Multifunctional Aspects of the Synthesized Pyrazoline Derivatives for AP1 5L X60 Steel Protection Against MIC and Acidization: Electrochemical, In Silico, and SRB Insights.

The inhibitory impact of low-cost synthesized pyrazoline derivatives named Pz series (Pz1 and Pz2) on the corrosion of API 5L X60 carbon steel in 5 M HCl was inspected to serve as corrosion inhibitors against such a solution for its usage in the oilfield well acidization process. Also, the same compounds were unitized as biocides for sulfate-reducing bacteria (SRBs) to inhibit the microbial-induced corrosion effect. This study was conducted via several electrochemical techniques, namely, electrochemical potentiodynamic polarization (EP) and electrochemical impedance spectroscopy (EIS), in addition to computational density functional theory (DFT). The inhibition efficiency (IE) of Pz series on the corrosion of 5L X60 carbon steel in 5 M HCl was found to increase whenever the Pz series molecule concentration was increased. EP measurements revealed that Pz1 and Pz2 have both cathodic and anodic features (mixed inhibitor) and their corrosion IEs were around 90%. The physicochemical properties of the Pz1 and Pz2 compounds were studied using Langmuir adsorption isotherms, where the equilibrium adsorption data were found to follow it accurately. EIS outputs were found to comply with the values obtained from EP. Scanning electron microscopy was used to examine the topographic anisotropy between the inhibited and uninhibited 5L X60 carbon steel samples to double-check the electrochemical findings. DFT calculations and Monte Carlo simulations were utilized to predict the behavior of inhibitors and to rationalize the experimental results. The serial dilution bioassay technique was used to assess the Pz series as potential biocides to counter the effect of SRBs in compliance with the TM0194-2014-SG standard test method, and the results showed the potency of Pz series in inhibiting such bacterial growth.

Introduction

Numerous metals and alloys utilized in various human activities are vulnerable to several types of corrosion as a result of exposure to different destructive media. Among the most utilized alloys in the oil and gas industry is 5L X60 carbon steel, which can be used in locations such as well tubing and trunklines.[1] Such places are constantly subjected to corrosion attacks due to different harsh operating environments.[2]

Specifically, mineral acids are an example of a very corrosive material yet extensively used in various industrial processes such as tubing pickling and oil well acidization.[3] Oil well acidization is considered as a routine technique for increasing the lifetime of producing oil wells. It generally comprises the direct injection of 10–15% HCl solutions into the well tubing, so that it can increase the wellbore permeability by simply dissolving the downhole sediments and rocks such as limestone, dolomite, and calcite which stimulate the oil production.[4−7] On the other hand, corrosion induced by hydrochloric acid well stimulation job is considered as a major problem in the oil and gas industry as it leads to significant loss due to the severe corrosive act and high maintenance cost.[8]

Chemical corrosion inhibitors are widely applied in inhibitive methods such as the well acidization process and acid pickling baths due to their ability to form a protective film on metal surfaces that can further diminish the corrosion rate (CR) to a tolerated level. Numerous investigations have been carried out to formulate a more convenient corrosion inhibitor to be utilized and consumed for different metal and alloy protection in various environments.[9−13]

The distinctive unsafe impacts resulting from different corrosive media via interchanging mechanisms can sometimes lead to synergy in accelerating the CR, and this has inspired the utilization of a more powerful inhibitor that works on every aspect.[14,15] Such multifunctional corrosion inhibitors are supposed to counter different attacks and works through different mechanisms, for instance, microbial induced corrosion (MIC) and under deposit corrosion. A few studies have been accounted for utilizing such a versatile class of corrosion inhibitors for carbon steel protection.[16,17] Hence, significant and considerable attention was devoted to the design of new and efficient structures of novel organic molecules that can fulfill such multipurpose roles.

Recently, much interest has centered on pyrazole as scaffolds for innovative moieties of further developed analogues with a wide range of unique properties in the arena of corrosion science.[18] Aside from being an approved corrosion inhibitor in many studies, pyrazole derivatives are well known for their antimicrobial activity, which can counter the effects of sulfate-reducing bacteria (SRBs) causing MIC.[19−21]

In view of the preceding findings and in connection with our research program in the synthesis, reactions of 1,3-diketones and their applications in industrial chemistry,[22−29] this work is aimed to inspect the multifunctional applicability of the prepared and structured pyrazoline derivatives in the field of protection of 5L X60 carbon steel against corrosion in a simulated acidic medium for the oil well acidization process, as well as their potential use as biocides for MIC parallel to the recently published studies in the filled outlines in Table .

Table 1

Comparison between Parallel Corrosion and Biocidal Activities of Recent Multifunctional Inhibitors and This Work

		corrosion inhibition		SRB inhibition
compound name	corrosive medium	optimum dose	max. inhibition %	optimum dose	max. inhibition %	refs
TOS3	10% HCl	50 ppm	87.6	50 ppm	80	(4)
MA-1156	1 M HCl	18 μM	86.4	16 μM	80	(30)
SPT	formation water	200 ppm	79.0	80 ppm	100	(16)
Pz2	5 M HCl	2000 ppm	89.0	20 ppm	100	this work
Pz1	5 M HCl	2000 ppm	91.0	20 ppm	83.3	this work

Results and Discussion

Synthesis of the Inhibitors Pz1 and Pz2

The synthetic approach for the synthesis of azo pyrazolinones Pz1 and Pz2 is presented in Scheme as we previously studied and described.[31−34] Treatment of the low-cost starting material, 3-(2-(4-methylphenyl) hydrazono)-2,4-chromandione (1), with hydrazine hydrate in refluxing ethanol yielded the corresponding 2-pyrazolin-5-one (Pz1). Diazo coupling of Pz1 with diazotized 4-nitroaniline in basic solution produced bis-azo pyrazolin-5-one (Pz2) as depicted in Scheme .

Scheme 1

Synthesis of the Pz Series Compounds

Electrochemical Measurements

Open Circuit Potential

The open circuit potential (OCP) change with time is portrayed in Figure for the inhibitor Pz2 with an immersion time of 30 min. The OCP plots reached a steady-state area after a rapid increase in the negative direction of the potential, where it implies that an equilibrium state at all concentrations of the added Pz series inhibitors was achieved between both the oxidation and reduction processes with a zero bias potential.[10] The behavior of both the blank and other inhibited samples was nearly the same in terms of the direction of the OCP or the values of the EOCP, where the differences were no more than 85 V and were comparatively within the same range of the listed Ecorr values in Table .[10] This signifies the ability of Pz series molecules to inhibit both the cathodic and anodic reactions and they can be designated as mixed-type inhibitors.

Figure 1

OCP–time curves for the API 5L X60 sample at different concentrations of Pz2 in 5 M HCl at 25 °C.

Table 3

Electrochemical Polarization Spectroscopy Parameters at Different Concentrations of Pz Series

sample (ppm)		E mV	icorr mA/cm2	βa mV vs SCE	βc mV vs SCE	CR mm/Y	Rp Ω·cm2	θ	IE
	blank	–539	3.3	124	–191	38	9.98
Pz1	100	–532	2.6	406	–221	31	23.89	0.21	21
	200	–542	2.3	375	–230	27	26.91	0.30	30
	500	–548	1.9	376	–250	22	34.31	0.42	42
	1000	–532	1.0	254	–167	12	43.75	0.70	70
	1500	–560	0.5	274	–182	5	109.40	0.85	85
	2000	–564	0.3	272	–168	4	150.31	0.90	90
Pz2	100	–548	1.8	356	–215	21	32.33	0.46	46
	200	–525	1.4	237	–169	17	30.59	0.58	58
	500	–564	1.3	288	–179	15	36.87	0.61	61
	1000	–544	1.0	242	–167	11	42.90	0.71	71
	1500	–555	0.6	260	–177	5	130.64	0.82	82
	2000	–561	0.3	262	–172	4	150.28	0.90	90

Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) offers a robust tool for quantifying the rate of metal corrosion and hence alleviating its impacts. The electrochemical impedance of the used 5L X60 carbon steel electrode over 5 M HCl at 298 K without and with Pz1 and Pz2 as additives with a dosing range between 100 and 2000 ppm is indicated in the recorded Nyquist plots (Figure ). Both uninhibited and inhibited curves are represented by a capacitive semi-circle shaped plot, which represents the charge exchange over the metal–solution interface in the corrosion reaction. The imperfect semicircular shaped curves for Nyquist plots are due to the inhomogeneity of the surface of the metal sample.[35] Furthermore, the growth of the radius of the semicircle is associated with an increase in charge transfer resistance (Rct) through the metal–solution interface.[36] The concentration of the Pz series inhibitors was increased from 100 to 2000 ppm, and a corresponding increase in the semicircle width was noticed as depicted in Figure . In order to analyze the obtained plots, the obtained impedance data were fitted to the closest equivalent electrical circuit using ZSimpWin software. The components of the equivalent electrical circuit parameters were identified such as solution resistance (Rs) and charge transfer resistance (Rct) as presented in Figure . The latter term can be utilized to measure the corresponding CR and IE using the following eq .[37]where Rct0 and Rct are the charge transfer resistances for the uninhibited and inhibited tests, respectively.

Figure 2

Nyquist plots for API 5L X60 steel at different concentrations of (a) Pz1 (b) Pz2 in 5 M HCl at 25 °C.

Figure 3

Equivalent electrical circuit for API 5L X60 steel in 5 M HCl.

In addition to these parameters, constant phase element (CPE) was present in the equivalent circuit, which represents an imperfect capacitor and is correlated with the electrochemical double-layer capacitance (Cdl). The impedance value of the CPE can be calculated using the following eq .[38]where ZCPE is the impedance of the CPE, Y0 is the CPE constant, ω is the angular frequency, j is the imaginary number, and “n” is the phase change between those circlet components because of the surface heterogeneity; the n value is represented by the relation 0 < n < 1, where at unity (n = 1), it represents a true capacitor and at n = 0 it represents pure resistance. The double-layer capacitance of a system including the CPE is given by eq .

The acquired parameters from the nonlinear square fitting for impedance information are tabulated in Table . The preliminary investigation of this table implies that the Rct values were increasingly changed when the added concentrations of both Pz1 and Pz2 were increased where Pz2 scored 91% IE, while Pz2 scored 89%. The greatest inhibition value was accomplished for the concentration of 2000 ppm at 298 K for both Pz series compounds, which was due to the structuring of the layer at the metal–solution interface.[39] The double-layer capacitance might have been diminished altogether by increasing the Pz series inhibitor concentration. This is mainly due to the presentation of inhibitor molecules in the solution, where a quasi-substitution process takes place, replacing the profoundly dielectric nature of water particles with the low dielectric natural inhibitor molecule and in-time it expands in thickness.[40] Accordingly, this would prevent the formation of the electric double layer and decrease the number for transferred electrons, which decreases the CR. We can account for these changes through the Helmholtz model eq for the calculation of the double-layer capacitance.[41]where εs is the dielectric constant of the medium, d is the thickness of the double layer, A stands for the surface area of the cathode, and ε0 is the vacuum permittivity. The phase-change “n” value was found to be around 0.54–0.75, which also denotes the association of charge exchange between the metal surface and the solution, thus preventing metal corrosion.[42]

Table 2

EIS Parameters at Different Concentrations of Pz Series

	sample	Rs Ω·cm2	Q	n	Rct Ω·cm2	Cdl mF/cm2	θ	IE	X2
Pz1	blank	2.26	0.006693	0.71	19	2.88
	100	1.97	0.003455	0.61	39	0.96	0.51	51	0.00117
	200	2.39	0.003023	0.54	46	0.56	0.59	59	0.00130
	500	2.24	0.002133	0.55	53	0.36	0.64	64	0.00397
	1000	2.2	0.000855	0.76	60	0.33	0.68	68	0.00990
	1500	2.7	0.000338	0.75	180	0.13	0.89	89	0.00406
	2000	4.8	0.000278	0.75	191	0.10	0.91	91	0.00589
	100	2.83	0.004132	0.63	34	1.30	0.44	44	0.00308
Pz2	200	4.22	0.003191	0.59	46	0.84	0.59	59	0.00317
	500	2.53	0.002492	0.59	48	0.57	0.60	60	0.00242
	1000	3.60	0.000971	0.76	54	0.83	0.65	65	0.00762
	1500	2.70	0.000475	0.71	114	0.14	0.83	83	0.00647
	2000	4.30	0.000123	0.76	162	0.13	0.89	89	0.00697

Electrochemical Polarization (EP) Measurements

The current response of both anodic and cathodic reactions that occur throughout the corrosion process could give a chance for the CR to be probed by polarization investigation.[43] The impact of Pz series inhibitors on the polarization of 5L X60 carbon steel samples immersed in 5 M HCl with and without various concentration ranges of the inhibitors is illustrated in Figure .

Figure 4

Tafel plots for API 5L X60 at the different concentrations of (a) Pz1 and (b) Pz2 in 5 M HCl.

The cathodic and anodic parts of the polarization curves were moved to a lower current density value (i) with no significant shift in the potential (Ecorr). This behavior is typically expected due to the adsorption of the inhibitor over the metal surface which suppresses both the cathodic and anodic reactions (mixed-nature inhibitor).[44] Different electrochemical corrosion parameters were elicited from the Tafel curve extrapolation, namely, corrosion potential (Ecorr), corrosion current density (Icorr), cathodic and anodic Tafel slopes (βc, βa), and corrosion efficiency (IE) which was measured using Icorr by utilizing eq and such parameters are listed in Table .[45]where Icorr0 and Icorr represent the current density values in the absence and presence of the Pz series inhibitors, respectively.

The CR and Icorr were found to decline with the increase in the Pz series inhibitor concentration, which can be translated to their ability to prevent more anodic Fe metal dissolution and simultaneously prevent the cathodic hydrogen evolution reaction by simply isolating the exposed area of the metal. Concerning the Ecorr, the difference between the Ecorr values for uninhibited and inhibited solutions was lower than 85 mV. Accordingly, no significant potential shift was noticed, which further proves the mixed-nature inhibition action.[46] The polarization resistance (Rp) can be elicited from the calculated Tafel parameters utilizing the Stern-Geary mathematical eq .[47,48]

Adsorption Isotherm

Different adsorption isotherm modules were employed, yet the linearized fitting data indicated that the best fitted module was the Langmuir isotherm based on the calculated regression coefficient (R2) for Pz series molecules.

The Langmuir adsorption isotherm quantitatively describes the formation of a monolayer of an adsorbate on the surface of an adsorbent and afterward no further adsorption would take place. Thereby, the Langmuir represents the equilibrium distribution of metal ions between the solid metal and the liquid solution. The Langmuir adsorption isotherm can be expressed by the mathematical relation 7.[49]where Ci is the added inhibitor concentration, Kads is the adsorption/desorption equilibrium constant, and θ represents the calculated surface coverage. The used surface coverage data represent those extracted from the electrochemical polarization technique (EP). From Figure and the information from Table , it was obvious that the R2 values were 0.98 and 0.96 for Pz1 and Pz2 respectively, which emphasizes the near unity high-fitting aspect achieved by the Langmuir isotherm model. It is also clear that inhibitor II has a higher value of Kads than inhibitor I, which refers to the strong adsorptive activity. The calculated Kads values were in the order Pz1 > Pz2. This may confirm the increased ability of the Pz1 inhibitor to be adsorbed on the surface more than Pz2.

Figure 5

Langmuir isotherm module for (a) Pz1 and (b) Pz2 at different concentrations.

Table 4

Langmuir Adsorption Isotherm Parameters for Pz1 and Pz2

comp.	R2	slope	intercept	Kads (L/mol)	ΔGads (kJ/mol)
Pz1	0.98	1.049	0.7	1499	–28
Pz2	0.96	0.88	1.2	739	–26

The standard adsorption free energy (ΔGadso) can be calculated using the mathematical eq .[50−52]where R is the universal gas constant, and the value 55.5 represents the theoretical molar concentration of water. The calculated negative ΔGads values listed in Table showed that the Pz1 and Pz2 adsorptions would be of a spontaneous nature. Generally, a value of ΔGads around −20 kJ mol–1 or lower would result from an electrostatic attraction between the adsorbate particles and the adsorbent metal (physisorption). However, for the values of ΔGads over −40 kJ mol–1, it would result from the exchange of electrons between the inhibitor particles and the metal surface to form a chemical bond (chemisorption).[53,54] The listed ΔGads values in Table showed that the adsorption system of the Pz series inhibitors over the 5L X60 carbon steel surface immersed in 5 M HCl is of a mixed physical and chemical nature.[55] From the listed values of Kads and ΔGads, the adsorption of Pz1 was clearly superior to that of Pz2 on the surface in terms of the formed bonds between the Fe metal surface and the electron-rich adsorption centers in the Pz1 designed structure which can endure both Van der Waal forces and coordination bonds. What may support this claim is the presence of a nitrophenylazo moiety in the Pz2 compound that has a high electron-withdrawing effect, which diminishes the electron availability for bonding to the metal surface and hence lowers its adsorption ability compared to Pz1.[56] This claim can be better investigated through computational study for the electron density of both Pz series structures.

Computational Studies

Figure shows the electron density distributed on the optimized molecular structure of Pz1 and Pz2 compounds and their respective highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) surfaces in the gas phase. This image represents the optimized structure of Pz series, which was obtained through repetitive cycles of calculations by which we bring the energy on the coordinated atoms to the lowest possible value. Eventually, we can determine the frontier molecular orbital’s energies which comprises the HOMO that designates the regions prone to electron donation and metallic bonding, unlike the LUMO sites which can host electrons from the d-orbitals of Fe (back donation).[57,58] The energy gap (ΔE) between the HOMO and LUMO of Pz1 and Pz2 mainly depends on the energy value of its occupied and unoccupied orbitals. The smaller the energy gap (ΔE), the higher the ability of the molecule to be adsorbed on the Fe surface because it means it has lower LUMO and higher HOMO.

Figure 6

Density functional theory (DFT) outputs for Pz1 and Pz2, HOMO (left) and LUMO (right).

For compound Pz1, the HOMO is distributed on the phenol ring and extended to the pyrazoline moiety which indicates its high ability to donate electrons. On the contrary, the LUMO level for the same compound appears to be concentrated on the azo toluene moiety, which denotes its ability to receive electrons. However, in Pz2 and after the introduction of the electron-withdrawing nitrophenyl azo group, the situation was inverted. The HOMO level was shifted to the azo toluene moiety (from being previously on the phenol group) due to the high electron-withdrawing effect of the nitrophenyl azo group. What makes it interesting is that the LUMO level was changed to be on the nitrophenyl azo group itself, confirming its thirst for electrons. Therefore, Pz1 is confirmed to be of a richer electron cloud than Pz2. Table shows the values of some computed quantum chemical parameters for Pz1 and Pz2 in the gas phase and in case of the presence of a solvent. It is worthy to note that the HOMO energy of Pz1 is more than Pz2 in both phases, which explains the higher tendency of Pz1 to donate electrons compared to Pz2. Also, the LUMO energy for Pz2 was lower than that of Pz1, which confirms its higher tendency to accept electrons. ΔE is one of the characteristic parameters which represents the reactivity of molecules, where a compound with lower value (Pz1) is usually more reactive and has higher inhibition performance.[59] The electronegativity (χ) can also be considered as an index for molecular reactivity, which illustrates a compound’s tendency to hold its electrons. Usually, the lower the χ value, the higher the probability of electron donation and vice versa. This parameter’s values suggest that Pz1 is more prone to electron donation property and hence shows more ability to protect the steel surface. Most notably, the ΔN value symbolizes that the fraction of the transferred electrons either from Pz1 or Pz2 to the Fe substrate were correlated with the trend of the experimental IEs. In case the values of ΔN > 0, the electron will be transferred from Pz1 or Pz2 to the substrate surface. In addition, Pz1 has a higher ΔN value, which again confirms that Pz1 has better inhibition performance than Pz2.[60] The use of the calculated dipole moment (μ) value to confirm the experimental IE results is often a debatable point. Many studies discussed that an increase in the μ interaction value will point to a higher inhibition performance, while others support the counter opinion, with the explanation that a lower value of μ will support a better inhibitor adherence to the metal surface.[61] In our study, the μ trend is more compatible with the second point of view. To sum up, there is an agreement between the computed quantum parameters and the experimental one, also the quantum parameters show why Pz1 has a higher corrosion IE than Pz2.

Table 5

Computed Quantum Chemical Parameters of the Investigated Pz1 and Pz2

compound	EHOMO (eV)	ELUMO (eV)	ΔE (eV)	A (eV)	χ (eV)	η (eV)	ΔN (e)	μ (debye)
	gas phase
Pz1	–4.849	–3.306	1.543	3.306	4.078	0.772	1.892	4.556
Pz2	–4.939	–3.366	1.573	3.366	4.153	0.787	1.810	6.693
	solution phase
Pz1	–5.0858	–3.5293	1.557	3.529	4.308	0.778	1.730	5.075
Pz2	–5.1402	–3.4939	1.646	3.494	4.317	0.823	1.630	7.260

Monte Carlo Simulation

The Monte Carlo (MC) simulation method recently became a very supportive way to predict the interaction level between the prepared materials and the desired surface.[62]Table shows the outputs of the carried out simulation of loading either Pz1 or Pz2 on a constructed Fe surface. These parameters represent the calculated outputs of such a simulation, which includes the adsorption energy, rigid adsorption energy, and deformation energy. Adsorption energy represents the amount of energy released by the molecules’ adsorption on the surface, which is the sum of the rigid adsorption energy and the deformation energy. Rigid adsorption energy expresses the initially released energy when the unrelaxed molecules lie on the surface, whereas deformation energy represents the required or released energy for the molecular structure’s geometrical relaxation on the surface. The tabulated adsorption energy values in both the gas phase and in the presence of water molecule—Table —indicated that Pz1 has higher adsorption energy than Pz2, which showed its higher affinity to adhere to the Fe (110) surface. The term dEad/dNi represents the differential adsorption energy and its absolute value describes the required energy of removing a particular component of all the adsorbed species on the surface.[63]

Table 6

Adsorption Simulation Energy Outputs Calculated by MC Simulation for Pz1 and Pz2 on Fe (110)

compound	adsorption energy (kcal/mol)	rigid adsorption energy (kcal/mol)	deformation energy (kcal/mol)	inhibitor: (dEads/dNi) (kcal/mol)	water: (dEads/dNi) (kcal/mol)
	gas phase
Pz1	–924.45	–54.79	–869.66	–256.70
Pz2	–132.00	–38.55	–93.45	–132.00
	solution phase (500 part. H2O)
Pz1	–13707.23	–11076.22	–2631.01	–1004.50	–25.03
Pz2	–12750.49	–10884.68	–1865.81	–204.86	–28.26

The listed data interpretation in Table shows that the dEad/dNi value for the inhibitor is much higher than that of the water molecules, indicating the high adherence of the Pz molecules compared to that of the water particles. Figures and 8 show Pz1 and Pz2 adsorption modes’ side and top views on a cleaved Fe(110) crystal surface. For the gas phase simulations, Figure shows that both compounds were adsorbed in a near-flat orientation on the Fe(110) crystal surface. However, repeating the same simulations with H2O molecules—Figure —in the adsorption vicinity, shows that Pz1 maintained the flat orientation, yet Pz2 became bulkier to maintain such configuration. This might add to the fact that Pz1 was of better behavior in the adsorption approach and accordingly is a better corrosion inhibitor.

Figure 7

Side (left) and top (right) views of the adsorption mode of Pz1 and Pz2 on steel (110) surface.

Figure 8

Side (left) and top (right) views of the adsorption mode of Pz1 and Pz2 on steel (110) surface in the presence of 500 H2O molecules.

Surface Morphology Analysis

The formation a surface protective layer of inhibitor molecules over the steel substrate was investigated using the scanning electron microscopy (SEM) technique. Figure depicts the recorded SEM images of 5L X60 carbon steel surfaces that were dipped in 5 M HCl with and without an inhibitor dose of 2000 ppm. The carbon steel surface in Figure a represents the morphological effects by the HCl attack without an inhibitor, the surface might have been greatly destructed by corrosion, as clearly seen in areas that have been coarsened and extremely consumed on the surface with reference to that of the free metal without being subject to any corrosive attack (Figure b). However, after the addition of 2000 ppm dose of Pz series inhibitors, as portrayed in Figure c for Pz2 compound, the surface was smoother with a minimal trace of destructive attack. This may give a clue on how the rate of corrosion might have been controlled due to the adsorbed film of the inhibitor molecules on the carbon steel surface.

Figure 9

SEM photographs of API 5L X60 steel in (a) 5 M HCl, (b) free polished sample, and (c) in the presence of 2000 ppm of Pz1.

The protective feature of the layered film confirms the IE of the Pz series inhibitors that was discussed above by the electrochemical and computational approaches. Hence, SEM images affirm the ability of the Pz inhibitors in diminishing the CR of the 5L X60 carbon steel surface through the adsorption mechanism.

Microbial-Induced Corrosion

Microbial corrosion is a type of corrosion that arises from the presence of some undesirable coexisting factors, particularly, a suitable aqueous medium for the specified bacterial growth, the vulnerability of the targeted material, aerobic/anaerobic condition, and finally the strain of bacterium which will drive the reaction.[64] SRBs have demonstrated its power in MIC under anaerobic conditions for different grades of iron and its alloys.[65−67] This test was performed in order to study the effect of the structured pyrazoline derivatives (Pz series) as proven antibacterial agents against SRB microbes that causes MIC under anaerobic conditions. The utilized methodology in the assessment of the biocidal activity of the Pz series compounds adopts the standard test method NACE TM0194-14-SG. This standard practice is broadly used in the oil and gas wells as a routine test for determining the microbial growth rate in anaerobic water locations such as well trunklines and different phases of treatment plants such as enclosed water at locations such as the bottom of oil storage tanks.[68] Concurrently, this technique can be utilized to evaluate the efficacy of the presently used aldehyde-based commercial biocides.[69]Figure represents the employed sets of the specially synthesized culture media to measure the microbial cell count of SRB for the collected water sample from a previously known infected well formation against added doses of Pz1 and Pz2. These images depict isolated glass vials in order to achieve the desired anaerobic state. The inhibitor was injected into the vials in a serial dilution manner. Two sets of serial dilution series were tested, each at different concentrations. The two concentrations were 20 and 40 ppm, and the test was carried out for both Pz1 and Pz2. The Pz series inhibitors showed encouraging antimicrobial activity against SRB bacteria. Figure portrays the taken images for the sets of the injected vials after the 28 day incubation period along with the blank. The biocidal activity was evident as the number of black vials (infected vials) was reduced compared to that of the blank.

Figure 10

SRB growth quantification media kits injected with infected water samples with different concentrations of Pz1 and Pz2.

Table represents the data explication obtained from the carried-out test. It shows that Pz2 has strongest activity for the inhibition of the bacterial growth than Pz1 (up to 100%) as they clearly scored 101 and 0 cell/mL in the 20 ppm and 40 ppm doses, respectively. While noticing Pz1 scores, it was obviously lower since it scored 102 and 101 cell/mL in the 20 ppm and 40 ppm doses, respectively. The higher biocidal behavior of Pz2 compound than Pz1 against SRB may be connected with the extra nitrophenyl azo moiety present in the Pz2 structure—Figure —which had an electron-withdrawing effect that reduced its corrosion inhibition action, yet it increased its action against SRB for its antibacterial behavior as reported.[70]

Table 7

SRB Growth Count for Pz1 and Pz2 at 40 °C for 28 days.

compound	conc.	SRB count (cell/mL)	reduction in SRB count (cell/mL)	efficiency (%)
blank		106
Pz1	20	103	103	50%
	40	101	105	83.3
Pz2	20	102	104	66.7
	40	100	106	100

Figure 11

Extra nitrophenyl azo moiety in Pz2 that may cause its robust action against SRB.

Mechanism of Corrosion Inhibition

The inhibitive action for the corrosion of API 5L X60 carbon steel in 5 M HCl was examined by using the designed derivatives of pyrazoline, Pz1 and Pz2. The efficacy of such inhibitors depended on numerous variables such as concentration, quantity of adsorption centers, electron densities, and optimized geometrical structure. Particularly, the presence of heterocyclic moieties with extraordinary nucleophilic character such as pyrazoline rings and heteroatoms (N, and O) tends to suppress the corrosive attacks on the metal surface by their higher affinity to be adsorbed on the surface, obstructing both cathodic and anodic reactions on the surface of the carbon steel. Generally, the corrosion mitigation mechanism was about covering the surface with an isolating layer by sharing electrons with unfilled d-orbitals of the carbon steel (coordination), then gradually replacing the formerly adsorbed water particles from the surface. The structure of Pz1 inhibitor was clearly of a better effect for the inhibition than Pz2 due to its higher adsorption affinity as proved by the outputs of the adsorption isotherm model of Langmuir. Computational analysis, particularly DFT, revealed that the presence of the extra nitrophenyl azo group in Pz2 had an electron-withdrawing effect that limited the electron donation ability of Pz2 to the metal surface as presented in Figure .[56] The designated nitrophenyl azo group has limited the electron availability on the other adsorption centers, which had an adverse effect on the Pz2 bonding ability to the surface as marked with a red line in Figure . In addition, MC simulations showed that Pz1 was of almost a planar orientation to the surface, yet Pz2 was hindered by its energetically optimized geometry on the same surface, so Pz1 had higher adherence ability to the surface with maximum action.[71−73]

Figure 12

Proposed mechanism of action of Pz2 compared to Pz1 on the steel surface, outlining the electron-withdrawing action of the nitro phenyl azo group.

Experimental Section

Synthesis of the Inhibitors

Two selected pyrazoline derivatives Pz1 and Pz2 were prepared according to our previous protocol.[31,32]

Electrochemical Investigations

The three-electrode electrochemical glass cell was constructed which comprises a platinum sheet as the auxiliary electrode (CE), saturated calomel electrode (SCE) as the reference electrode, while the 5L X60 carbon steel specimen under evaluation was collected from a previously damaged pipe, cut and placed as the working electrode for an uncovered area of 1 cm2 for exposure to the corrosive solution.

Electrochemical examinations were accomplished using a Volta Lab/PGZ300 potentiostat by Tacussel-Radiometer Analytical, and data were analyzed by the aid of the Volta Master 4 V. 8.0 software package. The OCP immersion time was set to 1 h, within which a stable plateau (steady-state) was achieved. Accordingly, Tafel polarization measurements were performed with a ±200 mV potential sweep relative to this steady-state potential (EOCP). The examined scan rate was 1 mV/s which was more suitable to our experimental conditions in such highly acidic corrosive environment (5 M HCl), so as not to cause any damage to the electrode surface or produce local changes in the electrolyte which might interfere with the outputs.[74,75] The EIS technique was conducted by applying two small AC sin waves (current and potential) with a 10 mV PK–PK amplitude versus EOCP. The current responses were potentiostatically measured with a frequency change between a range of 100 kHz to 10 MHz. The added Pz series molecules were found to dissolve directly in the prepared HCl acid solution.

Computational Studies

The Materials Studio software package V. 6.0 from Dassault Systemes BIOVIA, Inc. was used to conduct all the DFT calculations for the inhibitor compound structures after being energetically minimized using the embedded DMol3 module with a selected basis set of GGA/BOP/DNP inputs. For reliable theoretical calculations, it is necessary to consider the solvation effects rather than executing the calculations in the gas phase, since the corrosion phenomena takes place in an aqueous environment. Thus, DMol3 with a specific COSMO solvation model was implemented to study the effect of the solvent. Using such technique, quantum chemical descriptors such as energy of Frontier molecular orbitals (HOMO and LUMO respectively), the fraction of electrons transferred (ΔN), the electron density, and the dipole moment were taken in consideration based on the utilized conformations with the lowest energy, using the following eqs and 10.[76]where ηFe, χFe, and ηinh, χinh denote respectively the global hardness and the electronegativity of Fe and the prepared compounds.[77]

MC simulations were used to investigate the adsorption ability of Pz1 and Pz2 on the Fe surface by calculating their adsorption energy. Fe(110) plane was selected to perform such adsorption imitations as it represents about 63% of the main crystal growth plane for Fe crystal, in addition, it is a bulk terminated plane without any further surface reconstructions.[78−80] Fe(110) surface was cleaved with 3 Å thickness, and the forcefield (COMPASS) was selected to minimize its energy before loading the inhibitor molecules under study via simulated annealing algorithm. The cleavage plane was expanded to a (15 × 15) supercell, with a 25 Å vacuum slab built above the Fe(110) crystal under periodic boundary conditions to allow homogeneous loading on the surface.[81] Throughout the simulation annealing study, the adsorbed molecule was in a restriction-free state while the Fe surface was fixed.[82]

Microbial-Induced Corrosion

Infected water samples with SRB were collected from the bottom of the oil production tank, where water usually settles after production. This location is perfect for the colonization of SRB which grows under anaerobic conditions that this location provides.[83,84] SRB can cause severe MIC impacts in such location which may lead to catastrophic consequences.[85] Specific media for the quantification of the SRB bacterial growth rate was commercially obtained from Egyptian Petroleum Research Institute (EPRI), which was prepared and sealed in isolated vials in accordance with NACE TM0194-14-SG.[86] The methodology of the testing was followed in accordance with NACE TM0194-14--SG standard test and our previous study.[4]

Conclusions

Structurally designed and synthesized pyrazoline-derived Pz series (Pz1 and Pz2) were utilized as powerful inhibitors for the protection of API 5L X60 carbon steel samples against both the acidization matrix of 5 M HCl and SRB microbial attacks. The Langmuir model was the best-fitted isotherm and the most appropriate for the adsorption modeling of such series inhibitors. It also proved their mixed cathodic/anodic nature. Both EIS and EP techniques have demonstrated the IE of either Pz1 or Pz2 inhibitors. Comparing Pz1 and Pz2, due to the presence of the electron-withdrawing nitrophenyl azo group in Pz2, its corrosion IE was inferior to that of Pz1. Computation quantum calculation has also proved this claim through DFT calculation and MC visualization that affirmed the electrochemical findings. MIC IE was assessed for both Pz1 and Pz2 against SRB infected water from the bottom of oil storage tanks. The collected data showed the superiority of Pz1 over Pz2 due to the biologically active nitrophenyl azo group, and in general, both had a strong biocidal effect against SRBs. Thus, Pz series compounds may be utilized as both acidization corrosion inhibitors and biocides for infected water areas such as bottom of the storage tanks and annulus water in the oil well. To sum up, Pz series inhibitors can serve as an economic option for oil-producing companies due to its dual action.