor molecules and the steel substrate, rather than with the neighboring molecules or the solvent.

4.1.2 Electrode rotation speed

Also, from Fig. 4, it is possible to note that the inhibition efficiency, within the experimental error, is practically constant, and it is higher than 80% for an inhibitor concentration larger than 25 ppm, independent of the electrode rotation speed. This is an important finding because it indicates that the cephalothin molecules remain on the electrode surface even when it rotates at 1000 rpm. Notwithstanding, it is important to state that with a RDE the hydrodynamic conditions imposed at this electrode angular velocity correspond to a non-turbulent flow, which can explain why the inhibition efficiency remains practically constant.