Contribution to understanding synergistic effect of Punica granatum extract and potassium iodide as corrosion inhibitor of S355 steel

2.1 Preparation of extracts and solution

The PG also called pomegranate was collected from plants cultivated in the region of the city of Marrakech, Morocco. The corresponding extract was prepared by the maceration method by using ethanol as a solvent; the dried PG was powdered and then added to 100 mL of ethanol by utilizing magnetic agitation for 24 h at room temperature (298 K). After filtration, the liquid extract was concentrated and used as a corrosion inhibitor. The solution of 1 M HCl was prepared using analytical grade hydrochloric acid (37%) with distilled water, and the inhibitor presented a good level of solubility in acidic media.

2.2 Electrochemical analysis

The electrochemical study was performed using a potentiostat PGZ 301 piloted by Voltamaster 4 software. The electrochemical cell was equipped with a double-walled to adjust the temperature of the medium, and the investigation was performed with three electrode configurations. The platinum electrode with a large surface was employed as a counter electrode; additionally, the potential was applied using Ag/AgCl as a reference electrode. S355 steel was used as a working electrode with an exposed surface to the electrolyte of 0.49 cm². The table below (Table 1) shows the composition of the substrate.

Before starting the electrochemical experiment, the working electrode was polished with emery paper with different granulometry (400, 1000, 2000) until a mirror-like surface was obtained, then, the surface was vigorously washed with water and distillate water.

Subsequently, the polarization curves were performed in the potential range of −0.6 to −0.2 V versus Ag/AgCl with a scanning rate of 1 mV.s⁻¹. Before all of the experiments, the open circuit potential was stabilized for 30 min, and they were repeated at least three times to ensure their reproducibility. The standard deviation was reported as well.

2.3 Characterization techniques

The surface of the steel was analyzed both with and without the inhibitor using a TESCAN VEGA3- EDAX SEM coupled with energy-dispersive X-ray spectroscopy (EDX) with an accelerating voltage of 20 KV. Examination of the Raman spectra were conducted using a Raman spectrometer (Confotec MR520), and 532 nm of radiation from the diode solid-state laser was used for irradiation. The UV-visible absorption spectra were obtained using a UviLine 9400 SECOMAM UV–vis spectrophotometer at room temperature.

3.1 Electrochemical analysis

Figure 1 shows the potentiodynamic polarization curves for the steel in 1 M hydrochloric acid solution that contains different concentrations of PG extract at 293 K. Table 2 presents the electrochemical kinetic parameters, such as corrosion current density i_corr (mA/cm²), corrosion potential E_corr, cathodic and anodic Tafel slopes (bc and ba), and inhibition efficiency η (%). These parameters were determined via the extrapolation method of the polarization curves. The inhibition efficiency η (%) was calculated using the following equation:

where icorr and i′corr are respectively the corrosion current density without and with inhibitor (PG).

Figure 1:

Potentiodynamic polarization curves of steel immersed in 1 M HCl without and with PG extract.

Further investigation of these results shows clearly that the corrosion current density decreases when the concentration of the inhibitor is increased, and the inhibition efficiency rises relatively until reaching a maximum of 36.30% at 2 g/L. Meanwhile, analysis of the polarization curves and the electrochemical parameters shows that the corrosion potential is moving towards anodic values with the inhibitor moderately affecting the cathodic slope. Nevertheless, the difference in E_corr values is less than 85 mV versus the blank, which confirms that the inhibitor is mixed in nature (Idouhli et al. 2018a; Kumar et al. 2017). The Tafel slopes’ (bc, ba) values vary in an irregular way, suggesting that the mechanism of inhibition is due to the mixed factor influencing the active sites (Deng et al. 2011).

Furthermore, scrutiny of the polarization curves and electrochemical parameters shows that the current density decreases significantly with the addition in inhibitor in the acidic medium, leading to an increase of the inhibition efficiency. From Figure 1 and the values of Tafel slopes (Table 2), it is clear that the anodic Tafel lines are parallel, inferring that the mechanism of the dissolution of steel remained the same after the addition of the inhibitor, yet we did note a slight inhibiting effect (36.30% at 2.0 g/L).

3.2 Thermodynamic parameters

In order to examine the corrosion process and the thermodynamics of its dissolution related counterpart, we studied the effect of temperature on the electrochemical parameters. The corrosion reaction can be studied as an Arrhenius-type process using the relations below (Haque et al. 2018):

where i_corr is the corrosion current density of steel, E_a is the apparent activation energy, A is the constant, R is the universal gas constant and T is the absolute temperature.

In addition, the current density of the current (i_corr) can be expressed in the following manner:

where h is the Planck constant, N is the Avogadro number, ΔSa∗ is the activation entropy and ΔHa∗ is the activation enthalpy, T is the absolute temperature and R is the perfect gas constant.

Parameters obtained from polarization curves at different temperatures are summarized in Table 3. In keeping with these limitations, the inhibition efficiency increases with rising temperatures.

We reported in Figure 2 the variation of ln (i_corr/T) as a function of the inverse of the temperature in the absence and the presence of the inhibitor.

Figure 2:

Arrhenius plots of ln (i_corr/T) vs. 1/T (K − 1) in 1 M HCl with and without 2 g/L of PG extract.

The slope of the line obtained was (ΔHa∗/R) and the intercept was (lnR/Nh+ΔSa∗/R). From these equations, we can calculate the values of ΔHads∗ and ΔSa∗.

From the values of ΔHads∗ and ΔSads∗ (Table 4) we are able to note that:

• The positive sign of ΔHads∗ (41.56–26.36 kJ/mol) shows that the dissolution process of S355 steel is endothermic (Matad et al. 2014).

• The rise in the entropy value in the presence of the inhibitor compared to the blank indicates that there is an increase in the disorder during the transformation of the reagents to activate complex in the acidic media. Also, the low inhibition performance of the inhibitor molecules onto the steel surface leads to adsorption-desorption, creating a chaotic distribution on/in the interface of the steel/solution (Matad et al. 2014).

3.3 The study of the effect of potassium iodide on the corrosion of S355 steel in HCl medium

The polarization curves of the steel in 1 M of HCl with 10 mM of KI and increasing amounts of PG extract are reported in Figure 3. From this curve, we can highlight the electrochemical parameters that are listed in Table 5.

Figure 3:

Tafel curves of S355 steel in hydrochloric medium, In the presence of different concentrations of pomegranate bark extract and 10 mM KI.

Here, it is obvious that at a fixed concentration of KI, the effect of increasing PG concentration is an increase in the inhibition efficiency, which is demonstrated by the decrease of corrosion current density; hence, our results comply well with those of Eyu (Eyu et al. 2016). Due to the electrochemical parameters that were obtained, we can also observe the variation in the anodic and cathodic slopes with a displacement of corrosion potential towards anodic values. This suggests that the inhibitor studied can be considered to be a mixed inhibitor type with anodic predominance. Moreover, we noted a decrease in the corrosion current density with the addition of inhibitor.

3.4 Thermodynamic parameters

The electrochemical parameters of steel in 1 M hydrochloric acid media with 2.0 g/L inhibitor and potassium iodide (KI = 10 mM) at different temperatures are summarized in Table 6.

In this section, we will extract thermodynamic parameters by studying the evolution of ln (i_corr) and ln (i_corr/T) versus 1000/T, as given by Equations (2) and (3) (Khadom et al. 2018).

The attained straight-line curves (Figures 4 and 5) present a good correlation coefficient, and the enthalpy ΔHa∗ and entropy ΔSa∗ activation parameters are summarized in Table 7.

Figure 4:

Representation of the Arrhenius equation by ln i_corr versus 1000/T without and with 2.0 g/L pomegranate bark extract and 10 mM KI.

Figure 5:

Representation of the Arrhenius equation by ln (i_corr/T) as a function of 1000/T without and with 2.0 g/L of pomegranate bark extract and 10 mM of KI.

When observing the results in Table 7, we note that the addition of 10 mM of KI to the solution containing HCl and 2 g/L of PG extract causes:

• A decrease in apparent activation energy suggesting the facility of forming a protective film on the exposed surface of the steel S355 compared to the blank (44.11–21.52 kJ/mol). As a result, the molecules of the inhibitor adsorb to the surface via strong bonds (chemisorption) (Idouhli et al. 2018b; Preethi Kumari et al. 2017).

• The positive sign of ΔHa∗ (41.56–19.561 kJ/mol) shows that the dissolution process of S355 steel is endothermic (Koumya et al. 2019b).

• There is a decrease in the ΔSa∗ entropy value in the presence of the inhibitor with respect to the blank solution. This means that there is an growth in the order during the transformation of the reagents to the activated iron-molecule complex (Ali and Mahrous 2017).

From the analysis of the thermodynamic data obtained before and after the addition of 10 mM of KI, it is worth remarking on the fact that the adsorption process remains the same (chemisorption process). Furthermore, the process of dissolution of steel retains its endothermic nature.

3.5 Adsorption isotherm

The study of the adsorption isotherms elucidates the nature of interaction between inhibitor and steel surfaces (Idouhli et al. 2016). Accordingly, the surface coverage was evaluated using the inhibition efficiency values determined from the potentiodynamic polarization studies that relate to the expressions below:

The surface coverage with different concentrations of PG extract was evaluated using the adsorption isotherm, which assumes that the amounts of adsorption (surface coverage) were directly correlated to the inhibition performance (Equation (4)). A survey of relevant literature indicates that the largest amount of isotherm adsorption described the adsorption of the inhibitor onto the steel surface with Langmuir, Freundlich and Temkin isotherms.

An inspection of the figures relating to isotherms (Figure 6) clearly indicates that the adsorption of the inhibitor (PG) presents a low coefficient correlation for all adsorption isotherms and reaches 0.655 as a maximum value with Freundlich isotherm adsorption. This could be explained via low-inhibition efficiency with regards to the extract being adsorbed onto the steel surface. However, the addition of KI to the inhibitor shows that the Freundlich isotherm was the best fit in terms of facilitating the optimal adsorption behavior of the studied inhibitor with a correlation coefficient (R²) equal to 0.973. This type isotherm complimented the adsorbed molecules in a heterogeneous surface with possible lateral interactions in the adsorbed inhibitor and exponential distribution function of the adsorption energy (Quiñones and Guiochon 1998).

Figure 6:

Langmuir, Freundlich and Temkin isotherms for different concentrations of PG extract and PG extract + 10 mM KI at 298 K.

In order to evaluate the thermodynamic parameters, it is necessary to know the molecular weight of the inhibitor (PG). In our case, we could not simply assume the value of the molecular weight of the extract of the inhibitor (Idouhli et al. 2019).

3.6 Surface morphology (SEM/EDS)

To study the morphology of the steel surface and prove that the inhibitor had adsorbed onto the steel surface, we employed scanning electron microscopy (SEM). During this part of study, we were focused on determining the atomic percentage of carbon, iron, oxygen and iodine present by using EDX analysis (Table 8).

As described in Figure 7, it is possible to see that the surface of the steel after 72 h of immersion in HCl 1 M at 25 °C was damaged by the presence of grey clusters and pit sites (Figure 7a). The addition of inhibitor results in the formation of a uniform layer that is poorly adherent as shown in Figure 7b. We report in Figure 7c the morphology behavior of steel samples immersed in solution containing HCl 1 M with 10 mM of KI, where we can clearly see that the shape of the presented surface is different from those visible in Figure 7a–b. Also, we note that the metallic surface covered by a protective film, as scattered islands, is low. In addition, we underline the appearance of streaks relating to mechanical polishing. Otherwise, a mixture of KI and inhibitor (Figure 7d) gives better coverage to the surface exposed to the aggressive media and in instances where it is presented in a specific shape. The formed film is thicker and multilayered which can significantly reduce the active surface of the steel and reduce the access of the electrolyte to the surface (Popova et al. 2003). Nonetheless, the film is not uniform. This, may be due to the presence of different sites of adsorption on the steel surface, which corroborates with the Freundlich adsorption mechanism of the inhibitor with iodide ions.

Figure 7:

Micrograph (SEM) of the steel surface S355.

(a) After 72 h of immersion in HCl 1 M at 25 °C, (b). 2.0 g/L inhibitor, (c) HCl + KI 10 mM, and (d) KI + 2.0 g/L inhibitor.

Optical analysis of the microstructures of the steel surface was completed using light microscopy to identify the distributions and various phases present on the surface of the studied sample (Samuels 1999).

The surface exposed to HCl 1 M bath consists of grains of ferrite in grain boundaries. Furthermore, the development of pitting between the grain boundaries is due to the presence of halide ions, which are typically chlorides (Figure 8a). The addition of KI in the media implies that the grain boundary disappears; a phenomenon that is accompanied by the random distribution of inclusions (Davis 2000; Samuels 1999). Meanwhile, adding inhibitor into the media revealed the formation of the adsorbed layer with the appearance of some islands of oxide. However, by including KI and inhibitor in the media, the surface of the sample exhibited more coverage with a protective film. The micrograph observations are in good agreement with the microscopy analyses of the steel in hydrochloric media that are described below.

Figure 8:

Micrograph (SEM) of the steel surface S355.

(a) After 72 h of immersion in HCl 1 M at 25 °C, (b), 2.0 g/L inhibitor, (c) HCl + KI 10 mM, and (d) KI + 2.0 g/L inhibitor (PG).

3.7 Raman spectroscopy

Raman spectra for treating steel samples were performed with 532 nm excitation sources, and all of the obtained spectra are shown in Figure 9. Using this figure, the presence of many bands is noteworthy, and their shape and intensity depend on the way that samples were treated. Fives bands are seen on the samples immersed in HCl, in HCl with inhibitor, and also on the steel placed in HCl with the presence of KI and inhibitor extract. Those bands are centered on 218, 274, 390, 474 and 598 cm⁻¹. The four first bands may be attributed to the vibration mode E_g type; on the other hand, bands at 598 cm⁻¹ could be a result of the A_1g vibration mode. As reported in the literature, these vibrations modes are relative to iron oxide α-Fe₂O₃ (Justin et al. 2018; Wang et al. 2014).

Figure 9:

Raman spectra of steel samples treated at different solution, with a LASER source of 532 nm.

The addition of P. granatum extract to HCl 1 N increases band intensity (Figure 9), an increase that is attributable to the thickening of the oxide film. Moreover, the inhibitor contributes to a greater recovery rate of the surface exposed to an aggressive bath, because the contribution of metallic substrate in Raman spectra disappears. The Raman spectra of the steel sample, in HCl 1 M, to which we have added 10 mM of I⁻, revealed the occurrence of new bands centered at 248, 344, 548, 683 and 710 cm⁻¹. The band observed at 248 cm⁻¹ is ascribed to a weak vibration E_g of Hematite (α-Fe₂O₃) and the other four bands are relative to the vibration mode of Fe₃O₄ as reported by Dar and Shivashankar (2014). Bonds at 344, 548, 683 and 710 cm⁻¹ appeared when KI was added to the solution containing HCl and inhibitor. In summary, we can conclude that the effect of KI being added is the formation of a mixture of Fe₂O₃ oxide and Fe₃O₄ reactive oxide in acidic media, which ameliorates the corrosion behavior of the steel in an acidic solution.

3.8 UV-visible

The UV-visible spectroscopy technique is based on the property of a substance, particularly certain molecules, absorbing certain wavelengths (Ali and Mahrous 2017; Koumya et al. 2019a). The transitions observed for organic compounds were due to the presence of electrons σ or π and n non-binding doublets of atoms such as H, C, N, O. Additionally, iron ions moving in solution can interact with a species through a donor-acceptor mechanism. In order to examine the iron -inhibitor interaction in a 1 M HCl medium both with and without the inhibitor and KI, analyses were conducted as shown in Figure 10.

Figure 10:

UV–vis spectrum of inhibitor in 1 M HCl without and with KI after 24 h of immersion of steel.

The inspection of the spectra revealed that the inhibitor, and the inhibitor immersed with the steel, exhibit two common peaks (233 and 240 nm) alongside the disappearance of the other peaks. This change in terms of the spectra can be linked to the adsorption of some compounds on the steel surface and the formation of the complexes. The same observations are noted for spectra in relation to KI solution and when the steel is immersed in KI media of HCl; two common peaks at 231 and 243 nm are observed in this instance, and the band at 230 nm could be due to π–π* transition (Ali and Mahrous 2017). An absorption limit of 243 nm relates to the iodide (Gardner et al. 2009). Additionally, the mixture of inhibitor with KI involves the disappearance of these two peaks, and one maximum absorption peak at 260 nm appeared in the presence of the inhibitor with the steel and with the mixture of inhibitor and KI. These results support the probability of a complex formation between Fe²⁺ and inhibitor in the 1 M HCl and suggest that the sorption of iodide onto the steel surface also facilitates the adsorption of inhibitor. The formation of these complexes would seem to be the origin of the protective layer on the surface of the steel.