Pitting corrosion and effect of Euphorbia echinus extract on the corrosion behavior of AISI 321 stainless steel in chlorinated acid

3.2.1 Potentiodynamic polarization measurements

The anodic and cathodic polarization curves of the electrochemical influence of EEE on the SS dissolution in H₂SO₄ (H₂SO₄+400 ppm Cl⁻) are shown in Figure 2. The evaluated parameters of Tafel polarization curves including corrosion potential (E_corr), corrosion current density (i_corr), cathodic Tafel constant (β_c), and inhibition efficiency (η) are presented in Table 1. The corrosion current density (i_corr) was obtained by extrapolating the linear segments of the cathodic Tafel curve to the corrosion potential (E_corr). The inhibition efficiency η (%) was calculated by

Figure 2:

Tafel polarization curves for 321 SS in (1 m H₂SO₄+ 400 ppm Cl⁻) in the absence and presence of different concentrations of EEE at 293 K.

where i_corr and i′ corr are corrosion density without and with the addition of EEE, respectively.

From the results shown in Figure 2 and Table 1, it is evident that EEE acted significantly on both anodic and cathodic processes by decreasing their current densities. The addition of the EEE reduced significantly the corrosion current density. It is clearly observed that the inhibition efficiency of EEE increases by the increase of inhibitor concentration, and the maximum inhibition efficiency was observed at 3.5 g l⁻¹ EEE. Furthermore, the decreased value of i_corr in the presence of inhibitors particularly at higher inhibitor concentration is mainly attributed to the adsorption on inhibitor molecules over the SS surface. Also, it is clearly seen that the corrosion potential shows a positive shift with the addition of EEE. Generally, the inhibitor type is determined by its E_corr displacement; if the value of E_corr for inhibited sample is displaced more than 85 mV with respect to the blank, then the inhibitor can be categorized as anodic or cathodic type (Loto et al., 2015). However, if the shift in the E_corr value is less than 85 mV, then the inhibitor can be categorized as mixed-type inhibition; thereby, EEE acts as a mixed-type inhibitor with an anodic character. The Tafel slopes of the cathodic reaction β_c vary slightly with the change in the concentration of the inhibitor.

3.2.2 EIS measurements

EIS is used to investigate the corrosion behavior of SS in aggressive acid (1 m H₂SO₄+400 ppm Cl⁻) in the absence and presence of the EEE and to explore even more the metal-solution processes. All experiments were conducted at OCP and at 293 K.

The inhibition efficiency was calculated by the equation as follows:

where R_ct and R′ct are the charge transfer resistances of SS in the absence and presence of EEE, respectively.

The Nyquist plots of SS in H₂SO₄ with and without various concentrations of EEE are given in Figure 3. The parameters derived from EIS measurements and the corresponding inhibition efficiencies are listed in Table 2. As shown in Figure 3, all EIS graphs had two time constants; a well-defined capacitance loop at higher frequencies arises from the charge-transfer resistance and the electric double layer followed by a Warburg tail at lower frequencies. The presence of the Warburg time constant can be linked to ion diffusion through the passive film. Such semi-infinite Warburg diffusion process may also confirm that the mass transport is limited by the surface passive film (Li et al., 2014) and thereby indicates that both charge transfer and diffusion processes are controlling the corrosion mechanism. It is observed that Warburg tail tends to curve over the real axis with intersection at sufficiently lower frequencies. These diffusion tails and arcs were found to bear a close relationship with anomalous diffusion of the electroactive species (Ehsani et al., 2011). EIS spectra presented in Figure 3 are described by a simple equivalent circuit as shown in Figure 4, and all the circuit components were determined by the impedance fitting procedure. R_s is the solution resistance, R_ct is the charge transfer resistance, C_dl is the double layer capacitance, and W is the Warburg impedance.

Figure 3:

Nyquist plots for 321 SS in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of different concentrations of EEE at 293 K.

Figure 4:

Equivalent circuit used to model impedance data for 321 SS.

Compared to the blank, the larger capacitive loop in the inhibited solution indicates the adsorption of inhibitor compounds on the SS surface. From the results depicted in Table 2 and Figure 3, it is evident that the charge transfer resistance increases as the concentration of EEE increases, whereas the double layer capacitance was found to decrease as the inhibitor concentration increases. The lower C_dl values in the inhibited solution confirm the enhancement of the adsorption of the inhibitor on the SS surface (Seifzadeh, 2014). The decrease in C_dl is attributed to an increase in the thickness of the electronic double layer due to the adsorption of EEE organic compounds. Furthermore, Bode diagrams presented in Figure 5 show clearly the presence of two time constants as well as the impedance increases with increasing inhibitor concentrations at low frequencies. All the electrochemical parameters clearly propose that the corrosion control depends on the concentration of the inhibitor and its addition increases the impedance but does not change the other aspects of the SS behavior in aggressive medium. It is clearly observed that both EIS and polarization measurements show the same trend, only the difference in the inhibition efficiency may be due to the surface status, and impedance tests were run at the last reached OCP value, whereas in potentiodynamic measurements the electrode was polarized at higher potential.

Figure 5:

Bode diagrams for 321 SS in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of different concentrations of EEE at 293 K.

3.2.3 Cyclic voltammetry

To study the effect of EEE on the pitting corrosion of SS, cyclic voltammograms for SS in 1 m H₂SO₄+400 ppm Cl⁻ have been recorded in the absence and presence of different concentrations of EEE. Figure 6 shows that the pitting potential has been obtained as the onset of the peak observed at 1 V corresponding the anodic oxidation of Cr(III) to Cr(VI). The obtained data depicted in Figure 6 demonstrates that the pitting potential remains almost the same regardless of the tested concentration of the inhibitor. Therefore, the cyclic voltammetry method shows that EEE has almost no effect on the pitting corrosion.

Figure 6:

Cyclic voltammograms for 321 SS in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of different concentrations of EEE at 293 K.

3.2.4 Current time potentiostatic transients

To describe the kinetic behavior of SS passivation and pitting, chronoamperometric transients were recorded by polarizing the working electrode at 1000 mV vs. SCE in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of 3.5 g l⁻¹ inhibitor. To study the early stages of passive film growth and pitting, the current time curves were plotted in logarithmic scale as presented in Figure 7 (inset).

Figure 7:

Current time transients for 321 SS polarized for 1 h at 1000 mV vs. SCE in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of 3.5 g l⁻¹ EEE at 293 K.

For both cases, the current density decrease with time is related to the passive film growth as given by the MacDonald model [Eq. (19; Chen et al., 2009]. In the absence of the inhibitor, the current density decreases rapidly to a minimal current density value corresponding to the induction time at which the initiation of pitting corrosion starts. The continuous increase in current density taking place after the induction time is indicative of the pitting initiation as described by the Engell stollica model [Eq. (18; Albrimi et al., 2011]:

where B and b are constants that depend on the anodic applied potential of polarization and the solution concentration. b corresponds to the rate of pitting initiation.

where A and n are constants that depend on the constant anodic potential and the electrolyte concentration. The n constant yields the rate of the passive film growth and is given by the first slope of the descending current density as depicted in Figure 7. However, in the presence of the inhibitor, the pitting initiation domain has been reduced and no clear current density increase has been observed. From this fact, it could be stated that the tested inhibitor acts on the pitting initiation. A possible explanation for this observation is that the adsorption of the inhibitor on the metal surface blocks the penetration process that is the main step responsible for passivity loss induced by the pitting corrosion. As further inspection of Figure 7, for both cases, the steady value attained after the initiation of the metastable pits corresponds to the repassivation of the passive film. This fact indicates that no pitting growth could be seen as no second linear straight line with a different slope has been observed after the initiation stage.

3.2.5 Adsorption isotherm

The inhibition efficiency of a corrosion inhibitor is related mainly to its adsorption nature on the metal surface; hence, the study of the adsorption isotherms can provide further data on the nature of the interaction between the inhibitor polar functions and the metal surface and could therefore give precious information on the inhibition mechanism (Idouhli et al., 2018a,b). The graphical study of adsorption process was done by fitting the surface coverage and the inhibitor concentration to different adsorption isotherms, i.e. Langmuir, Freundlich, and Temkin. The plot of C/θ against C gives a linear plot as shown in Figure 8 with a correlation coefficient close to unity (R²=0.997), suggesting that the adsorption of E. echinus compounds on the SS surface is consistent with the Langmuir isotherm. Moreover, the Langmuir isotherm is based on the assumptions that the inhibitor adsorption is a monolayer with no lateral interaction between the adsorbed species (Karthik et al., 2014).

Figure 8:

Langmuir adsorption isotherm plots for 321 SS in 1 m H₂SO₄+400 ppm Cl⁻ with different concentrations of EEE at 293 K.

The adsorption process of the inhibitor on the metal surface is considered as a substitution between water molecules and the organic molecules of the inhibitor (Lopes-Sesenes et al., 2012).

where Org_sol and Org_ads are the organic molecules of the inhibitor dissolved in the aqueous solution and adsorbed onto the metallic surface, respectively. H₂O_ads is the water molecules adsorbed on the metal surface and n is the ratio that represents the number of water molecules replaced by a single inhibitor organic molecule (Umoren et al., 2013).

The surface coverage (θ) is calculated using the following relation:

The Langmuir isotherm is given by the following relation:

Temkin isotherm:

Freundlich isotherm:

where θ is the surface coverage, C_inh is the concentration of the inhibitor, and K_ads is the standard adsorption equilibrium constant that related to the standard free energy of adsorption ΔGads° by the following relation:

where 55.5 is the molar concentration of water in the solution, R is the gas equilibrium constant, and temperature T is 293 K.

To describe in more detail the adsorption process, the standard free energy (ΔG°) was determined according to Eq. (26) and presented in Table 3. The negative sign of ΔG° values suggest the spontaneous adsorption of EEE on SS surface (Loto, 2013). In contrast, the calculated ΔG° values are more than −20 kJ mol⁻¹, indicating the physical interaction between the charged molecules and the charged metal (Idouhli et al., 2016).

3.2.6 Thermodynamic parameters for the corrosion process

The temperature factor has a tremendous effect on the inhibitor-metal interaction; it is well known that the variation in the medium temperature may considerably promote the change in the inhibition mechanism such as the adsorption of the inhibitor and its stability in the aggressive medium (Idouhli et al., 2018a,b). For this reason, polarization measurements were performed at different temperatures. It is clearly shown in Table 4 that current density increases significantly with the rise of temperature for both inhibited and uninhibited solutions. We also note that the inhibition efficiency depends on the temperature and decreases with the rise of temperature from 293 to 323 K, indicating that a higher temperature dissolution of SS predominates on the adsorption of EEE at the metal surface. This can be explained by the decrease of the strength of the adsorption process at high temperature and can suggest a physical adsorption mode (Daoud et al., 2015).

For a better understanding of the inhibition process, the activation energy parameter was determined from Arrhenius equation:

where E_a is the apparent activation energy of the corrosion reaction, R is the gas constant, T is the absolute temperature, and A is the Arrhenius pre-exponential factor. Figure 9 presents the evolution of ln(i_corr) against 1/T in the absence and presence of inhibitor. The linear regression allows us to determine the slope (−E_a/R) of the straight line, which enables to determine the apparent activation energy for both cases.

Figure 9:

Arrhenius plots for 321 SS in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of 3.5 g l⁻¹ EEE at 293 K.

The analysis of Table 5 shows that the magnitude of apparent activation energy for the inhibited medium (46.32 kJ mol⁻¹) is much greater than its value in the absence of the inhibitor. The increase value of E_a in the presence of the inhibitor with respect to the blank may be due to a physical adsorption.

According to Szauer and Brandt (1981), the activation energy value increase could be assigned to an appreciable decrease in the adsorption of the inhibitor on the metal surface that occurs with the increase of temperature. Due to the fact that the desorption process is in equilibrium with the adsorption of molecules on the metal surface, as adsorption decreases, more desorption process of inhibitor molecules happens, subsequently leading to more dissolution with the increase in temperature. Furthermore, the whole process was controlled by the surface reaction as the energy of the activation corrosion process in both absence and presence of inhibitor was greater than 20 kJ mol⁻¹ (Herrag et al., 2010).

The enthalpy and entropy of activation (ΔHa∗, ΔSa∗) were calculated based on the transition state theory, being an alternative formula of Arrhenius [Eq. (28)]:

where h is Planck’s constant and N is Avogadro’s number.

Figure 10 shows a plot of ln(i_corr/T) versus 1/T. Straight lines are obtained with a slope of  −ΔHa∗/R and an intercept of lnRhN+ΔSR from which the values of ΔSa∗ and ΔHa∗ are calculated and are given in Table 5. From the analysis of the obtained data, the positive values of ΔHa∗ in the absence and presence of the inhibitor mean the endothermic nature of the SS dissolution process (Obi-egbedi and Obot, 2012). The negative values of entropy of activation in the absence and presence of inhibitor 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 from the reactants to the activated complex (Heakal et al., 2011). We can notice that E_a and ΔHa∗ values vary in the same way, which verifies the thermodynamic equation between E_a and ΔHa∗ as presented in Table 5.

Figure 10:

Transition-state plots for 321 SS in 1 m H₂SO₄+400 ppm Cl⁻ in the absence and presence of 3.5 g l⁻¹ EEE at 293 K.

3.2.7 Fourier transform infrared spectroscopy (FTIR) analysis

To improve our knowledge about the inhibition provided by the crude extract constituents of E. echinus plant, spectral analyses were made using a vertex 70/70v FTIR spectrometer. Infrared spectroscopy is a valuable method for the detection and identification of chemical functional groups; the FTIR spectra were recorded in the range of 400–4000 cm⁻¹. Figure 11 shows the FTIR spectrum of the EEE powder. The assignments of vibrational bands are achieved by comparing the band positions observed in IR spectra to the wave numbers of the studied known pure compounds in the literature. The spectra can be assigned generally as in the following:

Figure 11:

FTIR spectra of E. echinus powder.

The strong band at 3399 cm⁻¹ corresponds to N-H and O-H stretching, the band at 2924 cm⁻¹ is attributed to an -CH₂ asymmetrical stretching vibration, and the strong band at 1926 cm⁻¹ is related to C=O stretching vibration. The peaks at 1426 and 1383 cm⁻¹ correspond to C=C and N-O stretch, respectively. The band at 1254 cm⁻¹ is the C-O stretch and the vibrational bands at about 1034 and 596.08 cm⁻¹ correspond to C-N and C=C-H stretching. Based on the assignment of the FTIR spectra, it is observed that the extract tested meets the structural criteria of green corrosion inhibitors that have been widely reported and justifies the inhibition provided.

3.2.8 Analysis of ultraviolet (UV)-visible absorption spectra

To study the metal-inhibitor interaction, UV-visible analysis was conducted. The UV-visible spectra of EEE solution without and with the exposed SS are shown in Figure 12. As can be observed, the absorption spectra exhibit a single peak at 280 nm with a higher change in the absorbance maximum for the EEE solution resulting from SS immersion. The increase in absorption intensity indicates the formation of the Fe²⁺-inhibitor complex (Abuthahir et al., 2014) and subsequently corroborates the efficiency of the inhibitor.

Figure 12:

UV-visible absorption spectra of (A) solution containing EEE and (B) solution containing EEE-SS.

3.2.9 SEM-energy-dispersive X-ray (EDX) analysis

Using SEM-EDX analyses, the chemical composition and morphology of the SS surface have been studied for the electrode polarization at the critical value of 1000 mV vs. SCE in H₂SO₄ containing different concentrations of Cl⁻ in the absence and presence of inhibitor. The EDX elemental analysis of the weight percentage of the SS alloying elements obtained in different condition is summarized in Table 6. It is clearly observed from the SEM micrographs (Figure 13) that pitting frequency increases with the increase in Cl⁻ concentration. In the presence of 10,000 ppm Cl⁻, the SS surface appears to be highly pitted with the maximum number of pits. It is worthwhile noting that even for lower concentrations some localized pits could be observed on SEM micrographs. It should be added that for lower concentration most nucleated pits die immediately and do not apparently produce metastable pits. After the addition of 3.5 g l⁻¹ inhibitor, the SS surface is shown to be less pitted for all Cl⁻ concentrations. Table 6 shows that the carbon content increases with the addition of inhibitor. These facts could be justified by the inhibitor adsorption onto the metal surface. These findings substantiate the claim that pitting initiation could be blocked in the presence of EEE as described earlier using current time transients.

Figure 13:

SEM images of polarized 321 SS in H₂SO₄ containing (A) 400 ppm Cl⁻, (B) 400 ppm Cl⁻+3.5 g l⁻¹ EEE, (C) 1000 ppm Cl⁻, (D) 1000 ppm Cl⁻+3.5 g l⁻¹ EEE, (E) 10,000 ppm Cl⁻, and (F) 10,000 ppm Cl⁻+3.5 g l⁻¹ EEE at 293 K.