Corrosion Mitigation Performance of N80 Steel in 5% Sulfamic Acid Medium by Applying Novel Tetrahydro-1,2,4-triazines Including Triazene Moieties: Electrochemical and Theoretical Approaches.

We observed our newly developed tetrahydro-1,2,4-triazines, including triazene moieties (THTA), namely, 6-((1E)-1-((2E)-(4-(((Z)-1-(2,4-diphenyl-2,3,4,5-tetrahydro-1,2,4-triazin-5-yl) ethylidene) triaz-1-en-1-yl)piperazin-1-yl) triaz-2-en-1-ylidene) ethyl)-2,4-diphenyl-2,3,4,5-tetrahydro-1,2,4-triazine (THTA-I), and 1-((E)-((E)-1-(2,4-diphenyl-2,3,4,5-tetrahydro-1,2,4-triazin-6-yl) ethylidene) triaz-1-en-1-yl) naphthalen-2-ol (THTA-II), as effective inhibitors for the corrosion protection of N80 carbon steel metal in 5% sulfamic acid as the corrosive medium via electrochemical approaches such as potentiodynamic polarization and electrochemical impedance spectroscopy. Furthermore, the tested steel exterior was monitored using X-ray photoelectron spectroscopy after the treatment with the investigated components to verify the establishment of the adsorbed shielding film. The investigated compounds acted as mixed-type inhibitors, as shown by Tafel diagrams. The compounds considered obey the Langmuir adsorption isotherm, and their adsorption on the steel surface was chemisorption. When the tested inhibitors were added, the double-layer capacitances, which can be determined by the adsorption of the tested inhibitors on N80 steel specimens, decreased compared with that of the blank solution. At 10-4 M, the inhibitory efficacy of THTA-I and THTA-II achieved maximum values of 88.5 and 86.5%, respectively. Density-functional theory computations and Monte-Carlo simulation were applied to determine the adsorption attributes and inhibition mechanism through the studied components. Furthermore, the investigated inhibitors were considered to adsorb on the Fe (1 1 0) surface. The adsorption energy was then measured on steel specimens.

Introduction

Corrosion progression plays a crucial role in the economy and preservation of alloys and metals. N80 carbon steel is extensively operated as a structural material in industries, therapeutic applications, propulsion applications, petroleum applications, and food manufacturing. The corrosion of carbon steel petroleum lines is a complex process that leads to considerable problems owing to localized corrosion because certain solutions (i.e., media) are aggressive. Corrosion can be mitigated by the addition of inhibitors in smaller amounts. Organic compounds including heteroatoms like S, N, and O are the most effective inhibitors.[1−5] As per the present data, organic components with functional groups such as −OH, −NH2, −OCH3, −COOH, and −SO3H as well as benzene moieties containing heteroatoms may be effective inhibitors because they can react with the studied metal or alloy to form a protective film on the inspected exterior.[6−9]

Although their corrosion properties are weaker than those of H3PO4, HCl, and H2SO4, the aquatic solutions of sulfamic acid have been examined as a forceful acid; their dissolved maximal industrial accumulation is determined by creating soluble components through sulfamic acid.[10] To prevent the corrosion of investigated metals, researchers should add organic inhibitors to the acid purification of pipes, cylinders, and apparatus composed of several types of steels.[11,12]

Various amino acids,[13] tetra-pyridinium components,[14] azo dye-particles,[15] aquatic root extract of Salvadora persica,[16] monomeric and Gemini surfactants,[17] and cationic surfactants,[18] were reported as corrosion inhibitors for carbon steel in corrosive sulfamic acid environments. Consequently, triazine derivatives indicate a great assurance as inhibitors for biological and pharmaceutical applications, for example, antibacterial,[19] analgesic,[20] antiviral,[21] antimalarial,[22] antitumor, and cytotoxic activities.[23] Organic compounds, including thiazole moieties, have previously been explained as biologically active operatives, such as vitamin B1 (thiamine), thus improving the nervous system function,[24] and multiple general thiazole-containing antibiotics.[25]

Quantum chemical concepts have been extensively used to understand corrosion mechanisms and clarify experimental data and interpret chemical ambiguity. The use of such concepts is an appropriate technique for examining the corrosion reaction mechanisms of organic inhibitory compounds with the examined surface.

This paper aimed to examine the influence of tetrahydro-1,2,4-triazines including triazene derivatives, on the corrosion prohibition of N80 steel in 5% sulfamic acid solutions through potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) proportions. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to scan the surface morphology to demonstrate the deposited protective coating on the N80 steel. To demonstrate energetic cores and prohibition mechanisms of corrosion processes for the investigated THTA compounds, we applied quantum chemical calculations (DFT) using Fukui indices, Mulliken atomic charges, molecular electrostatic potential (MEP) mapping, and Monte-Carlo (MC) simulations.

Materials and Methods

Materials and Reagents

In this study, an N80 carbon steel sample [constitution (weight percent): C (0.31%), S (0.008%), P (0.01%), Si (0.19%), Mn (0.92%), Cr (0.20%), and the remainder comprised Fe] with an effective surface of ≈1 cm2 was utilized as the working electrode and scraped using emery papers of various grades (up to 1000 grade), degreased with propanone, rinsed with doubly distilled water, and desiccated with faint tissues. Trials were conducted in 5% sulfamic acid solution in the absence and presence of various portions of the investigated tetrahydro-1,2,4-triazines incorporating triazene derivations outlined in previous work[26] (Table ). For each experiment, the concentrations of the analyzed components ordered from 1 × 10–6 M to 1 × 10–4 M; a freshly discarded solution was used.

Table 1

Molecular Weight and Chemical Composition of Inhibitors under Investigation

Electrochemical Measurements

An N80 carbon steel sample was utilized as the working electrode, which was attached to the epoxy resin of polytetrafluoroethylene, a saturated calomel electrode (SCE) as the auxiliary electrode, and a Pt wire as the assembly conductor. To reduce the IR drop, we linked the auxiliary conductor to a Luggin capillary tube, and the head of the capillary was located near to the working electrode. The reaction chamber was released in the atmosphere; all tests were conducted at 25 ± 1 °C. The potential standards were calibrated vs SCE. Before every measurement, the N80 carbon steel surface was scratched with various emery papers, cleaned with a basic solution (15 g Na2CO3 + 15 g Na3PO4 by liter),[27] rinsed with deionized water, and then wiped.

The electrode potential was adjusted between −0.8 and 0.2 V versus SCE using a scan rate of 1 mVs–1 at an open circuit potential to achieve the PP results. The Stern Geary process[28,29] was used to determine the corrosion current by calculating the cathodic and anodic Tafel figures of charge transfer restrained corrosion to a spot that obtains (log icorr) and the equivalent corrosion potential (Ecorr) for 5% sulfamic acid medium, and the concentrations of the studied triazene inhibitors (THTA). Subsequently, icorr was used to assess the prohibition capability and surface coverage (θ) using the following equation:wherever icorr(free) and icorr(inh) are the corrosion current densities in the presence and absence of the examined THTA components, respectively.

EIS investigations were executed in the frequency range of 100,000 to 0.1 Hz, thus resulting in a peak-to-peak voltage of 10 mV through AC signals at open circuit potential.

Impedance experiments were conducted, and the result for the constructed circuit was explained. The polarization resistance Rp and double-layer capacity of Cdl, which were determined, were the intended variables concluded with the examination of Nyquist semicircles as follows:wherever fmax is the angular frequency when the imaginary part of the impedance expands the most. The following equation was used to compute the impedance method’s prohibition capability and surface coverage (θ):

The electrode potential was permitted to stabilize for 30 min prior to each measurement, which was performed at 25 °C. A Gamry Reference 3000 was used to perform electrochemical measurements using a potentiostat/galvanostat/ZRA with Gamry Framework version 6.33 software set up on a PC for leading and result registration. Echem Analyst version 6.33 software was used for statistic planning and analysis. For the agreement and guarantee of investigations, all tests were conducted on a regular basis in a similar state. Each electrochemical test was repeated three times to emphasize data duplicability.

Surface Investigation by XPS

High-resolution XPS examination was carried out through K-ALPHA (Thermo Fisher Scientific, USA) with monochromatic X-ray Al K-alpha radiation to unrestrained and restrained N80 specimens in 5% sulfamic acid using 1 × 10–4 M 6-((1E)-1-((2E)-(4-(((Z)-1-(2,4-diphenyl-2,3,4,5-tetrahydro-1,2,4-triazin-5-yl) ethylidene) triaz-1-en-1-yl)piperazin-1-yl) triaz-2-en-1-ylidene) ethyl)-2,4-diphenyl-2,3,4,5-tetrahydro-1,2,4-triazine (THTA-I) for 24 h at 25 ± 1 °C.

Density-Functional Theory (DFT) Calculations and MC Simulations

To consider the corrosion mechanism, the association within theoretical characteristics, and experimental results, the inhibition efficiencies were efficiently analyzed. Computerized analyses were completed using Accelrys Materials Studio 7.0[30] including the DMol3 module for DFT calculations and the adsorption locator module for MC simulations. The configurations of triazine components were optimized in DFT calculations utilizing the GGA/BLYP functional with set DNP foundation and COSMO solvation controls.[30] The best proper adsorption preparations of the triazine derivative inhibitors on the Fe (1 1 0) surface were demonstrated using the adsorption locator module in MC simulations to estimate the prohibition tendency of the tested inhibitors.[31] The THTA component adsorption with water particles and examined surface of Fe (1 1 0) was obtained using a simulating box (32.27 × 32.27 × 50.18 Å3) and the assigned COMPASS force field.[32] Moreover, all the theoretical analyses’ entries, results, and computations were reflected in our recently published publications.[30,31]

Results and Discussion

Eocp vs Time and PP Studies

Effect of Triazene Component Concentrations

Figure A and B reveals the change of Eocp vs time for inhibited and uninhibited N80 steel in 5% sulfamic acid solutions during 30 min exposure time. As shown in Figure BA, the N80 steel in uninhibited and inhibited solutions reaches a stable state after ≈ 400 s immersion, and the values of Eocp exhibit a small adjustment (< 0.001 V vs SCE). The primary value of the blank specimen was −0.474 V vs SCE, and it reached steady state near −0.485 V vs SCE. For inhibited specimens with triazene derivatives, the values of Eocp shifted less than 85 mV compared to the uninhibited specimen for the period of the immersion indicating that triazene derivatives serve as mixed-type inhibitors. The values of Eocp slightly reduced with the immersion period up to ≈400 s. The disturbed period could be ascribed to desorption and adsorption of the triazene derivatives at the interface of N80 steel/sulfamic acid. After the exposure period ≈400 s, the Eocp values predisposed to steady, demonstrating that the adsorption and desorption of triazene derivatives had affected a stable equilibrium.

Figure 1

Eocp vs time measurements (A,B) and Tafel diagrams (C,D) for N80 carbon steel corrosion in corrosive medium containing 5% sulfamic acid in the absence and presence of different doses of THTA-I (A,C) and THTA-II (B,D) at 25 ± 1 °C

The corrosion performance of the N80 steel in 5% sulfamic acid solutions using the triazene derivatives as corrosion inhibitors was investigated using the PP approach at 25 ± 1 °C. Cathodic–anodic curves were determined with and without the examined inhibitors, as shown in Figure C,D for THTA-I and THTA-II, respectively. The corrosion potential (Ecorr), corrosion current density (icorr) acquired utilize polarization plots by extrapolation, anodic and cathodic Tafel slopes (βa and βc), the exposure surface (θ), and prohibition efficiency (%IE) are recorded in Table and acquired by eq .

Table 2

Tafel Polarization Parameters for Corrosion of N80 Carbon Steel in 5% Sulfamic Acid Solution with and without Different THTA Compound Concentrations at 25 ± 1 °C

inhibitor	conc., (M)	–Ecorr mV (vs SCE )	icorr, μAcm–2	βa, mV dec–1	βc, mV dec–1	corrosion rate mpy	Θ	IE %
5% sulfamic acid (BLANK)		486	257.0 ± 13.7	170.1	217.3	117.6
THTA-I	1 × 10–6	485	92.1 ± 4.9	119.1	205.4	80.43	0.642	64.2
	5 × 10–6	482	88.6 ± 4.5	126.1	220.4	69.09	0.655	65.5
	1 × 10–5	448	73.9 ± 3.5	85.8	168.9	59.65	0.712	71.2
	5 × 10–5	488	46.8 ± 2.2	139.7	187.8	42.87	0.818	81.8
	1 × 10–4	495	39.2 ± 1.9	119.5	167	38.03	0.847	84.7
THTA-II	1 × 10–6	420	98.5 ± 5.2	56	181.7	84.28	0.617	61.7
	5 × 10–6	433	90.0 ± 4.7	64.1	179.8	55.19	0.650	65.0
	1 × 10–5	455	75.0 ± 4.0	74.3	147.2	47.74	0.708	70.8
	5 × 10–5	489	49.7 ± 2.6	92.1	228.5	43.76	0.807	80.7
	1 × 10–4	446	42.4 ± 2.3	56.5	117.4	39.32	0.835	83.5

With the increase in the inhibitor concentrations of analyzed inhibitors, Tafel polarization plots demonstrated that the investigated compounds[33] displaced either anodic or cathodic curves to small current densities, indicating that THTA molecules are mixed-type inhibitors. Moreover, the corrosion current density was reduced from 275 μA cm–2 to 39.2 and 42.4 μA cm–2 in the presence of 1.0 × 10–4 M THTA-I and THTA-II inhibitors, respectively.

Table shows that the rising of the tested inhibitor concentrations from 1 × 10–6 M to 1 × 10–4 M enhanced the inhibition capacity percentage (% IE). The corrosion potentials (E) were slightly changed by the addition of THTA components, indicating that these molecules were mixed-type inhibitors. The anodic and cathodic Tafel slope (βa and βc) values were altered by the increase concentrations, indicating that THTA compounds affected the dissolution mechanism and evolution of hydrogen reactions.[34,35]Table shows that the inhibitory power of THTA-I was greater than that of THTA-II at various dosages.

Adsorption Isotherm

Based on the adsorption mechanism of THTA compounds on N80 specimens, all examined THTA components limited the corrosion approach. Equation shows the adsorption of the THTA compound at the metal/solution interface, which occurred when water molecules were replaced by THTA compounds.[36]where THTA(sol) and THTA(ads) denote the THTA components in the solution and adsorbed on the N80 carbon steel exterior, correspondingly, and x is the number of water molecules that have been replaced by THTA particles.

The following formula can be used to calculate the surface exposure (θ) by THTA particles:wherever θ is the different adsorption isotherm classes obtained by determining the best adsorption isotherm type. We also examined these investigated inhibitors by different isotherms as the Flory–Huggins isotherm (R2 = 0.228, for THTA- I and R2 = 0.224 for THTA-II), Temkin (R2 = 0.899, for THTA-I and R2 = 0.889 for THTA-II), and Frumkin (R2 = 0.817, for THTA-I and R2 = 0.800 for THTA-2). All the THTA inhibitors fitted the Langmuir isotherm type (R2 = 0.999, for THTA-I and R2 = 0.999 for THTA-II). eq , which provides a straight line with a coefficient constant (R2) of singularity at 25 ± 1 °C and slope equal to unity, explains the relationship between C and C/θ (Figure ).wherever C stands for the inhibitor’s concentrations and Kads refers to the adsorption process equilibrium constant, which is linked to the standard free energy of adsorption method (ΔGads0) and can be derived using eq :[37]whenever 55.5 is the apparent concentration of water solution expressed in (mol/L), (T) is the absolute temperature expressed in K (°C + 273.15), and (R) is the universal gas constant. The values of (θ) obtained from the polarization method were compared with a chart to determine the best adsorption isotherm, indicating that the adsorption of the tested THTA compounds on the N80 surface (Figure ) followed the Langmuir adsorption isotherm.[38,39]

Figure 2

Langmuir adsorption isotherm fitting for corrosion data for of N80 steel in 5% sulfamic acid within different portions of the investigated THTA compounds at 25 ± 1 °C.

Using eq , the standard free energy of adsorption (ΔGads0) for THTA-I was −42.9 kJ mol–1 (Kads = 6.17 × 105 L mol–1) and that for THTA-II was −43.5 kJ mol–1 (Kads = 7.53 × 105 L mol–1). Charge transfer with the evaluated THTA compounds against the steel surface to create a coordinate bond (chemisorption adsorption)[40] and a high negative criterion of (ΔGads0) (i.e., −40 kJ mol–1 or above) demonstrated that the tested THTA compounds were spontaneously adsorbed on the steel surface.[41] Moreover, the creation of coordinate bonds with active locations of the investigated THTA (N atoms and π-orbital of double bonds) with vacant d-orbitals of the steel surface maintained the ΔGads0 values of THTA molecules nearly stable for the studied steel surface.

EIS Measurements

The Nyquist and Bode graphs for N80 carbon steel in 5% sulfamic acid corrosive medium with and without various dosages of THTA particles are demonstrated in Figure A,B and Figure A,B. The semicircle radius is used to express charge transfer resistance; hence the values of Rct for N80 steel in the corrosive medium of 5% sulfamic acid were low compared to when THTA inhibitors (1 × 10–4 M) were added (i.e., the radius increased).

Figure 3

Nyquist diagrams for the corrosion of N80 CS in 5% sulfamic acid medium including different portions of THTA compounds (A) THTA-I and (B) THTA-II at 25 ± 1 °C.

Figure 4

Bode diagrams for the corrosion of N80 carbon steel in 5% sulfamic acid medium involving different doses of THTA components (A) THTA-I, and (B) THTA-II at 25 ± 1 °C.

Figure A,B and Figure A,B demonstrate an impaired arch figure, an impedance smoothing within the center below the real axle, and depressed loops expanding significantly with captivation time, which is a normal action for the N80 steel electrode that displayed frequency dispersion for the impedance results,[42,43] which are confirmed with abnormality and extra inhomogeneity of the N80 surface.[44,45]

For inhibited solutions, two-time constants appeared in the EIS plots at small frequencies (Nyquist and Bode diagrams), whereas the blank solution demonstrated the configuration of a prohibitive barrier on the N80 carbon steel surface.

To examine the aforementioned achieved impedance calculations, we proposed the corresponding electric circuit structure in Figure a,b, wherever Rs is the solution resistance, Rf is the examined compound film resistance, CPEf is the constant-phase element of the tested component film, CPEdl is the constant-phase element of the double layer, and Rct is the charge transfer resistance.

Figure 5

Equivalent circuit employed to fit the data obtained from EIS technique (a) using a blank solution, and (b) with different portions of the examined inhibitors.

The protection efficacy for the corrosion of N80 steel (%IE) in the corrosive solution of 5% sulfamic acid was received with the polarization resistance RP by the following formulas:[46]

The polarization resistances of N80 samples in 5% sulfamic acid media with and free THTA components are RPo and RP, respectively.

The impedance ZCPE can be considered using the formula:[47,48]wherein Y0 is the CPE admission, j is the imagined digit, ω is the angular frequency (ω = 2πf), and n is the CPE index determined as a phase shift. Whenever n = 0, CPE designates a resistor; whenever 0 < n < 1, it designates a not perfect capacitor; whenever n = 1, it represents a perfect capacitor.

The nonideal capacitive action is assigned to the heterogeneousness of metal specimens because of the unevenness of the N80 steel, dispersion of energetic positions, metal dissolution, contaminants, and the adsorption of the tested components on the N80 exterior, which is represented by the standards of the n index change within 0.451 to 0.994.

Table lists the electrochemical variables discovered during EIS experiments. The higher Rct standards of N80 steel in corrosive media containing 5% sulfamic acid contain THTA components, demonstrating that the inhibition efficiency of N80 steel specimens was demonstrated by THTA inhibitor adsorption on N80 steel specimens as well as the permutation of adsorbed water particles on the N80 surface.

Table 3

Electrochemical Kinetic Variables Acquired by EIS for N80 Carbon Steel in 5% Sulfamic Acid Media Including Diverse Doses of THTA at 25 ± 1 °C

concentration, M		Rs, Ω cm2	Y0,f × 10–6, sn Ω–1 cm–2	nf	Rf, Ω cm2	Y0,dl c10–6, sn Ω–1 cm–2	ndl	Rct, Ω cm2	θ	IE/%
5% sulfamic acid		3.77				233.2	0.827	28.8 ± 1.8
THTA-I	1 × 10–6	5.01	105.5	0.903	13.5	106.4	0.838	112.7 ± 7.9	74.5	74.5
	5 × 10–6	4.16	52.57	0.978	7.4	167.6	0.790	113.40 ± 8.1	74.6	74.6
	1 × 10–5	4.49	94.92	0.902	24.1	86.26	0.828	121.8 ± 8.8	76.4	76.4
	5 × 10–5	4.69	65.28	0.880	13.7	47.37	0.770	240.6 ± 16.2	88.0	88.0
	1 × 10–4	5.77	91.28	0.848	26.4	3.305	0.994	250.5 ± 17.4	88.5	88.5
THTA-II	1 × 10–6	5.27	272	0.776	70.1	631.90	0.704	93.0 ± 6.5	0.691	69.1
	5 × 10–6	6.94	395	0.731	66.2	371	0.834	96.9 ± 6.8	0.703	70.3
	1 × 10–5	6.03	594	0.669	165.0	77.16	0.669	120.0 ± 8.6	0.760	76.0
	5 × 10–5	6.86	498	0.641	177.7	193.20	0.986	131.6 ± 9.5	0.781	78.1
	1 × 10–4	5.26	122	0.786	6.6	1328	0.451	215.0 ± 14.5	0.866	86.6

The consequential reduction in the access of CPE standards was assigned to the THTA inhibitor adsorption on the N80 specimens, which expanded the width of the electrical double layer and inhibitor layer barrier based on THTA particle adsorption on the N80 surface. The investigations exhibited the corrosion inhibition capability of THTA-I > THTA-II at different concentrations of THTA components.

Surface Morphology by XPS Examinations

XPS tests were used to identify, discriminate, and confirm the chemical bonding and configuration of the THTA-I component and the adsorption mechanism of THTA-I particles on the N80 steel exterior. As shown in Figure , the XPS spectra were recorded for the N80 steel surface corroded in 5% sulfamic acid with the THTA-I compound. The peaks for C 1s, Fe 2p, O 1s N 1s, and S 2p were discovered in samples treated with the THTA-I compound, indicating the THTA-I molecule adsorption on the N80 exterior. The binding energies (Bes; eV) and corresponding appointments of each peak ingredient are recorded in Table .

Figure 6

XPS plots of C 1s, Fe 2p, O 1s, N 1s, and S 2p for N80 carbon steel in 5% sulfamic acid solutions with 1 × 10–4 M of the THTA-I compound.

Table 4

Binding Energy (eV) and Their Assignments for the Best Core Lines Noted for the N80 Carbon Steel Surface in 5% Sulfamic Processed with THTA-I Inhibitor

5% sulfamic acid treated with 1 × 10–4 M THTA-I inhibitor
core element	BE, eV	assignments
C 1s	285.41	–C–H, −C–C–, −C=C–
	286.89	–C–N
	287.55	–C=N
	288.95	–C=N+
Fe 2p	711.26	Fe 2p3/2 of Fe2+ in FeO FeCl2
	713.10	Fe 2p3/2 of Fe3+ in Fe2O3, Fe3O4, FeOOH
	719.01	satellite Fe 2p3/2 of Fe2+ in FeO
	721.09	satellite Fe 2p3/2 of Fe3+ in Fe2O3, Fe3O4
	724.36	Fe 2p1/2 of Fe0 in FeO
	726.65	Fe 2p1/2 of Fe2+ in FeO
	729.65	Fe 2p1/2 of Fe3+ in Fe2O3
	734.00	satellite Fe 2p1/2 of Fe2+ in Fe2O3, Fe3O4
O 1 s	530.21	FeO, Fe2O3
	531.70	FeOOH
	532.25	adsorbed water molecules
N 1s	399.21	sp3 bonding (C–N)
	400.17	sp2 bonding (C=N)
	401.07	C=N+
S 2p	168.63	–SO32–

The simple spectra of C 1s presented four peaks (Figure ), with that at 285.41 eV assigned to the existence of −C–C–, −C=C–, and C–H bonds; that at 286.89 eV was credited to the presence of the −C–N bond; that at 287.55 eV was attributed to the C=N bond; and that at 288.95 eV was assigned to the C=N+ bond.[49,50] The XPS spectra of Fe 2p for the treatment with the THTA-I compound demonstrated eight typical peaks (Figure ), with 711.26 eV assigned to the Fe 2p3/2 of Fe2+, 713.10 eV to the Fe 2p3/2 of Fe3+,[51] 719.01 eV to Fe 2p3/2 satellites of Fe2+, 721.09 eV to Fe 2p3/2 satellites of Fe3+,[52] 724.36 eV to the Fe 2p1/2 of Fe0, 726.65 eV to the Fe 2p1/2 of Fe2+, 729.65 eV to the Fe 2p1/2 of Fe3+, and 734.49 eV to the Fe 2p1/2 satellites of Fe2+.[53,54] Moreover, the high-resolution O 1s spectrum for the THTA-I compound revealed three peaks (Figure ): the first (530.21 eV) was attributed to O2– and may be associated with O– atoms bonded to Fe2+and Fe3+ in FeO and Fe2O3 oxides,[55,56] respectively; the second (531.70 eV) was attributed to OH– that can be bonded to Fe3+ in FeO.OH and the third (532.25 eV) was assigned to the adsorbed water molecules.[57,58]

Moreover, the N80 steel in 5% sulfamic containing THTA-I exhibited N 1s spectra with three peaks (Figure ) at 399.21 eV for sp3 bonding (C–N), 400.17 eV for sp2 bonding (C=N), and 401.07 eV for C=N+, all of which are present in the THTA-I molecule.[59,60] Furthermore, the S 2p spectra included one characteristic peak (Figure ) at 168.63 eV, and it was assigned to the −SO32– group exhibited in the sulfamic acid solution.[57] We confirmed the mechanism of THTA-I adsorption on the N80 surface in 5% sulfamic acid solution, which was in agreement with the XPS data.

DFT Computations

The interaction range in the active locations of triazine derivative components with the N80 steel surface was examined using DFT simulations. Furthermore, quantum chemical variables are designed in Table . Figure displays the optimal constructs, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) for the examined inhibitors. HOMO and LUMO energies were allocated to the studied inhibitor’s provider and receiver capabilities at the interface of inhibitor/metal exterior, in accordance with the FMO hypothesis.[61] Consequently, components with large EHOMO and small ELUMO values were considered to be better corrosion inhibitors.

Table 5

Intended Quantum Chemical Variables for THTA Derivatives

inhibitor	THTA-I	THTA-II
EHOMO, eV	–4.50	–4.70
ELUMO, eV	–2.99	–2.46
ΔE, eV	1.52	2.24
I	4.50	4.70
A	2.99	2.46
Χ	3.75	3.58
Η	0.76	1.12
Σ	1.32	0.89
ΔN	0.70	0.55
ΔEback-donation, eV	–0.19	–0.28
dipole moment value, debye	8.77	6.89
molecular surface area, Å2	696.15	454.51

Figure 7

Optimized molecular configuration, HOMO and LUMO of the THTA inhibitors via the DMol3 module

Table shows that the THTA-I compound had a larger EHOMO value of −4.50 eV than the THTA-II molecule (−4.70 eV). The HOMO point was placed on the triazine, tetraazene, phenyl, and naphthalenol rings for the inhibitor compounds (Figure ), indicating that N and O atoms were selected as sites for the electrophilic approach on the N80 steel exterior. Such arguments confirmed capability of the analyzed compounds to adsorb on N80’s exterior and, consequently, increased the prohibition productivity along with experimental results. Furthermore, the ELUMO values for THTA-I (Table ) were −2.99 eV lower than for THTA-II (−2.46 eV), which is consistent with experimental results, and THTA-I had a higher inhibitory potency than THTA-II. Moreover, the energy difference (ΔE) is an important indicator of prohibition productivity, which increases as the ΔE value reduces.[62]Table shows that THTA-I has a smaller ΔE value (1.52 eV) than THTA-II (2.24 eV) confirming that THTA-I has a greater tendency to be adsorbed on the N80 steel exterior. Furthermore, the low values of electronegativity (χ) provide a high potential reactivity for the examined inhibitors to allow electrons to the investigated surface; however, THTA-I has a greater value of χ (3.75) than THTA-II (3.58), signifying agreement with experimental conclusions; moreover, THTA-I has greater inhibition tendency than THTA-II.[63] Furthermore, the compound stability and susceptibility can be determined by hardness (η) and softness (σ), that is, soft compounds have additional protected potency than hard compounds owing to the smooth delivery of electrons to the N80 steel exterior during the adsorption, thus, they were evaluated as effective protective inhibitors.[64] As evidenced in Table , THTA-I has lower η values and greater σ values than THTA-II, indicating that THTA-I has a high potency for donating electrons to the examined steel and more prohibition potency.

The fraction of the charge transfer, ΔN values indicates how the compound possibly donates electrons to the steel sample; the higher the ΔN value, the more likely the compound is to donate.[65] As per the computed ΔN values in Table , THTA-I (0.70) has a higher ΔN value than THTA-II (0.55). Thus, THTA-I is more prone to deliver electrons to the N80 sample than THTA-II. Moreover, when η > 0 the ΔEback-donation will be <0, indicating that an electron will then be transported to a compound afterward, a back-donation from the inhibitor and it will be better selected.[32] The calculated ΔEback-donation standards for triazine compounds in Table are negative (−0.19, −0.28), indicating that back-donation was favored for the investigated inhibitors that formed a strong connection with the steel samples.[66]

Furthermore, the dipole moment is a significant indicator for predicting inhibition efficiency.[67] The increase in dipole moment increased the deformation energy and chemical adsorption on the examined steel. Consequently, increasing the dipole moment increased the corrosion inhibition potency.[69,70]Table shows that THTA-I has a higher dipole moment value (8.77 debye) than THTA-II (6.89 debye), indicating that THTA-I has a greater potency to be adsorbed on the tested N80 steel which improves the protection efficiency.

Furthermore, the propensity of the examined particles for preserving the N80 surface in corrosive media was linked to the molecular surface area. Given the contact region within the inhibitor compounds and the increased surface area of the N80 steel, the prohibition effectiveness boosted with the molecular surface area increments. As shown in Table , THTA-I demonstrated the greatest molecular surface area, which is supported by experimental evidence, whereas the highest prohibition potency was observed for THTA-I (696.15 Å2) than THTA-II (454.51 Å2).

Furthermore, using the Dmol3 module, MEP mapping may be employed to examine the higher energetic locations of THTA inhibitors. The MEP mapping is a 3D visual descriptor that has been proposed to identify the whole electrostatic effect of a molecule based on general charge dispensation.[68] The red-colored image in Figure shows the highest electron density area in the MEP maps; the MEP is always more negative (nucleophilic reaction). The blue colors, however, denote the highest positive domains (electrophilic reaction).[70] The highest negative domains are typically above triazine and phenyl moieties, as per the optical analysis in Figure . However, the electron density over the tetraazene moieties in the studied components was lower. For THTA compounds, MEP mapping demonstrated the most available positive locations over hydrogen atoms. The positions with high electron density (i.e., red areas) in the tested inhibitors may be most suited for correlations inside the steel, thus yielding a firmly adsorbed protecting film.

Figure 8

Visual diagram for MEP of the THTA inhibitors via the DMol3 module.

MC Simulations

The MC simulations aimed to indicate any evident adsorption mechanism and distinguish the correlations between the examined particles and steel specimens. Figure shows the most acceptable adsorption formation, which is virtually flat, for the THTA particles on the N80 steel specimens, indicating an increase in adsorption locator and maximum surface covering.[71]Table shows the obtained values for the adsorption energies derived from MC simulations. THTA-I (−2696.48 kcal mol–1) has a larger negative value of the adsorption energy than THTA-II (−2671.58 kcal mol–1), indicating that THTA-I has a strong adsorption on the N80 surface; moreover, it forms a strongly adsorbed barrier that inhibits the corrosion of N80 steel. These obtained data support the experimental results.[72]Table demonstrates that the adsorption energies for THTA-I in the pregeometry optimization stage, that is, unrelaxed (−2845.38 kcal mol–1) are higher negative than THTA-II (−2803.64 kcal mol–1) and in the postgeometry optimization stage, that is, relaxed (148.90 kcal mol–1) is greater than that of THTA-I (132.06 kcal mol–1), declaring a greater prohibition potency for THTA-I compared with THTA-II.

Figure 9

Best appropriate configurations for the adsorption of THTA inhibitors on Fe (1 1 0) substrate acquired by the adsorption locator module.

Table 6

Statistics and Descriptions Intended by the MC Simulation for the Adsorption of the Investigated Inhibitors on the Fe (1 1 0) Surface

structures	adsorption energy/kcal mol–1	rigid adsorption energy/kcal mol–1	deformation energy/kcal mol–1	dEads/dNi: inhibitor kcal mol–1	dEads/dNi: water kcal mol–1
Fe (110)	–2696.48	–2845.38	148.90	–190.31	–13.97
THTA-I
water
Fe (110)	–2671.58	–2803.64	132.06	–173.09	–13.84
THTA-II
water

If the adsorbed investigated compounds or water molecule is ignored, the dEads/dNi values can be used to explain the metal-adsorbate structure energy.[73]Table shows that the dEads/dNi values for THTA-I (−190.31 kcal mol–1) is higher than that for THTA-II (−173.09 kcal mol–1), indicating that THTA-I has a better adsorption than THTA-II. Furthermore, the water (dEads/dNi) values were close (−13.91 kcal mol–1) to the triazine derivative values, indicating that the examined particles have strong adsorption relative to the 250 water molecules used, which helps in the swapping of water molecules by THTA compounds. Hence, the investigated compounds were powerfully adsorbed on steel samples, thus providing a strongly adsorbed preventive barrier that offered a steel sample prohibition in 5% sulfamic acid medium, as determined by experimental and computational examinations.

Inhibition Mechanism of THTA Components

The results of different electrochemical techniques revealed that THTA compounds protected the tested N80 carbon steel surface from corrosion in 5% sulfamic acid solutions. The THTA inhibitors’ prohibitive effectiveness can be related to the examined components’ adsorption on the evaluated steel surface, which reduced the exposed surface area to the corrosive medium.

THTA inhibitors have more active sites for π-electrons as aromatic phenyl moieties and several heteroatoms, such as O and N; therefore, the adsorption of the investigated THTA inhibitors increased in the sequence of THTA-I > THTA-II.[74]THTA inhibitors existed in the protonated form in the sulfamic acid medium.[74] In sulfamic acid solutions, N80 specimens are positively charged.[75] Hence, the adsorption process can occur via −SO3NO2– anions, which are initially adsorbed onto the protonated N80 surface, to yield a negatively charged film. Therefore, the adsorption of the protonated form of THTA components will be restrained by the anion’s concentration on the N80 surface. THTA compounds are likewise adsorbed by donor–acceptor correlations within the undivided electron pairs of (N and O) heteroatoms and π-electron of each (C=C) to create a coordinate bond through the vacant d-orbitals of Fe atoms on the N80 metal, which acts as a Lewis’s acid and the development of a preventive chemisorbed barrier.[76−78] This result was confirmed with ΔGads0 values, which were greater than −40 kJ/mol confirming the chemisorption adsorption of THTA compounds of N80 carbon steel in 5% sulfamic acid medium. The improved inhibition tendency of THTA-I was ascribed to the presence of an additional phenyl moiety, which has active spots that are prone to electrophilic attack and the smallest values for HOMO energy, ΔE, and hardness.[79]

Conclusions

The tested inhibitors were efficient in inhibiting corrosion of N80 carbon steel metal in 5% sulfamic acid media, indicating that they were mixed-type inhibitors.

The electrochemical data obtained from whole measurements confirmed that the inhibition potency improved with the concentrations increment. The %IE was accepted with the sequence of THTA-I > THTA-II.

Double-layer capacitances diminished with reference to 5% sulfamic acid solution before the tested THTA inhibitors were added. Such a result may be confirmed by the adsorption mechanism of THTA compounds onto the N80 steel surface.

The Langmuir adsorption isotherm was the appropriate isotherm for the adsorption of the tested THTA compounds on the N80 steel surface in 5% sulfamic acid solutions.

The negative values of ΔGads0demonstrated the spontaneity of the adsorption.

XPS was performed to support the adsorption of THTA-I onto the N80 steel surface.

The inhibition capacity values obtained from the whole electrochemical methods (PP and EIS) demonstrated the authenticity of the received data, and an upstanding agreement was observed within the experimental and theoretically obtained data.