Justicia brandegeeana as a green corrosion inhibitor for carbon steel in sulfuric acid

ABSTRACT

The anti-corrosive effect of the extract in ethanol of Justicia brandegeeana (Acanthaceae) against carbon steel AISI 1020 in sulfuric acid medium 1 mol L⁻¹ was investigated. In addition to the effect of seasonality, several techniques were used to investigate the anti-corrosive activity, such as electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PP), linear polarization resistance (LPR), and weight loss. The chelating effect was analyzed by UV-Vis spectrophotometry and the surface was analyzed by scanning electron microscopy (SEM). The results showed that the inhibition efficiency is dependent on the concentration of the extract, reaching its maximum at 1500 ppm and despite having good thermal stability (82.41% inhibition at 70°C), it decreases with increasing temperature, indicating a physisorption behavior. The extract obeyed the Langmuir isotherm and the Tafel curves characterized it as a mixed inhibitor, with a tendency to cathodic region. It also presented a 52% chelating effect against Fe⁺² ions, and the SEM analysis indicated a protection with the presence of plant extract, thus minimizing surface roughness. These results indicated the J. brangeeana ethanol extract as a green anti-corrosive agent.

GRAPHICAL ABSTRACT

KEYWORDS:

• Justicia brandegeeana (Acanthaceae)
• green anti-corrosion agent
• electrochemical methods
• weight loss
• chelating effect

Introduction

Corrosion is a crucial problem worldwide that heavily affects natural and industrial environments. Countless production areas, such as the chemical industry, civil construction, automotive sector, dentistry, and others, are directly impacted by the corrosive process, which mostly affects metals. It is estimated that in 2016 the global annual cost of corrosion consisted of 3.4% of GDP, equivalent to approximately US 2.5 trillion (1).

Among the various materials that can corrode, carbon steel is the most used industrially due to its low cost and ease of welding and forming. Carbon steel pipes are responsible for transporting large volumes of oil, derivatives, and natural gas, but this material has limitations, such as low resistance to corrosion during prolonged exposure to high temperatures, which can cause serious problems in the industry (2, 3).

Corrosion can be controlled by several means, from modifying the process or the corrosive medium to applying protective coatings. The use of corrosion inhibitors is one of the more effective ways to prevent or control corrosion in several materials (4). The use of natural inhibitors has been intensified due to their renewable origin, biodegradability, low cost and not hazardous to human health, and the environment (5). Furthermore, the inhibitors, to be considered green or eco-friendly need to meet three requirements: (i) they must not be bioaccumulative, (ii) they must be biodegradable, and (iii) they must have zero or very low marine toxicity (6, 7).

Plant extracts have been widely studied as inhibitors due to their low cost, abundant availability, and biodegradability. The different parts of plants are useful, such as bark, stem, root, leaves, flowers, and seeds, or even waste and by-products of agroindustry; however, they may present different inhibition efficiencies due to the amounts of secondary metabolites and other organic substances (8–12).

Considering the range of existing studies on vegetal extracts with corrosion-inhibitory characteristics, it is possible to exemplify with the citation of some works. The corrosion inhibition efficiency of 89.2% by the aqueous extract of coffee husks was obtained at 800 ppm in HCl 1.0 mol L⁻¹ with 24 h of immersion (4). In the same corrosive medium, Dehghani et al. (13) evaluated the aqueous extract of the Chinese gooseberry (Kiwi) fruit peel and obtained 92% efficiency in 2.5 h of immersion at 1000 ppm (13).

Hydroalcoholic extracts at 400 ppm of orange, mango, passion fruit, and cashew peels, in addition to papaya seed and grape pomace, were evaluated in two different media: neutral and HCl 1.0 mol L⁻¹ medium. The extracts did not show good inhibition in a neutral medium, except for 2% concentrated grape pomace, which showed 74% efficiency in 72 h of immersion by the gravimetric technique. However, they obtained a variation of 93%–97% of inhibition efficiency in an acid medium (14).

Santana et al. (15) studied the aqueous extract of castor bean, also at 800 ppm and obtained 97.8% efficiency by the gravimetric method in HCl 1.0 mol L⁻¹ medium, showing that castor bean is a mixed inhibitor, attributed to the existence of ricinoleic acid in its composition (15).

The hydroalcoholic extract of Malvaviscus arboreus was tested against H₂SO₄ 1.0 mol L⁻¹ and it showed an inhibition efficiency of 97.5% at 3, 24, and 48 h at 500 ppm (16). The aqueous extract of Coriandrum sativum seeds was evaluated in two different acid media (H₂SO₄ 0.5 mol L⁻¹ and HCl 1.0 mol L⁻¹) and showed good inhibition efficiencies for both media at a concentration of 1000 ppm, being 96.7% and 93.7%, respectively (17).

Alvarez et al. (18) evaluated the methanolic extract of Rollinia occidentalis in 1.0 mol L⁻¹ of the HCl medium and obtained 85.7% anti-corrosion efficiency with the extract at 1000 ppm (18).

Recently, the aqueous extract of Brassica rapa was evaluated regarding the anti-corrosion effect in Q235 steel in a sulfuric acid medium. The observed results revealed an inhibition efficiency of 94.3% and 92.5% by polarization potentiodynamic (PP) and electrochemical impedance spectroscopy (EIS), respectively (19). The review of natural plant extracts shows the performance of Fraxinus excelsior (FE), Zingiber zerumbet (ZZ), and Isatis tictoria (IT) as efficient anti-corrosion agents of the mild carbon steel in phosphoric acid (2 mol L⁻¹). The observed inhibitory efficiencies by the weight loss method were 95.2%, 92.8% and 91.7%, respectively, to FE, ZZ, and IT aqueous extracts with the concentration of 3 g L⁻¹ (20).

Furthermore, Gu et al. (21) evaluated the efficiency of copper protection in sulfuric acid (0.5 mol L⁻¹) by the aqueous extract of Lycium barbarum leaves. The electrochemical methods showed corrosion inhibition efficiencies of 92.9%, 92.8%, and 94.1% at 298, 303, and 308 k, respectively (21). Golshani et al. (22) investigated the Dracocephalum extract as a green corrosion inhibitor for mild steel in sulfuric acid (0.5 mol L⁻¹) and hydrochloric acid (1.0 mol L⁻¹) media. Interestingly, the authors observed an inhibition efficiency of 94% at 75 ppm of concentration and 89% at 200 ppm in the H₂SO₄ solution, but in the HCl medium the value of 88% at 100 ppm and 90% at 400 ppm. The results indicated that extract acts in cathodic and anodic processes by the Langmuir adsorption isotherm (22).

Because of the range and importance of studies with extracts of vegetal species, it is proposed, then, the analysis of Justicia brandegeeana, an ornamental plant, as the object of study of this work. J. brandegeeana shown in Figure 1 is popularly known as red shrimp, and belongs to the Acanthaceae family, originally from Mexico. It is a bushy plant, being used for ornamental purposes in tropical and subtropical climates (23, 24).

Figure 1. Aerial parts of Justicia brandegeeana.

Source: author’s own.

The first phytochemical investigation of J. brandegeeana was carried out by Jiang et al. (25), where its essential oil was analyzed, identifying 32 compounds, with the main class being alcohols (68.42%), followed by aldehydes and ketones (9.92%), esters (6.59%), and alkanes (1.10%) (25). Another study demonstrated the effectiveness of J. brandegeeana petroleum ether extract at 1200 ppm as a biocontrol agent against Rhynchophorus ferrugineus which is a pest of rapid dispersion in several species of palm tree. 98.47% of total fatty acids was detected by GC/MS, with 11.98% saturated fatty acids, mainly palmitic acid; 86.49% unsaturated fatty acids, the main component being linoleic acid with 37.4% (26).

Thus, with the advancement of the search for new eco-friendly corrosion inhibitors, especially the use of plant extracts, the choice of J. brandegeeana for this work was outlined due to the existence of few biological and phytochemical studies related to it, and no studies on its use as an anti-corrosive agent, in addition to being an easy handling ornamental plant. The anti-corrosive effect of the ethanolic extract of J. brandegeeana was then evaluated in carbon steel in an H₂SO₄ medium at 1.0 mol L⁻¹ because is a corrosive medium usually used in industrial processes. The analyses occurred by the gravimetric method, electrochemical analysis as electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PP) and linear polarization resistance (LPR), and surface analysis by scanning electron microscopy (SEM).

Experimental

Plant extract

The aerial parts (leaves, flowers and stems) of Justicia brandegeeana Wassh. & L. B. Sm. were collected in autumn and spring, to compare the effects of seasonality in the extracts. The collection took place in the garden of the Chemistry Institute and the samples were identified in the Herbarium of the Department of Botany which deposited a voucher specimen RBR 56243, both Federal Rural University of Rio de Janeiro

were collected 500.0 g of material that after drying at room temperature presented 70% and 69% of moisture for the June and October collections, respectively. The dry, crushed, and heavy mass was extracted by static maceration in 2.0 L of ethanol for 7 days, generating a fluid extract of J. brandegeeana with 3470 ppm that was stored in an amber container under refrigeration of approximately 4°C. The concentration of extract was obtained by the difference in mass the first weighing of the material dry before extraction and the material dry after extraction, obtaining the concentration of the material in the solvent. From the fluid extract were performed dilutions with ethanol to obtain concentrations in the range of 250–1500 ppm of the inhibitor.

Both extracts were analyzed by FTIR (ATR, Bruker – VERTEX 70), ¹H NMR and ¹³C, obtained in DMSO-d₆ (Bruker – AVANCE – 500 MHz) spectra to compare the seasonality effects on the main organic compound classes found in the ethanol extract of J. brandegeeana.

Acid solution

The acid solution used was prepared from concentrated sulfuric acid in analytical grade and Milli-Q water. A stock solution was prepared at the concentration of 2.0 mol L⁻¹ and the required concentration of 1.0 mol L⁻¹ was obtained by dilution.

Electrochemical assays

The AUTOLAB PGSTAT 302N potentiostat was used for the analyses, with an electrochemical cell composed of three electrodes: the reference electrode of Ag/AgCl (3 mol L⁻¹ KCl), the counter electrode of platinum, the working electrode of carbon steel AISI 1020 inserted in epoxy resin, with an exposed surface area of 2.1 cm². All measurements were performed in a Faraday cage and the results were analyzed in the software NOVA 2.1.5. The working electrode, before the analysis, was sanded with sandpaper of three different grades (400, 600, and 1200 mesh), washed with distilled water to remove impurities, defatted with ethanol and dried.

Initially, the blank solution contained only ethanol and H₂SO₄ 1.0 mol L⁻¹. Subsequently, the solutions containing the ethanol extract of J. brandegeeana at 250, 450, 750, 1000, and 1500 ppm were analyzed at a temperature of 32°C with a total volume of 50 mL in the cell for each reading. The analyses were performed in triplicate to examine the reliability and reproducibility of the measurements, and one of the rejoinders was selected to represent the data graphically.

The electrochemical analysis consisted in the stabilization of the open circuit potential (OCP) in 5400 s, thus obtaining the OCP corresponding to the corrosion potential (E_corr) of the working electrode. Then, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and finally potentiodynamic polarization (PP) were performed.

The EIS spectra were performed in a frequency range of 10 kHz–0.1 Hz with 10 mV peak-by-peak amplitude and the inhibitor efficiency (η_EIS) was calculated using Equation 1(1) ${\eta }_{\mathrm{EIE}}(\text{\%})=\left({\displaystyle \frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}}\right)x100$(1) : (1) ${\eta }_{\mathrm{EIE}}(\text{\%})=\left({\displaystyle \frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}}\right)x100$(1) where R_ct is the charge transfer resistance in solution with inhibitor and R⁰_ct is the charge transfer resistance in the blank solution.

The LPR assays were performed using a sweep rate of 1.0 mVs⁻¹ in the potential range of ±10 mV around the open circuit potential (E_ocp), and the inhibition efficiency (η_LPR) was calculated from Equation 2(2) ${\eta }_{\mathrm{RPL}}(\text{\%})=\left({\displaystyle \frac{R_p-R_p^0}{R_p}}\right)x100$(2) : (2) ${\eta }_{\mathrm{RPL}}(\text{\%})=\left({\displaystyle \frac{R_p-R_p^0}{R_p}}\right)x100$(2) where R_p and R⁰_p are the polarization resistance in the solution with inhibitor and blank solution, respectively.

The PP assays were performed in the ±200 mV potential range around the open circuit potential (E_ocp), with a sweep rate of 1.0 mVs⁻¹, and the inhibition efficiency (η_PP) was calculated with Equation 3(3) ${\eta }_{\mathrm{PP}}(\text{\%})=\left({\displaystyle \frac{i_{\mathrm{corr}}^0-i_{\mathrm{corr}}}{i_{\mathrm{corr}}^0}}\right)x100$(3) : (3) ${\eta }_{\mathrm{PP}}(\text{\%})=\left({\displaystyle \frac{i_{\mathrm{corr}}^0-i_{\mathrm{corr}}}{i_{\mathrm{corr}}^0}}\right)x100$(3) where i_corr,0 and i_corr are the corrosion current densities in the solution with inhibitor and blank solution, respectively.

Weight loss measurements

The weight loss tests were performed with carbon steel coupons AISI 1020 with dimensions of 3.0 × 3.0 × 0.15 cm in 50 mL of H₂SO₄ 1.0 mol L⁻¹ solution and the ethanol extract of J. brandegeeana in a concentration range of 250–1500 ppm.

The coupons received the same treatment performed for the working electrode in electrochemistry, then were weighed in an analytical balance and immersed in the solution in the absence (blank) and in the presence of the extract under temperature control with the aid of a thermostatic bath. After immersion, the coupons were again cleaned with distilled water and ethanol, dried, and weighed.

The gravimetric assays were performed in triplicates with variations of immersion time (3, 6, 24, and 48 h), temperature (30°C, 40°C, 50°C, 60°C, and 70°C) and inhibitor concentration (250, 450, 750, 1000, and 1500 ppm).

The inhibition efficiency (η%) of the plant extract of J. brandegeeana for carbon steel AISI 1020 was calculated by Equation 4(4) $\eta (\text{\%})=\left({\displaystyle \frac{W_{\mathrm{corr}}^0-W_{\mathrm{corr}}}{W_{\mathrm{corr}}^0}}\right)x100$(4) : (4) $\eta (\text{\%})=\left({\displaystyle \frac{W_{\mathrm{corr}}^0-W_{\mathrm{corr}}}{W_{\mathrm{corr}}^0}}\right)x100$(4) where W⁰_corr and W_corr are the corrosion rates in the blank and the presence of the inhibitor, respectively.

Chelating effect

To determine the chelating effect of the ethanol extract of J. brandegeeana, the UV-VIS spectrophotometry method (Shimadzu, model UV-1800) was used. The spectra were obtained in quartz cuvette (1.5 mL) and to determine the interaction between the plant extract and the Fe⁺² ions FeSO₄ solution was used at the final concentration of 1.87 × 10⁻⁶ mol L⁻¹, and solutions of the extract at 125 ppm, concentrations that provided the best curves.

The analysis was performed after a time-out for 10 min at room temperature. EDTA was used as a positive control for the chelating effect at the concentration of 2.32 × 10⁻⁴ mol L⁻¹.

The analysis was performed after 10 min of rest at room temperature. EDTA was used as a positive control for the chelating effect at the concentration of 2.32 × 10⁻⁴ mol L⁻¹. The chelating effect of the ethanol extract of J. brandageeana was calculated and expressed as a percentage from Equation 5(5) $\mathrm{Chelating}\mathrm{effect}(\text{\%})=-\left[1-\left({\displaystyle \frac{A_s}{\mathrm{Ac}}}\right)\right]x100$(5) : (5) $\mathrm{Chelating}\mathrm{effect}(\text{\%})=-\left[1-\left({\displaystyle \frac{A_s}{\mathrm{Ac}}}\right)\right]x100$(5) where A_s and A_c are the maximum absorbances of the sample and the control, respectively.

Surface analysis

The analysis of the metallic surface was performed by scanning electron microscopy (SEM) in the presence and absence of the ethanol extract of J. brandegeeana. Three coupons of carbon steel AISI 1020 were analyzed: metallic surface without immersion in any medium (Coupon I), metallic surface after immersion in the corrosive medium H₂SO₄ 1.0 mol L⁻¹ in the absence of the extract in ethanol of the plant species for 24 h at 30°C (Coupon II) and metal surface after immersion in the corrosive medium H₂SO₄ 1.0 mol L⁻¹ in the presence of the plant extract in ethanol species at 1000 ppm for 24 h at 30°C (Coupon III).

The treatments used in the coupons before and after the analysis were the same ones performed in the weight loss tests. The micrographs were obtained with an acceleration voltage of 15 kV and with 4000× magnification. The analysis was performed using the HITACHI TM 3000 Tabletop Microscope.

Results and discussion

Plant extract

The effects of seasonality for the ethanolic extracts of J. brandegeeana collected on two different occasions were evaluated for the presence of the main functional groups present in the metabolites through infrared spectroscopic analysis, ¹H NMR, and ¹³C NMR for each of the samples (27).

Figure 2 shows the infrared spectrum for the extract obtained from the June and October collects, presenting broadband at 3299 cm⁻¹ relative to the presence of OH with intermolecular interactions derived from hydrogen bonds. Bands in 2923 and 2854 cm⁻¹ were attributed to stretch asymmetric and symmetric strains of aliphatic C–H, respectively. The band at 1733 cm⁻¹ can be attributed to carbonyl groups of ester or carboxylic acids (28). The average intensity band at 1614 cm⁻¹ may indicate vibrations of C = C conjugated to carbonyl. At 1400 cm⁻¹, it was observed the absorption relative to the OH folding, and at 1051 cm⁻¹ the stretch relative to a primary alcohol. The presence of these bands indicates the existence of heteroatoms such as oxygen, which are desirable in organic corrosion inhibitors. The infrared spectrum of the extract of the second collection (October) showed great similarity with that collected in June, indicating that the effect of seasonality showed change only in the intensity of the bands of the spectrum, being in this case less intense not having change like the classes of extracted substances.

Figure 2. Overlapping of infrared spectra, in ATR, for extracts of J. brandegeeana obtained in June 2021 and October 2021.

The ¹H NMR spectra for the ethanol extracts of J. brandegeeana in the collects of June and October are presented in Figure 3 they were very similar, only with a difference in the intensity of the signals, being the extract collected in June the one that presented the highest intensity in the signals, as well as in the ¹³C-DEPTQ NMR spectra showing in Figure 4.

Figure 3. Overlapping of ¹H NMR spectra in DMSO-d₆, at the power of 500 MHz for the extracts of J. brandegeeana obtained from the collections of June 2021 and October 2021.

Figure 4. Overlapping of ¹³C-DEPTQ NMR spectra in DMSO-d₆, at the power of 500 MHz for the extracts of J. brandegeeana obtained from the collections of June 2021 and October 2021.

The ethanol extract prospection was performed also through the ¹H and ¹³C-DEPTQ NMR spectra enabling the identification of the carbohydrate class presence through the signals in the range of 4.0–5.5 ppm (¹H NMR) and 60–80 ppm (¹³C-DEPTQ NMR), with the presence of methoxy groups (3–4 ppm for ¹H and 52.6 ppm for ¹³C). In addition, signals at 10.14 and 10.56 ppm in the ¹H NMR spectra indicated the OH moieties’ presence of carboxylic acids and/or alcohols according to the bands observed in the infrared spectra. Chemical displacements in the ¹³C-DEPTQ NMR spectra in the range of 100–60 ppm suggested the presence of disaccharides or glycosylated metabolites according to the literature (29, 30), highlighting the absorptions in 99.17 ppm: anomeric carbon of β-D-fructopyranose, 97.21 ppm: anomeric carbon of β-D-glucopyranose, 92.31 and 92.63 ppm: anomeric carbon of α-D-glycopyranose, 82.15 and 82.87 ppm: CH (C5) of α-D-fructofuranose, 68.77–77.60 ppm: -CHOH of gluco or fructopyranose, 60.88–63.50 ppm: CH₂-OH of gluco or fructopyranose.

Electrochemical tests

Analysis of the OCP

Figure 5 shows the evolution of the open circuit potential for 5400 s, until its stabilization, in the presence and absence of the plant extract at different concentrations. Such stabilization is necessary so that there is no potential variation during measurements, thus, the blank stabilized at −0.427 V, while in the presence of the J. brandegeeana extract the OCP varied in the range of −0.438 to −0.432 V in the concentrations of 250–1000 ppm, with values more negative than the blank, as shown in Table 1. Already at 1500 ppm, stabilization occurred at −0.420 V, with potential more positive than blank.

Figure 5. OCP variation obtained in the absence and presence of J. brandegeeana ethanol extract at various concentrations.

Table 1. EIS parameters for carbon steel AISI 1020 in H₂SO₄ 1.0 mol L⁻¹ solution in the absence and presence of various concentrations of the J. brandegeeana ethanol extract at 32°C.

Inhibitor (ppm)	OCP Ag/AgCl (V)	Rct (Ω cm^2)	Rs (Ω cm^2)	fmax (Hz)	N	Y^0 (Ω^−1 cm^−2 s^n)	Cdl (µF cm^−2)	χ^2	θ	ηEIS (%)
Blank	−0.427 ± 0.002	11.49 ± 0.06	1.81 ± 0.329	100.00	0.894 ± 0.007	65.24 ± 0.476	32.95 ± 0.604	0.0186 ± 0.001	–	–
250	−0.438 ± 0.005	31.73 ± 0.07	2.68 ± 0.191	50.12	0.884 ± 0.006	43.14 ± 0.591	22.74 ± 0.152	0.2488 ± 0.053	0.619 ± 0.011	61.90
450	−0.432 ± 0.002	50.43 ± 0.16	1.93 ± 0.303	39.81	0.860 ± 0.007	36.09 ± 0.842	16.66 ± 0.754	0.0941 ± 0.041	0.746 ± 0.013	74.57
750	−0.432 ± 0.001	63.62 ± 0.03	2.15 ± 0.446	39.81	0.850 ± 0.008	32.86 ± 0.875	14.35 ± 0.486	0.1514 ± 0.057	0.795 ± 0.006	79.54
1000	−0.435 ± 0.003	85.70 ± 0.03	4.04 ± 0.446	31.62	0.873 ± 0.006	28.05 ± 0.909	14.32 ± 0.924	0.2169 ± 0.030	0.849 ± 0.015	84.94
1500	−0.420 ± 0.002	115.70 ± 0.54	3.91 ± 0.305	19.95	0.848 ± 0.007	29.29 ± 0.269	14.05 ± 0.997	0.2136 ± 0.033	0.885 ± 0.009	88.49

Electrochemical impedance spectroscopy – EIS

The ethanol extract of J. brandegeeana was evaluated for its inhibitory activity at concentrations of 250, 450, 750, 1000, and 1500 ppm using EIS, against carbon steel AISI 1020 immersed in a solution of H₂SO₄ 1,0 mol L⁻¹ at 32°C.

After the potential stabilization, the Nyquist and Bode diagrams, shown in Figure 6(A,B), respectively, were obtained. The Nyquist diagram is an alternative to represent the results in characteristic frequencies, using the charge transfer function, varying from zero to infinity (31).

Figure 6. (A) Diagram of Nyquist and (B) Bode diagram, obtained in the absence and presence of the ethanol extract of J. brandegeeana for carbon steel AISI 1020 in H₂SO₄ 1.0 mol L⁻¹.

The graph obtained for the Nyquist diagram shows a single semicircle, which corresponds to a capacitive arc, with displacement along the real impedance axis (Z^‵) for the blank and the variations of the concentration of the plant extract. The displacement for higher values of Z^‵ indicates that the corrosion reaction is under the control of load transfer and double-layer capacitance, thus, the higher the concentration, the larger the diameter of the semicircle, indicating adsorption of plant extract substances to the metal surface promoting an inhibitory barrier to the charge transfer reactions occurring at the metal-inhibitor interface (32).

There is no change in the shape of the curves in the presence and absence of the inhibitor, which, according to Carlos et al. (33), indicates that the adsorption of the extract on the surface occurs without altering the mechanism of corrosion of the metal. In addition, curves in the absence and presence of the inhibitor present a flattening that may be related to non-homogeneity and the presence of roughness on the metal surface, caused by the corrosive process itself and the formation of a protective layer, promoting the dispersion of the frequency (33–35).

When analyzing the Bode diagrams, we have the first portion log f vs phase angle, in which we can observe the presence of a single constant phase for the corrosion process, represented by a single crest in the presence and absence of the inhibitor (36).

With the increasing frequency there is an increase in phase angle in the absence and presence of the inhibitor; however, the blank has a maximum angle of 42.7°, while in the presence of the inhibitor, there is a variation in the range of 50.4°–57.1° in 250 and 1500 ppm, respectively, still displaying a single time constant (37). This fact indicates that with the increase of the time of contact of the extract with the carbon steel, and the concentration of the inhibitor there is an increase of the phase angle and capacitance, suggesting a modification in the microstructure in the metallic surface, due to the formation of a protective film, whereas an ideal capacitor occurs with a maximum phase angle of 90° (33, 38).

When observing the portion log Z vs log f it is noticed that, in low frequency there is an increase of impedance in the presence of the inhibitor, being demonstrated by progressive parallel lines according to the increase of the extract concentration, suggesting the protection of the metallic surface (35). Both results observed in the Bode diagram corroborate the results observed in the Nyquist diagram.

The equivalent circuit model that fit all curves was Randle’s circuit, presented in Figure 7, in which the resistance of the solution (R_s) is in series with the resistance to load transfer (R_ct), and this is in parallel with the constant phase circuit (CPE) (39).

Figure 7. The equivalent circuit model adjusted for the EIS experimental data.

Each component of the equivalent circuit provides information about what occurred at the metal/inhibitor interface, and, according to Ma et al. (38), the R_ct value is influenced by the inhibitor concentration and by the formation of a film on the metal surface. This film generates a new layer at the metal/electrolyte interface which is rated by the electrical capacitance of the double layer (C_dl) (38).

The constant phase circuit (CPE) model is inserted to justify the heterogeneous nature of the inhibitor coating, in which the capacitance is not ideal, thus seeking a more precise adjustment (38). An n parameter is also inserted, which includes surface roughness, inhibitor adsorption, and/or desorption and formation of porous film, with the deviation of the ideal behavior between 1 and –1 (34, 37).

The C_dl was calculated by Equation 6(6) $C_{\mathrm{dl}}=Y_o(2\pi f_{\max }{)}^{n-1}$(6) , through constant phase elements (40). (6) $C_{\mathrm{dl}}=Y_o(2\pi f_{\max }{)}^{n-1}$(6) where Y_o is the magnitude of the CPE and f_max is the frequency where the imaginary component of the impedance is the maximum.

The degree of surface coverage (θ) was obtained in Equation 7(7) $\theta ={\displaystyle \frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}x100}$(7) , followed by the calculation of the corrosion inhibition efficiency described in Equation 1(1) ${\eta }_{\mathrm{EIE}}(\text{\%})=\left({\displaystyle \frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}}\right)x100$(1) , where R_ct is the load transfer resistance in the blank and R⁰_ct is the load transfer resistance in the presence of the inhibitor. Thus, Table 1 shows the results obtained by EIS. (7) $\theta ={\displaystyle \frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}x100}$(7) When observing the results obtained by EIS, it is noted that with the increase of the concentration of the J. brandegeeana extract, there is an increase in the efficiency of corrosion inhibition, reaching 88% in 1500 ppm. Together with this data there is an increase of R_ct, indicating that the resistance to load transfer is dependent on the concentration of the inhibitor, being justified by the increase of θ and the resistance of the solution.

There is also a decrease in C_dl, which suggests a decrease in the local dielectric constant and/or increase in the viscosity of the electric double layer, demonstrating a slow substitution of water molecules and ions by inhibiting molecules, thus creating a protective film on the metal surface (38, 41).

Potentiodynamic polarization – PP

The potentiodynamic polarization (PP) assays were performed under the same conditions as EIS for the ethanol extract of J. brandegeeana and the same concentrations. In Figure 8 the PP curves are obtained to identify the behavior of the plant extract, classifying it as a cathodic, anodic, or mixed inhibitor.

Figure 8. PP curves obtained in the presence and absence of the J. brandegeeana ethanol extract in varied concentrations for carbon steel AISI 1020 in H₂SO₄ 1.0 mol L⁻¹.

There was a decrease in current density with the presence of the extract in ethanol, both in the anodic and cathodic portions, suggesting the adsorption of inhibitors to the metal surface. The decrease was constant for all concentrations studied, being higher in the cathodic portion in relation to the blank.

There is a slight displacement of the E_corr for more negative values, being more pronounced in 250 ppm with 13.85 mV of displacement, indicating a blockage of the cathodic areas of the metal surface. According to Fernandes et al. (36) and Ma et al. (38) to have a classification of the inhibitor as cathodic or anodic this displacement must reach values greater than 85 mV (36, 38). In this case, the results indicate that the ethanol extract of J. brandegeeana can be considered as a mixed inhibitor, inhibiting both the dissolution of the metal in the anode and the reduction of hydrogen ions in the cathode, but with cathodic tendency (41, 42).

In the anodic area, for a potential above −0.3 V, the formation of a plateau in high current density, which corresponds to a passivation process, which can be explained by the adsorption of inhibiting molecules to the metal surface simultaneously the desorption of these molecules, caused by the dissolution of the metal in the corrosive medium. In this plateau then, the desorption rate of the inhibitor is higher than the adsorption rate, being observed by the increase of the current density with the increase of the potential (34, 43, 44).

Table 2 describes the electrochemical parameters of corrosion potential (E_corr), corrosion current density by surface area (j_corr), anodic (β_a) and cathodic (β_c) constants obtained by Tafel extrapolation. The inhibition efficiency η_PP was obtained from Equation 3(3) ${\eta }_{\mathrm{PP}}(\text{\%})=\left({\displaystyle \frac{i_{\mathrm{corr}}^0-i_{\mathrm{corr}}}{i_{\mathrm{corr}}^0}}\right)x100$(3) .

Table 2. PP parameters for carbon steel AISI 1020 in H₂SO₄ 1.0 mol L⁻¹ solution in the absence and presence of the extract in ethanol of J. brandegeeana.

Inhibitor (ppm)	Ecorr vs. Ag/AgCl (mV)	jcorr (mA cm^−2)	ba (mV dec^−1)	bc (mV dec^−1)	CR^a (mm/year)	θ	ηPP (%)
Blank	−419.81 ± 0.609	1.195 ± 0.054	70.01 ± 0.700	151.73 ± 0.488	14.30 ± 0.601	–	–
250	−433.66 ± 0.636	0.445 ± 0.022	70.03 ± 0.099	143.24 ± 0.700	3.38 ± 0.299	0.628 ± 0.018	62.79
450	−425.61 ± 0.969	0.306 ± 0.010	65.46 ± 0.234	139.72 ± 0.403	3.29 ± 0.119	0.744 ± 0.008	74.39
750	−429.95 ± 0.886	0.292 ± 0.011	70.94 ± 0.781	169.40 ± 0.636	3.22 ± 0.127	0.756 ± 0.023	75.57
1000	−432.76 ± 0.106	0.230 ± 0.013	72.30 ± 0.655	148.77 ± 0.665	2.71 ± 0.152	0.807 ± 0.010	80.74
1500	−420.68 ± 0.806	0.186 ± 0.011	69.28 ± 0.388	144.53 ± 0.728	2.32 ± 0.127	0.845 ± 0.004	84.47

^a Corrosion rate.

The results show that with the increase of the inhibitor concentration occurs the decrease of the corrosion current density and consequently the increase of the inhibition efficiency, presenting 84% to 1500 ppm, indicating inhibition of the metal surface by adsorption of the J. brandegeeana extract (37). In addition, the cathodic and anodic Tafel lines show little variation in the presence of plant extract, and this fact linked to impedance results suggests the inhibition of corrosion without mechanism modification of the cathodic and anodic reactions (45).

Linear polarization resistance – LPR

The linear polarization resistance (LPR) was also evaluated under the same conditions as the other electrochemical analysis, and the inhibition efficiency was calculated from Equation 2(2) ${\eta }_{\mathrm{RPL}}(\text{\%})=\left({\displaystyle \frac{R_p-R_p^0}{R_p}}\right)x100$(2) . The values of the polarization resistance in the presence (R_p) and absence (R⁰_p) of the plant extract are described in Table 3.

Table 3. LPR parameters for carbon steel AISI 1020 in the solution of H₂SO₄ 1.0 mol L⁻¹ in the absence and presence of the ethanol extract of J. brandegeeana.

Inhibitor (ppm)	r^2Table Footnotea	Rp (Ω cm^2)	Θ	ηLPR (%)
Blank	0.999	13.06 ± 0.177	–	–
250	0.999	34.46 ± 0.665	0.621 ± 0.007	62.11
450	0.999	52.48 ± 0.511	0.751 ± 0.013	75.12
750	0.998	66.51 ± 0.400	0.804 ± 0.005	80.37
1000	0.999	88.89 ± 0.178	0.853 ± 0.014	85.31
1500	0.999	114.80 ± 0.584	0.886 ± 0.009	88.62

^a Linear correlation coefficient.

Good linear correlation coefficients were obtained for the blank and the other extract concentrations, that the R_p value is dependent on the concentration, complementing the results obtained in EIS and PP tests. At the highest concentration, 1500 ppm, the adsorption of the inhibitor to the metal surface was more efficient, with 88.62% inhibition, minimizing the corrosion intensity in the metal.

Weight loss measurements

The initial weight loss tests were performed in carbon steel coupons AISI 1020, with an area of 19.80 cm², in the presence of the ethanol extract of J. brandegeeana with variation of extract concentration and immersion time at a fixed temperature of 30°C. Table 4 shows the results of these tests with their respective standard deviations.

Table 4. Results of corrosion rates (W_corr in mg cm² h⁻¹) in the absence and presence of ethanol extract of J. brandegeeana in H₂SO₄ 1.0 mol L⁻¹ against carbon steel AISI 1020, and inhibition efficiency η (%) at 30°C with variation of immersion time and inhibitor extract concentration.

Inhibitor (ppm)	3 h		6 h		24 h		48 h
	Wcorr	η (%)	Wcorr	η (%)	Wcorr	η (%)	Wcorr	η (%)
Blank	2.688 ± 0.004	–	2.607 ± 0.028	–	1.534 ± 0.030	–	0.615 ± 0.067	–
250	0.676 ± 0.003	74.86	0.241 ± 0.004	90.74	0.158 ± 0.012	89.71	0.201 ± 0.005	67.31
450	0.398 ± 0.001	85.18	0.227 ± 0.005	91.28	0.148 ± 0.005	90.35	0.191 ± 0.043	68.91
750	0.359 ± 0.008	86.64	0.219 ± 0.006	91.60	0.136 ± 0.002	91.13	0.168 ± 0.066	72.73
1000	0.325 ± 0.001	87.89	0.210 ± 0.002	91.93	0.135 ± 0.004	91.22	0.133 ± 0.027	78.32
1500	0.252 ± 0.003	90.61	0.168 ± 0.001	93.54	0.133 ± 0.003	91.31	0.106 ± 0.014	82.72

When comparing the values of blank W_corr and extracts, it was observed that there was a considerable decay of the values of the inhibition rates, indicating an inhibitory effect of the plant extract. It is also noted that with the increase of extract concentration occurred the increase of corrosion inhibition, which was more pronounced after 6 h of immersion, may thus be considered that the increased concentration of the extract caused the formation of a protective layer on the metal surface (4).

When analyzing the values of η (%) in 24 h of immersion stability of the protective layer was formed, and at 48 h of immersion there was a decay in efficiency, which can be explained by a gradual desorption of the extract that interacted with the metallic surface (46, 47). Thus, among the results, the time of 6 h stands out as the best time evaluated, presenting 93.54% inhibition efficiency at 1500 ppm.

Among the concentrations studied, those that presented better inhibitory results were 1000 and 1500 ppm and considering the small difference between the inhibition efficiencies of both, it was decided to follow with the concentration of 1000 ppm for the other evaluations.

Weight loss with temperature variation

The temperature effect was also studied for the ethanol extract of J. brandegeeana, maintaining the concentration of 1000 ppm, and varying the temperature at 30°C, 40°C, 50°C, 60°C and 70°C, as shown in Table 5.

Table 5. Results of corrosion rates (W_corr in mg cm² h⁻¹) and efficiency of inhibition η (%) with 3 h of immersion in H₂SO₄ 1.0 mol L⁻¹ against carbon steel AISI 1020, and temperature variation in the absence and presence of the ethanol extract of J. brandegeeana.

Temp. (°C)	Blank	Extract (1000 ppm)
	Wcorr	Wcorr	η (%)
30	2.688 ± 0.004	0.325 ± 0.001	87.89
40	3.878 ± 0.055	0.527 ± 0.011	86.40
50	5.157 ± 0.005	0.791 ± 0.006	84.66
60	8.019 ± 0.010	1.319 ± 0.012	83.55
70	16.605 ± 0.198	2.918 ± 0.041	82.43

With the increase in temperature, there was a decrease in inhibition efficiency, and consequently an increase in W_corr; however, this decay can be considered mild, indicating thermal stability of the extract, with a variation of 6% considering if the increase in temperature from 30°C to 70°C, providing a good inhibition efficiency at 70°C (η = 82.43%). This result also indicates that the corrosion inhibition process occurs by physisorption, that is, the adsorption between the plant extract and the metal surface, occurs mainly through interactions by van der Waals forces, indicating a reversible adsorption mechanism.

From the results of the weight loss obtained, the physicochemical parameters involved in the corrosion process were calculated for the ethanol extract from the equations of Arrhenius (Equation 8(8) $l{n}_{\mathrm{Wcorr}}=-{\displaystyle \frac{E_a}{\mathrm{RxT}}+\mathrm{lnA}}$(8) ) and Eyring (Equation 9(9) $\ln {\displaystyle \frac{W_{\mathrm{corr}}}{T}=\ln \left[{\displaystyle \frac{K_b}{h}{e}^{{\displaystyle \frac{\mathrm{\Delta }{S}^{\text{‡}}}{R}}}}\right]-{\displaystyle \frac{\mathrm{\Delta }{H}^{\text{‡}}}{\mathrm{RxT}}}}$(9) ), as shown previously. All the results are described in Table 6 and the graphs generated in Figure 9. (8) $l{n}_{\mathrm{Wcorr}}=-{\displaystyle \frac{E_a}{\mathrm{RxT}}+\mathrm{lnA}}$(8) where W_corr is the rate of corrosion in the presence of extract, E_a activation energy, R gas constant, T temperature in Kelvin, and A frequency factor. (9) $\ln {\displaystyle \frac{W_{\mathrm{corr}}}{T}=\ln \left[{\displaystyle \frac{K_b}{h}{e}^{{\displaystyle \frac{\mathrm{\Delta }{S}^{\text{‡}}}{R}}}}\right]-{\displaystyle \frac{\mathrm{\Delta }{H}^{\text{‡}}}{\mathrm{RxT}}}}$(9) where W_corr is the rate of corrosion in the presence of extract, T temperature in Kelvin, K_b Boltzmann constant, h Planck constant, R gas constant, ΔS^≠ apparent entropy of transition, and ΔH^≠ apparent enthalpy of transition.

Figure 9. Graphs of (A) Arrhenius and (B) Eyring for the corrosive process of carbon steel AISI 1020 in H₂SO₄ 1.0 mol L⁻¹ in the absence (blank) and in the presence of the ethanol extract of J. brandegeeana at 1000 ppm.

Table 6. Values of physicochemical parameters (E_a, ΔH^≠, ΔS^≠) associated with the corrosive process of AISI 1020 carbon steel in H₂SO₄ 1.0 mol L⁻¹ medium, in the absence and presence of the J. brandegeeana ethanol extract at 1000 ppm.

Parameters	Blank	1000 ppm
Ea (kJ mol^−1)	28.22	37.72
ΔH^≠ (kJ mol^−1)	25.60	35.10
ΔS^≠ (J mol^−1 K^−1)	−152.38	−138.53

Higher values of E_a and ΔH^≠ for the extract compared to blank indicated the need for a higher energy barrier to start the corrosive process, due to the formation of interactions between the extract and the metal surface (13, 32). Regarding ΔS^≠, a less negative value was observed with the addition of the plant extract, indicating a greater disorder in the system, due to the desorption of water molecules and sulfate ions of the metallic surface and adsorption of the plant extract (4). This result characterizes a relevant mass transfer to the system, corroborating with the higher inhibition efficiency after 6 h of immersion, by the gravimetric technique of weight loss, suggesting a slow mass transfer, with a high induction time.

To verify if the efficiency obtained for Justicia brandegeeana is consistent with other plant extracts, a comparative study was carried out, shown in Table 7, with other vegetal species found in the literature, for carbon steel in H₂SO₄ in two different concentrations, being analyzed by the same method, weight loss. The maximum concentration used for J. brandegeeana is acceptablecompared to the other species studied in the same corrosive medium (H₂SO₄ 1.0 mol L⁻¹) because it showed higher inhibition efficiency by the weight loss method and same immersion time. In addition, plant species employed in a less corrosive medium (H₂SO₄ 0.5 mol L⁻¹) required longer immersion time to obtain inhibition efficiencies close to that found for J. brandegeeana.

Table 7. Comparison of the corrosion inhibition efficiency of Justicia brandegeeana with other vegetal inhibitors against carbon steel in H₂SO₄ by the weight loss method.

Plant species	Conc. of H2SO4	Conc. of extract (ppm)	Immersion time (h)	η (%)	Reference
Sida cardifolia	0.5 mol L^−1	500	24	90.70	(59)
Solanum surattense		500	24	92.80	(58)
Artabotrys odoratissimus		1250	12	92.85	(60)
Ficus hispida	1.0 mol L^−1	250	2	86.56	(61)
Malvaviscus arboreus		500	3	97.50	(16)
Equisetum hyemale		1000	6	82.00	(62)
Euphorbia heterophylla		2000	6	92.72	(63)
Justicia brandageeana		1500	6	93.54*	This paper

*Value obtained by the weight loss method at 6 h of immersion at 30°C.

Adsorption isotherm

Corrosion inhibition is dependent on the adsorption capacity of the inhibitor on the metal surface, which can be evaluated by adsorption isotherms through different mathematical models, and it is necessary to calculate the degree of surface coverage by the inhibitor (θ). The values of θ can be obtained in a more reliable form using the gravimetric technique of weight loss through Equation 10(10) $\theta ={\displaystyle \frac{W_{\mathrm{corr}}^0-W_{\mathrm{corr}}}{W_{\mathrm{corr}}^0}}$(10) (16). (10) $\theta ={\displaystyle \frac{W_{\mathrm{corr}}^0-W_{\mathrm{corr}}}{W_{\mathrm{corr}}^0}}$(10) where w⁰_corr is the corrosion rate in the absence of an inhibitor and w_corr in the presence of an inhibitor.

The isothermal models of Frumkin, Temkin, Flory-Huggins, El-Awady, and Langmuir, Equations 11(11) $\mathrm{Frumkinisotherm}:\log \left({\displaystyle \frac{\theta }{\left({\displaystyle \frac{1-\theta }{C}}\right)}}\right)=\log K_{\mathrm{ads}}+\mathrm{g\theta }$(11) –15(15) $\mathrm{Langmuirisotherm}:{\displaystyle \frac{C}{\theta }=\left({\displaystyle \frac{1}{K_{\mathrm{ads}}}+C}\right)}$(15) , respectively, were used to evaluate which would best suit the results obtained for the anti-corrosive effect of J. brandegeeana extract. The equations of the first four applied models did not show good linear correlation coefficients, as can be seen in Table 8. (11) $\mathrm{Frumkinisotherm}:\log \left({\displaystyle \frac{\theta }{\left({\displaystyle \frac{1-\theta }{C}}\right)}}\right)=\log K_{\mathrm{ads}}+\mathrm{g\theta }$(11) (12) $\mathrm{Temkinisotherm}:\log \left({\displaystyle \frac{\theta }{C}}\right)=\log K_{\mathrm{ads}}+\mathrm{g\theta }$(12) (13) $\mathrm{Flory}-\mathrm{Hugginsisotherm}:\log \left({\displaystyle \frac{\theta }{C^0}}\right)=\log K_{\mathrm{ads}}+{\eta }_{\mathrm{FH}}\log (1-\theta )$(13) (14) $\mathrm{El}-\mathrm{Awadyisotherm}:\log \left({\displaystyle \frac{\theta }{1-\theta }}\right)=\log K_{\mathrm{ads}}+\mathrm{ylog}(C)$(14) (15) $\mathrm{Langmuirisotherm}:{\displaystyle \frac{C}{\theta }=\left({\displaystyle \frac{1}{K_{\mathrm{ads}}}+C}\right)}$(15) where θ is the degree of surface coverage by the inhibitor, C concentration, K_ads adsorption constant, g is the degree of lateral interaction between adsorbed molecules, ηFH is the number of metal ions occupying the adsorption sites on two membranes, and y is the parameter of occupation of the active site.

Table 8. Adsorption isotherms for the anti-corrosive effect presented by the ethanol extract of J. brandegeeana in carbon steel AISI 1020 in the solution of H₂SO₄ 1,0 mol L⁻¹.

Isotherms	r^2	y = ax + b
Frumkin	0.91290	7.8391x − 3.0675
Temkin	0.83843	0.1862x + 0.3252
Flory-Huggins	0.89252	1.6652x − 1.4774
El-Awady	0.90788	0.6020x − 0.9198

The best fit was obtained for the Langmuir isotherm, presenting a 0.99957 linear correlation coefficient and 1.06 slope, as shown in Figure 10. The value of the slope near the unit confirms the suitability to the Langmuir isotherm, indicating the adsorption of the plant extract in monolayer, with each site containing only one adsorbed molecule, which does not suffer side interactions or stereo impediment between them (16, 48).

Figure 10. Adsorption isotherm of Langmuir for the anti-corrosive effect presented by the ethanol extract of J. brandegeeana in carbon steel AISI 1020 in the solution of H₂SO₄ 1.0 mol L⁻¹.

The adsorption constant (K_ads) was calculated from the Langmuir isotherm, obtaining the value of 0.0160 L mg⁻¹ that is comparable to founds in the literature for other plant extracts such as aqueous extract of coffee grounds (0.0571 L mg⁻¹), the aqueous extract of Cumaru (0.0690 L mg⁻¹) and the extract in a solution containing ethyl acetate, distilled water, and ethanol in the ratio 8:22:70 of Equisetum arvense (0.0166 L mg⁻¹) (42, 49, 50).

After obtaining the K_ads, the adsorption standard free energy (ΔG⁰_ads) was calculated by Equation 16(16) $\mathrm{\Delta }{G}_{\mathrm{ads}}^0=-RxTx\ln ({\rho }_{H_{2}O}x{K}_{\mathrm{ads}})$(16) , which assists in the evaluation of the interaction of the inhibitor with the metallic surface: (16) $\mathrm{\Delta }{G}_{\mathrm{ads}}^0=-RxTx\ln ({\rho }_{H_{2}O}x{K}_{\mathrm{ads}})$(16) where R is the universal constant of gases (J K⁻¹ mol⁻¹), T is the temperature in K, and ${\rho }_{H_{2}O}$ is the density of water (mg L⁻¹) (51, 52).

The value of −24.39 kJ mol⁻¹ for ΔG⁰_ads was obtained, indicating that the adsorption process of the inhibiting extract on the metallic surface is spontaneous. Moreover, according to the literature, values of ΔG⁰_ads lower than −40 kJ mol⁻¹ indicate that the corrosion inhibition process occurs by chemisorption, while values close to −20 kJ mol⁻¹ or less negative indicate physisorption (13, 53–56). Thus, for this system, the value of ΔG⁰_ads indicates that the mechanism of corrosion inhibition must occur by physisorption, corroborating with the data found by the gravimetric method of weight loss with temperature variation, where it was observed a decrease in inhibitory efficiency with increasing temperature.

Chelating effect

The analysis of the chelating effect of the ethanol extract of Justicia brandegeeana against ions Fe⁺² occurred in three modes: (I) only the extract in ethanol at 125 ppm; (II) extract solution in ethanol at 125 ppm and FeSO₄ at 1.87 × 10⁻⁶ mol L⁻¹ in the ratio 1:1 and (III) solution of the extract in ethanol at 125 ppm and FeSO₄ at 1.87 × 10⁻⁶ mol L⁻¹ in a 2:1 ratio.

The ultraviolet–visible spectrum with the three modes of analysis is presented in Figure 11, where it is possible to observe in modes II and III a slight bathochromic displacement, for the red region and a hypochromic displacement, for lower absorbance values when compared to mode I, being more pronounced in the 1:1 ratio between the ethanol extract and Fe⁺². This pronounced displacement demonstrates a change in the energy levels of the substances present in the extract, suggesting complexation with Fe⁺².

Figure 11. Ultraviolet-visible spectra for the ethanol extract of Justicia brandegeeana (125 ppm) in the presence and absence of Fe⁺² (1.87 × 10⁻⁶ mol L⁻¹).

The bands are weakly displaced to longer wavelengths, the effect caused by the yellowish-green staining of the solution, which absorbs in violet at 400–435 nm, where it is possible to observe the wavelength of maximum absorption (57). There is also the presence of two zones: (a) in the wavelength range of 399–403 nm that is related to the transitions π→π* of C = C; (b) connections, in the range of 664–670 nm related to n→π* transitions, assigned to groups C–O and OH that must be present in the substances that make up the plant extract (58).

The overlays of the spectra quantified the chelating effect of the extract in ethanol of J. brandegeeana against Fe⁺² ions. The positive control of EDTA at 2.32 × 10⁻⁴ mol L⁻¹ showed an 83% chelating effect, while the plant extract in ethanol showed 54% for mode II, and 22% for mode III, confirming that the extract in ethanol forms complex with Fe⁺², being more effective in the ratio 1:1. The chelating effect observed in the presence of Fe⁺² ions may be related to the effect of corrosion inhibition by the extract in ethanol of J. brandegeeana.

SEM analysis

The micrographs of the three analyzed coupons are shown in Figure 12. Coupon I, without immersion in any medium, has a smooth surface, visible with the marks of the treatment with the sandpapers.

Figure 12. Micrographs of SEM (2000× and 4000×) of carbon steel AISI 1020 without immersion in any medium (Coupon I); in corrosive medium H₂SO₄ 1.0 mol L⁻¹ in the absence of the extract in ethanol of the plant species for 24 h at 30°C (Coupon II); in corrosive medium H₂SO₄ 1.0 mol L⁻¹ in the presence of the plant extract in ethanol species at 1000 ppm for 24 h at 30°C (Coupon III).

In coupon II, immersed in the blank solution, a roughness and irregular surface related to the acid attack is observed due to the metal deterioration caused by the presence of sulfuric acid, it is not possible to observe the sanding marks.

In coupon III, with the presence of the inhibitor at 1000 ppm, even not observed sanding marks and less rough, there is a significant improvement in the morphological aspect of the metal surface, thus being more preserved compared to coupon II, indicating a protection on the surface due to the formation of the protective layer, minimizing the attack of sulfuric acid.

The surface analysis ratifies the weight loss measurements and the electrochemical results showing the effectiveness of J. brandegeeana extract in the protection of carbon steel in the presence of a corrosive medium with sulfuric acid (1.0 mol L⁻¹).

Conclusions

The aerial parts of Justicia brandegeeana presented high moisture contents, causing low yields. According to the infrared spectra performed for both collections, there is the presence of heteroatoms, such as oxygen. Likewise, the infrared spectra, together with the ultraviolet spectrum, confirmed the presence of classes containing electrons π, with their electronic transitions, as well as the NMR spectra that enabled the identification of the carbohydrate class.

The results of weight loss indicated that all concentrations studied for the extract in ethanol had good inhibitory efficiencies against carbon steel AISI 1020 in H₂SO₄ 1.0 mol L⁻¹ medium, being the best in the concentration of 1500 ppm (93.54% to 6 h of immersion) indicating that the increase in inhibition is dependent on the inhibitor concentration, as well as electrochemical analysis of EIS, PP, and LPR (88.49%, 84.47%, and 88.62%, respectively). It was also observed stability in the inhibition efficiency between 6 and 24 h of immersion, with decay at 48 h for all concentrations. It is noteworthy that the characteristics of the PP curves suggested that the extract behaves as a mixed inhibitor, with a tendency to the cathodic region. When evaluating the inhibition efficiency with temperature variation, the inhibitor showed good thermal stability in the wide range studied, presenting 82.41% inhibition efficiency at 70°C in 3 h of immersion, suggesting a physisorption process. The results were confirmed by the physicochemical parameters of E_a, ΔH^ǂ and ΔS^ǂ.

The adsorption of the extract in ethanol of J. brandegeeana to the metallic surface was observed according to the Langmuir isotherm, and the calculated K_ads was adequate to other values for plant extracts in the literature, and the calculated value of ΔG⁰_ads corroborated with the gravimetric results, confirming physical adsorption of the plant extract to the metallic surface. It was also observed that the ethanol extract showed chelating activity with Fe⁺² ions, in a 1:1 ratio (54% chelating effect), supporting the results found in the analysis of anti-corrosive activity. In addition, the SEM provided the visualization of the morphology of the metallic surface in the presence and absence of the inhibitor, showing that in the presence of the plant extract, there is greater preservation and less roughness of the metallic surface, evidencing the formation of a protective film.

Disclosure statement

No potential conflict of interest was reported by the author(s).