Protoporphyrin Extracted from Biomass Waste as Sustainable Corrosion Inhibitors of T22 Carbon Steel in Acidic Environments

Abstract

Carbon steel is one of the most employed materials in many industrial sectors due to its unique physical and mechanical properties. However, within a certain period of time, carbon steel-based materials are susceptible to corrosion under operating conditions and corrosion inhibitors are important to extending the limit of use of carbon steel. In this study, the influence of protoporphyrin from animal blood hemin as an eco-friendly corrosion inhibitor for T22 carbon steel in an acidic environment (0.5 M HCl) was conducted. The hemin isolated from animal blood extracts was modified to obtain the protoporphyrin. The dosage of protoporphyrin was varied between 40 and 200 ppm and the temperature influence were studied in the range of 298–318 K. The inhibition efficiency of protoporphyrin in 0.5 M hydrochloric acid reached up to 46.2% at a dose of 160 ppm at a temperature of 298 K. The inhibition efficiency (IE) value further decreases with increasing temperature, thereby showing the process exothermic in nature and the weakening of the inhibitor molecules to adsorb on the surface of the T22 carbon steel. The potentiodynamic polarization measurements indicate that protoporphyrin acts as a mixed-type inhibitor. The adsorption of protoporphyrin on the surface of T22 carbon steel obeys the Langmuir adsorption isotherm. The thermodynamic parameter of adsorption allows us to suggest the adsorption process was dominated by physical adsorption. Thus, these current results present a case study using protoporphyrin as a promising green inhibitor for carbon steel T22 in hydrochloric acid prepared from livestock waste.

1. Introduction

Metals and carbon steel are broadly utilized in several industrial applications as a construction material since they have excellent mechanical and chemical properties combined with their economic viability. However, the assortment of applications makes these metals and steels susceptible to progressive destruction by corrosion due to contact with various corrosive environments such as the use of acidic solution [1]. The hydrochloric acid (HCl) solution is generally used as a pickling agent adopted in manufacturing industries for rust and sticky oxide removal, cleaning, descaling, grease etc. on the surface [2]. The cleaning of metallic materials is carried out periodically to prevent damage and maintain normal operation efficiency [3]. The chemical-based cleaning is selected rather than that of mechanical cleaning method because there is no need to reassemble metal equipment to be cleaned [4]. However, during the pickling there will be a loss of materials caused by metal degradation via an electrochemical reaction that leads to a huge economic loss, shortens the lifetime of equipment, produces product losses, wasting of resources, brittleness, environmental pollution, health impact, and other undesirable changes [5]. Due to their negative effects, corrosion processes ought to be prevented to ensure reasonable lifetime. Various strategies have been used to develop an excellent method for preventing the corrosion process and the rates of the corrosion processes. One of the acceptable practices for metal protection from corrosive attack is utilization of corrosion inhibitors. A corrosion inhibitor is depicted as the chemical which, when added in small concentration to the media, can effectively reduce the corrosion rate [6]. Corrosion inhibitors are extensively used owing to their economic viability, simplicity of use, and high efficiency in reducing extensive corrosion damage [7]. Corrosion inhibitors prevent the occurrence of corrosion by adsorbing onto the metal surface and blocking one or more of the electrochemical reactions occurring at the solution/metal interface [8]. In other words, the inhibitors provide a protective barrier film, which in turn restricts further corrosion [9].

Many studies have been conducted to find effective and suitable inhibitors from inorganic and organic compounds for various corrosive media, because an inhibitor that works well in one medium may not work in another. Therefore, the continuous development of new formulae or new inhibitors for different environment has been explored to find the most practical and cost effective method in fighting corrosion [10]. Alongside the corrosion environment, the effectiveness of some inhibitors also depends on their properties. Several common inhibitors from inorganic compounds include chromic salt, nitrite, and silicates, which are widely used for various metals [11]. Other synthetic inorganic inhibitors from the group of phosphites (M(HPO₃)) have been observed to inhibit well the corrosion of carbon steel in saline water with the order of inhibition being as follows: Mn(HPO₃) > Co(HPO₃) > Cu(HPO₃) [12]. It has been assumed that the action mechanisms of phosphate inhibitors are based on their adsorption on the metal surface, forming a good protective layer, which isolates the surface from the aggressive environment [13].

Different approaches have been used to explore the utilization of organic corrosion inhibitors. Many reports have demonstrated promising inhibitors derived from organic compounds containing nitrogen, oxygen, sulfur, or nitrogen-hetero cyclic compounds with a polar group [14]. The development of corrosion inhibitor-based organic compounds was found to effectively protect from corrosion through strong adsorption on the metal surface [15]. This strong interaction involves the charge sharing between empty d-orbitals of the surface metallic atoms and heteroatoms (nitrogen, oxygen, sulphur, etc.), as well as through double or triple bond or aromatic rings. The study of the adsorption of furan, pyrrole, and thiophene on the iron surface were studied by density functional theory (DFT) calculations which inferred that the inhibitors containing hetero atoms are found to have high electron-donating ability and their inhibition efficiency increases in the order O < N < S [16]. The presence of multiple N–H and C=O groups has contributed to a great extent inhibition corrosion of mild steel by polybutylene succinate extended with a 1,6-diisocynatohexane–L-histidine composite (PBSLH) in 1 M HCl [17].

Heterocyclic compounds are widely distributed in nature and used extensively as an active component of corrosion inhibitors due to the ease of subtly engineering their structures to reach a desirable function. Interestingly, the increased number of N atoms in the N-heterocyclic ring has increased the strength of adsorptions. For instance, tetrazole compounds can strongly adsorb on the steel surface through the five-membered tetrazole ring containing four N-hetero atoms [18]. The contribution of aromatic compounds was evidenced by the better inhibitive performance of the triazole-based compound (efficiency >90%) than that of the imidazole-based compound (~85%), at a concentration of 850 μM [19]. In addition, the substitution of the phenyl group attached to benzimidazole obviously increased corrosion inhibition propensity [20]. Other organic compounds such as thiophene and hydrazine derivatives have shown special affinity to inhibit corrosion of metals in acidic environments [21]. A study of the inhibitory properties of 5-Amino 1,3,4-thiadiazoles (5-ATT), in which the substituted groups include amino groups, has shown that the addition of 5-ATT in the corrosive solution promotes the formation of a stable and insoluble oxide layer (Fe₂O₃, FeOOH) that can reduce ion diffusion, and therefore improves the corrosion resistance of mild steel in 1 M HCl [22].

Another study explored corrosion inhibitors of organic origin from macrocyclic nitrogen compounds such as porphyrins as a promising class of corrosion-inhibiting agents [23]. Numerous reports on the corrosion inhibition of porphyrins and their derivatives have revealed their ability to form the conjugated systems that led to the formation of ordered molecular layers on electrode surfaces [24]. These molecular layers serve as protective films that avoid the diffusion of electroactive species towards the electrode surface, which may lead to electrode fouling [25]. Some factors contribute to the performance of organic inhibitor such as the functional group, p-orbital character, electron density at the donor atom, and the electronic structure of the molecule [21]. The need to develop cheap, safe, sustainable, and eco-friendly inhibitors has urged researchers to extensively explore the use of natural inhibitors. Porphyrins are naturally found in nature and play various roles. There are three main sources of porphyrins in nature: (1) hemoglobin, a material that gives the red color to the blood; (2) chlorophyll, a material that gives leaves their green color; and (3) cytochrome, which is an enzyme used to catalyze a redox reaction in respiration cells [26]. Porphyrin easily reacts with various types of metal ions forming metalloporphyrin. Many complexes of porphyrin compounds have been synthesized with iron, magnesium, zinc, manganese, cobalt, nickel, silver, lead, and cadmium. Among such complex compounds, iron–porphyrin complex compounds are the most important, as hemoglobin (red blood cells) contain one such complex, namely hemin [25].

Hemin consists of four isoindoles which are bonded to each other through an azo group (–N=) in a position to form a cyclic carbon-α [26] (Figure 1). Therefore, hemin has a planar bulk molecular structure, so when it is applied as a corrosion inhibitor on the metal, it is expected to cover efficiently the metal surface. In addition, hemin is easily obtained compared to chlorophyll and cytochrome. Hemin can be extracted from blood of animal slaughter waste such as cattle and poultry [27].

According to the Ministry of Agriculture of the Republic of Indonesia, the national chicken consumption reaches 1.4 million tons per year, with a blood content of 2.5%. Therefore, about 35,000 tons of chicken blood will be wasted annually. The previous study even estimated the potential yearly of hemin production extracted from 22.9 × 10⁹ L animal blood to be 1.7 × 104 kg or 2.2 × 108 kg [28] calculated by using either 0.75 mg L⁻¹ (trioctylmethyl ammonium salicylate) [29] or 10 g L⁻¹ (acetic acid/acetone/HCl) [30], respectively. This work is part of on-going efforts to utilize livestock waste for the attainment of value-added protoporphyrin for corrosion inhibition. The capability of the protoporphyrin IX derivative, dimegin, to form complexes with metals from different groups has inhibited the dissolution of metal (in this case, iron) and is capable of stabilizing the passive state of steel at high concentrations. This study was observed that the introduction of 10 μmol/L of dimegin to the solution increases the potential of free corrosion of steel and reduces the passivation current density by three. The inhibition of active anodic dissolution with increasing dimegin occurs with further increases in concentration [31].

The present study reports the modification of hemin isolated from chicken blood to acquire the protoporphyrin and its performance as a green inhibitor for T22 carbon steel in 0.5 HCl by means of electrochemical techniques and theoretical calculations. The protoporphyrin compounds were obtained from hemin by removing ferric ions contained in the hemin molecules. Hence, the amine and imine groups in protoporphyrin molecules become freer and lead the molecules to be prospective for use as a metal corrosion inhibitor. The utilization of livestock waste is expected to serve as a novel source for production of renewable corrosion inhibitors, provide value-added products in form of natural corrosion inhibitors, and help to diminish the disposal cost and environmental pollution cause by blood from slaughterhouse waste.

2. Materials and Methods

2.1. Isolation and Modification of Hemin

The tested inhibitor was synthesized according to the previous study with some modifications [32]. Glacial acetic acid was mixed with SrCl₂·6H₂O and acetone to obtain a fluid extractor. The chicken fresh blood was added into the liquid extractor and the mixture was shaken. The was mixture left for 30 min before heating it to a boil, and then the mixture was filtered. The filtrate was heated to 100 °C, centrifuged, washed with acetic acid and water, and then washed once more with ethanol and ether. The hemin extract obtained was dissolved in formic acid and boiled. The iron powder was added to the hemin extract mixture and left for 30 min. Afterwards, the hemin extract mixture was shaken for 40 min and filtered. The solids acquired from filtration were filtered, washed with water, and dried.

2.2. Protoporphyrin Characterization

The formation of protoporphyrin was characterized by infrared spectroscopy (FTIR) and atomic absorption spectrometry (AAS), as reported in the previous study [33]. The dried protoporphyrin was crushed homogenously with potassium bromide (KBr) to obtain powders. These samples were compressed into pellets using 10 tons pressure using a hydraulic pellet press to obtain transparent pellet samples that allow the IR radiation to pass through them. The spectrum from the infrared spectrophotometer (FTIR Prestige 21 Shimadzu, Kyoto, Japan) were recorded in the range of 400 cm⁻¹ to 4500 cm⁻¹. The protoporphyrin conversion was also confirmed with atomic absorption spectroscopy (AAS) carried out in a Perkin Elmer 1000 Spectrophotometer using a conventional air–acetylene flame [34].

2.3. Electrochemical Measurement

The ability of protoporphyrin as a corrosion inhibitor of carbon steel in 0.5 M HCl was studied by the potentiodynamic polarization (Tafel plots) method [35]. The working solutions of protoporphyrin were prepared by diluting the stock solution of 20 × 10³ ppm. The stock solution was prepared by diluting the 0.20 g of protoporphyrin powdered in 10 mL of 98% formic acid. The polarization was performed using an electrochemical apparatus (Tacussel-Radiometer PGZ 3O1) and controlled with Tacussel corrosion analysis software (Voltamaster 4). Measurements were made using a conventional three-electrode glass cell with a platinum counter electrode and a saturated calomel electrode (SCE) as reference with a Luggin capillary bridge [36]. Prior to the potentiodynamic polarization measurements, the working electrode was immersed in the test solution for 25 min for stabilization of the OCP.

The anodic and cathodic polarization curves were recorded by a constant sweep rate of 0.5 mVs⁻¹, potential range ± 75 mV, with respect to open circuit potential [37]. The corrosion rate (CR) was determined by the Tafel extrapolation method and fitting the linear part of the curve, and the calculated using the following equation (Equation (1)) [38]:

$$\mathit{CR}=3.27\times {10}^{-3} \frac{I_{corr}\times \mathit{EW}}{\mathsf{\rho }}$$

(1)

where CR is the corrosion rate (mm per year), I_corr is the corrosion current density (mA/cm²), EW is the equivalent weight of the corroding species (g) and ρ is the density of the corroding material (g/cm³), whereas the inhibition efficiency (IE%) was calculated by using the equation as follows (Equation (2)):

$$\mathrm{IE}\%=\left(1-\frac{I_{corr}}{I_0}\right)\times 100$$

(2)

where I₀ and I_corr are the corrosion current density in the absence and presence of inhibitor, respectively [37].

3. Results and Discussion

3.1. Characteristic of Hemin and Modified-Hemin (Protoporphyrin)

Blood is one of the natural sources of hemin, which is located within each globin of hemoglobin. Hemoglobin is the main protein component of red blood cells, consisting of four separate polypeptides known as globin, with a molecular weight as a tetramer of 68 kDa [27]. Hemin or ferriprotoporphyrin is a complex of iron(III) protoporphyrin comprising four substituted pyrrole rings linked by –CH groups [29]. Hemin formation started from the acid hematin produced by the action of glacial acetic acid on blood in the presence of strontium chloride, assisting decomposition of hemin and hemoglobin into a globular protein that is soluble in hot glacial acetic acid [39]. The protein test against hemin indicated that hemin was successfully separated from hemoglobin.

The hemin product was further modified by removing the ferric ions contained in the hemin structure through a reduction process by adding hemin in an organic acid medium to produce protoporphyrin. In this method, iron powder and formic acid were used. Fe powder acts as a reducing agent that will reduce Fe³⁺ to Fe²⁺ in the structure of hemin, while formic acid serves as a medium for Fe²⁺ ions, which has been separated from the structure of hemin, as well as the H⁺ ions provider to nitrogen pyrrole to the central atom of Fe regardless of the porphyrin ring [33]. The removal of ferric ions contained in the hemin was traced by atomic absorption spectroscopy (AAS) and the results shown in

Table 1. Levels of iron in hemin and product modification (protoporphyrin).

Sample	(Fe)/ppm	Note
Hemin extracted from blood	49.5003
Product modifications hemin (protoporphyrin)	10.1064	Rest of Fe 20%

. AAS is well known as a reliable method with which to determine the concentration of metal-ion containing compounds [34] by means of absorbance of iron content [29].

It can be seen, in Table 1, that ferric ion content in the protoporphyrin is abruptly lower than those of the hemin, suggesting the conversion process of hemin to protoporphyrin was occurred. Simultaneous deposit formation was observed when acetate buffer was added into the solution system of modified hemin, indicating the isoelectric point of protoporphyrin compound has been reached (data not shown). Thus, data supported the formation of protoporphyrin.

The FTIR spectroscopy analysis of hemin and protoporphyrin can be identified considering their structures. Hemin contains a protoporphyrin ring system with the Fe (III) metal ion attached at the cavity center [40]. Figure 2 demonstrates the most prominent spectral region of both compounds in the fingerprint region (600–1650 cm⁻¹) and at high frequency (4000–2900 cm⁻¹). As can be seen in Figure 2, the spectra of the hemin and the protoporphyrin closely resemble each other, but some important obvious differences could be very informative, if they are properly assigned [41].

The FTIR experimental data show the O–H stretching vibration of the hydroxyethyl side chain of hemin is absorbed weakly at 3487.30 cm⁻¹ (Figure 2a). The Fe (III) center is square pyramidal in geometry, due to the high spin penta coordination of ligands. The porphyrin ring has several side chains such as carboxyethyl, vinyl, and methyl groups [42]. Band spectra of medium to weak intensity are found in the 3100–2800 cm⁻¹ regions, and these can be assigned to C–H stretching modes. The features at 2916.37 cm⁻¹ are attributed to antisymmetric of C–H stretching vibrations. Stretching vibrations of the C–H groups are interesting because of their role in non-radiative transitions in porphyrins [41]. The IR absorption intensity of the C–H vibrational modes in the ring, which do not contain NH-groups, is higher than in the protonated [33]. The strongest absorption peak is detected in the spectra at 1701.22 cm⁻¹ and can be associated with the methyl propionate ester carbonyl stretch in hemin and at 1720.50 cm⁻¹ in the protoporphyrin. A sharp peak spectrum at 846.75 cm⁻¹ for hemin and 835.18 cm⁻¹ can be assigned to the methane C–H bond for hemin and protoporphyrin, respectively [43].

The C=C and C=N stretching vibrations of the hemin were identified at 1411.89 and 1300.02, while in the protoporphyrin they were designated at 1612.49 to 1406.11 and 1381.03 (Figure 2b). The complex of protoporphyrin demonstrates an absorption band close to 1620 cm⁻¹ which is relatively more intense than those of the hemin complex. The enhancement in absorption may contribute through the C=C stretching vibration of the vinyl substituent. The C–N stretching model in hemin and protoporphyrin was difficult to appoint since it overlaps with the strong alkyl bending absorption.

Comparison FTIR spectra of protoporphyrin with hemin show a weak spectra band at 3320 cm⁻¹ that can be associated with an N–H stretching vibration in the protoporphyrin spectra. As expected, the absence of N–H in the Hemin spectra suggesting the characteristic of protoporphyrin in its metalated form. There two peak absorption bands detected in the protoporphyrin at 989.48 and 910.40 cm⁻¹ which were assigned to an in-plane porphyrin deformation mode. In addition, the decrease in the intensity of the absorption peaks is significant in the area of 900 and 400 cm⁻¹, and it was thought to have come from uptake of Fe-N and Fe-Cl. A decrease in the intensity of the absorption indicates that the conversion process of hemin into protoporphyrin due to the loss of iron ions contained in the hemin structure [44]. The spectra absorption peaks which are independent of the metal ions can be readily assigned and are in general agreement with those recently reported for a number of metal-free porphyrins [45]. Furthermore, synergetic measurement of the chemical structure with NMR spectroscopy is interesting to explore to confirm the FTIR results.

3.2. Potentiodynamic Polarization Measurement of Protoporphyrin as Candidate of Green Corrosion Inhibitor

The performance of protoporphyrin as a corrosion inhibitor to carbon steel in 0.5 M HCl was studied by the potentiodynamic polarization (Tafel plots) method [46] in order to fully understand this process. Tafel extrapolation was carried out to assess the cathodic Tafel slopes (β_c), the anodic Tafel slopes (β_a), and the corrosion current densities (I_corr) of the current-potential line at the resembling corrosion potentials (E_corr) at different variations of temperature. The potentiodynamic parameters were calculated by extrapolating the Tafel regions of the curves to the corrosion potential. Furthermore, the data were analyzed to determine the corrosion rate (CR) and the inhibition efficiency (IE%) using Equations (1) and (2), respectively. Polarization curves for T22 carbon steel in 0.5 M HCl at various concentrations of protoporphyrin and temperatures are shown in Figure 3 and the corrosion kinetics parameters such as cathodic Tafel slopes (β_c), anodic Tafel slopes (β_a), corrosion current densities (I_corr), corrosion potentials (E_corr), corrosion rate (CR) and the inhibition efficiency (IE%) are given in

Table 2. Corrosion parameters obtained from potentiodynamic polarization result of T22 carbon steel in 0.5 M HCl at various temperatures in the presence and absence of different concentrations of protoporphyrin.

Temp. (K)	Cinh (ppm)	Ecorr (mV)	Icorr (μA.cm−2)	βa (mV.dec−1)	βc (mV.dec−1)	CR (mmpy)	IE (%)	θ
298	blank	−460.0	267.31	60.0	−70.8	55.93	-	-
	40	−463.5	219.12	59.3	−65.1	45.89	18.0	0.1803
	80	−465.6	178.32	57.5	−63.7	38.45	33.3	0.3329
	120	−467.5	156.93	58.5	−61.5	32.98	41.3	0.4129
	160	−468.3	143.93	56.4	−60.5	30.15	46.2	0.4615
	200	−470.6	142.16	55.8	−59.3	29.75	46.8	0.4682
308	blank	−467.1	312.23	88.0	−97.3	86.06	-
	40	−483.3	272.01	73.6	−86.2	74.98	12.9	0.1288
	80	−485.5	258.21	70.2	−83.2	71.34	17.3	0.1730
	120	−486.2	230.43	67.2	−80.1	63.33	26.2	0.2620
	160	−487.1	216.21	68.3	−78.2	60.13	30.8	0.3075
	200	−485.8	214.34	66.3	−76.8	58.83	31.4	0.3135
318	blank	−496.6	324.94	70.5	−89.1	101.9	-
	40	−513.4	300.73	68.3	−83.2	94.3	7.5	0.0745
	80	−508.4	288.65	67.3	−81.8	90.6	11.4	0.1137
	120	−501.5	276.65	66.7	−80.6	88.8	14.9	0.1486
	160	−506.5	271.99	65.2	−79.2	86.3	16.3	0.1630
	200	−509.3	272.91	65.1	−80.1	88.6	16.0	0.1601

.

From Figure 3 and Table 2, it is shown that the polarization curves obtained in the presence of animal blood hemin protoporphyrin at a temperature of 298 K were below those of the non-inhibited solution, indicating the addition of protoporphyrin reduce I_corr that leads a decrease in the corrosion rate (CR) of carbon steel. The increase in surface coverage degree (θ) at 298, 308, and 318 K (Table 2) suggests that the inhibition efficiencies increased at all temperature ranges. Furthermore, the shape of the branches of the potentiodynamic curves is similar to the plots of the blank solution, suggesting the corrosion mechanisms have not changed and the protoporphyrin acts by an adsorption process onto both anodic and cathodic active sites of the surface [4]. This results were in line with previous studies which reported the increase in the inhibition efficiencies by the derivative tryptophan (inhibitor with an amine group) along with the increase in the inhibitor concentrations [46].

From Table 2, it can be observed that the corrosion rate (CR) decreases and the inhibition efficiency (IE%) increases with addition of different concentrations of protoporphyrin at all temperatures; this evidence confirms the inhibitory character of protoporphyrin. The alteration of the potential ΔE_corr = E_corr − E°_corr, with E°_corr being the potential of the blank, gives an indication of the electrochemical type of the inhibitor [47]. According to the literature [48,49,50], generally, if ΔE_corr transcends ± 85 mV, the inhibitor acts as either an anodic or cathodic type. On the contrary, for ΔE_corr < ±85 mV, the inhibitor is considered as a mixed type. In the presence of protoporphyrin, the corrosion potential switches slightly (ΔE_corr = −10.6 mV at 298 K, −18.7 mV at 308 K, and −12.7 mV at 318 K) towards the lowest values, which demonstrates that the protoporphyrin is a mixed-type inhibitor which acts on the inhibition of both the anodic dissolution of the steel and reduction of hydrogen ions on the metal [51]. In addition, the protoporphyrin causes no change in the Tafel slopes (βa; βc).

It is also observed that the inhibition efficiency increased with increasing protoporphyrin concentration at all temperatures and exhibited both cathodic and anodic inhibition through adsorption on the T22 carbon steel surface blocking active sites. The addition of protoporphyrin decreased the number of exposed active sites that could be involved in steel dissolution through the formation of protective films of the inhibitor on the steel surface. These results support the structural hypothesis of the possibility of protoporphyrin as a porphyrin derivative to behave as a mixed-corrosion inhibitor [24]. The molecules of porphyrins can reconfigure the electron distribution of the aromatic ring and change their properties in order to create ordered molecular layers. The electrochemical measurements (Table 2) demonstrate that the T22 steel surfaces have protected by the film barrier avoiding the diffusion of electroactive species towards the T22 steel surface. The results of this current work confirmed previous studies which displayed increases in inhibition efficiency and surface coverage of all studied porphyrins with increasing concentrations [23].

It is obvious from the potentiodynamic polarization data that inhibition efficiency (IE) and surface coverage (θ) increases with increase in concentration of the inhibitor at all temperatures. The corrosion rate (CR) decreases with increase in inhibitor concentration. The optimum inhibition efficiencies of 46.2% at 298 K, 30.8% at 308 K, and 16.3% at 318 K are all obtained from a 160 ppm solution of protoporphyrin. Comparison analysis of corrosion potential and carbon steel corrosion current density of protoporphyrin in 0.5 M HCl indicated the excitation of the energy level of the charged orbitals on the carbon T22 steel surface exceeding that of the energy level of empty orbitals of the solution. Temperature is another important factor to be studied to better understand the mechanism of the inhibitor–metal surface system. The influence of temperature on the inhibition process was studied by calculating values of inhibition efficiency for T22 carbon steel in 0.5 M HCl in the absence and presence of different concentrations of protoporphyrin at three different temperatures 298, 308, and 318 K. The influence of temperatures was recorded in Table 2. Based on Table 2, it is shown that, as the temperature increases, the corrosion rate (CR) is increased while the inhibition efficiency is decreased. These occurrences are due to the weakening of the adsorbed inhibitor molecules on the surface of the T22 carbon steel, allowing the corrosion process.

The types of interactions between protoporphyrin molecules and the T22 carbon steel surface could be evaluated by different isotherms. The process is influenced by several factors such as the property and charge of the metal surface, adsorption of solvent and other ionic species, electronic characteristics of the metal surface, a temperature of corrosion reactants, and the electronic potential of the metal–solution interface [17]. During this process, the adsorption isotherms of Langmuir (Equation (3)), Freundlich (Equation (4)), Temkin (Equation (5)), and Frumkin (Equation (6)) were tested by using the following equations [52]:

-

Langmuir:

$$\frac{C_{\mathrm{inh}}}{\mathsf{\theta }}=\frac{1}{K_{\mathrm{ads}}}{+C}_{\mathrm{inh}}$$

(3)

-

Freundlich:

log θ = log K_ads + n log C_inh

(4)

-

Temkin:

exp(−2αθ) = b × C_inh

(5)

-

Frumkin:

$$\frac{\mathsf{\theta }}{1-\mathsf{\theta }}\exp (-2\mathsf{\alpha }\mathsf{\theta })=\mathrm{b}\times C_{\mathrm{inh}}$$

(6)

where C_inh is the concentration of the protoporphyrin, θ is the surface coverage, and K_ads is the adsorption equilibrium constant.

The evaluation of corrosion inhibitor mechanisms and the interaction of inhibitor and the metal surface of T22 steel was determined by observing the straight line generated from graphs plotting between log C_inh/θ and C_inh for four model isotherms as represented by Equations (3)–(6). The value of surface coverage (θ) at different concentrations of protoporphyrin in HCl solutions have been made to explain the best isotherm with which to determine the adsorption process [13]. It was found that the best fit for the protoporphyrin adsorption on the T22 steel obeys the Langmuir isotherm. Figure 4 shows that the Langmuir adsorption isotherm fits the data well in the 0.5 M HCl solution at all temperatures. It presents the interactions between molecules of protoporphyrin and T22 carbon steel via monolayer adsorption over a homogeneous surface of the adsorbent [53]. The r² values are close to unity, suggesting strong adherence to the Langmuir adsorption isotherm [12].

The values of K_ads were calculated from the intercepts of the straight lines on C/θ axis. The values of K_ads can be used to evaluate the standard free energy of adsorption (ΔG°_ads) using the following equation (Equation (7))

$$\mathsf{\Delta }{G}_{\mathrm{ads}}^{\circ }=-\mathit{RT}\ln (C_{{\mathrm{H}}_2\mathrm{O}}\times K_{\mathrm{ads}})$$

(7)

where $C_{{\mathrm{H}}_2\mathrm{O}}$ = 10³ g/L = 55.5 M, R is the universal gas constant, and T is absolute temperature.

The values of K_ads and ΔG°_ads are given in

Table 3. The values of K_ads and ΔG°_ads protoporphyrin on the surface of T22 carbon steel at all temperatures.

Temperature (K)	Kads	ΔG°ads (kJ/mol)
298	6.41	−14.55
308	4.12	−13.90
318	2.74	−13.28

. The positive values of K_ads depict the adsorption process as more favorable than the desorption at all temperatures. Even though the desorption process is relatively more common at higher temperature than at low temperature. The values of ΔG°_ads are negative, which demonstrates the spontaneous adsorption, and stable interactions occur between protoporphyrin and T22 carbon steel. In general, ΔG°_ads values of −40 kJ/mol or more negative ones are related to the chemical adsorption, while those around −20 kJ/mol are related to electrostatic interactions between charged molecules and metal charges (physical adsorption) [54]. In the present study, the value of ΔG°_ads is about −14 kJ/mol and it shows that the protoporphyrin is physisorbed on the surface of T22 carbon steel.

4. Conclusions

This study investigated the modification of hemin isolated from chicken blood to acquire the added value of protoporphyrin and its performance as a green inhibitor for TT22 carbon steel in 0.5 M HCl in the absence and presence of different concentrations of protoporphyrin at three different temperatures, i.e., 298, 308, and 318 K. The protoporphyrin was demonstrated to be a promising inhibitor for the corrosion of T22 carbon steel in 0.5 HCl solution. The maximum inhibition efficiency at 160 ppm is 46.2% at a temperature of 298 K. The potentiodynamic kinetics experiment showed that the introduction of protoporphyrin increased the inhibition efficiency of T22 carbon steel with proper dosages of protoporphyrin but decreased the inhibition efficiency when the temperatures of the solutions were raised. The results suggest that proper dosages of protoporphyrin may be used as a corrosion control agent. Polarization measurements demonstrate that the presence of protoporhyrin affects both the anodic and cathodic half-reaction and validated the assumption that protoporphyrin may behave as a mixed-type inhibitor with a predominantly cathodic inhibitive property. Adsorption of the molecules of protoporphyrin on the T22 carbon steel surface was found to obey the Langmuir adsorption isotherm at all temperatures. According to the thermodynamic adsorption parameter, the adsorption mechanism of protoporphyrin on the T22 carbon steel surface is dominated by physical adsorption. These current works demonstrate protoporphyrin as an alternative promising green corrosion inhibitor originating from waste materials. Though recently the application of nanomaterials as corrosion inhibitors have been explored, the use of green inhibitors is still considered as an eco-friendly and cost-effective method for protection of steel. The computational simulation of the corrosion inhibitor adsorption becomes a prospective area of research, since it may provide in silico screening for potential green corrosion inhibitors and their molecular interaction.