Novel triazole derivatives as ecological corrosion inhibitors for mild steel in 1.0 M HCl: experimental & theoretical approach.

The present paper illustrates the investigation of two novel ecological triazole derivative corrosion inhibitors, namely ethyl 2-(4-phenyl-1H-1,2,3-triazol-1-yl) acetate [Tria-CO2Et], and 2-(4-phenyl-1H-1,2,3-triazol-1-yl) acetohydrazide [Tria-CONHNH2]. The studied inhibitors were investigated against the corrosion of mild steel in 1.0 M HCl solution using different electrochemical techniques. Potentiodynamic polarization experiments indicated that the [Tria-CO2Et], and the [Tria-CONHNH2] acted as mixed type inhibitors. Electrochemical impedance spectroscopy measurements revealed that both inhibitors presented a high inhibition performance, achieving an inhibition efficiency of 95.3% for [Tria-CO2Et] and 95.0% for [Tria-CONHNH2] at a concentration of 1.0 × 10-3 M. Based on the Langmuir isotherm model and the activation parameters, these triazole derivatives were adsorbed onto a steel surface by physical and chemical bonds. Density functional theory based on B3LYp6-311G(d,p) was also carried out to correlate the inhibition efficiencies obtained experimentally with the theoretical descriptors of the studied molecular structures. This journal is © The Royal Society of Chemistry.

Introduction

Generally, triazole is a five-membered ring containing three nitrogen atoms, and acts as a building block for many compounds that have various applications, especially in medicine.[1] These triazole derivative compounds have attracted wide interest from many researchers because of their exceptional properties. They have diverse agricultural, industrial, and biological properties, as well as anti-microbial, anticonvulsant, anticancer, anti-inflammatory, diuretic, antibacterial, hypoglycemic, antitubercular, and antifungal activities.[2]

Various industrial fields use construction materials such as mild steel, since it is of low cost, high availability, and good physicochemical characteristics.[3,4] However, mild steel can be easily weakened and causes wide human and economic costs when it is in contact with an aggressive acidic solution. Therefore, the best way to protect the steel surface is by applying inhibitors that act as a wall between the steel surface and the aggressive medium.[5,6] Moreover, this film barrier can be explained by the adsorption of these molecules on the metal surface using several heteroatom centers such as N, S, and O heteroatoms, and π-electrons. Furthermore, this adsorption can be achieved through physical adsorption, chemisorption, or both (physical and chemical).[7,8]

Recently, many researchers have focused on the application of eco-friendly corrosion inhibitors. These compounds can be considered as ecological inhibitors since they have low toxicity and characteristics of strong chemical activity.[9,10] These triazole derivatives are amphoteric in nature, forming salts with acids and bases, and have special affinity to metal surfaces with moving water molecules on the surface. Moreover, they have abundant p-electrons and unshared electron pairs on the nitrogen atom that can combine with d-orbitals of the metal to afford a protecting film.[11,12] Therefore, several previous works have focused on the application of 1,2,4-triazole derivatives as corrosion inhibitors.[13,14] For instance, El Belghiti et al. showed that two 3,5-bis (disubstituted)-4-amino-1,2,4-triazole derivatives (T1 and T2) have a corrosion inhibition efficiency of 86% for mild steel when used at a concentration of 1.0 × 10−3 M in 2 M H3PO4.[15] More recently, newly synthesized heterocycles, namely (1-p-tolyl-1H-1,2,3-triazol-4-yl) methanol (TTM)[16] and 5-hexylsulfanyl-1,2,4-triazole (HST),[17] have been investigated in inhibiting steel corrosion in 1.0 M HCl. These compounds displayed excellent inhibition performance. The inhibition efficiencies reached 97% for HST and 81% for TTM based on electrochemical data at 1.0 × 10−3 M. In addition, the effect of heteroatoms on the corrosion inhibition of structurally similar azomethine-based organic molecules (FMT and TMT) showed that both molecules had good efficiency (>90%) at 5 mmol L−1 concentration in 1.0 M HCl medium.[18]

Many authors have reported that the quantum chemical calculations can offer broad information about structural properties and relate inhibitors' adsorption ability with their structural aspects.[19] Y. El Aoufir et al.[20] have established a correlation between two 1,2,4-triazole derivatives (TR8 and TR10) and their electronic properties. This investigation confirmed the strong adsorption of these inhibitors on the mild steel surface through active centers distributed over the triazole moiety and the carbon chain of the studied compounds. In addition, some other authors have used quantum chemistry calculations (density functional theory, DFT) to understand inhibitor interactions with the metal surface.[21-24] As an example, Table 1 reports the percentage inhibition efficiency for some selected triazole derivatives used as corrosion inhibitors against the corrosion of mild steel in an acidic medium.

Percentage inhibition efficiency for some selected triazole derivatives used as corrosion inhibitors against the corrosion of mild steel in an acidic medium

Triazole derivative	Inhibition efficiency (%)	Medium	Ref.
	85.05% at 3.2 mM	0.5 M HCl	14
(3-Bromo-4-fluoro-benzylidene)-[1,2,4] triazol-4-yl-amine (BFBT)
	72.83% at 3.2 mM	0.5 M HCl	14
(2-Fluoro-4-nitro-benzylidene)-[1,2,4] triazol-4-yl-amine (FNBT)
	86.81% at 1.0 × 10−3 M	2 M H3PO4	15
3,5-Bis(4-methoxyphenyl)-4-amino-1,2,4-triazole (T1)
	86.20% at 1.0 × 10−3 M	2 M H3PO4	15
3,5-Bis(4-chlorophenyl)-4-amino-1,2,4-triazole (T2)
	89.9% at 1.0 × 10−4 M	1.0 M HCl	16
3,5-Bis(3-aminophenyl)-4-amino-1,2,4-triazole (3-APAT)
	24% at 1.0 × 10−2 M	1.0 M HCl	17
5-Amino-1,2,4-triazole (5-ATA),
	92% at 1.0 × 10−2 M	1.0 M HCl	17
5-Amino-3-mercapto-1,2,4-triazole (5-AMT)
	82% at 1.0 × 10−2 M	1.0 M HCl	17
5-Amino-3-methylthio-1,2,4-triazole (5-AMeTT)
	82% at 1.0 × 10−2 M	1.0 M HCl	17
1-Amino-3-methylthio-1,2,4-triazole (1-AMeTT)

In this work, we have investigated the effect of two novel synthesized compounds derived from triazole, namely ethyl 2-(4-phenyl-1H-1,2,3-triazol-1-yl) acetate [Tria-CO2Et], and 2-(4-phenyl-1H-1,2,3-triazol-1-yl) acetohydrazide [Tria-CONHNH2], as corrosion inhibitors. This investigation was performed on mild steel substrates using various electrochemical techniques, such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). DFT based calculations in the gaseous as well as in the aqueous phase were executed to correlate the quantum chemical descriptors of the triazole-derived compounds used and their experimental inhibition efficiency.

Experimental

Inhibitor synthesis

The click coupling of phenylacetylene (1) with ethylazidoacetate (2), in the presence of sodium ascorbate and copper sulfate as a catalyst in a mixture of t-BuOH : H2O (1 : 1), gave the targeted ethyl 2-(4-phenyl-1H-1,2,3-triazol-1-yl) acetate (1) with 96% yield after stirring at room temperature for 4 h.

The structure of the 1,2,3-triazole (3) was elucidated based on its spectral data (IR, and 1H and 13C-NMR). Its 1H-NMR spectrum revealed the absence of the characteristic alkyne proton (CH) and the presence of a distinct singlet at δH = 8.50 ppm assigned to the triazolyl C5–H proton, confirming the success of the 1,3-dipolar cycloaddition reaction. The spectrum also revealed the presence of a triplet at 1.24 ppm and a quartet at 4.18–4.22 ppm attributed to the ethyl ester protons (CH3) and (OCH2), respectively. In the 13C-NMR spectrum, the carbon signals belonging to CH3, NCH2 and OCH2 resonated at δC 14.78, 51.31, and 63.40 ppm, respectively. The sp2-carbons were recorded at their appropriate chemical shifts. Thermal hydrazinolysis of the resulting 1,2,3-triazole based-ester (3), with hydrazine hydrate for 4 h, afforded the corresponding acid hydrazide (4) in excellent yield (90%) (Scheme 1). The success of the hydrazinolysis reaction has been clearly evidenced based on the spectral data of compound (4), which revealed the disappearance of the ethyl ester protons and carbons of its starting material (3). The 1H-NMR spectrum also confirmed the presence of the diagnostic hydrazide NH2 and NH protons as two singlets at δH 4.62, and 9.58 ppm, respectively. All carbon signals related to the proposed structure of compound (4) resonated in their appropriate regions.[25] The structures and the IUPAC names of the studied compounds are given in Table 2.

Scheme 1

Synthesis of the 1,2,3-triazole based-ester and/or hydrazide (3)/(4).

Abbreviations, structures, and IUPAC names for the studied triazole derivatives

Abbreviations	Structures	IUPAC name
[Tria-CO2Et]		Ethyl 2-(4-phenyl-1H-1,2,3-triazol-1-yl)acetate
[Tria-CONHNH2]		2-(4-Phenyl-1H-1,2,3-triazol-1-yl)acetohydrazide

The measurement of the melting points was performed with a Stuart Scientific SMP1. The functional groups were identified using a SHIMADZU FTIR-Affinity-1S spectrometer in the range of 400–4000 cm−1. The measurement of the 1H-NMR (400 MHz) and 13C-NMR (100 MHz) spectra was performed with a Bruker spectrometer (400 MHz). Elemental analyses were performed using a GmbH-Vario EL III Elementar Analyzer.

Synthesis and characterization of ethyl 2-(4-phenyl-1H-1,2,3-triazol-1-yl)acetate (3)

A mixture of phenylacetylene (1) (10 mmol), CuSO4·5H2O (0.20 g), sodium ascorbate (0.30 g), and ethylazidoacetate (1) (12 mmol) in t-BuOH : H2O (1 : 1, v/v) (20 mL) was stirred at room temperature for 4 h. After completion of the reaction, ice cold water (100 mL) was added to the reaction mixture. The formed precipitate was collected by filtration, washed with a saturated solution of ammonium chloride, and recrystallized from ethanol to give the targeted 1,2,3-triazole (3).

Yield: 96%, mp: 101–102 °C, IR (ν, cm−1): 1550 (CC), 1740 (CO), 2985 (C–H), 3060 cm−1(C–H). 1H-NMR (400 MHz, DMSO-d6): δH = 1.24 (3H, t, J = 4.0 Hz, CH3), 4.18–4.22 (2H, q, OCH2), 5.14 (s, 2H, NCH2), 7.32–7.40 (m, 3H, Ar–H), 7.82–7.90 (m, 2H, Ar–H), 8.50 (s, 1H, CH-1,2,3-triazole). 13C-NMR (100 MHz, DMSO-d6): δC = 14.78 (CH3); 51.31 (NCH2); 63.40 (OCH2); 122.57, 125.70, 127.89, 128.31, 130.98, 146.45 (Ar–C), 166.24 (CO). Calcd for C12H13N3O2: C, 62.33; H, 5.67; N, 18.17. Found: C, 62.50; H, 5.59; N, 18.06.

Synthesis and characterization of 2-(4-phenyl-1H-1,2,3-triazol-1-yl)acetohydrazide (4)

Compound (3) (10 mmol) was dissolved in ethanol (30 mL) containing hydrazine hydrate (12 mmol). The mixture was heated under reflux for 4 h. After cooling, the crude product was collected by filtration and recrystallized from ethanol to afford the targeted acid hydrazide (4).

Yield: 90%, mp: 185–186 °C, IR (ν, cm−1): 1540 (CC), 1710 (CO), 2960 (C–H), 3080 (C–H), 3080 cm−1(N–H). 1H-NMR (400 MHz, DMSO-d6): δH = 4.62 (s, 2H, NH2), 5.08 (s, 2H, NCH2), 7.33–7.45 (m, 3H, Ar–H), 7.84–7.87 (m, 2H, Ar–H), 8.54 (s, 1H, CH-1,2,3-triazole), 9.58 (s, 1H, NH). 13C-NMR (100 MHz, DMSO-d6): δC = 51.24 (NCH2); 123.14, 125.45, 128.12, 129.45, 130.98, 146.97 (Ar–C), 165.15 (CO). Calcd for C10H11N5O: C, 55.29; H, 5.10; N, 32.24. Found: C, 55.05; H, 5.18; N, 32.13.

Materials preparation

The steel used in the present paper is a mild steel composed of Fe (99.21), C (0.21), Mn (0.05), Si (0.38), S (0.05), P (0.09), and Al (0.01). Prior to each experiment, the steel samples were polished with emery paper (until 1500 grid size), washed with distilled water, degreased with acetone, and dried. The molar hydrochloric acid solution was prepared by dilution of analytical grade 37% HCl. The concentration of the studied inhibitors ranged from 5.0 × 10−5 M to 1.0 × 10−3 M.

Electrochemical study

The electrochemical tests were performed using a potentiostat type VersaSTAT 4, controlled with versa studio analyses software. The various electrochemical experiments were conducted using a three-electrode glass cell. Platinum as the counter electrode, Ag/AgCl as a reference electrode, and mild steel samples as the working electrode. The surface area of the steel electrode used for the electrochemical tests was 1.00 cm2, and the volume of the solutions used in the glass cell was 50 mL. Prior to the experiments, the potential of the working electrode was stabilized for 30 min until it achieved a stable open circuit potential. The polarization curves were carried out with a scan rate of 1 mV s−1 with a potential range of ±250 mV according to the open circuit potential (OCP). The inhibition efficiency (ηpp%) was calculated from the corrosion current density values using eqn (1).[26]where and icorr are the values of the corrosion current densities in the absence and presence of inhibitors, respectively.

On the other hand, the EIS technique were performed in the frequency range from 100 kHz to 100 mHz with 10 points per decade. In this case, the Nyquist plots were plotted and analyzed using a suitable equivalent circuit. The inhibition efficiency was calculated using eqn (2).[27]where and Rp are the polarization resistance of the mild steel electrode in the presence and absence of inhibitors, respectively.

Theoretical approach

Several reactivity descriptors were extracted, such as the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), dipole moment (μ), and energy gap (ΔEgap), etc. In addition, the reactive sites from electrophilic or nucleophilic attacks were extracted using Fukui indices calculations. These calculations were performed using the Gaussian 09 program[28] at the DFT/(B3LYP) level with the 6-311G (d,p) basis set.

The fraction of electrons transferred (ΔN110) from the inhibitor to the (110) surface of the metal was evaluated as reported by Pearson theory:[29]where the work function (Φ) is the theoretical value of the electronegativity on the (110) surface and it presents a dense surface package (Φ = χFe(110) = 4.82 eV). The global hardness corresponding to the metallic bulk is ηFe(110) = 0 eV.

The Fukui indices indicate a tendency of the molecule to give or obtain electrons. Therefore, these functions have been modeled to detect the most nucleophilic interactions in a molecule.[30] Generally, electrophilic (f−k) and nucleophilic (f+k) attacks are calculated using eqn (8) and (9):

Nucleophilic attack

Electrophilic attackwhere Pk is the natural population for the atom k site in the cationic (N − 1), anionic (N + 1), or neutral molecule (N).

Results and discussion

Concentration effect of the studied triazole derivatives

Open circuit potential

The variation of the mild steel potential versus the elapsed time during 30 min for the uninhibited solution and the highest-tested concentration of the [Tria-CO2Et] and [Tria-CONHNH2] inhibitors is illustrated in Fig. 1.

Fig. 1

Evolution of the open circuit potential (OCP) versus time for mild steel in 1.0 M HCl at the highest-tested concentration of [Tria-CO2Et] and [Tria-CONHNH2] at 298 K.

It was noticed that the addition of the studied molecules induces a shift in OCP (i.e., the corrosion potential Ecorr). Based on the plots presented in Fig. 2, it can be observed that the mild steel sample could achieve a quasi-stable open circuit potential in under 30 min. Therefore, 30 min of OCP measurement was assumed prior to performing all electrochemical measurements in this work.

Fig. 2

Polarization curves of mild steel immersed in 1.0 M HCl without and with various concentrations of [Tria-CO2Et] and [Tria-CONHNH2] at 298 K.

PDP polarization curves

The polarization curves for the mild steel in the presence and absence of [Tria-CO2Et] and [Tria-CONHNH2] in 1.0 M HCl at 298 K are presented in Fig. 2. Tafel parameters such as the corrosion potential (Ecorr), corrosion current density (icorr), cathodic Tafel slope (bc), and percentage inhibition efficiencies (ηPP%) are summarized in Table 3.

Polarization parameters for mild steel in 1.0 M HCl without and with various concentrations of [Tria-CO2Et] and [Tria-CONHNH2]

Medium	Conc. (M)	−Ecorr (mV vs. Ag/AgCl)	i corr (μA cm−2)	−βc (mV dec−1)	η PP%
1.0 M HCl	—	413	944	139	—
[Tria-CO2Et]	5.0 × 10−5	435	230	138	75.6
	1.0 × 10−4	427	109	138	88.4
	5.0 × 10−4	402	29	136	96.9
	1.0 × 10−3	388	25	130	97.3
[Tria-CONHNH2]	5.0 × 10−5	440	261	138	72.3
	1.0 × 10−4	431	111	138	88.2
	5.0 × 10−4	414	29	137	96.9
	1.0 × 10−3	402	27	137	97.1

It can be seen from this figure that the cathodic Tafel slope in the presence of inhibitors decreased obviously to lower values compared to the blank cathodic branches. Also, all curves rise to parallel lines, indicating that our inhibitors do not alter the hydrogen evolution mechanism.[31] In other words, the studied molecules can reduce the hydrogen ions by covering the active reaction sites at the steel surface forming, therefore, a protective film. Moreover, the cathodic slope (bc) values did not show a large change with the increase of the inhibitor concentration, which indicates that the reduction of hydrogen reaction is investigated according to the pure activation mechanism.[32]

Usually, when the Ecorr displacement is larger than 85 mV, corresponding to that of the uninhibited solution, the inhibitor is regarded as a cathodic- or anodic-type inhibitor. On the other hand, when the displacement is less than 85 mV, the inhibitor is classified as a mixed-type one.[33] In the present paper, the maximum Ecorr displacements were 25 mV with [Tria-CO2Et] and 27 mV with [Tria-CONHNH2], suggesting that both inhibitors acted as mixed-type.

EIS measurements

To gain more information about the corrosion mechanisms and confirm the previous results obtained from polarization measurements, EIS measurements were performed. Thus, the Nyquist plots and Bode diagrams (experimental and fit) of the samples in 1.0 M HCl in the presence and absence of the [Tria-CO2Et] and [Tria-CONHNH2] inhibitors are shown in Fig. 3. In addition, the electrochemical parameters obtained from this technique, and grouped in Table 4, were extracted after a good simulation in the EC-Lab V10.02 software using the electrical equivalent circuit presented in Fig. 4. It can be observed that the presented circuit has a CPE instead of a pure capacitance element since the obtained plots showed a depressed semicircle, non-ideal with their center located below the real axis, which is related to different physical phenomena such as surface heterogeneity.[34]

Fig. 3

Nyquist and Bode plots for mild steel in 1.0 M HCl with and without various [Tria-CO2Et] and [Tria-CONHNH2] concentrations.

EIS parameters obtained for mild steel in 1.0 M HCl with and without inhibitors

Medium	Conc (M)	R s (Ω cm2)	R p (Ω cm2)	CPE		C dl (μF cm−2)	θ	η imp %
				Q (μF Sn−1)	n dl
1.0 M HCl	—	1.7	33.0	312.7	0.784	89.1	—	—
[Tria-CO2Et]	5.0 × 10−5	1.7	114.2	160.7	0.828	70.1	0.711	71.1
	1.0 × 10−4	1.6	237.0	116.4	0.838	58.3	0.860	86.0
	5.0 × 10−4	1.7	627.8	64.8	0.854	37.5	0.947	94.7
	1.0 × 10−3	1.6	702.7	58.9	0.855	34.3	0.953	95.3
[Tria-CONHNH2]	5.0 × 10−5	1.7	105.0	165.5	0.824	69.8	0.685	68.5
	1.0 × 10−4	1.8	214.9	132.5	0.831	64.3	0.846	84.6
	5.0 × 10−4	1.8	607.5	62.3	0.848	34.6	0.945	94.5
	1.0 × 10−3	2.1	660.9	57.2	0.866	34.5	0.950	95.0

Fig. 4

Electrochemical equivalent circuit used to fit the EIS data.

Moreover, it is clear from Fig. 3 that all of the Nyquist fitted diagrams show a single capacitive loop and the size of these plots increased with the rise of inhibitor concentration, indicating that the corrosion reaction is principally controlled by a charge transfer process.[35] Therefore, this phenomenon is generally shown when we have the dispersal frequency attributed to the surface heterogeneity and roughness of the steel surface.

On the other side, the EIS measurements are presented also in Bode diagrams. The Bode phase angle plots show a single peak at intermediate frequencies, indicating the presence of one time constant. Moreover, the Bode plots obtained in the presence of our inhibitors displayed only one phase maximum, indicating only one relaxation process. Thus, the charge transfer process could have taken place at the metal/electrolyte interface.[36] It is also observed from the Bode plots that a linear relationship between log|Z| vs. log(f) was shown in the intermittent frequency region, indicating that the phase angle is less than −90° and the slope value is close to −1. These results justified the equivalent circuit obtained.[37]

From Table 4, it can be observed that the Rp values increased with an increase in the [Tria-CO2Et] and [Tria-CONHNH2] concentration, as well as the inhibition efficiency, which achieved a maximum value of 95.3% for [Tria-CO2Et] and 95.0% for [Tria-CONHNH2] at the highest-tested concentration (1.0 × 10−3 M). On the other hand, the values of Q and Cdl decreased as the concentration of both compounds increased, indicating adsorption on the mild steel surface. Moreover, the ndl values obtained are less than unity in both the inhibited and uninhibited solutions, which indicates that the CPE element acts as a pseudo capacitor.[38] From these results, it can be seen that both studied inhibitors showed a close efficiency despite the replacement of the CO2Et group by CONHNH2.

Table 5 reports the percentage inhibition efficiency for some selected heterocyclic compounds used as corrosion inhibitors in 1.0 M HCl compared with our compounds (Tria-CO2Et and Tria-CONHNH2). The values of inhibition efficiency, given in Table 5, were obtained using EIS measurement after 1/2 h of immersion in 1.0 M HCl solution containing 1.0 × 10−3 M of other derivatives. By comparing these data, we can show that our triazole derivatives, Tria-CO2Et and Tria-CONHNH2, are the most effective inhibitors in 1.0 M HCl. Moreover, triazole derivatives Tria-CO2Et and Tria-CONHNH2 remain effective against the corrosion of steel at high temperatures (90% at 328 K).

Percentage inhibition efficiency for different heterocyclic compounds in 1.0 M HCl (the concentration used is 1.0 × 10−3 M)

Heterocyclic compound	Highest inhibition efficiencya (%)	Metal exposed	Reference
	95.3	Mild steel	This work
Ethyl 2-(4-phenyl-1H-1,2,3-triazol-1-yl)acetate (Tria-CO2Et)
	95.0	Mild steel	This work
2-(4-Phenyl-1H-1,2,3-triazol-1-yl)acetohydrazide (Tria-CONHNH2)
	92.4	Mild steel	39
2,3-Diphenylquinoxaline (Q-H)
	86.3	Mild steel	40
Benzo[d]thiazole-2-thiol
	88.0	Mild steel	41
N-(1-Methyl-2,4-dioxo-1,2,3,4-tetrahydroquinazoline-3-carbonothioyl)propionamide
	94.0	Mild steel	42
2-(1,4,5-Triphenyl-1H-imidazol-2-yl)phenol (P1)
	84.2	Mild steel	43
2-(Phenylthio)phenyl-1-(o-tolyl)methanimine (PTM)

The inhibition efficiency values were determined using EIS measurements after ½ h of immersion.

Isotherm adsorption

In order to comprehend the adsorption mechanism of [Tria-CO2Et] and [Tria-CONHNH2] onto the mild steel surface in the inhibited medium, various isotherm models were tested (Langmuir, Temkin, and Freundlich) using the electrochemical spectroscopy impedance data (Fig. 5). The linear equations of various isotherms are as follows:

Fig. 5

Langmuir, Freundlich, and Temkin adsorption isotherms of [Tria-CO2Et] and [Tria-CONHNH2] on the mild steel surface.

Langmuir isotherm:

Freundlich isotherm:

Temkin isotherm:where: θ = the degree of surface coverage. Cinh = the inhibitor concentration. K = the equilibrium constant of the adsorption/desorption process. a = the molecular lateral interactions: (a > 0; attraction), (a < 0; repulsion).

The expression for the standard Gibb's free energy of adsorption, , was calculated using eqn (13)[44]where 55.5 is the molar concentration of H2O in solution, R is the universal gas constant (8.314 J mol−1 K−1), T is the absolute temperature, and K is the equilibrium constant of adsorption/desorption.

Firstly, it is clear from Table 6 that both [Tria-CO2Et] and [Tria-CONHNH2] obey the Langmuir adsorption isotherm, since they achieve the best regression coefficient (0.999) and a slope close to 1 (1.032 for [Tria-CO2Et], and 1.033 for [Tria-CONHNH2]).[45] In addition, the adsorption constant values Kads for the Freundlich isotherm are too small to show any significance. Thus, these inhibitors disobey the Freundlich isotherm model. On the other side, the high value of Kads in the Temkin model led us to propose that our compounds might be exhibiting a repulsive interaction, since they have a negative value of the parameter (a) but the regression coefficient is too small compared to those obtained in the Langmuir isotherm, which allowed us to report that [Tria-CO2Et] and [Tria-CONHNH2] disobey the Temkin isotherm.[46]

Parameter results from different isotherm models tested

Isotherms	Inhibitors	R 2	Parameters		K
Langmuir	[Tria-CO2Et]	0.999	Slope	1.032	6.65 × 104	−37.5
	[Tria-CONHNH2]	0.999		1.033	5.90 × 104	−37.2
Freundlich	[Tria-CO2Et]	0.907	n	11.19	1.83	−11.4
	[Tria-CONHNH2]	0.908		9.98	1.97	−11.6
Temkin	[Tria-CO2Et]	0.920	a	−6.68	4.88 × 108	−59.5
	[Tria-CONHNH2]	0.923		−6.06	1.46 × 108	−56.5

Many studies have reported that electrostatic interaction happens between charged molecules and charged metals (physical adsorption) when ΔGads is around −20 kJ mol−1. Meanwhile, a coordinated bond (chemisorption) is achieved when the ΔGads values are around −40 kJ mol−1 or more.[47,48] In the present work, the ΔGads values are −37.5 kJ mol−1 for [Tria-CO2Et] and −37.2 kJ mol−1 for [Tria-CONHNH2], indicating that our inhibitors adsorbed onto the steel surface by creating a strong barrier film. It has previously been demonstrated that the tested triazole compounds have good corrosion inhibition performances due to their ability to form significant interactions with the iron atoms. It can also be highlighted that in an acidic solution, the surface of the steel electrode takes a positive charge. These actions imply three types of interaction: (i) the interaction of the non-bonding electron pairs on the hetero-atoms with the vacant d-orbitals of the Fe-atoms and hence responsible for chemical adsorption. (ii) The interaction occurring between the negatively charged Cl− ions on the mild steel surface and the positively charged protonated forms of [Tria-CONHNH2] and [Tria-CO2Et]. (iii) π-electron clouds on the aromatic ring also participating in the donor–acceptor kind of interaction (retro-donation) with the ionized Fe atoms on the surface. These interactions result in the minimization of metal dissolution in the acidic medium by protective film formation of the inhibitor molecules on the mild steel surface.

Temperature effect of the studied triazole derivatives

Temperature is a valuable parameter in studying the metal corrosion behavior because it can change the electrode/electrolyte interface, such as the dissolution of the adsorbed molecule barrier.[49] Therefore, the effect of temperature on the corrosion inhibition of mild steel in 1.0 M HCl in the absence and presence of 1.0 × 10−3 M [Tria-CO2Et] and [Tria-CONHNH2] has been investigated at temperatures ranging from 298 K to 328 K using the polarization curve technique. The polarization curves at the highest-tested concentration (1.0 × 10−3 M) are presented in Fig. 6, and the various electrochemical parameters are listed in Table 7.

Fig. 6

Polarization curves for steel surfaces without and with the highest-tested concentration of [Tria-CO2Et] and [Tria-CONHNH2] (1.0 × 10−3 M) at various temperatures.

Electrochemical parameters for steel surfaces with and without the studied inhibitors at temperatures ranging from 298 K to 328 K

Medium	Temp. (K)	−Ecorr (mV vs. Ag/AgCl)	i corr (μA cm−2)	−βc (mV dec−1)	η PP%
1.0 M HCl	298	413	944	139	—
	308	410	1690	137	—
	318	411	2328	126	—
	328	412	3387	120	—
[Tria-CO2Et]	298	388	25	130	97.3
	308	410	46	136	97.2
	318	416	170	121	92.6
	328	428	270	117	92.0
[Tria-CONHNH2]	298	402	27	137	97.1
	308	417	49	137	97.1
	318	418	86	125	96.3
	328	428	216	119	93.6

From the temperature analysis, it can be seen that the icorr values in the presence of the studied inhibitors are less than those obtained in the blank solution, signifying that these compounds have considerably inhibited the corrosion reaction of mild steel. As shown in Table 7, when the temperature is increased from 298 to 328 K, the icorr values are increased from 25 μA cm−2 to 270 μA cm−2 for [Tria-CO2Et] and from 27 μA cm−2 to 216 μA cm−2 for [Tria-CONHNH2]. In addition, it can be noted that the inhibition efficiency decreases slightly in the presence of the inhibitors, so that the two inhibitors remain effective against the corrosion of the steel in hydrochloric acid. Thus, the examined compounds still show superior inhibition performance to protect mild steel from corrosion by forming a firm adsorption film on the steel surface.[49,50]

The Arrhenius plots of Ln(icorr) vs. 1000/T and Ln(icorr/T) vs. 1000/T of mild steel in 1.0 M HCl medium containing [Tria-CO2Et] and [Tria-CONHNH2] are presented in Fig. 7. The corrosion kinetic parameters, such as activation energy (Ea), enthalpy of activation , and entropy of activation for the corrosion of mild steel in acidic solution without and with the highest-tested concentration of the inhibitors (1.0 × 10−3 M) at temperatures ranging from 298 K to 328 K were calculated from the Arrhenius eqn (14) and the transition state eqn (15).[51] The activation parameters for MS in 1.0 M HCl with and without the studied triazole derivatives are presented in Table 8.

Fig. 7

Arrhenius and transition state plots for mild steel in 1.0 M HCl solution with and without the optimum concentration (1.0 × 10−3 M) of the studied inhibitors.

Thermodynamic parameters of the activation parameters for [Tria-CO2Et] and [Tria-CONHNH2]

Activation parameters	1.0 M HCl	[Tria-CO2Et]	[Tria-CONHNH2]
E a (kJ mol−1)	33.8	68.7	55.1
	31.2	66.1	52.5
	−82.7	3.0	−42.1

The activation parameters for mild steel in 1.0 M HCl solution without and with the [Tria-CO2Et] and [Tria-CONHNH2] compounds were obtained from linear square fits of ln Icorrvs. 1000/T, while the ΔH* and ΔS* values were obtained from linear square fits of ln Icorr/T vs. 1000/T (Fig. 7).where N is Avogadro's number, T is the absolute temperature, R is the gas constant, and ℏ is Plank's constant. From the activation parameter results, it can be seen that the Ea values of the solution containing [Tria-CO2Et] and [Tria-CONHNH2] are higher than those in the case of the uninhibited solution, which may be attributed to the formation of a compact barrier film on the mild steel surface.[52] The higher energy barrier for the corrosion process in the case of the inhibited solutions suggests that the adsorbed inhibitor film prevents the charge/mass transfer reaction occurring on the surface,[53,54] thus protecting the metal from dissolution. The positive values for the activation enthalpy reflect the endothermic nature of the mild steel dissolution process.[55]

The value of activation entropy increases and is negative in the presence of the inhibitor [Tria-CONHNH2], which means a decrease in the disorder during the transformation of the reagents into an activated complex;[56] in the case of [Tria-CO2Et] the value of was high and positive meaning an increase in the disorder.[53,57]

DFT study[1]

DFT has been mainly useful to correlate the electronic properties to the inhibition performance obtained experimentally, i.e. understanding the adsorption mechanism of the molecules used.[58] Quantum descriptor calculations were extracted using the DFT method at the B3LYP/6-311G (d,p) level (Table 9). The optimized geometries of [Tria-CO2Et] and [Tria-CONHNH2], as well as their frontier molecular orbitals (LUMO and HOMO), are shown in Fig. 8. The Fukui functions have also been calculated using the natural populations in order to find the most reactive sites of the studied molecules.

Quantum chemical descriptors for [Tria-CO2Et] and [Tria-CONHNH2] in the gas and aqueous phases

Parameters	[Tria-CO2Et]		[Tria-CONHNH2]
	Gas	Aqueous	Gas	Aqueous
E HOMO (eV)	−6.1183	−6.4269	−6.4427	−6.3763
E LUMO (eV)	−0.7997	−1.0482	−1.0792	−1.0242
ΔEgap (eV)	5.3185	5.3787	5.3634	5.3520
σ (eV−1)	0.3760	0.3718	0.3728	0.3736
η (eV)	2.6592	2.6893	2.6817	2.6760
χ (eV)	3.4590	3.7375	3.7609	3.7002
μ (D)	5.2803	7.3222	2.6494	6.3022
ΔN	0.2558	0.2012	0.1974	0.2092
Ω	2.2496	2.5971	2.6372	2.5582
ε	0.4445	0.3850	0.3791	0.3908

Fig. 8

Optimized structures, HOMO and LUMO and ESP maps for [Tria-CO2Et] and [Tria-CONHNH2] in neutral form.

From the HOMO and LUMO, the orbital distribution is localized principally in the aromatic and triazole rings showing that [Tria-CO2Et] and [Tria-CONHNH2] inhibitors can create bonds with the vacant orbital of iron because they have many reactive sites distributed along the inhibitors' structures. Moreover, the ESPM distributions show that the total density (in red color) is located on the oxygen and nitrogen atoms.[59,60] It could be concluded that the present inhibitors can favor the adsorption phenomenon onto the surface of mild steel. The values of the theoretical descriptors obtained for [Tria-CO2Et] are close to those obtained with [Tria-CONHNH2]. These findings are in good agreement with the experimental results. According to the obtained EHOMO and ΔEgap values (Table 9), it can be observed that [Tria-CO2Et] is very reactive in the gas phase, while it is less reactive in the aqueous phase. In addition, it can be suggested that the similar inhibition proprieties of the investigated compounds create this contradiction between the two studied phases. Also, the lower values of ELUMO obtained for both studied molecules indicate the ability of these molecules to accept electrons in the aqueous phase.[61]

Furthermore, the value of ΔN110 < 3.6 according to Lukovist's study, signifying the increase in electron-donating ability to the metal surface and this can decrease the corrosion rate of mild steel for both inhibitors.[62] According to the literature, small electronegativity values cause molecules to easily reach electron equilibrium so that the molecules get more reactive. In contrast, high electronegativity values show the opposite. In this study, the electronegativity value of [Tria-CO2Et] calculated in the gas phase is the lowest (3.45 eV) compared to the electronegativity value for [Tria-CONHNH2], which is 3.76 eV. Based on the dipole moment values in the corrosion field, some authors reported that the dipole moment increases with the efficiency but others say the opposite. In our case, we found that the dipole moment increases with the inhibition efficiency.[63,64]

The most active sites of the Fukui indices for the studied molecules have been extracted in the gas and aqueous phases and are listed in Table 10. It can be seen from these results that the calculated values of for [Tria-CO2Et] are typically localized on C11, C5, C6, and O16. While, O16, O14, and N4 are the most active sites for electrophilic attack, since the highest values of f−k were recorded.[65,66] For [Tria-CONHNH2], the highest values of f+k are situated on the C15, C10, and C5 atoms, which further suggests that these atoms are responsible for forming a back bond by accepting the electron coming from the mild steel surface. However, superior values of f−k are on O16, N2, N1, and C15. It can be observed that these responsible sites are also remarked in the aqueous phase and suitable for donor–acceptor interactions, and thus facilitate the adsorption of the inhibitor on the metal surface.

Most active sites of f+k, f−k for [Tria-CO2Et] and [Tria-CONHNH2] in the gas and aqueous phases

Molecule	Atoms	Phase	P(N)	P(N − 1)	P(N + 1)	f + k	f − k
[Tria-CO2Et]	N 1	G	7.265	7.175	7.298	0.033	0.090
		A	7.308	7.221	7.343	0.034	0.087
	N4	G	7.157	7.144	7.161	0.003	1.270
		A	7.144	7.132	7.155	0.011	0.011
	C5	G	6.097	6.014	6.178	0.081	−1.046
		A	6.071	5.968	6.170	0.099	0.103
	C6	G	6.069	6.023	6.140	0.070	0.055
		A	6.077	6.017	6.216	0.139	0.059
	C 11	G	6.205	6.099	6.318	0.113	0.032
		A	6.211	6.103	6.388	0.177	0.108
	O14	G	8.568	8.520	8.598	0.029	3.392
		A	8.562	8.529	8.573	0.011	0.033
	O16	G	8.540	8.446	8.615	0.075	2.500
		A	8.588	8.499	8.611	0.022	0.089
[Tria-CONHNH2]	N 1	G	7.266	7.176	7.296	0.030	0.089
		A	7.311	7.218	7.346	0.034	0.093
	N2	G	7.022	6.903	7.051	0.029	0.118
		A	7.051	6.915	7.078	0.026	0.135
	C 5	G	6.097	6.019	6.186	0.088	0.078
		A	6.072	5.983	6.168	0.095	0.088
	C 10	G	6.069	6.033	6.145	0.076	0.035
		A	6.076	6.046	6.235	0.158	0.029
	C15	G	6.206	6.108	6.324	0.118	0.097
		A	6.212	6.136	6.408	0.196	0.075
	O 16	G	8.574	8.458	8.6200	0.0458	0.115
		A	8.631	8.472	8.6373	0.0059	0.158

Conclusion

In the present study, the corrosion inhibition and adsorption characteristics of two triazole derivatives ([Tria-CO2Et] and [Tria-CONHNH2]) in 1.0 M HCl solution were investigated by various electrochemical techniques and a theoretical approach. The polarization curves display that these compounds acted as a mixed-type inhibitor. The electrochemical impedance spectroscopy results indicate that the inhibition efficiency reaches a maximum value of 95.3% for [Tria-CO2Et] and 95% for [Tria-CONHNH2]. The temperature study did not show a remarkable effect of the two studied inhibitors in the range of 298–328 K. The adsorption behavior shows that these triazole derivatives suit the Langmuir isotherm model. The obtained global and selective descriptors are in good correlation with the experimental part.

Conflicts of interest

There are no conflicts to declare.