1 Introduction to corrosion of mild steel in acidic media
Mild steel, one of the most common materials used in industry (such as petrochemicals, construction, and manufacturing), is very prone to corrosion in acidic conditions. Sulfuric acid (H2SO4) is one of the most used acids during chemical processes like acid pickling, descaling, and oil well acidification [1]. The corrosion process is characterized by the anodic dissolution of iron and cathodic hydrogen evolution, resulting in great material deterioration with economic losses of billions yearly [2]. Conventional inorganic anti-corrosion compounds, such as chromates and phosphates, worked well, but their use has been limited due to both environmental concerns and health hazards caused by their being toxic and non-biodegradable [3,4]. The research has thus been directed towards eco-friendly and green inhibitors that can be derived from natural sources, offering sustainable solutions with low toxicity and renewability [5], see Figs. 1-3. Green corrosion inhibitors, mainly derived from plants, have only become popular as potential alternatives to synthetic ones since the mid-2010s [6,7]. These inhibitors are usually composed of organic compounds such as alkaloids, flavonoids, tannins, saponins, terpenoids, and so on, which adsorb on the metallic surface, forming a film that retards corrosive ion attack. They are effective in acid solutions, and the efficiencies of inhibition were higher than 80 - 90% at optimum concentrations according to reported studies. For example, some studies show that the corrosion rate of mild steel in H2SO4 is inhibited by physisorption or chemisorption on plant inhibitors with efficiency up to 96% and follows Langmuir or Temkin isotherms [8]. On the other hand, green products obtained from leaves and fruits of some plants and agro-wastes have been found to act as mixed-type inhibitors, i.e., inhibiting both anodic and cathodic reactions [9]. The advantages of these inhibitors are based on their availability, cheapness, and correspondence with the demands for environmental protection, driving out dangerous chemicals in favor of biodegradable ones. Among the plant extracts, however, a less explored but promising source for corrosion inhibition is that of fibers and agro-industrial wastes derived from lignocellulosic materials containing both lipophilic and hydrophilic compounds [10]. Fibers derived from plants such as sugar beet, banana, bamboo, and hemp contain pectins, hemicelluloses, and waxes, which can deposit those substances on metal surfaces, creating hydrophobic barriers. For example, enzymatic and acid-extracted pectin from sugar beet pulp was shown to have a maximum inhibition efficiency of up to 92% for mild steel in 1 M H2SO4 at 500 ppm due to polysaccharide chains' adsorption onto the metal surface, which blocks active sites [11]. This is in accordance with the general trend that biomass wastes such as fruit peels and leaf waste residues act as green inhibitors, showing 85 - 95% efficiencies in HCl and H2SO4 media, because of their polyphenol contents. An overview of plant and biomass wastes is presented to highlight their role in creating chelate complexes with iron ions for increased passivation in an acidic medium. Especially for water-repellent protection in aqueous acidic media, the lipophilic extractives, fatty acids, sterols, and hydrocarbons are of importance. These nonpolar compounds promote strong adsorption through van der Waals interactions, leading to a decrease in water and acid penetration. Recent studies investigating fiber-based lipophiles highlight their potential as mild steel protectors, though so far, less use has been made of them when compared to aqueous extracts [12]. Recent advancements in the field have highlighted the significance of utilizing diverse biomass sources for corrosion mitigation, demonstrating that complex phytochemical compositions can yield superior inhibition performance compared to single-component synthetic inhibitors [13-15]. Furthermore, the development of green inhibitors from agricultural by-products aligns with circular economy principles, offering a dual benefit of waste reduction and material protection [16]. Agro-wastage such as those from the Agave sisalana a perennial succulent, commercially grown in Tanzania well known for its fibers (sisal), is a rich source of lipophilic extractives, viz., Fatty acids (30% of total lipids) which included α-hydroxyfatty acids (CH3−(CH2)n −CH(OH) − COOH) and ω-hydroxyfatty acids (HO-(CH2)n-COOH) (10%), fatty alcohols (CnH2n+1OH) (20%), free sterols (CnH2n-8O) (11%) where n is typically 27, 28 0r 29, alkanes (11%) and several ferulic acid esters of long chain alcohols. These compounds are solubilized in solvents such as hexane and account for 0.5 - 1% w/w of dry fiber, possessing amphiphilic properties which are conducive to adsorption at surfaces. In addition to fatty acids, trace levels of diglycerides and sterol esters were reported along with sterol hydrocarbons and ketones, monoglycerides, aldehydes, waxes, and sterol glycosides [17]. Previous researchers reported on the contents of sisal, new insights about the sisal’s application potentials beyond its traditional use, e.g., antioxidants or anti-inflammatory agents, suggesting bioactivity relevant to corrosion. For example, the agro-based sisal leaf extracts proved to be biofilm inhibitors for metals, thus indirectly confirming antimicrobial corrosion control [18]. Few direct uses of sisal extractives as corrosion inhibitors are arising. Similar fiber extracts, such as loquat leaves, obtained 89% inhibition in 0.5 M H2SO4, and the lipophilic phenolics were deposited on mild steel, forming a barrier film over it. Likewise, H2SO4 extracts of Tetradenia riparia leaves achieved an efficiency of 92% by a mixed inhibition mechanism where lipophilic terpenoids improved adsorption [19]. This study aimed to assess sisal extract’s ability to suppress corrosion on mild steel in an aggressive 1 M H2SO4 medium.
Fig.1 Inorganic and organic inhibitors for corrosion prevention. |
Fig.2 Schematic presentation of the methodology part. |
Fig.3 (a) The FTIR Spectrum of Agave sisalana extracts. (b) Mechanism via which the extract provides a protective layer. |
2 Research gap and novelty
Although plant-based extracts are becoming more well-known as environmentally friendly corrosion inhibitors, the majority of current research is on how well they work under mildly acidic conditions [20]. Agave sisalana and its abundance of lipophilic chemicals (waxes, terpenoids) are also "scarcely addressed" for corrosion prevention reasons, despite being a readily available agro-industrial byproduct. In particular, less is known about the effectiveness and inhibitory mechanisms of the Agave sisalana extract at more stringent, highly concentrated acidic conditions, which are crucial for industrial uses like pickling. This study presents a new, environmentally friendly mild steel corrosion inhibitor made from Agave sisalana extract, an underused and regenerative agro-waste product. This work is novel in a number of ways. First, by valuing sisal fiber, a particular agro-industrial byproduct, it offers a unique source, going beyond the well-researched leafy plant extracts and including a component of resource sustainability. Additionally, it explores unique application conditions by thoroughly assessing the inhibitor's efficacy in a highly aggressive 1 M H2SO4 environment. Compared to the less concentrated acids that are usually described, this represents a major improvement and tackles a more difficult and practical industrial problem. Lastly, the work adopts a novel mechanistic approach by concentrating on the role of unique lipophilic compounds (such as waxes and terpenoids) in sisal extract, which forms a strong protective barrier on the metal surface.
3 Methodology
The Agave sisalana fibers were collected, cut, desiccated, and finely pulverized. After being dried, the leaves were ground into small particles and sieved to achieve a consistent 300 μm particle size. To prevent any possible temperature-related changes, the resultant leaf powder was placed in plastic containers and kept at room temperature for 120 h. The powder was then split into six parts, each weighing 250 g, and steeped in 500 mL of ethanol for 24 h. Each 250 g of powder produced a yield of roughly 60 g. Whatman filter papers and a Buchner funnel attached to a vacuum pump were used to filter the mixture twice after it had been soaked. A rotary evaporator (Model RE-2000A, Shanghai, China) operating at 60 °C, 140-190 rpm, and 170 mbar of pressure was then used to concentrate the lipophilic molecules extract. The functional groups involved in the adsorption process were then verified using FTIR (Perkin Elmer UATR Two, HTDS, Waltham, MA, USA).
To evaluate the inhibitory effect of extracted sisal inhibitor (ESI), mild steel coupons (API 5 L X70 of 50 mm diameter) were prepared. Before any experiment, the specimen surface coupons were mechanically polished sequentially using various grades of emery papers (grades 600-1200) to ensure a smooth, standardized surface finish, and continuously cooled with deionized water to prevent overheating. The specimens were chemically degreased with acetone immediately after polishing, and before the corrosion test, they were rinsed with distilled water to prevent additional dissolution. The coupons were immersed in a highly aggressive 1 M H₂SO₄ solution to replicate industrial pickling conditions. The concentration of 1 M H₂SO₄ was selected to simulate the severe corrosive environment typical of industrial acid pickling and descaling processes, providing a rigorous test for the inhibitor's efficacy under demanding operational conditions. Electrochemical techniques like PDP and EIS were employed to assess corrosion rates, inhibition efficacy, and adsorption behavior. Both treated and untreated coupons underwent SEM surface examination. The EIS measurements were performed in the frequency range of 100 kHz to 10 m Hz, with a sinusoidal perturbation of 10 mV amplitude.
4 Results and discussion
4.1 FTIR characterization
The Agave sisalana extract’s FTIR spectrum demonstrates distinctive absorption peaks that correlate to functional groups found in its lipophilic components, such as terpenoids and waxes, which are essential to its unique inhibitory action. Peaks in the 2800 - 3000 cm−1 range may be attributed to C-H stretching vibrations from long-chain fatty acids and aliphatic hydrocarbons, which help establish a hydrophobic protective layer on the steel surface [3]. Also, different literature reports that the C 
O stretching from esters or carbonyl groups found in terpenoids and waxy components may cause a peak to form between 1700 and 1750 cm−1, which is responsible for improving adsorption onto the metal surface through polar interactions [3,21]. Bands in the 1000 - 1300 cm−1 range most likely represent C-O stretching vibrations, a suggestion obtained from various literature associating the stretch with esters, ethers, or alcohols that promote physisorption or chemisorption on the steel contact [3,21]. Additionally, peaks about 1600 - 1650 cm−1 may reflect C 
C stretching from unsaturated terpenoids, which are reported to increase π-electron interaction with the metal surface. Thus, these functional groups highlight the role of underutilized lipophilic constituents in improving corrosion inhibition under industrially relevant, aggressive environments and support the extract's ability to form a strong, adsorbed barrier layer in highly concentrated acidic conditions.
O stretching from esters or carbonyl groups found in terpenoids and waxy components may cause a peak to form between 1700 and 1750 cm−1, which is responsible for improving adsorption onto the metal surface through polar interactions [3,21]. Bands in the 1000 - 1300 cm−1 range most likely represent C-O stretching vibrations, a suggestion obtained from various literature associating the stretch with esters, ethers, or alcohols that promote physisorption or chemisorption on the steel contact [3,21]. Additionally, peaks about 1600 - 1650 cm−1 may reflect C C stretching from unsaturated terpenoids, which are reported to increase π-electron interaction with the metal surface. Thus, these functional groups highlight the role of underutilized lipophilic constituents in improving corrosion inhibition under industrially relevant, aggressive environments and support the extract's ability to form a strong, adsorbed barrier layer in highly concentrated acidic conditions.4.2 Electrochemical performance
The results in Table 1 show that the sisal extract has a distinct and potent inhibitory effect. The most important factor, the corrosion current density (icorr), falls sharply as the inhibitor concentration rises, from 767.60 µA cm−2 for the blank to a low of 104.80 µA cm−2 at 750 mg L−1. This instantaneous drop in icorr indicates a reduction of the overall corrosion reaction [22,23]. Consequently, the inhibition effectiveness (IE%) rises and reaches a remarkable peak of 96.03% at 750 mg L−1. This remarkable efficiency in such a demanding 1 M H2SO4 environment immediately answers the research gap concerning performance under highly concentrated acidic conditions. The corrosion potential (Ecorr) shifts slightly in the negative (cathodic) direction as concentration rises, from − 516.45 mV to − 490.05 mV [24]. These shifts, however, are relatively small (less than 26 mV overall). A shift in Ecorr of less than 85 mV with respect to the blank potential is a well-established scientific criterion indicating that the inhibitor functions as a mixed-type inhibitor, affecting both anodic metal dissolution and cathodic hydrogen evolution reactions [25]. A change in Ecorr of less than 85 mV indicates that the inhibitor functions as a mixed-type inhibitor, which means it delays both the anodic metal dissolution and the cathodic hydrogen evolution events [26]. The changes in the Tafel slopes (βa and βc) further support this. There is no discernible trend for one slope to be disproportionately more affected than the other, but both show significant alteration in comparison to the blank. For example, βa significantly drops from 145.05 to 125.22 mV/dec at 750 mg L−1 (96.03% IE), and βc changes from 126.54 to 10.63 mV/dec. This validates the mixed inhibition mechanism, in which both half-reactions' active sites are blocked by the organic molecules in the extract adhering to the steel surface [27]. The corrosion rate decreases from 3.770 mm/year for the blank to a very low 0.373 mm/year at 750 mg L−1, following the anticipated inverse trend to inhibition efficiency. This measures the extraordinary protection performance in practical terms of the Agave sisalana extracts. However, a crucial observation is the performance reduction at 900 mg L−1, where icorr and corrosion rate rise, and IE% declines to 81%. This shows an ideal concentration (750 mg L−1) beyond which solubility difficulties, molecular aggregation, and or the desorption events may diminish the efficiency of the protective coating generated by the lipophilic chemicals in the sisal extract.
Table 1 The potentiodynamic polarization parameters for 1 M H2SO4. |
| Conc. of inhibitor (mg/L) | -Ecorr (mV)± σ | icorr (µA/cm2) ± σ | Βa (Mv/dec) | Βc (Mv/dec) | Inhibition eff (%)± σ | Corrosion rate (mm/year) |
|---|---|---|---|---|---|---|
| Blank | 516.45 ± 1.05 | 767.60 ± 3.05 | 145.05 | 126.54 | − | 3.770 |
| 250 | 512.58 ± 3.51 | 229.19 ± 1.42 | 145.43 | 118.92 | 80.14 ± 1.32 | 1.973 |
| 450 | 510.48 ± 2.61 | 151.83 ± 1.81 | 140.16 | 107.08 | 90.22 ± 1.20 | 1.281 |
| 600 | 506.64 ± 2.35 | 136.59 ± 1.95 | 149.51 | 107.96 | 92.21 ± 1.22 | 0.698 |
| 750 | 494.95 ± 2.29 | 104.20 ± 1.15 | 125.22 | 103.63 | 96.03 ± 1.22 | 0.373 |
| 900 | 490.05 ± 4.22 | 107.80 ± 1.95 | 139.40 | 120.20 | 81.00 ± 1.31 | 1.925 |
Key: βa - Anodic Tafel, βc - Cathodic Tafel, Ecorr - corrosion potential, icorr - corrosion current density. |
The Tafel curves in Fig. 4a visually depict the extract's inhibitory activity. On the current density (log i) scale, the Blank (1 M H2SO4 only) curve is at the top, indicating the most aggressive corrosion kinetics. Each succeeding curve clearly moves to the right and downward when the inhibitor is added. The lower icorr values in the table are closely correlated with the downward shift, which indicates a significant drop in both anodic and cathodic current densities (Log I This validates the inhibition of both metal dissolution and hydrogen evolution processes. Because both the anodic and cathodic branches of the curves are clearly suppressed without one being disproportionately dominant, the rightward shift (toward higher positive potentials) corresponds with the negative shift in Ecorr values, visually validating the inhibitor's categorization as a mixed-type. The polarization curves were analyzed by extrapolating the Tafel lines in the linear region, typically extending over one decade of current density, to the point of intersection near Ecorr. Care was taken to select the linear Tafel regions (approximately ±20 mV from Ecorr to ensure accurate determination of kinetic parameters, avoiding irregular regions at higher overpotentials. The comprehensive analysis of these polarization parameters, particularly the evolution of Tafel slopes, provides insight into the alteration of the reaction kinetics, confirming the adsorption mechanism suggested in recent literature [28-30].
Fig.4 (a) Potentiodynamic polarization plots, (b) The variation of inhibition efficiency (IE %) against the concentration of inhibitor. |
The Electrochemical Impedance Spectroscopy (EIS) data displayed in Table 2 provide crucial information on the interfacial changes and corrosion prevention mechanism of the Agave sisalana extract. The charge transfer resistance (Rct), the most important metric, shows a significant and concentration-dependent increase from a low value of 27.41 Ω cm2 for the uncontrolled blank to a high of 189.78 Ω cm2 at 750 mg L−1. This substantial increase in Rct, which shows a markedly impeded electron transmission across the metal-solution interface, immediately reflects the formation of a highly resistant protective barrier by the deposited inhibitor molecules. This trend is therefore followed by the inhibitory efficiency determined from Rct values, which reaches an outstanding 95.56% at 750 mg L−1 and strongly supports the 96.03% efficiency derived from the potentiodynamic polarization data. At 750 mg/L, the double-layer capacitance (Cdl) drops consistently and sharply from 1776.10 µF cm−2 for the blank to 552.70 µF cm−2. One conventional measure of successful adsorption is this decline. The local dielectric constant is lowered and/or the protective electrical double layer is thickened as a result of the inhibitor molecules, particularly the lipophilic waxes and terpenoids, displacing water molecules and other ions at the metal surface. Additionally, the solution resistance (Rs) shows a little but steady increase with inhibitor concentration, which could be explained by slight variations in the ionic conductivity of the solution due to the dissolved organic molecules. Importantly, Table 2 shows a performance barrier at 900 mg L−1, where Rct plateaus and slightly decreases, and the inhibition efficiency abruptly drops to 81%, echoing the trend reported in the polarization data. This consistent result from two different electrochemical methods strongly suggests that the optimal concentration for monolayer surface coverage is 750 mg L−1, over which molecular aggregation or desorption processes compromise the integrity of the protective coating. Overall, the EIS results demonstrate that the inhibition mostly occurs via a surface adsorption mechanism, providing strong corrosion protection in harsh environments like 1 M H2SO4. The low standard deviations (±σ) of Rs and Rct, as well as the narrow range of the constant phase element (CPE) exponent (n), which ranges between 0.82 and 0.87, were used to evaluate the goodness of fit of the EIS data. These findings show a good agreement between the experimental data and the fitted equivalent circuit model, with minimum dispersion and accurate parameter estimates. The minor fitting errors and constant n values close to unity indicate that the chosen model accurately describes the electrochemical system, and that the derived impedance parameters are statistically significant and physically meaningful.
Table 2 The electrochemical impedance spectroscopy results for 1 M H2SO4. |
| Conc. of inhibitor (mg/L) | Cdl (µFcm−2) | Rs (ꭥ cm2) ± σ | Rct (ꭥ cm2) ± σ | τ (s) | CPE Exponent (n) | Inhibition Eff (%) |
|---|---|---|---|---|---|---|
| Blank | 1776.10 | 1.2398 ± 0.007 | 27.41 ± 0. 891 | 0.0487 | 0.82 | - |
| 250 | 980.53 | 1.9175 ± 0.006 | 103.75 ± 2.050 | 0.1017 | ~0.83 | 83.58 |
| 450 | 710.46 | 2.4984 ± 0.011 | 143.13 ± 2. 864 | 0.1017 | ~0.84 | 90.85 |
| 600 | 653.20 | 2.9320 ± 0.012 | 151.50 ± 3.085 | 0.0989 | ~0.86 | 91.91 |
| 750 | 552.70 | 3.3587 ± 0.013 | 189.78 ± 3.101 | 0.1049 | 0.87 | 95.56 |
| 900 | 543.67 | 3.2641 ± 0.012 | 180.91 ± 3.099 | 0.0983 | ~0.86-0.87 | 81 |
The relaxation time constant (τ), derived as the product of Rct and Cdl, helps understand the kinetics of electrochemical reactions at the metal/electrolyte interface. Table 2 shows a considerable increase in τ from 0.0487 s for the blank solution to around 0.10-0.105 s in the presence of the inhibitor. The rise in τ with inhibitor concentration suggests a slower interfacial charge transfer mechanism, corresponding with the creation of a protective adsorbed coating on the metal surface. The increased τ values at 250-750 mg L⁻¹ indicate that the inhibitor effectively slows the corrosion reaction by extending the time needed for charge redistribution at the interface. This behavior corroborates the observed increase in Rct and decreases in Cdl, indicating improved surface coverage and barrier development. At 750 mg L⁻¹, τ reaches its maximum (~0.105 s) with the highest inhibitory efficacy, indicating optimum adsorption and electron transport hindrance. However, at 900 mg L⁻¹ , τ reduces somewhat (~0.098 s), indicating a minor decline in protective effectiveness, possibly due to molecular aggregation or partial desorption at higher doses.
The protective characteristics of the Agave sisalana extract on the mild steel surface are proven by the electrochemical impedance spectroscopy (EIS) data. Nyquist and Bode plots, which give insight into the features of the protective layer generated and the corrosion kinetics, serve as the foundation for the investigation. The electrochemical processes at the electrode-electrolyte interface are portrayed by the Nyquist plots, represented by the imaginary component of impedance (Z'') vs the real component (Z′). Every semicircle represents a distinct sisal extract inhibitor concentration. Equations 1 through 3 in the work by Marwa 2025 show a direct correlation between the diameter (or real-axis span) of these semicircles and the polarization resistance (Rp), which is the sum of the Rct and any film resistance (Rf) contributed by the adsorbed layer of sisal compounds.
Fig. 5a displays a sequence of depressed semicircles, each corresponding to a different inhibitor concentration, with their diameters along the real impedance axis (Z′) growing dramatically compared to the blank. This increase in semicircle diameter is the most direct illustrative indication of the extract's inhibitory properties/activity. According to electrochemical impedance theory, the diameter of the semicircle in the Nyquist plot indicates the Rct at the metal-solution interface. The Figure reveals that the semicircle for the Blank solution is the lowest, suggesting a low Rct and fast, which signifies unconstrained corrosion kinetics. Upon addition of the extract, the semicircles get gradually bigger with increasing concentration up to 750 mg L−1. This reveals a large rise in Rct, suggesting that the transport of electrons essential for the corrosion process (Eqs. 4 and 5) is being severely inhibited.
Fig.5 The plots of the behavior of the Mild steel in the presence and absence of inhibitor in 1 M H2SO4 media (a) Nyquist plot with insert of fitted curves as well as the simplified Randles equivalent circuit model (b) Bode plot, and (c) Phase angle plot. |
The reason behind this improved resistance is the adsorption of the extract's organic compounds (waxes, terpenoids, phenolics) onto the steel surface. By physically obstructing the active sites and raising the electrical impedance for Rct, this adsorption creates an insulating protective layer that functions as a barrier. This interpretation is supported by the fitted equivalent circuit model inset (inset in Fig. 5a), which displays Rs, Cdl, and Rct. The depressed nature of the semicircles necessitated the use of a CPE instead of a pure capacitor in the equivalent circuit to account for surface heterogeneity and roughness. The variation of the CPE exponent 'n' (Table 2) is crucial; the increase in 'n' from 0.82 (blank) to 0.87 (750 mg L⁻¹) indicates a decrease in surface roughness and heterogeneity due to the formation of a smoother, more uniform inhibitor film on the metal surface. In other words, we can say this effect is due to the adsorption of inhibitor molecules, which form a protective coating on the metal surface, reducing surface imperfections and suppressing localized corrosion sites. With this, therefore, we can say that the comprehension of the extract's performance profile is strengthened by two significant findings from the Nyquist graph that exactly match the quantitative EIS data. Firstly, the semicircle corresponding to the 750 mg L−1 is obviously the greatest in diameter, providing direct visual proof that this dose gives the highest Rct and, hence, the strongest protection. Secondly, the performance loss at 900 mg L−1 is obvious, as its semicircle is noticeably smaller than that of the 750 mg L−1 sample. This graphically substantiates the contemporaneous decline in both Rct and I E % noted in Table 2. This incident certainly suggests that going beyond the optimal concentration threshold compromises the authenticity of the protective film, conceivably due to molecular aggregation or desorption processes that reduce the effectiveness of the adsorbed inhibitor layer. The Bode magnitude and phase angle graphs in Figs. 5b and 5c, respectively, provide additional information about the corrosion inhibition mechanism of the Agave sisalana extract and validate the findings from the Nyquist plot and EIS data. The Bode magnitude plot (Fig. 5b) shows the log of impedance modulus (|Z|) vs log frequency. As shown in the figure, the impedance in the low-frequency band, which is most representative of the corrosion process, clearly increases with inhibitor dosage up to 750 mg L−1. The Blank exhibits the least level of corrosion resistance, as seen by its lowest curve. Each curve rises as concentration increases, with the 750 mg L−1 curve reaching the highest |Z| value. This indicates that the formation of a protective adsorbed coating has significantly increased the system's overall resistance. The upward shift is strongly associated with an increase in Rp and Rct values, confirming that the extract enhances the barrier properties of the interface. The curve for 900 mg L−1, which shows a little drop in the low-frequency impedance relative to 750 mg L−1, provides visual confirmation of the decline in performance beyond the optimal dose.
The Bode phase angle diagram provides crucial details about the protective layer's homogeneity and capacitive behavior. The broad and comparatively low Blank's phase angle peak suggests that active corrosion is causing the capacitive contact to be poorly defined [31]. When the inhibitor is administered, the phase angle peaks rise and move to lower frequencies. The development of a more capacitive, protective layer with better dielectric properties is shown by this increase in the maximum phase angle, which is most pronounced for the 750 mg L−1 concentration. Additionally, when the inhibitor adsorbs and forms a uniform coating, the peak's broadening and expansion over a wider frequency range indicate that the surface is becoming more homogenous [32]. This is a sign of an efficient barrier that uses a single dominating time constant to control the corrosion process. Although the phase angle for 900 mg L−1 is still higher than the blank, it has somewhat dropped from 750 mg L−1, indicating that the adsorbed layer's quality or coverage may have declined at this higher concentration.
The comparative Table 3 shows that Agave sisalana extract achieves a high maximum inhibition efficacy of 96.03% at the current experimental circumstances (1 M H2SO4, 750 mg L−1), making it one of the most powerful plant-based corrosion inhibitors known. Using a low concentration and an easily accessible, renewable plant source, this performance is on par with or better than a number of other recent inhibitors, including banana peel extract (98%) and loquat leaf extract (96%). Crucially, the ability of Agave sisalana to suppress both anodic and cathodic corrosion events is demonstrated by its mixed-type inhibition mechanism, which is comparable to other highly effective inhibitors like Moringa oleifera and sugar beet pectin.
Table 3 Describing a comparative study between the current study and other recent studies. |
| Inhibitor Source | Acid Conc | Optimal Conc | IE (%) | Inhibition Type | Reference |
|---|---|---|---|---|---|
| Agave sisalana | 1 M H2SO4 | 750 mg L−1 | 96.03 | Mixed-type | The current study |
| Loquat leaf extract | 0.5 M H2SO4 | 100% v/v | 96 | Modest cathodic | [33] |
| Tetradenia riparia leaf extract | 0.5 M H2SO4 | 500 ppm | 90.6 | Mixed-type | [34] |
| Sugar beet pectin | 1 M HCl | 1gL−1 | 91.6 | Mixed-type | [35] |
| Banana peel extract | | 1gL−1 | 98 | Adsorption barrier | [36] |
| Bamboo shoots | 0.5 M H2SO4 | 0.4 gL−1 | 93 | Anodic predominance | [37] |
| Moringa oleifera Leaf Extract | 1 M HCl | 500 mg L−1 | 99 | Mixed-type | [38] |
| Ginkgo leaf extract | 1 M HCl | 200 mg L−1 | 90 | - | [39] |
| Glycyrrhiza glabra leaves extract | 1 M HCl | 800 ppm | 88 | Mixed-type | [40] |
4.3 SEM characterization
The SEM images seen in Fig. 6 provide strong visible proof of the corrosive attack and the Agave sisalana extract's ability to preserve mild steel in 1 M H2SO4. The pristine mild steel surface prior to any exposure is seen in Fig. 6a. This serves as a baseline for comparison, as seen, it is established by the surface's apparent uniformity, smoothness, and lack of significant flaws, as well as the visible grinding and or polishing marks. A striking difference can be seen in Fig. 6b, which shows the uninhibited sample following exposure to the corrosive media. The image reveals a very porous, rough, and fractured morphology; the surface has been extensively and evenly degraded, which is no other than an indicator of extreme corrosion. The significant corrosion rate reported electrochemically for the blank is confirmed by this widespread deterioration, which is apparent at high magnification and is the direct result of the strong acid attacking the steel without protection. A notable protective effect is seen in Fig. 6 c, which shows the steel sample coated with an inhibitor (Agave sisalana extract) following the same exposure as that of (b). However, compared to the uninhibited sample, the surface morphology of Fig. 6 c is very different. The deep holes and fissures observed in (b) are mostly absent, and it seems much smoother and more intact. Despite the appearance of a thin, adsorbed covering or some little roughness, the surface's overall integrity is preserved. This graphic proof shows that the adsorbed layer of lipophilic compounds (waxes, terpenoids) from the Agave sisalana extract successfully forms a cohesive barrier, and it exactly matches the electrochemical data. By physically keeping the aggressive electrolyte from coming into contact with the metal substrate, this barrier greatly reduces the corrosive attack and maintains surface integrity.
Fig.6 SEM of the Mild steel iron (a) before immersion in a corrosive medium (1 M H2SO4), (b) after immersing it in a corrosive medium without coating it with an inhibitor, (c) after immersing it in a corrosive medium with an inhibitor coated on it. |
4.4 Social, industrial implications and future perspectives
The utilization of Agave sisalana waste as a corrosion inhibitor offers significant social and industrial benefits. Socially, it promotes the valorization of agricultural waste, potentially providing an additional income stream for sisal farmers and contributing to rural economic development. Also, it reduces the dependency on toxic synthetic inhibitors, thereby lowering the ecological footprint of industrial processes. For the industrial sector, particularly in metal pickling and descaling, this green inhibitor presents a cost-effective, sustainable, and high-performance alternative that complies with increasingly stringent environmental regulations, facilitating safer and greener operational practices.
That aside, the study's findings offer several promising avenues for more research and practical applications. Future research should focus on combining Agave sisalana extract with additional green inhibitors or biodegradable polymers to enhance synergistic effects, improve film durability, and extend protection duration in dynamic industrial circumstances. Pilot-scale testing in industrial pickling baths or acid cleaning processes is recommended to confirm the inhibitor's effectiveness under real working conditions, such as temperature variations, flow conditions, and prolonged exposure times. Extensive eco-toxicological studies are also necessary to confirm the biodegradability and environmental safety of the inhibitor and its byproducts in order to guarantee adherence to green chemistry principles. Furthermore, adsorption energies and interaction processes might be predicted by DFT calculations and molecular dynamics simulations, which would help in the logical development of more potent bio-based inhibitors.
5 Conclusion
This study shows that Agave sisalana fiber extract is an efficient and eco-friendly corrosion inhibitor for mild steel in a 1 M H2SO4 solution. The extract, which contains lipophilic ingredients such as waxes, terpenoids, and fatty acids, promotes adsorption onto the metal surface, establishing a protective barrier that inhibits both anodic metal dissolution and cathodic hydrogen evolution reactions, confirming its mixed-type inhibitory activity. Electrochemical experiments, such as potentiodynamic polarization and EIS, repeatedly showed that increasing inhibitor concentration resulted in a significant improvement in corrosion resistance. At 750 mg L⁻¹ , the greatest inhibition efficiency was 96.03% for polarization and 95.56% for EIS, indicating optimum surface coverage. The increase in Rct, decrease in Cdl, and increase in τ suggest the production of a stable and compact adsorbed film, effectively retarding charge transfer processes at the metal/electrolyte interface. Furthermore, the increase in the CPE exponent (n) approaching unity indicates increased surface homogeneity due to inhibitor adsorption. Surface morphology investigation using SEM corroborated the inhibitor's protective function, exhibiting a smoother and less damaged surface than the badly corroded blank sample. A modest decrease in inhibitory efficacy at higher concentrations (900 mg L⁻¹) may indicate molecular aggregation or partial desorption, emphasizing the need for optimum dosage. The data obtained in this study support the Agave sisalana extract as a potential, low-cost, and long-lasting corrosion inhibitor generated from agro-industrial waste.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Marwa Emmanuel: Writing - review & editing, Writing - original draft, Visualization, Validation, Software, Project administration, Methodology, Formal analysis, Data curation, Conceptualization. Joseph Fugo: Data curation, Software. Petro Karungamye: Formal analysis, Methodology, Writing - review & editing.