A Study of the Inhibition Capacity of a Novel Ilex guayusa Green Extract for Preventing Corrosion in Mild Steel Exposed to Different Conditions

Abstract

Corrosion is a critical industrial problem. To solve this problem, the present research analyzed the influence of corrosive media on the efficiency of a guayusa inhibitor. Therefore, guayusa extract was obtained, and five groups of ASTMS A36 steel test tubes were prepared, each with variable extract concentrations (200 ppm, 400 ppm, 600 ppm, 800 ppm, and 1000 ppm) that were exposed to different corrosive media (5% NaCl, 5% NaCl + acetic acid, 1% HNO₃, and 10% HNO₃). The results obtained were compared to determine the percentage efficiency of the inhibitor in each of the corrosive media. This study provides a detailed understanding of how the corrosive environment influences the effectiveness of a guayusa inhibitor, which is used as a green inhibitor for the first time, allowing its viability and performance to be assessed under various conditions.

1. Introduction

Corrosion in metals is a natural and inevitable process caused by several agents, such as rain, temperature, humidity, salinity, acids, and pollution [1]. It causes economic problems in industries due to the deterioration of infrastructure, equipment, and machinery [2]. Consequently, direct annual costs are estimated to range between USD 64.3 billion and USD 10.15 billion, and they are accompanied by indirect costs due to traffic delays and loss of productivity that exceed direct costs by ten times [3]. In addition to economic effects, corrosion generates pollutants and waste that cause damage to aquatic and terrestrial ecosystems [4].

Additionally, metallic pipe materials and their corrosion products in drinking water distribution systems promote antibiotic resistance [5]. The assessment of oil and gas pipeline repair techniques indicates that non-metallic composite repair methods have the greatest environmental impacts, unlike welded patching techniques, which have the lowest impacts according to life cycle analysis [5]. ASTM A36 steel, also known as black iron, is a material that is not heat-treated because it forms parts of structures with an alloy composed mainly of iron (98%), carbon content of less than 1%, and traces of minerals such as manganese, which help improve its strength, as well as phosphorus, sulfur, and copper, which contribute to enhancing its weldability and resistance to atmospheric corrosion, giving it advantages such as high resistance, uniformity, ductility, durability, toughness, and fatigue resistance [6].

In response to this problem, corrosion inhibitors, which are essential for the protection of metallic materials in aggressive environments, have been developed; they include indole derivatives, which have been shown to be effective in inhibiting the corrosion of low-carbon steel under hydrochloric acid solutions [7]. New trends in the development of corrosion inhibitors include the use of amino acids with the most efficient organic compounds being those containing heteroatoms such as N, S, P, and O. It is reported that organic compounds that contain -OH, -COOH, and NH₂ are excellent corrosion inhibitors, mainly in acidic media [8].

However, these types of inhibitors present significant problems related to toxicity and environmental damage. Because some inhibitors contain hazardous chemicals, such as certain acrylamide derivatives and other organic compounds, they can be toxic to both the environment and human health [9]. In addition, it is essential to evaluate the toxicity of synthetic inhibitors based on sulfonamides before their application in industry [9].

To solve these problems, industries have started to have an interest in green inhibitors as a safer and more sustainable alternative [10]. To this end, some authors, such as [11,12,13], reported corrosion protection efficiency above 90% when different green inhibitors, such as carbon dots, were used. Unfortunately, these methods are more sophisticated and difficult to produce under our laboratory conditions; thus, it is necessary to obtain green inhibitors that can be obtained by simple methods available in Ecuador and that are comparable with previous reports. Additionally, studies have shown that corrosion inhibitors based on plant extracts and food additives are efficient and not harmful to the environment and health, making them promise ecological alternatives that minimize toxicity and reduce the impact associated with synthetic inhibitors [10].

For example, seaweed extracts are estimated to have the potential to prevent corrosion in acidic media [14]. Furthermore, bioactive substances present in plant extracts, such as flavonoids, tannins, polyphenols, phenolic acids, and glycosides, stand out for their potential for corrosion inhibition [15]. Polyphenols, due to their easy extraction method and low cost, show promise in the protection of metal surfaces [16]; therefore, they have gained attention in the industry due to their environmental compatibility, biodegradability, and safety, with inhibition efficiencies ranging from 80% to 90%, making them stand out as a viable alternative to control corrosion [17].

Ecuadorean plants have the potential to serve as green inhibitors; however, there are currently no publications specifically on the use of guayusa extract as a green corrosion inhibitor, thus highlighting the need to explore its potential as an inhibitor [18]. This absence highlights a gap in knowledge that could be valuable to explore due to the success of other plant extracts as corrosion inhibitors in acidic and saline media [19], thus contributing to the development of new friendly and effective solutions to reduce corrosion [20]. In addition, there are no studies that have established the influence of the corrosive medium on the efficiency of the inhibitor, leading to the need for further research to develop inhibitors that maintain their effectiveness in various corrosive media, thus optimizing their industrial application [21]. The plant Ilex guayusa (Loes) was selected for this study due to its richness in bioactive compounds and its origin in the Ecuadorian Amazon region, where it holds significant cultural and medicinal relevance. Previous focus has been on flavonoids, as these, along with other compounds such as tannins, polyphenols, phenolic acids, and glycosides, have demonstrated high potential as corrosion inhibitors [16].

The effectiveness of the active compounds in guayusa extracts has been proven in previous pharmaceutical studies. For example, [22] highlight in their research on Ecuadorian medicinal plants that guayusa leaves contain polyphenols and alkaloids, phytochemical compounds capable of inhibiting corrosion on metal surfaces, as evidenced in recent studies [23].

For these reasons, in the present study, we pursued to analyze the influence of the corrosive environment on the corrosion efficiency of green inhibitors derived from plants, for which guayusa (Ilex guayusa Loes) was selected as the inhibitor of interest.

2. Materials and Methods

2.1. Materials and Reagents

In the present research, the following chemicals were used: nitric acid (LCQ at 70%, from Merck, Germany), sodium chloride (NaCl—S271-1 Sodium Chloride Certified ACS crystalline, from Merck, Germany, 99% purity), acetic acid (C₂H₄O₂ acetic acid glacial at 99.9%, Merck, Germany), alcohol (1L—96% purity, Merck, Germany), and distillated water.

2.2. Preparation of the Extract

We prepared the Soxhlet extract using standard equipment. First, 100 g of guayusa leaf obtained from local producers in Puyo, Ecuador, was introduced in a cellulose thimble, which was closed with cotton. Secondly, the extractor part was filled with 0.5 L of ethanol at 96% concentration, and the cellulose thimble was introduced into the extractor until it was soaked by the alcohol. Thirdly, we assembled the rest of the equipment and turned on the heating system, as depicted in Figure 1. We adjusted the heating system to raise the mixture’s temperature to 333 K. We carried out the Soxhlet extraction for 24 h, dividing it into 4 days. Fourth, the cellulose thimble was taken off the extract and weighed again. The round-bottom flask containing the alcohol–guayusa extract mixture was then taken off and put in a rotary evaporator for one hour to remove the ethanol. The solvent recovery part was then saved for later use. Finally, we weighed the Soxhlet guayusa extract (Guayusa-SE) and stored it at room temperature for future experiments.

2.3. Preparation of the Metallic Samples

We fabricated ASTM A36 steel plates into cubic forms, each measuring 20 mm in height and width and 2 mm in thickness. We fabricated a total of 60 steel plates. After being polished, the steel plates were put through four different corrosion environments for 168 h. These were 5% NaCl (T1), 5% NaCl + 10% acetic acid (T2), 1% HNO₃ (T3), and 10% HNO₃ (T4), as shown in Figure 2. Subsequently, the samples were removed, desiccated, and weighed. The samples were reintroduced into identical settings, but this time, varying amounts of guayusa inhibitors were incorporated: 200 ppm (E1), 400 ppm (E2), 600 ppm (E3), 800 ppm (E4), and 1000 ppm (E5). Subsequently, after 168 h, the samples were removed, desiccated, and weighed for subsequent analysis.

2.4. Chemical Characterization of the Extract

The chemical characterization of the obtained extract was necessary to determine the main components that protect the metal against corrosion. FT-IR measurements were taken with a spectrometer (type Vector 22 from Bruker, Germany) to find out about the chemical properties of the extract. Moreover, the main components of the extract were evaluated using Wagner, Drage Dorff, and Mayer’s reaction to determine if the extract contains alkaloids (the main anticorrosive component of the present inhibitor). Lastly, ferric chloride (FeCl₃) reactive was used to see if the inhibitor had polyphenols, which are another anticorrosive substance.

2.5. Calculation of the Corrosion Rate

One of the simplest and most widely used methods to quantify corrosion rates (C_R) is the immersion test. The usual approach is to assume that the initial surface area of the corroding object remains constant over time [24]. In this sense, the corrosion rate for each of the metallic samples was calculated according to the following equation:

$${\mathrm{C}}_{\mathrm{R}}={\displaystyle \frac{\mathrm{k}·{\mathrm{m}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}{\mathrm{A}·\mathrm{t}·\mathsf{\rho }}}$$

(1)

where k is a constant 8.76·10⁴ so that C_R is in [mm/y], m_loss is the mass loss [g] of the metal (m₀ − m_f) in time t [hours], A is the surface area of the material exposed [cm²], and ρ is the density of the material [g/cm³] [25]. Moreover, the density of the metal plates was calculated by measuring the average weight of the plates and divided it by the average volume of the plates.

The corrosion rate allows us to calculate the inhibition efficiency of the prepared extract to protect the metallic plates in different corrosion medium. The inhibition efficiency was calculated according to the following equation [26]:

$$\mathrm{Efficiency} ,\mathrm{\%}={\displaystyle \frac{{\mathrm{C}}_{R0}-{\mathrm{C}}_{Rf}}{{\mathrm{C}}_{R0}}}\ast 100$$

(2)

where C_R0 is the corrosion rate without an inhibitor, and C_Rf is the corrosion rate with the addition of the inhibitor.

2.6. Statical Analysis

A statistical analysis utilizing the analysis of variance (ANOVA) test was conducted to evaluate the significance of the differences in efficiency between the inhibitor and the corrosive medium, as presented in

Table 1. Statistical models of the samples immersed in different environments with the addition of guayusa inhibitor.

Treatment	Essay
T1 (5% NaCl)	T1E1 (200 ppm 1)
	T1E2 (400 ppm)
	T1E3 (600 ppm)
	T1E4 (800 ppm)
	T1E5 (1000 ppm)
T2 (5% NaCl + 1% Acetic Acid)	T2E1 (200 ppm)
	T2E2 (400 ppm)
	T2E3 (600 ppm)
	T2E4 (800 ppm)
	T2E5 (1000 ppm
T3 (1% HNO3)	T3E1 (200 ppm) T3E2 (400 ppm) T3E3 (600 ppm) T3E4 (800 ppm) T3E5 (1000 ppm)
T4 (10% HNO3)	T4E1 (200 ppm) T4E2 (400 ppm) T4E3 (600 ppm) T4E4 (800 ppm) T4E5 (1000 ppm)

¹ Concentration of the guayusa inhibitor.

. The ANOVA test results elucidated the differences among the groups, and the subsequent application of the Tukey test will determine whether the corrosive medium significantly impacts the inhibitor [27]. Measurements were conducted in triplicate for each trial to enhance the statistical analysis of the data. The results facilitated the assessment of the variation in inhibitor efficiency relative to the pH of the corrosive medium. The ANOVA test was employed on the efficiency results from each treatment, allowing for the identification of statistical differences among the data groups and the formulation of statistically supported conclusions.

Moreover, we conducted Tukey’s statistical test to determine if the corrosive medium significantly affects the efficiency of the guayusa inhibitor. The test was performed by using the following equation [27]:

$$q_s={\displaystyle \frac{\overline{Y_A}-\overline{Y_B}}{SE}}$$

(3)

where $\overline{Y_A}$ and $\overline{Y_B}$ are the two means being compared, and SE is the standard error for the sum of the means. The value q_s is the sample’s test statistic.

To learn more about the inhibitor’s behavior, the effect of the corrosive environment (specifically, the pH) on its effectiveness was also studied. This is because most studies that have been conducted on inhibitors have only measured how well they work in a single corrosive medium (mostly saline and acidic) rather than testing each inhibitor separately in corrosive media with different oxidative capacities, as shown in

Table 2. pH values for the different corrosion mediums.

Corrosion Medium	pH
5% NaCl
Without Inhibitor
5% NaCl	5.72
With Inhibitor
200 ppm	4.63
400 ppm	4.21
600 ppm	3.92
800 ppm	3.71
1000 ppm	3.53
5% NaCl + Acetic Acid
Without Inhibitor
5% NaCl + Acetic Acid	2.55
With Inhibitor
200 ppm	2.59
400 ppm	2.62
600 ppm	2.63
800 ppm	2.64
1000 ppm	2.66
1% HNO3
Without Inhibitor
1% HNO3	0.54
With Inhibitor
200 ppm	0.61
400 ppm	0.61
600 ppm	0.78
800 ppm	0.79
1000 ppm	0.81
10% HNO3
Without Inhibitor
10% HNO3	0.044
With Inhibitor
200 ppm	0.018
400 ppm	0.072
600 ppm	0.080
800 ppm	0.108
1000 ppm	0.176

.

2.7. Corrosion Inhibition Mechanism

To determine the inhibition mechanism of the prepared inhibitor, it was evaluated to be linear polarization and interpreted by Tafel curves. In this sense, the voltage position allowed us to determine if the protection is anodic or cationic. The potentiodynamic polarization measurements were carried out over a potential range of ±200 mV vs. OCP with a scan rate of 0.5 mV s⁻¹. The recorded measurements were plotted and analyzed as Tafel plots. These plots allowed the estimation of the corrosion kinetic parameters such as corrosion potential (E_corr), corrosion current density (i_corr), and anodic (ba) and cathodic (bc) Tafel slopes. Moreover, the inhibitor efficiency was calculated according to the following equation [28]:

$${\mathrm{I}\mathrm{E}}_p \left(\%\right)={\displaystyle \frac{{i^0}_{corr}-i_{corr}}{{i^0}_{corr}}}\times 100$$

(4)

3. Results

3.1. Chemical Characterization of the Plant Extract

The chemical groups were characterized by FT-IR measurements of the extract, as indicated in Figure 3. The spectrum shows the presence of alkaloids in the green extract. The figure shows that the 3282.25 cm⁻¹ bandwidth is caused by the O-H stretching of alcohol, phenol, and carbohydrates as well as the N-H stretching of amines. A band at 2923 cm⁻¹ depicts alkane C-H stretching, whereas a band at 1550 cm⁻¹ symbolizes C=C stretching and C=N stretching, as well as imine or oxime, amide, or δ-lactum C=O stretching and amine N-H bending, which are the principal functional groups of the alkaloids [29]. Moreover, aromatic C=C bending and the N-H bending of amine are denoted by a sharp band at 1550 cm⁻¹. The band at 1450 cm⁻¹ is caused by carboxylic acid O-H bending, and the strong peak at 1336 cm⁻¹ is caused by alcohol, phenol, gem dimethyl, or aldehyde C-H bending. A strong peak at 1029 cm⁻¹ backs up the idea that the band at 1238 cm⁻¹ is caused by aromatic amine C-N stretching. The 1164 cm⁻¹ absorption band is caused by the C-O stretching of aromatic ethers, third alcohols, esters, and the C-N stretching of amines. The 1164 cm⁻¹ band is caused by the C-O stretching of secondary alcohols, ethers, and amines. Because of the alkene’s C=C bending, there is another band at 860 cm⁻¹ [30,31].

Additionally, the analysis focused on the alkaloid content of the green extract, as shown in

Table 3. Alkaloids content analysis of the guayusa Ile green inhibitor extract.

Reactive	Composition	Use	Results
Wagner	Iodide (I2) and potassium iodide (KI)	Detection of Alkaloids	+
Dragendorff	Bismuth (Bi) and Chloride acid (HCl)	Detection of Alkaloids	+
Mayer	Chloride acid (HCl) and copper chloride (CuCl2)	Detection of Alkaloids	+

. Positive results for all determinations confirm the presence of alkaloids in the extract, indicating its quality and potential uses. In line with what the author [32] says, the ability of these phytochemicals to stop corrosion is largely affected by the presence of heteroatoms that follow a general trend of O < N < S < P. The guayusa extract will act as a corrosion inhibitor because it has a lot of these groups, as shown by the FT-IR analysis, which will be explained in more detail below.

3.2. Analysis of the Rate of Corrosion

The experiment plan involved exposing the test pieces to various corrosive media for 168 h.

Table 4. Analysis of the corrosion rate after 168 h of immersion in different environments.

Corrosion Rate Without Inhibitor, mm/year					Corrosion Rate with Different Concentration of Inhibitor, mm/year
T1
E1	E2	E3	E4	E5	E1	E2	E3	E4	E5
0.060	0.066	0.030	0.034	0.246	0.044	0.021	0.020	0.015	0.032
0.055	0.041	0.052	0.056	0.062	0.032	0.009	0.016	0.012	0.014
0.077	0.044	0.064	0.037	0.116	0.029	0.018	0.012	0.012	0.011
T2
E1	E2	E3	E4	E5	E1	E2	E3	E4	E5
0.475	0.535	0.533	0.406	0.449	0.244	0.213	0.223	0.206	0.180
0.541	0.564	0.489	0.471	0.513	0.295	0.273	0.248	0.261	0.175
0.511	0.543	0.484	0.536	0.490	0.302	0.267	0.193	0.169	0.169
T3
E1	E2	E3	E4	E5	E1	E2	E3	E4	E5
1.594	1.545	1.752	1.584	1.383	1.249	1.237	1.248	1.271	1.260
1.686	1.704	1.581	1.718	1.562	1.231	1.237	1.203	1.303	1.237
1.531	1.301	1.664	1.548	1.421	0.996	1.197	1.220	1.254	1.324
T4
E1	E2	E3	E4	E5	E1	E2	E3	E4	E5
11.01	12.28	12.79	11.37	12.80	10.99	11.67	11.48	9.98	11.34
13.18	13.01	13.70	13.20	12.95	11.40	11.14	10.62	10.88	11.11
13.32	12.78	12.63	11.45	13.28	10.80	11.12	10.01	12.25	11.00

shows the corrosion rates of those pieces with and without an inhibitor. The highest corrosion rate occurred in T4 (nitric acid) with a corrosion rate value of 1.67 mm/y. T1 recorded the lowest corrosion rate, measuring 0.049 mm/y. The results show that the prepared extract works better in a salty environment and less well in acidic ones. However, it is still necessary to figure out how well it stops corrosion in order to fully understand how the reaction works or what kind of inhibition the prepared extract provides.

3.3. Analysis of the Efficiency of Corrosion Protection

Table 5. An analysis of corrosion resistant efficiency after 168 h of immersion in different environments.

Sample	Essay (Concentration of Inhibitor)
T1
	E1	E2	E3	E4	E5
1	25.8507	68.2367	33.0840	56.6896	86.9495
2	41.9454	78.0075	67.4620	78.8260	77.4012
3	61.4100	59.1584	80.9406	68.2390	89.9961
Average	43.0687	68.4675	60.4955	67.9182	84.7822
T2
	E1	E2	E3	E4	E5
1	48.5289	60.0606	58.0631	49.2212	59.8975
2	45.4251	51.4909	49.2290	44.5618	65.8362
3	40.8638	50.7614	60.0946	68.4245	65.4974
Average	44.9393	54.1043	55.7956	54.0692	63.7437
T3
	E1	E2	E3	E4	E5
1	21.6519	19.9480	28.7764	19.7252	8.9083
2	26.9845	27.4232	23.9018	24.1786	20.8210
3	34.9027	7.9502	26.7209	19.0152	6.7816
Average	27.8464	18.4405	26.4664	20.9730	12.1703
T4
	E1	E2	E3	E4	E5
1	0.2037	4.9602	10.1764	12.1909	11.4141
2	13.5093	14.3576	22.4763	17.5095	14.2172
3	18.9050	12.9742	20.7159	6.9489	17.1454
Average	10.8727	10.7640	17.7895	7.5838	14.2589

displays the percentage of inhibition efficiency for specimens exposed for 168 h. T1 exhibited the highest inhibition efficiency with a percentage of 84.78% in the E5 concentration. T4 recorded the lowest inhibitive efficiency with a percentage of 12.17% in the E4 concentration. As indicated in Section 3.1, the extract response is better in the saline environment, and it does not respond adequately when the samples are immersed in an acidic environment. This is due to the hydrolysis that the extract suffers when it is exposed to acid conditions, which will be explained in greater detail in follow parts.

The bar graph in Figure 4 shows how the protective effect of the guayusa extract inhibitor in ASTM A36 steel increases as the extract concentration increases (ppm). This occurs because the inhibitors’ chemical groups absorb them on the surface of the metal plates. These factors contribute to the presence of antioxidant groups in the guayusa extract, which are the active compounds responsible for stopping corrosion. However, exposure to acid conditions hydrolyzes these compounds in the extract. Therefore, while the prepared extract may not be effective in protecting metal under extreme conditions, it can effectively shield metal pieces in saline environments, making it a suitable choice for the construction industry. The results obtained confirm this. It is clear from the research results that as the pH of the corrosive environment rises, it has a big effect on how well the guayusa inhibitor works. This is because the hydrogen ions in the acidic medium are competing with the inhibitor molecules for the steel’s surface, which is where the action is [33]. As the acidity rises, the corrosive medium can become more aggressive, which can break down some of the guayusa inhibitor. This implies that the guayusa inhibitor loses some of its original chemical structure, making it less effective in protecting the metal from corrosion [34]. Researchers found that the green inhibitor extract from Jatropha Curcas leaves worked less well in HCl and H₂SO₄ solutions because the acidic mediums caused some of the molecules to break down. At high pH, they can experience significant changes in their stability and efficacy. Some phytochemicals may break down or lose their ability to stick to metal surfaces in alkaline environments, which makes them less effective at stopping corrosion [35]. According to [36], phytochemicals in Pueraria tuberosa showed remarkable chemical stability under high pH conditions over the 15-day period. Therefore, we can establish that certain phytochemicals can withstand degradation in alkaline environments.

3.4. Analysis of the Effect of the Environment on Corrosion Resistance

Table 6. An analysis of the correlation between the environment and the efficiency of corrosion after 168 h of immersion.

Tukey
$\overline{{\mathrm{Y}}_o}$		$\overline{{\mathrm{Y}}_{o highest}}$ − $\overline{{\mathrm{Y}}_{o lowest}}$
$\overline{{\mathrm{Y}}_1}$	64.95	$\overline{{\mathrm{Y}}_1}-\overline{{\mathrm{Y}}_4}$	51.98
$\overline{{\mathrm{Y}}_2}$	54.53	$\overline{{\mathrm{Y}}_1}-\overline{{\mathrm{Y}}_3}$	43.77
$\overline{{\mathrm{Y}}_3}$	21.18	$\overline{{\mathrm{Y}}_1}-\overline{{\mathrm{Y}}_2}$	10.42
		$\overline{{\mathrm{Y}}_2}-\overline{{\mathrm{Y}}_4}$	41.56
$\overline{{\mathrm{Y}}_4}$	12.97	$\overline{{\mathrm{Y}}_2}-\overline{{\mathrm{Y}}_3}$	33.35
		$\overline{{\mathrm{Y}}_3}-\overline{{\mathrm{Y}}_4}$	8.212
Tukey comparison value
q(a,k,v) = 4.05	CME = 80.66	√(CME/n) = 4.02	q(a,k,v)*√(CME/n) = 16.27
Tukey result to identify if the corrosive medium affects the inhibitor
51.98 > 16.27	There is a significant difference between the means
43.77 > 16.27	There is a significant difference between the means
10.42 > 16.27	There is not a significant difference between the means
41.56 > 16.27	There is a significant difference between the means
33.35 > 16.27	There is a significant difference between the means
8.212 > 16.27	There is not a significant difference between the means

and Figure 5 demonstrate how well the guayusa extract stopped corrosion in different corrosive environments with different amounts of the inhibitor at the end of the exposure time (168 h with and without inhibitor). Treatment T1 (saline medium; 5% NaCl) had the best inhibition when the inhibitor was at its highest concentration, E5, 84.78% of the time. At the same time, the 800 ppm inhibitor (E4) had the lowest ability to stop the reaction in the corrosive medium with the lowest pH level (10% HNO₃). Its recorded efficiency was 7.58%. Statistical tests showed that the inhibitor does not react the same way in all corrosive environments. This was due to significant differences in the tested concentration means compared to the Tukey comparator value (16.27). The inhibitor works best in purely saline media (5% NaCl) with an average percentage of 64.95% across all concentrations. On the other hand, as the pH of the corrosive medium drops, the inhibitor’s effectiveness goes down with an average percentage of 12.97% across all concentrations in the most acidic environment (10% HNO₃).

3.5. Results of the Corrosion Inhibition Mechanism

The Tafel curves (Figure 5) showed how the electrochemical behavior of ASTM A36 steel changed when it was put in different types of corrosion media with and without an inhibitor. It was observed that there was a rightward shift in the corrosion potential (E_c) when we decreased the pH of the solution, indicating a decrease in the steel’s tendency to corrode. Also, when the inhibitor was added to saline conditions, the corrosion current density (ic) went down significantly compared to the reference curve (blank) that did not have the inhibitor, as shown in

Table 7. Electrochemical parameters for corrosion measurements on the mild steel after adding different concentrations of Guayusa SE in 10% NaCl.

Sample	ic (A/cm2)	Ec (V) vs. Ag/AgCl/KClsat	bc (V/dec)	ba (V/dec)	Corrosion Rate (mm/year)	IE (%)
Blank	0.027	0.227	0.131	0.140	4.86	-
Mild steel in 5% NaCl	0.0032	0.344	0.202	0.217	0.37	88.15
Mild steel in 5% NaCl + 10% Acetic Acid	0.0052	0.309	0.181	0.195	0.43	80.74
Mild steel in 1% Nitric Acid	0.0075	0.275	0.161	0.173	1.29	72.22
Mild steel in 10% Nitric Acid	0.013	0.234	0.137	0.147	2.58	51.85

. These results suggested that the guayusa extract acted effectively as a corrosion inhibitor, being more effective in saline environments. The decrease in i_c and the positive shift in E_c confirmed that the resulting decrease in the corrosion current in the presence of an inhibitor indicates the blockage of the reacting sites by the inhibitor molecules [37]. Moreover, Table 7 shows the electrochemical parameters of the mild steel samples that were put into different environments with and without 1000 ppm of the guayusa extract. So, corrosion works 30% better when the medium is more basic, which means there is a potential shift of more than 85 mV between the different pH environments. This shows that the guayusa extract in the corrosion medium acts as an anodic-type inhibitor, which mainly stops the cathodic reaction [38,39,40].

4. Conclusions

We accurately and quickly measured the corrosion rate by immersing ASTM A36 steel samples in various corrosive media for 168 h. This enabled the evaluation of the inhibitor’s effectiveness in various corrosive media. The guayusa inhibitor works best in saline environments with not too much acid because as the pH of the corrosive environment rises, it becomes less effective. It was found to be 64.94% effective in 5% NaCl, 54.53% effective in 5% NaCl + acetic acid, 21.17% effective in 1% HNO₃, and 12.96% effective in 10% HNO₃. So, the guayusa inhibitor’s ability to slow down corrosion in media that is not very aggressive suggests that it might be useful in situations where corrosion is not a big problem. The Tafel curves also show that the inhibitor is an anodic one that mainly stops the anodic reaction in different environments. The highest level of corrosion protection was found in salty environments.