Sustainable Corrosion Inhibition of Mild Steel in Hydrochloric Acid Using Extracts of Phaseolus Vulgaris and Vicia Faba

Compounds extracted from natural products as corrosion inhibitors have garnered significant attention in recent years due to their cost-effectiveness and environmental benefits [1, 2]. Usually, the extracts from roots (e.g., Rheum ribes [3]), leaves (e.g., Acanthopanax senticosus [4]), flowers (e.g., Camellia chrysanth [5]), fruits (e.g., Punica granatum [6]), and seeds (e.g., Persea americana [7]) of different plants are used [8, 9]. This also opens a new way for the usage of food residues and biomass-derived products [10].

Common beans (Phaseolus vulgaris) and fava beans (Vicia faba) have been extensively cultivated for both human and animal consumption, given that their beans are rich in several vitamins and polyphenols [11, 12]. Boiling in water is a typical culinary preparation method for these beans, a process that results in the extraction of a portion of the phenols and vitamins into an aqueous phase. The boiling extracts, despite their potential usefulness, are typically discarded and remain underutilized in the food and fodder industries.

Recent studies indicate that polyphenols [13, 14] and vitamins [14] exhibit properties that inhibit metal oxidation. It is thus plausible to hypothesize that the boiling extracts of both Phaseolus vulgaris and Vicia faba beans could serve as efficient and environmentally friendly corrosion inhibitors. However, the inhibitory properties of bean extracts on metal corrosion have been sparingly studied [15, 16].

Specifically, one study [15] investigated the effects of Vicia faba hexane or acetone extracts on the corrosion of mild steel in a NaCl solution, while another [16] studied the impact of the winged bean (Psophocarpus tetragonolobus) hexane extract on the corrosion of reinforced steel in hydrochloric acid. Noticeably, these previous investigations involved the use of organic solvents, leaving the potential of water extracts from beans unexplored. The extensive usage of toxic organic solvents, moreover, raises sustainability concerns for such natural corrosion inhibitors [17].

Given the easy availability of water extracts during the bean cooking process, and the current tendency to simply discard such extracts, the present study aims to investigate the inhibitory properties of the boiling extracts of the black Russian varieties of Phaseolus vulgaris and Vicia faba on the corrosion of mild steel EN Fe37-3FN in a 0.5 M hydrochloric acid medium. This research hopes to uncover new, sustainable ways of utilizing these bean boiling extracts.

3.1 Polarisation studies

The polarisation tests were performed in hydrochloric acid with and without the additions of inhibitors. Electrodes were polished using the Р2500 emery paper and degreased by ethanol. The measurements were conducted in a standard three-electrode electrochemical cell, consisting from the working electrode (steel sample), auxiliary graphite electrode, and the silver-silver chloride reference electrode. The cell was placed into the Faraday shield cell. An open circuit potential was recorded during 30 min. The results are presented in Figure 1. Polarisation curves were recorded in the potential range from –500 to +500 mV relatively to the measured open circuit potential with the potential sweep rate of 10 mV/s, which allows the clear estimation of the Tafel slopes. Each experiment was performed in triplicate. The obtained polarisation curves were presented in the coordinates E(lg i), and the Tafel slopes and the corrosion current density were evaluated from them [14]. The inhibitory ability of the compound was estimated from the ratio of the corrosion current densities in the absence (i₀) and in the presence (i) of the inhibitor: IE=(i₀–i)/i₀ 100% [14]. The results are presented in Figure 2 and in Table 1.

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(a) The Phaseolus vulgaris bean extract

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(b) The Vicia faba bean extract after 30 min of exposure

Figure 1. The open circuit potential of steel in 0.5 M HCl with the different additions (g/L)

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(a) The Phaseolus vulgaris bean extract

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(b) The Vicia faba bean extract after 30 min of exposure

Figure 2. The polarisation curves of steel in 0.5 M HCl with the different additions (g/L)

3.2 EIS studies

The EIS tests were performed in hydrochloric acid with and without the additions of inhibitors. Electrodes were polished using the Р2500 emery paper, and degreased by ethanol. The measurements were conducted in a standard three-electrode electrochemical cell, consisting from the working electrode (steel sample), auxiliary electrode from the porous graphite, and the silver-silver chloride reference electrode. The cell was placed into the Faraday shield cell. An open circuit potential was recorded during 30 min. An impedance values were recorded at the open circuit potential value in the alternate current frequency interval from 100 mHz to 10 kHz with the potential amplitude of 10 mV. Each experiment was performed in triplicate. The obtained results were presented in the form of Bode and Nyquist plots [18]. For the estimation of the impedance parameters, a simplified Randles equivalent electrical circuit [18], containing the solution resistance R_s, the consecutive charge transfer resistance R_ct of the passivation layer, and the parallel constant-phase element representing the double electric layer, was employed. The fitting of the equivalent circuit parameters to the experimental impedance values was performed using the free software EIS Spectrum Analyser [19]. The inhibitory ability of the compound was estimated from the ratio of the charge transfer resistances in the presence (R) and in the absence (R₀) of the inhibitor: IE=(R–R₀)/R^.100% [14]. The results are presented in Figure 3 and in Table 2. The impedance spectra of both inhibitors are very similar to each other.

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(a)

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(b)

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(c)

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(d)

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(e)

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(f)

Figure 3. The Bode plots of steel in 0.5 M HCl with the different additions (g/L) of (a), (c) the Phaseolus vulgaris bean extract, (b), (d) the Vicia faba bean extract, and the Nyquist plots of steel in 0.5 M HCl with the different additions (g/L) of (e) the Phaseolus vulgaris bean extract, (f) the Vicia faba bean extract after 30 min of exposure

3.3 Langmuir adsorption model

The description of the adsorption of the bean extract components on the steel surface was performed in terms of the Langmuir adsorption model, which assumes that an adsorbate behaves like an ideal gas at isothermal conditions [20]. The Langmuir adsorption isotherm equation was linearised in the form $\mathrm{c}_{\mathrm{inh}} / \theta=1 / \mathrm{K}_{\mathrm{ads}}+\mathrm{c}_{\mathrm{inh}}$, where $\mathrm{c}_{\text {inh }}$ is the concentration of the Phaseolus vulgaris bean or Vicia faba bean extract solution (g/L), $\mathrm{K}_{\mathrm{ads}}$ is the adsorption-desorption equilibrium constant (L/g), and θ is the percentage of the surface covered by the inhibitor, which assumed to be equal to the inhibition efficiency. The dependencies of $\mathrm{c}_{\mathrm{inh}} / \theta$ on $\mathrm{C}_{\text {inh }}$ are presented in Figure 4 and in Table 3. The data were processed using the least squares technique [21], and the equilibrium constants $\mathrm{K}_{\mathrm{ads}}$ together with their confidence intervals were estimated as the intercepts of the regression equations. The Gibbs energy changes of the sorption and their confidence intervals were estimated from the equation $\Delta_{\mathrm{ads}} \mathrm{G}=-\mathrm{RT} \ln \left(\mathrm{K}_{\mathrm{ads}} \cdot \mathrm{c}_{\text {water }}\right)$, where c_water=10³ g/L is the water concentration in the extracts. The results are presented in Table 3.

Table 1. The results of the electrochemical measurement of the corrosion rates

Table 2. The results of the EIS measurement of the corrosion rates

Table 3. The parameters of the Langmuir adsorption model

The inhibitory ability of the Phaseolus vulgaris bean and Vicia faba bean boiling extracts on the corrosion of mild steel EN Fe37-3FN in 0.5 M hydrochloric acid medium was investigated using electrochemical methods and EIS. The inhibition efficiencies at the different inhibitor concentrations were estimated. The adsorption of the extract components on a steel surface follows the Langmuir adsorption model, and the nature of adsorption is mostly physical. The Phaseolus vulgaris bean and the Vicia faba bean extract, as the discarded by-products of bean cooking, are identified as promising and environmentally friendly substances for reducing the steel corrosion rate in acidic environments. Given the increasing need for environmentally friendly solutions in industry, the discovery that these commonly discarded by-products can act as effective corrosion inhibitors is of significant importance. The future studies on exploring other potential environmentally friendly inhibitors based on commonly discarded natural products are beneficial.