Synergistic inhibition effects between leaves and stem extracts of Sida acuta and iodide ion for mild steel corrosion in 1 M H₂SO₄ solutions

2.1 Materials

The mild steel sheets used for this study were sourced locally. Each sheet was 0.04 cm in thickness and has the following composition (wt%): C = 0.13; Si = 0.18; Mn = 0.39; P = 0.40; S = 0.04; Cu = 0.025 and the balance Fe. It was mechanically press cut into 5 × 4 cm coupons. They were abraded with different grades (# 400, 600, 800 and 1000) silicon carbide paper, degreased in absolute ethanol, dried in acetone and stored in a moisture-free desiccator prior to use. The corrosive medium was 1 M H₂SO₄ prepared from 98% analytical grade supplied by Sigma–Aldrich. Deionized water was used for the preparation of all reagents.

2.2 Preparation of the S. acuta extracts

The leaves and stem of S. acuta were sun dried and ground to powder using a manual blender. One kilogramme of the dry powdered samples was extracted using absolute ethanol for 48 h. The extract was concentrated initially using vacuum evaporator and finally by evaporation to dryness on a steam bath to obtain a solid residue devoid of ethanol. From the solid residue, different concentrations (0.1–0.5) g were weighed and then dissolved in one litre of 1 M H₂SO₄ solution for weight loss and hydrogen evolution measurements.

2.3 Weight loss measurements

Weight loss measurements were conducted under total immersion in stagnant aerated condition using 250 mL capacity beakers containing 200 mL test solution at 30–60 °C maintained in a thermostated water bath. The mild steel coupons were weighed and suspended in the beaker with the help of rod and hook. The coupons were retrieved at 2 h interval progressively for 10 h, immersed in a solution of 20% NaOH containing 200 g/L zinc dust at room temperature, scrubbed with bristle brush under running water, dried in acetone and reweighed. The weight loss, in grammes, was taken as the difference in the weight of the mild steel coupons before and after immersion in different test solutions. Tests were performed for the free acid solution, solution of 0.5 g/L leaves and stem extracts of S. acuta alone and this concentration of the extracts in combination with different concentrations (0.5–5 mM) KI at different temperatures. The experiments were repeated three times to ensure good reproducibility. The standard deviation values among parallel triplicate experiments were found to be smaller than 5%, indicating good reproducibility. From the weight loss values, corrosion rates were computed using the expression:

The inhibition efficiency (η%) was computed using Eq. (2)

2.4 Hydrogen evolution measurements

Hydrogen evolution measurements were performed using gasometric assembly. Detailed description of the assembly has been reported elsewhere (Umoren and Ekanem, 2010). The procedure followed was similar to that reported by other authors (Umoren et al., 2006; Eddy et al., 2009b; Obot et al., 2009; Aytac et al., 2005; Ebenso and Oguzie, 2005). The test solution was kept at 100 mL. The progress of corrosion in the absence and presence of test inhibitor was monitored by careful measurements of the volume of hydrogen gas evolved at fixed time intervals. The experiment was performed for 1 M H₂SO₄ (blank), solution of 0.5 g/L leaves and stem extracts of S. acuta alone and this concentration of the extracts in combination of different concentrations (0.5–5 mM) KI at 30–60 °C was maintained using a thermostated water bath.

Hydrogen evolution rate (RV_H) that can be correlated to the mild steel corrosion rate was computed based on the volume of H₂ evolved using the expression:

The inhibition efficiency (η%) was computed using the equation:

3.1 Weight loss, corrosion rates and inhibition efficiency

The weight loss method as a means of evaluating corrosion rates of metals in corrosive environments and inhibiting potentials of studied inhibitors is widely reported in the literature, such that the technique forms the baseline method of measurement in many corrosion monitoring programmes (Afidah and Kassim, 2008). Results of the present investigations involving leaves and stem extracts of S. acuta indicate that the inhibition efficiency increases with increase in extracts concentration and the maximum inhibition efficiency was obtained with 0.5 g/L concentration within the range of concentrations studied. This concentration was, therefore, chosen to evaluate the effect of iodide ion additive. Fig. 1 shows the weight loss-time curves for mild steel in 1 M H₂SO₄ without and with 0.5 g/L leaves (LV) of S. acuta extract and different concentrations of iodide ion in combination with 0.5 g/L leaves extract of S. acuta at different temperatures (30–60 °C). Similar plots for the stem (ST) of the plant extract are depicted in Fig. 2 at the same temperatures.

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Figure 1

Plot of weight loss against time for mild steel in 1 M H₂SO₄ without and with S. acuta leaves extracts and in combination with different concentrations of KI at 30, 40, 50 and 60 °C.

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Figure 2

Plot of weight loss against time for mild steel in 1 M H₂SO₄ without and with S. acuta stem extracts and in combination with different concentrations of KI at 30, 40, 50 and 60 °C.

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Inspection of the figures show that the weight loss of mild steel varies linearly with time and was reduced in the presence of the inhibitor (plant extract) compared to the free acid solution. Further inspection of the figures reveals that weight loss increases with temperature and greater weight loss was obtained at 60 °C for extracts of both plant parts. Also, the extent of reduction in weight increases with the increase in concentration of the iodide ion at all temperatures studied.

The computed values of corrosion rate and the inhibition efficiency for the additives at different temperatures for the two plant parts are given in Table 1. Results in the table show that the corrosion rates of mild steel in 1 M H₂SO₄ are lower in the presence of the plant extracts compared to the blank acid solution. Again corrosion rates are observed to be lower in the inhibiting solutions containing extracts–KI combination than that of the extracts alone. Examination of the table further reveals that the corrosion rate of mild steel in solutions of leaves extract–KI combination is lower than that of the stem extract–KI combination at all the temperatures. This indicates that the LV extract is a better inhibitor than the ST extract of S. acuta. Results in Table 1 also show that corrosion rate increases with increase in temperature with the highest values obtained at 60 °C for all the systems investigated. Inhibition efficiency values also presented in Table 1 show a decreasing trend with increasing experimental temperatures for all the systems studied. This suggests possible desorption of some of the adsorbed inhibitors from the metal surface at higher temperatures. Such behavior shows that the additives were physically adsorbed on the metal surface (Oguzie, 2007). The inhibition efficiency was found to be synergistically enhanced in the presence of KI additive and increased with the increase in KI concentration with the most profound effect noted with 5 mM KI concentration. For instance at 30 °C, the inhibition efficiency obtained for 0.5 g/L leaves and stem alone were 85.00% and 52.14%, respectively (Table 1). Addition of 5 mM KI to extracts of both plant parts increased the values to 91.97% and 77.42% for the leaves and stem, respectively.

The inhibitive action of leaves and stem of S. acuta extract toward the acid corrosion of mild steel can be attributed to the adsorption of the leaves and stem extract components onto the steel surface. Corrosion inhibition is initiated by the displacement of adsorbed water molecules by the inhibitor species leading to specific adsorption on the metal surface (Solmaz et al., 2008). Some studies (Edeoga et al., 2005; Akaneme, 2007) have shown that extracts of S. acuta are a complex mixture of various photochemicals components like saponins, flavanoids, tannis, alkaloids, organic acid and anthraquinones in the leaves and tannins, alkaloids and anthraquinones only present in the stem extract. Some of these components have heteroatoms, such as N and O in their molecules. Thus, the corrosion of mild steel may be attributed to the adsorption of S. acuta components through these atoms that are regarded as centers of adsorption onto the metal surface thereby creating a barrier for mass and charge transfer and thus isolating the metal from further attack of the corrosive anions. Decrease in inhibition efficiency with increase in temperature indicates that the components of the plant extracts were physically adsorbed on the mild steel surface. It is noted that the inhibition efficiency of the leaves extract at all experimental temperatures was higher than that of the stem extract both in the presence of the extracts alone and on the addition of iodide ions. This seems to suggest that the leaves extract is a better corrosion inhibitor than the stem extract and may be attributed to higher concentration of the phytochemical constituents of the leaves extract compared to that of the stem extract.

3.2 Hydrogen evolution measurements

By immersion of mild steel alloy in aqueous solutions, the sources of hydrogen are water decomposition and reaction of water with the metal (Lunarska and Chernyayeva, 2006). Evolved hydrogen may recombine and leave the surface as a gas or may enter the metal causing the hydrogen-induced degradation of the metal. In order to reduce the susceptibility to hydrogen uptake, the modification of solution by the addition of inhibitors or the modification of the metal surface may be applied. The requirement for effective inhibition of hydrogen uptake is to inhibit the hydrogen evolution, to promote the hydrogen gas recombination and to inhibit the hydrogen entry. Since hydrogen evolves in corrosion processes, the inhibition of corrosion should have inhibited also the hydrogen evolution. On the other hand, the inhibition of hydrogen evolution does not necessarily mean a decrease in the hydrogen charging of the metal. The presence of corrosion inhibitors may substantially affect the hydrogen ingress processes. Therefore, the mutual influence of inhibitors on the corrosion and on the hydrogen evolution should be studied.

The corrosion rates of mild steel in the absence and presence of leaves and stem extracts of S. acuta and the extracts–KI combination were assessed by monitoring the volume of hydrogen gas evolved at fixed time intervals. Fig. 3 shows representative plots of the volume of the hydrogen gas evolved as a function of reaction time at 30, 40, 50 and 60 °C for mild steel in 1 M H₂SO₄ without and with 0.5 g/L LV extract and this concentration of the extract in combination with different concentrations of KI. Similar plots for the uninhibited and inhibited solutions containing the stem (ST) and ST–KI mixtures are depicted in Fig. 4. It is observed from the figures that the volume of hydrogen gas evolved varies linearly with time and was less in the presence of S. acuta extracts compared to the blank solution. Further reduction in the volume of hydrogen gas evolved is noticed on the introduction of iodide ions to inhibited solutions containing the extracts of the plant parts. Also the volume of H₂ evolved increases with the temperature rise.

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Figure 3

Plot of volume of H₂ evolved against time for mild steel in 1 M H₂SO₄ without and with S. acuta leaves extracts and in combination with different concentrations of KI at 30, 40, 50 and 60 °C.

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Figure 4

Plot of volume of H₂ evolved against time for mild steel in 1 M H₂SO₄ without and with S. acuta stem extracts and in combination with different concentrations of KI at 30, 40, 50 and 60 °C.

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Table 2 presents the computed values of the hydrogen evolution rate and the inhibition efficiency of the various additives. Results in the table reveal that the rates of hydrogen evolution were reduced on the introduction of the S. acuta plant parts extract to the uninhibited solution. The increase of the rate of hydrogen evolution in the uninhibited solution can be rationalized on the basis that sulphuric acid reacts with iron and forms metal sulphates that are soluble in aqueous media (El-Meligi and Ismail, 2009). There are two reactions occurring, namely the anodic reaction and cathodic reaction. The following equations represent iron reaction in acidic solutions (El-Meligi and Ismail, 2009):

Anodic reaction (oxidation reaction):

Cathodic (reduction reaction or hydrogen evolution reaction):

The decrease in the hydrogen evolution rate observed for the extract–KI combination for both plant parts indicates enhancement of inhibitory action of S. acuta in the presence of the iodide ions. The highest values for the hydrogen evolution rate were obtained at 60 °C for all the systems investigated and the stem extract had the highest values in comparison to the leaves extract even with the iodide ion additive. This suggests that the components of the plant parts extract are adsorbed onto the metal surface and block the electrochemical reaction efficiently by decreasing the available surface area.

The values of inhibition efficiency are also listed in Table 2 and can be seen to follow the trend reported for weight loss measurements. It synergistically increased on addition of iodide ion to both the leaves and stem S. acuta extracts but decreased with increasing temperature. The inhibition efficiency of the two independent methods used in this study is different. The values for the hydrogen evolution method are lower than those of the weight loss technique. This may be attributed to the difference in immersion time needed for the inhibiting species to get adsorbed and form a protective film on the mild steel thereby insulating the metal from the attack of the aggressive anions present in the solution. In addition, it has been argued that corrosion rate values from gasometric method represent instantaneous values while those from the gravimetric method represent average values (Umoren and Ekanem, 2010).

3.3 Synergism between S. acuta extracts and iodide ions

Results obtained in the present study clearly show that the inhibition efficiency of the S. acuta leaves and stem extracts was greatly enhanced on the addition of iodide ions and could be ascribed to a synergistic effect. The anodic dissolution of steel in aqueous solution is known to be significantly facilitated by hydroxyl ions that form intermediate catalytic complexes on the metal surface (Bockris et al., 1961). Halide ions, at some concentrations, may replace hydroxyl ions adsorbed on the surface of the metal, thus leading to a reduction in the catalytic effect of the hydroxyl ions as follows (Bala, 1984; Chin and Nobe, 1972):

Two approaches were adopted to characterize the synergistic inhibitive effect of iodide ions addition to both the leaves and stem extracts of S. acuta. In one approach, the concentration of the extracts was kept constant and the concentration of iodide ions varied while the reverse was adopted in the second approach. Fig. 5 shows the plot of inhibition efficiency (η%) as a function of concentration for (a) leaves and (b) stem of S. acuta extract in the presence of fixed KI concentration (5 mM). Similarly, Fig. 6 shows the plot of η% vs. KI concentration added to a fixed extracts’ concentration (0.5 g/L) for (a) leaves and (b) stem extracts at different temperatures. Fig. 6 clearly reveals the existence of synergism phenomenon between iodide ions and the S. acuta parts extract considering that η% was increased on addition of the iodide ions to the extracts in comparison to the extract alone.

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Figure 5

Plot of inhibition efficiency against extract concentration in combination with fixed 5 mM KI for mild steel in 1 M H₂SO₄ at different temperatures for (a) LV and (b) ST extracts.

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Figure 6

Plot of inhibition efficiency against KI concentration in combination with fixed 0.5 g/L extract concentration for mild steel in 1 M H₂SO₄ at different temperatures for (a) LV and (b) ST extracts.

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To further confirm whether synergism is taking place, one has to determine the synergism parameter (s), as initially proposed by Murakawa et al. (1967) for describing the combined inhibition behavior of amines and halide ions. Generally, for the interaction of inhibitors A and B this synergism parameter (S₁) is defined as follows:

3.4 Adsorption considerations

For the plant extract to inhibit the corrosion of mild steel, there must have to be an adsorption of the components of the plant part extracts on the metal surface leading to greater surface coverage as the first step of the inhibition mechanism. The values of the degree of surface coverage (θ) defined as the fraction of the mild steel surface that was covered by the inhibitor are quite useful for the determination of inhibitor adsorption characteristic. Assuming direct relationship between surface coverage and inhibition efficiency, the surface coverage can be evaluated from the expression: η% = θ × 100. Attempts were made to fit θ values obtained from the weight loss measurements to various isotherms and correlation coefficient (R²) values were used to determine the best-fit isotherm. By far, the best result was obtained for Freundlich isotherm that has a general form (Kliskic et al., 2000):

The logarithm of surface coverage (θ) plotted against logarithm of concentration (C) is shown in Fig.7 for (a) leaves and (b) stem extracts alone and in combination with iodide ions at 30 °C. Linear plots were obtained indicating that the experimental results fit the Freundlich isotherm. The calculated values of K_ads and $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ were found to be 7.335 g L⁻¹ and −15.138 kJ mol⁻¹, respectively, for leaves extract alone and 1.311 g L⁻¹ and −10.799 kJ mol⁻¹ for leaves extract in combination with 5 mM KI. Values of 1.256 g L⁻¹ and −10.691 kJ mol⁻¹ as well as 1.347 and −10.868 kJ mol⁻¹ were obtained for stem extract and stem extract–KI mixtures, correspondingly. It is seen that the values of the $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ in all the cases are negative. The negative values of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ indicate the spontaneous adsorption of the additives on the mild steel surface and stability of the adsorption layer on the metal. Literature survey reveals that negative values of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ around 20 kJ mol⁻¹ or lower are consistent with the electrostatic interaction between charged molecules and the charged metal (physisorption) while those around 40 kJ mol⁻¹ or higher involve charge sharing or transfer from organic molecules to metal surface to form co-ordinate type of bond (chemisorption) (Oguzie et al., 2010; Obot et al., 2009). Results obtained indicate that the values of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ for all the systems indicate spontaneous adsorption of the additives onto the mild steel surface via physisorption mechanism.

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Figure 7

Freundlich adsorption isotherm plot for mild steel in 1 M H₂SO₄ with S. acuta extracts alone and in combination with KI for (a) leaves and (b) stem at 30 °C.

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3.5 Effect of temperature

Temperature can modify the interaction between the mild steel electrode and the acidic medium in the absence and the presence of inhibitors. To assess the influence of temperature on corrosion and corrosion inhibition processes, the weight loss measurements were carried out at 30–60 °C temperature range. The results obtained indicated that corrosion rates increase with rise of temperature both in uninhibited and inhibited solutions. More so, inhibition efficiency for the extracts alone and in combination with iodide ions decreases with increase in temperature. The dependence of corrosion rate on the temperature can be regarded as an Arrhenius-type process, the rate of which is given by:

The Arrhenius plot is shown in Fig. 8 for mild steel in free acid solution and inhibited solution containing (a) leaves extract and leaves extract–KI mixtures and (b) stem extract and stem extract–KI mixtures. Linear plots were obtained from the slope (−E_a/2.303R), activation energy (E_a) values are deduced and listed in Table 4. The temperature dependence of the inhibiting effect and the comparison of the values of the apparent activation energy of the corrosion process in the absence and presence of inhibitors can provide further evidence (Szauer and Brandt, 1981; Ivanov, 1986) concerning the mechanism of the inhibiting action. The decrease of the inhibitor efficiency with temperature rise, which refers to a higher value of E_a, when compared to that in an acid with no inhibitor, is interpreted as an indication for an electrostatic character of the inhibitor’s adsorption. The lower value of E_a in an inhibited solution when compared to that of an uninhibited one shows that strong chemisorption bond between the inhibitor and the metal is highly probable (Zerga et al., 2010). Activation energy, E_a values in the table are higher for inhibited solution (for the different systems) than the uninhibited one, indicating a strong inhibitive action of the additives by increasing energy barrier for the corrosion process, emphasizing the electrostatic character of the inhibitor’s adsorption on the mild steel surface (physisorption).

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Figure 8

Arrhenius plot for mild steel in 1 M H₂SO₄ without and with S. acuta extracts and in combination with different concentration of KI for (a) leaves and (b) stem.

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Experimental corrosion rate values obtained from weight loss measurements for mild steel in 1 M H₂SO₄ in the absence and presence of S. acuta (leaves and stem) extracts and S. acuta extracts–iodide ion combination were used to further gain insight on the change of enthalpy (ΔH*) and the entropy (ΔS*) of activation complex in the transition state using transition state equation (Noor and Al-Moubaraki, 2008)

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Figure 9

Transition state plot for mild steel in 1 M H₂SO₄ without and with S. acuta extracts and in combination with different concentrations of KI for (a) leaves and (b) stem.

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Further insight into the adsorption mechanism is offered by considering the thermodynamic functions for the mild steel dissolution in 1 M H₂SO₄ in the presence of S. acuta leaves and stem extracts and extracts–KI mixtures. In this regard, heat of adsorption (Q_ads) was evaluated from the variation of surface coverage with reciprocal of temperature from the kinetic/ thermodynamic model (Solmaz et al., 2008; Avic, 2008)

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Figure 10

Plot of log (θ/1 − θ) vs. 1/T for mild steel in 1 M H₂SO₄ containing S. acuta extracts and in combination with different concentrations of KI for (a) leaves and (b) stem.

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