Using Organic Substances as Green Corrosion Inhibitors for Carbon Steel in HCl Solution

Abstract

Acid pickling is a vital stage in metal manufacturing during which the material is susceptible to corrosion if the process is not appropriately managed. Adding green corrosion inhibitors to the acidic solution used is one solution to this critical problem that the industry faces today. This paper examines the application of two organic substances, tea tree essential oil and the expired drug Sinecod, as green corrosion inhibitors for carbon steel in concentrated chlorohydric acid. Corrosion behavior is evaluated using the weight loss method, potentiodynamic polarization, and electrochemical impedance spectroscopy for three inhibitor concentrations (1%, 2%, 3%, and 4%) and a Blank sample. SEM analysis was performed for surface analysis. The mechanism of inhibition was also investigated by fitting the electrochemical data to adsorption isotherms such as the Langmuir and the Freundlich models. The optimum concentration proved to be 4% for both substances, with inhibition efficiencies up to 90% in the case of tea tree essential oil and up to 60% in the case of expired Sinecod, showing that the inhibitor concentration and inhibitor efficiency are directly correlated in this case. The findings of this study show the possibility of using expired pharmaceutical compounds or natural extracts as corrosion inhibitors for the concentration of acid solutions used for industrial processing.

1. Introduction

Sustainable development has been one of the biggest challenges in the past decades from multiple viewpoints, including government, industry, and research [1]. Sustainability was first defined in 1987 by the United Nations Brundtland Commission as “meeting the needs of the present without compromising the ability of future generations to meet their own needs.” [2] (p. 9). This framework has evolved over the years into many legislations aimed at environmental protection and has fostered new research pathways for the development of innovative green technologies [3]. One of the sectors that these changes have actively targeted is the metallurgical industry, namely carbon steel manufacturing. Due to its numerous applications, there is a great demand for this high-quality material at the lowest cost [4,5]. Chemical descaling, also known as acid pickling, is a crucial step in manufacturing carbon steel, which cleans the metal surface by removing contaminants and impurities [6]. This method, however, raises environmental and economic concerns due to the highly corrosive acids used (sulfuric acid [7,8], hydrochloric acid [9,10,11], phosphoric acid [12]). Being a chemical method, the byproducts (spent acid, sludge, and spent rinse water) are considered hazardous to human health and to the environment [13]. During this processing stage, corrosion and overcorrosion of the metal surface can occur, resulting in material loss, durability issues, and increased maintenance costs. Mitigating all these drawbacks has been the forefront of research in the field, with results showing that adding corrosion inhibitors to the pickling solution can reduce the adverse effects of corrosion by up to 25% [14].

Right now, the industry relies heavily on synthetic corrosion inhibitors; however, their toxicity, rapid changes in legislation, and environmental protection practices require green solutions that are sustainable and environmentally friendly [15]. Thus, the research and development of green corrosion inhibitors have moved forward in recent years [16,17,18,19]. These substances adhere to green chemistry practices by using renewable feedstock, reducing the need for hazardous chemical synthesis, and designing safer chemicals that are effective and friendly to the environment [20]. Several organic substances have been identified as potential green corrosion inhibitors (Figure 1) for several metal substrates (copper [21,22], aluminum [23,24,25], steel [26,27,28]) and corrosive solutions (H₂SO₄ [29], HCl [23,30], seawater [28,31]).

Two of the seven organic compounds are the focus of this research. Plant extracts and expired drugs are substances easy to obtain with high inhibition potential due to numerous heteroatoms (e.g., S, N, and O) and functional groups found within their chemical structure, which enhance the absorption of the inhibitor to the metal substrate, protecting it from the acidic media [18]. According to the literature, the inhibition efficiency of several plant extracts and expired drugs has already been tested on steel in hydrochloric acid. These studies include Citrus x clementine peel extract [32], Fennel fruit essential oil [33], Lawsonia inermis extract [27], expired bupropion [34], expired lactulose drugs [35], and metamizole sodium drug [36].

Despite the extensive literature on this topic, previous research has shown that the interaction between various green corrosion inhibitors and the metal substrate is influenced by the concentration of the acidic solution, ranging from 0.1 M to 1.5 M. This research focuses on tea tree essential oil (TEO) and expired Sinecod (SIN) drug, which have been previously reported as efficient corrosion inhibitors for low carbon steel in 0.5 M H₂SO₄ solution [37], respectively, on aluminum in simulated sea water [31]. Industrial pickling is carried out with solutions of a much higher concentration, 5 M (18%) when hydrochloric acid is used, and 2 M (20%) for sulfuric acid [38]. This paper aims to enrich the knowledge on the topic of green corrosion inhibitors by testing two substances, tea tree essential oil and the expired drug Sinecod, in solutions of industrial concentration. Thus, the research evaluates the potential transferability from controlled laboratory environments to industrial settings, as well as the inhibition efficiency and adsorption mechanisms of the two substances.

2. Materials and Methods

2.1. Materials

As the literature shows, when choosing suitable substances for corrosion tests, their molecular structure can give information on their inhibitive potential [39]. On one hand, the expired Sinecod drug has as its main active ingredient butamirate citrate (Figure 2a), which contains polar groups such as hydroxy, ketone, ether, carboxyl, and tertiary amine in its chemical formula. On the other hand, the tea tree essential oil is a complex substance that has in its composition multiple compounds previously identified [40], out of which Figure 2b shows the six main ones. Here, the presence of the hydroxyl and ether groups can be observed along with multiple double bonds and benzene derivatives. The existence of these groups within the chemical structure of both tested substances suggested their inhibitory potential. For this work, both substances were commercially acquired from a local business; the Sinecod drug expired in 2023. To confirm the existence of functional groups, the substances were characterized by Fourier-transform infrared spectroscopy (FTIR).

Steel grade 35 is widely used in the machine-building and construction sectors due to its excellent performance and affordable cost. This steel is used to make a variety of shafts, cylinders, gears, axles, discs, and traverses, as well as steel structures (shaped and reinforcing bars). Steel coupons used for corrosion testing were of C35 carbon steel provided by a local steel manufacturer. The chemical composition of the samples is as follows: 0.37% C, 0.24% Si, 0.56% Mn, 0.022% Ni, 0.018% S, 0.011% P, 0.20% Cr, 0.15% Cu, and balance Fe.

These coupons were tested in a solution of 5.6 M HCl, obtained by mixing 36% HCl (purchased from Sigma Aldrich, St. Louis, MO, USA) with distilled water in a 1:1 ratio.

One of the most widely used acids for acid pickling is hydrochloric acid because the working temperature as well as the solution concentration are lower than those of sulfuric acid [41]. The chemical equations that govern the pickling process when using carbon steel as a metallic substrate are shown in Equations (1)–(3), while the overcorrosion is represented by Equation (4), where the corrosion inhibitor intervenes to protect the substrate [42].

$${Fe}_3{O}_4+8HCl \to {2FeCl}_3+{FeCl}_2+{2H}_{2}O$$

(1)

$${Fe}_2{O}_3+6HCl \to {2FeCl}_3+{3H}_{2}O$$

(2)

$$FeO+2HCl \to {FeCl}_2+H_{2}O$$

(3)

$${Fe+2HCl\to FeCl}_2+H_{2 }\uparrow$$

(4)

2.2. Molecular Anaysis—FTIR

FTIR (Fourier-Transform Infrared Spectroscopy) measurements were performed to analyze the functional groups present in the green corrosion inhibitors in their liquid state. The spectra were recorded using an Bruker Tensor 27 FTIR (Bruker, Viena, Austria), over the range of 4500–500 cm⁻¹ with a resolution of 4 cm⁻¹. A small amount of the liquid inhibitor was placed directly onto the ATR (Attenuated Total Reflectance) crystal for analysis. The measurements were conducted at room temperature without any prior dilution or chemical treatment.

2.3. Gravimetric Analysis

Before being immersed in the pickling solution, the steel coupons (50 mm × 50 mm × 1 mm) were measured, washed of all impurities, and weighed. The total immersion time was 8 h; every 2 h, each sample was removed from the solution, washed, dried, and weighed again on an analytical balance to track the evolution of the corrosion rate within the experiment’s time frame. The inhibitor concentration used was calculated as a volumetric percentage (1%, 2%, 3%, and 4%) for both inhibitors, and the results were compared with a sample containing no inhibitor called Blank. This method of assessment follows the guidelines of ASTM NACE/ASTM169/G31-12a standard [43].

2.4. Electrochemical Analysis

An OrigaFlex 500 equipment (OrigaLys Electrochem SAS, Rillieux-la-Pape, France) was used for electrochemical tests, which included electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). All electrochemical measurements were carried out at room temperature (20 °C). The EIS measurements were carried out over a frequency range of 10 kHz to 10 mHz, with 5 points per decade, at an amplitude of 10 mV around the open-circuit potential value. The electrochemical cell used consisted of three electrodes: a carbon steel coupon as the working electrode, a reference electrode (Ag/AgCl), and a platinum sheet as the counter electrode. Before each measurement, the exposed surface of the working electrode was polished with emery paper, then cleaned with alcohol. To obtain the open circuit potential (E_OCP), the electrode was left in each tested solution for one hour prior to further EIS and PDP testing. The scanning rate used for the PDP test was 10 mV/min, with a potential range of ±500 mV vs. E_OCP. For data interpretation, OrigaMaster 5, ZView 4.1b, and OriginLab 2025b software were used.

The experiments were performed three times for each electrolyte solution, with the results being consistent and within the same range. The reported results are from the second experiment across the board, and it was preferred over the average value due to the low number of experiments.

2.5. Surface Analysis

Morphological studies of the C35 surface were conducted in the form of SEM (Scanning Electron Microscopy) microscopy, with a Jeol 5600-LV microscope(JEOL GmbH, Freising, Germany). The steel samples were immersed for 24 h in solutions with and without the green corrosion inhibitor at the concentration that yielded the best inhibitor efficiency. During the immersion, the temperature was maintained at 20 °C. Following exposure, the samples were rinsed with distilled water, air-dried at room temperature, and examined without any additional treatment.

3. Results and Discussion

3.1. Molecular Characterization

Figure 3 presents the FTIR spectroscopic analysis of expired Sinecod and tea tree essential oil. For the expired Sinecod, the characteristic bands detected at 3285 cm⁻¹ are attributed to the stretching vibrations of -OH groups present in alcoholic compounds. The small peak observed at 2944 is also attributed to hydroxyl groups; however, it indicates a weaker bond. The 1638 cm⁻¹ peak is indicative of an aromatic compound paired with the 909 cm⁻¹ peak, which indicates an out-of-place C-H bond characteristic of an aromatic substitution. The 1408 cm⁻¹ peak shows the presence of carboxylic acid. The aliphatic ether can be characterized by the 1029–1113 cm⁻¹ peaks. Looking at the spectra of the tea tree essential oil (Figure 3b), the peaks related to C-H and CH₃ bonds at 2956 cm⁻¹ and 2873 cm⁻¹ can be seen. There are also peaks attributed to the presence of alcohol (O-H bending at 1383 cm⁻¹) and tertiary alcohol (C-O stretching at 1159 cm⁻¹). The peak around 1064 is indicative of the C-O stretching of an aliphatic ether. The medium peaks between 954 cm⁻¹ and 856 cm⁻¹ indicate the presence of alkenes, while the peaks around 794 cm⁻¹ correspond to the C-H bending from a 1,4-disubstituted benzene. Overall, both substances contain functional groups that could facilitate the adsorption on the metal surface and hint towards the possibility of using them as green corrosion inhibitors.

3.2. Gravimetric Data Analysis

Gravimetric measurements, commonly referred to as the weight loss method, are the most straightforward approach for determining the efficacy of an inhibitor and the optimal concentration of the tested substance. This technique is extensively employed in corrosion inhibitor research [44,45,46]. Due to its cost-effectiveness and the absence of complex equipment requirements, it is also a preferred method for industrial settings. Based on the mass loss data, the corrosion rate can be calculated (Equation (5)) and its evolution observed.

$$V_{corr}={\displaystyle \frac{\mathsf{\Delta }m}{tS}}$$

(5)

where Δm is the difference between two successive measurements (g), t is the time of immersion (h), and S is the contact surface (cm²). Having the corrosion rate, the inhibitor efficiency can be calculated with Equation (6) [33,37,47].

$${IE}_{GAV} \left(\%\right)={\displaystyle \frac{V_{corr}^B-V_{corr}^i}{V_{corr}^B}}\times 100$$

(6)

where $V_{corr}^B$ is the corrosion rate of the Blank sample and $V_{corr}^i$ is the corrosion rate of the samples containing the tested corrosion inhibitor.

In Figure 4, the corrosion rate for both substances decreases once the inhibitor concentration is increased from 1% to 4% volumetric concentration, with the corrosion rate approaching 0 for all samples at the last measurement, after 8 h. This demonstrates that SIN and TEO have the ability to inhibit the corrosion of carbon steel at the studied concentration. However, the proximity to the 0 value of the corrosion rate does not mean an eliminated corrosion attack on the metallic substrate. By removing the samples from the solution and washing them, the protective film that forms on the metallic surface is also removed, resulting in reduced corrosion inhibitor consumption during the experiment time frame. This phenomenon can be observed by examining the 1% SIN sample in Figure 4a, which shows an increase in corrosion rate between the 6 h and 8 h marks. This sample also exhibited the lowest inhibitor efficiency across the board, at around 10%, which can be attributed to the chemical nature of the expired drug. The following test concentration showed an increase in the IE parameter (from 10% to 60%), demonstrating a direct correlation between the inhibitor’s concentration and its efficacy in inhibiting corrosion. The same can be observed for the TEO inhibitor, which reached the best inhibitor efficiency at 80%.

When comparing the two substances, it can be concluded that the tea tree essential oil is a better inhibitor for carbon steel immersed in highly concentrated hydrochloric acid at the same volumetric concentration (Figure 5). This can be linked to the very different chemical compositions of the two, and to the fact that tea tree essential oil has multiple molecules in its structure compared to the Sinecod expired drug, whose central molecule is butamirate.

3.3. Electrochemical Impedance Spectroscopy Data Analysis

The Nisquist plots are presented in Figure 6, while Figure 7 shows the equivalent electrochemical circuit used to fit the experimental data. The impedance spectra are of similar allure across the board and are characterized by a single capacitive loop with varying diameters. The lack of a perfect impedance semicircle can be attributed to the fact that, despite sanding the samples prior to each measurement, total surface homogeneity is unattainable under the conditions of this experiment, as well as in an industrial setting [48]. The diameter of the semicircle increases with the increase in inhibitor concentration, which can be attributed to the adsorption of the corrosion inhibitor on the metallic substrate, thereby creating a protective layer at the metal/electrolyte interface. Consistent with the gravimetric findings, tea tree essential oil is a more effective inhibitor for carbon steel than expired Sinecod, achieving a maximum value of the recorded polarization resistance of 70.87 Ω cm², compared to 25.61 Ω cm² for SIN.

The same conclusion can be drawn after fitting the Nisquist plots with the help of ZView software and quantifying the various electrochemical parameters such as solution resistance R_s, resistance of polarization R_p, and the value of a constant phase element CPE. All calculated parameters from the fitting of the EIS data exhibit errors of less than 2%, according to the ZView software’s output.

This element was used in place of a pure capacitor (C_dl) to account for the lack of homogeneity of the working electrode. This is an acceptable simplification of the circuit because, in practical cases, such as industrial pickling, no external current is applied. The impedance function of the CPE element can be mathematically expressed by Equation (7). These parameters were further used for the calculation of the double-layer capacitance with the help of Burg’s formula (Equation (8)) [49].

$$Z_{CPE}=\left[Y\right(j\omega {)}^n{]}^{-1}$$

(7)

$$C_{dl}=({R_p}^{1-n}Y{)}^{{\displaystyle \frac{1}{n}}}$$

(8)

where Z_CPE is the capacitance of CDE calculated by the software, ω = 2πf is the angular frequency, j = √(−1) is the imaginary unit, Y is the magnitude of the CPE, and n is a fitting parameter (0 ≤ n ≤1), which measures the element deviation from the ideal capacitive behavior (showing n = 1), and R_p is the polarization resistance.

Having these parameters, the inhibitor efficiency from the EIS data can be calculated with Equation (9). The values of the inhibitor efficiency are consistent with the ones obtained with gravimetric data 86% being the best inhibitor efficiency in the case of 4% TEO and 61% when using SIN as corrosion inhibitor (

Table 1. Electrochemical parameters estimated from the EIS spectra of carbon steel in HCl solution in the presence and absence of different concentrations of TEO and SIN.

Sample	Concentration (%)	Rs (Ω cm2)	Rp (Ω cm2)	CPE (μFsn−1 cm−2)	n	Cdl (μFcm−2)	IEEIS (%)
Blank		3.19	9.86	903.63	0.73	2611.563	-
SIN	1	3.20	11.26	549.30	0.81	425.733	12.43
	2	3.27	17.47	638.95	0.77	1034.041	43.56
	3	3.20	20.56	491.13	0.78	661.594	52.04
	4	3.18	25.61	946.70	0.82	864.341	61.49
TEO	1	2.98	21.78	660.67	0.75	1606.936	54.72
	2	3.38	37.85	496.57	0.74	1576.421	73.94
	3	3.15	43.944	458.89	0.74	1493.270	77.56
	4	3.05	70.84	667.20	0.73	2282.710	86.08

).

$${IE}_{EIS}\left(\%\right)={\displaystyle \frac{R_p-R_p^B}{R_p}}\times 100$$

(9)

where R_p and $R_p^B$ The polarization resistance is measured with and without the inhibitor present in the electrolyte solution.

The increase in the R_p parameter from 11.26 Ω cm² (1% SIN) to 25.61 Ω cm² (4% SIN), respectively, from 21.78 Ω cm² (1% TEO) to 70.84 Ω cm² (4% TEO) implies an increased adsorption of the inhibitor molecules on the steel surface which leads to a the formation of a protective film on the metal surface and in some cases to the thickening of the film, both phenomena providing a better protection against the corrosive electrolyte. Such findings have been previously reported in the case of organic coatings [50] and other organic substances [51,52,53] tested as green corrosion inhibitors.

Due to the fact that a minimum number of measured values per point is 50, no measuring data statistical analysis was performed (e.g., dispersion, etc.), so the maximum value for the measurement error was considered in terms of maximum absolute value given by the potential/current accuracy of the instrument, which is lower than 0.1%.

3.4. Polarization Data Analysis

The polarization curves (i vs. E) of the working electrode (carbon steel) in 5.6 M HCl, with and without various inhibitor concentrations, are presented in Figure 8. It can be observed that, in the case of both inhibitors, their presence in the pickling solution shifts the cathodic branch to lower current values compared to the Blank sample, while the anodic branch remains within the same values. This decrease in the cathodic current indicates the inhibitors are effective on the cathodic process. Compared to the Blank sample, the slope of the cathodic branch (β_c 

Table 2. Electrochemical parameters obtained from the Tafel extrapolation of polarization measurements of carbon steel in HCl solution in the presence and absence of different SIN and TEO concentrations.

Sample	Concentration (%)	Ecorr (mV vs. Ag/AgCl)	icorr (mA/cm2)	|βc| (mV/dec)	βa (mV/dec)	IETaf (%)
Blank	0	−409.9	8.9812	444.6	268.8	-
SIN	1	−427.5	6.7666	223.7	123.1	24.64
	2	−428.4	4.7714	195.5	105.0	46.86
	3	−427.5	3.7849	216.4	185.5	57.85
	4	−413.2	2.8895	190.4	126.2	67.82
TEO	1	−427.6	3.5047	207.0	133.3	60.97
	2	−433.9	2.4443	216.3	144.3	72.78
	3	−445.1	1.6449	241.5	159.5	81.68
	4	−497.7	0.9956	200.1	145.5	89.36

) changes, from 444 mV/dec to 200 mV/dec, which indicates an adsorption of inhibitor molecules at the cathodic reaction sites. This slows the electron transfer between the metallic surface and the electrolyte, contributing to the inhibition of the corrosion process. The anodic branches and the anodic slopes remain within the same parameters (260–100 mV/dec), showing that the presence of the tested green corrosion inhibitors did not affect the kinetics of the anodic reaction.

The electrochemical parameters obtained from these measurements, such as corrosion current density (i_corr), corrosion potential (E_corr), and anodic and cathodic slopes (β_a, β_c), were computed by the Tafel extrapolation method and are presented in Table 2. Using Equation (10), the inhibitor efficiency was calculated.

$${IE}_{Taf}\left(\%\right)={\displaystyle \frac{i_{corr}^B-i_{corr}}{i_{corr}^B}}\times 100$$

(10)

where $i_{corr}^B$ and $i_{corr}$ are the corrosion currents in the absence and in the presence of the green corrosion inhibitors, respectively.

As the previous tests have shown, the inhibitor efficiency increases with the concentration of the inhibitors. The corrosion current density (i_corr) reaches 2.89 mA/cm² when using expired Sinecod as a corrosion inhibitor, and 0.99 mA/cm² when adding tea tree essential oil to the acidic solution. The decrease in this parameter can be directly correlated to the increase in inhibitor concentration, showing a direct link between the two. This behavior can also be associated with the adsorption of the inhibitor molecules to the metallic surface [54]. In line with previous findings, TEO is a more effective overall corrosion inhibitor under experimental conditions, achieving an 89% corrosion inhibition efficiency. Research data shows that a shift in the corrosion potential E_corr higher than ±85 mV against the Blank sample translates to a predominantly anodic or cathodic inhibitor [55]. A lower shift in this parameter suggests a mixed-type inhibitor that influences both reactions and their reaction rates, with a dominant anodic effect [56]. In the present study, the Sinecod expired can be considered a mixed type inhibitor, ΔE_corr being 20 mV towards more negative values, while the tea tree essential oil can be classified as a cathodic type inhibitor since the difference between the corrosion potential between the Blank sample and the 4% TEO sample is 87 mV. This supports the notion of TEO affecting the kinetics of the cathodic reaction previously discussed.

3.5. Inhibitor Efficiency in the Context of Existing Literature

Overall results of inhibitor efficiency across all tests included in this study are presented in

Table 3. Inhibitor efficiency across all measurements.

Sample	Concentration (%)	IEGAV (%)	IEEIS (%)	IETaf (%)	IEavg (%)
SIN	1	9.68	12.43	24.64	15.58
	2	37.37	43.56	46.86	42.59
	3	47.97	52.04	57.85	52.62
	4	59.49	61.49	67.82	62.93
TEO	1	50.68	54.72	60.97	55.45
	2	68.34	73.94	72.78	71.68
	3	71.27	77.56	81.68	76.83
	4	82.92	86.08.	89.36	86.14

, as well as a calculated average inhibitor efficiency. The difference values from one test to another remain in the ±10% range, except for samples tested in solutions containing 1% expired Sinecod. These differences can be attributed to the fact that the electrochemical methods are localized from a surface point of view and sensitive to the heterogeneities of the surface sample. Gravimetric analysis considers the entire sample, minimizing the effect of pitting corrosion on the results. At the same time, gravimetric analysis is a long-term method, with effects over time getting integrated in the same final value, while EIS and PDP require a shorter time, capturing the electrochemical system in its early or transient stages. These are reasons why such methods are used complementarily for a comprehensive assessment of the inhibitive performance of the studied compounds [57].

Comparing the results of the present study with previously reported results, it can be concluded that these inhibitors prove to be efficient in more than one corrosive medium across different metallic substrates. Al-Ghaban et al. tested the butamirate drug on aluminum in artificial sea water obtaining a maximum corrosion inhibition efficiency of 84% at 24 mL/L concentration and at a temperature of 54 °C [31]. As reported by this research expired Sinecod has an average efficiency of 62% at an approximate 40 mL/L concentration in a highly corrosive hydrochloric acidic solution (5.6 M) at room temperature (20 °C). These results are complementary in nature as they both offer information on butamirate behavior in different testing conditions and offer transferability data in terms of the inhibitor used.

Tea tree essential oil has been reported as a successful corrosion inhibitor for mild steel in diluted H₂SO₄ solution. The best inhibitor efficiency (96%) was obtained at a concentration of 3.5% volumetric concentration. In the present study, the best inhibitor efficiency was obtained at 4% with an average inhibitor efficiency of 86%. The 10% difference in efficiency could be attributed to the acidic nature and concentration (5.6 M HCl vs. 0.5 M H₂SO₄) [37].

Corrosion is a complex process that is dependent on the experimental conditions, so a comprehensive assessment of the corrosion inhibitor’s behavior needs to consider multiple metallic substrates and corrosive media of different natures and concentrations. This information facilitates decision making when choosing corrosion inhibitors and shows industrial stakeholders that the option of green corrosion inhibitors obtained from recycling (e.g., expired drugs) or natural extraction (e.g., essential oils) is a viable option. Reports on such substances make it easier for them to compare existing data from within their processes and consider such alternatives.

3.6. Surface Analysis

A standard surface analysis technique used in corrosion inhibitor studies is SEM microscopy, used to characterize the surface morphology of the metallic surface and to visually show the effectiveness of the corrosion inhibitors studied. Presented in Figure 9 are the results of the SEM analysis on samples immersed in electrolytic solutions for 24 h and before immersion (Figure 9a). This sample presents a smooth surface with fine parallel polishing marks and minor corrosion sites (pitting corrosion circled), showing that the polishing was not performed in an in-depth manner, as is often the case in industrial settings during the intermediate stages of the production process. Figure 9b is characterized by a rough and cracked surface with evident corrosion damage due to the lack of protection and the high concentration of the acid. The deep etching features that can be observed confirm heavy material loss during immersion. On the contrary, sections (c) and (d) of Figure 9 show that the steel specimens withstand corrosion to different degrees. When using expired Sinecod as a corrosion inhibitor, Figure 9c, the surface shows signs of non-uniform partial protection (reported inhibitor efficiency being at 60%). Localized areas are evidence of partial film formation; however, pitting corrosion still occurs (circled area). The best inhibitor efficiency was obtained when using the tea tree essential oil. It is apparent from Figure 9d that the surface is relatively smoother as compared to the other samples, the layer formed on the metallic surface appearing to be more continuous, and smaller pitting corrosion regions can be observed (circled).

SEM analysis demonstrates a progressive decrease in corrosion severity from the sample immersed in solution containing 4% TEO to that containing 4% SIN, with the most extensive corrosion attack observed on the uninhibited (Blank) sample. The formation of a protective film is morphologically evidenced by the reduced surface degradation and the development of a more uniform surface layer, particularly in the sample exposed to the 4% TEO solution. These morphological findings corroborate the proposed inhibition mechanism, which is primarily attributed to the adsorption of inhibitor molecules onto the metal surface, resulting in the formation of a protective barrier.

3.7. Adsorption Isotherms

Literature analysis reveals that corrosion inhibition primarily occurs through the development of a protective film on the metal surface, a phenomenon governed by the adsorption of inhibitor molecules onto the metallic substrate. There are three primary methods by which these molecules can be adsorbed: physical adsorption, chemical adsorption, and mixed-type adsorption. To determine which is the case when using the expired drug Sinecod or the tea tree essential oil as green corrosion inhibitors for carbon steel in highly concentrated hydrochloric acid solution, adsorption isotherms were drawn following the Langmuir, Frundlich, and Temkin adsorption models (

Table 4. Adsorption isotherm modelling of SIN and TEO in HCl solution.

Isotherm Model	Linear Form	Plot
Langmuir	${\displaystyle \frac{C_{inh}}{\theta }}={\displaystyle \frac{1}{K_{ads}}}+C_{inh}$	${\displaystyle \frac{C_{inh}}{\theta }} vs. C_{inh}$
Frundlich	$log\theta =Log{K}_{ads}+{\displaystyle \frac{1}{n}}\ast log{C}_{inh}$	$log\theta vs.log{C}_{inh}$
Temkin	$\theta =-{\displaystyle \frac{2.303}{2\alpha }}{C}_{inh}-{\displaystyle \frac{2.303}{2\alpha }}ln{K}_{ads}$	$\theta vs.log{C}_{inh}$

where C_inh is the inhibitor concentration, K_ads is the absorption coefficient, n is a coefficient of value between 1 and 10 that indicates the ease of the adsorption process, and α is a parameter that describes the molecular interactions.

). The surface coverage θ is expressed as the inhibitor efficiency obtained from the EIS data analysis. From the drawn plots, the adsorption constant (K_ads) was determined.

Figure 10 shows the results in each case, also displaying the correlation coefficient R² used to select the most suitable model. The defining parameter of the adsorption characterization is the value of the free adsorption energy, calculated with Equation (11).

$${\mathsf{\Delta }G}_{ads}=-RTln\left(55.5K_{ads}\right)$$

(11)

where R is the universal gas constant, T is the temperature, and K_ads is the absorption coefficient.

Three types of interactions can occur between the charged metal surface and the protonated inhibitor molecules, characterized by the value of the free energy. If the attractions are weak (ΔG_ads < −20 kJ/mol), the primary mechanism is that of physical adsorption as opposed to a stronger interaction between the metal surface and the inhibitor molecule (ΔG_ads > −40 kJ/mol) characterized by the presence of covalent, hydrogen, or electrovalent bonds. Having a ΔG_ads between −20 kJ/mol and −40 kJ/mol shows the presence of both types of adsorptions, making the interaction a mixed type one [58].

Based on the values of the correlation coefficient (R² = 0.995), it can be determined that the expired drug Sinecod follows the Temkin adsorption model. The value of parameter α in this case is positive, at 0.79, indicating an attraction at the molecular level between the inhibitor and the metal surface, as opposed to a negative value of α, which translates to repulsion between the two. Looking at the free energy parameter, ΔG_ads has a value of −21.62 kJ/mol, which indicates that the inhibitor’s adsorption on the metallic surface is spontaneous, as well as the presence of both chemisorption and physisorption at the metal surface.

Tea tree essential oil follows the Langmuir adsorption isotherm (R² = 0.992), indicating the presence of chemical bonds between the corrosion inhibitor and the carbon steel surface, resulting in the formation of a passivation layer that protects the metal from corrosive media. The calculated free energy is −20.91 kJ/mol, which supports the mixed-type adsorption characterization, with slightly more physisorption sites, bringing the value close to the −20 kJ/mol threshold.

3.8. Corrosion Inhibition Mechanism

Based on the experimental results and the existing research on the topic of green corrosion inhibitors and their inhibition mechanism [59,60,61], it can be concluded that the inhibition mechanism of both substances tested in this study is a complex one involving both physical and chemical adsorption processes. The defining factors of these interactions are the molecular structure and the composition of the used electrolyte. Therefore, this paper proposes an inhibition mechanism for carbon steel immersed in a 5.6 M HCl solution with the tested inhibitors. Figure 11 shows the schematic diagram of the proposed inhibition mechanism. Both inhibitors have aromatic rings and functional groups with isolated pairs of electrons and π electrons in their molecular structure, which interact well with iron. This is because its valence electron arrangement is 3d⁶4s², allowing it to accept electrons in its d orbitals, facilitating the adoption process. At the same time, the d-orbital of Fe interacts with the delocalized π electrons of the aromatic rings, specifically the carbon double bond (C=C), forming a donor-acceptor connection, also known as retrodonation.

4. Conclusions

This work contributes to the extensively researched topic of green corrosion inhibitors by testing known inhibitive substances under industrial-like conditions, bringing the research one step closer to industrial transferability. Expired Sinecod and tea tree essential oil proved to be efficient corrosion inhibitors for carbon steel immersed in a hydrochloric acid solution of industrial concentration. The findings show that TEO is a better overall inhibitor than SIN, with an efficacy of up to 90% at the optimal concentration (4% wt), compared to the 60% reached with SIN at the highest concentration (4% wt). Potentiodynamic polarization investigations revealed that both inhibitors can be considered mixed-type inhibitors, with a slight preference for affecting the anodic reaction. Electrochemical impedance spectroscopy spectra demonstrated that the inhibitors successfully adsorb on the carbon steel surface, decreasing the capacitance of the double layer and increasing the resistance to the charge transfer. These findings were corroborated by morphological assessment by means of SEM microscopy. This adsorption mechanism was further explored using adsorption isotherms, of which SIN followed the Temkin model while TEO followed the Langmuir model. With all this data, a corrosion inhibition mechanism was proposed that considers both physical and chemical adsorption of the inhibitor molecules on the metal surface. These findings demonstrate that expired Sinecod and tea tree essential oil can be utilized as green corrosion inhibitors for C35 steel in industrial pickling solutions.