1 Introduction
Aluminum is regarded as one of the most prevalent metals found in the Earth’s crust. Its affordability, substantial strength, low density, and excellent thermal and electrical conductivity make aluminum and its alloys suitable for a diverse range of applications, including marine industries, aerospace, food packaging, electronics, heat exchangers within the petroleum sector, scrubbers, and storage tanks [1,2]. However, despite these beneficial properties, aluminum is particularly susceptible to corrosion in acidic conditions, especially in hydrochloric acid solutions that are frequently utilized in industrial processes such as pickling, cleaning, and descaling, typically under temperature conditions above room temperature [1]. Under such harsh environments, the passive oxide layer that naturally develops on the aluminum surface can dissolve, resulting in accelerated dissolution of the metal and deterioration of its structural integrity.
Given the adverse effects of corrosion, various measures have been undertaken to safeguard the aluminum surface from degradation in aggressive acidic or other corrosive environments [3−5]. The utilization of corrosion inhibitors stands out as one of the most effective and cost-efficient strategies for mitigating metal corrosion in acidic settings [3,6]. Organic inhibitors demonstrate particular efficacy due to their ability to adsorb onto metal surfaces, thereby creating protective films that shield the metal from corrosive agents. The incorporation of heteroatoms such as nitrogen, oxygen, and sulfur, in conjunction with aromatic structures and π-electron systems [7], significantly boosts the adsorption potential of inhibitor molecules via donor-acceptor interactions [3,8,9]. For example, research has highlighted the effectiveness of Pyrazolocarbothioamide derivatives [8], triazinedithiol inhibitors [3], N1, N1’-(ethane-1,2-diyl)di(ethane-1,2-diamine) [10], and amine compounds [7] as inhibitors for aluminum in hydrochloric acid. Although the majority of organic compounds demonstrate efficacy as corrosion inhibitors, certain compounds necessitate the utilization of expensive and hazardous chemicals during their synthesis, thereby highlighting the need for alternative solutions. The development of deterioration inhibitors that are eco-friendly, efficient, and affordable have been given higher priority by researchers [11].
Recently, pharmaceutical compounds have garnered considerable interest as corrosion inhibitors owing to their effectiveness, accessibility, and relatively low toxicity profiles [12]. Trimethoprim (TMP) and sulfamethoxazole (SMX) are pharmaceutical entities characterized by multiple heteroatoms and conjugated systems (Fig. 1), rendering them promising candidates for corrosion inhibition. Given the structural features of TMP and SMX, they are anticipated to provide protection against aluminum corrosion in acidic environments. Moreover, the synergistic application of TMP and SMX may yield even greater protective efficacy. Trimethoprim-sulfamethoxazole (TMP-SMX), commonly referred to as co-trimoxazole, represents a combination of two antimicrobial agents that exhibit a synergistic effect against a diverse range of bacteria.
Fig. 1 (a) Structure of SMX (b) Structure of TMP. |
Naser et al. (2024) [13] conducted a study on the synthesis and characterization of trimethoprim-metal complexes, evaluating their potential as corrosion inhibitors for carbon steel in a hydrochloric acid (HCl) environment. Two derivatives were synthesized, and their effectiveness in mitigating corrosion was assessed using potentiodynamic polarization (PDP), which indicated their suitability as deterioration inhibitors for carbon steel alloys [13]. Abdullah et al. (2022) [14] examined the potential of expired metheprim as a corrosion inhibitor for aluminum in a 1 M HCl solution, employing potentiodynamic polarization alongside theoretical studies, specifically density functional theory (DFT), to ascertain quantum chemical descriptors relevant to the inhibitor's performance. However, this investigation lacked electrochemical impedance spectroscopy (EIS), a critical technique for determining essential parameters such as charge transfer resistance [14]. Also, significant computational methodologies, including Monte Carlo and molecular dynamics simulations, were not utilized in the research. Ibrahim et al. (2019) [15] reported the synthesis of chromium (III), manganese (II), cobalt (II), nickel (II), and copper (II) complexes of trimethoprim, which were also explored as corrosion inhibitors. The effectiveness of corrosion inhibition was evaluated by measuring weight loss, which validated the effectiveness of the complexes for carbon steel in 1 M HCl [15]. A review of the existing literature indicated a lack of information regarding the evaluation of TMP-SMX as an inhibitor for the degradation of aluminum in HCl, particularly using integrated methodologies such as Electrochemical Impedance Spectroscopy (EIS), Potentiostatic Polarization (PDP), Monte Carlo (MC) simulations, and Molecular Dynamics (MD) simulations. Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP) play an essential role in studies focused on corrosion inhibition [16], as they yield complementary perspectives. PDP elucidates the mechanism of the inhibitor, whether anodic, cathodic, or mixed, and provides insights into corrosion rates, whereas EIS assesses the characteristics of the protective film, such as charge transfer resistance, thereby validating the film's efficacy as a barrier. The integration of both methodologies offers a comprehensive understanding of how inhibitors safeguard metals through the examination of kinetic processes and film formation. In addition, contemporary computational approaches, including DFT, MC and MD, offer in-depth molecular-level insights into the mechanisms involved in corrosion inhibition [17]. Specifically, DFT facilitates the assessment of electronic characteristics and molecular reactivity; MC simulations estimate adsorption energy and configurations; while MD simulations evaluate the stability of adsorbed inhibitor layers under varying dynamic conditions. This study employs electrochemical techniques (i.e EIS and PDP) alongside a combined DFT-MC-MD framework to elucidate the inhibition mechanisms of TMP and SMX on aluminum in hydrochloric acid.
2 Methodology
2.1 Computational analyses
DFT calculations were executed using Gaussian '09 and GaussView 6.0, employing the B3LYP/6-311 + +G(d,p) theoretical framework in accordance with the procedure reported by Oyeneyin et al. (2024) [18]. Gaussian '09 serves as a computational chemistry software utilized for performing calculations, whereas GaussView 6.0 functions as a graphical user interface that facilitates the setup of these calculations and the visualization of results.
$I = - E_{HOMO}$
$A= - E_{LUMO}$
$ΔE = E_{LUMO} -E_{HOMO}$
$\chi =\frac{I+A}{2}$
Monte Carlo (MC) and Molecular Dynamics (MD) simulations were conducted utilizing the Forcite and Adsorption Locator modules within Materials Studio 2020. The geometries of TMP and SMX were enhanced utilizing a high-precision optimization technique within the Forcite module [18]. The Al structure was designed as a 10 × 10 supercell, cleaved along the (11 1) Miller plane, and situated within a simulation box that measured 24.8 × 24.8 × 38.1 Å ³ , which incorporated a solvent layer of 30 Å. This layer comprised 280 water molecules, along with 10 H3O+ and 10Cl-, all of which were pre-optimized using the same theoretical framework. To evaluate the adsorption properties of TMP and SMX on the Al surface, MC simulations were performed on both the Al (11 1)-TMP/280H2O/10H3O+/1OCl- and Al (111)-SMX/280H2O/10H3O+/1OCl- systems. The MD simulations concentrated on optimizing the Al (111)-TMP/280H2O/10H3O+/1OCl- and Al (11 1)-SMX/280H2O/10H3O+/1OCl- systems, engaging high-precision COMPASS II simulations by means of Ewald summation while maintaining constraints on the Cartesian coordinates of the Al atoms. The MD simulations for the optimized systems, were conducted using fine-quality Forcite quenching over a simulation period of 500 picoseconds, with a time step of 1.0 femtoseconds. The temperature of the adsorbate-adsorbent system was maintained at 298 K [18]. Also, the radial distribution function (RDF) for TMP and SMX on the Al (111) exterior was examined. The Fukui function calculations were performed using Gaussian '09, GaussView 6.0, and the UCAFUKUI V 2.0 package [19].
2.2 Electrochemical investigations
The Trimethoprim-sulfamethoxazole (i.e 80 mg of trimethoprim and 400 mg of sulfamethoxazole) employed in this study was obtained from Ibadan, Oyo state and utilized without purification, while the Aluminum sheet employed in this study was obtained from Port Harcourt, Rivers state, Nigeria. Analyses were carried out in a 1 M HCl solution, identified as a corrosive electrolyte. The open circuit potential (OCP) was measured over a period of 1800 s (30 min) to facilitate the dissipation of the charging current and to establish system stability. This process is essential for the metal to dissolve at its equilibrium or free potential, which is attained at a stable OCP. Subsequently, all electrochemical investigations were performed under OCP conditions. These tests utilized a cell configuration comprising three electrodes: a 1 cm² exposed aluminum surface functioning as the working electrode (WE), a silver/silver chloride (Ag/AgCl) reference electrode (RE) for potential measurements, with all potentials reported relative to the Ag/AgCl reference, and a platinum wire serving as the counter electrode (CE). Electrochemical impedance spectroscopy (EIS) was executed with a signal amplitude perturbation of 10 mV across a frequency spectrum ranging from 100 kHz to 10 mHz [21]. The potentiodynamic polarization (PDP) study was carried out within a potential range around the OCP, employing a scanning rate of 0.2 mV s⁻¹ . The resulting data were subsequently analyzed using appropriate electrochemical analytical software; Zsimpwin 3.2 was utilized for EIS analysis with a fitted equivalent circuit tailored to the specific metal/electrolyte system, while EC-lab software was employed for the PDP analysis [20,22,23]. Rct and Icorr values were utilized to compute the inhibition efficiency, according to the equations presented in previous studies [22,23].
2.3 Weight loss analysis
The weight loss study involved immersing pre-weighed aluminum coupons in glass containers filled with 200 mL of a 1 M HCl solution, conducted under two conditions: a blank experiment without any additives and an inhibited experiment featuring TMP/SMX. A temperature-controlled water bath was utilized for both experimental setups at temperatures of 303 K, 313 K, 323 K, and 333 K. Following a 5-hour exposure period, the coupons were extracted from the corrosive solutions, thoroughly rinsed in ethanol, de-oiled using acetone, and subsequently allowed to air dry. The final mass, measured using an analytical balance, was employed to calculate the weight loss (∆w = final mass-initial mass) which was in turn utilized to compute corrosion rate, CR (i.e CR = ∆w/At where A is area of coupon and t is immersion time).
3 Results and discussion
3.1 Fukui analysis
Fukui indices serve as a tool for identifying the most reactive atomic sites within a corrosion inhibitor molecule, thereby indicating potential bonding areas with a metal surface. Analyzing these indices facilitates predictions regarding the molecule's orientation and efficacy during the adsorption process. The index fk -, which reflects the potential for electrophilic attack by the inhibitor (i.e., electron donation to the metal), indicates that a high fk - value for a particular atom k signifies a strong tendency for that site to donate electrons to the vacant orbitals of the metal surface [24]. Conversely, the index fk+, which pertains to nucleophilic attack by the inhibitor (i.e., electron acceptance from the metal), indicates that a high fk+ value for atom k implies a greater likelihood of accepting electrons from the filled D-orbitals of the metal, a process known as back-donation [24].
In the case of TMP, the five atomic sites ranked highest in their capacity to donate electrons to the vacant orbitals of the metal surface, in descending order, are C7, N11, N12, C4, and O16 (as presented in Table 1). The top five sites exhibiting the highest values of fk+, thereby demonstrating a significant propensity for electron acceptance, are identified as H39, H38, H32, H35, and H36. In the case of SMX, the leading five sites with the greatest inclination to donate electrons to the vacant orbitals of the metal surface are N12, C8, N5, C13, and C10, listed in descending order. Conversely, the top five sites showing the strongest tendency to accept electrons from the metal surface are H26, H25, H20, H21, and H19.
Table 1 Fukui analysis of TMP and SMX. |
| TMP | | SMX | | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Atom | | | | Δf(r) | Atom | | | | Δf(r) |
| C1 | -0.0035 | 0.0002 | 0.0017 | -0.0034 | C1 | -0.0092 | 0.0198 | 0.0053 | 0.0106 |
| O2 | 0.0165 | 0.0024 | 0.0095 | -0.0141 | C2 | 0.034 | 0.0046 | 0.0193 | -0.0294 |
| C3 | 0.0038 | 0.0165 | 0.0101 | 0.0127 | C3 | 0.0107 | 0.0088 | 0.0097 | -0.0019 |
| C4 | 0.0762 | 0.0041 | 0.0401 | -0.072 | C4 | 0.0014 | 0.0032 | 0.0023 | 0.0018 |
| C5 | 0.0032 | 0.0084 | 0.0058 | 0.0052 | N5 | 0.0797 | 0.0074 | 0.0436 | -0.0723 |
| C6 | -0.0257 | 0.0227 | 0.0015 | -0.003 | S6 | -0.0196 | -0.001 | 0.0103 | -0.0187 |
| C7 | 0.1333 | -0.0098 | 0.0617 | -0.1235 | O7 | 0.0513 | 0.0154 | 0.0334 | -0.0359 |
| C8 | -0.0001 | 0.0116 | 0.0057 | 0.0115 | C8 | 0.1178 | 0.0213 | 0.0695 | -0.0965 |
| N9 | 0.0382 | 0.0068 | 0.0225 | -0.0315 | C9 | 0.0017 | 0.0144 | 0.0081 | 0.0127 |
| C10 | 0.0079 | 0.0091 | 0.0085 | 0.0012 | C10 | 0.0722 | 0.0082 | 0.0402 | -0.064 |
| N11 | 0.1147 | 0.0088 | 0.0618 | -0.1059 | C11 | 0.0311 | 0.0226 | 0.0269 | -0.0085 |
| N12 | 0.0933 | 0.0212 | 0.0572 | -0.0721 | N12 | 0.1706 | 0.0353 | 0.1029 | -0.1353 |
| C13 | -0.0055 | 0.0146 | 0.0045 | 0.0091 | C13 | 0.0748 | 0.0077 | 0.0413 | -0.0672 |
| C14 | 0.0188 | 0.0105 | 0.0147 | -0.0082 | C14 | -0.0038 | 0.0133 | 0.0047 | 0.0094 |
| C15 | 0.052 | 0.004 | 0.028 | -0.048 | O15 | 0.0565 | 0.0162 | 0.0363 | -0.0403 |
| O16 | 0.0746 | 0.0061 | 0.0404 | -0.0685 | N16 | 0.0449 | 0.0001 | 0.0225 | -0.0449 |
| C17 | -0.0119 | 0.0098 | 0.001 | -0.0021 | O17 | 0.0347 | 0.005 | 0.0199 | -0.0297 |
| C18 | 0.0493 | 0.0218 | 0.0355 | -0.0275 | H18 | 0.0165 | 0.0391 | 0.0278 | 0.0226 |
| O19 | 0.0267 | 0.0013 | 0.014 | -0.0254 | H19 | 0.0164 | 0.0489 | 0.0327 | 0.0325 |
| C20 | -0.0059 | 0.024 | 0.0091 | 0.0182 | H20 | 0.0149 | 0.1627 | 0.0888 | 0.1478 |
| N21 | 0.0615 | 0.0251 | 0.0433 | -0.0364 | H21 | 0.0188 | 0.0971 | 0.0579 | 0.0782 |
| H22 | 0.018 | 0.0154 | 0.0167 | -0.0026 | H22 | 0.0233 | 0.0178 | 0.0206 | -0.0055 |
| H23 | 0.0032 | 0.0059 | 0.0045 | 0.0027 | H23 | 0.0267 | 0.01 | 0.0184 | -0.0167 |
| H24 | 0.0047 | 0.0103 | 0.0075 | 0.0056 | H24 | 0.0286 | 0.0253 | 0.027 | -0.0033 |
| H25 | 0.016 | 0.0038 | 0.0099 | -0.0122 | H25 | 0.027 | 0.1639 | 0.0954 | 0.1369 |
| H26 | 0.0226 | 0.0044 | 0.0135 | -0.0182 | H26 | 0.0276 | 0.2114 | 0.1195 | 0.1839 |
| H27 | 0.0346 | 0.0287 | 0.0316 | -0.0058 | H27 | 0.0281 | 0.013 | 0.0206 | -0.0151 |
| H28 | 0.0185 | 0.0073 | 0.0129 | -0.0112 | H28 | 0.0231 | 0.0086 | 0.0158 | -0.0145 |
| H29 | 0.0213 | 0.0084 | 0.0148 | -0.0129 | | | | | |
| H30 | 0.0157 | 0.0027 | 0.0092 | -0.013 | | | | | |
| H31 | 0.0137 | 0.0012 | 0.0075 | -0.0124 | | | | | |
| H32 | 0.0221 | 0.1319 | 0.077 | 0.1098 | | | | | |
| H33 | 0.0114 | 0.0136 | 0.0125 | 0.0022 | | | | | |
| H34 | 0.0165 | 0.0116 | 0.014 | -0.0049 | | | | | |
| H35 | 0.0193 | 0.0796 | 0.0495 | 0.0603 | | | | | |
| H36 | 0.0028 | 0.0477 | 0.0252 | 0.0449 | | | | | |
| H37 | 0.0108 | 0.0213 | 0.0161 | 0.0105 | | | | | |
| H38 | 0.0203 | 0.145 | 0.0826 | 0.1247 | | | | | |
| H39 | 0.0115 | 0.2418 | 0.1266 | 0.2304 | | | | | |
As noted by Muhsinah et al. (2024) [19], the selectivity for nucleophilic or electrophilic attacks can be forecasted through the Dual Descriptor index ∆f (r), calculated as ∆f (r) = [f+ (r) - f-(r)]. A positive value of ∆f (r) (>0) indicates that the atom is preferentially selective for nucleophilic attack, while a negative value (∆f (r) < 0) suggests a preference for electrophilic attack [19].
3.2 Monte Carlo simulation and molecular dynamics simulation
The outcomes of the Monte Carlo simulations concerning the adsorption of TMP and SMX on the Al(111) surface are presented in Table 2. Precisely, the total energy (Etot), adsorption energy (Ead), rigid adsorption energy (Erigid_ad), deformation energy (EDef), and differential adsorption energy (dEad/dNi) are presented in Table 2. The significantly negative values of Ead indicate that the adsorption process for both inhibitors is spontaneous and thermodynamically favorable [24]. Comparatively, SMX demonstrates a more pronounced negative adsorption energy than TMP, suggesting a more robust interaction with the aluminum substrate [24]. The optimized configurations of adsorption, depicted in Fig. 2, reveal that both inhibitors maintain nearly parallel orientations with respect to the aluminum surface, thereby enhancing surface coverage and boosting interactions through heteroatoms and π-electron systems. Also, the calculated deformation energies (Table 2) imply that the inhibitor molecules experience structural modifications upon adsorption, which further corroborates the strong interactions between the inhibitors and the surface.
Table 2 Energy parameters (in kcal/mol) obtained from Monte Carlo simulation of adsorption of SMX and TMP on Al (111) in HCl medium. |
| Structure | Etot | Ead | Erigid_ad | EDef | dEad/dNi |
|---|---|---|---|---|---|
| Al (111)-SMX/280H2O/10H3O+/1OCl- | -5292 | -5096 | -5151 | 55.39 | -93.08 |
| Al (111)-TMP/280H2O/10H3O+/10Cl- | -5438 | -4965 | -5156 | 191.46 | -41.09 |
Fig. 2 Side and top views of the equilibrium adsorption configurations of SMX and TMP on Al (111) surface in hydrochloric acid medium. |
Table 3 outlines the evolution of total energy across the inhibitor-metal systems observed during the molecular dynamics (MD) simulations. The energy profiles illustrate initial fluctuations followed by a phase of stabilization, which suggests the establishment of stable adsorption systems. Fig. 3 presents side and top-view images of the adsorption configurations achieved at various simulation intervals. Throughout the 500 ps simulation duration, both TMP and SMX consistently exhibit firm adsorption on the aluminum surface [25].
Table 3 Total energies in kcal/mol of Al (111)-SMX and Al (111)-SMX system in hydrochloric acid medium at total simulation times of 500 ps. |
| SMX | | TMP | |
|---|---|---|---|
| System-Simulation time (ps) | Total Energy | System-Simulation time (ps) | Total Energy |
| 0 | -5453.838 | 0 | -5658.170 |
| 5 | -5773.868 | 5 | -5925.341 |
| 125 | -5873.027 | 145 | -6081.641 |
| 305 | -5907.281 | 460 | -6081.403 |
Fig. 3 Side and top views of equilibrium adsorption configurations obtained from MD at different ps of the SMX and TMP on the Al (111) surface in HCl medium at simulation times of 500 ps. |
The Radial Distribution Function (RDF) has demonstrated its efficacy in assessing the interactions between inhibitors and the surface [26]. Typically, in the RDF graph, the emergence of a peak within the distance range of 1 - 3.5 Å from the metal surface indicates chemisorption. In contrast, when RDF peaks occur at distances exceeding 3.5 Å, this is indicative of physisorption [26]. The RDF plots for SMX and TMP are illustrated in Figs. 4 and 5, respectively. Significant RDF peaks observed at distances under 3.5 Å between the heteroatoms of the inhibitor and aluminum atoms indicate the prevalence of chemisorption over weak physisorption [26].
Fig. 4 Radial distribution function of SMX and Al (111) surface in hydrochloric acid medium at simulation time of 406 ps. |
Fig. 5 Radial distribution function of TMP and Al (111) surface in hydrochloric acid medium at simulation time of 305 ps. |
3.3 Quantum chemical descriptors
The optimized structure of SMX and TMP are presented in Fig. 6, while the quantum chemical parameters calculated for SMX and TMP are detailed in Table 4 and Table 5, respectively. Both compounds demonstrate relatively elevated HOMO energies, which imply a strong propensity for electron donation to the aluminum surface [27]. TMP possesses a marginally higher HOMO energy (-5.94 eV) in comparison to SMX (-6.29 eV), indicating a somewhat enhanced capacity for electron donation [28]. The HOMO-LUMO energy gaps for both SMX and TMP are comparably small, reflecting a high level of molecular reactivity and a pronounced inclination to engage with the aluminum surface [28]. The energy gaps observed in this analysis are lower than those reported for the most reactive inhibitor identified in a similar investigation [28]. These findings imply that both inhibitors are capable of effectively participating in charge transfer mechanisms during the adsorption process.
Fig. 6 (a) Optimized structure of SMX. (b) Optimized structure of TMP. |
Table 4 Quantum chemical parameters of SMX. |
| SMX | Energy (a.u) | ELUMO (eV) | EHOMO (eV) | ∆E (eV) | IP | EA | η | σ | χ | CP | ω |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Vacuum | -1175.93 | -1.02 | -6.29 | 5.28 | 6.29 | 1.02 | 2.64 | 0.38 | 3.66 | -3.66 | 2.53 |
Table 5 Quantum chemical parameters of TMP. |
| TMP | Energy (a.u) | ELUMO (eV) | EHOMO (eV) | ∆E (eV) | IP | EA | η | σ | χ | CP | ω |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Vacuum | -989.27 | -0.64 | -5.94 | 5.29 | 5.94 | 0.64 | 2.65 | 0.38 | 3.29 | -3.29 | 2.04 |
Fig. 7 present the frontier molecular orbitals for SMX and TMP, respectively, while Fig. 8 present electrostatic potential map of SMX and TMP. The distributions of the highest occupied molecular orbitals (HOMO) are predominantly concentrated on heteroatoms and aromatic rings, suggesting that these areas play a crucial role in electron donation. In contrast, the distributions of the lowest unoccupied molecular orbitals (LUMO) imply an aptitude for accepting electrons from the metallic surface [29,30].
Fig. 7 (a) HOMO of SMX. (b) LUMO of SMX. (c) HOMO of TMP. (d) LUMO of TMP. |
Further examination through global reactivity descriptors corroborates the inhibitory efficacy of TMP and SMX. The significantly low η and elevated σ reflect the ability of both compounds to readily modify their electronic configurations upon adsorption. Moreover, the low χ facilitate electron donation to the metal orbitals, thereby enhancing their anticorrosion properties. Specifically, TMP (χ = 3.29 eV) and SMX (χ = 3.66 eV) exhibit a tendency to donate electrons to the metal substrate [31]. Notably, the χ for TMP and SMX are lower than the corresponding χ value identified for a previously studied inhibitor with a tendency for electron donation to the metal [31]. Also, the chemical potential (CP) values of SMX (-3.66 eV) and TMP (-3.29) reinforce the findings related to electronegativity [19].
Fig. 8 (a) Electrostatic potential map of SMX. (b) Electrostatic potential map of TMP. |
3.4 Open circuit potential
The plots depicting the open circuit potential (OCP) as a function of time for aluminum in a blank 1 M HCl solution and with varying concentrations of the inhibitor are illustrated in Fig. 9. The OCP-time trajectory demonstrates that the systems achieved a stable OCP [32,33] within the first 600 s. Notably, the OCP values observed in the presence of the inhibitor surpass those recorded for the blank solution. This observation is in agreement with another study [34,35]. This significant disparity between the OCP values of the blank HCl and the inhibited systems underscores the substantial impact the inhibitor has on the electrochemical processes occurring within the corrosive environment [36,37]. In the uninhibited medium at a concentration of 0.0 g/L, the OCP profile remains relatively constant but shows a slight movement towards more negative values, ultimately stabilizing at roughly −0.69 V vs Ag/AgCl. This pattern indicates ongoing corrosion activity at the metal surface in the absence of any protective layer, with no indication of passivation. The gradual shift towards more negative potentials signifies continuous anodic dissolution.
Fig. 9 OCP analysis of aluminum for blank uninhibited system in comparison with the inhibited, tested in 1 M HCl. |
Upon introducing the inhibitor, a significant alteration in the OCP behavior is observed, highlighting its interaction with the metal/electrolyte interface [32]. At a concentration of 0.2 g/L, the potential initially shifts in the positive (less negative) direction, indicating a temporary reduction in corrosion activity attributed to the initial adsorption of inhibitor molecules. Within the first approximately 400-600 s, a swift decline towards more negative potentials is noted, followed by stabilization around −0.66 V. This pattern suggests that while the process of adsorption occurs rapidly, the protective layer formed at this concentration is insufficiently stable or compact, resulting in some degree of desorption or rearrangement of the adsorbed species. At a concentration of 0.4 g/L, a more pronounced positive shift in the OCP is observed during the initial phase, as compared to 0.2 g/L, indicating enhanced adsorption and surface coverage. The subsequent gradual decline in potential over an extended period (approximately 1200s) before achieving a quasi-steady state indicates a continuous rearrangement of inhibitor molecules on the surface, ultimately leading to the formation of a more stable and cohesive protective film. This also clearly suggest that time has a substantial influence on steady-state potential [32]. The extended time required to reach stabilization suggests stronger interactions between the inhibitor and the metal surface. Conversely, at the highest concentration of 0.6 g/L, the OCP curve displays relative stability throughout the immersion period, characterized by only minor fluctuations around −0.65 V. This behavior indicates the swift formation of a consistent and stable inhibitor layer that effectively protects the metal from corrosive elements. The minimal fluctuations in potential suggest that the dynamics of adsorption and desorption are predominantly suppressed, facilitating a rapid attainment of equilibrium within the system. Therefore, the introduction of the inhibitor leads to a modest shift in the OCP values toward more positive potentials when compared to the blank solution, although this shift remains under 85 mV which is not enough to classify TMP-SMX as cathodic or anodic [38]. Such findings imply that the inhibitor acts as a mixed-type inhibitor, affecting both the anodic dissolution of the metal and the cathodic evolution of hydrogen [39], all while preserving the integrity of the fundamental corrosion mechanism.
3.5 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) plays a vital role in the study of corrosion inhibition, as it elucidates the mechanisms by which inhibitors function by examining the metal-electrolyte interface. Fig. 10 presents the Nyquist plots for aluminum submerged in both inhibited and control solutions, while the electrical equivalent circuit (EEC) utilized for the analysis of the EIS data is depicted in Fig. 11. The EEC encompasses solution resistance (Rs), pore resistance (Rpo), charge transfer resistance (Rct), and a constant phase element (CPE) to represent the effects of double-layer capacitance [22]. The analysis identified two non-ideal capacitances, designated as CPE1 and CPE2. Rs represents the resistance faced by the electrolyte solution, while Rct reflects the resistance associated with the specific electrochemical reaction at the electrode interface. In this investigation, it is observed that the Rs values exhibit only negligible fluctuations, while both the Rct and Rpo values (as shown in Table 6) demonstrate an increase in the presence of TMP-SMX. This finding indicates a potential blockage of certain active sites and a reduction in Rct. The Nyquist plots, illustrated in Fig. 10, exhibit a single semicircular capacitive loop, both in the presence and absence of TMP-SMX [29]. Moreover, the plots indicates that the diameter of the recorded semicircles expands when the inhibitor was incorporated and the observation is consistent with other studies [40,41]. This observation suggests an increase in charge transfer resistance, as detailed in Table 6, when the inhibitor is present. This rise, particularly regarding charge transfer resistance Rct, serves as a crucial indicator of successful corrosion inhibition [21]. The two n values represent homogeneity factors for CPE, quantifying surface roughness, non-ideality, or porous film properties. The n values increased upon introducing TMP-SMX and the increase is consistent with increase in TMP-SMX dosage, suggesting that TMP-SMX has an influence on electrochemical behavior of Aluminum in HCl. Ranging from 0 to 1, n = 1 suggests an ideal capacitor, while lower values reflect surface heterogeneity. Values of n obtained in this investigation are less than 1 [22].
Fig. 10 Nyquist plot of aluminum for blank uninhibited system in comparison with the inhibited, tested in 1 M HCl. |
Fig. 11 Electrochemical equivalent circuit employed in the study. |
Table 6 EIS data relating to aluminum for blank uninhibited system in comparison with the inhibited, tested in 1 M HCl. |
| System | Rs (Ω. cm2) | Rpo (Ω. cm2) | Q1 × 10−5 (μΩ−1 sn cm−2) | n1 | Rct (Ω cm2) | Q2 × 10−5 (μΩ−1 sn cm−2) | n1 | θ | IE % |
|---|---|---|---|---|---|---|---|---|---|
| 0.0 g/L (Blank) | 14.51 | 20.0 | 121.8 | 0.88 | 220.0 | 23.15 | 0.86 | - | - |
| 0.2 g/L | 15.11 | 50.0 | 87.8 | 0.89 | 400.0 | 17.31 | 0.88 | 0.450 | 45.0 |
| 0.4 g/L | 16.10 | 70.0 | 72.3 | 0.94 | 610.0 | 16.19 | 0.89 | 0.639 | 63.9 |
| 0.6 g/L | 17.21 | 60.0 | 61.7 | 0.98 | 490.0 | 12.37 | 0.96 | 0.551 | 55.1 |
The semicircular arcs observed in the Nyquist plots exhibited imperfections, which are believed to stem from frequency dispersion induced by the surface roughness or inhomogeneity of the aluminum [29,42]. Nevertheless, it is evident that the semicircular shapes obtained both with and without TMP-SMX are comparable. The pronounced similarities in the impedance plots of aluminum recorded in both blank 1 M HCl and in the presence of the inhibitor strongly suggest that TMP-SMX did not change the corrosion mechanism of aluminum in this aggressive environment [42].
In addition, Fig. 12 illustrates the Bode plots that depict the impact of increasing inhibitor concentration on the corrosion of aluminum in 1 M HCl. These plots demonstrate an increased area under the curves when the inhibitor is present, in contrast to the absence of the inhibitor [42]. The impedance modulus is more enhanced at higher dosages of TMP-SMX [43]. Also, there is a significant rise in the impedance modulus at low frequencies [43], where the Rct is predominant. The observed increase in Rct values in the presence of the inhibitor may be attributed to the adsorption of inhibitor molecules onto the surface of aluminum, which leads to enhanced surface coverage.
Fig. 12 Bode plots (a) modulus and (b) phase angle for EIS analysis of aluminum for blank uninhibited system in comparison with the inhibited, tested in 1 M HCl corrosive environment at room temperature. |
3.6 Potentiodynamic polarization
Potentiodynamic polarization (PDP) plays a vital role in corrosion research, enabling rapid evaluation of inhibitor performance and facilitating the understanding of corrosion kinetics, encompassing rates and mechanisms, by analyzing variations in current in relation to applied potential. This method elucidates active, and passive behaviors, while also supplying essential parameters such as corrosion current density and corrosion potential.
Fig. 13 illustrate the PDP curves and the corresponding data, respectively. In the absence of the inhibitor, the corrosion current density (Icorr) was significantly high, signifying a swift dissolution rate of aluminum [44,45]. In contrast, with the application of the inhibitor, Icorr demonstrated a substantial decrease. Specifically, Icorr dropped from 1776 µA/cm² to 941 µA/cm², corresponding to an IE of 47% (Table 7). The Icorr values further decreased to 564 µA/cm² when the concentration of the inhibitor was increased to 0.4 g/L, which corresponds to an efficiency of 68.2%. This pronounced decrease in Icorr could be due to adsorption of TMP-SMX on the exterior of the metal [17]. The reduction observed in both anodic (βa) and cathodic (βc) Tafel slopes within the inhibited systems suggests that the inhibitor molecules impede the processes of metal oxidation and hydrogen ion reduction [46]. Also, the corrosion potential (Ecorr) experienced a slight shift of less than 85 mV. Such a minor shift indicates that the inhibitor acts as a mixed-type inhibitor [47], effectively reducing both anodic and cathodic reactions [36,42].
Fig. 13 Potentiodynamic polarization (PDP) analysis of aluminium for blank uninhibited system in comparison with the inhibited, tested in 1 M HCl corrosive environment at room temperature. |
Table 7 PDP parameters relating to aluminum for blank uninhibited system in comparison with the inhibited, tested in 1 M HCl. |
| System | Ecorr | Icorr | βa | βc | θ | IE |
|---|---|---|---|---|---|---|
| mV/Ag/AgCl | μAcm−2 | mVdec−1 | mVdec−1 | | % | |
| 0.0 g/L (Blank) | -740 | 1776 | 75 | 101 | - | - |
| 0.2 g/L | -710 | 941 | 62 | 91 | 0.470 | 47.0 |
| 0.4 g/L | -720 | 564 | 54 | 87 | 0.682 | 68.2 |
| 0.6 g/L | -721 | 672 | 48 | 74 | 0.622 | 62.2 |
In a related investigation, Abdullah et al. (2022) [14] assessed the effectiveness of expired Metheprim, comprising 40 mg of Trimethoprim and 200 mg of Sulfamethoxazole, as a corrosion inhibitor for aluminum in a 1 M HCl solution. The trends in the results of the present study align closely with those reported by Abdullah et al. (2022) [14]. Both studies identified a deviation from the expected pattern where inhibition efficiency typically increases with higher inhibitor dosages. Specifically, in the electrochemical analysis, the inhibition efficiency was observed to rise with increasing concentrations of the inhibitor for certain dosages, followed by a decline at elevated inhibitor concentration. For instance, in our findings, IE improved from 47% at a concentration of 0.2 g/L of inhibitor to 68.2% at 0.4 g/L of inhibitor; however, it subsequently decreased to 62.2% at 0.6 g/L of inhibitor. Similarly, Abdullah et al. (2022) recorded an increase in IE from 42.05% with a 0.1 M inhibitor concentration to 59.26% at 0.2 M, which was followed by a reduction to 53.13% with a 0.3 M inhibitor dosage, all at the same temperature [14].
3.7 Effect of temperature
At various temperatures (303 K, 313 K, 323 K, and 333 K), the effectiveness of TMP/SMX as a corrosion inhibitor for aluminum in 1 M HCl was evaluated using different concentrations of the inhibitor (0.2 M, 0.3 M, 0.4 M, and 0.5 M) through weight loss analysis. Throughout all tested temperatures, the introduction of TMP/SMX at varying concentrations led to a notable decline in the corrosion rate. It was observed that the corrosion rate (CR) increased with rising temperature; however, it decreased with higher inhibitor concentrations, as illustrated in Fig. 14. This observed CR increase with temperature, alongside a decrease in IE, may be attributed to potential desorption of the inhibitor molecules that were already adsorbed. In addition, the significant reduction in aluminum corrosion in 1 M HCl observed with TMP/SMX, as well as the enhanced efficiency at greater concentrations, may result from an increased number of inhibitor molecules adsorbing onto the aluminum surface [4,23].
Fig. 14 Variation of CR with TMP/SMX dosage at different temperatures. |
3.8 Adsorption mechanism and thermodynamic parameters
Adsorption isotherms within corrosion research serve to illustrate the adhesion of corrosion inhibitors to metallic surfaces, elucidating the underlying mechanisms of adsorption (physisorption or chemisorption) and their effectiveness by correlating experimental data (such as weight loss) with theoretical models including Langmuir, Temkin, and Freundlich, (Eqs. 8-10 respectively) [4]. This analysis aids in the assessment of surface coverage and thermodynamic parameters, such as Gibbs free energy (∆G). The Langmuir plots (Fig. 15) demonstrated a strong correlation indicated by high values of the correlation coefficient (R), supporting the conclusion that the adsorption of TMP-SMX conforms to the Langmuir model. Temkin (Fig. 16) and Freundlich (Fig. 17) plots showed less correlation. The ∆G⁰ values (Table 8) obtained from Eq. 11 (where R is universal gas constant and T is absolute temperature) were consistently negative and fell within the range indicative of physical adsorption, suggesting that TMP/SMX spontaneously adheres to aluminum via physisorption. Figs. 16 and 17
$\frac{\mathrm{C}}{\mathrm{\theta }}=\mathrm{C}+\frac{1}{\mathrm{K}}$
$\theta =-\frac{1}{2a}\mathrm{l}\mathrm{n}C-\frac{1}{2a}\mathrm{l}\mathrm{n}K$
Fig. 15 Langmuir plots for Al in bare 1 M HCl and inhibited medium. |
Fig. 16 Temkin plots for Al in bare 1 M HCl and inhibited medium. |
Fig. 17 Freundlich plots for Al in bare 1 M HCl and inhibited medium. |
Moreover, the activation enthalpy (ΔH*) and the activation entropy (ΔS*) were calculated using the slopes and intercepts derived from the transition state plot (Fig. 18) in accordance with Eq. 12, where N represents Avogadro’s number (6.0225 × 10²³ mol⁻¹), and h denotes Planck’s constant (6.6261 × 10⁻³⁴ Js), with T indicating the absolute temperature, CR denotes corrosion rate, while ΔH* refers to the enthalpy of activation and ΔS* signifies the entropy of activation. The observed positive values of ΔH* suggest that the process of metal dissolution is endothermic. In addition, the observed increase in ΔH* correlating with higher concentrations of the inhibitor indicates that the decrease in the corrosion rate of aluminum is predominantly affected by kinetic activation parameters [10]. Elevated ΔH values imply a more robust interaction between TMP/SMX molecules and the aluminum surface. In addition, changes in entropy indicate a rise in order within the system during the corrosion mitigation process facilitated by TMP/SMX. Table 8
$\mathrm{l}\mathrm{n}\left(\frac{CR}{T}\right)=\left(\mathrm{l}\mathrm{n}\left(\frac{R}{Nh}\right)+\frac{∆S}{R}\right)-\frac{∆H}{RT}$
Fig. 18 Transition state plots for Al in bare 1 M HCl and inhibited medium. |
Table 8 ∆G and other parameters determined from Langmuir model. |
| Isotherm | Temperature (K) | R | K | ∆G0 (kJ/mol) |
|---|---|---|---|---|
| Langmuir | | | | |
| | 303 | 0.93109 | 4 | -13.61 |
| | 313 | 0.97564 | 5.26 | -14.77 |
| | 323 | 0.96902 | 4.35 | -14.73 |
| | 333 | 0.95457 | 5.26 | -15.72 |
| | 343 | 0.98906 | 6.67 | -16.86 |
3.9 Energy of activation
In terms of activation energy (Ea), Eq. (13), which represents the Arrhenius equation [48,49], was employed to determine the Ea by plotting In(CR) versus 1/T (Fig. 19). An increase in the Ea associated with the corrosion process in the presence of TMP/SMX as shown in Table 9 suggests the formation of a protective barrier on the aluminum surface following the introduction of TMP/SMX [10], thereby hindering the corrosion process, particularly as Ea rises with the increased dosage of TMP/SMX.
$\mathrm{I}\mathrm{n}\left(CR\right)=\mathrm{I}\mathrm{n}\mathrm{A}-\frac{E\mathrm{a}}{RT}$
Fig. 19 Arrhenius plots for Al in bare 1 M HCl and inhibited medium. |
Table 9 Table Enthalpy of activation, entropy of activation, and activation energy values. |
| Transition state plot | ∆S (kJ/mol.K) | ∆H (kJ/mol) | Ea (kJ/mol) |
|---|---|---|---|
| Blank | -0. 1455 | 27.69 | 30.10 |
| 0.2 | -0.13635 | 32.26 | 35.00 |
| 0.3 | -0.13460 | 33.00 | 35.67 |
| 0.4 | -0.12745 | 35.42 | 37.33 |
| 0.5 | -0.9761 | 45.56 | 48.22 |
| 0.6 | -0.10775 | 42.48 | 45.39 |
4 Conclusions
The impact of Trimethoprim-Sulfamethoxazole on aluminum corrosion in hydrochloric acid was examined using both electrochemical and computational techniques. TMP-SMX simultaneously reduced both anodic and cathodic reactions on Aluminum surface. The charge transfer resistance increased significantly in the presence of TMP-SMX, whereas the corrosion current density saw a marked reduction. Computational analyses revealed strong interactions between TMP-SMX and the aluminum substrate, aligning with the experimental outcomes. The optimized configurations of adsorption, revealed that TMP and SMX maintain nearly parallel orientations with respect to the aluminum surface, thereby enhancing surface coverage and boosting interactions through heteroatoms and π-electron systems. As environmental regulations become stricter, the pharmaceutical-based inhibitor could be a friendly alternative to traditional inhibitors. Therefore, TMP-SMX holds promise as an environmentally benign corrosion inhibitor for aluminum in HCl environment.
Future studies should include surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) to better understand the interaction mechanisms between the inhibitor and aluminum. In addition, exploring the TMP-SMX’s effects on various metals and alloys in different media could be valuable.
Declaration of Competing Interest
The authors declare that they have no known competing interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Nnenna Winifred Odozi: Writing - review & editing, Supervision, Software, Conceptualization. Msenhemba Moses Mchihi: Writing - review & editing, Writing - original draft, Supervision, Data curation, Conceptualization. Ojo Abdullah Olasunkanmi: Project administration, Investigation. David Abujah: Visualization, Software.