Novel Synthesized Benzothiazole Bearing a Pyrazole Moiety as Corrosion Inhibitor for Copper in 1 M Nitric Acid Solution: Experimental and Theoretical Modeling

Abstract

In this study, we are looking into how to prevent corrosion of copper in 1 M HNO₃ by synthesizing a novel benzothiazole bearing a pyrazole moiety, namely, 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA), which was confirmed by weight loss (WL), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), atomic force microscopy, and FTIR measurements. As the temperature rises, the inhibitory efficacy (IE) falls and grows with an increase in BTPA dosage. At higher inhibitor concentrations, WL data showed improved BTPA adsorption on the copper surface with a maximum effectiveness of 92.5% at 15 μM concentration and 25 °C. The Langmuir isotherm provided the most accurate description of the adsorption of the studied derivatives on the copper surface. The calculated values of the standard free energy change of adsorption (ΔG _ads ^o) and the adsorption equilibrium constant (K _ads) suggested the spontaneous character and exothermic nature of the adsorption phenomenon. The investigated BTPA performs as a mixed-type inhibitor according to the polarization results. The BTPA molecule demonstrated an effective adhesion to the Cu surface, as demonstrated by techniques using atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Computational chemical methods, including quantum chemical and molecular dynamics simulations, yielded relevant results that aligned with the experimental findings. The quantum chemical parameters (E _LUMO, E _HOMO, and ΔE) exhibit a significant correlation with the protective efficacy of the examined BTPA. Monte Carlo (MC) simulations were also used to predict the inhibitor conformational adsorption alteration on the copper surface. Finally, this investigation demonstrated that the experimental and theoretical data had a strong correlation.

1. Introduction

Corrosion is a principal process necessary for economic and safety reasons, especially for metals. Copper is used extensively in many industries because of its exceptional mechanical, biological, anticorrosive, and physical qualities, as well as its great electrical conductivity, affordability, and other comparatively noble properties. Copper can readily corrode due to a variety of environmental factors. Many studies work with nitric acid as a copper corrosive solution because it is the most commonly used one. Anodic and cathodic reactions on metal surfaces are part of the electrochemical process of corrosion. It can also be defined as the degradation of a metal’s properties due to physicochemical interactions with its surrounding environment, leading to changes in its physical structure.

Corrosion is costly and hazardous. Every year, billions of dollars are spent fixing corroded machinery, components, and infrastructure. At the same time, premature failure can endanger lives and compromise safety. Adding chemical compounds to corrosive environments is one effective way to mitigate metal corrosion. Corrosion inhibitor substances, which significantly reduce corrosion rateseven in small quantitiesare among the most efficient solutions. Recent research focuses on developing highly effective inhibitors and ensuring they are low-cost, environmentally friendly, and easy to produce. −

One of the best strategies to stop copper from corroding is the use of inhibitors. Organic compounds with nitrogen (N-heterocyclic), sulfur, long carbon chains, aromatic, and oxygen atoms are the most well-known excellent inhibitors. Among these, organic inhibitors offer several benefits, including easy synthesis, low cost, minimal toxicity, and high inhibitory efficiency. −

Despite being widely prohibited and ecologically unfavorable, benzotriazole (BTA) is copper’s most effective corrosion inhibitor because of its well-known inhibitive function and low cost. The rise of environmental protection discourses has sparked a growing interest among researchers in safe inhibitors whose actions do not harm the environment. Pyrazole pyrimidine and its derivatives are regarded as green organic compounds and are employed as affinity reagents in medicinal chemistry. Numerous investigations have identified the pyrazole pyrimidine’s capacity to reduce corrosion, particularly in steel − and Cu in H₂SO ₄ and HCl acid medium, respectively. Moreover, pyrimidine − and pipyrazole have been investigated as corrosion protection of Cu in sodium chloride media. Yet, no study has been done on the corrosion hindrance of Cu in sodium chloride media by pyrazolo pyrimidine. Heterocycle-containing pyrimidine is a safe inhibitor at some degree of concentration and has excellent corrosion inhibition effects on copper metal in acidic media. The corrosion protection of PPO as a corrosion hindrance for Cu in a 3% sodium chloride solution was evaluated by different tests. According to the results, the chemical under investigation had good inhibitory activity, with a maximum value of 96.5% at 10^–3 M.

The corrosion inhibition of copper in 1 M HCl with pyrazole was investigated by using the PDP test with a rotating disc and rotating ring disc electrode (RRDE). The addition of pyrazole alters the mechanism of copper corrosion in hydrochloric acid, achieving a maximum inhibition efficiency of approximately 74% at 0.1 M Pz.

Six chemical compounds were synthesized, developed, and were used as corrosion protection, namely, the corrosion resistance of Cu in basic environments was estimated using 3-amino-1-(2,4-dinitrophenyl)-5-pyrazolone, 3-amino-1-phenyl-5-pyrazolone, 1-H-pyrazole-3,5-diamine-4-(2-phenyldiazenyl), 1H-pyrazole-3,5-diamine, 4-[2-(4-methylphenyl) diazenyl], 1-phenyl pyrazole-3, 5-diamine, 4-[2-(4-methylphenyl) diazenyl], and 5-amino-1,3-diphenyl-1H-pyrazolecarbonitrile.

Pyrazole derivatives were employed as a novel corrosion inhibitor of different metals in HCl, H₂SO₄, H₃PO₄, and NaCl media, and they were studied using mass reduction, PDP, and impedance studies, achieving a maximum inhibition efficiency of approximately 90% at a concentration of 5 × 10^–4 M.

In comparison to the inhibitor-free control, the relative percentages of 3,5-dimethylpyrazole and 3,5-diethylpyrazole were 71.5 and 85.0%, respectively. The unsubstituted 1,2,4-triazole (89.6%) outperformed the alkyl-substituted triazoles (77.7% for 3,5-dimethyl-1,2,4-triazole and 82.9% for 3,5-diethyl-1,2,4-triazole) in terms of corrosion protection.

A synthesized pyrazole derivative (BM-01), as an inhibitor of mild steel in a solution of HCl, yielded inhibition efficiencies of at a BM-01 dose of 10^–3 M, reaching 90.40% (WL) and 90.0% (EIS). Organic chemicals have been widely used for corrosion resistance and prevention. Heterocycles, including motifs like isoxazole, pyrazole, and pyrazoline, have created a lot of interest among organic molecules. Recently, we examined the inhibiting action of organic derivatives containing nitrogen atoms (Table ).

1. Comparing the Effectiveness of Organic Derivatives Enclosing Nitrogen Atoms Used as Corrosion Hindrance for Copper, and in This Study.

Organic derivatives	Sample	Medium	%IE	refs
5-Methyl-2,4-dihydropyrazol-3-one and 5-methyl-2-phenyl-2,4-didhydropyrazol-3-one	copper	0.1 M H2SO4	76
(2Z)-1-(4-chlorophenyl)-2-[(3E)-3-[2-(4-chlorophenyl)-2-oxoethylidene]-3,4-dihydro quinoxalin2 (1H)-ylidene]ethanone (Q4)	copper	2 M HNO3	90.2
3-methyl-6-oxo-4,5,6,7-tetrahydro--2H-pyrazolo[3,4-b]pyridine-5-carbonitrile	Copper	0.5 M HCl	92
5-phenyl, 2,4-dihydro-3H-pyrazol-3-one (Py)	Copper	0.5 M H2SO4	84.9
3-((2,4-dinitrophenyl)thio)-5-(4-methoxyphenyl)-4H-1,2,4-triazol-4-amine	copper	1 M HNO3	91.6
Imidazole derivatives: 4-methylimidazole, 4-methyl-5-hydroxymethylimidazole, 1-(p-tolyl)-4-methylimidazole	copper	0.5 M HCl	45–45
Aryl pyrazolo[3,4-b]pyridine derivatives (APP I)	copper	0.5 M HCl	92.3
Pyrazolylindolenine compounds: InPzTAm, InPzTH, InPzPh	copper	1 M HCl	InPzTAm 94
			InPzTH 91
			InPzTH 79
Three novel imidazole derivatives: PDI, PAI, PMI	copper	3.5 wt % NaCl,	PDI 95.9
			PAI 69.6
			PMI 20.4
4-(benzo[d]thiazol-2-yl)-1H-pyrazol-5-amine (BTPA)	copper	1 M HNO3	94.1	Current work

4-(benzo­[d]­thiazol-2-yl)­1H-pyrazol-5-amine serves as a foundational building block in medicinal chemistry and organic synthesis, particularly for developing novel compounds with potential antiproliferative, anticancer, and antimicrobial activities.

BTPA has never been tested as a corrosion inhibitor. The current study results show that BTPA, a novel benzothiazole bearing a pyrazole moiety, has excellent corrosion inhibition properties and that further research on this compound would be valuable. Furthermore, in light of the fundamental ecological issues, these compounds’ nontoxic properties and high solubility in the test solution increase their inhibitory efficacy and enhance their inhibitory efficacy.

The objective of this study is to provide additional insight into the corrosion-inhibiting characteristics of this recently studied BTPA. Low toxicity, environmental friendliness, ease of preparation, low odor, lack of corrosive bleaching impact, and the presence of N, S, and π-bonds are among the advantages of BTPA. Additionally, it works with lower concentrations, which helps reduce costs while maintaining adequate protection. Advanced methods are employed, and atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to examine the surface. Molecular dynamics simulations and quantum chemical computations were analyzed and are discussed. The study also sought to illustrate how molecular structure affects a molecule’s adsorption capacity on a metal surface.

2. Experimental Procedure

2.1. Materials and Solution

Copper specimens with dimensions of (20 mm × 20 mm × 2 mm) and a composition (in weight percentage) of 0.0023 Pb, 0.005 Zn, 0.004 Ni, 0.0023 P, 0015 Si, 0.0018 Al, Co 0.0019, and 0.0011 S, and the remainder being Cu, were polished to achieve a mirror-like surface finish, subsequently degreased using acetone, and then employed for the weight loss (WL) study. Furthermore, a specimen area of 10 mm × 10 mm was utilized for PDP, EIS, and surface investigation.

2.1.1. Inhibitor

As shown in Scheme , the examined 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA) was synthesized. , Figure lists the molecular formulas and chemical structures. A stock solution of dosage inhibitors with a concentration of 1 × 10^–3 M was created by dispersing the synthesized molecule in a mixture of 1 mL DMSO and 25 mL EtOH. Furthermore, the required concentrations (3 × 10^–6, 6 × 10^–6, 9 × 10^–6, 12 × 10^–6, and 15 × 10^–6 M) were prepared by dilution of stock solutions of BTPA with doubly distilled water. The presence and absence of the examined inhibitors were studied at several concentrations. Every experiment was carried out under thermostatic conditions.

1. BTPA Synthesis Route.

1.

Molecular structures, formulas, and weights of BTPA.

2.1.2. Preparation of 4-(Benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine

4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA) compound was created by heating (E)-2-(benzo­[d]­thiazole-2-yl)-3-(dimethylamino)­acrylonitrile (0.5 g, 2 mmol), hydrazine hydrate (0.25 mL, 5 mmol), dry dioxin (20 mL), and potassium hydroxide (0.12g, 1 mmol), as a catalyst, for 2 h under the control of TLC. The reaction mixture was then cooled and added to ice water to create the BTPA compound; the precipitate that resulted from this was filtered out, cleaned with water, and then recrystallized from ethanol/DMF to afford 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine in yellow crystals; 90% yield, mp 240–245 °C.

IR (KBr) (ν/cm^–1) = 3421 (NH₂), 3016 (NH), 1624 (CN), 1601 (CC), 1153 (C–S–C). ¹H NMR (300 MHz, DMSO-d ₆); (ppm) = 6.89, 7.56, and 11.44 (3S, 4H, NH2, CH and NH), 7.07–7.24 (m, 4H, Ar). MS (EI, 70 ev): m/z(%) = 216 (M⁺, 21%), 204 (32%), 198 (48%), 133 (38%), 95 (62%), 82 (51%), and 68 (100%). Anal. Calc. for C₁₀H₈N₄S (216.26): C, 55.54; H, 3.73; N, 25.91; S, 14.82%. Found: C, 55.50; H, 3.69; N, 25.87; S, 14.83%.

2.2. Weight Loss Tests

Copper specimens for the WL investigation were obtained in compliance with ASTM G31–72 guidelines. The specimens underwent precise quantification for corrosion rate (CR) measurement and were suspended by using glass hooks. The initial weight of the specimens was documented before their immersion in a 100 mL test solution contained within a beaker, in both the absence and presence of the BTPA. eq was utilized to determine the inhibitors’ inhibition efficiency when copper coins were preweighed and tested in 1 M HNO₃ solution in the presence and absence of these inhibitors. The duration of immersion was measured at different temperatures. After 3 h, the samples were taken out, dried, and measured. Samples are expelled from acidic solutions and reweighed at equal intervals, i.e., 30 min. Lastly, the following equations can be utilized to determine the inhibition efficiency (%η) and surface coverage degree (θ)

$$\%\mathit{\eta }=\theta \times 100=\left(1-\left(\frac{W}{W^{°}}\right)\right)\times 100$$ 1

W is the WL data for the solution, including the inhibitor, and W° is the WL value for the blank.

2.3. Electrochemical Tests

The electrochemical methods used the Volta-Master 4 software package V7.8 with the Volta-Lab model PGZ402 potentiostat. The Polarization analysis was conducted using three-electrode electrochemical cells. The specimen was prepared utilizing a low-density metallic substrate, presenting a cathode surface area of 1 cm² on one face, with the remaining area being coated by a red enamel layer. A saturated calomel electrode (SCE) was utilized as the reference electrode, while a large platinum foil with a rectangular shape was positioned as the counter electrode (area: 0.625 cm²). The counter electrodes underwent a significant environmental change, resulting in the formation of a considerable cathode. The platinum counter and working electrodes were immersed in distilled water to eliminate potential inhibitory substances. The working electrode’s exposed surface area measured 1 cm². One side of it was welded to a Cu wire, which served as the electrical connection. After the samples were inserted into a glass tube that was only slightly bigger than the samples, the samples were adhered to the glass tube using epoxy resin. Every experiment was conducted at a temperature of 25 °C. Before starting the electrochemical measurements, the working electrode (WE) was immersed in the test solution for 30 min to achieve a stable open-circuit potential (OCP).

The experimental procedure involved using a saturated calomel electrode (SCE) as the reference electrode. Tafel polarization experiments were performed at a controlled temperature with a scan rate of 0.5 mV/s, within the potential range of ±250 mV with respect to the open-circuit potential (OCP). The interplay of the correlation anodic and cathodic sections of Tafel plots in the presence and absence of changed inhibitor concentrations was used to calculate the corrosion potential (E _corr) and corrosion current density (i _corr). EIS tests were performed in the frequency range from 100 kHz to 0.01 Hz, with an excitation signal provided by a 10 mV sine wave voltage. The resistance of charge transfer (R _ct) and the double layer’s capacity (C _dl) are the key variables that come from the examination of the Nyquist diagram. Every experimental procedure was carried out three times to achieve better data reproducibility.

2.4. Miromorphology Analysis

The morphology of the corrosion was also examined by utilizing Cu samples. The copper samples were immersed in HNO₃, with and without adding 15 × 10^–6 M of BTPA inhibitor. Following a soaking period, the samples underwent removal and were repeatedly washed using double-distilled water and ethanol. After drying, the corrosion morphology was examined by employing Pico SPM2100 AFM equipment for AFM tests, which was used to analyze the surface roughness of the film in the presence and absence of BTPA. The scanning scope measured 5 μm × 5 μm. Devices manufactured by Thermo Fisher Scientific in Waltham, Massachusetts, USA, were utilized for SEM and to obtain FTIR spectra through the identification of peaks associated with specific functional groups within the inhibitor’s chemical composition. These surface morphologies were ascertained to corroborate the electrochemical results.

2.5. Quantum Calculations

Researchers utilized a software suite called Materials Studio version 7.0 for chemical analysis. This software employs a semiempirical approach based on density functional theory (DFT) to perform simulations on various materials. Materials Studio is an effective tool with features for computational chemistry, molecular dynamics, bioinformatics, chemical informatics, and quantum mechanics. In this study, version 7.0 was leveraged to conduct advanced research on the materials of interest, including polymers and carbons. A semiempirical method was used to calculate the trajectories. Molecules were classified using the B3LYP function (Becke-3-Parameter-Lee–Yang–Parr) and the DNP function during the default setting. Efficient use and reduction of the Earth’s orbits, as well as the formation of water, were used as a solution affecting purification through COSMO.

2.6. Monte Carlo Simulation

Materials Studio version 7.0 (Accelrys Inc., San Diego, CA, USA) was utilized to conduct MC simulations in a simulation box with recurring boundary conditions. The most stable (lower energy) plane (111) was used to cleave the pure copper crystal, creating a 30 Å vacuum slab. The surface of Cu (111) has been extended to a supercell (10/10), after the planar Cu surface has been relaxed by lowering its energy. The high-quality force field known as COMPASS was assigned to combine inorganic and organic parameters.

3. Results and Discussion

3.1. Weight Loss Measurements

3.1.1. Effect of Concentrations and Temperature

The weight loss (WL) curves for copper submerged in a 1 M HNO₃ solution with and without BTPA are shown in Figure . We used these curves to determine the inhibition efficiency (% IE) and corrosion rate (k _corr) for copper at different temperatures between 25 and 40 °C. Table provides a summary of the collected data. The Table shows a distinct pattern: as the temperature rises, the corrosion rate increases, while %η _WL increases with an increase in BTPA dosage. The enhanced corrosion inhibition efficacy was credited to the integration of the heteroatom onto the metallic surface of the copper, thereby impeding corrosion. The outcomes demonstrated that BTPA exhibited favorable anticorrosive characteristics. The discoveries of this research agreed with the results from earlier investigations involving other BTPA. The adsorption of the BTPA active molecules onto the Cu surface caused this effect. The percentage η_WL drops from 77.1–92.5 at 25 °C to 71.1–88.5 at 40 °C. Adsorption of additives on copper surfaces causes the percentage ηWL to rise, which in effect causes a protective layer to build up on the Cu surface. As the temperature increases, this layer decreases.

2.

Time vs WL diagrams of Cu in 1 M HNO₃ without and with altered doses of BTPA at 25 °C.

2. %η _WL and k _corr of the Inhibitor at Temperatures (25–40 °C) at 120 min, Dipping in the Presence and Absence of Altered Doses of BTPA.

temperature °C	Conc, μM	Wt loss mg/cm	k corr ×10–2 mg/cm2/min	θ	%ηWL
25	0.0	1.811	1.50916667
	3	0.41478	0.34565	0.771	77.1
	6	0.300686	0.25057167	0.834	83.4
	9	0.22227	0.185225	0.877	87.7
	12	0.179533	0.14961083	0.901	90.1
	15	0.136668	0.11389	0.925	92.5
30	0.0	3.372	2.81
	3	0.944457	0.7870475	0.720	72
	6	0.660812	0.55067667	0.804	80.4
	9	0.478031	0.39835917	0.858	85.8
	12	0.396114	0.330095	0.883	88.3
	15	0.307844	0.25653667	0.909	90.9
35	0.0	6.578	5.48166667
	3	1.862587	1.55215583	0.717	71.7
	6	1.342032	1.11836	0.796	79.6
	9	0.987045	0.8225375	0.850	85
	12	0.845114	0.70426167	0.872	87.2
	15	0.66762	0.55635	0.899	89.9
40	0.0	12.589	10.4908333
	3	3.639655	3.03304583	0.711	71.1
	6	2.761761	2.3014675	0.781	78.1
	9	2.124823	1.77068583	0.831	83.1
	12	1.769017	1.47418083	0.859	85.9
	15	1.407532	1.17294333	0.888	88.8

3.1.2. Thermodynamic Activation Parameters

It is widely accepted that corrosion follows the Arrhenius equation, where the activation energy (E*_a) is determined using the corrosion rate constant (k _corr)

$$\log k_{\mathrm{corr}}=\left(\frac{-E_{\mathrm{a}}^{*}}{2.303\mathit{RT}}\right)+\log A$$ 2

where A is the Arrhenius pre-exponential factor, plotting (log k _corr) against (1/T) for the studied BTPA yielded straight lines (Figure ), with the slope representing (E*_a/2.303R). From these plots, the activation energy (E*_a) was determined. Table reveals that as the inhibitor concentration increased, the energy barrier and E*_a values also rose, indicating physical adsorption of the BTPA compound onto the copper surface. , Additionally, the activation entropy (ΔS*) and standard enthalpy (ΔH*) were calculated utilizing the transition state equation , as

$$\log k_{\mathrm{corr}}=\log \left(\frac{R}{\mathrm{Nh}}\right)+\frac{\mathrm{\Delta }{S}^{*}}{2.303R}+\frac{\mathrm{\Delta }{H}^{*}}{2.303\mathit{RT}}$$ 3

where h equals Planck’s constant; straight lines were obtained by plotting (log k _corr/T) against (1/T) for all additives (Figure ). The intercept, (ln (R/Nh) + ΔS*/R), was used to determine ΔS*, while the slope (−ΔH*/R) provided ΔH* values (Table ). The positive ΔH* values confirm that copper dissolution is an endothermic process. − Additionally, the negative sign of ΔS* at higher additive concentrations suggests an associative (rather than dissociative) mechanism in the rate-determining step of the activated complex formation.

3.

Arrhenius bends for Cu dissolution in the 1 M HNO₃ solution, both uninhibited and with varying inhibitor BTPA concentrations.

3. Kinetic Parameters for Cu Dissolution in 1 M HNO₃ Solution, Both Uninhibited and with Varying Inhibitor Concentrations.

	activation parameters
	E a*	ΔH *	–ΔS*
Conc., × 10-6 M	kJ mol–1	kJ mol–1	J mol–1 K–1
Free Acid (1 M HNO3)	100.5	97.9	48.9
3	111.6	109.1	74.3
6	114.0	111.6	80.0
9	116.2	113.6	84.3
12	118.2	115.6	89.1
15	120.4	117.9	94.6

4.

log k _corr/T vs 1/T for Cu without and with BTPA in 1 M HNO₃.

3.1.3. Adsorption Study

The adsorption of BTPA molecules onto the metal surface, which results in enhanced surface coverage as the first step of the inhibition mechanism, is necessary for the medication to have an inhibitory impact on the corrosion of copper. For determining the inhibitor’s adsorption properties, the data for the degree of surface coverage (θ), which is the percentage of the Cu surface that the inhibitor occupies, are quite useful. Assuming that inhibition efficiency and surface coverage are directly correlated, the surface coverage can be calculated by using the following expression: IE% = θ × 100. Correlation coefficient (R ²) values were used to find the isotherm with the best fit to the θ values obtained from the weight loss measurements for several isotherms. In the case of adsorption, a high R ² value (close to 1), interestingly, the Langmuir isotherm, which has a generic form, produced the best results. It was noted that the Langmuir adsorption isotherm is the optimal one for testing the adsorption of BTPA (Figure ). The constant K _ads is given from eq

$$(\theta /1-\theta )=\mathit{KC}$$ 4

where K _ads equals the equilibrium adsorption constant. The standard Gibbs free energy of adsorption (ΔG _ads ⁰) was computed from eq

$$K_{\mathrm{ads}}=\frac{1}{55.5}\exp \left(\frac{-\mathrm{\Delta }{G}_{\mathrm{ads}}^{\mathrm{o}}}{\mathit{RT}}\right)$$ 5

It was discovered that as the temperature rose, the adsorption parameters ΔG°_ads and %η increased. The BTPA appears to be physically adsorbed into the Cu surface, based on the ΔG°_ads values of around 20.6 to 19.5 kJ mol^–1 (Table ). − Also, the heat of adsorption ΔH _ads ^o is planned from the van ’t Hoff balance ,

$$\ln K_{\mathrm{ads}}=\frac{-\mathrm{\Delta }{H}^o}{\mathit{RT}}+\mathrm{constant}$$ 6

A linear relationship was obtained from the plot of log K _ads against 1/T for BTPA (Figure ), where the slope corresponds to ΔH _ads ^o /2.303R. The standard adsorption entropy (ΔS _ads ^o) values were then derived from the fundamental thermodynamic eq − using the obtained ΔG _ads ^o and ΔH _ads ^o values at various temperatures.

$$\mathrm{\Delta }{G}_{\mathrm{ads}}^o=\mathrm{\Delta }{H}_{\mathrm{ads}}^o-T\mathrm{\Delta }{S}_{\mathrm{ads}}^o$$ 7

5.

Langmuir plots for the dissolution of Cu in 1 M HNO₃ with optimum doses of BTPA.

4. Langmuir Parameters for BTPA Adsorbed on the Cu Surface.

Temp. °C	K ads M–1	–ΔG ads ° kJ mol–1	–ΔH ads ° kJ mol–1	–ΔSads ° J mol–1 K–1
25	73.3	20.6	40.8	68.9
30	49.9	19.9		65.7
35	40.2	19.7		63.9
40	32.9	19.5		62.3

6.

Plot of log K _ads against 1/T for BTPA adsorbed on the Cu surface.

3.2. Potentiodynamic Polarization Measurements

Potentiodynamic polarization (PDP) curves were generated for Cu within an HNO₃ environment, both with and without varying doses of BTPA (3–15 μM) at a temperature of 25 °C. As illustrated in Figure , the cathodic and anodic curves are shifted to lower current density data in the tested BTPA, thereby reducing the Cu corrosion rate. Attending to BTPA significantly reduces the corrosion current density. The shift of the Tafel bends toward the cathodic region was observed.

7.

PDP bends for Cu metal in the 1 M HNO₃ solution at different BTPA doses at 25 °C.

Tafel polarization (TP) parameters, including i _corr, E _corr, βc, βa, %η _PDP , and θ, are documented in Table . Based on the data presented in Table , the corrosion current density (i _corr) in the presence of BTPA is lower than when it is absent, implying that the BTPA initially adsorbs onto the Copper surface before exerting its protective effect through a simple blocking mechanism involving its active sites.

Figure displays the PDP curves for Cu corrosion in 1 M HNO₃ with and without varying doses of BTPA at 25 °C, showing that both anodic and particularly cathodic current densities are reduced and E _corr moved to more cathodic potentials, indicating that the inhibitor is mixed-type with predominantly cathodic action.

According to the Stern–Geary theory, a stronger corrosion resistance should also result from higher (β_a) and (β_c) values, as depicted in Table , corresponding to lower i _corr values. In the presence of the inhibitor, anodic Tafel slopes did not change significantly, which indicates that the inhibitor inhibits the rates of this reaction without affecting its mechanism. On the other hand, cathodic Tafel slopes are reduced, which suggests that the inhibitor affecting this reaction most probably by changing the mechanism.

Finally, the observation of a plateau-like range in the cathodic potential indicates that the electrochemical reaction is limited by the rate of diffusion of reactants to the electrode surface. In this diffusion-controlled behavior, the current remains relatively constant despite further changes in the electrode potential, as the reaction is no longer limited by the charge transfer kinetics but by the supply of reactants. −

The η% was calculated using the relation

$${\%\eta }_{\mathrm{PDP}}=\left(\frac{i_{\mathrm{corr}}-i_{\mathrm{corr}}^{\prime }}{i_{\mathrm{corr}}}\right)\times 100$$ 8

where i′_corr and i _corr refer to the corrosion current densities of the metal copper attendance and lack of varying BTPA inhibitor, respectively, it is noticed that with decreasing the (i _corr), %η _PDP values rise as inhibitor dose increases, as observed in Table . Furthermore, the E _corr displacement was less than 85 mV across all concentrations, and a reduction in both cathodic and anodic partial currents was observed; these findings indicate the mixed nature of the BTPA inhibition, with a predominant cathodic effect under the conditions investigated. In our study, the observed potential shift was less than −37 mV vs SCE, indicating that BTPA functions as a mixed-type inhibitor.

5. PDP Parameters for Cu Metal in 1 M HNO₃ in Dissimilar Doses of BTPA at 25 °C.

Conc., μM	E corr. mV vs SCE	i corr, μA cm–2	β a, mV dec–1	β c, mV dec–1	k corr, ×102 mpy	θ	%η
Free Acid (1 M HNO3)	19.9	589	129	297	442
3	12.6	115	118	228	213	0.805	80.5
6	1.3	101	128	245	172	0.829	82.9
9	2.9	72	115	229	114	0.878	87.8
12	–16.8	56	125	197	84	0.905	90.5
15	–16.9	35	119	250	28	0.941	94.1

3.4. Electrochemical Impedance Spectroscopy Measurements

Figures and present the electrochemical impedance spectroscopy (EIS) data analyzed using the proposed equivalent circuit model for copper immersed in nitric acid solutions, both with and without varying concentrations of BTPA. The figures display the Nyquist and Bode plots characterizing copper dissolution in 1 M nitric acid under different BTPA inhibitor concentrations. − Figure highlights the low-frequency impedance response, showing higher values when BTPA is present. The increasing diameter of Nyquist semicircles with BTPA concentration reflects greater charge transfer resistance, attributable to BTPA adsorption at the Cu/solution interface. − A high resistance has been obtained. The Bode plots (Figure ) reveal a consistent increase in phase angle shift, directly corresponding to enhanced BTPA adsorption on the copper surface. These plots also demonstrate a concentration-dependent rise in overall impedance, as evidenced by both (log Z vs log f) and (phase angle vs log f) relationships. Notably, the Bode spectra for BTPA exhibit a single-phase maximum, indicating one dominant relaxation process characteristic of charge transfer at the metal/electrolyte interface. The Bode plots reveal that increasing concentrations of BTPA lead to higher total impedance (Z) and a shift in the phase angle toward greater values. This shift is attributed to the adsorption of BTPA onto the Cu surface. However, the Bode phase plots (log f vs phase) demonstrate a continuous increase in the phase angle shift, which can be attributed to the increasing adsorption of the compounds onto the Cu surface. The phase angle values rise with improving PEE concentration, demonstrating enhanced capacitive behavior of the Cu electrode due to the BTPA molecule adsorbed on the surface of Cu. The recorded phase angles were 42.41, 55.37, 56.44, 58.64, and 60.72°, with the maximum value of 71.18° observed at 15 × 10^–6 M. These findings show a direct correlation between the gradual rise in phase angle and the rising PEE concentration.

9.

Nyquist bends for Cu in 1 M HNO₃ without and with an altered dose of BTPA at 25 °C.

10.

Bode diagram for Cu in 1 M HNO₃ at varying BTPA concentrations at 25 °C.

Figure illustrates the equivalent circuit used to model the copper/electrolyte interface, comprising: (1) solution resistance (R _u), (2) charge transfer resistance (R _ct), (3) a constant phase element (CPE), and (4) Warburg impedance (W _d). This single-time constant circuit accurately describes our experimental data. For improved fitting accuracy, the model employs a constant phase element instead of an ideal double-layer capacitor. The CPE impedance parameters (Y ₀ and n) are defined in the following equation.

$$Z_{\mathrm{CPE}}=1/Y^o{(j\omega )}^n$$ 9

where Y ₀ is the CPE amplitude and n (−1 ≤ n ≤ 1) characterizes its nonideality, with­(ω_max = 2πf _max), f _max(maximum frequency). Table presents fitted Nyquist parameters. BTPA adsorption on Cu blocks active sites and increases surface homogeneity (rising n values).

8.

Model uses an equivalent circuit to fit the EIS, incorporating Warburg diffusion impedance.

6. EIS Parameters Calculated for Copper Metal in 1 M HNO₃, in the Presence and Absence of Varying BTPA at 25 °C.

Conc., μM	Y O, (μ Ω–1 s n  cm–2)	n	R CT, Ω cm2	C dl, μF cm–2	W d Ω cm2	Θ	% IE	Goodness of fit (χ2)
Free Acid (1 M HNO3)	518	0.971	19	451	78.47 × 10–3			20.32 × 10–5
3	319	0.979	163	299	17.51 × 10–3	0.883	88.3	22.47 × 10–5
6	229	0.983	187	217	15.74 × 10–3	0.898	89.8	17.75 × 10–5
9	187	0.985	204	177	14.17 × 10–3	0.907	90.7	20.14 × 10–5
12	108	0.989	224	103	8.99 × 10–3	0.915	91.5	19.45 × 10–5
15	93	0.995	266	91	3.89 × 10–3	0.929	92.9	14.77 × 10–5

The excellent agreement with the equivalent circuit is confirmed by the low χ² values (0.000224–0.000147) presented in Table .

The observed decrease in Y ₀ (CPE) values with increasing inhibitor doses can be attributed to a decrease in the local dielectric constant. This behavior confirms that inhibitor molecules adsorb at the Cu/HNO₃ interface, thereby suppressing copper corrosion. The charge transfer resistance (R _ct) consistently increased with BTPA doses, demonstrating values significantly higher than those of the uninhibited system. This enhancement confirms the formation of a protective BTPA adsorption layer on the copper surface. The maximum Rct value occurred at 15 μM BTPA concentration, corresponding to the lowest corrosion rate.

Conversely, the double-layer capacitance (C _dl) decreased progressively with increasing inhibitor concentration, as expected from the adsorption mechanism. The C _dl values were calculated using the following equation

$$C_{\mathrm{dl}}=\frac{1}{{2\pi f}_{\max }{R}_{\mathrm{ct}}}$$ 10

The %η_EIS was calculated by using eq

$${\%\eta }_{\mathrm{EIS}}=\left(\frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^{*}}{R_{\mathrm{ct}}}\right)\times 100$$ 11

In this notation, R _ct ^o denotes the native charge transfer resistance (without inhibitor), while R _ct represents the resistance with inhibitor present. The progressive enhancement of η% directly correlates with additive concentration, reflecting both the initial adsorption and subsequent thickening of the protective surface layer. , Notably, the inhibition efficiencies derived from EIS measurements agreed with those determined by gravimetric (WL) and electrochemical (PDP) tests.

3.5. Surface Analysis

3.5.1. Scanning Electron Microscope Test

SEM was employed to examine the surface morphology of copper samples after 24 h immersion in 1 M HNO₃/solution, both with and without inhibitor treatment. Figure a reveals significant surface corrosion from aggressive acid attacks on the unprotected copper specimen. In contrast, the sample containing 15 μM BTPA inhibitor (Figure b) exhibits markedly smoother topography, − demonstrating the inhibitor’s protective effect. SEM analysis of copper surfaces in 1 M HNO₃ showed: (a) severe corrosion in uninhibited solution (Figure a), versus (b) protected, smooth morphology with 15 μM BTPA (Figure b). The dramatic difference confirms that BTPA forms an effective protective film that inhibits acid corrosion. These results suggest that BTPA molecules form an adherent coating film on the surface of copper, significantly mitigating acid-induced corrosion.

11.

SEM image of Cu in 1 M HNO₃ (a) without and (b) with BTPA inhibitor treatment.

3.5.2. Atomic Force Microscope Analysis

By examining the surface’s morphology, AFM is one of the most effective techniques for determining how BTPA affects copper surfaces. Before this analysis, the copper surface was polished, and the copper pieces were immersed in HNO₃ solution for 24 h to determine whether 15 μm BTPA was present. Figure shows the copper surface’s topographic maps, including 3D photos.

12.

(a) 3D AFM image of copper after inundation in 1 M HNO₃ for 24 h (b) 3D AFM after 24 h immersion in 1 M HNO₃ + 15 μm ppm BTPA.

Surface roughness analysis provides critical insights into the inhibitor performance and the characteristics of adsorbed protective films. Figure compares (a) a severely corroded copper surface with deep cracks from 1 M HNO₃ exposure, versus (b) a smoother surface with an apparent protective film in the presence of an inhibitor.

Atomic force microscopy (AFM) quantification revealed significant differences in the roughness parameters. The mean roughness (Ra) was measured:

• 592 nm for unprotected copper after 24 h immersion in 1 M HNO₃.

• 76 nm for samples protected with 15 μM BTPA.

This 87% reduction in surface roughness demonstrates effective inhibitor adsorption, forming a protective layer that substantially decreases the corrosion rate.

3.5.3. Fourier Transform Infrared Spectroscopy

Spectra of stock BTPA and copper surface after engagement in 1 M HNO₃ + 15 μm ppm BTPA for 3 h at 25 °C are presented in Figure . To make it easier to define the effective groups, each peak in the spectrum has a specific value. It is also used to identify the kind of response that occurs between the copper surface and BTPA. Figure shows the FTIR spectrum of the BTPA inhibitor. The N–H bend at 1627 cm^–1 deviated to 1618 cm^–1, indicating a reaction between the inhibitor and the Cu surface. The shifted frequencies identify the bond between BTPA and the Cu surface. These shifts would have been caused by the decrease in electron density of the N–H, C–N bond, and CO bond due to the change in electron cloud density from N and O to Cu²⁺. A broadband in the range 3370 to 3357 cm^–1(O–H stretch) indicates the existence of coordinated/lattice water molecules designed on the cathodic sites of the copper surface.

13.

FTIR spectra of PTBA stock and BTPA adsorbed layer on the surface of copper.

3.6. Quantum Calculations

DFT is a powerful computational tool for exploring the interactions between the inhibitor molecule and the metal interface. Calculated quantum parameters revealed the following. (1) The energy gap (ΔE = E _LUMO – E _HOMO), representing the energy required for electron excitation from the highest occupied molecular orbital. Smaller ΔE values typically correlate with higher inhibition efficiency, facilitating electron transfer between the inhibitor and the metal surface. It would be ideal to show a direct comparison between experimental results and the quantum chemical parameters (such as corrosion rate reduction and a lower energy gap). (2) The dipole moment (debye) quantifies the molecular polarity and covalent bond characteristics. Larger dipole moments promote metal surface adsorption through stronger electrostatic interactions.

The computational results presented in Figure and Table demonstrate that BTPA exhibits the lowest energy of the studied BTPA inhibitor, indicating both superior adsorption characteristics, evidenced by its: (1) lowest ΔE, (2) highest softness (σ), and (3) optimal quantum parameters (Ip), the ionization (Ip- E _HUMO), (E _A) and the electron affinity (E _A = −E _LUMO). Additional calculated propertiesglobal hardness (η), softness (σ), and chemical potential (μ)further confirm its inhibition potential through established quantum chemical relationships from the following equations.

$$\mu =-\chi =-\frac{I_{\mathrm{p}}+E_{\mathrm{A}}}{2}$$ 12

$$\chi =\frac{I_{\mathrm{p}}+E_{\mathrm{A}}}{2}$$ 13

$$\eta =\frac{I_{\mathrm{P}}-E_{\mathrm{A}}}{2}$$ 14

$$\sigma =\frac{1}{\eta }$$ 15

14.

Frontier molecular orbitals (HOMO and LUMO) of the tested inhibitors reveal their respective electron density distributions.

7. Theoretical Chemical Parameters of the Studied Organic Inhibition of BTPA.

parameters (Variable)	BTPA
E HOMO (eV)	–4.985
E LUMO (eV)	–2.986
ΔE (eV) (E L–E H)	1.999
E A (eV)	4.985
I p (eV)	2.986
(eV)χ (electronegativity)	3.99
μ	–3.99
η, eV	1
σ, eV	1
dipole moment (debye)	8.599
molecular surface area, Å2	248.48

The above results reveal that the hetero atoms (N and S) in the structure of inhibitor molecules greatly influence quantum chemical parameters. The theoretical study demonstrates that the heteroatom (S) could cause effects on the adsorption of the inhibitor molecules on the Cu metal. In the following, we perform further molecular dynamics calculations on the adsorption of the inhibitors on the copper surface.

3.7. Monte Carlo Simulation

MC simulation is a perfect simulation tool for finding the most stable adsorption conformations in 1 M HNO₃ of substituted 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA). The simulation results for the investigated derivatives are presented in Figure and summarized in Table . Figure illustrates the most stable adsorption configuration of the inhibitor molecules on the Cu (111) surface. The observed molecular orientation results from the electron-rich moieties of these inhibitory compounds preferentially interacting with the metal surface.

15.

Adsorption Locator module reveals the most stable molecular orientation and binding geometry of the BTPA molecule on the Cu (111) surface.

8. MC Simulation Data for Adsorption of BTPA Molecule on Cu (111).

structures	adsorption energy, kcal mol–1	rigid adsorption energy, kcal mol–1	deformation energy, kcal mol–1	compound dE ads/dN, kcal mol–1	H2O dE ads/dNi, kcal mol–1
Cu (111)/ BTPA/H2O	–9084.528	–51.548	–9032.98	–245.615	–4.5

The interfacial interactions between the occupied orbitals of the studied derivative and unoccupied orbitals of Cu (111) are quantitatively characterized by several energetic parameters: adsorption energy (E _ads), rigid energy (E _rigid), deformation energy (E _def), and energy ratios (dE _ads/Eni). These values are given in Table . These quantify orbital interactions between inhibitor molecules and the metal surface. The remarkable adsorption energy (−9084.528 kcal/mol) of BTPA stems from its -N- and -S- functional groups, directly correlating with its inhibition performance. By displacing corrosive species, BTPA forms a protective surface film that minimizes metal dissolution. ,

Table presents the adsorption energies of H₂O and NO₃ ^– species on the Cu (111) surface. Notably, the studied inhibitor demonstrates substantially higher adsorption energies (|Eads|> 100 kcal/mol) compared to these corrosive species (|Eads| < 10 kcal/mol). This pronounced difference confirms the superior ability of BTPA to displace preadsorbed corrosive agents from the copper surface, subsequently forming an effective protective film. MC calculations show that there is a strong adsorptive interaction that takes place between the BTPA inhibitor and the iron surface. Theoretical adsorption energies derived from molecular simulations showed a strong correlation with experimentally determined inhibition efficiencies, confirming the predictive capability of our computational model.

4. Inhibition Mechanism

As demonstrated through combined experimental investigations and theoretical computations, the adsorption behavior is governed by physicochemical properties (including electron density distribution and functional group composition) and copper surface charge characteristics. We observed the creation of a film that was concentrated randomly on the whole surface of the copper. Due to the adsorption of 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA) molecule on the surface of Cu, blocking the active center’s existence on the surface of Cu. Also may be due to the association of BTPA molecule in the interaction with the reaction center of Cu surface, lead to a breakdown in the contact among Cu and the aggressive medium (1 M HNO₃) and hence, hindrance excellent protection influence. Figure shows the possible inhibition mechanism of 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA) on the Cu alloy surface. Multiple studies confirm that copper surfaces in HNO₃ solutions acquire a positive charge. , This positively charged surface preferentially adsorbs NO₃ ^– anions, creating a negatively charged interface that attracts cationic species in solution. The inhibitor 4-(benzo­[d]­thiazol-2-yl)-1H-pyrazol-5-amine (BTPA) can undergo protonation through lone electron pairs on its nitrogen and sulfur atoms. The protonated BTPA molecules adsorbed via electrostatic interactions (physisorption) to the metal surface, as illustrated in Figure .

16.

Mechanism of corrosion protection of Cu dipped in 1 M HNO₃ with BTPA.

5. Conclusions

Based on the results obtained, the following conclusions can be drawn:-

The BTPA tested proves to be an excellent corrosion inhibitor for Cu corrosion in nitric acid solutions.

The inhibition efficiency consistently increases with concentration and reaches a high value of 94.1% for a concentration of 15 μM of BTPA.

BTPA adsorption follows the Langmuir adsorption isotherm.

According to PDP research, the BTPA influences both the anodic and cathodic processes, acting as a mixed-type inhibitor.

AFM, SEM, and FTIR measurements verify that the investigated BTPA successfully adsorbs on the Cu surface.

Furthermore, both DFT and MC simulation results confirmed that BTPA strongly adsorbed to the Cu surface.

A.A.O.Y., Z.E.A., I.R., A.M.O., and M.M.M. contributed equally to this article, as follows: conception, design of the work, analysis, method and statistical analysis, interpretation of data, drafting the work, and substantively revising it. All authors reviewed the manuscript.

The authors declare no competing financial interest.