Paprika extract: a green inhibitor for mitigating carbon steel disintegration in 1 M HCl pickling solution

ABSTRACT In 1 M HCl pickling media, the paprika extract was used as an ecofriendly anti-corrosive material for carbon steel. Many strategies were utilized to estimate the mitigation efficacy such as potentiodynamic polarization (PP), frequency modulation (EFM), electrochemical impedance spectroscopy (EIS), and mass loss. Surface analyses were accomplished by scanning microscopy (SEM) and dispersion X-ray (EDX) techniques. The mitigation efficacy ameliorates with the concentration of extract and temperature. The inhibition is ascribed to the building of a protective coating at the surface of carbon steel that reduces its dissolution. The adsorption of Paprika extract on steel metal satisfied Langmuir, s model. The adsorption belongs to the physical and chemical types. The adsorption of the extract affected both anodic and cathodic reactions. Tafel curves demonstrated that the Paprika is a mixed inhibitor. The Nyquist curves confirmed that Paprika extract prohibits the disintegration of steel in acid media without changing the dissolution reaction mechanism. The adsorption of the Paprika extract on the carbon steel metal was confirmed by ATR - FTIR analysis and SEM examination. Using 300 ppm of Paprika extract, a maximum inhibitory efficacy of 95% was observed at 313 K. GRAPHICAL ABSTRACT


Introduction
Using carbon steel as a structural material in industrial sectors has been a major challenge for chemical engineers and researchers in recent years. The majority of the acidic industry applications like pickling, industrial spotless, refining crude oil, descaling, and petrochemical processes utilize carbon steel as an essential material. Also, alloys of carbon steel are cheap and have good characteristics over the other metal alloys. In the industrial sector, HCl is widely applied for scrubbing and descaling processes. The main problem concerned with this process is the fact that carbon steel rusts when exposed to the acid environment. As a result, the use of inhibitors to prevent metal disintegration would be unavoidable if steel was to be memorized in acid environments.
The majority of organic compounds contain heteroatoms and exhibit anti-corrosive properties; however although, the preparation of these compounds is costly and venomous to humans and the environment. Because eco-friendly compounds are substantially less expensive than classic compounds, scientists have focused on their utilization. One of these categories of compounds is plant extracts. The plant extracts are biodegradable, easily obtainable, and renewable resources of materials. Besides, they are green inhibitors and free from heavy metals or other venomous compounds [1][2][3].
Paprika extracts are generally utilized as natural flavoring and coloring agents in many foods. Also, it is used dried or fresh in different pharmacological preparations. It has long been used as a source of biologically effective compounds, like flavonoids, carotenoids, phenols, capsaicinoids, and vitamins. Capsicum fruits contain coloring pigments, resins, protein, cellulose, mineral elements, and quite small volatile oil, whilst the seeds contain fixed (non-volatile) oil. Triglycerides make up the majority of the fixed oil (approximately 60%), with linoleic acid and other non-saturated fatty acids dominating.
The active ingredient in Paprika is capsaicin, a homovanillic acid derivative (8-methyl-N-vanillyl-6-nonenamide, Figure 1). The quantity of the capsaicin in the Paprika is about 0.025%, and in the hot pepper attains 0.25% [30]. It is a many-sided agent, and it is utilized in a diversity of scopes like pharmacology, nourishment, chemical arms, and shark fend off [31]. Capsaicin has a high molecular mass (305.418 g/mol) and various groups such as NH, OH, and C = O that can coordinate with different metals via free electrons. As a result, we hypothesized that Capsaicin, which is present in Paprika extract, could serve as a useful inhibitor for reducing carbon steel dissolution in pickling HCl solution.
The goal of this research is to examine how Paprika extract reduces carbon steel disintegration in a 1M HCl pickling solution. The adsorption performance of ecofriendly environment Paprika extract on carbon steel was investigated. The study also comprises the impact of temperature on inhibition efficacy. In addition, the surface of carbon steel samples devoid of and with Paprika extract was examined.

Steel coupons
Carbon steel with the following composition (wt. %): 0.19 C, 0.0026 Si, 0.024 P, 0.34 Mn, and 99.4 Fe was used for the measurements. Carbon steel sheets with dimensions 2 × 2 × 0.2 cm 3 were polished with various grades of silicon carbide emery boards, degreased with petroleum ether, and scrubbed with bi-distilled water. Eventually, the specimens were desiccated at room temperature and weighed by analytical balance [14]. The working electrode utilized in PP, EIS & EFM measurements having an exposed surface area of 1 cm 2 .

Preparation of pickling solution
1 M HCl is used as a corrosive medium for carbon steel specimens. It was prepared from analytical grade condensed 32% HCl, and doubly distilled water.

Making the Paprika extract
The Paprika plant was dried and milled to a fine powder using an electrical mill at room temperature. 200 mg of Paprika powder was soaked in 800 mL of methanol (100%). Under vacuum, the extract was removed and dried by a rotary evaporator. The extract was liquefied in absolute ethanol (1 g/L) and kept in the refrigerator [32].

Mass loss measurements
The weighed steel coupons were submerged in 100 mL of 1 M hydrochloric acid devoid of, and with different quantities of Paprika extract for 180 min. At different time intervals (30 min), the specimens were taken away from a solution, washed, dried, and finally weighed precisely. To ensure reproducibility, each experiment was repeated at least three times. The corrosion rate (CR) was determined by the subsequent equation: Where CR is the rate of corrosion (mg/cm 2 /min), W 1 is the mass of coupon before submersion, and W 2 is the mass after. S is the area of the coupon, & t is the exposition time.
The mitigation efficacy (η%) can be calculated from the calculated CR values through the given equation [33]: Where CR 1 , and CR 2 are the corrosion rate values devoid of, and with distinct quantities of Paprika extractor, respectively.

Electrochemistry studies
The electrochemistry studies were accomplished by using Potentiostat / Galvanostat / ZRA. A standard glass cell with a Pt-counter electrode (CE), and a saturated Hg 2 Cl 2 electrode (SCE) was utilized. The working electrode (WE), carbon steel rod, was pressed into a poly tetra fluoro ethylene (PTFE) holder, exposing 1 cm 2 of surface area to the corrosive media. Before any experiment, the WE was immersed in corrosive acid for 30 min to obtain a stationary-open potential. The polarization curves were swept at a rate of 1 mV s −1 from a starting potential of −500 mV to a final potential of +500 mV vs. SCE [34]. EIS tests were established in a wide scope of frequency 100 kHz -10 MHz using a sine wave with a magnitude of 10 mV [35][36][37][38][39][40]. Frequency modulation (EFM) is a promising technique with high sensitivity and accuracy due to the measurement of corrosion parameters at harmonics and intermodulation of input frequencies. Two different frequencies were used for electrochemical modulation: 2, and 5 Hz. The undermost frequency was 0.1 Hz that the waveform renews every one second. The spectrum includes current responses, ascribed to harmonically, and inter-modification current peaks. The larger humps were accustomed to estimate the current of corrosion (i), the slopes of Tafel, and also the causality factors [41].

Attenuated total refraction infrared (ATR-FTIR) analysis
ATR-FTIR spectrum of the Paprika extractor and the coating created at the steel surface was investigated using Thermo Fisher Scientific (Nicoleti S10 model) within a wavenumber scope of 400-4000 cm −1 .

Surface examination
SEM analysis was carried out to gain insight into the changes at the carbon steel surface before and after the addition of Paprika extract. The coupons were polished, washed, and immersed in 1M HCl for 24 h in the absence and presence of 300 ppm Paprika extract at 298 K. A JEOL JSM-5500 scanning electron microscope was used.

Mass loss tests
The mass loss of steel coupons versus time was studied in the pickling solution, 1M HCl, utilizing various quantities of Paprika extract for 120 min. According to Table 1, the mass loss of steel coupons decreases with increasing the extract concentration. Along the same line, the mitigating efficacy also increases with the concentration of extract [42]. Raising the temperature between 298 and 313 K improves the inhibition efficacy. The improvement of mitigating efficacy with increasing temperature is chiefly assigned to the chemical adsorption of Paprika extract from the steel surface at elevated temperatures.

Type of adsorption and chemical thermodynamics approach
The inhibition steps and the thermodynamic variables of the disintegration process can be elucidated and determined by adsorption isotherms. The Langmuir isotherm only fitted experimental data. Langmuir isotherm can be described by the subsequent relation: Where θ is the coverage of the surface; determined from the mass loss experiments, C inh is the inhibitor concentration and K ads is the adsorption equilibrium constant. The latter is computed from the intercept of the straight line on the (C inh /θ) versus C inh plot ( Figure 2). K ads estimated from the inverse of the intercept of isotherm lines are listed in Table 2. The Paprika extract is mightily adsorbed on carbon steel at different temperatures. The standard Gibbs energy change for the adsorption process, ΔG o ads , is concerned with adsorption equilibrium constant (K ads ) via the subsequent equation: Where R is the gas constant, T is the absolute temperature and 55.5 is the water concentration in the molarity unit. ΔG o ads magnitudes for the extract are negative, referring that the adsorption process proceeds from its own accord and the steadiness of the adsorbed layer. The magnitudes of ΔG o ads are smaller than -20 kJ mol −1 when the temperature is 308 K or less, indicating that the adsorption is physisorption. On the other hand, as the temperature attains 313 K, ΔG o ads turns into -24.39 kJ mol −1 , indicating that the adsorption is changed to chemisorption. As a result, the adsorption of Paprika extract on the surface of carbon steel can be classified as both physical and chemical adsorption. The adsorption heat, ΔH o ads , can be predestined in the light of the Van't Hoff equation: ΔH o ads magnitude is estimated from the slope of the linkage between ln K ads vs. 1/T, which is equal to -ΔH o ads /R. The magnitude of ΔH o ads is negative, glimpsing that the adsorption of the extract is accompanied by an evolution of heat. Entropy change of the inhibitor adsorption (ΔS o ads ) was estimated utilizing the subsequent equation: The acquired ΔS o ads values are scheduled in Table 2. As the temperature rises from 298 to 313 K, ΔS o ads increase from −25.1-7.64 J/K. This result indicates that the disordering increases with temperature as a result of the replacement of high numbers of water molecules by Paprika extract [43][44][45][46][47][48][49].

Inhibition mechanism
Adsorption of the inhibitor onto the metal surface is the first step in any corrosion inhibition reaction. The adsorption can occur via electron transfer to the metal surface (chemisorption) or via charge transfer from the charged inhibitor to the charged metal surface (physisorption).
Several studies on the adsorption of a corrosion inhibitor onto a metal's surface have found that the presence of hetero atoms (N, S, O, or P) in an aromatic system or long carbon chain, as well as π-electrons, will aid the adsorption of the inhibitor onto the metal surface. The adsorption of Paprika extract's components onto the steel metal surface may explain its inhibitory effect on corrosion. Because the adsorbed layer acts as a barrier between the metal surface and the acidic solution, the corrosion rate is reduced. According to the chemical characteristics of Paprika extract, its constituents include chemical organic compounds with hetero atoms and carbonyl groups rich in electrons, which serve as an excellent adsorption site on metal. The adsorption of these components resulted in a reduction  in the reactivity between steel metal and acid media (HCl), and hence a reduction in the rate of corrosion.

Potentiodynamic tests
The impact of Paprika extract on the V-I plots of carbon steel in 1M HCl is depicted in Figure 6. The Paprika extract inhibits the cathodic and anodic reactions. The corrosion current diminishes greatly with extract concentration. The extract effectively inhibits the disintegration of carbon steel metal in the pickling HCl solution. The cathodic polarization curves in Figure 6 show an analogous trend, referring that the adsorption of Paprika extract at the metal surface did not alter the reaction mechanism of the cathode. Meanwhile, the slopes of the anodic curves are varied; pointing out that the mechanism of the anodic reaction has altered. This alludes to that the adsorption of Paprika extract at the metal surface can mightily prohibit the dissolution process.
In the absence of Paprika extract, Clions played an important role in iron disintegration, as shown below: (FeCl) ads (FeCl) + + e (8)  (FeCl) + Fe 2+ + Cl Because the experiments were carried out in aerated acid solution, the cathodic reactions comprise both oxygen reduction and hydrogen evolution.
Even though, it has been reported [50] that the impact of dissolved O 2 reduction on iron dissolution is only noteworthy at pH values above 4. As a result, the cathodic H 2 evolution reaction was the main one in this work. Table 3 shows the magnitudes of the disintegration current (i), disintegration potential (E corr ), cathodic, and anodic Tafel slopes (β c ), (β a ), respectively. The presence of the Paprika extract in the pickling media reduces the disintegration current of steel metal. The prepared extract can be classified as a mixed-type [51,52]. Because the anodic and cathodic polarization curves are a symmetrical about E corr , The sum of β c and β a does not equal 1.
The mitigating efficacy can be estimated in step with the equation: Where, i (free) and i (inh) are the disintegration current densities devoid of and with extract, respectively. In step with Table 3, the mitigating efficacy ameliorates with the concentration of extract [53,54]. These results are in good consent with the results of the mass loss technique, which has been argued previously.

Impedance study
The purpose of EIS tests was to study the mechanism of the dissolution process. Nyquist & Bode-phase angle plots of carbon steel in 1M HCl devoid of, and with distinct quantities of Paprika extract is given in Figure 7. The Nyquist curves found a consistent trend, confirming that Paprika extract hinders steel disintegration in acidic media without modifying the dissolution reaction mechanism, Figure 7(a). The high-frequency loops are not ideal semicircles as anticipated from the hypothesis of EIS. The discrepancy can be expounded by the roughness and heterogeneity of the electrode surface [55]. It may be noticed in Figure 7(b), the Bode plot, that there is one peak only, appears within the phase angle, proving that the extract produces a time constant  when adsorbed to the surface carbon steel. The equivalent Randles circuit is demonstrated in Figure 8. The circuit comprises the charge transfer resistance R ct that is connected in parallel to the double-layer capacitance C dl [56,57]. The output of both is attached to the solution resistance R s . Our experimental findings are completely consistent with this model. The mitigation efficacy was estimated via R ct as follow: Where, R ct and R o ct are the charge transfer resistance devoid of and with Paprika extract, respectively. The impedance variables are illustrated in Table 4. The magnitude of R ct raises however, C dl values drop with increment the extract concentration. This is mainly due to the progressive exchange of H 2 O molecules by the extract molecules at the steel surface. This will minify the dissolution reaction. As the R ct values become higher the disintegration of the steel gets slower. The decrease in C dl values is assigned to the decreasing of the dielectric constant and rising of the electrical double-layer thickness. This result indicates that the extract molecules work by adsorption on the metal/solution boundary. The adsorption occurs via the interaction of the Paprika extract with adsorbed Clanions. Also, the interaction may be between the π-electrons or unpaired electron pairs of Capsaicin molecules and the empty orbitals of the metal [58,59].
The difference in the magnitudes of mitigation efficacy obtained from PP & EIS techniques is concerned with the variation in the surface state of the working electrode in the two techniques.

Frequency modulation tests
The modulation spectra that got from EFM tests for the disintegration of carbon steel in 1 M HCl devoid of and with distinct concentrations of Paprika extract is depicted in Figure 9.
The input frequencies and frequency constituents are included in the current response. The latter is the product of the multiples, differences, and sum of the two input frequencies. There are two major peaks at 0.2, and 0.5 Hz, which are the response to the excitation frequencies. The major peaks are used to compute the corrosion current (i), the Tafel slopes, and also the causality factors (CF-2&CF-3). Table 5 displayed the inhibition efficacy, Tafel constants, and the causality factors for distinct concentrations of the Paprika extract at 298 K. The corrosion current density decreases as the concentration of Paprika extract in the corrosive medium increases. This is due to the formation of a protective coating on the surface of carbon steel, which reduces dissolution. The interaction between the Paprika extract and the adsorbed Clanions causes the adsorption [60,61]. The values of the causality factors directly correlate to the theoretical ones 2 and 3. This indicates that the results obtained are of high quality.

ATR-FTIR analysis
ATR-FTIR spectroscopy is a rapid, nondestructive, noninvasive, reagent-free, cheap, sensitive, and highly reproducible physicochemical tool for characterization molecules. This technique is used to identify the adsorbed functional groups on the metal surface. Figure 10 illustrates the ATR-FTIR spectra of Paprika extract and the surface of carbon steel metal after soaking for 6 hrs in 1 M HCl in the presence of 300 ppm Paprika extract. The spectrum of Paprika extract includes the stretching -OH frequency at 3382 cm −1 , The stretching -C = O frequency at 1649 cm −1 , the bending SP3 -C-H frequency at 1398 cm −1 , and stretching -C-O frequency at 1054 cm −1 . The spectrum of steel metal after immersion in 1 M HCl + 300 ppm Paprika extract, on the other hand, is free of the previous functional groups. This suggests that the active functional groups in Paprika  extract (primarily Capsaicin) bind to the surface of the carbon steel metal and prevent it from dissolving.

Scanning electron microscope (SEM) examination
This investigation is carried out on the metal surface before and after immersion in a 1M HCl solution devoid of and containing 300 ppm of Paprika extract. Figure 11(a) depicts the morphology of a carbon steel metal surface after 6 h of exposure to a 1 M HCl solution.
It is possible to see that the entire steel surface is severely corroded due to the pitting action of chloride ions. The morphology of the steel surface treated with the 300 ppm Paprika extract improved significantly in Figure 11(b). The Paprika extract is responsible for the formation of a shield layer on the steel surface that prevents it from corrosion.

A comparison of the inhibition efficiency of
Paprika extract and other plant extracts in the literature for steel corrosion. Table 6 summarizes the comparison of the investigated Paprika extract and other plant extracts reported in the

Conclusions
In 1 M HCl, the paprika extract is an effective carbon steel disintegration inhibitor. The concentration of extract and temperature affect the effectiveness of inhibition.
As the Paprika extract concentration and temperature are increased, the inhibitory efficacy increases. Using 300 ppm of Paprika extract at 313 K, the maximum inhibitory efficacy of 95% was attained. The Langmuir model describes the adsorption of the extract on the carbon steel surface. Chemical and physical types of adsorption exist. The Paprika extract is strongly adsorbed on the carbon steel surface, as indicated by the negative sign of standard free energy change.
Paprika is a mixed inhibitor. The adsorption of the Paprika extract on the carbon steel metal was confirmed by ATR -FTIR analysis and SEM examination.
Other plant extracts are less effective as corrosion inhibitors than paprika extract.

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