An Experimental and Theoretical Investigation of the Efficacy of Pantoprazole as a Corrosion Inhibitor for Mild Steel in an Acidic Medium

Abstract

The corrosion behavior of mild steel in a 1 M aqueous sulfuric acid medium in the presence and absence of the drug Pantoprazole was investigated using potentiodynamic polarization and quantum chemical calculations as well as Monte Carlo and molecular dynamic simulations. The potentiodynamic experiments indicated that this molecule, as a result of its adsorption on a mild steel surface, functioned as a mixed inhibitor. The goal of the study was to use theoretical calculations to acquire a better understanding of how inhibition works. The adsorption behavior of the examined compounds on the Fe (1 1 0) surface was calculated using a Monte Carlo simulation. Furthermore, the molecules were studied using density functional theory (DFT), especially the PBE functional, to determine the relationship between the molecular structure and the corrosion inhibition behavior of the chemical under research. The adsorption energies of Pantoprazole (in its three different protonation states) iron were calculated more precisely using molecular mechanics with periodic boundary conditions (PBC). The predicted theoretical parameters were found to be in agreement with the experimental data, which was a considerable help in understanding the corrosion inhibition mechanism displayed by this chemical.

1. Introduction

The physical and chemical properties of mild steel as well as its availability and cost have made it one of the most extensively utilized materials in a wide range of industrial fields [1,2,3,4,5,6]. This material is used in a range of applications, including mineral processing equipment, petroleum refining, and other building materials. Despite having a wide range of uses, the material has a low corrosion resistance when subjected to different atmospheric conditions. Many metal materials degrade as a result of this exposure, resulting in substantial economic consequences. As a result, corrosion prevention and protection have become unavoidable requirements. Several methods have been explored to protect metals but one of the most successful and cost-effective has been the inclusion of inhibitors into the corrosion media. According to the literature, the most efficient corrosion inhibitors are organic molecules containing unsaturated bonds, aromatic rings, and heteroatoms such as O, N, and S [7,8,9,10]. Using mild steel in 1 M H₂SO₄, we investigated the inhibitory impact of the drug Pantoprazole [6-(difluoromethoxy)-2-(((3,4-dimethoxypyridin-2-yl)methyl)sulfinyl)-1H-benzo[d]imidazole] on the corrosion of this material. This drug is a possible corrosion inhibitor due to its chemical structure, which contains heteroatoms (O, N, S, F) and numerous double bonds. Theoretical calculations (DFT, MC, and MD) were used to complete the experimental study.

2. Materials and Methods

2.1. Instrument, Material, Electrode Preparation, and the Corrosive Solution

A PalmSens3 potentiostat was used in combination with a three-electrode cell at 298 K. A graphite rod (d = 3 cm, l = 4 cm) was employed as an auxiliary electrode, and a saturated calomel electrode (SCE) was used as a reference electrode. Scanning the electrode potential vs. E_OCP at a 1 mV/s sweep rate yielded potentiodynamic polarization curves. The measurements were taken in an atmospheric environment. To guarantee reproducibility, each experiment was carried out three times.

Table 1. Composition of mild steel (in %).

Fe	C	P	Mn	Si	Cr	S	Mo	Ni
99.54	0.1252	0.0316	0.1836	0.0561	0.0124	0.0282	0.0125	0.0015

shows the mass percentages of mild steel (in weight percent) [11]. The corrosive solution utilized was a 1 M sulfuric acid solution that was prepared by diluting the solution with bi-distilled water.

The electrode for the electrochemical studies was created by embedding a mild steel rod (d = 1.6 mm, l = 12 cm) into a Teflon^® tube (d = 0.8 cm, l = 10 cm) using epoxy glue and allowing it to cure. The electrode was polished on silicon carbide abrasive paper with a medium grain diameter of 6.5–15.3 microns, then on a (DP-Nap) cloth soaked in an aluminum oxide solution with a particle size of 0.3 microns, then rinsed and sonicated in water.

2.2. DFT Calculations

The DFT calculations were fulfilled by Dmol3 software from Biovia [12,13]. A meta-generalized gradient approximation [14,15] employing M11-L [15,16,17] and a double numeric quality basis set (DNP) [18] were used for the geometry optimizations. A lower than 0.00001 kcal/mol convergence standard for the self-consistent field (SCF) was used for the SCF. The energy minima were validated by carrying out a vibrational analysis and establishing that there were no imaginary frequencies present in the data [19,20,21].

2.3. Monte Carlo (MC) and Molecular Dynamic (MD) Simulation

The simulation of the interaction of a mild steel surface with the drug Pantoprazole was performed in a corrosion environment by utilizing a ten atom-thick layer unit cell of Fe (1 1 0) surface as the basis for the experiment (under a periodic boundary condition). The sizes of the slab model were 19.859 Å × 19.859 Å × 18.342 Å with an enclosed addition of a 30 Å vacuum layer at the C axis that was introduced with 1 Pantoprazole molecule/350 H₂O molecules/5 sulphate + 10 hydronium ions. In advance of the MD step, the geometry of the simulation boxes was optimized (energy converge tolerance of 1 × 10⁻⁵ kcal/mol) using the Forcite module in the Biovia software package.

MD was performed at a temperature of 25 °C [22] using the constant volume/constant temperature (NVT) canonical ensemble with a simulation period of 1000 ps [23,24,25,26,27]. The Berendsen thermostat conserved the temperature. The calculations for MC and MD were carried out using the Condensed Phase Optimized Molecular Potential II (COMPASSII) forcefield [26,28,29,30,31,32,33,34,35,36]. A radial distribution function (RDF) analysis included the entire MD trajectory [2,20,21,23].

3. Results

3.1. Polarization Measurements

The anodic and cathodic polarization curves of a mild steel electrode in a 1 M H₂SO₄ solution are shown in Figure 1 in the absence and presence of the drug Pantoprazole at 298 K. The IE (in percent) was determined using Equation (1):

$$IE(\%)=\frac{i_{absence of inhibitor}^{corr.}-i_{presence of inhbitor}^{corr.}}{i_{absence of inhibitor}^{corr.}}100 .$$

(1)

The electrochemical parameters of the corrosion potential (E_corr) and corrosion current density (i_corr) were determined from the intersection of the anodic and cathodic Tafel slopes and are presented in

Table 2. Electrochemical parameters of mild steel at various concentrations of Pantoprazole molecules in 1 M sulfuric acid.

C (ppm)	Ecorr (V)	Icorr (μA/cm2)	bc (mV/dec)	ba (mV/dec)	IE (%)
-	−0.502	0.4891	−153.1	84.4	-
10	−0.480	0.2908	−155.2	92.1	48.34
50	−0.491	0.1933	−166.1	96.4	72.12
100	−0.495	0.1689	−166.9	98.1	78.27
250	−0.472	0.1014	−172.2	99.4	94.53

.

The Tafel plot in Figure 1 reveals that the adsorption of the Pantoprazole molecules onto the mild steel surface significantly reduced the corrosion current of the mild steel in this hostile environment, indicating a high degree of corrosion inhibition of up to 94.53 percent.

When Pantoprazole was present in the solution, the values of the cathodic and anodic Tafel slopes (bc, ba) changed. The changes in the Tafel slopes indicated that the molecule had an effect on the kinetics of the hydrogen evolution process [10]. This resulted in a greater energy barrier for the proton discharge and, therefore, less gas evolution. The investigated compound had no discernible effect on the corrosion potential, suggesting that it acted as mixed-type inhibitor [6].

3.2. DFT, MC, and MD Results

The microspecies distribution of the Pantoprazole molecules was performed using Chemaxon software prior to the computation to take into consideration the pH influence on the protonation/deprotonation of the Pantoprazole molecule. As may be observed in Figure 2A, Form A, C, and E of the chemical structures were below a pH of 7.

According to [37,38], charge density profiles are seen as a map of the charge density distribution across the surface of the molecule and give information about the molecule as well as information about its solubility. To create the charge density curve, referred to as the sigma profile, it is essential to perform calculations centered on the conductor-like screening model (COSMO) (Figure 3). COSMO operates on molecular-shaped cavities, which are characterized by partial atomic charges and serve as a representation of the electrostatic potential [15,39,40,41,42]. As seen in Figure 3 (peaks in the σ-profile at the screen charge density values near −0.01 and 0.01 e/Ų), the inhibitors operated as an acceptor/donor of H-bonds in the process. As a result, the solubility of the various protonated forms of Pantoprazole was dependent on its capacity to interact with water molecules via this type of contact.

When it comes to molecular simulations and mechanism verifications, both HOMO and LUMO (Figure 4) are frequently employed as recommendations to determine if a reaction occurs normally and which components of the molecules are responsible for the reaction [6,29,30]. The HOMO displayed the electrons being donated to the electron-accepting portions of the molecule and the LUMO illustrated the electrons being donated to the net-donor sections of the molecule.

Figure 4 depicts the HOMO and LUMO OM and ESP of the Pantoprazole medication, and

Table 3. Calculated theoretical chemical parameters for the different protonated forms of the Pantoprazole inhibitor.

Theoretical Parameters	Pantoprazole Form A	Pantoprazole Form C	Pantoprazole Form E
HOMO	−5.9710	−6.2020	−6.7640
LUMO	−2.1580	−2.7930	−3.2990
∆E (HOMO-LUMO)	3.813	3.409	3.465
Ionization energy (I)	5.9710	6.2020	6.7640
Electron affinity A)	2.1580	2.7930	3.2990
Electronegativity (Χ)	4.0645	4.4975	5.0315
Global hardness (η)	1.9065	1.7045	1.7325
Chemical potential (π)	−4.0645	−4.4975	−5.0315
Global softness (σ)	0.5245	0.5867	0.5772
Global electrophilicity (ω)	4.3326	5.9336	7.3062
Electrodonating (ω−) power	6.6032	8.3954	10.0385
Electroaccepting(ω+) power	2.5387	3.8979	5.0070
Net electrophilicity (∆ω±)	2.3872	3.7788	4.9074
Fraction of transferred electrons (∆N)	−0.2189	−0.3718	−0.5199
Energy from Inhib to Metals (∆N)	0.0913	0.2356	0.4683
∆E back-donation	−0.4766	−0.4261	−0.4331

lists the most often seen DFT indices. The majority of the HOMO and LUMO densities were localized in the region of the molecule that contained the imidazole moiety. When it comes to Pantoprazole, the HOMO and LUMO were significant due to their ability to interact with the Fe (1 1 0) surface via electron donation and acceptation, respectively [2,7,21,24,25,26,27,29,30,31,36,43,44,45].

When the electron affinity and ionization potential of the material were computed, a comparable capacity was obtained. Additionally, the proclivity of the forms of Pantoprazole for adsorption on metal surfaces were enhanced by an appropriate softness value and a relatively low hardness ratio [9,11,46,47].

3.3. MC and MD Results

As seen in Figure 5, the lowest energy configurations of the distinct protonated forms of the Pantoprazole molecule on the metal surface in the simulated corrosion environment were identified. It appeared that the heteroatoms (mostly oxygen and nitrogen atoms) were involved in the adsorption process of the inhibitor based on the adsorption geometries. The creation of a protective anticorrosion layer on the metal surface was a result of this adsorption affinity.

The quantitative determination of the interaction of the inhibitor molecule with the metal surface was accomplished via the computation of the adsorption energies using the following equation [48]:

$$E_{adsorption}=E_{Fe\left(110\right)|inhibitor }-\left(E_{Fe\left(110\right)}+E_{inhibitor}\right)$$

(2)

where E_{Fe(110)|inhibitor} is the total energy of the simulated corrosion system and E_Fe(110) and E_inhibitor are the total calculated energy of the Fe (1 1 0) surface and that of the free inhibitor molecule, respectively.

For each MC, random molecular configurations as large as possible (ions or molecules) were chosen and deposited in the simulation box. Using the graph in Figure 6, we observed that when more of these systems were used, the average energy of the system approached a plateau, indicating that the system had reached its maximum energy balance (after 200,000 steps).

The results of the Monte Carlo simulations were consistent with those of the experiments. Due to the strong negative value of the adsorption energies (as seen in Figure 7), the adsorption process was considered to be spontaneous [11,24,25,33].

It is possible to watch and record the dynamics of the adsorption of an inhibitor on metal surfaces using MD simulations [30]. Monitoring the actual temperature change during the MD run is one method to ensure that the system uses the least amount of energy. From Figure 8, it can be seen that the temperature drift was modest, suggesting that the MD of our system was successfully conducted.

Figure 5 depicts the most recent inhibitor configurations on a metal surface. In accordance with the MC calculations, the N and O atoms were responsible for the adsorption of the Pantoprazole molecule. Using the adsorption energy calculated from the MD simulations (Figure 9), we observed that the inhibitor, regardless of its protonation form (A, C, or E), interacted strongly with the surface; it laid nearly flat on it, limiting the ability of the corrosion species to reach the surface, consequently reducing the severity and rate of the mild steel corrosion.

This deduction was also sustained by the analysis of the radial distribution function (RDF), which calculated the oxygen atoms from the horizontal of the metal surface presented in Figure 10.

It has been commonly established that when a peak appears in the RDF graph of a particular atom(s) and the surface between 1 and 3.5 Å, it is a strong indication that chemisorption occurred whereas the presence of physisorption RDF peaks is estimated at greater distances (typically > 3.5 Å) [22,23,25,31,32,33,34,43].

The RDF for the O, N, S, and F atoms (Figure 10) of the inhibitor advocated the chemisorption of the inhibitor on the metal surface [28,29]. The accomplished results from MD and the corresponding RDF analysis validated the well-founded propensity of the inhibitors to adsorb and protect the metal.

3.4. The Study of the Film Density and the Self-Diffusion Coefficients of Pantoprazole

After 1000 ps of MD, the density values of the Pantoprazole-formed films were obtained; the average density values are presented in

Table 4. Parameters for the corrosion thin-film model construction.

Pantoprazole	a = b = c (Å)	α = β = γ (°)	Density (g/cm3)	Number of Molecules	Number of Particles
Form A	31.814	90	1.407	50	1
Form C	31.842	90	1.382	50	1
Form E	31.869	90	1.425	50	1

.

The fractional free volume (FFV) of the inhibitor films were evaluated from the free and the occupied volumes of the Connolly surface using the following equation [29]:

$$FFV=\frac{V_{free}}{V_{free}+V_{occupied}}$$

(3)

where V_free is the free volume and V_occupied is the occupied volume of the PBC box containing the inhibitor film.

The determination of the self-diffusion coefficient (SDC) was calculated by [29]:

$$D=\frac{1}{6}\underset{t\to \infty }{lim}{\displaystyle \sum }_{i=1}^{N\alpha }\langle {\langle r_i\left(t\right)-r_i\left(0\right)\rangle }^2\rangle$$

(4)

where 〈〈r_i(t) − r_i(0)〉²〉 is the mean squared displacement (MSD) values obtained from the MD trajectory (Figure 11).

The FFV of the corrosive simulation media was calculated and is shown with other parameters in

Table 5. Parameters for the corrosion thin-film model construction.

Pantoprazole	Form A	Form C	Form E
(Self-diffusion coefficient) (cm2/s)	2.67 × 10−7	1.16 × 10−8	8.33 × 10−9
Free volume (Å3)	2814.56	3288.26	2276.13
Occupied volume (Å3)	20,010.63	19,868.82	20,180.50
FFV	0.123309	0.141998	0.101357

.

An important characteristic that determines the mobility of corrosive ions (such as hydronium and sulphate ions) at the interface of a produced inhibitor film is the mobility of the inhibitors themselves. van der Waals and Coulomb forces work on the film-forming inhibitor molecules, causing them to interact and become displaced in the corrosive solution. This causes the volume and shape of the cavities in the film to change as well as the size of the cavities [29,48]. This has an effect on the mobility of the corrosive ions within the film. A high mobility film suggests that the corrosive species has a higher diffusion coefficient than normal. It was proposed that the SDC be used to assess the mobility of the inhibitor film. The self-diffusion coefficients for the Pantoprazole (Form A, C, and E) inhibitors are shown in Table 5. These values were determined by the balance between the flexibility of the molecule and its capacity to interact with nearby molecules in the surrounding environment. Although the protonation appeared to have no effect on the tight packing of Pantoprazole, this actually strengthened the interactions between its molecules. The reflection of the inhibitory performance of an inhibitor molecule is a complicated process that is the result of an accumulation of a number of different combined effects (volume of the molecule, adsorption ability, adsorption energy, the FFV of the film, the mobility of the film-forming molecules, the interaction of the corrosive species with the film, etc.).

4. Conclusions

The investigated compound was an effective mild steel inhibitor when used in an acidic environment. Using polarization measurements, it was discovered that this compound functioned as a mixed inhibitor. The adsorption centers of the inhibitors were determined by the use of DFT simulations. The MC and MD simulations further confirmed the strong adsorption contact of the inhibitor with the metal surface, giving molecular proof of the adsorption behavior (geometry) and adsorption energy of the Pantoprazole molecule on the iron surface, which was consistent with previous findings. Several theoretical results (DFT, MC, and MD) confirmed the results of the experiments.