Investigation of the corrosion inhibition potential of Axonopus compressus (Carpet grass) Extract on Mild steel in 〖C0〗_2corrosive media

Corresponding author: Olusegun Peters Akinyemi Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Investigation of the corrosion inhibition potential of Axonopus compressus (Carpet grass) Extract on Mild steel in 𝐶02corrosive media Olusegun Peter Akinyemi 1, *, Favour Oluwadarasimi Emmanuel 1, Oghenemetega Divine Edoja 1 and David Asuquo Ante 2 1 Department of Chemical Engineering, Faculty of Engineering, Lagos State University, Ojo, Nigeria. 2 Department of Chemistry, Louisiana State University, USA. World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 Publication history: Received on 17 June 2025; revised on 22 July 2025; accepted on 25 July 2025 Article DOI: https://doi.org/10.30574/wjarr.2025.27.2.2640 Abstract Corrosion remains a formidable challenge in process plants, necessitating eco-friendly mitigation strategies. This study explores the potential of Axonopus compressus (Carpet grass) extract as a corrosion inhibitor for mild steel in a 𝐶𝑂2 saline environment. The aim is to advance the understanding of environmentally friendly corrosion inhibitors in a CO₂ saline environment, containing 3.5% NaCl. The project's objectives encompass phytochemical extraction, characterization, and corrosion inhibition potential evaluation. The extraction of Axonopus compressus was conducted through the solvent extraction method employing methanol. Various concentrations of the extract were subjected to testing within a temperature of 30°C and 60°C, pH of 6.0, and for an immersion period of 163 hours. The corrosion inhibition assessment utilized the gravimetric (Weight Loss) method. The results of the experiments revealed a remarkable corrosion inhibition efficiency of 93% at a temperature of 30°C and concentration of 2.8g in 100ml of solution for the Axonopus compressus extract. At 60°C, the corrosion rate was found to increase significantly, and the inhibition efficiency decreased slightly, particularly at lower concentrations, though high concentrations still maintained substantial protective effects. Notably, the inhibition efficiency exhibited a positive correlation with the concentration of the extract. As the concentration increased, the inhibitive effect on corrosion demonstrated a corresponding enhancement. The adsorption isotherm for the adsorption of Axonopus compressus extract on the steel surface was found to follow the Langmuir adsorption isotherm. Keywords: Green corrosion inhibition; Axonopus compressus Extract;𝑪𝑶𝟐 saline environment; Mild Steel; Langmuir Adsorption Isotherm 1. Introduction Corrosion is a term that describes the transformation of pure metals and their alloys to a more stable thermodynamic state as a result of a reaction with their surroundings, and this stable state may include their various sulfides, hydroxides, and oxides. The effect of corrosion on properties is quitea lot as they may cause damage like leakages, facility downtime, and collapse of a system which can be injurious and hazardous [1]. The fundamental element that drives corrosion is the potential energy difference between the potential energy of the corroding metal and the corrosion product. In the extraction of metals from their ores, there is a required amount of energy used to achieve this and after a while, this metal finds a way of returning to its original energy level when it finally begins to interact with its environment [2]. World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 742 𝐶02corrosion,also referred to as sweet corrosion, is commonly associated with mild steel and represents a significant challenge within various production sectors, notably in the oil and gas industry, where the presence of both water and oil is prevalent [3]. The presence of acidic gases such as 𝐶02 and 𝐻2𝑆 in produced fluids aggravates corrosion tendencies in production facilities and can cause localized or pitting corrosion in steels [4]. The emergence of the use of 𝐶𝑂2 injection for enhanced oil recovery makes 𝐶𝑂2 corrosive media a common issue in the exploration of gas reservoirs [5]. 𝐶𝑂2 is soluble in the formation water that comes to the surface with production fluid and eventually forms a weak carbonic acid (𝐻2𝐶𝑂3) which is extremely corrosive to carbon steel. 𝐹 𝑒𝐶𝑂3 is believed to be the main corrosion product formed with carbon steel in a corrosive media and some cathodic mechanisms have been proposed to soothe the 𝐶𝑂2 corrosion behavior with 𝐶𝑂2 believed to accelerate the cathodic reaction [6]. 1.1. 𝐂𝟎𝟐 Corrosion Mechanism on Mild Steel 𝑪𝟎𝟐corrosionis a significant concern in industries like oil and gas, where mild steel is commonly used due to its costeffectiveness and strength. The corrosion process involves several key steps: The process begins with the dissolution of carbon dioxide 𝑪𝟎𝟐in water (𝑯𝟐𝑶(𝒍)) that accompanies production fluids. 𝑪𝟎𝟐, a gas, is highly soluble in water, leading to its conversion into aqueous 𝑪𝟎𝟐 (𝒂𝒒) [7]. 𝑪𝟎𝟐 (𝒈)⇌𝑪𝟎𝟐 (𝒂𝒒) ………………… (1) 𝐶02 (𝑎𝑞) reacts with water to form carbonic acid (𝐻2𝐶𝑂3). This reaction occurs due to the interaction between 𝑪𝟎𝟐 molecules and water molecules. According to a study by Nešić [8], as cited in the study by Fazal [6],only a minor portion, approximately 0.2%of the dissolved CO₂ reacts with water to form carbonic acid. The reaction is described by (Eq 1) and (Eq 2), respectively. 𝑪𝟎𝟐 (𝒂𝒒)+ 𝑯𝟐𝑶(𝒍)⇌𝑯𝟐𝑪𝑶𝟑……………………….. (2) Carbonic acid is a weak acid but is corrosive to mild steel because it initiates the corrosion process. Carbonic acid undergoes dissociation into hydrogen ions (𝐻+) and bicarbonate ions (𝐻𝐶𝑂3−) in an equilibrium reaction: 𝑯𝟐𝑪𝑶𝟑⇌𝑯++ 𝑯𝑪𝑶𝟑−…………………….. (3) The presence of hydrogen ions is a critical factor in the corrosion process. Cathodic Reaction: Reduction of Protons: One of the primary cathodic reactions involves the reduction of protons 𝐻+ 𝟐𝑯+ + 𝟐𝒆−⇌𝑯𝟐 …………………….. (4) This reduction reaction produces hydrogen gas (𝐻2) as a byproduct. While the reduction of protons is commonly accepted as the main cathodic reaction as in (Eq 5), a proposed 'buffer effect mechanism' suggests that the equilibrium reaction of carbonic acid primarily serves to supply more hydrogen ions (𝐻+) rather than directly participating in the reduction process [6, 9]. 𝑯𝟐𝑪𝑶𝟑 + 𝟐𝒆−⇌𝑯𝑪𝑶𝟑−+ 𝑯𝟐 …………………….. (5) This mechanism posits that the primary reduction occurs with protons (𝐻+). Role of Temperature: It's important to note that the rate-controlling step for cathodic reaction kinetics is often the hydration of carbon dioxide (𝐶02 (𝑔)) to carbonic acid (𝐻2𝐶𝑂3), especially at temperatures below 50–60 °C [8]. Anodic Reaction - Iron Dissolution: To balance the cathodic reactions, mild steel (iron) undergoes anodic dissolution: 𝑭𝒆(𝒔)→ 𝑭𝒆𝟐+ + 𝟐𝒆− …………………….. (6) World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 743 The dissolved iron ions enter the solution as part of the corrosion process. Adsorption and Oxidation of Iron Ions: Iron combines with water (𝐻2𝑂) to form adsorbed intermediate products: 𝑭𝒆(𝒔)+ 𝑶𝑯−→ 𝑭𝒆𝑶𝑯 + 𝒆− (adsorption) …………………….. (7) The adsorbed intermediate products then oxidize further: 𝑭𝒆𝑶𝑯→ 𝑭𝒆𝑶𝑯𝟐+ + 𝒆− …………………….. (8) These reactions contribute to the overall anodic dissolution of iron. For the case of mild steel one can write the overall reaction as; 𝑭𝒆(𝒔)+ 𝑪𝑶𝟐 + 𝑯𝟐O → 𝑭𝒆𝟐+ + 𝑪𝑶𝟑𝟐− + 𝑯𝟐 …………………….. (9) In essence, 𝐶02 corrosion on mild steel is a complex electrochemical process involving the dissolution of 𝐶02 (𝑔) in water, the formation of carbonic acid, the reduction of protons, and the anodic dissolution of iron [10]. The specific reactions can vary depending on factors such as temperature, pH, and the presence of inhibitors [11]. 1.2. Green Corrosion Inhibitors Green Corrosion Inhibitors can be natural or synthetic, as seen in figure 1. The idea of the use of plants has been recognized as a potential replacement for organic compounds and there have been numerous research in the last decade and more to be done in years to come, it is believed that the use of naturally occurring compounds is of interest because of their cost-effectiveness, easy accessibility of raw materials and eco-friendliness. When selecting an inhibitor, several factors merit consideration, including the inhibitor's cost, potential toxicity impacting humans and other living organisms, its availability, and its environmental friendliness [12]. Plant extracts serve as viable options for environmentally friendly alternatives [13]. According to Sapunyo W.L. [14], the aqueous methanol, and dichloromethane: methanol (1:1) extracts of Axonopus compressus (carpet grass) possess phytochemical compounds with anticandidal activity. The extracts have also been classified as non-toxic [14]. Figure 1 Natural and Synthetic green corrosion inhibitor Given the growing environmental and economic concerns surrounding corrosion, especially in CO₂-rich saline environments such as oil and gas pipelines and process plants, there is an increasing need for eco-friendly corrosion mitigation strategies. Conventional inhibitors are often synthetic and toxic, posing environmental and safety risks. As a result, attention has shifted towards plant-based green inhibitors that are biodegradable, cost-effective, and sustainable. World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 744 This study explores Axonopus compressus as a novel green corrosion inhibitor for mild steel in a CO₂-saturated 3.5% NaCl medium. The aim is to advance the understanding of the inhibition mechanism of this plant extract by evaluating its phytochemical composition, adsorption behaviour, and inhibition performance under different environmental conditions. 2. Materials and Methods 2.1. Preparation of Carpet grass extract Axonopus compressus, sourced from the vicinity of Epe in Lagos, Nigeria, underwent a series of preparation steps. Initially, it was subjected to washing and subsequent air-drying. Following this, it was exposed to an oven set at 45°C where it remained for 48 hours. The resulting dried carpet grass was then processed into a powdered form using a miller. To achieve a finer consistency, the powder was sieved through a mesh with a size of 10mm. Finally, the fine particles were carefully stored in an airtight container and placed inside a desicator for safekeeping. Cold extraction was used. A quantity of one hundred grams (100g) of the milled carpet grass was measured in 100% methanol for 48 hours, shaking sporadically every two hours. After filtering the extract through Whatman filter sheets No. 42 (125 mm), the solvent was recovered using steam distillation with the water bath set at 70𝑜𝐶 and the resulting wet residue was dried in an electric oven set at 40𝑜𝐶. The crude extract was stored in sealed plastic container at 4 °C in a refrigerator until when needed. Using the following formula, the extract's percentage yield was determined. Ratio for Cold Maceration = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑟𝑝𝑒𝑡 𝑔𝑟𝑎𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑜𝑙𝑣𝑒𝑛𝑡 …………………….. (10) Ratio of Methanol Recovered = Volume of Recovered Methanol Volume of solvent used …………………….. (11) Yield = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝐸𝑥𝑡𝑟𝑎𝑐𝑡 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑟𝑝𝑒𝑡 𝑔𝑟𝑎𝑠𝑠 ˟ 100 …………………….. (12) 2.2. Preparation of Metal Specimen Mild steel sheets of 1 mm thickness were cut into eleven identical coupons with dimensions of 3 cm × 3 cm × 0.1 cm. To ensure surface cleanliness, each coupon was polished using sandpaper to remove surface oxides and impurities, then rinsed with analytical-grade ethanol. Degreasing was subsequently carried out using acetone to eliminate residual oil or grease that could interfere with surface interactions during testing. One of the coupons was selected and submitted to the Quality Control Laboratory for elemental analysis. The remaining ten coupons were individually labeled and reserved for the corrosion inhibition experiments. Each coupon was weighed using a precision balance (±0.01 g), and the initial weights were found to be approximately 6.43 g, confirming uniformity in dimensions and material density. Figure2 The Energy Dispersive X-ray Spectroscopy (EDS) shows the elemental analysis of mild steel before corrosion study. The EDS compositional analysis results obtained from the spectra show a weight percent of Carbon (3.0 wt. %), Oxygen (20.0 wt. %), Iron (54.0 wt. %), Aluminum (2.00 wt. %), Magnesium (4.00wt%), Zinc (8.40), Calcium (1.30wt%), Potassium (2.00wt%), Silicon (1.20wt%) and Titanium (4.00wt %) for the mild steel before corrosion experiment. World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 745 Figure 2 Energy Dispersive X-Ray Spectroscopy Analysis of Mild Steel After drying the specimens in an oven at 40 °C, their initial weights were re-measured. The volume of the test solution was deliberately kept large enough to ensure the detection of any significant changes in its corrosive properties, whether due to the depletion of active corrosive species or the accumulation of corrosion byproducts. To preserve the prepared mild steel in a dust-free environment, the specimens were stored in desiccators and used immediately for the corrosion experiments after preparation. 2.3. Evaluation of Corrosion inhibition of mild steel (Weight Loss) CO₂ gas was continuously purged into a sealed bottle containing distilled water for approximately 15 minutes to ensure saturation, as illustrated in Figure 3. Following this, a solution was prepared by combining 100 mL of the CO₂-saturated water with 3.5% NaCl, adjusted to a pH of 6.0. This solution was then evenly distributed into ten separate beakers. Subsequently, varying concentrations (10%, 20%, 30%, and 40%) of the methanolic plant extract (as shown in Figure 3) were added individually to designated beakers. Each beaker received a pre-weighed mild steel coupon, after which all beakers were placed in water baths set at 30 °C and 60 °C, respectively. The immersion process lasted for 163 hours. After the exposure period, the steel samples were carefully removed, cleaned, dried, and reweighed. The mass loss was recorded meticulously for further analysis of corrosion rates and inhibition efficiency. The weight loss (Δw), corrosion rate (CR), inhibition efficiency (IE) and degree of surface coverage were calculated using standard equations ∆W = 𝑊𝑖 − 𝑊 𝑓 …………………….. (13) 𝐼𝐸% = 𝑊0−𝑊1 𝑊0 …………………….. (14) 𝜃 = 𝑊0 − 𝑊1 𝑊0 …………………….. (15) CR (mmpy) = K x Weight loss D x A x t (in hours) …………………….. (16) World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 746 Figure 3 Photographs showing preparation of Axonopus compressus Extract and solution. Where 𝑊𝑖 and 𝑊 𝑓are the initial and final weight of mild steel samples respectively; 𝑊1 and 𝑊0 are the weight loss values in presence and absence of inhibitor respectively. A is the area cm2, t is the time K = 8.76x104 (constant)), D is density in gm/cm3 (7.86). 3. Result and discussion 3.1. Yield and Characterization of Methanolic Extract Table 1 Qualitative Phytochemical Analysis of Extract Plant Phytochemicals Plant Metabolite Cardiac glycosides +++ Steroid glycosides ++ Saponins + + Tannins ++ World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 747 Alkaloids +++ Phlobatannins + Terpenoids + Flavonoids ++ Anthraquinones +++ -= Absent, + = Present, ++=Moderate, +++ = Abundant The methanolic leaf extract of Axonopus compressus had a greenish color and yielded 7% w/w of the dry matter. The composition of the phytochemical extract is summarized in Table 1. The phytochemical screening of Axonopus compressus indicated the presence of steroids (steroid glycoside), alkaloids, saponins, tannins, cardiac glycosides, flavonoids, phlobatannins, anthraquinones, and terpenes. Quantitative analysis revealed high levels of flavonoids, and alkaloids, along with significant amounts of polyphenolsconsistent with findings reported by Afolabi [15]. 3.2. Weight Loss Measurement Table 2 – Table 3 show the various samples, their respective concentrations, weight loss values, inhibition efficiency (η %) and surface coverage (θ) for mild steel at different concentrations of Axonopus compressus extract. These values were calculated after immersion in CO₂ corrosive media. From Table3, the analysis shows that the minimum weight loss was observed when the mild steel samples were immersed in the test solution containing Axonopus compressus extract for 163 hours, with concentration ranges from 10% (w/w) to 40% (w/w) at 30°C. In particular, Sample C (40% w/w) exhibited the least weight loss (0.01 g), followed by Sample B (30% w/w) with 0.04 g, indicating effective inhibition at higher concentrations. Table 2 The labeling of all extract samples and there corresponding concentration(w/v). Sample Concentration (w/v) Percentage (%w/w) Sample A (Blank) 0 0 Sample B 2.1 30 Sample C 2.8 40 Sample D 0.7 20 Sample E 0.4 10 Table 3 The data for varying weight of mild steel with different concentration of Axonopus compressus extract at the two temperatures studied. Time (hr.) Sample A (g) Sample B (g) Sample C (g) Sample D (g) Sample E (g) 30 0C 60 0C 30 0C 60 0C 30 0C 60 0C 30 0C 60 0C 30 0C 60 0C 0 6.43 6.43 6.43 6.43 6.43 6.43 6.43 6.43 6.43 6.43 163 6.29 4.68 6.39 5.83 6.42 6.13 6.38 5.68 6.37 5.53 In contrast, the blank sample (Sample A), which had no inhibitor, recorded a weight loss of 0.14 g at 30°C and 1.75 g at 60°C, confirming the absence of any protective barrier against corrosion. Thus, compared to the result of the blank, i.e., without the presence of inhibitor, it can be observed that the Axonopus compressus extract (Inhibitor) caused a reduction in weight loss and consequently reduced the corrosion rate at both 30°C and 60° World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 748 From the analysis above, it is evident that the carpet grass extract significantly improved corrosion resistance of mild steel by reducing the weight loss across all concentrations when compared to the blank. The higher the concentration of the extract, the greater the inhibition efficiency, with optimal results seen at 40% and 30% w/w. 3.3. Corrosion Rate The corrosion rate of mild steel in CO₂-saturated corrosive media was evaluated in the presence and absence of Axonopus compressus (carpet grass) extract, and the results are presented in Figure4 and Table 4. It was observed that the corrosion rate decreased with increasing concentration of the extract, indicating the effectiveness of Axonopus compressus as a corrosion inhibitor. At 30°C, the blank sample (Sample A), which contained no inhibitor, exhibited the highest corrosion rate of 9.57 mm/year, while the lowest corrosion rate of 0.68 mm/year was recorded for Sample C (2.8 g/ml or 40% w/w extract concentration). A similar trend was observed at 60°C, where the corrosion rate for the blank rose significantly to 119.66 mm/year, compared to 20.52 mm/year for the same highest concentration of inhibitor. This reduction in corrosion rate is attributed to the adsorption of phytochemical constituents of the carpet grass extract onto the surface of the mild steel, forming a protective barrier that hindered further interaction with the corrosive medium. Table 4 The Corrosion rate (CR), Inhibition efficiency (IE%) and surface coverage (θ) with different concentration of Axonopus compressus extract at the two temperatures studied. Sample Conc. (g/ml) Weight Loss (g) CR (mmpy) IE% (θ) 30 0C 600C 300C 600C 300C 600C 300C 600C Sample A 0 0.14 1.75 9.57 119.66 0 0 0 0 Sample E 0.4 0.06 0.9 4.1 61.54 57.14 48.57 0.57 0.49 Sample D 0.7 0.05 0.75 3.42 51.28 64.29 57.14 0.64 0.57 Sample B 2.1 0.04 0.6 2.73 41.02 71.43 65.71 0.71 0.66 Sample C 2.8 0.01 0.3 0.68 20.52 92.86 82.86 0.93 0.83 Figure 4 The variation of Corrosion Rate with extract concentration at the two temperatures studied. 3.4. Effects of Concentration on Inhibition Efficiency The inhibition efficiency of Axonopus compressus extract increased progressively with rising inhibitor concentration as shown in Figure 5. This trend was evident at both 30°C and 60°C, indicating that the protective action of the extract on mild steel is strongly dependent on its concentration in the corrosive medium. At 30°C, inhibition efficiency increased from 57.14% at 0.4 g/ml (Sample E) to 92.86% at 2.8 g/ml (Sample C), while at 60°C, a corresponding increase from World Journal of Advanced Research and Reviews, 2025, 27(02), 741-754 749 48.57% to 82.86% was observed. This steady improvement suggests that more inhibitor molecules are adsorbed onto the steel surface at higher concentrations, thereby enhancing surface coverage and reducing access of the corrosive species to the metal substrate. The slight reduction in inhibition efficiency at elevated temperatures may be attributed to increased desorption of the inhibitor molecules due to thermal agitation, which weakens the integrity of the adsorbed film. Despite this, the extract still retained considerable efficiency, even at 60°C, confirming its potential as a viable green corrosion inhibitor under varying thermal conditions. Figure 5 The variation of inhibition efficiency with extract concentration at the two temperatures studied. 3.5 Effects of Surface Coverage (θ) Surface coverage (θ) is a dimensionless parameter that represents the fraction of the metal surface covered or protected by the inhibitor molecules. It provides insight into the extent of adsorption of the inhibitor onto the metal surface, where a value of 1 indicates complete coverage and 0 implies no adsorption. A higher surface coverage value suggests more effective barrier formation against corrosion [16]. In this study, surface coverage increased with the concentration of Axonopus compressus extract, consistent with the observed trends in corrosion rate and inhibition efficiency. At 30°C, θ increased from 0.57 at 0.4 g/ml (Sample E) to 0.93 at 2.8 g/ml (Sample C). A similar increase was recorded at 60°C, where θ rose from 0.49 to 0.83 across the same concentration range. These results indicate that the extract’s active components adsorb more effectively onto the mild steel surface at higher concentrations, leading to enhanced protective film formation. Although surface coverage values were slightly lower at 60°C compared to 30°C, the extract still demonstrated significant adsorption capabilities under elevated temperature conditions. 3.6 Effect of Temperature Temperature plays a significant role in understanding the mechanism of adsorption and corrosion inhibition efficiency. As shown in Table 5, increasing the temperature from 303 K to 333 K leads to a decrease in the adsorption equilibrium constant (K ads) and a less negative Gibbs free energy of adsorption (∇G0ads) suggesting a reduction in the adsorption strength at higher temperatures. This trend indicates that the adsorption of the Axonopus compressus extract on the mild steel surface is predominantly physisorptive, as physical adsorption typically decreases with increasing temperature due to the weakening of intermolecular forces[17]. To further interpret the temperature effect, the Arrhenius equation will be employed: 𝑙𝑜𝑔𝐶𝑅 = log 𝐴 − 𝐸𝑎 2.303𝑅𝑇 …………………… (17) where CR is the corrosion rate, A is the pre-exponential factor, 𝐸𝑎is the activation energy, R is the gas constant, and T is the absolute temperature. A plot of log (CR) vs 1/T (with which multiplication by 1000 is usually done for graph plotting convenience) will provide the activation energy (𝐸𝑎) from the slope; (𝐸𝑎= −𝑠𝑙𝑜𝑝𝑒 × 2.303𝑅) ………………… (18)